Journal of Undergraduate Research - The Journal of Undergraduate

Transcription

Journal of Undergraduate
Research
A Refereed Journal for Undergraduate Research in the Pure & Applied Sciences,
Mathematics, and Engineering
July 2015
Editor: Robert F. Klie
http://jur.phy.uic.edu/
Volume 8
Number 1
On the Cover (from left to right):
1. Illustration of Concentrated Solar Power plant layout
consisting of a solar field, thermal electric storgae, and
power block subcomponents. (see E. Flores, page 28);
2. A92V ftFabI with residue 92 as valine. (see N. Silas et
al., page 4);
3. Photo of traditional electrical meters. (see A. Gilbert,
page 22).
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University of Illinois at Chicago
Journal of Undergraduate Research 8 (2015)
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Journal of Undergraduate Research
A refereed journal for undergraduate research in the pure & applied sciences, mathematics and engineering.
Founding Editor: Robert F. Klie
Department of Physics
845 W Taylor Street, M/C 273
Chicago, IL 60607
email: [email protected]
312-996-6064
The journal can be found online at: http ://jur.phy.uic.edu/
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Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
E. Kocs and G.W. Crabtree
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Triclosan Resistance in Francisella tularensis: A Site-Directed Mutagenesis Study of the FabI Enzyme . . . . . . . . . . . . . . . . . 1
N.Silas, R.Demissie, and L.W.M. Fung
Startup energy storage technologies for the building level: A review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
J. Choi
Onsite Energy Storage in Todays Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
B. Loeding
Piezoelectric Energy Harvesting for Powering Micro Electromechanical Systems (MEMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
A. Majeed
The Smart Grid and its Effects on Utilities, Consumers and Energy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
A. Gilbert
A Review of Latent Heat Thermal Energy Storage for Concentrated Solar Plants on the Grid . . . . . . . . . . . . . . . . . . . . . . . . . 28
E. Flores
Automotive Lithium-Ion Batteries: Dangers and Safety Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
E. Lee
Storage and Generation for Clean Renewable Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
J. Barrett
Energy Storage Methods - Superconducting Magnetic Energy Storage - A Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
R.V. Holla
Hydrogen Storage Technologies for Transportation Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
R. Todorovic
Microgrids with Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
T. Nelson
Electricity Deregulation: California and Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
T. Mlynarski
An Analysis of Current Battery Technology and Electric Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
R. Sprague
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Introduction
Dear Colleagues,
Welcome to the eighth edition of the Journal of Undergraduate Research at the University of Illinois at
Chicago, largely devoted to papers for the LAS 493 course on Energy Storage for the Grid and Transportation
in Fall 2014. The class consisted of mostly graduate students, with a few advanced undergraduates as well,
surveying the motivations, challenges and opportunities for storing energy for the electricity grid and electric
cars. The course focused on next generation uses of electricity storage, going well beyond the personal
electronics, such as cell phones, music players, camcorders, laptops and tablets that dominate battery use
today. Currently, transportation and the grid account for 67% of all energy consumed in the United States,
compared to only 2% for personal electronics. In energy terms, a fraction of the energy used in the grid and
transportation represents an order of magnitude increase over the present market for personal electronics.
The batteries needed for the grid and transportation are likely to be qualitatively different from the lithiumion batteries now in widespread use. Although lithium-ion batteries increase in performance and decrease
in cost by 5%-10% per year, they cannot yield the transformational advances in cost and performance
needed for widespread deployment of electric cars and of electricity storage on the grid. The papers in this
volume review and explore various topics and technologies that could emerge to meet these needs, and the
game-changing impact these technologies may have on the way we think about transportation and electricity.
Many of the papers were inspired by the lectures presented in the class, given by electricity storage experts
drawn from the University of Illinois at Chicago, the Joint Center for Energy Storage Research (JCESR),
the battery research programs at Argonne National Laboratory, Illinois Institute of Technology and other
universities and companies, including General Motors. The lectures covered beyond-lithium-ion battery
technology, electrochemical capacitors, solar and wind generation, electric cars of the future, long-distance
electricity transmission, distributed energy in microgrids, and the electricity regulatory environment. Nextgeneration electricity storage is a rich emerging area, ripe with promising science and technology and with
transformational impact. This volume of the Journal of Undergraduate Research begins to divulge some of
its secrets.
Elizabeth Kocs
George Crabtree
Argonne National Laboratory
July 2015
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Acknowledgements
We would like to acknowledge support for this publication from the National Science Foundation (Grant No.
DMR-1408427). Also, we would like to thank Ayesha Riaz and Evan Lussier for their help with typesetting
this issue of the Journal of Undergraduate Research, and Evan for his help in maintaining the journal’s
website.
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Journal of Undergraduate Research 7, 1 (2015)
Triclosan Resistance in Francisella tularensis: A Site-Directed Mutagenesis Study of
the FabI Enzyme
Nicholas Silas, Robel Demissie, and Leslie W.-M. Fung
Department of Chemistry, University of Illinois at Chicago, Chicago, IL
An NADH-dependent enoyl-acyl carrier protein reductase, FabI, catalyzes the final step of bacterial fatty acid biosynthesis, reducing the double bond of trans-2-enoyl-ACP to a single bond forming
acyl-ACP. Given its importance in bacterial fatty acid synthesis, FabI has become a recognized drug
target. Triclosan, a diphenyl ether, targets the FabI, inhibits its activity, and stops bacterial growth.
However, as a consequence of triclosan’s popularity, and thus its overuse, bacterial resistance to triclosan has been reported. The mutation G93V in Escherichia coli (E. coli) FabI allows E. coli to
resist the action of triclosan. We have identified the equivalent residue of G93 in Francisella tularensis FabI (ftFabI) as A92, and prepared a mutant A92V. E. coli cells, transformed with a plasmid
containing the ftFabI-A92V gene, were grown, and the gene was overexpressed. From two growths
(6 G of cells), 62 mG of protein, with a histidine tag, at a purity of 85% were obtained. Enzymatic
activity was assayed by monitoring the absorbance of NADH at 340 nm. In the presence of triclosan,
the wild-type protein was almost completely inhibited after NADH was converted to NAD+ in the
enzymatic reaction; however the A92V mutant exhibited similar activity with and without triclosan,
demonstrating that triclosan resistance may also develop in Francisella tularensis.
Introduction
Francisella tularensis (F. tularensis or ft) is a gramnegative facultative intracellular bacterium and causes
Tularemia in infected hosts. Many cell types can be infected by F. tularensis, yet infection most commonly occurs in macrophages. F. tularensis has a very low infectious dose and can be potentially life threatening without
proper treatment. Thus, F. tularensis is categorized by
the United States Centers for Disease Control as a “Category A Bioterrorism Agent”.1 It was even developed as
a standardized biological weapon in the United States
biological weapons program.2
F. tularensis synthesizes its fatty acids with the FASII
pathway, which differs from the eukaryotic pathway,
FASI, used by humans.3 The FASII pathway uses several small proteins, each individually coded by discrete
genes, while the FASI pathway utilizes a single large protein. Thus, enzymes in the FASII pathway are potential
antibacterial drug targets.4 One of these enzymes, FabI,
catalyzes the final step in the FASII pathway in many
bacteria to reduce trans-2-enoyl-ACP to acyl-ACP.5
The commonly used antimicrobial agent triclosan used,
for example, in many hand soaps, has been found to
inhibit the FabI enzyme. After NADH is converted to
NAD+ , triclosan and NAD+ bind to the enoyl substrate
site with high affinity to stop FabI from further acting
on its substrate, enoyl-CoA.6
Triclosan resistant mutations in E. coli and a few other
bacterial species have been reported.7 This is a particular
concern as triclosan resistance has been noted to cause
cross-resistance to other antibiotics.8 One such mutation,
G93V, in E. coli was found to exhibit resistance to triclosan, with a 95-fold increase in the minimum required
inhibitory concentration of triclosan as compared to the
wild-type.9 Due to the structural similarities between E.
coli FabI and F. tularensis FabI (ftFabI),10 a corresponding mutation in F. tularensis may also lead to triclosan
resistance. We have carried out studies on the triclosan
effect in ftFabI wild-type and a mutant A92V.
Methods
Sequence Alignment
Sequence alignment was carried out with ClustalW2.11
Structural Analysis
Preliminary structural analysis was carried out using
the PyMOL Molecular Graphics System.12 The 3D structure of ftFabI12 (PDB code: 3NRC) with bound triclosan
and NAD+ was examined, noting relative distances between triclosan and the FabI active site residues. Using
PyMOL software, a mutation was carried out at residue
92 to replace A with V to give A92V. The specific side
chain rotamer used for valine was the one with the highest percentage occurrence in proteins, specifically with
75.8% occurrence in ftFabI.
Protein Preparation
BL21+ E. coli cells were transformed with a pET-15b
vector containing either the wild-type or the ftFabI-A92V
gene and an N-terminal His-Tag (a gift from the Center of Pharmaceutical Biotechnology at the UIC). Cells
were grown in Lysogeny Broth (2L) containing ampicillin. When the optical density at 600 nm (OD-600) was
between 0.5 - 0.7, the cells were induced with 0.5 mM
isopropyl-β-D-1-thiogalactopyranoside (IPTG) to overexpress the gene. Cells were grown until the OD-600
value reached about 1.3 (about 4 more hours), harvested
and stored at -20 ◦ C, if not processed immediately.
Protein was extracted following standard methods.13
Phosphate buffer (50 mM) at pH 8 with 300 mM NaCl,
10 mM imidazole and 1% Triton-X 100 was used as the
lysis buffer. Cells were suspended in the lysis buffer and
sonicated. Similar buffers, except with 10 mM, 20 mM
and 250 mM imidazole, but without triton, were also
prepared and used for column loading, washing and elution, respectively. The sonicated mixture was centrifuged
at 34,541 g for 30 minutes, and then the supernatant
was loaded onto a Ni-affinity column equilibrated with
the loading buffer. Subsequently, washing and elution
buffers were used. The washing buffer served to elute
non-specific proteins not bound to the resins in the column. The elution buffer served to elute the His-tagged
protein (ftFabI) bound to the resin. The protein was
collected in time-controlled fractions. The 280 nm absorbance of each fraction was measured and an elution
profile was obtained. Fractions with high absorbance
readings were combined and dialyzed overnight in a 50
mM Tris buffer at pH 8 with 100 mM NaCl, and 1 mM
DTT. The purity of the samples (the wild-type and the
mutant A92V ftFabI) was analyzed for purity by 16%
polyacrylamide gel electrophoresis (PAGE).13
FIG. 1: Alignment of the protein sequences of E. coli
FabI, residues 1-120, and ftFabI, residues 1-119,
indicating the equivalent residue of G93 in E. coli FabI
is A92 of ftFabI. Residue 93/92 is indicated with a blue
arrow.
activity of ftFabI is calculated using units of NADH instead as shown below:
Specific Activity =
AV
t εl
m
(1)
where At is the absolute value of change in absorbance
per min, V is the assay volume (100 µL), ε is the extinction coefficient of NADH (4660 M−1 cm−1 ), l is the path
length, or the depth of the well with 100 µL liquid (0.291
cm), and m is the amount (mG) of enzyme. The unit for
specific activity is (µmol NAD/min)/mG FabI.
Results
Sequence Alignment
Specific Activity Assay
Sequence alignment indicated A92 in ftFabI corresponds to G93 in E. coli FabI (Figure 1). The sequence of
E. coli FabI from residue 90 to 96 is HSIGFAP, while the
sequence of ftFabI from residue 89 to 95 is HSIAFAP. We
have carried out studies on the triclosan effect in ftFabI
wild-type and A92V mutant.
Enzyme activity was carried out using a plate reader
(Model: Victor3 V; Perkin Elmer) by monitoring NADH
absorbance at 340 nm (A340 ).14 NADH absorbs at 340
nm, while its oxidized form NAD+ does not. The
decrease in absorbance over time therefore indicates a
change from NADH to NAD+ as the reaction proceeds.
NADH solution, ftFabI (wild-type or mutant A92V), and
assay buffer (50 mM Tris at pH 8 with 100 mM NaCl)
were prepared to make the final concentrations of NADH
and ftFabI in the reaction mixture 200 µM and 1 µM, respectively. Samples containing 5 µM triclosan dissolved
in DMSO (approximately 2 µL triclosan in DMSO per
200 µL to create a 5:1 molar ratio of triclosan to FabI
in the reaction mixture) were also prepared. To account
for changes in absorbance due to the addition of DMSO,
2 µL DMSO was added into the control (no triclosan).
However, it was found that the addition of DMSO had
a negligible effect on A340 . The reaction mixture (98 µL
was added to the wells of a 96 well plate and A340 was
monitored over approximately 3 minutes to determine a
baseline absorbance reading. 2 µL of 10 mM crotonylCoA (an analog of enoyl-CoA) was added to the reaction
mixture. The A340 values were then measured for 14
minutes.
Specific activity was calculated in enzyme units per
mG enzyme. However, since the rate of NAD+ generation equals the rate of crotonyl-CoA use, the specific
Protein Characterization
The elution profile of the affinity column of the over
expressed, N-terminal His-tagged wild-type ftFabI fusion
protein is shown in Figure 2.
The protein was observed on a 16% SDS gel. The
molecular mass, from sequence, of wild-type ftFabI with
the His-Tag is 29,968.5 Da, and for ftFabI-A92V with
the His-Tag is 29,996.5 Da. A 30,000 Da standard band
(Figure 3 a and b), Lane 1) was used to identify ftFabI.
The intensity of this band represents the purity of the
protein in the sample. Both wild-type ftFabI (Figure 3
a), Lane 3) and ftFabI-A92V fusion proteins (Figure 3b),
Lanes 2 and 3) were obtained at a purity of about 85%.
Activity Assays
Wild-type ftFabI samples with and without triclosan
were prepared and run in parallel (Figure 4). As the reac2
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FIG. 2: ftFabI-A92V column elution profile after
addition of elution buffer. About 80 mL of eluate was
collected to be dialyzed and concentrated ranging from
60 mL and 140 mL of original eluate, having an average
absorbance of 1.30 at 280 nm.
FIG. 4: Wild-type activity assays with (red) and
without (black) triclosan monitoring absorbance at 340
nm. The first 20 readings (from 0 to about 3 minutes)
were taken with no substrate to generate an absorbance
baseline. Substrate was added, and additional 99
measurements were taken.
FIG. 3: 16% polyacrylamide gel with low molecular
weight standards (LMWS) in Lane 1. (a, left)
Wild-type cell lysate (Lane 2), wild-type ftFabI (Lane
3). (b, right) ftFabI-A92V (Lanes 2 and 3), A92V cell
lysate (Lane 4). Wild-type and A92V both showed a
purity of about 85%.
FIG. 5: ftFabI-A92V activity assays with (red) and
without (black) triclosan monitoring absorbance at 340
nm. The first 20 readings (from about 0-3 minutes)
were taken with no substrate to generate an absorbance
baseline. Substrate was added, and additional 99
measurements were taken.
tion proceeded, the specific activity of the sample without
triclosan decreased due to limiting substrate concentrations. The specific activity of the sample with triclosan
had a very similar initial activity to the sample with no
triclosan; however, the specific activity of this sample
decreased much more rapidly. This is due to the generation of NAD+, through oxidation of NADH, as the
reaction proceeded. This generation of NAD+ allowed
triclosan to complex with the NAD+ which inhibited the
enzyme ftFabI (Figure 4). The specific activities taken
at 1 minute intervals of both wild-type samples with and
without triclosan are shown in Table I.
The results for ftFabI-A92V are shown in Figure 5 and
Table II.
FIG. 6: The change in specific activity for wild-type
ftFabI and A92V ftFabI, with and without 5 µM
triclosan. Each point represents the activity value
during that time interval. For example, for time = 3
min, the slope from 3 to 4 min was used to calculate the
specific activity for each sample. See Tables I and II.
Both Figures 4 and 5 show that the change in the rate
of NADH disappearance, or in substrate turnover, decreased steadily as a function of time in the wild-type
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TABLE I: Summary of the wild-type assays, with and
without triclosan, including the slope and specific
activities for each 1 minute time interval. Both assays
showed a decrease in activity through the duration of
the assays; however, the decrease was greater in the
assay with triclosan. The decrease in activity in the
assay without triclosan can be attributed to
consumption of NADH. The decrease in activity for the
assay with triclosan can be attributed to triclosan
inhibition as NAD+ gets generated.
Specific Activity of wild-type FabI
without Triclosan
Time Slope
Spec. Activity ([µmol
FIG. 7: Wild-type ftFabI with residue 92 as alanine.12
with 5 µM Triclosan
Slope
(min) (Abs/ NADH/min]/mG FabI) (Abs/ NADH/min]/mG FabI)
min)
min)
3-4
0.028
0.678
0.026
0.632
4-5
0.031
0.753
0.007
0.169
5-6
0.024
0.596
0.002
0.062
6-7
0.019
0.465
0.001
0.016
7-8
0.016
0.384
0.001
0.036
8-9
0.012
0.305
0.001
0.022
9-10
0.010
0.244
0.001
0.014
TABLE II: Summary of the ftFabI-A92V trials with
and without triclosan. Specific activities were
calculated from the slopes at each 1 minute interval.
For both samples (with and without triclosan) the
specific activates at each interval varied slightly;
however, they were generally similar.
FIG. 8: A92V ftFabI with residue 92 as valine.
Specific Activity of A92V FabI
without Triclosan
Time Slope
with 5 µM Triclosan
Slope
while that for the ftFabI-A92V was about 0.3 [µmol NAD
/min]/mG FabI. The specific activity leveled off to about
0.2 [µmol NAD /min]/mG FabI for both the wild-type
and ftFabI-A92V. However, in the presence of triclosan,
the specific activity for wild-type leveled off to about 0.02
[µmol NAD/min]/mG FabIwhile the ftFabI-A92V leveled
off to about 0.15 [µmol NAD/min]/mG FabI. Thus, in the
presence of triclosan, ftFabI-A92V showed about 7-fold
more activity than that of the wild-type.
(min) (Abs/ NADH/min]/mG FabI) (Abs/ NADH/min]/mG FabI)
min)
min)
3-4
0.012
0.284
0.008
0.203
4-5
0.007
0.183
0.008
0.193
5-6
0.009
0.224
0.007
0.161
6-7
0.008
0.189
0.007
0.165
7-8
0.006
0.154
0.006
0.150
8-9
0.007
0.165
0.006
0.154
9-10
0.007
0.181
0.005
0.131
sample (Figure 6). However, in the presence of five-fold
molar excess triclosan, the change in the rate dropped
dramatically, and leveled off quickly, suggesting progressive inhibition by triclosan during the initial period, after sufficient amounts of NAD+ species appeared. In
contrast, the A92V mutant maintains a steady turnover
rate throughout the course of the assay, similar in magnitude in both the absence and presence of triclosan, indicating that the A92V mutant catalyzes the oxidation of
NADH at 40% of the rate of the wild-type, and it resisted
triclosan-NAD+ binding by acting at that approximate
rate in the presence of triclosan.
In comparison, the wild-type exhibited an initial specific activity of about 0.7 [µmol NADH/min]/mG FabI,
Structural Interpretation
We hypothesize that the resistance mechanism stems
from increased steric hindrance from residue Val-92. Triclosan binds the enoyl substrate site at a distance of 3.8
Å away from Ala-92 (Figure 7). However, for triclosan to
be in its binding site with valine as residue 92, it would
have to be 2.5 Å away from Val-92 (Figure 8). This is not
possible since the sum of the van der Waal radii of the 2chloro of the 2,4-dichlorophenoxy group of triclosan and
the 3-carbon of valine is larger than 2.5 Å (approximately
3.6 Å), demonstrating that a valine should prevent successful triclosan binding and inhibition.
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tinue to proliferate. This study provides further evidence
towards the possibility of homologous mutations evolving
in other bacterial species, leading to similar resistances.
This study also provides information on how to design
the next generation of antibiotics to overcome the mutations effects.15
Conclusion
The ftFabI-A92V mutant showed reduced activity as
compared to the ftFabI wild-type. This is probably
due to the bulkier valine residue affecting the binding of
crotonyl-coA. However, this reduced activity was maintained in the presence of triclosan, allowing the bacterial
cells to resist triclosan. These results suggest the potential for growth in the presence of triclosan for a strain of
Francisella tularensis harboring the ftFabI-A92V.
Another central implication resulting from this study
is the importance of developing new antimicrobials.
Through overuse, it is generally known that microbes
harboring resistant mutations get selected for and con-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Acknowledgements
The authors would like to acknowledge S. Mehboob
and M.E. Johnson at the Center of Pharmaceutical
Biotechnology at the University of Illinois at Chicago for
the wild type plasmid.
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Startup energy storage technologies for the building level: A review
Joanne Choi
University of Illinois at Chicago, Chicago, IL 60607
The integration of renewable energy at the building level is increasing in popularity but brings
many challenges. Energy storage, or batteries, is a potential solution to these challenges, but
no currently available technology is objectively the best battery. This paper reviews promising
startup company energy storage technologies at the building level, and organizes them into the
following categories: cheaper, better, and greener. The amount of funding, which could be an
indicator of future success, is also reviewed. Six energy storage technologies and funding for the
respective startup companies were reviewed. The technologies reviewed were all found to have both
advantages and disadvantages, and Aquion Energys “green” battery was found to have received the
most funding. However, with recent policy trends and a quickly moving startup culture, the energy
storage industry will most likely see more activity with new technologies and companies emerging
quickly.
I.
INTRODUCTION
Renewable energy from wind or solar is increasing in
popularity as a means of supplementing or replacing the
traditional energy source (the grid) at the building level.
However, there are many challenges that need to be addressed when incorporating renewable energy sources,
such as reliability issues and intermittency. Renewable
energy sources vary with time, weather, and seasons,
and the patterns are not consistent enough to be reliable.
The intermittent nature of wind and solar energy prevent
these renewable sources from becoming large staples of
the energy sector on their own, due to their inability to
provide firm, dispatchable power. Natural production
times for wind and solar also do not necessarily coincide
with peak demand times. Furthermore, electricity producers are apprehensive of the idea of incorporating mass
amounts of unreliable energy sources to the grid because
of the potential for grid instability.
h̄
π·β
(1)
A solution to these concerns is to use energy storage, or
batteries, at the building level for the energy being produced onsite. Batteries provide the capability of storing
electricity during peak energy production periods, and
dispatching the energy when the renewable energy source
is no longer available. This method is called energy arbitrage or time shifting, and can turn inherently variable
energy resources into dispatchable energy sources. As
seen in Equation 1.
In addition, energy storage can be economically beneficial to the consumer by allowing for peak shaving. Since
the majority of wind power is generated during the night
when energy demand is low, and solar power is produced
intermittently when the sun is shining, the energy produced during those times may be stored and dispatched
during peak demand times. This method allows for less
grid electricity use, which means lower demand rates and
usage rates for the consumer. Peak shaving is also beneficial to the grid or power generators by decreasing the load
on already congested transmission lines and leveling out
demand fluctuations on power generating equipment. Although a myriad of potential energy storage technologies
do currently exist in various research or startup stages, a
particular technology has yet to take over the market. It
seems as though no product is commercially or economically viable enough to be widely implemented. However,
recent trends in policy decisions supporting the addition
of energy storage to the grid around the world, as well as
the increasing popularity of local renewable energy production, are putting pressure on the industry to accelerate the development of cost-effective technologies. Coupling these pressures with the currently booming startup
culture, it is only a matter of time before commercially
viable energy storage options are readily available and
accessible.
The goal of this paper is to review the close-to-market
technologies for energy storage at the building level, their
advantages and disadvantages, and their potential for
success. Many promising technologies will fail simply
due to the lack of ability to transition from the lab to
the market. As such, this paper will focus on established
energy storage startup companies, which have the most
potential in bringing a new technology to market quickly.
The startup companies that were reviewed were chosen
based on recent media coverage. Since the startup world
is run by investors that are not necessarily scientists, it
can be said that the potential for success of a startup
company boils down to pitches to investors as opposed
to perfect science. With almost all startups, pitches will
mostly likely fall into the following categories: cheaper,
better, or greener. As such, the startup companies being
reviewed will be organized within these three categories.
It is important to note that there will inevitably be overlap within these categories. Startup funding, which is an
indicator of marketability and potential for success, will
also be reviewed.
II
CHEAPER
TABLE I: Acronyms used in Figure 1
Energy Storage Technology
Acronym
Pumped Hydro
PH
Compressed Air Energy Storage, CAES (BG)
Below Ground
Compress Air Energy Storage, CAES (AG)
Above Ground
Sodium-Sulfur
Na-S
Advanced Lead Acid
Adv L
Zinc-Bromine
Zn-Br
Vanadium Redox Flow
Va Redox
Iron-Chromium Redox Flow Fe-Cr Redox
Lead Acid 2,200
L 2,200
Zinc-Air
Zn-Air
FIG. 1: Levelized Cost of Energy for Energy Storage
Technologies used for Renewable Energy Integration or
Time Shifting Compared to Combined Cycle Gas
Turbine. The LCOE is in December 2010 dollars.
Adapted from1
A.
II.
Eos Energy Storage
CHEAPER
Eos Energy Storage (Eos) provides zinc-air batteries,
which fall under the category of metal-air electrochemical cell technology. Metal-air batteries generate electric
current by coupling an electropositive metal with oxygen
in the air. The typical metals used are zinc, aluminum,
magnesium or lithium. A great advantage of zinc-air battery technology is that the metal used is relatively low in
cost due to abundance. In addition, the energy density
can be up to three times that of lithium-ion batteries.2
Zinc-air batteries are also very stable and do not contain
or produce toxic materials or gases. In fact, the primary material in the zinc-air battery, zinc-oxide, is 100%
recyclable.2
Unfortunately, zinc-air batteries typically have poor
recharging capabilities because of degradation issues.
Carbon dioxide from the air impacts the electrolyte and
cathode, zinc dentrite formation changes the battery
makeup, and thermal conditions must be managed. The
performance of zinc-air batteries turns out to be a roundtrip efficiency below 50 percent2. Exposure to ambient air conditions such as humidity or the presence of
airborne contaminants can also affect the performance.
Lastly, while the metals may be cheap, the technology
for the air electrodes is relatively complex and expensive.
Eos zinc-air battery uses a zinc hybrid cathode technology called Znyth TM and claims to have addressed typical
zinc-air battery issues while still maintaining its low cost
and increasing its life cycle to 30 years.3 Zinc dentrite
formation is deterred in ZnythTM batteries because the
electrolyte is aqueous and near neutral pH, and does not
absorb carbon dioxide.3 The current collector is titanium
with proprietary ceramic coating, which does not lose
conductivity, is non-corrosive, and self-heals.3 Most importantly, Eos improved the zinc-air batterys energy density and roundtrip efficiency by using proprietary electrolyte additives, buffering agents, cathode chemistries,
and electro-active catalysts.3 In addition to these many
In order to have market viability, a potential battery
option needs to be cost-effective, which can come from
low manufacturing costs or low implementation costs.
Cost may be the only deciding factor for some customers,
and there will most likely always be a market for low-cost
options. The price at which a potential energy storage
technology may be competitive in the market is typically determined by using its Levelized Cost of Energy
(LCOE), which is a normalized cost that allows for comparison across technologies with different efficiencies, capacity factors, and useful lives. The LCOE in dollars
per kilowatt-hour ($/kWh) of the potential technology
is compared with that of the currently used technology.
For energy storage technologies used for renewable energy integration and time shifting, the LCOE of potential technologies would be compared to that of a combined cycle gas turbine (CCGT), which is the least expensive and current option for this use. The Electric
Power Research Institute (EPRI) calculated these numbers in 2010.1 EPRI calculated LCOE ranges by dividing
the total costs (including construction, finance, operation, and maintenance) by the technologys useful output, and normalizing to have a consistent baseline for
the comparison. As shown in Figure 1, the LCOE range
for a CCGT is approximately $0.10-0.15 per kWh,1 and
is represented by dotted lines in the graph. In order for a
startup energy storage technology to be competitive, its
LCOE must be close to that range. Pumped hydro (PH)
and below ground compress air energy storage appears
to have comparable LCOE numbers.
The next sections will review two startup companies
that tout low-cost technologies.
7
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III
reasons, including a long life cycle, high efficiency, reliability, or even customizability.
The performance qualities of an energy storage technology are usually reported in the following categories:
energy storage capacity or size in megawatts (MW); discharge duration, or the amount of time energy can be
discharged; the number of cycles or the number of uses;
response time, which is typically less than one minute;
and the desired lifetime.
The International Energy Agency (IEA) and EPRI
compiled performance criteria needed for energy storage
systems for different applications.1,5 The characteristics
needed for the integration of renewable energy were compiled in Table II. These characteristics would specifically
be useful for photovoltaic integration, time shift, voltage sag, and rapid demand support; and home energy
management, which requires efficiency, cost-savings, and
reliability.
advances, Eoss battery manufacturing costs are kept low
by a highly standardized manufacturing process.3
Overall, the greatest advantage of Eoss battery is its
low capital cost. Eos claims a LCOE of $0.12-0.17 per
kWh of peak electricity,3 which is lower than the LCOE
range given in EPRIs report in 2010 of approximately
$0.20-0.25 per kWh.1 Eos LCOE is also comparable to
conventional combined cycle gas turbines.
B.
Ambri
According to Ambris website, Ambris battery is the
only all-liquid metal battery currently on the market or
even in development.4 The active components in an allliquid metal battery cell include two liquid electrodes and
a molten salt electrolyte that are not separated but selfsegregate due to differences in density and immiscibility
levels. While discharging, the electrons move from one
electrode to the other, causing the liquid ions to follow
and create an alloy with the ions on the other side. While
charging, the electrons are moved back to their original electrode, which also separates the alloy and returns
the system to its original state with three liquid layers.
The initial technology consisted of magnesium and antimony electrodes that created a magnesium-antimony
alloy; however, Ambri has since switched to a cheaper
and higher voltage chemistry that has not been released
to the public.4
The technology is low cost due to the use of materials that are inexpensive and abundant, and the use of
economies of scale during manufacturing. As for performance, Ambris battery has been demonstrated in the lab
to last over 1,600 cycles at full depth of charge, with a
0.0002% fade rate per cycle 4. Liquid electrodes typically exhibit longer life cycles than conventional solid
electrodes that typically degrade over time. For example,
the fade rate of a lead-acid battery at the same operating conditions is approximately 0.1% per cycle.4 Ambri
forecasts that their liquid metal battery will eventually
have a life cycle of over 15 years with tens of thousands of
cycles.4 Lastly, this liquid metal battery is emissions-free
and can store up to 4-6 hours of energy.4 This battery
must be operated at high temperatures, which may seem
to be a drawback at first, but the elevated temperatures
are provided by self-heating from charging and discharging.
Unfortunately, a LCOE for Ambris battery was not
available for inclusion in this paper.
III.
BETTER
TABLE II: Performance Criteria Needed for Renewable
Energy Integration.1,5
Size
Discharge
Cycles
Response Desired
(MW) duration
Time
Lifetime
1-5 15 minutes - >4000 total
<15 min 10-15 years
4 hours
or 0.5-2 per day
The following two startup companies claim to have
high performing batteries.
A.
UniEnergy Technologies
UniEnergy Technologies (UET) uses a vanadium reduction and oxidation (redox) flow battery. In flow batteries, energy is stored in liquid electrolytes instead of
in conventional electrodes. A flow battery consists of
external tanks and half-cells with an ion-selective membrane. When needed, the electrolytes from the external
tanks are pumped to the half-cells, in which the ions are
exchanged through the membrane. When charging, the
ions at the negative electrode accept electrons, while the
ions at the positive electrode release electrons. The electrical energy is stored chemically via this process. When
discharging, the chemical energy is released in the form
of electrical energy as the reactions reverse. UETs flow
batteries use vanadium as the ion.
The basic structure of a redox flow battery allows for
customer customization regarding the power/energy ratio specifications. The capacity of each battery is based
on the tank size, which is not fixed. Similar to Ambris
battery, the vanadium redox flow battery also basically
has an unlimited life cycle because of the avoidance of
conventional insertion and de-insertion of ions at electrodes. As such, the integrity of the physical structure
BETTER
Many products that are better usually come at a higher
cost; however, good performance is sometimes a must
depending on the application. Startup companies may
claim their battery is better because of many different
8
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B
Primus Power
IV
two tanks and a separator. In addition, the EnergyPods
electrode is metal and thus has a higher conductivity,
whereas a typical flow battery would have felt or plastic
electrodes.
EnergyPods are modular, so a custom system can be
built at increments of 280 kW to meet size requirements,
and energy can be discharged for a period of 4 to 5 hours.8
EnergyPods also have a long life cycle of 20 years that is
independent of depth of discharge.8 Primus Powers battery appears to have the performance qualifications in
terms of size, discharge duration, and desired lifetime.
In addition to those characteristics, Primus Powers EnergyPod also has a 70% AC/AC efficiency and is reportedly emission-free and cost-effective compared to natural
gas and diesel energy sources.8 Finally, unlike the other
batteries, the EnergyPod also includes computing and
software. The entire price for the EnergyPod complete
with computing and software is $500 per kWh.8
such as the tanks and plumbing, as well as the electronics and controls is maintained. The only limiting factor
for life cycle is the cell stack, which has a useful life of
approximately 10 years,2 and a cycle life that is normally
rated at 10,000 cycles.2 In addition, electrolyte management is reduced with vanadium redox flow batteries because fully discharged electrolytes are identical, making
shipping and storage of these batteries fairly inexpensive
and simple; and self-discharge is not an issue because
the electrolytes are stored in separate tanks. Lastly,
these batteries can ramp from zero to full output within
milliseconds.2
The vanadium redox flow battery does have a few disadvantages in terms of performance, such as a low energy
density, stability issues, and toxicity. The energy density
of these batteries is approximately ten times lower than
that of mobile lithium ion batteries. The active material form is not relatively stable because the vanadium
irreversibly precipitates out of the battery if in operation outside of its optimal temperature range (10 to 40 ◦
C). Finally, the solid ion exchange cell membrane is toxic
when exposed.
However, UET uses a new technology that originated
from research at the Pacific Northwest National Laboratory (PNNL), and the sulfate-chloride based complex
chemistry used in UETs vanadium redox flow battery
has reportedly greatly improved the performance6. First,
this battery has double the energy density of a typical
vanadium redox flow battery6. With UETs modular design, a UET energy storage system can have hold up to
23 megawatts (MW) per acre, or 46 MW per acre with
a stacked configuration.6 Second, the battery response
time is down to 0.8 milliseconds, and the total response
time from fully charging to fully discharging (or the reverse) is down to 50 milliseconds, which is limited only by
communication and controls.6 And third, this battery reportedly has no reduction in calendar life after continuous
cycling.6 UETs battery seems to have the performance
qualifications in terms of size, cycles, and response time.
On top of those performance characteristics, UETs
battery energy efficiency is up to 65-70% in AC/AC efficiency, 80% in electrochemical efficiency, and 97.5% in
AC/DC conversion efficiency).6 The battery also has an
improved stability with an operation temperature range
of -40 to 50 ◦ C.6
B.
GREENER
IV.
GREENER
“Green” seems to be the popular word in the market,
even in the battery world. The first benefit of “green”
batteries is the level of safety of these environmentally
friendly technologies. “Green” batteries typically use
technologies that are less toxic and less prone to combustion. The second benefit of “green” batteries is the marketability. “Green” or environmentally friendly batteries
may be conventionally counter-intuitive and receive more
attention, which can raise its marketability and chances
of success. The next two startup companies have environmentally friendly or “green” batteries.
A.
Aquion Energy
Aquion Energy (Aquion) provides an Aqueous Hybrid Ion (AHITM ) battery that uses a 200-year old ecofriendly technology: saltwater batteries.9 These aqueous
hybrid ion batteries use a unique saltwater or waterbased electrolyte with sodium lithium, and hydrogen ions
to store and release energy; an activated carbon anode
with a high surface area that uses capacitive interaction
for charging; a manganese oxide cathode with intercalation reactions instead of the more corrosive and conventional electrode surface reaction; a synthetic cotton separator that separates the electrodes and electrons, but
allows the sodium ions to flow through; and stainless
steel current collectors for the electrons to flow from the
electrodes.9 All these materials are nonhazardous, noncorrosive, and noncombustible, as well as relatively abundant and hence, inexpensive.9
Although these batteries consist of such innocuous
materials, unlike most other energy storage technologies, the performance is still very competitive with other
technologies. According to Aquions website, their MLine products have a life cycle of 3,000-plus cycles, a
Primus Power
Primus Powers product is called an “EnergyPod”,
which is a zinc-halogen flow battery. This technology
is similar to UETs flow battery technology, except that
the electrolyte is zinc-based and the system structure is
different. According to an interview with CEO Tom
Stepien,7 the EnergyPod structure is different in that
it only consists of one tank, one flow loop, one pump,
and no separator. This simple design makes the EnergyPod more efficient than the typical flow battery with
9
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Aquion Energy
VI
companies that was available on CrunchBase.11
UET funding information was not available because
UET is a privately held company. UET was still considered for this review, however, because they are building
on federal research. The technology was licensed earlier
this year from the Department of Energys Pacific Northwest National Laboratory.12
deep discharge depth of 100%, and a roundtrip DC efficiency of 85% at a 20 hour discharge that is higher than
most in the industry.9 These batteries are also sturdy
and not tolerant of abuse, not greatly affected by selfdischarge, and requires no thermal management or regular maintenance.9 On top of these great performance
characteristics, Aquions battery is relatively cheap to
produce at $250 per kWh9 (note: this price is not a
LCOE).
B.
Imergy Power Systems
Imergy Power System (Imergy) also has a vanadium
redox flow battery like UniEnergy Technologies battery; however, Imergys battery is produced from recycled
vanadium. Recycled vanadium is lower in cost than virgin vanadium by 40 percent.10 The industry benchmark
for vanadium redox flow batteries is $500 per kWh, but
with the cheaper recycled vanadium, Imergys goal is to
be under $300 per kWh.10
Recycled vanadium is most cost-effective because the
vanadium is recycled from environmental waste materials
such as mining slag, oil field sludge, and fly ash, which
have little to no market value. It would seem as though
using recycled vanadium would lower the performance of
the battery, but a purity of 98 % can actually be reached
via this process10. Conventional vanadium flow battery
manufacturers typically use virgin vanadium and obtain
a purity of 99%.10
According to Imergy, this relatively low-grade vanadium also has the ability to store more than twice the
energy per kilogram than regular vanadium flow batteries, and has a round-trip efficiency of 70 %.10
V.
FIG. 2: Funding Amounts of Energy Storage Startup
Companies.11
VI.
CONCLUDING REMARKS
The energy storage startup companies reviewed in this
paper all have battery technologies that could each do
well in a specific market. The“cheaper” batteries could
have a market in which cost is the deciding factor. Since
most projects are underfunded and/or over budget, this
market will potentially be the largest. There could also
be a niche market for “better” batteries for applications
that need higher performing batteries. As for “green”
batteries, its potential market could be for applications
that demand safety and nontoxicity.
However, with startup companies, the existence of a
market is not the only factor that determines the fate
of the company or its technology. Startup companies
also require funding, and thus, investor support, in order
to be successful. While different technologies have their
own advantages and disadvantages, if amount of funding
is an indicator of future success, Aquions “green” battery
is leading the pack.
It is important to note that the energy storage industry
will likely see more activity because of policies and mandates regarding renewable energy and energy storage for
the grid. In addition, the startup world moves so quickly
that new technologies still in research laboratories may
reach the market sooner than anticipated.
FUNDING
Funding is an integral part of the success of any startup
company. Without a steady flow of revenue or an established presence, a startup company relies completely on
funding in order to develop a product and bring it to market. The amount of funding a startup company receives
may be an indicator of the markets need or demand.
For the purposes of this study, funding amounts were
researched on the CrunchBase database. This free
database reports on technology companies, including
funding amounts for startup companies. While guidelines exist to protect content accuracy and quality, anyone can technically contribute to the database. Figure 2
shows the funding information for energy storage startup
1
CONCLUDING REMARKS
D. Rastier, Electricity Energy Storage Technology Options:
A White Paper Primer on Applications, Costs, and Benefits (Electric Power Research Institute, Palo Alto, CA,
2
10
2010).
G. Akhil, A. A. Huff, A. B. Currier, B. C. Kaun, D. M.
Rastler, S. B. Chen, A. L. Cotter, D. T. Bradshaw, and
c
2015
VI
3
4
5
6
7
8
9
10
11
12
CONCLUDING REMARKS
W. D. Gauntlett, DOE/EPRI 2013 Electricity Storage
Handbook in Collaboration with NRECA (Sandia National
Laboratories, Aluquerque, NM, 2013).
EOS
Energy
Storage,
URL
http://www.
eosenergystorage.com/.
Ambri, URL http://www.ambri.com/.
Technology Roadmap Energy Storage (International Energy Agency, Paris, France, 2014).
UniEnergy
Technologies,
URL
http://www.
uetechnologies.com/.
K. Fenrenbacher, A Next-gen Battery Is Coming Soon to
the Power Grid (Gigaom, 2014).
Primus Power, URL http://www.primuspower.com/.
Aquion Energy, URL http://www.aquionenergy.com/.
Imergy Power Systems, URL http://www.imergy.com/.
Crunchbase, URL http://www.crunchbase.com/.
F. White, Federal Research Spurs Washington State to
Store Energy (Pacific Northwest National Laboratory,
Richland, WA, 2014).
11
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Onsite Energy Storage in Todays Market
Bryan Loeding
Renewable energy has gained increasing popularity and public interest over the past decade. New
regulations on conventional coal and nuclear power generation has demanded a market turn to
renewable energy generation from solar and wind. Fluctuations from these renewable sources make
it difficult to utilize all the power generated since conventional power is needed to maintain demand
on the grid. To efficiently utilize all generation from renewable sources, a way to store the energy for
future use would be ideal. Such energy storage systems can be located on the grid or onsite where
electric is actually consumed. Energy storage systems provide wholesale market opportunities plus
other benefits such as energy arbitrage, reduced transmission congestion, low local marginal prices,
and both voltage and frequency regulation. Onsite energy storage devices, as a load-management
tool, can bring considerable value by reducing costs for individual customers and for the utility.
Introduction
The electricity produced has a characteristic of the
electric power sector, in that the amount of electricity
that can be generated is relatively fixed over short periods of time even though demand for electricity fluctuates
throughout the day. Energy storage can help balance the
grid system and improve the capacity factor from unstable renewable generation by storing electricity when not
needed and acting as a shock absorber to smooth out
fluctuations. Energy storage systems can provide reliability and power quality to commercial, industrial and
residential end users that have a need for premium power
supply. It can also improve the economics of power supply to end users who have time of use rates or high demand charges and low load factors, such as in industrial
or commercial applications. Having technology to store
electrical energy so it can be available to meet demand
whenever needed is a great asset in electricity distribution. Industry experts believe energy storage will play
a key role in supporting frequency regulation, ancillary
services, renewable energy integration, relieving transmission and distribution congestion, and improving the
balance of supply and demand.
Energy Use in America
In 2013, the United States consumed about 4,058 billion kilowatt-hours of electricity.1 As shown in Figure 1, a
majority of electricity consumed was from coal and other
fossil fuels. This accounted for around 67% of the electricity consumed. With the focus of the current U.S.
Federal and State Governments attention on reducing
greenhouse gas emissions, they have strongly supported
expanding many forms of renewable power generation in
recent years. As a result, renewable power generation
has doubled since 2003 with the exception of hydro. The
most successful renewable source has clearly been wind
power as shown in Figure 2.
Renewable power generation capacity has been sup-
FIG. 1: Electric Power Energy Consumption (adapted
from Reference 2)
ported by a number of government policies and subsidies.
The most successful forms of support in recent years appear to be the Federal Renewable Energy Production
Tax Credit (PTC) and the State Renewable Portfolio
Standards (RPS). The PTC renewable power subsidies
were originally created 20 years ago under the EPACT
of 1992.1 The subsidies have been extended numerous
times up to the current 2.3 cent/KWh Wind Power PTC,
which expired Dec. 31, 2013.1
The primary objective of expanding renewable power
generation capacity is to reduce the need for fossil fuel
power and the associated carbon emissions. U.S. carbon emissions (from consumption of fossil fuels) peaked
in 2007 at 6023 million metric tons per year (MMT/yr.)
and total emissions have since declined by about 12% in
2012.1 This is largely due to the stricter regulations on
current coal generation plants and the shift to natural
gas consumption actually increased by almost 10% between 2007-12.1 The individual State RPS or Renewable
Electricity Standards have been adopted by 36 States,
which collectively generate up to about 70% of total U.S.
net power.3 The average of these 36 State RPS targets
is to supply up to about 20% of total power consumed
able generation from wind and solar generators.Various
types of existing or potential storage technologies are
adapted for different uses. All storage technologies are
designed to respond to changes in the demand for electricity, but on varying timescales.4
Unlike other commodities, there are not significant
stocks or inventories of electricity to cushion differences
in supply and demand. Electricity must be produced at
the level of demand at any given moment, and demand
changes continually. Without stored electricity to call
on, electric power system operators must increase or decrease generation to meet the changing demand in order
to maintain acceptable levels of power quality and reliability.
Electricity markets are structured around this reality.
Currently, generating capacity is set aside as reserve capacity every hour of every day to provide a buffer against
fluctuations in demand. In that way, if the reserve capacity is needed, it can be dispatched or sent to the grid
without delay. There are costs, at times significant, to requiring the availability of generating capacity to provide
reserves and regulation of power quality. However, economic storage of electricity could decrease or eliminate
the need for generating capacity to fill that role.
Energy management storage systems are used for long
timescales operation where daily, weekly, and seasonal
variations in electricity demand are fairly predictable.
Higher-capacity technologies capable of outputting electricity for extended periods of time, such as pumped
hydroelectric storage or compressed air energy storage,
moderate the extremes of demand over these longer
timescales. These technologies aid in energy management, reducing the need for generating capacity as well as
the ongoing expenses of operating that capacity.4 Variations in demand are accompanied by price changes, which
lead to arbitrage opportunities, where storage operators
can buy power when prices are low and sell when prices
are high.
Shorter timescales maintain the power quality of the
electricity on the grid. These demand fluctuations are
on shorter timescales from sub-hourly, from a few minutes, down to fractions of a second. Management of these
fluctuations require rapidly-responding technologies like
flywheels, super-capacitors, batteries, or SMES which often have smaller capacity. Responding to these shorttimescale fluctuations keeps the voltage and frequency
characteristics of the grid’s electricity consistent within
narrow bounds, providing an expected level of power
quality. Power quality is an important attribute of grid
electricity, as poor quality electricitymomentary spikes,
surges, sags, or outagescan harm electronic devices.4
FIG. 2: Renewable Energy Consumption (adapted from
Reference 2)
from renewable power by 2020.3 The total U.S. carbon
emissions could be reduced significantly in the future if
these States accomplish their RPS targets.
Demand for access to low-cost electric power is the
primary physical limits of the transmission grid system
with the result of voltage drops and possible blackouts.
An alternative solution instead of adding more capacitors or more costly transmission lines is to deploy energy
storage systems at key points in the transmission network or at the site of the energy user. When voltage sags
occur, energy storage systems can supply real and reactive power, resulting in stabilization of the grid. The use
of energy storage can help balance the grid system and
improve the capacity factor from unstable renewable generation by storing electricity when not needed and acting
as a shock absorber to smooth out fluctuations. Energy
storage systems can provide reliability and power quality
to commercial, industrial and residential end users that
have a need for premium power supply. Having technology to store electrical energy, so it can be available
to meet demand whenever needed, is a great asset in
electricity distribution. Industry experts believe energy
storage will play a key role in supporting frequency regulation, ancillary services and renewable energy integration, relieving transmission and distribution congestion,
and improving the balance of supply and demand.
Types of Energy Storage
Grid-scale energy storage technologies are currently
limited in use but may see increased adoption in the future. Currently, the vast majority of existing storage is
pumped hydroelectric storage.2 A wide variety of technologies can serve an array of functions around the electric power system. These storage systems can be deployed from assuring power quality to deferring electric
power system infrastructure upgrades to integrating vari-
Onsite Energy Storage
Power grids depend on a stable power supply by optimally balancing supply and demand. As the use of solar
power and other renewable energy sources which have un13
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more severe than wind ramps because they are much
faster. Energy storage systems can provide local voltage
and VAR (volt-ampere reactive) support, and manage
intermittent variation in photovoltaic loads while reducing costs for customers by reducing energy consumption,
peaking costs, and capacity costs.
stable output continues to increase, power supply to the
entire grid could become unstable. This presents a variety of challenges.Energy storage can serve customers as
a controllable demand-side management option that can
also provide premium services, including power quality
for sags or surges, uninterruptible power supply for outages, and peak demand reduction to reduce electricity
bills.
Storing electrical energy can equalize the electrical load
on the grid and promote efficient use of energy. Energy
storage systems can also serve as back-up power sources
in an emergency. These systems can be highly versatile,
can meet the needs of various users, and be utilized in
diverse fields. These fields include power generators that
use renewable energy, grid equipment like energy transmission and distribution equipment, as well as commercial facilities, factories and homes.
Electricity pricing is moving from a standard flat rate
tariff to a Time-of-Use (TOU) pricing structure, where
rates are dependent on when electricity is consumed.
With TOU pricing, the cost of electricity during peak
daytime hours when most small and mid-sized businesses
operate is often 400% higher than it would be for the
same amount of electricity at night.With the motivation
of financial incentives, many consumers have the option
of shifting their electricity usage profiles to take advantage of TOU pricing. With storage, energy management
systems can automatically store electricity when prices
are low, and then discharge this energy on demand when
prices are high.
In addition to the advantages that energy storage provides to end-users,there are simultaneous benefits to the
electrical grid for the utility company. In order to maintain an adequate supply of power throughout the day,
utilities will typically rely on fossil fuel (coal, natural gas)
power plants to produce energy for those peak daytime
hours when electricity consumption is highest. Not only
does peak energy cost more to produce, it is also more
carbon intensive than baseline electricity. However, with
energy storage systems, the balance of electricity production can be tilted away from peak periods, thereby saving
money for utilities and benefiting the environment.
Electric Power Research Institute (EPRI) estimates
that 10% to 25% of utility generation, transmission and
distribution assets are needed for less than 400 hours per
year.1 This large capital investment are costly to build
and operate in comparison to the short time there used at
peak capacity. Energy storage systems offer utility companies a new option to improve capital utilization, defer
investment and increase reliability throughout the system, particularly in rural and congested urban areas. If
the utilities were to add less expensive energy storage systems, the addition of costly transmission lines for quick
response generation would not be needed.Utility distribution engineers anticipate increasing challenges managing high penetrations of solar photovoltaic on the local
distribution system. The effects of photovoltaic voltage
sags and demand shifts due to cloud effects are even
Battery Energy Storage
Batteries are a well-known technology that has seen
increasing improvements and advancements. A battery
energy storage system in commercial, residential and industrial buildings allows users to store electricity from
the grid during off peak or from local generations such
as PV panels and wind turbines. The energy stored provides potential value as a load-management tool to reduce costs for the individual customer and for the utility. The battery storage will bring a reduction in energy consumptions during high demand periods, bringing down the peaking costs, capacity costs and demand
charges.Utilizing the system reduces a users daily peak
demand load by peak shaving, or smoothing out their
energy consumption. For many users, demand response
charges can be a significant component of their electricity costs and leveling energy loads across a day provides
real savings. When the grid falters, batteries are ready
to provide hours of backup power supply.
The batteries most commonly used for grid storage are
lithium-ion. The batteries are rated in terms of their energy and power capacities. For most of the battery types,
the power and energy capacities are not independent and
are fixed during the battery design. Some of the other
important features of a battery are efficiency, life span,
operating temperature, depth of discharge, self-discharge
and energy density.
Currently, significant development continues with battery technology. Different types of batteries are being developed, of which some are available commercially while
others are still in the experimental stage. The batteries
used in power system applications such as grid applications so far have been deep cycle batteries with energy capacity ranging from 17 to 40 MWh and have efficiencies of
about 70%80%. Lithium ion batteries have a small footprint and require minimal maintenance, ensuring that the
system is cost effective to install and operate. With the
capability to be fully integrated into an existing building
energy management system, battery storage helps ensure
power supply is balanced with building loads, and congestion and peak demands.
Superconducting Magnetic Energy Storage
Superconducting Magnetic Energy Storage systems is
a device for storing and discharging large quantities of
power. The system is independent of capacity and size
a SMES system always includes a superconducting coil,
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changes, expansion the use of time-of-use (TOU) energy
prices, or substantive changes in energy storage, renewable energy, or conventional energy resource prices.
Minnesota Department of Commerce contracted with
Strategen and the Electric Power Research Institute
(EPRI) to investigate the potential costs and benefits
of grid-connected electrical energy storage technology located at the customer in the State of Minnesota. The
investigation included both a standalone storage system
and a storage system, which integrated solar PV. They
considered applications for both commercial and residential customer sites. Four different general operational use
cases for energy storage were identified and investigated
which included customer controlled for bill savings, utility controlled for distribution system benefits, utility controlled for distribution and market benefits and shared
customer and utility controlled for bill savings and market revenue.
The project team modeled each case to calculate
project lifetime costs and benefits, performed an analysis
of key barriers to implementation, and provided recommendations to address key gaps to energy storage implementation.
This case study found that utility controlled, customersited storage in Minnesota has the potential to provide
benefits to the grid greater than the systems cost. Several different combinations of benefits were required to
achieve benefit to cost ratios greater than one. While a
number of different value streams were investigated, the
value of each use case was primarily driven by the following grid services and incentives;Distribution upgrade
deferral, frequency regulation, system capacity, and Federal Investment Tax Credit (FITC) for solar and favorable accelerated depreciation schedules.
a refrigerator, a power conversion system (PCS), and a
control system.The superconducting coil, the heart of the
SMES system, stores energy in the magnetic field generated by a circulating current. The maximum stored energy is determined by two factors. The size and geometry
of the coil, which determines the inductance of the coil is
one factor. The larger the coil, the greater the stored energy.3The second is the characteristics of the conductor,
which determines the maximum current. Superconductors carry substantial currents in high magnetic fields.
There are several reasons for using superconducting
magnetic energy storage instead of other energy storage
methods. A key advantage that SMES has over other
energy-storage technologies is its ability to rapidly release stored energy. It can do what no other technology
can do, go from a full charge to a full discharge. Rapid
discharge makes SMES attractive for quickly stabilizing
high-voltage transmission lines during periods of heavy
use. Power is available almost instantaneously and very
high power output can be provided for a brief period
of time. Other energy storage methods, such as pumped
hydro or compressed air have a substantial time delay associated with the energy conversion of stored mechanical
energy back into electricity. Thus if a customer’s demand
is immediate, SMES is the most viable option. Another
advantage is that the loss of power is less than other storage methods because electric currents encounter almost
no resistance. Additionally the main parts in a SMES
are motionless, which results in high reliability.2
There are several small SMES units available for commercial use and a number of larger test projects. Commercial one- MW units are used for power quality control
in installations around the world, especially to provide
power quality at manufacturing plants requiring ultraclean power, such as microchip fabrication facilities. Due
to the energy requirements of refrigeration and the high
cost of superconducting wire, SMES is currently used for
short duration energy storage. Therefore, SMES is most
commonly devoted to improving power quality. If SMES
were to be used for utilities it would be a diurnal storage device, charged from base load power at night and
meeting peak loads during the day.SMES systems have
about the same life expectancy as pumped hydro and
compressed air systems: 10-20 years, as opposed to 1-10
years for batteries and 8-12 years for although the cost of
superconducting wire has dropped significantly in recent
years.1
California Incentives
There has been increasing awareness and government
leaders to address the need for renewable generation integration in our electric grid. States have added incentive programs to help drive change. Leading this push
is the state of California. In 2003, the state started the
Self-Generation Incentive Program (SGIP) to promote
a cleaner energy solution.The Self-Generation Incentive
Program offers incentives to renewable and emerging
technology projects and non-renewable fueled conventional CHP projects. The Self Generation Incentive Program (SGIP) provides financial incentives for the installation of new qualifying technologies that are installed
to meet all or a portion of the electric energy needs of
a facility. The purpose of the SGIP is to contribute to
Greenhouse Gas (GHG) emission reductions, demand reductions and reduced customer electricity purchases, resulting in the electric system reliability through improved
transmission and distribution system utilization.The program gives the customer financial motivation to pursue
these technologies. For investment in energy storage, the
Case Study
Minnesota On-site Storage
As previously mentioned, energy storage has the potential to provide multiple sources of value for customers
and utilities, from economic to grid reliability. As such,
the results from energy storage are subject to change
based upon federal and state tax policy changes, tariff
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site energy storage systems such as lithium ion batteries
or SMES can have a positive impact for both the customers and the utilities. These energy storage systems
are used to correct different problems which is effecting
the quality of power on the electric grid. SMES systems
have the ability of fast response which they can switch
from charge to discharge state within seconds, and they
can be charged/discharged rapidly and entirely. This is
a promising advantage for this energy storage systems,
which will be expected to quickly stabilize power quality when wind or solar resources experience regular and
sometimes precipitous plunges in output. The deregulation of the electricity market and the requirements to
enhance the power capacities of the present grids bring
the opportunity to use SMES in the protection against
power quality problems such as voltage sags and against
power reliability problems, such as voltage instability and
low voltage. Lithium ion Batteries provide the capacity
to realize load leveling. This can have both a financial
and operational value to the consumer. A variety of loads
ranging from industrial installations to residential require
energy to provide power quality and backup power. This
energy is used for a variety of conditions such as when
momentary disturbances require real power injection to
avoid power interruptions.
state is offering Advanced Energy Storage 1.62/W. with
a maximum incentive of 5 million of which 40% of the
project as customer investments. Typically, SGIP rebates pay up to 60 % of the cost of installed systems.
California is the first state to mandate storage as part
of the big utilities energy portfolios.Californias Governor authorized renewal of the states SGIP, which was
originally due to end next year, through 2020. Legislation requires the states investor-owned utilities to procure 1.325 gigawatts of energy storage by 2020.The state
is also requiring 200 megawatts of energy storage be installed behind the meter on customer sites and authorizes
83 million per year for the on-site storage technologies
and includes rebates for stand-alone storage systems and
those paired with solar or other distributed generation
systems.
Conclusion
The growing need in todays energy markets to integrate renewable energy sources such as wind and solar
is increasing. As seen in California with the incentive
programs, energy storage is a growing markets which addresses the need for clean and affordable electricity. On-
1
2
3
4
Monthly energy review (2014), http://www.eia.gov/
totalenergy/data/monthly/pdf/sec7_5.pdf.
EPRI, Handbook for Energy Storage for Transmission or
Distribution Applications, Report no 1007189, Techincal
Update (2002).
J.
Miller,
How
effective
are
us
renewable
power
policies?
(2013),
http://
theenergycollective.com/jemillerep/311406/
how-effective-are-us-renewable-power-policies.
M.
Goldes,
Superconducting
magnetic
energy
storage
and
future
ultraconducting
magnetic
energy
storage
(2008),
http://www.
energycentral.net/blog/08/01/superconducting-,
magnetic-energy-storage-and-future-ultraconducting-,
magnetic-energy-storage.
16
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2015
Piezoelectric Energy Harvesting for Powering Micro Electromechanical Systems
(MEMS)
Abdul Majeed
Recent advancements in wireless technology and low power electronics such as micro electromechanical systems (MEMS), have created a surge of technical innovations in the field of energy harvesting. Piezoelectric materials, which operate on vibrations surrounding the system have become
highly useful in terms of energy harvesting. Piezoelectricity is the ability to transform mechanical
strain energy, mostly vibrations, to electrical energy, which can be used to power devices. This paper
will focus on energy harvesting by piezoelectricity and how it can be incorporated into various low
power devices and explain the ability of piezoelectric materials to function as self-charging devices
that can continuously supply power to a device and will not require any battery for future processes.
Introduction
Innovations in the field of electronics has led to the
ever-decreasing size of electronics and the significant reduction in energy consumption. These innovations have
also steered the way to the possibility of self-powered
electronic devices through implementation of energy harvesting techniques.Exponential growth of wireless electronics, such as smart phones and tablets, has transformed such devices from simple communication platform
to possessing advanced computational capabilities,for example sending and receiving mails, taking pictures and
videos, and allowing the integration of global positioning
system technology in addition to many other functions.
Due to the high functionality of electronic devices, batteries are not always able to meet the demands of these
devices on a regular basis.This limitation forces people
to frequently charge their devices, as these devices have
become basic needs, for communication, entertainment
and other uses. On a daily basis, large amounts of useful
energy are being wasted, such as the energy from human
movement and movement of vehicles.Harvesting this type
of energy (energy from human and/or vehicular movement) uses piezoelectric materials,which convert a compressive or tensile strain to a voltage.1 A way to mitigate
or support human desire for fully charged personal devices is to harness their own energy. Materials exist that
can harness and release human and vehicular energy for
use.
These are piezoelectric materials and generation, and
include current applications, such as electric cigarette
lighters and gas stove burners, which when activated produces a spark resulting in a flame.Piezoelectric energy
originates from an environment surrounding a material.
This material utilizes the ambient energy from the environment such as mechanical vibrations and transforms
them to useful electrical energy. Piezoelectric materials
have uniquely high power densities, which make them
suitable for energy harvesting. It is because of their high
power densities, compared to other energy sources, such
as thermoelectric, electromagnetic, fuel cells and solar
cells, which has led to advanced research being conducted
in the field of piezoelectricity. Thus making them suitable candidates for energy sources for low power electronics, such as wireless sensing and actuating.Piezoelectric
generation is not like other sources of energy, which are
intermittent, and can require constant forecasting for example, sources like wind, solar or tidal energy. Piezoelectric generation takes only mechanical factors such as
pressure, strain or vibrations into account. These mechanical vibrations translates into power being generated
by a piezoelectric material. The magnitude of frequency
and strength of vibrations (useful energy harnessed) are
directly proportional to the power generated.The only
drawback for this type of generation is the low power
which is generated.This can be resolved by novel techniques, which capitalize on the maximum power generated by piezoelectricity.This paper will focus on these
techniques specifically for macro-scale power generation
applications.2
Direct and Converse Piezoelectric Effect
Direct Piezoelectric Effect
The ability of a piezoelectric element to convert mechanical energy to electrical energy is defined as the direct piezoelectric effect. When a piezoelectric crystal is
deformed by applying mechanical strain or vibrations, an
electric charge is produced. The direction of current can
be reversed by reversing the direction of pressure applied
to the piezoelectric element.3 (See Figure 1)
Converse Piezoelectric Effect
When a piezoelectric element transforms electrical energy to mechanical energy, it is called converse piezoelectric effect (see Figure 2). When an electric field is applied
to a piezoelectric element or when an element is subjected
to an electric field, it will lead to a deformation in the
FIG. 3: Standard Electrical Interface. Adapted from
Reference 6
FIG. 1: Direct Piezoelectric Effect.Adapted from
Reference 3
Storage and Efficiency
To obtain the maximum power output from piezoelectric power generation, it is necessary to have better storage techniques and methods to increase the efficiency.
This paper will not focus on complex technologies, only
about the AC-DC circuit followed by the DC-DC converter, which is one of the simplest technologies in generating power from a piezoelectric material.
FIG. 2: Converse Piezoelectric Effect.Adapted from
Reference 3
Standard AC-DC Interface
piezoelectric material. The direction of deformation can
be reversed by changing the direction of electric current
applied to the material.3
The standard AC-DC interface is one of the simplest
ways to generate electric voltage from a piezoelectric element. The electrical energy supplied by a piezoelectric
element is an AC voltage, whereas the battery connected
to the element requires a steady DC voltage. To ensure
that the electrical circuit has proper compatibility, an
AC-DC interface is installed between the piezoelectric element and the battery. The AC-DC interface consists of a
bridge rectifier, and a controller. The function of a bridge
rectifier is to achieve full-wave rectification. The controller that is installed is generally a DC-DC converter,
whose function is to optimize the power being delivered
to the battery as well as regulate the voltage, such that
the voltage being delivered is suited to the requirements
of the battery(see Figure 3).6
Principles of Generation
When a crystalline material, having the properties
of piezoelectricity, undergoes a mechanical deformation
such as strain or vibrations an electric voltage is developed across the material. This is due to the asymmetric
polarity inside the piezoelectric element. When mechanical strain is applied to a piezoelectric element, opposite
charges are induced on either side of the element, resulting in an electrical voltage developed across the element.
The voltage is present as long as the material experiences
mechanical deformation, and disappears completely once
the mechanical strain applied is removed. The amount
of voltage produced depends on the amount of strain applied to the element. The magnitude of deformation is
directly proportional to the voltage produced.4
Piezoelectricity generated by an element depends on
the following:
DC-DC Converters
The energy produced by a piezoelectric material is low
compared to other forms of energy harvesting techniques.
The efficiency obtained by the DC-DC converter when
used in low power applications is high. Efficiencies can
reach the range of 80%-99%. Another unique characteristic of the DC-DC converters are dynamic controlling of
power and voltage regulation.6
1. Crystal symmetrical properties
2. Magnitude of applied mechanical strain
Storage Devices
3. Polar orientation inside the crystalline structure
The energy generated from piezoelectric devices is extremely low, and can be stored in temporary power storage devices such as capacitors or rechargeable batteries.
Supercapacitors are being investigated as low power storage devices, as they offer higher energy densities than
For proper utilization of piezoelectric generated power, it
is best to have better efficiency and storage techniques for
the power generated, as the power produced is extremely
low.5
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ergy. To reiterate, the amount of power generated from
piezoelectricity depends on the magnitude of strain applied. Typically vehicles are heavy, and the amount of
power generated would be significantly higher than the
amount of power generated by a human being.Harvesting
potential from a one kilometer stretch of a highway (one
lane, one way) can produce approximately 250 kWh per
hour, when an estimated 20 cars and/or trucks travel
over that one kilometer stretch of a highway in one
minute.10 There are many current applications which utilize piezoelectric generation, but most of them are in their
test/experimental stages. There is not much faith in the
power generated from piezoelectricity. Though through
some techniques depicted in the next section we can hope
for better prospects for piezoelectric power generation.
regular capacitors. Their performances are comparable
to that of Lithium-Ion batteries and have better characteristics, such as cycle life (the number of times it can
be charged and discharged). There has been less implementation of supercapacitors as energy storage devices.
But there can be more to utilization of supercapacitors
as storage devices with piezoelectric generation.7
Applications
Piezoelectric generation from floors
Piezoelectric generation depends on the amount of mechanical deformation applied on the element for a certain
period of time. This led to the development of piezoelectric tiles that generate power from footsteps in places
with significant amounts of foot traffic such as shopping malls, airports, train stations, and night clubs. The
tiles are designed in such a way that they contain piezoelectric elements underneath the surface, leaving enough
room for the materials to undergo mechanical deformation. Upon deformation, potential difference is created,
which is converted and stored by an electrical interface.
This stored energy can be utilized to power electronic
devices.One of the important factors in using this type
of electric production is that it is self-generating. It has
no external power requirement. Moreover, they have no
health risks or hazards. Several nightclubs in Europe
have designed dance floors with piezoelectric materials,
thereby generating considerable amounts of electricity.8
For example, the nightclub WATT in Rotterdam, Netherlands, is estimated to generate between 2-20 watts of electricity from a single person, depending on their impact
with the surface.9 The East Japan Railway Company in
Tokyo was the first to harness energy from foot movement
by implementing piezoelectric tiles in railway stations. In
2006, they generated 10,000 watt-seconds per day from
a 6 m2 tile, which is equal to powering a 100 W bulb
for 100 seconds. An average person weighing about 60kg
can produce 0.1 Watt-second per second. In 2008, the
East Japan Railway Company repeated the same experiment, increasing the area from 6m2 to 90m2 , with hopes
to generate close to 500kWs per second, which is equal
to powering a 100 W bulb for 80 minutes.9
Future of Power Harvesting
Piezoelectric power generation makes use of the power
that is considered wasted. As it utilizes the power from
mechanical movements and strain. If there arises a possibility of increasing the efficiency of the power generated,
it is considerable to capitalize on this opportunity. Certain techniques on increasing the power output of piezoelectric power generation are depicted below.
Power generation using Synchronous Switch
Harvesting Inductor (SSHI) technique
In the Synchronous Switch Harvesting on Inductor
(SSHI) technique, an electronic switching device is placed
in parallel and an inductor is connected in series with
the piezoelectric element. The schematic representation
of this technique is depicted in Figure 4. The electronic
switching device is triggered on the maximum and minimum displacements and the piezoelectric voltage is inverted at these occasions when the switching device is
triggered. Through this process,the amount of electrical energy developed increases significantly due to the
presence of an inductor which creates a resonance circuit
along with the capacitor and increases the maximum output power for any mechanical deformation occurring on
the piezoelectric element.
Piezoelectric generation from roads
Advantages
Roads that generate electricity by having vehicles drive
over them are innovative ways to implement piezoelectricity for continuous power generation. Piezoelectric
materials are embedded in the road, approximately a few
centimeters below the surface, such that there is enough
compressional strain from traveling vehicles.When a vehicle travels on a road, the vehicle creates compressional
stress on the road. This stress deforms the piezoelectric material underneath the surface, and produces en-
There are some advantages to using energy harvesting devices utilizing SSHI technique that are worth noting here.The electrical energy generated by such devices
using SSHI technique is found to be anywhere between
250%-600% greater than standard piezo energy harvesting circuits.These systems also optimize the load with
the help of DC-DC converters and thereby increasing
efficiency of energy generated. Additionally, the inclusion of the SSHI technique decreases the vibrational dis19
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positive and negative energy bands. Alteration in the
band can significantly increase the rate of recombination,
which is required for improving efficiency in photovoltaic
cells and LEDs.This could be instrumental in the further advancement of photovoltaic cells, since the output
voltage of these photovoltaic cells can be controlled by
modifying the potential of the piezoelectric materials.15
Limitations
FIG. 4: Energy harvesting device with SSHI
technique.Adapted from Reference 11
Although piezoelectricity has its advantages, as noted
above, there are certain limitations that prevent its development for commercial use. One of the limitations
is the amount of output voltage that is generated from
a piezoelectric material. The output voltage is significantly low compared to other forms of energy sources
and is not suitable for application on electronic devices
on the market currently. Piezoelectric materials can produce a power of high magnitude if the material has a
large area of mechanical strain, but an increase in the
area can lead to the material being more prone to damage and failure due to its brittleness. Larger surface areas
cause more wear on the material due to the large deflection that it undergoes for each mechanical strain. Of
importance to note is that not all piezoelectric materials
have the inherent properties of piezoelectricity.For certain materials, these properties are brought about by a
poling process.The piezoelectric material loses its polarity when subjected to high electric or magnetic field. A
piezoelectric material is also temperature dependent. A
material will lose its properties of piezoelectricity once
the temperature exceeds the Curie temperature. The
Curie temperature is the maximum temperature point
for any piezoelectric material. The performance and life
of piezoelectric materials decreases at high temperatures.
Additionally, the possibility of decrease in efficiencies due
to mechanical and dielectric losses.16
Though there are limitations which prevent the wide
use of piezoelectric generation, those problems can be
resolved by techniques depicted in this paper. By this
techniques the possibilities of piezoelectric power generation will be more.
placement without causing a decrease in charge developed.These advantages show how important in making
piezoelectric power generation an integral part of power
generation in the near future.12
2. Piezoelectric Nanogenerators
Piezoelectric nanogenerators help convert mechanical
strain to electrical energy at the nano-level. Recent developments in this area of nanotechnology have led to
improvements in efficiency and durability of materials,
including piezoelectric nanowires. Although the energy
produced by a single nanowire is insufficient for any practical use, the integration of a large number of these piezoelectric nanowires can lead to a wide range of applications, such as wireless sensors and energy from bodily
movements.13
Advantages and applications
The advantages of utilizing nanogenerators is that the
energy from mechanical movements, like movement of the
body and/or muscle movement can be utilized to generate energy. Even vibrational energy, such as energy from
ultrasonic waves and/or acoustic waves can be used to
produce power from piezoelectric generation. Also, minuscule bodily functions such as the energy due to the
flow of blood, body fluids and/or movement of blood vessels can be made to good use.14
Conclusion
Batteries power most portable electronic devices today.
They are still the most economical solution for powering micro electromechanical systems, with Lithium-ion
having the best energy densities, efficiencies, and charge
and discharge rates compared to other batteries. While
self-powering devices, powered by photovoltaic cells, exist and have certain advantages such as integration and
easy availability of resources, they nonetheless have a
high cost and are dependent on sunlight. The potential for vibrational-based energy for applications in electronic devices appears to be of interest, and certainly
3. Piezo-Photronics
Piezo-phototronic effect is the ability to modify the
carrier properties at the p-n junction (the interface
between the negatively and positively charged materials), particularly in photovoltaic cells and/or LEDs, and
thereby it increases the performance of these devices.
These effects are generally produced by piezoelectric materials, which have semiconductor properties. This material modifies the electronic band present between the
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and storage devices has not yet been realized. This can
be highly applicable for powering low-powered electronic
devices. By this method, the dependence of piezoelectric
and battery systems separately would be reduced, and
this would increase the life and efficiencies of both systems. The techniques presented in this paper when utilized would help in implementing piezoelectric systems in
wide range of applications for realization of self-powering
devices.Piezoelectric power generation will play a significant role in the realization of self-powered devices in the
near future.
have the advantage of harnessing ambient energy from
its surroundings. The integration and components associated with it is much simpler. Research is being widely
conducted to realize the potential of piezoelectric generation in real world applications. The only downside
to powering electronic devices is the insufficient amount
of charge produced to power them completely. The two
areas where developments are required includes: increasing the efficiency of power generated, so that maximum
power can be entirely converted from mechanical strain,
and increasing the capability of storage for piezoelectric
devices. The idea of combining both piezoelectric devices
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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C. Baur, D. Apo, D. Maurya, S. Priya, and W. Voit, Polymer Composites for Energy Harvesting, Conversion, and
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The Smart Grid and its Effects on Utilities, Consumers and Energy Storage
Aiden Gilbert
U.S. electrical utilities have started a major renovation of the electrical grid infrastructure to
integrate IT technology to the existing grid, creating a Smart Grid. Until this recent conversion, the
electric grid has remained relatively unchanged for over 100 years.As the Smart Grid is installed,
there will be benefits such as higher reliability, increased system efficiency and the integration of new
greener technology. Energy storage technology could have the opportunity to provide relief on the
grid, especially during peaks of demand. However, the smart grid is not without its opponents due to
concerns of public health, cost and privacy.The purpose of this paper is to explore the conversion of
the existing grid infrastructure to a Smart Grid and how it will affect electricity consumers, utilities
and the integration of energy storage technology on the grid.
Introduction
Recently, electric utilities like Commonwealth Edison
have initiated programs to replace their customers existing electric meters with the Advanced Metering Infrastructure (AMI) to create a smarter power grid. This
smart grid is composed of the AMI (or Smart Meters),
communication networks and information management
systems.1 The current grid system provides electricity
to millions of people across the country with a power
generation capacity around one million megawatts, but
the technology has remained virtually unchanged since
its conception over a hundred years ago.2 By combining
Information Technology (IT) with the existing grid technology in place, the smart grid will increase the reliability
of power delivery, increase system efficiency, and create
bidirectional communication between the consumer and
the grid operator, which can open doors for new innovations with technologies like distributed power generation
and energy storage.2 Integrating a variety of energy storage technologies on the grid would create higher stability
to serve different functions, such as frequency regulation,
load swings, load leveling, and back up power.3
A Brief History of the U.S. Electrical Grid
In 1882, the first electric utility provided electricity to
85 customers, lighting 400 lamps in New York City.4 Not
long after that, numerous electric companies were started
across the country, creating a highly competitive, unregulated environment in the field, which caused the price of
electricity to drop.5 Without the presence of regulation,
electric companies installed mechanical meters to determine usage accurately, allowing the companies to charge
their customers based on the amount of energy used instead of a flat rate per month like the gas companies did
in that era.5
The device works like an odometer in a car, accumulating over time with energy consumption, and on a regular
basis, a utility employee must read the meter to know
how much to charge their customer. This type of meter
FIG. 1: Photo of traditional electrical meters
is still in place for much of the country. Figure 1 shows
an example of this traditional meter in a multi-family
building.
While this technology remained the same, the industry evolved drastically. The politically connected Samuel
Insull of Chicago Edison pushed for what he called the
Natural Monopoly until his goal was realized.5 In this system, one business controlled a regions generation and distribution with a regulated structure by generating large
amounts of energy for a low cost and then sold power
from the largest plants possible5.Consequently, the system became extremely corrupt leading to raising prices,
drops in production and shady dealings involving multilayered holding companies.5 When the market crashed
and led to the Great Depression, the bubble of this model
burst. Insull was left broke and fled the country. This led
to the Public Utilities Holding Company Act (PUHCA)
of 1935, which created stronger regulations and created
the Federal Energy Regulatory Commission (FERC).6
The industry was then stabilized, but there was little
incentive to innovate the technology. But in the 1990s,
the FERC deregulated interstate transmission and some
of the more industrialized states started to pass laws to
deregulate their electric utilities, causing the power companies to split the generation side from the transmission
side.6 The transmission side would continue under the
natural monopoly model, but the generation side would
be comprised of private entities entering a free market.6
At the present, due to political, economical, reliability,
consumer and environmental drivers, utilities are upgrading their existing technology to the smart grid.6
also no longer be used as well. All the data pertaining to a
customers usage will be transmitted wirelessly from each
smart meter to the utility to produce a more accurate
bill. There will no longer be the hassle of a monthly
meter reader visit and estimate charges, which should
lead to improved customer satisfaction.
The consumer will have the ability to become a power
supplier as well, instead of just a consumer.The smart
grid allows bidirectional communication between the
smart meters and the grid.2 The consumer can install
distributed generation technology at their home like photovoltaic (PV) solar panels to reduce their electric bill.
Additionally, there are net metering programs offered
by utilities outside the scope of smart meters. Through
these programs, any excess distributed power generated
can be sold back to the grid at retail prices, further reducing their bill.
One of the most valuable benefits to the consumer
could be the reduction of emissions from fossil fuels. Fuels like coal emit toxic pollutants that can be harmful to
a persons health. Fossil fuels also emit greenhouse gases
(GHGs) that can lead to climate change.Coal is still the
largest source of energy on the grid today, creating massive quantities of GHGs and toxic pollutants that are
harmful to the environment and public.Renewable energy
is a growing market, but due to its variable production, it
is not able to meet its full potential in a traditional grid
system.The smart grid uses technology that can properly
manage these variable power sources.
As utilities convert the old technology of the grid to
a smarter system, the customer experience will be reshaped, improving how the customer thinks about and
consumes energy. A more reliable system will ensure that
energy is delivered to the customer with less outages and
shorter durations of outages. Additionally, the grid is not
the only thing becoming smarter. The customer will be
able to have a stronger knowledge of their energy usage
with real time data, which can lead to more energy efficient behavior and lower electric bills. The customer will
also be less dependent on the grid for their energy needs
as customers will be able to generate distributed power.
Lastly, the smart grid allows for a cleaner system due
to its ability to support the variable nature of renewable
power generation like wind and solar. This wide array of
benefits for the customer appears to strongly justify the
merits of such a large endeavor.
Smart vs. Smarter Grid
The phrase, smart grid, has been thrown around
loosely as the word smart is being used as a buzzword
to emphasize new technologies. Consequently, the meaning of a smart grid has been clouded. But in its essence,
the smart grid is the combination of the existing electrical grid infrastructure with wireless communication
and IT technologies that allow the grid to operate with
higher automation capabilities.6 Some in the industry
may take offense at the phrase because it implies that
the current grid is dumb despite the fact that it has provided electricity to millions of people for over a century.6
The traditional grid has been deemed one of the greatest engineering achievements in history, but there is
room for improvement.7 With the upgrade, there will be
widespread benefits for all those involved.
Customer Benefits of a Smarter Grid
There are many different benefits associated with the
upgrade to a smarter grid for the general public, but
the reliability of energy delivery is a point of emphasis
as modern technology, like data centers, demands consistent, uninterruptible power.2 Reliability is the one metric
that is a universal concern for all electricity consumers.
If someone plugs in an appliance, there is an expectation
that electricity will be supplied to power the appliance.
Of course, people have come to expect the occasional
outage due to storms and other weather events. In the
old system, however, if there was a power outage, the
utilities relied upon its customers to report it in order to
know that a repair was needed.2 Smart meters use Internet technology to communicate with grid operators in
real time and can notify the utility if there is an outage anywhere on the system.2 This will cut down on the
amount of outages and the duration of those outages.6
The benefits for the energy consumer go beyond just
higher reliability. Customer satisfaction is expected to increase with the shift to the smart grid.6 The customer will
have the opportunity to lower bills due to an increased
knowledge of their own energy usage by tracking the real
time data.6 By analyzing this data, the customer can
determine how much energy they are using and change
their behavior to be more energy efficient by turning off
appliances and using them during lighter demand periods. In order to get the full potential of those savings,
the customer may need to switch to real time pricing.
In addition, there will be more accurate billing that
will not require a meter reader to visit each customers
meter to determine energy usage. Estimate billing will
Real-Time Pricing
As smart grid technology is installed, customers will be
able to have a higher awareness of their energy consumption and may look to demand side management techniques as a solution to lowering their energy cost.8 But
in order to reap the full economic benefits of demand side
management, customers might need to purchase energy
with real-time pricing.8
As shown in Figure 2, the price of electricity varies
23
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sumption will hopefully lead to more energy efficient behaviors and practices. This in turn would reduce the
demand on the system, especially during peak periods.
A steadier electrical load on the system would be a great
benefit for the grid operators by eliminating the peaks
and troughs that are typically experienced throughout a
day. The utility can even go further by offering demand
side management programs such as peak shaving and real
time pricing programs to help stabilize the demand on the
grid.
The utilities will benefit from the increased operational
efficiencies, as the grid operators will be able to follow
the flow of electricity in real time. Grid operators will
work with a higher degree of accuracy in the amount
of power to transmit to customers, which will reduce instances of oversupplying the system.6 Automated balancing systems will assist in this process to ensure that the
power supply on the system meets the demand.
The efficiency of grid operation will also increase with
the increased level of voltage control on the smart grid
system. In 2008, 6% of all power generation is lost on the
transmission lines.6 One factor that led to this was the
reactive power on the lines when the voltage and current
move out of phase from one another due to the presence
of inductive and capacitive loads on the system. As the
current travels through the transmission lines, reactive
power dissipates the energy and consequently some of
the current is not delivered to the load, therefore creating a less efficient system. With the smart grid, there
will be stronger voltage control and power factor optimization to diminish the amount of reactive power on
the transmission lines. These controls, in conjunction
with the automated balancing systems, will reduce these
line losses by approximately 30%, and consequently cut
down on overall demand.6
Utility will see many benefits as the smarter grid system is installed. By strengthening their reliability, the
utility will be able to respond to outages in a more
quickly and cost-effective manner during storms and
other weather events. A smarter customer base will also
benefit the utilities with more energy efficient behavior
that can assist in stabilizing the demand and creating
steadier electricity flow on the grid. Operation of the grid
will also become more efficient with automated balancing
systems to prevent oversupplying and increased voltage
control to diminish the presence of reactive power. With
these benefits, it is clear why the utilities are making the
move to this smarter system.
FIG. 2: The real time pricing of energy on November 4,
2014 in Northern Illinois. Adapted from Reference 9
throughout the day depending on the supply and demand
on the grid at each moment in time. Even though the
installation of smart meters is still in the early stages for
Illinois, real time pricing for energy has been available
for years. Illinois was the first state in the US to create a
real-time pricing program in 2007, but only 25,000 people
in the state take advantage of the program.10 Despite the
low numbers of participants, on average these customers
saved 28% on their electric bill.10 Also, with the immediate feedback and information on ones energy use and the
associated price, the average real-time pricing participant
reduced their total amount of kilowatt-hours of electricity consumed by 18%, which in turn creates a steadier
flow of electricity that is more easily maintained.10 Thus,
utilities and customers could both benefit from using real
time pricing.
Benefits to the Utilities of a Smart Grid
The benefits will not only apply to the consumer, but
to the utilities as well. The issue of reliability is not only
a concern for the customer. During storms, utilities have
to work around the clock to find and repair outages on
the system. This can be time consuming and costly, especially in the case of severe storms. With the smart
grid, the utility will have access to data that can pinpoint the location of faults on transmission lines without
needing the customer to report an outage. As a result,
the repair process can be done more quickly and more
cost-effectively.
In many ways, the benefits for customers will in turn
lead to benefits for the utility. A customer base with
higher access to the data concerning their energy con-
Energy Storage on the Smart Grid
As the grid becomes smarter, energy storage could play
an increasingly large role on the grid. For one,frequency
regulation is a major concern for the utilities as it is the
indicator of whether or not there is balance between the
supply and demand of energy on the grid.3 The grid in
the U.S. operates at 60 Hz (50 Hz in Europe) and fluctu24
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available, but the cost is much higher than just putting
in a gas plant that has an equal capacity.12 Research is
currently being performed at the US Joint Center for
Energy Storage Research (JCESR) at the Argonne National Laboratory in Illinois to develop a battery that
is five times more energy dense and five times cheaper
than the lithium-ion battery.13 Whether or not this goal
is obtained, extensive research is necessary before energy
storage becomes a viable, commonplace solution to issues
of the electrical grid.13
ates as the balance of load and demand shifts throughout
the day.3 By maintaining this frequency, balancing authorities prevent blackouts and the malfunction of equipment on the grid. If there is an interruption on the system
like a power plant going offline or a fault in a transmission line, the frequency will drop almost instantaneously
within milliseconds.3 Storage technologies like flywheels,
capacitors and superconducting magnetic energy storage
can respond quickly to provide the power to meet this
sudden demand on the system and prevent catastrophe.3
These technologies are most effective in short burst of
time on the scale of seconds and could not support the
grid for long periods of time.Additionally, due to the
smart grids automation capabilities, this process will be
expedited to prevent chain reaction blackouts.6
Load swings occur daily on the grid as people wake up
and businesses begin their operation for the day. Energy
storage to support the grid for longer periods of time
can help put relief on the generation side as power plants
come online. This would require the storage to provide
energy on the scale of an hour or so. Battery and flywheel technology are among the strongest candidates to
fill this role.Community energy storage (CES) is a good
example of how the smart grid and energy storage can
work together to improve the system. The utilities would
put small energy storage units on the electrical feeders in
residential areas that would supply power for one to two
hours during peak demand periods.? This would help
with voltage control, reliability and meeting the demands
of a changing load profile that can be brought on by new
technologies like variable distributed generation and electric car charging stations.? CES can especially help with
residential PV panel generation because the peak production period precedes the peak demand period by two or
three hours.? By storing that energy during the peak
production, the overall peak demand on the grid can be
reduced and create a steadier power flow.
Looking at the grid with economic mindset, one can
see that energy has become a commodity. As a commodity, the economic principles of supply and demand apply to energy the same way it would on any other product. When the demand for energy goes up during the
day, the price increases to meet the demand. As the demand diminishes, the price does as well. Energy storage
can thus be used for an energy arbitrage (or behind-themeter) system. In such a system, energy is purchased
and stored during the low electricity demand periods during the night and then used (or resold) during high demand periods.8 This can cut down on the peak demand
of the grid and create a steadier power flow that is easier to maintain and operate.11 However, if enough people
adopted this practice, the demand would rise during the
night. As a result, this would drive up the cost, defeating
the purpose of an arbitrage system in the first place.
There are challenges to integrating energy storage onto
the smart grid. For one, the price of energy storage is
very high at the moment. Currently, lithium-ion batteries have the highest energy density (energy per weight)
A Case Study: Stem
Startup companies have already started to create business models based around the combination of smart grid
and energy storage technologies. Stem uses cloud-based
data to predict energy prices with dynamic demand response systems.11 Then based on the data, the system determines if energy should be consumed from grid and/or
stored or to use energy that was already stored on a battery.Their customers are able to perform their normal
operations without any changes in behavior, but still realize savings from the time-shift of energy consumption
from the grid.11 The company has received large amounts
of funding that have allowed the company to shift their
work fromsmall-scale battery storage for solar panels to
large grid-scale energy storage systems.11
Stem recently acquired $100 million in funding to start
a leasing program for energy storage systems to nonresidential customers with no upfront costs while earning a profit through energy arbitrage.13 Companies like
SolarCity and Sunrun used this leasing model with PV
solar panels in the residential sector, and it has created
significant growth in the solar market.12 Can this model
create the same growth with energy storage technology
as well? The answer is yet to be determined. Stem has
already started to partner with utilities to incorporate
storage technology into the utilitys operations.12 For example, Stem is working with Hawaiian Electric to start
a 1-megawatt pilot project in the island of Oahu where
transmission is limited.12 Hawaiian Electric has a longterm goal of a 200-megawatt grid-scale energy storage
system.12
Stems example demonstrates how utilities may need to
team up with energy storage startup companies in order
to stay relevant in the grid of the future. As the utility
market evolves, partnerships with companies like Stem
or the development of energy storage system by the utilities may become essential for the survival of the utilities,
especially as renewable energy sources with variable generation, such as wind and solar, become more prevalent
on the grid.6
25
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states have seen similar opposition to the smart grid because the consumers do not want to pay anymore than
they already are. Utilities hope this rate increase will
prevent further rate increases down the line that could
be the result of storm repairs and other operational difficulties that the traditional grid would bring on.
Additionally, there are people on the right wing against
the smart grid as well. With smart meters, the utility is able to track their customers energy use, which to
some is seen as an invasion of their privacy. The utilities can observe when a customer is using energy and
how much. These activists are afraid this infringes on
their liberty and that this information could be sold to
third parties for profit.15 Utilities, like PG&E of California, however claim that the information belongs solely
to the customer, and no information will be sold to any
third party.15 There are also concerns that the utilities
will be able to shut down appliances such as air conditioning units on hot days when the demand on the grid is
high. Currently, there are opt-in programs to participate
in these sorts of peak shaving practices, but it is completely voluntary on the part of the customer and can
lead to financial gains.
The Challenge of Microgrids for Utilities
The smart grid allows for both centralized and distributed power generation with multidirectional power
flow.2 In other words, customers will have the capability
to receive and send out energy if there is on-site generation such as PV solar panels or large diesel generators. With this on-site generation, a microgrid is formed
where power generation and the end user are directly
connected.14 Some people in the utility industry, like
David Crane, CEO of NRG Energy, believe that distributed generation and microgrids will bring about the
demise of the electric grid.14 Utilities lose revenues from
customers with PV panels while they still use the transmission lines for a back up power supplies and sell excess
solar power back onto the grid.14
However, due to the nature of variable generation,
home PV solar panels cannot be fully relied upon. The
reliability of the grid could be its saving grace that keeps
them afloat. Yet, this dilemma of microgrids for the utilities would be exacerbated with effective, affordable energy storage. Even when the sun is not shining, stored
solar energy could power homes and businesses. Regardless, with the established system going through drastic
changes, utilities will have to be innovative to find their
place in the grid of the future in order to survive.
Conclusion
As the country integrates smart technology onto the
electric grid, there will be dramatic changes to the system that, in some respects, has remained unchanged
for a hundred years.5 The benefits are extensive and
will be felt by the utilities, consumers and the environment. The utilities will have higher operational efficiency with stronger voltage control and reduced transmission lines losses.6 The consumers will have more reliable service and an increased awareness of their energy
usage, leading to greener practices and behavior.6 The
presence of the smart grid can also drive advancements
within the field of energy storage technologies to perform
functions such as frequency regulation, load leveling and
load stabilization.2,3 Companies like Stem have already
started to find success in combining smart grid and energy storage technologies. The smart grid does not resolve all energy issues however. As technology continues
to advance, utilities may face challenges with the emergence of microgrids. The utilities will need to be innovative in order to stay relevant in the world of the smart
grid. Additionally, there are customers that are strongly
against smart meters due to increased rates and claims
that smart meters cause health issues and invade peoples
privacy.However, the overall benefits of a smarter grid for
customers, utilities and the new technological market it
creates seem to outweigh the challenges.
Those Against the Smart Grid
While there are many benefits from the smart grid,
there are groups that are strongly against the installation of smart grid. There are claims that it can cause
health issues such as seizures, migraines and heart troubles due to the radio frequency electromagnetic emissions
from the wireless smart meters, similar to those found in
cell phones.15 However, in 2005, The World Health Organization stated that there is no proof that the wireless technology leads to any health problems.15 On top
of this, the emissions from a smart meter are about 5%
of the radio frequency emission from a cell phone.15 Despite this, anti-smart meter groups have lobbied to make
laws banning smart meters in parts of California,like the
town of Fairfax and Marin County.15 There are opt-out
programs offered by the utilities to accommodate those
who do not want smart meters, but there is a fee and a
monthly charge on the customers electric bill.15
The installation will also bring upon higher electricity rates for the customer to account for the cost of the
smart meter deployment. In Illinois, a bill was passed
that allowed its two major electrical utilities, ComEd and
Ameren, to increase their rates to raise 2.6 billion, half of
which going towards the smart grid deployment.16 Other
1
U.S. Department of Energy, Smart grid system report: Report to congress (2014), http://energy.gov/sites/prod/
files/2014/08/f18/SmartGrid-SystemReport2014.pdf.
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2015
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
U.S. Department of Energy, Tech. Rep. (2014),
http://energy.gov/sites/prod/files/oeprod/
DocumentsandMedia/DOE_SG_Book_Single_Pages(1).pdf.
U.S. Department of Energy, Tech. Rep. (2013),
http://energy.gov/sites/prod/files/2013/12/f5/
GridEnergyStorageDecember2013.pdf.
M. Landrieu,
Electric grid reliability,
Senate
Hearing
(2014),
http://www.gpo.gov/fdsys/pkg/
CHRG-113shrg87851/html/CHRG-113shrg87851.htm.
J.A. Damask, Tech. Rep., Buckeye Institute for Public
Policy Solutions (1999), http://heartland.org/sites/
all/modules/custom/heartland_migration/files/
pdfs/2934.pdf.
S. Borlase, Electric Power and Energy Engineering, Volume 1:
Smart Grids (CRC Press, London, GBR, 2012), http://site.ebrary.com/lib/uic/
docDetail.action?docID=10611458.
Tech. Rep., National Academy of Engineering (2000),
http://www.mae.ncsu.edu/eischen/courses/mae415/
docs/GreatestEngineeringAchievements.pdf.
B. B. Davito, H. Tai, and R. Uhlaner, Tech. Rep., McKinsey Institute (2010), https://las493energy.files.
wordpress.com/2014/09/the_smart_grid_promise_
demandside_management_201003.pdf.
Live Prices - ComEd RRTP (2014), https://rrtp.comed.
com/live-prices/.
J.
Tomich,
Comed
real-time
pricing
customers reaped big savings in 13 (2013), http:
//www.citizensutilityboard.org/news20140501_
EEPublishing_RTPKolata.html.
E. Wesoff, Stem, a reinvented storage startup,
leverage batteries and the cloud (2012), http:
//www.greentechmedia.com/articles/read/
stem-a-reinvented-storage-startup-leverages-batteries-and-the-cloud.
E. Wesoff, Stem banks $100m to finance nomoney-down
energy
storage
(2014),
http:
//www.greentechmedia.com/articles/read/
Stem-Banks-100M-to-Finance-No-Money-Down-Energy-Storage.
R. Van Noorden, Nature 507, 26 (2014).
C. Martin, M. Chediak, and K. Wells, Bloomberg Businessweek (2013).
S. Michels, California activists want smart meters banned,
claim theyre bad for health (2013), http://www.pbs.org/
newshour/bb/science-july-dec13-meters_08-27/.
S. Mufson, Washington Post (2011).
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2015
A Review of Latent Heat Thermal Energy Storage for Concentrated Solar Plants on
the Grid
Edwin Flores
Thermal Energy Storage has improved the dispatch ability of Concentrated Solar Power Plants
(CSP), a renewable source of energy on the grid. Furthermore, Latent heat Thermal Energy Storage
(LTES)shows potential as a storage technology by further reducing costs and improving efficiency
for CSP plants. This papers reviews the goals for LTES, the developments in phase change materials
for LTES, the types of system configurations possible, and the challenges that LTES face. From
the scientific literature available, LTES systems can meet TES goals, and research is progressing in
making it a scalable technology for CSP plants.
Introduction
Thermal Energy Storage (TES) has gained traction
over the years as a way to store energy from renewable
sources. Particularly, it has gained more attention in the
application of Concentrated Solar Power (CSP) plants.
CSP plants concentrate the suns rays to heat a heat
transfer fluid that is fed it into a turbine system connected to a generator to produce electricity (see Figure
1). TES can improve the dispatch ability of CSP plants
by allowing the plant to store the suns heat during offpeak times and discharge it during peak times in the
absence of the sun or during shorter cloudy times, and
thus, increases CSP plants annual capacity factor. For
an example, TES systems with 7 hours of storage can increase a CSP plants capacity factor from 25% to as high
as 43%.1 As a comparison to other renewable sources
without storage, onshore wind farms have been shown to
have capacity factors from 30% to 45% and solar PV have
16% to 28%.2 For this reason, increased research efforts
have focused on the relationship between TES and CSP.
National efforts have grown in support of high temperature TES in conjunction with CSP technology. In 2011,
the U.S. Department of Energy launched the SunShot ini-
FIG. 1: Illustration of CSP plant layout consisting of a
solar field, TES, and power block subcomponents.
Adapted from Reference 3.
tiative as a collaborative project to make unsubsidized solar energy cost competitive by the year 2020.4 The goal is
to reduce the installed cost of solar energy systems about
75% of 2010 costs in order to increase large-scale adoption of the solar technology.4 One focus of the program
involves CSP technology with TES. Its specific milestone
is to achieve a subsidy-free levelized cost of electricity of
0.06 kW/h by the end of 2020.4 Current records for 2013
show an achievement of 0.13 kW/h, which is about 62%
reduction from 2010s 0.21 kW/h.4 The Sun-Shot Initiative is benefiting research in lowering costs and improving
performances of TES in CSP plants.
With regard to TES technology, three types of energy storage exist:sensible heat, latent heat and thermochemical. Sensible heat Thermal Energy Storage
(STES)uses either a solid or liquid as storage media
by increasing the temperature of the medium as more
thermal energy is stored. Latent heat Thermal Energy
Storage(LTES or LHTES) stores energy by converting
storage material from one phase to another (typically
a solid to liquid to reduce volumetric costs). Materials used in LTES are commonly referred to as Phase
Change Materials (PCMs). Thermochemical Thermal
Energy Storage stores and discharges energy by making and breaking chemical bonds. Each Thermal Energy
Storage type has different research projections, with sensible heat TES (STES) being the most mature technology
for CSP plants, followed by LTES, and thermochemical
TES as being the least mature technology.1
Various reviews have focused on TES with regard to
different design types and their techno-economic challenges.1,3After the successful commercialization of Sensible heat TES, Latent heat Thermal Energy Storage(LTES) is the next concept that can hold better performance for CSP plants, if designed well. This review
paper will focus on current trends in LTES for application in CSP plants. The general workings of LTES
systems and their ideal qualities for TES are explained,
and the current status and research on storage media
for LTES is reviewed below. Possible design types, challenges and future research possibilities for LTES are also
presented. This paper will present the reader with the
status of LTES and its potential in improving CSP plants.
TABLE I: Key requirements for developing TES
systems for CSP plants.Table data taken from
References 1,6
1 High energy density capability of storage material
2 Efficient heat transfer between the storage material and HTF
3 Fast response to load changes in the discharge mode
4 Low chemical activity of storage material and HTF towards the
materials of construction
5 Good chemical stability of storage material/HTF and temperature
reversibility in a large number of thermal charge/discharge cycles
comparable to a lifespan of the power plant, 30 years
6 High thermal efficiency and low parasitic electric power for the
system
7 Low potential contamination of the environmental caused by an
accidental spill of large amount of chemical used in the TES system
8 Low cost of storage material, taking into account the embodied
energy (carbon)
FIG. 2: T-s diagram for steam production in a power
plant with thermal storage process. Adapted from
Reference 5
9 Ease of operation and low operational and maintenance costs
10 Feasibility of scaling up TES designs to provide at least 10 full
load operation hours for large-scale solar power plants of 50 MW
electrical generation capacity and larger
General LTES Systems Workings and Goals
Phase Change Materials and Enhancement Methods
Latent heat Thermal Electric Storage(LTES) works
through an isothermal process in which the storage material changes phases. This characteristic can provide
enhanced storage capacity when compared to Sensible
heat Thermal Energy Storage (STES)systems of the same
temperature range since STES is limited by the need to
store at higher temperatures to achieve the desired output temperature for the turbine systems. Also, LTES
can potentially enable a smaller, more efficient, and lower
cost alternative to STES due to storage capacity being
related to enthalpy of phase change of the material (see
Figure 2). For this reason, LTES commonly uses phase
changes between solid and liquid states, due in large part
to its lower volumetric expansion, compared to liquidgas, and its high latent heat compared to solid-solid transition.
After years of research development in overall TES
technology, beginning with sensible heat TES, various
characteristics have emerged to define an ideal TES.
DOEs requirements for TES under the SunShot Initiative seeks to improve heat transfer and thermal energy
storage medium, lower the systems costs to less than 15
kW/h, increase the exegetic efficiency to greater than
95%, and lower material degradation due to corrosion
to less than 15 m/year.4 Research in Europe proposed
a methodology that summarized the key requirements
for developing a thermal energy storage system for CSP
(see Table I).6 These are currently the targets that researchers in TES would like to achieve. LTES systems
show potential in fulfilling several of these categories including reduction of component materials and costs and
increasing high energy densities capabilities when combined with thermo-enhancing elements.
Phase Change Materials (PCMs)are the storage
medium for LTES systems. Their energy storage capability depends on the thermal properties of the materials. For PCMs, a high latent heat of fusion is desired
in order to function within the high operating temperatures for the power block (or turbine system of the CSP
plant). Additionally, high thermal conductivity for materials is desired since it positively affects the dynamics of
heat transfer and the performance of the system. It was
shown that a reduction of heat transfer tubes for concrete
TES systems was possible for concrete with higher thermal conductivities.7 Table II presents some of the potential PCMs for LTES systems. Although thermal physical
properties vary across material types, ways to improve
some of these properties to fit the desired ranges for a
CSP plant do exist.
Some research has focused on improving the low thermal conductivities of PCMs. Recent experiments have
demonstrated that adding high thermal conductivity materials such as graphite, metal fibers or metal/ceramic
matrices to PCMs can increase heat transfer.10 A recent study that added graphite foam to the PCM material, Magnesium Chloride (MgCl2 ),similarly indicated
that heat transfer was greatly enhanced due to graphites
high thermal conductivity, processing ability and chemical stability.11 Some of the key findings for graphite foam
LTES were that round trip energy efficiencies increased
from 68% to 97% and that the number of heat transfer pipes decreased by a factor of eight, which can lead
to significant cost reductions.11 By improving material
properties of PCMs, greater system performances can be
29
©2015 University of Illinois at Chicago
tween the tanks (common in sensible heat thermal energy
storage), or passive systems, where the storage medium
is solid, contained in one tank, and the heat transfer
fluid passes through the storage material only for charging and discharging.Most Latent heat Thermal Energy
Storage (LTES)systems are passive system types to ensure that phase changes occur in a controlled environment. The systems in this review are:embedded structures LTES,packed bed/thermocline LTES, and cascading LTES1,3 ,each with their own unique characteristics
in improving the overall performance of LTES.
TABLE II: Published data on potential latent heat
storage materials. Table data taken from from
References 1,8,9.
Tmelt Tmelt Material
[◦ C] [◦ F]
Latent
heat of
fusion
[J/g]
Thermal
cond.
[W/(m
K)]
307
585
NaNO3
177
0.5
318
604
77.2 mol% NaOH-16.2% NaCl-6.6% 290
Na2 CO3
320
608
54.2 mol% LiCl-6.4% BaCl2 -39.4% 170
KCl
335
635
KNO3
88
340
644
52 wt% Zn-48% Mg
180
348
658
58 mol% LiCl-42% KCl-33% NaCl 170
380
716
KOH
380
716
45.4 mol% MgCl2 -21.6% KCl-33% 284
NaCl
381
718
96 wt% Zn-4% Al
397
747
37 wt%
Li2 CO3
443
829
59 wt% Al-35% Mg-6% Zn
310
450
842
48 wt% NaCl-52% MgCl2
430
0.96
470
878
36 wt% KCl-64% MgCl2
388
0.83
487
909
56 wt% Na2 CO3 -44% Li2 CO3
368
2.11
500
932
33 wt% NaCl-67% CaCl2
281
1.02
550
1022
LiBr
203
632
1170
46 wt% LiF-44% NaF2 -10% MgF2 858
1.20
660
1220
Al
398
250
714
1317
MgCl2
452
149.7
Na2 CO3 -35%
0.5
0.5
Embedded Stuctures LTES
138
K2 CO3 - 275
One concept of embedded structures LTES employs
pipes embedded in the storage medium to transfer heat
effectively as the heat transfer fluid passes through the
pipes. One study used a shell and tube heat storage
unit as a LTES because of its high efficiency and relatively smaller volume.13 In this particular unit, the Phase
Change Material (PCM) fills the annular shell space
around the tube while the heat transfer fluid flows within
the tube and exchanges the heat with the PCM.
2.04
Another embedded structure LTES uses a sandwich
concept for PCM using different fin materials made from
graphite or aluminum.13 The fins are mounted vertical to
the axis of the tubes and the PCM is placed between fins.
The fins enhance the heat transfer between the PCM and
heat transfer fluid.13 Systems such as these have increased
the thermal power densities to 10-25 kW m−3 .1
achieved, leading to cost reductions in other components
such as heat exchangers.
Other methods to improve the heat transfer of PCM
materials are macro- and microencapsulation.
Encapsulation involves putting PCMs in capsules made
from higher conducting materials as a way to overcome difficulties of low heat transfer of large masses
of PCM. Moreover, encapsulation of PCMs reduces
the PCMs reactivity with the surrounding environment
and controls the changes in the PCMs volumes as
the phase changes occur.1 Macro-encapsulation involves
larger sized capsules, but requires labor-intensive manufacture processes.3 On the other hand, microencapsulation involves smaller sizes and cheaper batch manufacturing processes. For this reason, micro-encapsulation is
investigated more often, particularly for radius sizes such
as 0.5cm.12 By encapsulating PCMs, less storage medium
is needed while thermal conductivity increases, thus leading to potential cost reductions for LTES systems.
Another concept under consideration is the employment of embedded gravity assisted Heat Pipesor thermosyphons between a PCM and heat transfer fluid in
order to improve heat transfer (see Figure 3). Many researchers have focused their efforts on investigating the
system properties of these systems.14-17Heat pipes have
a high thermal conductivity, allowing them to increase
the rate of heat transfer between the heat transfer fluid
and PCM.18Additionally, heat pipes can be modified to
operate passively in specific temperatures ranges and can
be fabricated in a variety of shapes.
Of all these variations for embedded structures LTES,
heat pipes LTES has been studied closely for integration to a modeled Concentrated Solar Power (CSP) plant.
One study found that longer heat pipe condenser length
increased the surface area available for the phase change,
enabling higher energy storage.14 However, a tradeoff between storage costs and heat exchange rates between the
heat transfer fluid and PCM emerged due to optimum
longitudinal spacing between heat pipes.14 This study is
critical for understanding the methodology to determine
the optimum design for LTES embedded with heat pipes
when working in conjunction to CSP plant operation.
Latent Heat Thermal Energy Storage System Types
In order to achieve the desired efficiencies and energy capacities for Thermal Energy Storage (TES), various system designs have been proposed. In general,
these systems are categorized in two groups: active systems, where the storage medium is a fluid and flows be30
FIG. 4: Schematic illustration of: (a) latent thermocline
storage system (LTES), (b) capsule cross-section.
Adapted from Reference 14
FIG. 3: Schematic illustration of: (a) close up view of
HTFs pipe with gravity heat pipes (b) gravity heat
pipes embedded latent thermal energy storage
(HP-TES). Adapted from Reference 14.
different temperature ranges to convert liquid to gas (an
isothermal process) and a higher temperature to superheat the steam in order to increase the turbine efficiency
(see Figure 2). A cascading system would utilize certain
PCM materials in discharging energy during the liquidgas phase of the steam Rankine cycle. Then other PCMs
would provide the energy at the appropriate high temperature to superheat the steam in order to increase the
turbine efficiency.
Currently several options exist for LTES systems that
improve heat transfer between PCMs and heat transfer
fluid and lead to material and cost reductions. Moreover,quantitative studies have been performed on some
systems to estimate the LTES performance on the storage system by itself or connected to a CSP plant.14,15
These studies are paving the way to better understand
the performances and feasibility of LTES systems before
actual construction, investment, and testing of a fullscale LTES system connected to CSP plant.Continued
research on LTES system designs will help overcome the
challenges in construction of full-scale LTES system.
Packed bed systems
Packed bed systems use a storage tank that contains
material elements, in this case PCMs, positioned in various shapes and sizes. These elements help transfer heat
from the heat transfer fluid as it flows between these elements and stores the thermal energy for later use.These
systems act similar to a thermocline system, in which
thermal gradient develops within the tank (see Figure
4). The hot heat transfer fluid is pumped to the top
in charging mode and pushes the colder fluid to the bottom. These elements act as filler materials to maintain an
ideal thermal gradient and reduce the natural convection
within the liquid. These systems are mostly single tanks,
which reduce costs compared to sensible heat Thermal
Energy Storage systems that use two tank systems to
establish a thermal gradient.
Packed bed systems with encapsulated PCMs have
been studied in detail with regard to their dynamic
performance when incorporating operational conditions
found in aCSP plant.Researchers found that the use of
smaller PCM capsules (.0075 m or less) can greatly reduce the tank volume by up to 40% compared to similar
systems with sensible materials.15 Overall, the numerical
study showed the effectiveness of packed bed thermocline
systems and provided a procedure to design an optimum
system with operational requirements for a CSP plant.
Challenges of Latent Heat Thermal Energy Storage
Although research has established tools for understanding and evaluations on the potential of Latent
heat Thermal Energy Storage (LTES) systems, various
challenges still exist in the deployment of LTES systems.These challenges have been reviewed by Steklie et.
al, and are elaborated in this section on tank Phase
Change Material (PCM) TES systems and encapsulated
TES systems.3 Some of the setbacks are attributed to
poor heat performances, manufacturing difficulties, and
durability of LTES systems. Nonetheless, opportunities
do exist for future research to investigate these setbacks.
Challenges for Tank PCM TES systems include poor
heat transfer, low energy efficiency, and added costs of
Cascading Systems
Cascading systems utilize more than one PCM in a single tank in order to maximize the exergetic efficiency.16,17
Since CSP plants utilize a steam Rankine cycle, they need
31
to overcome poor heat performances of PCMs, to discover manufacturing methods for microencapsulation,
and to test the durability of LTES after various charging/discharging cycles. Moreover, determining and lowering the costs for design and construction processes
need further investigation before a full scale LTES is
implemented to a CSP plant.A study by Nithyanandam
et al. provides one example of research analyzing the
techno-economic feasibility of LTES systems and performance when coupled to CSP plants. This study found
that Encapsulated PCM TES and LTES with embedded
heat pipes with a two-tank Sensible heat thermal energy
storage system can meet SunShot requirements in costs
(6 ¢kWh−1 ), energetic efficiency, and performance for
Supercritcal-CO2 cycles rather than steam cycles.12 With
regard to steam Rankine cycle, the Levelized Cost Of
Electricity (LCOE)for both systems were higher, ranging
in 9-10 ¢kWh−1 .12 Research like this shows promise for
LTES systems to become a competitive TES technology,
even despite the above named challenges.
cascading system variations. Typically, large tanks filled
with a PCM (commonly a salt) suffer from a low heat
discharge rate due to a buildup of solids near the heat
transfer area.3 The poor energetic efficiency is due to the
fact that TES systems incorporating one PCM can only
operate at a given temperature. This limits the TESs
operation to specific parts of the steam Rankine cycle
for a Concentrated Solar Power (CSP) plant. Various
solutions have been proposed to counter these challenges,
although bringing with it an added cost to the system
as a whole. Further research is needed to ascertain if
the added cost of improvements to the tank PCM TES
systems pays off in the performance of the overall CSP
plant while reducing costs in other components such as
pipes, heat exchangers and PCM.
Encapsulated PCM TES systems has its own set of
challenges, which focus on the manufacturing methods
needed to create a durable capsule for high temperature
CSP plant. Since encapsulation of PCM requires high
temperature durability and a void space to allow for expansion of the material, methods of manufacturing encapsulated materials for these applications need further
studies.1,3 Moreover, higher costs are assumed in developing such manufacturing methods and testing their quality control to ensure the encapsulated PCMS last for 30
years as required for CSP plants. Further research on
analyzing manufacturing methods, testing the reliability
of the production, and lowering its costs is called for in
order for Encapsulated PCM TES systems to be a viable
model.
Other challenges that LTES systems face,in common
with other sensible heat TES, are corrosion at high temperatures and loss of mechanical strength at high temperatures. Standard materials of containment construction
such as carbon and stainless steel have corrosion rates
higher than 15 µm yr−1 at 650 ◦ C when containing solar
salt.1 Corrosion measurement techniques for high temperature applications are also time intensive in gathering
data (months to years) and limited to testing a set of
conditions at a time.3 Additionally containment materials lose mechanical strength at high temperatures. Methods such as internal insulation and shot peening (a type
of strain hardening)do exist to mitigate these strength
losses, but further research is suggested in evaluating
any side effects when containing various PCM for many
charging/discharging cycles and any associated costs for
maintenance.
This section clearly indicates more research is needed
1
2
3
Conclusion
The status of LTES systems in the areas of requirements, material components, systems types, and challenges has been reviewed above. Research on LTES systems continues to look toward enhancing PCMs thermophysical properties,while trying to be cost effective.
Moreover, several types of LTES systems indicates that
they are capable of increasing thermal storage and heat
transfer and at the same time offsetting other system
costs.The challenges for LTES present opportunities of
future investigations toward developing better tank storage materials with less corrosion and improving manufacturing practices for encapsulated phase change materials. Although full-scale LTES systems have not yet
been built, numerical models estimate that encapsulated
phase change materials TES and heat pipes TES have
potential in meeting performance and cost requirements
compared to todays sensible heat TES technologies. A
pilot plant for LTES is needed to validate numerical models and show further insight on the feasibility of LTES.
Research in LTES systems in general point toward material research and manufacturing research in order to set
the stage to build a full scale LTES system and prepare
it for commercialization in the future.
4
S. Kuravi, J. Trahan, D. Y. Goswami, M. M. Rahman,
and E. K. Stefanakos, Progress in Energy and Combustion
Science 39, 285 (2013).
Transparent cost database (2009-2014), URL http://en.
openei.org/wiki/Transparent_Cost_Database.
J. Stekli, L. Irwin, and R. Pitchumani, Journal of Thermal
Science and Engineering Applications 5, 021011 (2013).
5
6
7
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S. E. T. Office, SunShot initiative portfolio book. (????).
R. Tamme, D. Laing, and W. D. Steinmann, Journal of
Solar Energy Engineering-Transactions of the Asme 126,
794 (2004).
R. Adinberg (2010).
D. Laing, W.-D. Steinmann, M. Fi, R. Tamme, T. Brand,
and C. Bahl, Journal of Solar Energy Engineering 130,
8
9
10
11
12
13
14
15
16
17
011006 (2007).
M. M. Kenisarin, Renewable & Sustainable Energy Reviews 14, 955 (2010).
H. Garg, S. Mullick, and A. Bhargava, Compilation and
indexing terms (Elsevier, Inc., 2014), chap. Solar thermal
energy storage.
J. Wu, J. Li, X. Xu, L. Yang, J. Wu, F. Zhao, and C. Li,
Journal of Wuhan University of Technology-Mater. Sci. Ed.
24, 651 (2009).
W. Zhao, D. M. France, W. Yu, T. Kim, and D. Singh,
Renewable Energy 69, 134 (2014).
K. Nithyanandam and R. Pitchumani, Energy 64, 793
(2014).
W.-D. Steinmann, D. Laing, and R. Tamme, Journal of
Solar Energy Engineering-Transactions of the Asme 131
(2009).
K. Nithyanandam and R. Pitchumani, Applied Energy
126, 266 (2014).
K. Nithyanandam, R. Pitchumani, and A. Mathur, Applied Energy 113, 1446 (2014).
Y.-Q. Li, Y.-L. He, Z.-F. Wang, C. Xu, and W. Wang,
Renewable Energy 39, 447 (2012).
H. Shabgard, C. W. Robak, T. L. Bergman, and A. Faghri,
Solar Energy 86, 816 (2012).
33
Automotive Lithium-Ion Batteries: Dangers and Safety Measures
Eugene Lee
Due to various economic, environmental, and political pressures, the automotive industry is beginning to put greater research efforts towards developing alternative energy storage technologies for
vehicles, with a particular focus on lithium-ion batteries. This new technology poses new dangers
which requires new technology to mitigate these dangers. Current and immediately next-generation
safety concerns and technologies are reviewed.
Introduction
Much of what determines whether a particular energy
storage technology will gain traction in the automobile
industry is its technical capabilities - specifically density, capacity, and charge rate. However, a technologys
safety is another major and equally pertinent concern,
particularly in the realm of vehicles. There have already
been a number of highly publicized occurrences of energy
storage-related automotive failures,1–3 and each incident
has been the cause of concern for the companies, largely
due to the possible financial ramifications. Although no
information was found of cases resulting in injury, the
mere potential has the capability of impacting a companys financial workings.4 This paper will describe the
basic workings of a common lithium ion battery system
so as to understand the dangers inherent in the most frequently seen alternative energy storage system to gasoline, and will review currently implemented safety measures to counteract these dangers.
Overview
In larger formats such as those for vehicles, lithium ion
batteries are constructed from groups of electrochemical
cells, usually separated into groups that are wired in parallel (to increase capacity, since current draw per cell is
decreased), which are themselves wired in series (to increase voltage). The operation of a basic electrochemical
cell will be described so that the operation and risks of a
battery can be understood.
An electrochemical cell operates by taking advantage
of the free energy difference between two electrodes of
different materials, such that when under no polarization, one electrode acts as an electron donor (oxidizes)
and the other electrode acts as an electron acceptor (reduces). Both electrodes are in contact with an electrolyte
that allows ions to pass between them while preventing
electron flow. If the cell is constructed such that there is
a possibility of the electrodes coming in physical contact
with one another, a separator is added to the cell. The
electrodes are then joined to an external circuit with a
load, and since there is an overall transfer of electrons,
current is generated. In a common modern lithium ion
FIG. 1: Generalized example of a Lithium-ion cell.
Adapted from Reference 5
cell, the positive electrode is composed of lithium cobalt
oxide (LCO, LiCoO2 ), the negative electrode is composed
of graphite, the liquid electrolyte is a combination of organic solvents and lithium salts such as LiPF6 , and the
separator is made of a polymer plastic like polyethylene
or polypropylene.
A quick clarification on nomenclature: the negative
electrode and the positive electrode are often referred to
as the anode and the cathode respectively. This is not
strictly correct since lithium ion cells are rechargeable,
which means that the role of the anode (electron donor)
and cathode (electron acceptor) can be reversed depending on whether one is charging or discharging the cell.
However, this document will follow convention for the
purposes of simplicity, the result being that for this example, the graphite will be labeled the anode and the
oxide will be labeled the cathode.
It should be noted that there are a variety of different
materials that can be used for the electrodes (in particular the cathode), each with its own advantages and disadvantages related to energy capacity, charge/discharge
rate, and other characteristics. This paper will use the
LCO chemistry here as an example because it is one of
the most extensively used - and therefore well studied materials. Furthermore, even though the lithium cobalt
oxide chemistry is primarily used in smaller-form battery
packs such as those used in mobile electronics, the fundamental mechanisms remain similar for many different
continue to exothermically react with the anode surface.
This also exposes the intercalated lithium to the electrolyte at elevated temperatures. At lower temperatures,
lithium does not tend to react with the organic solvents
in the electrolyte, but as the internal temperature rises,
a hydrocarbon-generating reaction can proceed.9 For example, in the case of ethylene carbonate,
lithium compounds used in vehicles. The half-cell and
full reactions during discharge are as follows:6
Cathode:
Li1−x CoO2 + xLi+ + xe− → LiCoO2
(1)
Anode:
Lix C6 → xLi+ + xe− + C6
(2)
2Li + C3 H4 O3 → Li2 CO3 + C2 H4
Full Cell:
CoO2 + LiC6 → LiCoO2 + C6
(4)
The result is the presence of flammable gases such as
ethylene.
At higher temperatures, the cathode itself begins to
break down to produce oxygen. This is the point at which
the cell truly becomes its own heat source, and is referred
to as the onset temperature. In the case of LiCoO2 , the
reactions occur at ≈ 130 ◦ C and are as follows:9
(3)
In this reaction, lithium that was inserted between
graphite layers (a state called intercalation) detaches
from the carbons and oxidizes into lithium ions. The
electrolyte and separator act as a semi-permeable membrane, permitting the disassociated lithium ions to pass
through it and reassociate with CoO2 . Additionally, during the first charging process, the compounds that form
the makeup of the electrolyte decompose onto the anodes
surface, resulting in a layer called the solid electrolyte interphase (SEI).7 This layer of electrolyte material is quite
stable and crucial to the cells operation, since it prevents
more electrolyte from reducing onto the surface of the
anode while continuing to act as an ion-conducting path
between the graphite and electrolyte.8 The effect of the
SEI on cell safety will be further discussed in the Current
Safety Measures section .
Lix CoO2 → xLiCoO2 + 1/3(1 − x)Co3 O4
+1/3(1 − x)O2
Co3 O4 → 3CoO + 1/2O2
CoO → Co + 1/2O2
(5)
(6)
(7)
(8)
As the temperature climbs even higher, the separator
begins to melt, causing short circuits to form. The reaction rates and pressure continue to increase, eventually
resulting in a ruined cell, and possibly - depending on a
variety of factors, including initial charge state and cell
chemistry - explosion and fire.
Thermal Runaway
Causes of Thermal Runaway
Since the reactions in a lithium ion cell are inherently
exothermic, the greatest source of concern comes from
the possibility of thermal runaway, which is a situation
in which increasing temperatures cause a greater increase
of temperatures in other words, a thermal feedback loop.
Such a process can eventually result in catastrophic failure, usually in the form of structural damage, flames,
or in extreme cases, an explosion. This system can be
seen as nearly adiabatic because of the enclosed nature of
the environment, and can only happen when the internal
temperature of the cell reaches a chemistry-dependent
temperature threshold at which a series of exothermic reactions begin to take place in a cascading fashion. This
is not to say that these reactions happen in an orderly,
consecutive manner, but there are different temperature
thresholds that must be reached for each process to take
place, with each threshold varying with the materials involved. Once individual temperatures are reached, the
processes will contribute and receive energy from other
systems in an interconnected manner.9 These reactions
are detailed below.
The first component of the cell that breaks down is the
SEI. This can begin to happen at temperatures as low
as 69 ◦ C if LiPF6 is present in the electrolyte.10 Once
the SEI layer begins to disappear, the electrolyte can
The major potential thermal runaway initiators are
overcharge, external heat, mechanical damage, external
shorts, and internal shorts. For the purposes of this paper, only overcharge, external shorts, and induced internal shorts will be considered, since external heat is fairly
self-explanatory, and the mechanisms behind mechanical
damage and external shorts are similar to those involved
in external object-caused internal shorts.
The concept of overcharge is much like it sounds: the
act of charging a battery past the recommended voltage.
In the extreme case, the result is the complete removal
of all the lithium from the cathode. The result is that
any current passing through the electrochemical cell generates heat through joule heating (also known as Joules
First Law, Q ∝ I 2 . Eventually, the rising temperature
results in thermal runaway.11
A standard testing practice for simulating internal
shorts is to slowly drive a nail through the cell.12 For the
automotive industry, this would be analogous to hitting
metal debris on the road, which then punches through the
battery pack. As the nail passes through the cell, it eventually contacts both the cathode and anode after puncturing the separator, causing an immediate short circuit
35
c
2015
and a sudden release of heat due to joule heating. Much
like the case of overcharging, this causes thermal runaway
by rapidly rising internal temperatures. Here it should be
noted that pressure buildup resulting in explosion is mitigated by the breaching nature of the process,11 since the
opening created by the puncture relieves pressure. Mechanical stress cases (i.e. torsion, impact, crushing) are
quite similar in their mechanics, since the eventual result
is the tearing of the separation layer, resulting in a short
circuit followed by thermal runaway. Additionally, there
is often tearing of the cell casing, which relieves the pent
up pressure inside the case and prevents an explosion.
There is another pathway to forming an internal short
that is unique to lithium-ion cells, and that is the formation of what are called lithium dendrites. Under certain
conditions (usually involving high charge rates13 ), the
lithium being transported within the cell can deposit onto
the graphite anode, eventually creating thin, branching
threads of lithium.14 Though the exact mechanisms that
drive the formation of the dendrites is not yet clear, the
main cause is due to the low relative voltage differential
compared to Li/Li+. In conjunction with high charging
currents, the lithium will electroplate onto the graphite
anode,15 growing larger and larger with each deposition.
Much like the nail test, a dendrite that grows long enough
to contact the cathode creates a path with minimal resistivity, and thus can cause a short circuit and rapid release
of stored energy.In the case of an external short, the anode and cathode become connected through a material of
low resistance, resulting in a continuous, high discharge
rate.16 Unlike the cases of overcharge and internal shorts,
the internal heat generated from an external short stems
largely from the high rate of chemical reactions, since
any current-generated heat would be radiating from the
conductor outside of the cell.
It must be reiterated that larger battery packs are composed of hundreds or even thousands of electrochemical
cells to form one large power source. This means that if
a damaged cell begins thermal runaway, the entire pack
is likely to do so as well, since they are all in close spatial proximity to each other. The first cell would begin
heating up and become a heat source, causing a domino
effect. The released energy would then be correspondingly multiplied by the number of cells.
FIG. 2: Schematic of a bimetallic switch. Adapted from
Reference 20
External Measures
External safety measures are more macroscopic in nature than their internal counterparts, consisting of additional components attached to the cell, pack, or even the
vehicle itself.
The remedy for overcharging is to use protection circuits, which are common and installed in most rechargeable battery technology, regardless of chemistry. Fundamentally, the circuits operate by rerouting current past
the cell if the cell is detected as being fully charged.17
Fixes for external shorts are far more varied, and are
often employed in conjunction with one another to provide a more complete solution.18 The simplest of these
is the thermal fuse, which starts off as a conductor but
abruptly changes to an insulator at a temperature threshold, cutting off current flowing to the cell.19 This process
is irreversible, much like a blown fuse in a power box, so
it is not a preferred failure method since the fuse must be
manually replaced for the system to continue operation.
The natural extension to the concept of the thermal
fuse would be to have a reversible version, such that after the temperature restabilizes, the system could resume
normal operation. Such devices exist in the forms of
bimetallic switches and positive temperature coefficient
(PTC) devices.
Bimetallic switches work by taking advantage of the
differing thermal expansion rates of two attached pieces
of metal. Upon heating, the overall geometry will warp,
with the metals curling in a direction perpendicular to
the bonded surface.
This change in shape can be used as a switch, and
will disconnect itself when too much current is running
through the system.16 These switches can come in a variety of forms, such as a cantilever like Figure 22 or a
disc,21 but the basic concept of movement due to thermal expansion is the same.
The preferred solution for modern lithium ion battery
packs is the positive temperature coefficient (PTC) device. A material with a positive temperature coefficient
Current Safety Measures
Safety measures can be categorized into three broad
categories: education, prevention, and correction. The
latter two are presently the categories of interest, since
they can be directly controlled on the manufacturing level
and are relatively separate from the possibility of human
error. This paper will further divide safety measures into
two categories for the sake of organization: internal and
external, or inside-of-cell and outside-of-cell.
36
c
2015
will increase its resistance with increasing temperature.
In polymeric PTCs (PPTCs), this is done by using a material made of a combination of conductive particles and
a polymer that transitions between a crystalline state
and an amorphous state at a temperature threshold.22
Under normal operating temperatures, the conductive
particles are held closely enough in the polymer crystal to conduct current, but when heated, the polymer
expands and increases the interparticle distances, which
increases the resistance. This will eventually result in
a much decreased flow of current, and will return to its
initial state once the temperature drops,17 ensuring that
the cell does not resume operation until the environment
is stable. This device effectively acts like a reversible
thermal fuse. PPTCs hold a number of advantages over
bimetallic switches, including lack of mechanical wear,
decreased size, and lower operating resistances.23
On a larger scale, the solution for preventing foreign
objects from intruding into the cells is fairly straightforward, since the battery simply needs to be physically
protected. Tesla Motors, Inc. makes for an excellent case
study for this, having suffered from two such incidents in
2013,1,24 despite being equipped with a quarter inch of
ballistic grade aluminum plate armor on their Model S.25
The eventual adjustment was to add three more protective components to allow the vehicle to deflect, crush,
or roll over unexpected objects in the road: a rounded
aluminum bar, a titanium plate, and a slightly angled
aluminum extrusion. The efficacy of these measures is
unknown due to lack of data, though the video evidence
of simulated situations under controlled settings provided
by Tesla Motors, Inc. is reassuring.
The most direct and brute-force way to prevent thermal runaway is to use an active cooling method. In the
case of the Chevrolet Volt, this is addressed by using a
cooling system that consists of aluminum cooling plates
and a coolant composed of a solution of ethylene glycol
and water. Each individual cell is always in contact with
a fin which has coolant running through it, while they are
individually monitored using temperature probes.26,27
Even with such preventative measures in place, system
failures are inevitable so corrective measures are required
as well. In the case of absolute failure and rapidly rising
internal temperatures, the last resort is to purposefully
open a safety vent so as to prevent pressure buildup. This
is irreversible since it physically breaches the cell, but is
necessary to decrease the chance of explosive rupture.23
The automotive manufacturer can also design against
worst case scenarios, constructing battery pack fire walls
to trap flames in preferred areas.1
Cathode
Though widely used for mobile devices, the LiCoO2
chemistry that was used as an example at the beginning
of this article is unsuitable for vehicular applications due
to its low thermal stability. Its tendency to decompose at
relatively low temperatures to produce oxygen28 - which
goes on to react with the electrolyte and generate more
heat - is not acceptable, especially for a situation that requires greater dependability as in a motor vehicle. Alternative chemistries that have been proposed to solve this
issue generally fall under three structural types(see Table
I: layered, such as lithium nickel manganese cobalt oxide (NMC, LiNix Mnx Co1−2x O2 ); spinel, such as lithium
manganese oxide (LMO, LiMn2 O4 ); and olivine, such as
lithium iron phosphate (LFP, LiFePO4 ).28 These molecular structures determine how the lithium is stored in
the cathode, as well as how the cathode materials interact with their environment. Though this paper will not
cover the specifics of the either the structures or their effects, such knowledge is key to understanding the reasons
for the following results.
TABLE I: Comparison of cathode materials by
structure. Adapted from Reference 28
Structure
Material
Chemical Formula
Abbrev.
Lithium Nickel
LiNix Mnx Co1−2x O2 ,
NMC
Type
Layered
Manganese Cobalt LiNi(1−x−y) Mny Cox O2
Oxide
Spinel
Lithium Manganese
LiMn2 O4
LMO
LiFePO4
LFP
Oxide
Olivine
Lithium Iron
Phosphate
The NMC chemistry is most simply viewed as a highly
modified version of the LCO chemistry, which also falls
under the umbrella of layered structures.If all of the
cobalt is replaced with nickel, there is a significant increase of theoretical capacity, but the problems of decomposition at moderate temperatures are not solved.29
This can be remedied by adding Mn and Co as stabilizing structural elements.It is apparent from the vagueness
of the stated chemical formulae in Table I that Mn and
Co quantities can vary greatly, but for the purposes of
safety, it was seen that the greatest thermal stability was
achieved when Mn and Co were added in ratios greater
than 1 to 8 versus Ni (i.e. LiNi1−x−y Mny Cox O2 , both
x and y > 0.1).29 As an example, fully charged LCObased and NMC-based cells have been directly compared
through the same induced runaway process.30 The NMC
composition was LiNi1/3 Mn1/3 Co1/3 O2 . The onset temperature in the LCO cell was seen to be 131.5 ◦ C versus
the NMC cells notably higher 175.4◦ C. NMC is thus seen
Internal Measures
Internal safety measures are dominated by novel material chemistries, designed with properties such that the
cells components themselves (i.e. cathode, anode, etc.)
do not fail in the aforementioned modes.
37
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problems is lithium titanate (LTO, Li5 Ti4 O12 ). Until recently, it was believed that LTO did not form an SEI
at all, which would remove concerns of SEI decomposition forming flammable gases. Indeed, experiments were
run on a cell with an LMO cathode and LTO anode
where the cell was discharged and heated to 100C for
12 hours, and no trace of any flammable hydrocarbons
were detected.35 This has since been shown to be untrue; a stable SEI film develops on LTO anodes during
normal cycling processes.36 From a practical standpoint,
this finding has no impact on LTOs efficacy in terms of
safety. It does not void previous results, so the conclusion remains much the same as before, since experimental
results continue to show excellent thermal stability.
LTO also provides an effective solution to the problem of dendrite formation. Since lithium plating (and
therefore dendrite formation) is dependent on the close
relative voltage of lithium intercalation in graphite, the
simple solution is to have the lithium interjection process happen at a higher voltage separation. In the case
of LTO, this happens at around 1.55 V versus Li/Li+ ,37
effectively negating the possibility of dendrite formation.
There are two large downsides of LTO: first, in its
basic, unaltered form, it has a low conductivity, which
means that it has poor power characteristics.37 Equally
importantly, the cell has a smaller operating voltage, also
reducing the capacity.35 Research is currently being done
on either modifying the structure (by doping) or the surface (by using additives such as carbon nanotubes) to
increase conductivity,37 while the possibility of coating
graphite with a layer of LTO to combine the best features of both materials remains.38
as a promising next-generation material, but these stability improvements still have much room for improvement,
especially in light of the following two materials.
The LMO cathode type has a spinel structure that allows for the lithium ions to flow easily through the lattice,
resulting in high charging and discharging rates.31 More
pertinently, the material begins to decompose at much
higher temperatures than the layered materials due to
structural stability regardless of charge state. The result
is an impressive onset temperature of roughly 260◦ C.32
In addition, the total amount of released energy is less
than half of that of the tested layered cathode.32 The
main drawbacks of LMO as a cathode material stem
from issues with cycleability (i.e. short overall lifetime),
and less theoretical capacity than the layered cathode
chemistries.31
The LFP cathode type also boasts excellent safety
characteristics. Not only is the onset temperature comparable to the LMO spinel at around 240 ◦ C, the released
energy is also only slightly more than half of that of the
LMO material.32 This is attributed to the construction
of the olivine structure, which does not allow reactions
that release oxygen to proceed until temperatures as high
as 600 ◦ C.32 However much like the LMO cathode, the
most significant difficulty that the LFP chemistry faces is
a much decreased capacity compared to either the LCO
or NMC chemistry, showing a theoretical capacity only
two-thirds that of LCO.33
Cathodes which use integrate positive temperature coefficient devices as additives have also been suggested.34
As PTC devices are normally located outside of the cell,
they do not prevent internal shorts. This solution would
prevent the internal heat of the cell from ever surpassing
a certain temperature limit as set by the PTC elements,
even in the advent of a cell puncture or dendrite contact. Naturally, the largest hurdle for this material is
that substituting some of the active material with PTC
compounds results in a lower capacity.
Separator
Though the separators role in the lithium ion cell is
straightforward, it must contend with a stringent list of
requirements. It is crucial that it maintains structural
integrity throughout the cells operation. If there was
either mechanical or thermal breakdown, the electrodes
would come into direct contact with each other and start
thermal runaway. The separator can also be designed
such that if a runaway event begins, it can react at a
temperature threshold and prevent any further chemical reactions from taking place - a process referred to as
shutdown. Assuming reasonable ion conductivity, this
means that an ideal separator material must rate highly
in both puncture strength (for prevention of short circuits
during manufacturing) and chemical stability, all while
having an acceptable shutdown temperature (above optimal operating temperature and near the temperature
where thermal runaway begins), low thermal shrinkage,
and a high breakdown point. Though any and all of the
above can be improved, current separators have acceptable puncture strength and chemical stability, so most
research is being dedicated towards improving the latter
three properties.
Anode
Due to its low price, respectable capacity, and good
thermal stability, graphite is the de facto standard anode
material for lithium ion cells regardless of application.
There is great variation within graphite products, with
differences in structure, morphology, and particle size,
but the material itself remains unchanged. This creates
a large differential in amount of research being done on
the negative electrode versus the positive electrode since
battery performance, capacity, and safety is not bottlenecked by the anode. However, major concerns still exist:
as newer cathode materials with higher decomposition
temperatures are introduced into the cell, the SEIs role
in thermal runaway becomes a more notable issue. Additionally, since dendrite formation begins on the graphitic
electrode, that safety concern must be addressed as well.
The material with the greatest promise to solve both
38
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However, this salt can also decompose into slightly different pair:
Most modern separators are made of polyethylene
(PE), polypropylene (PP), or a combination of the two.39
PE has a lower melting point (135C) than PP (165C)
while PP has better mechanical properties, so combining
the two allows for a shutdown response at the lower temperature while still maintaining physical separation.39
This fairly small temperature buffer of 30C and a large
amount of material shrinkage39 are what are trying to be
improved.
One possible method is to coat the polymer. It was
found that coating a PE separator with a thin film of
SiO2 caused the PE to have hardly any shrinkage at
all, even when held for 20 minutes above PEs melting
point.40 However, because the stability of the separator
stemmed directly from the SiO2 layer, it was also theorized that shutdown would not occur at the normal PE
melting temperature either. That is, since the SiO2 was
reinforcing the separator, the separator would not be able
to melt until SiO2 melted, which would be at a much,
much higher temperature.
There have also been proposals in which the electrodes
themselves are coated with a separator material, forming a single electrode-separator unit. In one example, a
graphite anode was coated with a poly(p-phenylene oxide) and SiO2 layer.40 The resultant unit (referred to as
the Seperator/Electrode Assembly or SEA) showed negligible shrinking and no noticeable change in operation
at 170C. Though the SEA is unable to provide shutdown
functionality, the authors suggest that since it provides
such high thermal stability, such functionality is unnecessary.
Another proposed alternative is to fabricate a ceramic separator, which is made of tiny inorganic particles bound together by a chemical that keeps them
all together (called, unsurprisingly, the binder). In one
study, Al2 O3 was crushed into powder using a ball mill
and distributed into a poly(vinylidene-fluoride) (PVDF)
binder.41 This material was then heated to 150C and held
there for 30 minutes. The Al2 O3 -PVDF separator exhibited very little shrinkage despite the high temperature.
Though mechanical results were not listed for this study,
a common problem with many inorganic separators is a
lack of mechanical strength, making them incapable of
being tightly wound, which is a necessity for most cell
constructions.42
LiP F6 → LiF + P F5
PF5 is known to be a powerful Lewis acid, which can
result in both damage to the SEI10 and the direct decomposition of solvents like EC,44 resulting in flammable
gases. Since the primary concerns are the breakdown of
the SEI and the release of flammable gases, many proposed solutions revolve around adding compounds to stabilize the salt, as well as developing new solvent mixtures
to prevent the generation of flammable hydrocarbons.
Additionally, compounds can also be added to counteract
overcharge.
The simplest method to decrease the production of
PF5 is to increase the amount of present LiF, since that
would imbalance the equilibrium towards LiPF6 .43 However, there will always be some amount of PF5 present in
the cell, so in order to decrease the amount of PF5 that
will react with the SEI or the solvents, a small amount
of a weak Lewis base can also be added.45 Examples of
such a compound include tris(2,2,2-trifluoroethyl) phosphite (TTFP) and 1-methyl-2-pyrrolidinone (NMP).45
A promising group of solvent materials are liquid salts,
also referred to as ionic liquids (ILs). ILs are nonflammable, non-volatile, and highly stable, but have
lower conductivities than LiPF6 and form unstable SEI
layers with graphite.46 However, it was found that when
various ILs were mixed with traditional solvents (and
vinylene carbonate for assisting in SEI stability) at a percentage of around 40% IL by volume, it was possible to
achieve good conductivity while preventing combustion,
even under direct flame. Less than 3% weight loss was
observed even with the mixture held at 350C, demonstrating very good thermal stability.46
There are a wide variety of electrolyte compounds that
can also confer overcharge protection either by using the
extra electric current to progress a redox reaction, or
by increasing the resistance by insulating the cathode.43
In the case of the latter situation, a compound can go
through polymerization (in which individual molecules
can link together to form long chains) at the cathode,
forming a physical layer which increases the resistance of
the system.43
Electrolyte
Conclusion
Most commercial cells use lithium hexafluorophosphate (LiPF6 ) salts dissolved in organic solvents such
as ethylene carbonate (EC), propylene carbonate (PC),
dimethyl carbonate (DMC), and ethyl methyl carbonate
(EMC), among others.43 The salts can transport lithium
between the electrodes by splitting into a cation and an
anion:
LiP F6 → Li+ + P F6−
(10)
The current state of automotive energy storage safety
seems fairly well-considered, though advancements in
technology for capacity or energy transfer will require
parallel developments in safety technology. As society demands batteries with greater capacities and power, engineers and scientists will have to contend with the increase
in stored energy. Yet even with all the theoretical and
laboratory work, the true safety challenges in the space of
automotive lithium-ion batteries will only come to light
(9)
39
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after extensive use in real-world scenarios. Until electric vehicles achieve greater market penetration, thereby
increasing the sample size of failures, developments will
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2015
Storage and Generation for Clean Renewable Transportation
Joshua Barrett
Current technology advancements have made renewable power generation and electric vehicles
feasible in todays market. As these technologies continue to merge into our systems, they create
a need for energy storage and greater demand for clean power. The electric vehicle and the grid
are going to be integrated due to the charging need of the EV. By developing the technologies
together with smart communications, they can help solve issues with a reward or solution for each
industry. Vehicle and grid connectivity is of the upmost importance as Electric Vehicles (EV) come
online. Communications and infrastructure upgrades are going to be needed as renewables and
EV technology develops. Renewable energy production tends to be intermittent and will require
storage. Adaptation of the Electric Vehicle depends on a better battery. As we strive to reduce our
dependence on fossil fuels the electric vehicles are becoming part of our means of transportation.
These changes are creating a greater need for renewable electric generation to power these vehicles
and reduce fossil fuel usage. As additional renewable power generation comes onto the grid, the need
for storage is increased. Electric vehicles will also create a large demand on the grid for charging the
batteries. Utilizing smart charging, vehicle-to-grid, and improved communications can solve these
hurdles.
Introduction
The electric vehicle (EV) is not a new concept. The
first all-electric vehicle was built in the 1800s, but it was
not until the second half of the 19th century that practical electric vehicles found its way into the market place.
Move forward a hundred plus years and people are still
toying with the idea of an all-electric vehicle. Most of the
limitations that caused the masses to adopt the internal
combustion engine are still limitations for the modern
EV. Cost, driving range and charge times are critical
hurdles that consumers face when they opt to drive an
EV. Storage technology development has made progress,
but it is still the main obstacle that needs to be solved
so the masses will adapt. Additionally, if the goal of converting from the Internal Combustion Engine (ICE) to an
EV is to reduce emissions, then the grid power needs to
come from a clean renewable source. If we are charging
EVs with power that was generated from coal generation, this can actually have a negative impact over just
burning fossil fuel in an ICE. Therefore, charging the EV
with renewables is of the upmost importance.
Current power generation throughout the grid is primarily based on fossil fuel consumption. A large part
of energy put into producing power with fossil fuels is
lost as heat. There are ways to capture and utilize heat
loss, but the majority is lost in the process. Generally
speaking, once power is generated and put on the grid,
an estimated 6-8% is lost due to transmission and distribution. By the time the end user receives the power
at the point of use or the meter, the efficiency based on
energy to energy use is very low. Current base power
output on a coal plant is estimated at 33% efficient, and
if we replace one unit of energy on the grid by utilizing
wind, solar, hydro-electric renewables, we actually reduce
three times the amount of energy units of fossil fuel being
burned to generate that same unit of energy supplying
the coal generating station.
Additional issues with the grid that consequently impact EV integration and the electric utility come with the
time of use and the need for generation to happen synchronous with the use. The use profile, or demand profile, varies throughout the day and with seasonal changes.
For the grid to operate correctly, generation for the peak
demands is needed, but only used when the system demands. This plays havoc on the economics of the power
producer. Furthermore, backup generation is needed if
a plant goes down adding standby generation into the
equation. All these costs drive the out of pocket cost for
electricity that the consumer pays. This additional cost
adds to the amount of fossil fuel consumed to generate
electricity. If we can reduce the fossil fuel usage on the
grid and add capacity for charging EVs, we can improve
the amount we pollute our environment and decrease our
dependence on fossil fuels.
The need for renewables for power generation has never
been greater and is currently one of the big providers
of new power generation going online. The EV market
places even more pressure to focus on more efficient and
cost effective renewables if the goal is to reduce dependence on fossil fuels. Renewable power generation will
impact how the grid operates and the need for energy
storage will grow with this change. The current and upcoming need for storage on the grid and in EVs has never
been greater. Developing technologies to make a reliable, affordable, and practical EV will play a great role
in helping resolve some of our problems we face today.
As technology develops and the masses move to the EV,
the potential beneficial partnership between the grid and
EV have a real opportunity to develop.
As political and security issues are at an all time high,
moving to sustainable renewable energy offers protection
against outside influences. Integrating renewable power
power at a real time price or to negotiate prices with
large customers. This is where transmission, generation
and end-user start to play a role in the economics and
limitations. Furthermore, oversight of the grid for reliability and demand puts stipulations on generation; sales,
use and other requirements the users and producers have
to meet. For example: if a base load plant is running at
full capacity in July, the temperature rise will cause electric air conditioning will come online, and the demand
will start to peak. This base load generation plant will
typically not be able to meet these additional demands.
This additional demand will start to push up the value of
the electricity being produced. This is when the regulatory agency will dispatch additional generation to come
online. A gas turbine plant peaker plant may be dispatched to provide power to the grid. Conversely, in the
middle of the night in January the demand will be very
little. This may be an issue if the demand does not meet
the plants minimum output. The plant will be generating
surplus power that it needs to sell.
Historically, natural gas generation came at a higher
cost. Coal and nuclear plants have a long start time
and the cheaper fuel makes them the natural base load
provider. Hence, peaker plants have historically been
gas turbines with low efficiency and high priced fuels. As
hydraulic fracturing (fracking) has been successful with
recovering natural gas, this type of peaker plant usage is
rapidly changing. Natural gas prices have become stable and allowed electricity generation with natural gas to
be competitive with other forms of electric generation.
Combined cycle natural gas plants are becoming common
practice in the base load production. This technology utilizes the waste heat to generate behind the combustion
side of the turbines, which helps with efficiency of the system. This system is beneficial as they are flexible with
coming on and offline with fast start up times. However,
with the use of natural gas, it continues to promote using
fossil fuel plants, as they are flexible and cheap to operate
when supplementary power is needed to go to renewable
sources. Burning natural gas does not solve the issues
that come with burning fossil fuels, but it does assist in
utilizing intermittent renewable generation on the grid.
The ultimate goal is to obtain reliable, inexpensive
power that comes from renewable sources, such as solar, geothermal, wind, hydro, biofuels, etc. Cost has
historically been renewables main hurdle. Cost hurdles
are changing as wind has undergone rapid growth over
the last ten years. Solar is also on the cusp of breaking
through and supporting itself in grid applications. All of
these sources need to be considered when designing EVs
and the most cost effective means of production.
Solar has a great fit within the grid power production needs, or demand curve. Typically solar generation
has been costly and comes at a premium. However, this
is changing. The Department of Energy (DOE) SunShot
Initiative has set some aggressive goals to reduce the cost
of solar generation. These goals are to make solar competitive with other forms of generation by the end of the
generation and the development of electric vehicle technology at this time really makes for an exciting change.
A basic understanding is needed to fully grasp the importance of how electric vehicles and renewable energy production will integrate. This paper will determine what
role each plays, what barriers are faced and what solutions are possible when trying to develop some of these
technologies. An explanation of each independent role
and operation are given below. Once the foundation of
these processes is laid, this paper will move to discuss
the implications of the complex relationship of the EV
and renewable energy, including the grid, and finally will
examine the effects each will have on each other when integrated. Finally, this paper will examine some options
that are on the forefront of this historic change.
The Electrical Grid & Options for Renewables
The grid is one of the largest, most complex and sophisticated system humans have ever built. This system
consists of power generators that are then tied to transmission, distribution and it ends up at the consumer for
use. The current grid has very little storage capabilities,
meaning the power supplied is equal to power being generated. Usage is not consistent through the day or year.
Daily variations and seasonal variations will create a realtime demand profile, so the generation needs to happen
simultaneously with usage.
History
In its infancy stage, the grid was comprised of small
isolated generating stations in urban areas. Thomas Edison designed the Pearl Street Station in New York City
and was launched in 1882. This was a small 110 V Direct
Current (DC) generating station that served a few hundred lamps. Ten years, later Alternating Current (AC)
started to show its advantages for transmission. William
Stanley, Jr. built the first generator that used AC.
By the 1900s, the grid was dominated by small AC
generating stations in mainly urban areas. Public outcry
and the Great Depression pushed regulators to regulate
and develop service to rural areas of the United States.
By the 1930s regulated utilities provided a vertically integrated utility, and along with it came large regulated
monopolies.
Current Grid
Over the years and development of the grid, we have a
semi-deregulated market that opens the doors for some
consumers to purchase power on an open market. There
is even the opportunity to produce power for use or sell
it back on the grid and offset retail rates. Large producers have an open market that allows them to sell
42
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not an issue. This surplus power production may not be
needed to meet real time demand requirements. However
all this surplus generation may be needed at a different
time. This peak generation demand, which is also called
peak-demand matching, is not taken offline as the power
may be needed in the near future. This issue is exacerbated when a wind producer may have a large production
capability, yet the wind may not be blowing and may be
unable to go online for service when they are needed. The
wind power producer may also face a situation where the
wind is blowing in the middle of the night in January,
but there is no need for the power (low demand). This
will play into the economics of the real time pricing. A
power producer will most likely try to put the power online and sell it at reduced rate due to the lack of fuel cost.
This situation will compound the baseline production issues previously discussed. Adding excess power to the
grid may cause issues with the grid frequency, and may
be counter productive to producing renewable power because the baseline plants are already meeting the grid
demands.
Lack of predictability for wind production will significantly impact the grid when large amounts of this intermittent power are being used. Having surplus power
coming online is counter productive when we are trying
to replace fossil fuels power with renewable power and
the demand is not matching the generation. This creates
a greater need for power storage. The thought of power
storage is basically taking energy and delaying its use. If
renewable generation is geographically widespread, then
outages or down time of the renewables becomes less
problematic as generation in other areas are highly probable. The wind may be calm in Chicago, but blowing in
South Dakota. This is where the advantage of connecting to the grid and transmission over a micro grid will
be advantageous. If the lack of wind is short lived and
storage is integrated, the issue is not as important.
decade. The target goal is to have solar power being
produced at 0.06 per kWh. If this goal can be achieved,
it will enable us to have solar generate roughly 14% of
total electric generation by 2030, and 27% by 2050.1 By
integrating solar energy into the grid and with the assumption EVs are integrated into the grid, these solar
advancements can help the advancements of the EV by
allowing clean renewable power generation to power the
EV.
Finally, hydroelectric dams needs certain geographical
conditions and environmental concerns come with this.
Currently, hydroelectric dams provide large amounts of
power on the Western grid. This power production is
reliable, cheap, clean and flexible, but has proven to have
a negative environmental impact by destroying habitat
and hindering the ability of migration of aquatic life.
Power Production with Renewables
The majority of power produced for the Grid is based
on fossil fuels or nuclear reactors. As mentioned, these
methods have long start-up times and cannot be shut
down for short durations. Typically, these plants are very
large and provide the Grid with base load production.
When demand drops, the supplier needs to sell power
to continue operating or it will start to cause frequency
problems on the grid. This lack of demand will start
to drive the real time pricing to zero or even a negative
price, meaning the consumer will be paid for the power
being used. This is counter productive to the producer,
if they are burning fossil fuel at a low efficiency, putting
pollution into the atmosphere, and yet the have to pay
the consumer for the power because there is no demand
for the power. This lack of demand can create on opportunity for energy storage, which is needed to make power
generation more efficient and economical. The following
are a few renewable technologies that are in the forefront
of new production coming online:
Solar Generation
Wind Generation
Solar electric generation comes in differing forms. Concentrated solar power (CSP) and photovoltaic (PV) are
the two methods of electric generation. CSP utilizes mirrors to concentrate the solar energy (heat) to heat a solution. The thermal energy is used to heat a solution. The
system is then connected to a heat pump or mechanical steam turbines to power a generator. The generator
is used to generate alternating current (AC) electricity.
This type of system has historically been on large-scale
operations. CSP is typically connected to a mechanical
generator system that is used to generate the AC power.
This type of system has been used due to the historical
high cost of the PV cells.
PV modules tend to be more desirable over CSP due to
the ability to take the suns rays and convert them directly
to direct current(DC) power, making the system simple,
and low maintenance. To make the electricity match with
Wind power generation has had the greatest growth in
terms of renewable power production in the last 10 years.
The technology has had significant development and government financial incentives,which have made this market ripe for development. Wind energy is not reliable,
but is somewhat predictable in the short term. However,
the tradeoff for this is the fuel used for generation is free.
This type of generation really comes down to a capital
investment and maintenance cost. Once the capital investment is made, the producer needs to make power and
sell it to turn a profit. This need to sell power, generated
with free fuel, poses problems when trying to operate a
grid with dispatched power production. The generators
are going to sell the power at any price because it is a
source of revenue and the need to cover fuel expenses is
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research. Increasing the module efficiency is what has
had the most drastic effect on reducing the cost of PV
systems. However, the balance of system (BOS) is currently of main focus on reducing the installed cost. One
of the main financial hurdles are soft costs, referring to
permits, installation and inspections. As PV solar becomes mainstream, these costs will be naturally reduced.
Governing bodies can also look at ways to reduce permitting costs on installation projects. While the costs continue to decline, the number of installations continues to
increase, further supporting the projections. Adaptation
of electric vehicles will push the demand for additional
generation to come online and create additional demand
for solar power production.
Installing smaller solar arrays at the consumers location, or behind the electric meter is also practical. This,
combined with smart meters, start to offer the consumer
the advantage of solar production at peak pricing. This is
mutually beneficial for the consumer and the grid as the
transmission is reduced because the generation is happening where it is being consumed. Using solar technology comes with many benefits, but cloud cover cannot
be ignored.One way to overcome this hurdle is with diversification and having interconnection with adequate
capacity throughout the Grid. Spreading the generation
throughout a large area will naturally make the system
more resilient. On days that clear skies are everywhere,
then storage may be needed to store the excess power.
This is not a necessarily a critical flaw, but rather an
issue that supports the need for storage even more.
the grid power type the DC is then converted to AC by
utilizing an inverter. This phenomenon also comes as a
system with no moving parts, low maintenance and a long
life expectancy. Until recently the driving cost of PV has
been the cost of the panel module. Recent technology
development and manufacturing has driven the price per
watt to historic lows, to the point that the balance of
the system (BOS) (racks, inverters, installation, power
modules, tracking, etc.) is now driving the costs and
controlling the market. However, PV solar installs are
growing rapidly. As solar costs continue to decline, it
starts to make the power production competitive with
real market pricing.
One of the main benefits of PV is the production output typically follows the demand profile. One of the main
uses for electricity in the peak demand times is air conditioning, which is caused by solar radiation. When the
sun comes out and drives up the demand, the PV plants
are going to produce the most power. This is positive for
the economics of the systems output. The production follows the demand profile and along with that it produces
the most power at the peak real time pricing. Therefore, solar economics can support a premium price over
wind or other methods of production. Solar production
is very predictable (outside cloud cover) and is one of the
main reasons dry desert climates make the most sense for
large PV systems being they provide clear sky for most
days of the year. As large systems are constructed for
grid applications, the cost of land or the impact utilizing fertile agricultural land for PV plants also needs to
be considered. The cost of the land and the fertility are
typically directly related, meaning desert land will be less
expensive then Midwest farm land. Latitude is also another consideration when placing solar as the suns angle
impacts the solar irradiance and maximizes the output.
However, as the cost continues to decline, the economics
will allow for greater flexibility in the installations and
locations of these systems. With diversity in placement
of the system, the various geographical opportunities will
start to play into resiliency of the overall systems. For
example, if Chicago has a cloudy day and production is
low, but is connected to Kansas which is having a sunny
cool day, then power can be moved to the areas that it
is needed. EIA estimates that transmission losses, on
average, are only 6% in the US.2
The SunShot Initiative by the DOE has set some aggressive goals. The base load power production costs
projected to year 2020 and relating to coal generating
costs establishes a baseline or the goal of $0.06 per kWh.
In 2010, the cost of generating solar power was $0.214 per
kWh; by 2013 it had been reduced to $0.112 per kWh.
This is roughly a 45% reduction in just 4 years. This is
evidence the overall solar technology is well on its way to
compete with base load coal power production of $0.06
per kWh in 2020. Currently numerous module technologies, or solar panels, are being developed and have made
great advances. It has been the module technology advancements that have historically been the focus of PV
Energy Storage for Renewables
The need for energy storage is essential when intermittent generation creates excess electricity that may not be
needed at the time of generation. Additionally, this energy storage is critical when economics are not favorable,
and the energy needs to be saved until a later date when
it is favorable.
Energy storage has been previously done on large-scale
systems with hydro-storage facilities. Water is pumped
to high ground reservoirs by utilizing surplus energy,
where it is stored as potential energy. When electricity is needed, the water is released through turbines to
make electricity, which is put back on the grid. This
is currently the most economical storage technology utilized today. Hydro storage is limited to geographical constraints and it has some environmental concerns, as does
traditional hydropower production. There are some storage plants in use and also some are currently being built
today, but the capacity is relativity small in relation to
the amount needed as we continue to adapt a large portfolio of renewables on the grid.
Currently, battery storage is being researched in efforts
to provide a solution to this intermittency of generation
caused by renewables. There are two types of storage
being examined: grid storage and EV storage. Both need
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to be inexpensive, have a long life, be able to charge and
discharge rapidly, and have the ability to cycle numerous
times. Energy density is much more important for EVs
than in grid storage.
If EVs are going to be integrated and reliant on the
grid, one needs to consider the large impact on the transportation sector they will have in terms of pollution and
reliability. Renewables are essential if the goal is to use
clean power for transportation. Plug in electric vehicles
will have a major impact on the grid and the demand
curve as they continue to come online, necessitating upgrades to the current system. These challenges can be
major issue caused by the EV, or they can assist in leveling the demand curve issues that the grid faces today.
By integrating the EV correctly, they can be part of the
solution, or designed and integrated to minimize the implications the grid interfaces. A major benefit the EV
offers the grid is their large batteries and the likelihood
they are parked a large portion of the day. This opens
an opportunity to utilize the EVs battery storage to assist with some of the hurdles that come with renewable
power generation.
TABLE I: Table created with data provided by Federal
Highway Administration.4
Age
Male Female Total
16-19
8,206
20-34
17,976 12,004 15,089
6,873
7,624
35-54
18,858 11,464 15,291
55-64
15,859
7,780
11,972
64+
10,304
4,785
7,648
Average 16,550 10,142 13,476
Transportation with Electric Vehicles
FIG. 1: Numeric values in y-axis show drive miles
equivalent.(Image recreated from data provided by
Tesla Motors5 )
Electric vehicles have been around as long as cars
themselves. Lack of energy storage (batteries) and cost
are one of the main reasons the EV was not adapted
by the masses but rather overtaken by the internal combustion engine car. Today we are facing the same challenges. With a better understanding of implications that
come with the fossil fueled internal combustion engines,
the masses are in search for an EV that will give them
all the conveniences that we expect today. Coordinating
the adoption of the EV and having controlled charging,
renewable electricity generation can provide a low cost
solution, as little as $40 per vehicle a year more than current costs, in exchange for reducing emissions and fossil
fuel usage.3
People may still be concerned if they need to drive longer
distances and how long it will take to recharge the battery. If a battery could be charged in a very short time,
then recharge would not be a big concern. Most batteries have fairly long charge times, or need special charging stations to accelerate the charging process. This is
where Telsa Motors is leading the industry by strategically placing super charging stations throughout the US.
Figure 1 shows the difference with conventional charging
stations and the 120kW times to charge. Tesla Motors
even advertises the placement is near dining and shopping for convenience to the travelers, validating the issue
with charging time. The issue comes when someone driving a long distance does not want to stop to charge the
car every few hours, then and have to sit and wait for
30-60 minute during this time. In efforts to overcome
this issue Tesla Motors has what they claim is the fastest
charging station in the world. This graph shows the typical chargers compaired to the Tesla Supercharger. The
milage equivilant, or range, is given on the y axis for a 30minute charge at three differing stations.5 These stations
are being distributed throughout the world and they are
free to use for Tesla vehicles. Figure 1 shows the amount
of miles or charge rate equivilent (y axis) for 30 minutes
by comparing standard chargers versus the Tesla Supercharger.
Battery Storage
A typical persons daily driving consists of commuting
to work and running local errands. These tend to be
relatively close to our homes. The annual miles driven
per person are given in Table I from the Federal Highway
data. If one assumes this is an average per day, then the
35-54 male bin data would equate to driving 52 miles per
day (18,858m/365d), all days of the year. If one considers
this mileage from a work day commute it would be closer
to 95 miles (18,858m/200d) which is the highest category
given.4
Given this data, a battery that will go a distance of 100
miles would suffice the average driver for daily commutes.
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be connected to the grid for greater than 20 hours per
day.
Another developing technology is the Vehicle to Grid
(V2G) for charging and grid integration. At times, the
grid needs additional power, and other times it needs to
shed power as base load demand is minimal. V2G will
also allow for storage and supplement supply of intermittent generation of renewables.
By definition, V2G is the capability to have bidirectional flow of power from the grid to the vehicle.
This capability allows for the Grid to utilize the storage
that is in the EV when it is needed. The Department
of Defense (DOD) is currently working on a case study
to implement this technology for their use. The DOD
found in one study that frequency regulation alone could
offset the cost of an EV lease by as much as 72%. This
may start to change the way we look at our EV and
the economics of owning a car as a money savings decision. Integration of the EV with the Grid allows for a
mutual beneficial relationship and can allow the finance
savings to be passed on to the consumer. Once storage
is online, this could be used to offset some of the rolling
reserve that is in the grid system, but not being utilized
for power production. The reduction in redundancy generation or standby generators will help reduce the cost of
electricity.
Furthermore the EV has the capability to drastically
change the demand profile. This change can be good or
bad, depending on how it is implemented. If done incorrectly, consumers will return from work in the evening,
plug in, and charge their EV. This will be a huge demand
at one time on the grid and has the potential to have
catastrophic effects to the system. The rate of charging
that is needed, location of chargers, and number of EVs
that are being plugged in would all play into the issues.
If done correctly, the EV will have the capability to help
the grid rather than cause issues.
One way to effectively manage the demand profile
would be to allow the consumer control to set charging
and discharging with price points within a given software. When the desired price is cheap to the consumer,
then the EV would start to charge. Conversely, when the
price point was high enough, a consumer could discharge
or sell the power back to the grid. This necessitates a
minimum discharge point capability in the software so
the consumer would maintain a minimum charge in the
EV. It is assumed the pricing would follow the demand
based on simple supply and demand economics. As intermittent renewables are online, the supply and demand
may not be as simple as shown in Figure 2. Rather supply will be intermittent, requiring the need to dispatch
power or shed power when abundance is in the system.
This could be done in real time by utilizing supply and
demand economics with the consumer.
A typical charging and discharging cycle would look
something like the profile given in Figure 2.
Furthermore, the storage is eligible as rolling reserve
to the grid and frequency regulation all will be available
Battery Cost
The cost of the battery is one of the main drivers in the
economics of the electric vehicle. Argonne National Laboratory has assembled a multi-disciplinary team called
the Joint Center for Energy Storage Research (JCESR),
and they seek to reduce the cost of the battery by five
times, increase the energy density of the battery by five
times and do this within 5 years. Currently, the cost of
the battery is around $100 per kWh of storage capacity.
If this cost can be reduced by a factor of 5, it will be
closer to $20 per kWh.
Battery Energy Density
The energy density of the battery is also an issue with
driving a car that can go a long distance. Energy density
refers to the amount of power storage per unit weight of
battery. This is most important in an EV because the
vehicle has to move the weight of the battery within the
car. To illustrate, if a battery is fully charged, then the
car is driving with a mass that is full of energy. As this
battery drains, the vehicle is still carrying the weight of
the battery. This vehicle is now using energy to carry
the discharge battery or dead weight, which will make
the vehicle use more energy to travel, directly impacting
the distance the car can travel. In a typical internal
combustion engine, the energy density of the fuel is much
greater then that of a battery, and its weight is shed as
the fuel is burned and the tank empties. This means
the car becomes more efficient as you drive. The less
the battery can weigh, the less energy that is needed
to drive the car. This will have a compound factor on
the distance a vehicle can drive is the energy density is
increased. The real need for development of the storage
and implementation of the EV is driven by cost, charge
time, and distance, and consumer perceptions.
New Technology and Electric Vehicles
Autonomous vehicles are the future of transportation.
Google, among others, have numerous autonomous vehicles driving all over the world. They have been working with auto manufactures to integrate this technology
into new vehicles. California is working on rewriting laws
that allow for self-driving cars. This creates advantages
for safety and allows a person to enjoy the ride or do
something else during a commute that would otherwise
be spent on driving. This technology can be also used
to program a vehicle to park itself and connect to the
grid while being parked. This connection is vital if the
EV is going to be interfaced with the grid for charging
and discharging throughout the day. Assuming grid connections are available, and the typical commute is less
than 100 miles per day, one can assume a vehicle could
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is large. These connections will need to be installed at
residences, places of work and shopping locations. These
changes in distribution will change how power is currently
handled on the grid. The existing grid distribution system will change with the adaptation of the EV and V2G.
Argonne National Laboratory, in collaboration with
others, is currently working on standardizing the connections for charging stations and communications. Currently, most manufactures are participating within this
development. However, Tesla Motors is not. This may
be due to the fact Tesla offers free charging at their Super Charging Stations. This differing connection is not
compatible with other EVs, therefore not allowing other
non Tesla Motor vehicles to get a free charge.
FIG. 2: Typical charge discharge scenario. (Image
recreated from Reference 6)
Conclusion
within the V2G. With these abilities, a third party will
need to aggregate the consumers and communicate with
the grid operators for dispatch purposes and simplicity.
This communication will need to happen at two levels,
the grid to the aggregator, and the aggregator to the
consumers.7
Recent research performed a case study for California
2012-2027 found that renewable penetration can be used
to reduce emission by up to 90%, without severely raising the cost of electricity. The driving force behind this
stability in pricing comes from well-timed or controlled
charging of EVs. Without this control, the opposite effect
could take place. Policies to encourage smart controlled
charging are crucial. This control will allow customers
to switch demand times to a point when renewables are
generating electricity.
Our future needs to minimize pollution and hold a
sustainable renewable energy portfolio that will provide
for us all years to come. The move from ICE vehicles
to EVs comes with much excitement, but not without
hurdles to overcome. Renewable wind and solar are intermittent sources of energy and the EV will dramatically change the demand profile. All this comes with the
need for energy storage and puts the need at an all time
high. Recent consumers demands, governmental policy,
and government subsidies, alike have made this need in
the market place. This high demand has driven companies and consumers alike to push for the next best thing.
We do not need to make an EV that is comparable to
the internal combustion engine; rather we need to make
the electric vehicle of the future better then what we
currently have. This will ensure the success and adaptation of this technology. As renewables come online and
bring with them intermittency problems, electric vehicles
can offer large amounts of storage that can level the demand profile and dispatch power as needed. These all
are promising ventures but none of this will happen if
we are not proactive in developing the infrastructure and
software needed to do this. It is apparent that battery
technology needs to improve and come down in cost.
Examining the progress in renewable energy technology and battery technology, the future of transportation
will support EVs and create additional need for renewable power generation. This will offer clean transportation and security from having to depend on fossil fuels.
A major advantage of these renewable power systems is
free fuel, adding stability to the cost to generate electricity. Although EVs do not provide GHG reductions,
with controlled charging they do allow for load flexibility, which will allow reducing the cost of the renewable
electric generation.3
V2G is currently being studied; the research is limited
compared to the amount being invested in the research
for batteries and PV solar generation. This is logical, being the V2G will not work without having batteries that
support EVs, or still depending on fossil fuels for gener-
Vechicle-Grid Physical Connection
The integration of the EV and the grid is technically
logical, but the EV and grid need to actually physically
connect for this to work. This can be in the form of plug
in or wireless microwaves, but nonetheless the two need
to be married. The technology needs to address when
and how they will connect. If autonomous vehicles are
in our future, the wireless connections will more than
likely play into parking at work and commercial areas.
If wireless charging is going to be utilized, then RFID,
Bluetooth, or some type of wireless connections will be
needed, and more than likely within the plug in charging stations as well. With this come additional Internet
connections at the charging locations and further need
for infrastructure upgrades as consumers adapt the EV.
In addition to the transfer of power between the EV and
the grid, communications are needed to implement these
actions.
Based on the data provided by Tesla Motors charging
comparison, and looking at the size of the power connection, it clearly shows the amount of energy that needs
to be transferred at the point of connection (EV/grid)
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system more resilient and reliable. With this comes the
need for storage, generation, and communication hurdles
that need to be overcome. If researched and done properly we all will benefit from these advancements. Utilizing the EV batteries and integrating the EV into the
grid will have benefits to all in terms of renewable power
generation and clean transportation. The key to success is making a better grid system than currently exists
and supports renewable generation and developing electric vehicles that are better than the Internal Combustion
Engine vehicles we drive today.
ating power for the EV. However, at the rate these technologies are developing, V2G, controlled smart charging
and infrastructure upgrades will be the next challenge in
adaptation of the EV.
Solar generation and wind generation offer benefits to
reliable generation. These differing types of generation
will allow for dispatch of variable types of generation
when one or the other may not be available. This would
not be possible if only one type of renewable is being
utilized.8
Adaptation of renewable generation and electric vehicles will transform the way we live and make our overall
1
2
3
4
5
6
7
8
U.S. Department of Energy, Sunshot goals, Web Site (2014),
URL http://energy.gov/eere/sunshot/mission.
U.S. Energy Information Administration, Frequently Asked
Questions, Web Site (2014), URL http://www.eia.gov/
tools/faqs/faq.cfm?id=105\&t=3.
D. G. Choi, F. Kreikebaum, V. M. Thomas, and D. Divan,
Environmental Science & Technology 47, 10703 (2013).
U.S. Federal Highway Administration, Our nation’s highways (2000).
Tesla Motors, Supercharger the fastest charging station on
the planet.
M. Honarmand, A. Zakariazadeh, and S. Jadid, Energy
Conversion and Management 86, 745 (2014).
W. Kempton and J. Tomic, Journal of Power Sources 144,
280 (2005).
M. Fripp, Environmental Science & Technology 46, 6371
(2012).
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Energy Storage Methods - Superconducting Magnetic Energy Storage - A Review
Rashmi V. Holla
Energy storage is very important for electricity as it improves the way electricity is generated,
delivered and consumed. Storage of energy helps during emergencies such as power outages from
natural calamities, equipment failures, accidents etc. It is very challenging to balance the power
supply needs with the demand instantaneously within milliseconds. This makes power networks
more resilient and efficient. Storage of excess energy, to meet the ever increasing levels of primary
energy derived from renewable sources needs further development and advancement. The introduction of environmentally-conscious policies to lower greenhouse gas emissions and increase the
security of energy supplies heavily influences the market rules today. All these factors have led to
explore renewable energy sources, their use to meet the ever increasing energy demand and electrical
energy storage (EES). One of the energy storage methods, superconducting magnetic energy storage
(SMES),will be discussed in this paper.
Introduction
Energy storage plays an important role in the future
of renewable energy for the following reasons:
1. It helps the electrical grids to be more stable and
flexible, so that any surge in peak demand can be
addressed effectively and more efficiently, thereby
allowing balance in supply and demand of energy.
2. It assists in managing excess energy generated for
a later use.
3. It minimizes renewable energy curtailment, thereby
increasing the return on investment of renewable
energy generation.
4. It reduces the use of fossil-fuels.
5. It facilitates in maintaining power quality.
6. It defers or eliminates the need for additional generation or transmission infrastructure.
Many renewable energy sources (most notably solar
and wind) produce intermittent power. Wherever electricity generated from renewable energy sources such as
wind and solar are higher than what is required, energy
storage becomes an option to provide reliable energy supply. Individual energy storage projects augment electrical grids by storing excess electrical energy during periods of low demand in other forms until needed on an
electrical grid. The energy is later returned to the grid
as needed. Different electrical energy storage (EES) systems have been listed in Figure 1.
SMES technology stores electrical energy directly into
electric current.1
Superconductivity
The complete disappearance of electrical resistance
and expulsion of magnetic fields in various solids when
they are cooled below a characteristic critical temperature is known as Superconductivity.2 This phenomenon
was discovered by Dutch physicist Heike Kamerlingh
Onnes in 1911 in Leiden.
Expulsion of a magnetic field (which is known as Diamagnetism) from a superconductor during its transition
to the superconducting state is known as the Meissner
Effect.
Critical temperature, known as the transition temperature, differs for materials, as shown in Table I.3
TABLE I: Critical Temperatures of Different Materials
Material
Critical Temperature
Gallium
Aluminum
Indium
Tin
Mercury
Lead
Niobium
La-Ba-Cu-oxide
Y-Ba-Cu-oxide
Tl-Ba-Cu-oxide
1.1K
1.2 K
3.4 K
3.7 K
4.2
7.2 K
9.3 K
17.9 K
92 K
125 K
Cooper Pairs and BCS Theory
Explanation of why the materials exhibit superconductivity was put forward by three physicists, John Bardeen,
Leon Cooper and Robert Schrieffer in their theory known
as BCS theory (named in the honor of its three discoverers). This theory explains that the materials become superconductors when the electrons inside them join forces
to make Cooper pairs. Electrons generally are scattered
minium, Tin, Lead, Mercury, Uranium, Zinc, Cadmium,
Titanium, Gallium, Indium etc.
Type II Superconductors
They are made from alloys or complex oxide ceramics.
High temperature superconductors are Type II superconductors. Besides being mechanically harder than Type
I superconductors, they exhibit higher critical magnetic
fields. They usually exist in a mixed state of normal and
superconducting regions. This is called the vortex state,
because filaments or cores of normal material are surrounded by vortices of superconducting currents. In this
state, they exhibit incomplete Meissner effects.
Examples of Type II superconductors: Niobiumtitanium (NbTi), Niobium-nitride (NbN), Niobium-tin
(Nb3 Sn), Vanadium silicide (V3 Si) etc.
B. Superconductors are classified based on critical temperature into Low temperature and high temperature superconductors.
Low Temperature Superconductors
It was assumed that the superconductivity could occur
only at low temperatures.Ordinary or metallic superconductors have transition temperatures below 20K (-253.15
◦
C / -423.67 ◦ F).Electric currents encounter no resistance, so they can cycle through the coil of superconducting wire forever without losing energy.
FIG. 1: Different Types of Energy Storage
High Temperature Superconductors
due to the impurities, defects and vibrations of the material crystal lattice. All of these cause resistance to
the flow of electricity. But at low temperatures, when
the electrons pair up, they can freely move without being scattered, thus causing no resistance to the flow of
electricity.4
However, in 1986, German physicist J. Georg Bednorz
and Swiss physicist K. Alex Muller, discovered a ceramic
cuprate (a material containing copper and oxygen) that
could become a superconductor at transition temperature of around 30 K in LaBaCuO4 . They have been
awarded Nobel Prize for their discovery.Even though they
have been discovered more than 25 years ago, very little
is known about their mechanism.
Several Iron based compounds are now known to be
superconducting at high temperatures.5–7 High temperature superconductors are preferred for certain SMES
(such as one used in Brookhaven) as the magnetic field
H can be greater, and the energy stored in SMES is proportional to the square of the field (H2 ).
There is widespread use of superconducting materials
for various purposes including medical magnetic-imaging
devices, magnetic energy-storage systems, motors, generators, transformers, computer parts, and very sensitive
devices for measuring magnetic fields, voltages, or currents.
Classification of Superconductors
A. Superconductors are classified based on response to
a magnetic field into Type I and Type II superconductors.
Type I Superconductors
They are mainly metals or metalloids that display superconductivity.Identifying characteristics for this type
are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and
a critical magnetic field above which superconductivity ceases. Examples of Type I Superconductors: Alu50
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History of SMES
The initial proposal for SMES was introduced by Ferrier M. in 1969 in France. In 1971, the University of
Wisconsin launched research in U.S., which led to the
construction of the first SMES device. Following this
production SMES developed rapidly, where many companies established SMES systems, including Hitachi (1986),
Wisconsin Public Service Corporation (2000), ACCEL
Instruments GmbH( 2005) and many others. Over 100
MW of SMES units are now in operation worldwide,
though the deployment of SMES has been slow for the
last two decades due to its limitations.
FIG. 2: SMES System
Advantages of SMES
Superconducting Magnetic Energy Storage (SMES)
It is important to note some of the features of SMES
systems. Refer to Table II8 for the characteristics of
SMES.These features make SMES suitable for use in solving voltage stability and power quality problems for large
industrial customers.
These systems store energy in a magnetic field created
by the flow of direct current in a superconducting coil
which has been cryogenically cooled to a temperature
below its superconducting critical temperature.
To maintain the inductor in its superconducting state,
the coil is immersed in liquid helium contained in a cryostat that is vacuum-insulated. Usually, the conductor is
made of niobium-titanium, and the coolant is liquid helium at 4.2 K, or super fluid helium at 1.8 K. (Super
fluidity is the state in which matter behaves as a fluid
with no viscosity. In that state, matter self-propels and
travels against the forces of gravity and surface tension.
This phenomenon was initially discovered in liquid helium.)
The SMES system comprises three major components,
as shown in Figure 2, a superconducting unit (coil), a
cryostat system (acryogenic refrigerator and a vacuuminsulated vessel),and a power conversion system.Current
in the coil will not decay, once the superconducting coil is
charged and the magnetic energy can be stored for indefinite time. The stored energy can be released back to the
network once the coil is discharged. The power switching and conditioning system uses an inverter/rectifier to
transform alternating current (AC) power to direct current (DC) or convert DC back to AC power. The inverter
(also known as rectifier) accounts for about 23% energy
loss in each charging and discharging cycle.
The energy stored in the SMES coil can be calculated
by
E = 0.5 · LI 2 ,
TABLE II: SMES significant characteristics
Property / Parameter
Value
Specific energy
Energy density
Specific power
Power Density
Charge/discharge efficiency
Self-discharge rate
0.5 - 5 Wh/kg
0.2 2.5 Wh / L
500 2000 W/kg
1000 4000 W/L
95%
0% at 4 K
100% at 140 K
Unlimited cycles
ms to 8 s
$ 1000-10000 /kWh
Cycle durability
Discharge time
Energy Cost
It is important to take a look at why SMES developed
so quickly in such a short amount of time. SMES systems
have notable advantages, including:
1. SMES shows a very high energy storage efficiency
(typically > 97%).
2. SMES has very high power density.
(1)
3. Rapid response: The time delay during charge and
discharge is quite short, often in ms to 8 s. See
Figure 4 to demonstrate discharge time for various storage systems.9 Power is available almost instantaneously and very high power output can be
provided for a brief period of time. Thus if a customer’s demand is immediate, SMES is the suitable
option.
where, E is the energy measured in Joules, L is the inductance of the coil measured in Henry and I is the current
passing through it, measured in Ampere.
For high temperature superconductors, the coolant
used is liquid nitrogen, which is much cheaper and easier
to handle than liquid helium.
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4. The loss of energy in a charge-discharge cycle is less
than other storage methods because electric currents encounter almost no resistance and is greater
than 95%.1 Round trip efficiency refers to the energy that is not lost. Hence,Round trip efficiency
of SMES is high due to low losses.
5. SMES has a high cycle life and, as a result, is suitable for applications that require constant, full cycling and a continuous mode of operation.
6. Main parts in a SMES are motionless, which results in high reliability, as any loss due to friction
is avoided.
FIG. 3: Grid Storage Technology Maturity
7. The energy output of an SMES system is less dependent on the discharge rate, compared to batteries.
8. It improves power quality for critical loads and provides carryover energy during momentary voltage
sags and power outages.10
9. It does not rely on a chemical reaction and no toxins
are produced in the process.10
10. Ultra-high field operation enables both short and
long-term storage SMES systems in a compact device with cost advantages in material and system.10
11. SMES systems withsuper-fast response are used for
power qualities such as instantaneous voltage drop,
flicker mitigation and short duration of UPS. Power
rating for such systems is generally lower than 1
MW.10
Applications of SMES:11
FIG. 4: Graphical representation to demonstrate
discharge time for various storage systems. Adapted
from Reference 9
1. They are used in UPS, where the power quality and
response time required are very high.
2. They are used in Flexible AC Transmission systems
(FACTS).
Technical Challenges
3. They are used in Pulse power source such as in
Electromagnetic launcher, magnetic forming.
Although SMES systems have considerable advantages
when it comes to response time and negligible loss of
power, they also face some unique challenges.
Technical Comparison of Electrical Energy Storage
(EES) Methods
1. Energy storage and power are considered to be important criteria for an energy storage device. The
energy is given by the product of the mean power
and the discharging time. Energy storage is dependent on mean power and discharging time. Refer
Figure 4 for comparison of System power rating of
various EES plotted against their discharge time.
These two quantities depend on application.
Technological maturity of various energy storage systems is shown in 3.
SMES system is developed and currently being demonstrated. These systems are commercially available, however, their reliability is yet to be established.
Technical comparisons of various EES systems based
on Discharge Time, Power rating, Storage duration, capital cost, cycle efficiency, energy & power density , Life
time and cycle life are shown in Figure ??, as previously
reported by Chen et l.8 .
2. Shorter discharging times (milliseconds to seconds)
is desirable to protect an electric load from voltage
sags. SMES are referred in such conditions.In a
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power grid for leveling of load, the discharging time
should be large (hours to weeks). SMES does not
have larger discharging time.
An important project to improve the application of
SMES at Grid level is being sponsored by U.S Department of Energy (DOE), under Advanced Research
Projects Agency for Energy (ARPA-E). They aim to
develop 1-2 MWh commercial scale SMES systemthat
is cost competitive with lead acid batteries.11 Team:
Brookhaven National Laboratory, University of Houston,
ABB and SuperPower.12 $5.3 M, out of which $4.2 M
from DOE.12
A 30 MJ (8.4 kWh) SMES unit with a 10 MW
converter has been installed and commissioned at the
Bonneville Power Administration (BPA) substation in
Tacoma, Washington. This system, which is capable of
absorbing and releasing up to 10 MJ of energy at a frequency of 0.35 Hz.13
In 2000, in order to enhance the grid stability, American Superconductor Company installed six SMES unitsin
the grid in northern Wisconsin, USA.7,14
To provide high quality power for a synchrotron source,
1.4 MVA/2.4 MJ SMES was installed at Brookhaven National Laboratory (USA).15
In North Carolina, Owens Cornings extrusion and production lines have been protected by SMES from voltage
sags.16
In South Africa, a SMES has protected a paper machine against 72 voltage dips in 11 months.17
In Japan, to compensate the voltage dips in a liquid
crystal manufacturing facility, a 5 MW 7 MJ SMES was
installed in 2003.18
3. SMES have low energy density. This area requires
more research and development to make SMES systems competitive.
4. The energy content of current SMES systems is
very small. Methods to increase the quantity of
energy stored in SMES often resort to large-scale
storage units.
5. It is necessary to have a cryogenic system and
power cost required to maintain lower temperatures
are high.
6. A robust mechanical structure is usually required to
contain the very large Lorentz forces generated by
and on the magnet coils, without any degradation
of superconducting properties. (Lorentz Force is
the combination of electric and magnetic force on
a point charge due to electromagnetic fields.)
7. The substantial cost for SMES is its superconductor, associated cooling system and the mechanical
structure.
8. Existing and continued development of adequate
technologies using normal conductors.
9. Health issues associated with strong magnetic
field.Exposures to higher magnetic field often result in increased heat beat, changes in brain activities, effect on immune system, accelerated tumor
growth, skin problems etc. However, full life cycle
analysis for SMES systems is yet to be done for the
concluding the harmful effects.
Conclusion
By 2020, the current demand for energy is expected to
more than double globally. It would be a challenge to
meet the future demand with the existing power generation capacity. Energy storage will help to maintain a
backup to cater to the high future demand. With many
EES systems commercially available today, energy storage industry face many challenges such as price arbitrage,
energy balancing, transmission and distribution.
Existing SMES Projects
SMES systems have been deployed all over the world
and are functional. Some of them have been listed as
follows.
1
2
3
4
5
6
K. Cheung, S. Cheung, and S. N., Tech. Rep., Imperial College London (2007), http://www.homes.doc.ic.
ac.uk/~matti/ise2grp/energystorage_report/.
http://www.britannica.com/EBchecked/topic/574212/
superconductivity.
http://hyperphysics.phy-astr.gsu.edu/hbase/
solids/scond.html#c2.
http://www.explainthatstuff.com/superconductors.
html.
J. Timmer, Ars Technica (2012).
Z.-A. Ren, G.-C. Che, X.-L. Dong, J. Yang, W. Lu, W. Yi,
X.-L. Shen, Z.-C. Li, L.-L. Sun, F. Zhou, et al., EPL (Eu-
7
8
9
10
11
12
53
rophysics Letters) 83, 17002 (2008).
C. Q. Choi, Scientific American 298, 25 (2008).
H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li, and Y. Ding,
Progress in Natural Science 19, 291 (2009).
G. Crabtree and J. Misevich, Tech. Rep., American Physicl
Society (2010).
http://www.superpower-inc.com/content/
superconducting-magnetic-energy-storage-smes.
P. Tixador, IEEE/CSC & ESAS EUROPEAN SUPERCONDUCTIVITY NEWS FORUM 3, 1 (2008).
http://mydocs.epri.com/docs/
publicmeetingmaterials/1110/7TNRSL46577/
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13
14
15
16
17
18
12-SuperconductingMagneticEnergyStoragesystemforGRIDS(LehnerforLi)
.pdf.
H. J. Boenig and J. F. Hauer, Ieee Transactions on Power
Apparatus and Systems 104, 302 (1985).
T. Abel, Modern power systems 19, 28 (1999).
J. Cerulli, in Power Engineering Society 1999 Winter
Meeting, IEEE (1999), vol. 2, pp. 1247–1252.
J. Cerulli, G. Melotte, and S. Peele, in Power Engineering
Society Summer Meeting, 1999. IEEE (1999), vol. 1, pp.
524–528.
R. Schottler and R. Coney, Power Engineering Journal 13,
149 (1999).
S. Nagaya, N. Hirano, M. Kondo, T. Tanaka,
H. Nakabayashi, K. Shikimachi, S. Hanai, J. Inagaki,
S. Ioka, and S. Kawashima, Ieee Transactions on Applied
Superconductivity 14, 699 (2004).
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Hydrogen Storage Technologies for Transportation Application
Ruzica Todorovic
The objective of this paper is to provide a brief overview of the possible onboard hydrogen storage
options available today namely in fuel cell or ICE/electric hybrid vehicles. The issues associated
with increasing the hydrogen volumetric densities via solid state materials storage and liquid carriers
are the main focus with an overview of the current storage technologies. Possible onboard hydrogen
production is discussed based on water splitting technologies.
Introduction
Decreasing greenhouse emissions has been a topic of
interest in the recent decades. Key contributors to observable greenhouse effects are emissions from combustion of fossil fuel in industry and transportation sector.
Developing a viable technology that could provide a clean
energy source is of outmost importance for future generations. In addition to the environmental concerns, recent
research and development of the smart grids and electric
vehicles shows that the demand for electrification is on
the rise. Hydrogen is viewed as a potential fuel of choice
as it can be produced from renewable energy sources, is
non-polluting and its combustion produces no (or very
little) harmful by-products. Today most of the hydrogen is produced from syngas. Syngas (a mixture of various concentrations of carbon monoxide and hydrogen)
is easily obtained since it can be produced from many
non-petroleum resources such as natural gas, coal-bed
gas, landfill gas, coal or biomass, through the processes
of steam reforming, partial or autothermal oxidation.1,2
Diversion from carbon-based hydrogen production which
yields significant amounts of CO2 into a sustainable energy source utilizing wind or solar technology has been a
focus of numerous studies.
The term hydrogen economy has been used as a way to
describe an overall process of hydrogen utilization as an
FIG. 1: Schematic of the hydrogen fuel cycle
electricity source encompassing its production (from renewable resources), transportation, storage and electrical
generation (Figure 1).3 The current state of technology
has not reached levels where industrial commercialization
is viable, as the challenges such as hydrogen generation,
storage (onboard a vehicle, during delivery and in refueling stations) and high performance fuel cells still have to
be addressed before H2 as a fuel can be fully integrated
within the transportation sector. From a practical and
economic point of view, an advantage of hydrogen fuel is
that its incorporation into existing internal combustion
engines would require only slight modifications. Fuel cells
(ex. polymer electrolyte membrane) can convert chemical energy stored in hydrogen into electrical energy and
water as the only products4 with a significantly higher efficiency (60%) compared to currently used internal combustion engines (around 20%). System design (Figure 2)
can be tuned to control power of the vehicle by varying
the size of the fuel cell while the amount of energy stored
onboard can be controlled with the size of the fuel tank.5
While conceptually this design shows promise, the main
challenge still remains of how to develop the technology
if the root of the problem is in understanding and influencing the intrinsic properties of the fuel (hydrogen).
In its pure state hydrogen is a diatomic molecule that
is transparent, odorless and nontoxic. It is a solid below 13.8 K and gas above 20.3 K with a very small liquid
phase range.6 In gas phase it is flammable in a wide range
of temperatures, has strong buoyancy and high volatility.
Hydrogen is mainly found as a part of a compound (e.g.
water, hydrocarbons), which implies that a method of H2
production will determine if the electricity is generated
from the hydrogen cycle or from the carbon cycle. The
detriment of hydrogen as a fuel is its low energy content
by volume due to extremely low density under normal
conditions, which causes a need for large onboard fuel
tanks in order to provide for longer driving range capability. The benefit is high energy content by weight,
with the heat of combustion for gas phase hydrogen being
twice that of liquid gasoline (Figure 3). Research efforts
are focused on tailoring the system to reduce the volumetric requirements while maintaining the gravimetric
performance of hydrogen. These efforts are driven by the
goal set by DOE (Department of Energy) to develop the
storage capacity of 81 g H2 /L with 9 wt% by 2015.
To meet these goals requires advancement in the over-
FIG. 2: Engine schematics for a fuel cell vehicle
FIG. 3: Comparison of energy density by volume and
specific energy by weight for various fuels under
standard conditions. Values for methanol, methane and
diesel are for liquid phase, and values for hydrogen,
methane and natural gas are for the gas phase fuels.
Adapted from Reference 7.
all H2 storage system (tank, storage media, safety system, valves, regulators, piping, mounting brackets, insulation, added cooling capacity, etc.), and in the performance characteristics such as refueling time, discharge
kinetics and cycle life. Optimally, stored fuel should
be under atmospheric pressure and ambient temperature. However, most mature hydrogen storage technologies require either high pressures (pressurized cylinders)
or very low temperatures (liquefaction). Newer directions
focusing on the interaction between H2 and the material as a means of storage and transportations (adsorption, absorption, physisorption, liquid carriers, etc.) are
challenging with respect to thermodynamic and kinetic
restrictions upon the reaction between the nanostructured storage material and hydrogen. Currently these
materials need further research and development to increase their porosity, tuning of pore sizes, optimization
of adsorption potentials, and enhancement of volumetric
capacities.7 This review will provide an overview of the
most promising, renewable energy technologies for hydrogen generation with an emphasis on the most recent
advancements in hydrogen storage via chemisorption and
physisorption of pure hydrogen and with liquid carriers.
is very high. Further improvements in the efficiencies are
necessary, with the research also focusing on the charge
transfer ability and surface catalytic reactivity for halfreactions. In order for solar energy to be the major contributor to the generation of hydrogen, the efficiencies of
solar water-splitting devices need to be improved.8,9
The electrolyzer systems use electricity to produce hydrogen from water. Recently, electron-coupled-proton
buffers (ECPBs) have been developed that allow decoupling of the half-reactions of electrolytic water splitting
(H2 and O2 evolution proceed at separate times).10 The
process uses a redox mediator that is reversibly reduced
during the oxidation step, upon which it is transferred
to a separate compartment for catalytic H2 evolution.11
This system exhibits potential for onboard H2 production
as the electrochemical step is performed at atmospheric
pressure while the H2 evolution would be performed in
a different higher pressure compartment. Also, no H2
is produced in the electrolytic cell, H2 evolution would
not be directly coupled to the rate of water oxidation,
and the hydrogen produced has the potential to have an
inherently low O2 content.10
Hydrogen Production
As previously mentioned, industrially hydrogen is produced from syngas. New directions for an effective and
clean way to produce H2 are being explored. Some selected technologies that exhibit potential for onboard H2
production systems are water splitting via photoelectrochemical solar energy conversion and via electrolysis.
Hydrogen generation from solar energy conversion via
photochemistry processes, specifically solar water splitting, has shown great promise. In general, using catalyst,
water is split into hydrogen and oxygen via an oxidationreduction reaction. This is achieved when the incoming energy is high enough to excite an electron from the
valance band into the conduction band. A requirement
for the photo-catalyst is to have a bandgap higher than
2.43 eV, which is the energy needed for the splitting of
water. The speed of the reaction is the main determinant
in this process which could be addressed by reducing the
bandgap to a wavelength where the intensity in sunlight
Hydrogen Storage
Hydrogen storage is an important component in the
ongoing development and commercialization of hydrogen
and fuel cell technologies. Safe and economically feasible hydrogen storage technology needs to be developed
in order to compete with current fossil fuels driven economy. Storage is required onboard the vehicle, in production sites, during transportation and in refueling stations.
Challenge for onboard storage is to safely store enough
fuel to enable a driving range in excess of 300 miles which
usually corresponds to 47 kg of hydrogen. This requires
a significant increase in H2 energy density within the
storage media. This is further complication by ambient
temperature and atmospheric pressure conditions desired
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the molecule has to overcome a dissociation barrier. Once
a bond with surface metal is formed (chemisorption) hydrogen atom can either diffuse across the surface or into
the bulk. Inherent complexity associated with storage
in solid state materials is the heat produced during the
refilling process. Therefore, thermodynamic restrictions
may be difficult to overcome.22
for this process. In general, the density of a gas can
be increased with very low temperatures (below critical
point), by applying work to compress gas or by interaction with second material. When the second material is
utilized, uptake and process reversibility also become important. Based on these requirements, hydrogen can be
stored as pure H2 via compression or liquefaction,12,13
in solid state materials via chemisorption in metal hydrides and complex (meal-hydrogen) hydrides14–18 and
physisorption,19–21 or utilizing a liquid as a carrier.
Physisosorption
Physisorption is an adsorption process that binds hydrogen weakly to the surface. This process has shown
to exhibit excellent kinetic properties utilizing highly
porous materials (high surface area). Determinant to
physisorption are low temperature (< 273 K for carbon based materials) requirements for the Van der Waals
forces have any substantial effect on the process, which
implies low hydrogen adsorption unless the process can
be catalytically improved. Hydrogen storage on high surface area materials generally exhibits excellent kinetic
properties.
Carbon-based porous materials have been identified as
promising candidates for hydrogen storage via physisorption due to high surface area, large pore volume and good
chemical stability. The strength of the hydrogen carbon
interaction at room temperature can potentially be improved by doping activated carbons with nanoparticles of
elements that dissociate hydrogen easily, most predominantly researched element being platinum. The kinetics are improved via the spillover mechanism wherein at
room temperature, hydrogen dissociates on Pt and diffuses onto the surface. The results show significant hydrogenation and reversible dehydrogenation of a carbon
support suggesting routes to design improved catalysts
for hydrogenation and fuel cell applications.24 Surface
diffusion from the catalyst to its support was shown to
occur via a mobile chemisorbed phase and the reverse
mobility occurred by diffusion back to the catalyst. Significant research still needs to be performed, as the exact
spillover mechanism and the reaction rate limiting step
are still unknown.
Pure Hydrogen Storage
Compression
High pressure gas cylinders are the most mature technology for pure H2 storage, currently exhibiting the highest storage capacity and best overall performance compared to other storage methods.12,13 Significant increase
in energy density is achievable; however, a drawback is
the reduction in gravimetric density under high pressure
due to the increasing thickness of the cylinder walls. Additionally, safety concerns associated with pressurized hydrogen gas are significant.22 Full cylinder contains 4% by
mass hydrogen at 450 bar which can autoignite at ambient temperature, requiring additional safety measures
for thickness of the cylinder wall. This further increases
the size of the storage tank which is currently too large
to accommodatethe entire hydrogen storage unit capable
of providing a desired driving range. Cylinders are usually produced from materials such as austenitic stainless
steel, copper or aluminum alloy which are nonreactive
with hydrogen and have high tensile strength.23
Liquefaction
Liquefaction is a physical storage of cryogenic hydrogen in isolated tanks at -253 C and the pressures of 6-350
bar. This process requires large amount of energy for
liquefaction and the continuous boil-off due to heat leaks
which would not be practical or economically sustainable
in the transportation sector.12,13
Metal hydrides
Solid State Materials
Metal hydrides have been extensively studied as a possible hydrogen storage medium due to high hydrogen
content by weight and a potential for high adsorption
capacity under desired reaction conditions. The absorption process involves surface dissociation of H2 and its
consequent diffusion into the metal or metal alloy to
form metal hydrides. At ambient temperatures metal
hydrides tend to exhibit low gravimetric densities due to
very weak atomic hydrogen bonding (ex. AlH3 , VH2 ),
while metal hydrides exhibiting acceptable gravimetric
densities tend to bind atomic hydrogen very strongly (ex.
LiH, MgH2 ).25 Further improvements to reaction thermo-
Hydrogen storage based on solid state materials exhibits potential to significantly increase the volumetric
density of stored hydrogen under low pressure and ambient temperature conditions. The process involves surface
H2 dissociation and consequent diffusion of H atoms into
the bulk structure (absorption) or by surface diffusion
only (adsorption). The type of storage is determined
by the strength of interaction between hydrogen and the
solid material. Van der Walls forces lead to the initial physisorption state of the hydrogen molecule, upon which
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tion and reforming at atmospheric pressure and low temperatures (65 - 95C).29 Full conversion of all available
hydrogen has been observed (Figure 4) with the 3:1 ratio
of H2 to CO2 . Incorporation of this technology into fuel
cell vehicle design could provide an extremely high energy
density (3 times that of lithium ion batteries) and for an
extremely low (1 ppm) CO and CH4 concentration in
product.29 Additional research is needed to prevent the
deactivation of the catalyst because the base used for
activation of the process is preventing deprotonation of
methanol and formic acid (reaction intermediate species).
Therefore, the nature of the base and its concentration,
the water content and the temperature require further
optimization.
dynamics (enthalpy) are required. The aim is to decrease
the current 300-350C reaction temperature.
Complex hydrides
Light metals (e.g. Li, Mg, B, and Al) give rise to a
large variety of complex metal hydrides. The goal is
to form alloys using elements capable of affecting the
hydrogen dissociation rate. Metallic alloys have been
shown to have high hydrogen storage capacity in addition to good dehydrogenation properties allowing for the
reversible process.26 These compounds exhibit the highest gravimetric hydrogen densities at room temperature,
the most predominant being LiBH4 , with 18% by mass
gravimetric density and 121 kg m−3 volumetric density.25
However, the compounds are highly stable requiring temperatures in excess of 650 K to release H2 . Subsequent
research has provided a new crystalline phase that contains two polymorphs -LiHB (-LiN2 H3 BH3 ) at low temperature and -LiHB at high temperatures. This material
is more reactive and less stable upon heating than the
parent HB.27
Complexes of rare-earth and d-transition metals such
as Ln4 MHx have demonstrated unique synergy that exhibits increased hydrogen adsorption. These hydride
clusters are of particular interest as molecular models
for hydrogen storage alloys.28 However, well-defined rareearth/d-transition metal polyhydride complexes are currently very rare due to the lack of a strategy for efficient
synthesis and difficulty in catalyst characterization.
FIG. 4: Schematic pathway for a homogeneously
catalyzed methanol reforming process via three discrete
dehydrogenation steps. Adapted from Reference 29.
Conclusion
This review has looked at some new promising technologies that exhibit potential for onboard hydrogen production and storage, and technological issues that still
need addressing before full integration into the transportation sector is viable. Onboard H2 production systems such as water splitting via photoelectrochemical solar energy conversion or electrolysis offer an alternative
that could potential make for an easier transition into
the hydrogen economy. Hydrogen storage based on solid
state materials shows potential to substantially increase
hydrogen density in storage materials and improve the
kinetics of hydrogen uptake and release under the low
pressure and room temperature conditions. Unlike the
pure hydrogen storage technologies (compression and liquefaction) which are commercially available today, these
technologies still require significant research to further
understand the reaction mechanisms as well as to design
a concept that can be fully integrated into an electric vehicle design. However, storage materials such as complex
metallic hydrides and liquid energy carriers offer a safe
and effective way to store hydrogen which can be a way
towards substantial increase in hydrogen based fuel cell
electric vehicles on the roads.
Liquid Carriers
Recent advances have been made in developing liquid
energy carriers,29 utilizing molecules containing a high
percentage of hydrogen. The most promising non-fossil
resources are alcohols (e.g. ethanol, methanol) because
in the presence of water and, at relatively low temperatures, hydrogen-rich mixtures can be produced. Steam
reforming of methanol (CH3 OH + H2 O CO2 + 3H2 )
as a process for onboard H2 production is of special relevance. Liquid methanol has high energy density (under standard conditions), low boiling point, has no C-C
bonds and contains 12.6% by mass of hydrogen. These
properties indicate that under relatively mild reformate
reaction conditions H2 generation can be viable through
a process that does not contribute to a net addition of
CO2 to the atmosphere.
Ruthenium complex catalysts have been shown to exhibit potential for aqueous-phase methanol dehydrogena-
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T. Shima, Y. Luo, T. Stewart, R. Bau, G. J. McIntyre,
S. A. Mason, and Z. Hou, Nature Chemistry 3, 814 (2011).
M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler,
H. Junge, S. Gladiali, and M. Beller, Nature 495, 85
(2013).
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Microgrids with Storage
Travis Nelson
The existing electrical grid infrastructure is aging and based on century old systems and approaches to electrical delivery. Microgrid technology offers many advancements and benefits to the
future of the electrical grid. This paper looks at the inner makings of microgrids and their implementation. This paper examines various storage methods within microgrids, a key benefit to their
utilization, as well as a brief look at various microgrid control functions and issues. Regulatory
issues are covered to address the potential wide spread expansion of microgrids and their ability to
be the cornerstone of the electrical grid in the future.
Introduction
The electrical grid has grown over the decades of its existence from small independent systems with local power
generation and consumption, to a large interconnected
and very complex machine. Along with the growth of the
grid, the need for the energy that is transported along
its many millions of miles of wires has grown with it.
High voltage transmission lines moved power generation
to more remote locations and the growth of people connected to the grid has increased the grids interconnectivity across the map. Over the grids existence, however, innovation has been slow and many problems to date affect
the grids ability to transform into the required system to
power the fast advancing technological world. New technologies are available and can be utilized to tackle some
of the obstacles that face the grid.
The grid has a reliance on fossil fuel produced energy.
The production of energy from fossil fuel plants generates
greenhouse gasses that adversely affect the environment.
Peak demands and the overall peaking nature of the electrical grid have negative effects to the grid. It takes a lot
of capital cost to maintain and update the grids infrastructure which is determined by the peaking loads. To
date a large amount of the grids infrastructure is aging
and not equipped to handle the continually increasing
load. Also power plants need to be available to supply
the energy during the peak times and by nature these
plants tend to be fossil fuel driven. Renewable energy and
the technology associated to producing it has advanced
over the last few decades yet the ability to maximize its
potential on the grid has not been achieved. By nature
renewables are unpredictable at producing power, which
makes incorporating them in the current demand driven
grid system very difficult. The abundance of renewables
are also generally not located near large areas of energy
consumption and there is limited ability to transmit that
energy over the required distances.
Though no one technology or fix will be able to correct
all these problems that face the grid, smart effective uses
of current technology can improve and significantly aid
in the abilities and modernization of the grid. Energy
storage is a crucial component to the future of the grid
and its implementation needs to be expanded. Micro grid
infrastructures utilizing storage can aid in the resiliency
and stability of the larger grid. Smart controls are needed
to incorporate the end user with the micro grid and the
overall utility grid. Together these changes can not only
improve upon the existing energy delivery system to the
end user but also change the way power is produced and
used which will have beneficial affects to the environment
we live in as well.
Microgrid
People rely on energy in many forms every day. From
the gasoline in the car you drive to the electricity that
comes out of the wall outlet to power your appliances,
energy is a vital necessity. 40% of energy consumption
comes in the form of electricity. 30% of this is for commercial and residential needs.1 Even with energy efficiency initiatives and legislation, energy consumption is
growing and with this the needs of an improved grid to
get the electricity to the end user reliably and safely. Micro grids have shown to be a viable option in the grid
system and come with many advantageous benefits. Microgrids are defined by the Microgrid Institute as “A microgrid is a small energy system capable of balancing
captive supply and demand resources to maintain stable
service within a defined boundary.”2 A notable benefit
to a micro grid is its ability to be designed for its applicable purpose. Whether servicing a residential community or commercial shopping district a micro grid can be
designed and implemented accordingly in various ways
that also suit the geographical location. Microgrids are
designed and built with one or more distributed energy
resources (DER) in order to form a sytem.2 There are
many variations of the microgrid to include how it is
connection to the grid. They can be fully interconnected
to the grid and consume and supply power, they can be
fully isolated from the grid, often referred to as islanded,
and they can be connected but only capable of consuming
and not supplying power to the grid.2
A common element to microgrids is the local DER
which often utilizes environmentally friendly renewable
energy sources such as solar and wind. Generally microgrids are implemented in order to provide reliable power
term needs and is how much power the energy system can
supply per unit volume.6 The energy and power density
comprise the storage systems storage capacity which is
sized and designed to the overall microgrid and how it
operates.6
Though energy storage is not required within a microgrid it does have advantages. If the microgrid is connected to the main grid, storage allows time for DER
to start up and supply the microgrid should the main
grid go down.6 If the microgrid is an island microgrid,
or has become islanded, storage allows stable operation
of the DER independent of system load.6 If the microgrid utilized renewable energy sources, storage can supply needed power during the times that renewables are
not able to generate power. Finally storage can be used
to peak shave and therefore reduce the demand on the
main grid and demand charge for the customer for grids
connected to the main grid.6
The most prevalent form of storage is the battery. Batteries designed to support microgrids come in various
forms and materials.
to an area that does not have power, or power is not
reliable, and in some cases this can also minimizes the
energy cost from grid consumption. In all cases, local
power generation would be a requirement. There are
many draws to using renewable energy sources for microgrid applications. Renewables are a fast growing industry with costs associated with their implementation
dropping yearly. Compared to fossil fuels, which are resourced from concentrated areas around the world, almost any place a microgrid can be installed will have a
source of renewable energy that can be used for a DER.3
Renewable sources are also environmentally friendly and
do not have carbon emissions when producing electricity.
The lack of emissions from renewables greatly reduces
the overall production of greenhouse gasses.
Microgrids and their associated DER also provide benefits to the main grid. With the traditional grid setup,
generation is done at large scale and in remote locations from the energy end user. This means long distance transmission and energy losses along the way. The
loses can be as high as 10 percent.4 The close proximity
of microgrid power generation minimizes loses over long
distance. The grids transmission capabilities is designed
and built around peak demand for the associated loads.
This means added cost in infrastructure to build capacity
for areas. This capacity is only then utilized for a short
time out of the year.4 Due to microgrids serving a small
designated area, the infrastructure can be built accordingly and at relatively significantly less cost. Proper operation will also reduce peak demand energy taken from
the main grid and therefore reduce the required capacity
the main grid is required to handle. The main grid has to
balance electrical supply with demand over large service
areas with a constantly fluctuating demand.5 This poses
challenges and difficulty to the grid operators. Microgrid
generation and storage alleviates this fluctuation and allows for a more predictable energy demand from the grid
as the microgrid is seen as one overall load. The microgrid can handle various changes in demand by controlling
various DER and relying on the main grid for a constant
supplemental supply.
• Lead acid batteries have been used for nearly a century in automobiles and have also been applied to
individual house applications, however, they have
a low energy density and therefore large and heavy
for the amount of energy they can store. They also
do not hold up well to repeated charging-discharge
cycles.5
• Sodium-sulphur batteries have been used in microgrid applications as well. They offer higher energy
density then lead acid batteries and are capable of
thousands of charge-discharge cycles.
• Sodium-sulphur batteries suffer from high cost per
kW however.6 They also operate at 300◦ C, with
the sodium and sulphur in the molten state, and
will suffer irreparable damage if they are completely
discharged and grow cold.6,7
• Over the last few years a large amount of research
and development has gone into Lithium-ion batteries. Most prevalent in the automobile industry to
power the new wave of electric vehicles, lithium-ion
batteries have proven to be very capable batteries.
Lithium-ion battery technology is the fastest growing battery chemistry technology today.8 Advantages of lithium-ion batteries that make them suitable for microgrid applications are their high energy
density, and low maintenance. Lithium-ion batteries do have disadvantages as well. Lithium ion batteries have highly flammable electrolytes and pushing the boundaries on Li-ion technology poses the
risk of fires caused by these batteries. They are also
still very expensive to manufacture.3 Using lithiumion batteries in electric vehicles for grid storage is
also a possibility known as vehicle to grid (V2G).
Energy Storage on the Microgrid
A critical aspect of a well-designed microgrid is energy
storage. To date the current grid system has very limited
energy storage. The lack of energy storage and sufficient
large scale energy storage technology is a pressing matter
with the electrical grid. Building and implementing microgrids allows for the implementation of energy storage,
utilizing various technologies, near the consumption of
the energy. Storage on the microgrid, or distributed storage, aids the DER in meeting the needs of the microgrid.6
Microgrids have both an energy need and a power need.6
An energy need is for mid to long term electrical supply
and a storage system would have a good energy density
to supply this to the microgrid.6 A power need is for short
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• Another form of battery that is being revitalized
from 200 year old technology is the saltwater battery. Produced for microgrid and energy management systems, Aquion Energy is manufacturing an
aqueous hybrid battery based on this technology.
Aqueous Hybrid Ion (AHI) battery chemistry is
centered on a saltwater electrolyte.9 This allows the
battery to be nontoxic and non-combustible.9 This
makes the battery well suited for microgrids and
being a distributed storage source because it is environmentally friendly and safe to handle.9
ever lost from the main grid, the control system must be
able to manage the distributed generation, energy storage, and loads on the microgrid to maintain the reliable
power.6 Current microgrid control systems use either a
central controller or distributed controller among the various electrical components of the microgrid.6 Communication between the electrical components are done via
their electrical connection or by network communication
connections. Voltage, Frequency, and active and reactive
power are the main variables used to control microgrid
systems.11 A microgrid control system allows for the optimization of the distributed energy resources associated
within that microgrid.
Challenges to the design and implementation of microgrid control systems include:
• Flow batteries are being utilized for grid and microgrid storage. A flow battery is a rechargeable battery with two tanks and liquids that are pumped
past a membrane held between two electrodes. Due
to their large size flow batteries are suited more for
stationary use such as in a microgrid. Other benefits include long life cycles and quick charging.
• Network communication limitations
• Variable power supplies
• Predicting power generation from variable supplies
Many forms of batteries can be utilized for microgrids.
Research and development is being deployed throughout
the world to make them cheaper, safer, and last longer.
These are all necessary components of an energy storage
device serving a microgrid.
Flywheel energy storage is another energy storage option for microgrid applications. Flywheels work by storing energy in a spinning mass. The flywheel has both a
motor and a generator.5 The motor spins the flywheel
and the energy is stored within this kinetic energy.5
When electrical power is required the flywheel spins the
generator.5 The construction of the flywheel consists of
a large rotating cylinder on magnetic bearings within a
vacuum chamber.10 The amount of energy stored in a flywheel is proportional to its inertia and speed. Flywheels
have a long life span and require little maintenance which
makes them useful in microgrid systems. They have a
fast response; ability to absorb energy within seconds
or minutes, and give it back to the grid just as fast.5,6
This makes them well suited for frequency regulation, to
support the operation of the microgrid, and not the overall storage device of the microgrid.5 Therefore in microgrid application flywheels can be used to support battery
storage. Also,there are multiple flywheel approaches in
which ’flywheel farms’ have been implemented to store
megawatts of electricity for minutes of discharge time.10
Another drawback to the flywheel is the overall cost.
• Integration of various technologies
• Variability in design
All these aspects need to be accounted for when implementing a microgrid control system. Important features
of a microgrid control system include:
• Control of DER supply
• Integration of DER supply with main grid
• Control load/ demand
• Economic use of DER
• Smooth ability to transition between modes of operation
During operation with the microgrid attached to the
main grid, loads will get power from both the DER installed in the microgrid and the main grids supply.11
During this mode of operation, the frequency, of the microgrid is controlled by the main grid.12 If the microgrid is
disconnected from the main grid, either on purpose, due
to an outage, or malfunction of equipment, the microgrid control system needs to balance loads with supply to
maintain frequency.11 This operation is more challenging
than being connected to the main grid.11 Energy storage
systems can be used during temporary mismatches between power generation and demand, allowing for stable
microgrid operation in all modes as well as transitions
from one mode of operation to another.11
In September 2014, the United States Department of
Energy announced $8 million in funding to improve resiliency of the grid.13 The funding is going to seven
projects around the country that focus on microgrid technology and deal with microgrid control in one way or another. Each project is receiving $1.2 million and they will
bring together regional and state entities, private sector
Microgrid Control
To make the most out of environmentally friendly renewable energy sources and energy storage technology
within a microgrid system, a sophisticated microgrid
control or management system is required. One of the
most important aspects of the microgrid control system is
safety. The control system must be able to supply power
to the loads of the microgrid safely when it is attached
to the main grid and also when it is not.6 If power is
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stemmed from the benefits of reliability and resiliency.18
The overall benefits of renewable integration has not been
in the forefront of microgrid implementation. After the
devastating effects of Hurricane Sandy many policy makers examined ways to fortify and make the electric grid
more reliable during times of extreme weather when reliable electrical power was crucial. States along the east
coast, such as New York and Connecticut, have developed
incentive programs for microgrid deployment due to the
impact of recent hard hitting storms including Hurricane
Sandy.19 Microgrids also provide the ability to deploy
renewable energy sources, decrease emissions per unit
of power used, and allow the grid and institutions that
depend on the microgrid to become more efficient and
resilient.20 Storage on the microgrid is a key element of
utilizing the benefits of renewable and California recently
put in place a 1.3 gigawatts of energy storage program
by 2020 that could ultimately lead to financial incentives
for microgrids.18,19 California also allocated $26.5 million to microgrid projects that focus on integration of
renewables.18 The widespread implementation of microgrids still faces many challenges and obstacles. Technical,
financial, and regulatory issues must all be addressed.19
Three prevalent barriers to the more rapid and universal
deployment of microgrids are: convincing the public that
the overall benefits outweigh the costs required to install
them, figuring out how to put cost value on reliability,
and updating current regulations.20 Though challenges
do exist, the overall benefits are profound and microgrid
advancements are being made.
Many different rules may apply depending how a microgrid operates and many state’s laws are not clear
about the legal status of microgrids.21 In much of the
United States the electric utility system is owned and
operated by a monopoly distribution system operator
(DSO).22 This makes it hard for independent microgrid
developers to install infrastructure and make any modifications that would allow the microgrid and the main grid
to operate optimally together.22 Issues that need to be
addressed include: the connection of the microgrid to the
main grid, the location of the microgrid infrastructure,
how is the microgrid funded, the regulation of the power
generated on the microgrid, and how the utility adjusts
procurement and resource adequacy with consideration
to microgrid deployment.22 There is a growing number
of research projects that are being done in the development of microgrid technology and some states are addressing the regulatory issues. However, due to the current regulatory environment this is a slow process and
until the regulatory framework makes microgrid development simpler, microgrids simply will not grow beyond
niche applications.23
business and universities. Commonwealth Edison, known
as ComEd, the local utility for the northern Illinois area
recently obtained the $1.2 million grant to build a master
microgrid controller that will be able to operate multiple microgrids.14 Along with other entities, ComEd will
work to build a controller that will be able to optimize
the relationship of one or multiple microgrids with the
main grid.13 Alstom Grid, Inc. and Burns Engineering,
Inc. received the $1.2 million in funding also for work
on microgrid controls. The project will use the former
Philadelphia Navy Yard to perform the work.15 The Electric Power Research Institute, located in Knoxville, Tennessee, is slated to develop generic microgrid controller.13
General Electric will develop an advanced microgrid control system for Potsdam, New York.13 TDX Powers, Inc.
located in Anchorage, Alaska will install a microgrid control system on Saint Paul Island.13 The University of Californias Advanced Power and Energy Program will develop and test a standard microgrid controller utilizing
the University microgrid as a test bed.13 Finally Burr Energy was selected to construct and test microgrid control
systems for two Maryland suburbs.13 All these projects
are important as the controlling capabilities and operational features of microgrids will ultimately determine
the degree of proliferation in the utility power industry.16
Depending on the infrastructure and operational characteristics of the microgrid, the required control and operation strategies can be significantly different then that of
the conventional power system.16 These projects can aid
in speeding up the development and commercialization
of microgrid controllers.
Microgrid control brings new questions of grid security.
Physical grid security has always been a concern with the
grid, however, with implementation of microgrids and
their control architecture, cyber security is a growing
issue.17 Physical grid security is comprised around the
physical components that make up the grid such as power
plants, transmission and distribution lines, and sub stations. While this is still a threat with microgrids, having
distributed infrastructure makes it more difficult for large
scale affects from attacks. This same distributed infrastructure also aids the microgrid set up from large scale
effects of cyber-attacks to the grid. Microgrid control
systems rely on software based programs and communication networks to perform their functions.17 This makes
them targets for directed cyber-attacks. Therefore cyber security and cyber hardening is very important parts
to the design and construction of the microgrid control
systems.17
Microgrid Implementation
Currently in the United States there is about 1,051
megawatts of microgrids deployed.18 The majority of
these are located and facilitate factories, college campuses, hospitals, and military bases.18 Most of the current installed capacity and microgrid programs have
The Micro-Microgrid or Nanogrid
Critical facilities such as hospitals, fire stations, and
data centers depend on a reliable source of electrical
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through the use of a nanogrid. Renewable energy sources
can be installed for building microgrid DER in areas that
have an abundance of resources. The buildings electrical
and thermal loads can also be supplied by a combined
plant within the building. Combined heat and power
(CHP) plants supply a building heat loads as well as a
good amount of the electrical load. By combining the
thermal and electrical loads the efficiency of energy generation can increase by as much as 25 percent.28
power. To ensure consistent electrical supply critical facilities will often install their own equipment to facilitate the generation of electrical power should the main
grid go down. Along with improving the reliability, microgrid technology provides the opportunity and desired
infrastructure for improving the efficiency of energy consumption in buildings.24 Commonly referred to now as a
nanogrid.25 A nanogrid may range from a laptops battery
and plug to a buildings onsite storage and generation or
just the energy management system.25 Many of the same
benefits that come with microgrids can be found with installing nanogrids. Increased reliability is the largest factor leading to nanogrid installation. Another large benefit to nanogrid instillation is peak shaving.25 With current technology in energy management systems (EMS)
coupled with a storage device, usually a battery system,
there is potential for customers to save substantially on
electrical usage.
One company moving this technology forward on the
grid and building scale is Stem Inc., a start-up based out
of California. Stems system uses storage and management software to lower peak demand and therefore the
customers energy bill.26 Stem is taking a different approach to marketing the microgrid and nanogrid by focusing on the customers cost of electricity and their ability to reduce peak demand, a significant factor to business energy bill.26 Stem also promotes the added benefit
of the customers overall peak shaving on the main grid as
well as offering utility services.26 Recently Stem received
a $1 million dollar grand for a project that includes installing 1 MW of storage and control systems on the island of Oahu to help with the integration of renewables.27
With the proliferation of renewables in the commercial
and residential markets solutions such as Stems will increasingly become essential to maximize the benefits as
well as protect the main grid.
Energy efficiency within buildings can also be achieved
1
2
3
4
5
6
7
8
Conclusion
Over the next few years and decades the energy industry will be at the forefront of change and innovation. With strong pushes due to environmental concerns
the methods of generating the electricity the world uses
will be forced to ever cleaner sources. Also with the aging electrical transmission and distribution infrastructure
concerns over grid reliability will grow. The electrical
distribution system will be required to undergo advancements utilizing the latest technologies and practices. Microgrids provide a means to ultimately balance the needs
of the future with the technology of today. A properly
deployed microgrid provides improved grid reliability and
resiliency. The use of storage on the microgrid allows for
the full utilization of environmentally friendly renewable
sources of energy such as solar and wind. Challenges
are still prevalent in the wide spread implementation of
microgrids throughout the United States and the world,
however, the benefits are sound and progress is currently
underway. From the nanogrid to the microgrid all the
way to the macorogrid the way electricity is delivered to
end users is changing. The science is being developed in
full force. The regulation controlling the implementation
of that science needs to follow suit.
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review (2014), http://www.eia.gov/totalenergy/data/
annual/#consumption.
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businesswire.com/news/home/20140916006179/en/
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//www.burns-group.com/news/announcements/
article/alstom-and-burns-engineering-inc.
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IEEE Power & Energy Magazine 6, 54 (2008).
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cyber_secure_microgrid.
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Electricity Deregulation: California and Australia
Thomas Mlynarski
The twentieth century saw regulation and deregulation of many industries, including transportation, trucking, airline, banking and media. Toward the end of the century, there was an increased
interest in, and implementation of, liberalizing policies in the energy industry. The U.S. and Australian deregulation system has been left mainly in the hands of their states1,2 , which has led to
many interesting case studies. This paper will focus on two of these case studies, California and
Australia. California’s “deregulation crisis” is often cited as an argument against liberalizing energy
markets.3 Comparatively, the eastern provinces of Australia integrated into a single deregulated
market, demonstrating some early successes and some difficulties associated with the deregulation
model.
History and Background
The size and political composition of the United States
allows for a variety of regulatory environments, by region
and by state. The electric industry had been consistently
treated across the country as a utility, and this regulation encouraged price stability and network reliability.
This economic certainty allowed for financing large capital expenditures in energy generation and infrastructure.
It was not unusual for these projects to have large cost
overruns, with the costs being passed on to consumers. A
desire to produce a more efficient allocation of resources
and the resulting lower prices for consumers led policy
makers to begin considering liberalization of the energy
industry in the same way that transportation, airlines,
telecommunications and others were being successfully
deregulated.
The energy industry can be likened to other industries but has its own unique challenges. For example,
demand is not elastic. Demand needs to be instantaneously matched to supply, and the produced electricity
cannot be easily stored currently. This combination along
with limited production, or capacity constraints, can result in excess demand being curtailed by rolling blackouts rather than the typical reduction of usage by higher
prices.1 Service interruptions also define network reliability and blackouts can have political consequences, both
for politicians and the industry. Electricity production
is constrained by the available resources, which depends
on local conditions, or otherwise necessitates bringing in
fuel or electricity by high voltage transmission lines. Different production methods, such as coal, nuclear, solar,
etc., vary in risks and costs, so that some energy mix may
be more desirable at the national level.
The industry is composed of four parts: generation,
transmission, distribution, and retail. The electric utility paradigm has all of these components under one company. In order to foster more competition, the deregulation schema1 splits the industry both horizontally and
vertically, so that each component is separate and each
component also has multiple choices, where practical.
Generation is the production of electricity, and lends
itself well to this process. Under deregulation, generating stations would be privatized and would compete
to supply power. Typically an official exchange is created where the generators sell their electricity and the
electricity providers purchase it for consumers. The exchange functions like any market exchange, where generators compete on price and buyers may choose depending
on their requirements, usually cost, but possibly environmental considerations for example. An official exchange
facilitates oversight to analyze market performance and
prevent collusion. Transmission refers to the component
of the electric industry responsible for transporting produced energy to energy markets, and is one of the most
difficult areas to regulate. The infrastructure cannot be
easily reproduced, is difficult to price, and can localize
generation which will distort market behavior by not allowing generators to compete efficiently. Distribution
provides the infrastructure for the final connection to the
customer. The infrastructure is similarly difficult to duplicate. For this reason, transmission and distribution
continue to be regulated, with the preferred method being performance based regulation (PBR). PBR rewards
transmission and distribution firms for achieving some
metric, such as achieving a level of reliability while keeping costs below some index of expected costs.4 Failure to
properly implement transmission and distribution regulation can lead to reliability issues by under investment
or non-competitive behavior if a single retail utility monopolizes the local infrastructure. The retail component
procures electricity from the generators in the electricity
market and delivers it to the customers. These companies
are the face of the electricity industry for consumers and
handle billing, customer accounts, and offer electricity
products, such as green energy. Together these changes
constitute what is loosely called the textbook case for
deregulation in the electric industry. An implementation
of deregulation may pick the extent of applying the different aspects.1
California
The California regulatory energy environment was
dominated by investor owned utilities Pacific Gas and
Electric (PG&E) and Southern California Edison (SCE),
and two municipally owned utilities San Diego Gas and
Electric (SCG&E) and the Los Angeles Department of
Water and Power.5 Under regulation, the utilities were
vertically integrated, controlling their own generation,
transmission, distribution, and retail operations. The
system was managed so that the California Public Utility Commission (CPUC) set the electricity rates paid by
consumers by considering the investments required in infrastructure. The primary benefit of this type of regulatory system is the reliability of the electric grid. In 1995,
the CPUC found that the regulatory system was overly
complex, and under pressure from industrial customers
and independent generators, California voted to deregulate the industry to promote competition in 1996.5
Deregulation changed the California electricity landscape. Public utilities were required to divest forty percent of their generating capacity to a new set of five
major producers, who were to compete to provide lower
wholesale electric prices. A non-profit California Power
Exchange (PX) was created where the retail electricity
providers, in this case still the same utilities, and the new
generators could trade electricity at auction to create a
competitive price through supply and demand mechanics. Control of transmission and distribution was transferred to a new entity, the Independent System Operator,
who was responsible for keeping open access to the power
transmission system. The old utilities would continue to
function in the retail electric market, along with new entrants who could compete with them for customers by
purchasing electricity on the PX.
The initial electric rate for consumers in the new system was set by CPUC at ten percent below the prevailing
rate.5 This would build support among consumers and assuming lower wholesale electric prices on the PX, would
compensate utilities for the divested generation infrastructure. These rates would be in effect for individual
utility companies until full divesture of generation. The
relatively small number of generators created opportunities for collusion. Generators found that electric scarcity
allowed higher prices, which discouraged investment in
new generating capacity and encouraged restricting supply. This produced electricity price spikes and supply
shortages. The summer of 2000 saw rolling blackouts,
despite an installed capacity of 45 gigawatts and demand
peaking at 28 gigawatts. When the first utility (SCG&E)
fully divested generation, the electric rates it passed on
to customers tripled their electricity bills. Other utilities
were not so lucky and had to absorb price differences between the fixed rate they could charge customers and the
higher rate they were forced to buy on the PX. In January of 2001, electric utilities exhausted cash reserves
and suspended payments to producers. Generators had
anticipated this scenario and started taking generation
FIG. 1: The average retail price in Californias LA
County shows the initial price decline and steady
behavior, followed by a large jump during the crisis and
the locking in of higher rates. The average US city price
is included for comparison. Prices are in USD.7,8
offline earlier.5 PG&E could no longer afford to buy electricity on the PX and declared bankruptcy, forcing the
state to intervene and purchase electricity for delivery
to customers. The crisis was finally resolved when California entered a state of emergency, allowing the state to
sign a ten year contract to purchase electricity, a solution
criticized for locking in high electricity rates. (Figure 1)
shows this price behavior for LA County. The crisis was
estimated to have cost California $40 billion.6
California suffered at all levels of deregulation. The
lack of sufficient independent generators created market
power at the production level, which was magnified by
the retention of significant generation capacity by the
utilities themselves. In the wholesale market, generators
had a disproportionate ability to manage pricing by collusion and supply manipulation. The transmission system created electricity transportation bottlenecks which
allowed generators even more market power in local areas. The fixed rate did not allow customers to respond to
price fluctuations and priced out retail competition once
PX wholesale prices were higher than the fixed rate. The
uncertain regulatory environment perpetuated the crisis.
Electric utilities failed to sign their own long term contracts with suppliers, not knowing if such action would
be viewed by CPUC as circumventing the PX. There was
also the expectation that the fixed electric rate as was a
floor for customer rates, but CPUC refused to pass the
rising costs on to customers.5
Australia
Prior to World War II, Australia’s energy markets were
geographically localized, with regions producing their
own electricity and no way to transport the electricity
around the continent. The war brought increased industrialization that required better infrastructure. Australia’s large geographic size but relatively modest pop67
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FIG. 3: Electricity price change is plotted next to the
consumer price index. The plot is normalized for 1998
prices and shows that electricity prices started to grow
after 2007, primarily due to increased spending on
distribution infrastructure.13
FIG. 2: The National Electricity Market prices are
shown in AUD cents per kWh. The price stays within a
relatively narrow band relative to retail prices, with
volatility tied to prices in fuels such as gas.9
cial customers slightly behind.11 The problem was compounded by generous subsidies on photovoltaic equipment. Consumers installed nearly 2,600 MW of solar
capacity, or 1.3% of consumption, to avoid the increasing electricity prices, leaving a smaller customer base to
pay the fixed transmission and distribution costs.12 Industrial customers also canceled projects as they became
uneconomical in the new environment, further eroding
the base. Australia is attempting to remedy the crisis by switching to a performance based regulation for
the distributors, where infrastructure improvements are
evaluated against a benchmark and efficiency savings
are shared with consumers after review by a regulator,
rather than reimbursing infrastructure investments at a
fixed rate of return.12 Even today, the wholesale electricity market continues to provide decent electricity prices
given macroeconomic conditions in Australia. Nonetheless, the entire energy picture continues to develop.
ulation presented formidable challenges for a comprehensive continental infrastructure project. The improvements occurred throughout the fifties and sixties as rapid
growth in local coal power plants, a large hydropower
and storage project in the Snowy Mountains region,
and transmission interconnections between geographically close provinces that connected most of Eastern Australia in the nineties.2 This system spanned roughly 5,000
km North-South, and connected a population of 16 million. Australia experienced calls for internationally competitive electricity prices contemporaneously with the
U.S., despite boasting among the lowest electricity prices
in the world, and deregulation began in 1998.2 Eastern Australias integrated electric system provided a suitable environment for trading electricity among competing
generators through a National Electricity Market (NEM)
which would be established for this purpose. Each Australian state had an electric utility that comprised the regional electricity industry. These utilities were dissolved
into numerous generators, transmission and distribution
companies, and retail components. The generators were
required to sell their electricity on the NEM by submitting bids to the market every five minutes. The market
operator matched the required demand by sending automatic generation control signals to plants that were most
economically able to meet demand at the time, with average price behavior shown in (Figure 2). Transmission
and distribution networks were available to new generators. Customers were given choice to change their retail
electricity provider.
A 2006 government review of the deregulation progress
and results showed a decrease in electricity prices to consumers, and increased investment in transmission and
distribution networks.10 The rosy assessment proved premature. By 2013, as the transmission and distribution
investment costs were passed onto consumers, the average cost of electricity to retail customers had risen by
70% from 2006, as seen in (Figure 3), with commer-
Conclusion
Deregulation affords opportunities to unlock efficiencies from stagnant regulatory systems, or to disturb the
semi-functioning electricity production and delivery systems of today. Although status quo is the safe option,
regional competition may force a more efficient framework on this existing system. Currently, genuinely practiced deregulation offers the best alternative for consumers, with thirteen US deregulated states (Connecticut, Delaware, Illinois, Maine, Maryland, Massachusetts,
New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Texas) experiencing price increases
below inflation in the last year, while thirty US regulated
states had just the opposite effect.14 Although recovering the catastrophic cost of California’s electricity crisis
is not realistic through market efficiencies, enough cases
and solutions now exist, providing policymakers with
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guidance for many situations. In particular, the “textbook model” of an open wholesale market, performance
based regulation of transmission and distribution, and
1
2
3
4
5
6
7
8
9
10
11
12
13
14
allowing retail competition shows considerable promise
through its successes.
Paul L. Joskow, Nature 29, Lessons Learned From Electricity Market Liberalization, 33 (2008).
Frank Brady, CIGRE and AHEF, A Dictionary on Electricity, 20 (1996).
Illinois Chamber Of Commerce, Electricity and Natural
Gas Customer Choice in Illinois, A Model For Effective
Publuc Policy Solutions, 20 (2014).
The Future of the Electric Grid, 280 (2011).
James Bushnell, Energy Policy 32, California’s electricity
crisis: a market apart? 7 (2003).
Christopher Weare, Energy Policy 32, The Claifornia Electricity Crisis: Causes and Policy Options 140 (2003).
Bureau of Labor Statistics, U.S. City Average (2014),
http://www.bls.gov/.
California Energy Comission, Energy Almanac, Statewide
Electricity Rates by Utility (2014).
Average Price Tables, Australian Energy Market Operator 29, (2008), http://www.aemo.com.au/Electricity/
Data/Price-and-Demand/Average-Price-Tables.
Continuing Medical Education, Electricity Prices in Australia An International Comparison 16 (2012).
Kai Swoboda, Energy Prices The Story behind Rising
Costs 2 (2013).
Victoria Melbourne, Australian Energy Regulator State of
the Energy Market (2013).
Electricity CPI Australia Australian Bureau of Statistics,
Series A2328141J (2013).
Kathleen McLaughlin, Indianapolis Business Journal
Pence Cracks Door to Electricity Deregulation (2014).
69
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2015
An Analysis of Current Battery Technology and Electric Vehicles
Ryan Sprague
The purpose of this paper is to describe current uses of battery technology for internal combustion
engine vehicles and newer hybrid electric vehicle and battery electric vehicle alternatives. This paper
will also discuss the benefits and challenges to alternative vehicle adoption. As battery technology
and charging infrastructure continue to advance, and drivers become more informed about these
technologies, adoption rates for alternative vehicles have the potential to increase dramatically,
leading to a dramatic transformation of the auto and petroleum industries.
Introduction
Battery-powered electric vehicles have the possibility
to be one of the most disruptive technologies of the
early 21st century and can potentially alter two of the
largest and most influential industries of the world economy: automobile and petroleum. While electric vehicles
are not a perfect solution, they do offer some answers
to current concerns in society. The greatest challenges
for widespread adoption of electric vehicles are twofold.
First, the cost and energy density of battery technology
prevents electric vehicles from being comparable to internal combustion engine vehicles. Second, drivers perceptions and fears of the limitations of electric vehicles need
to be skillfully finessed.
This paper will explore the history and current state
of vehicle battery technology and its deployment, the
current use of batteries in vehicles, and different battery chemistries currently utilized. The benefits and
challenges of current battery technology will be assessed
considering performance characteristics and safety concerns. Further, perceptions of electric vehicles preventing
widespread electric vehicle adoption will be appraised.
Additionally, charging infrastructure and its benefits and
challenges will be explored.
Battery Fundamentals and History
Batteries in todays society are so prolific and easy to
use that it is easy to dismiss the effect they have on convenience, comfort, and technological advancement. They
have enabled the cell phone industry, portable electronics and computing, robotics, and the electric car industry,
just to name a few. Without the ability to store energy
electrochemically in a battery, many of todays advancements would not be possible. All electrical devices would
have to be plugged into a continuous supply of electricity,
typically supplied by the electric grid, and would therefore be tethered to their power source eliminating most
forms of reasonable portability. Because batteries are so
common and easy to use, it is easy to ignore the hidden
chemistry that enables our modern conveniences. While
varying types of batteries exist their basic process of storing electricity remains the same.1 Batteries that have
a single use, primary cells, are composed of a negative
terminal, anode, positive terminal, cathode, electrolyte
and casing or packaging.1 The electrolyte separates the
anode from the cathode while allowing ions, positively
charged particles, to pass freely between them. When
a load, the source that is using the electricity, is connected between the terminals the battery undergoes an
electrochemical reaction that sends electricity, electrons,
through the circuit and the connected load. This process
occurs when the anode undergoes an oxidation reaction
and releases electrons through the terminal.1 Simultaneously, the cathode undergoes a reduction reaction in
which the cathode material, reacts with ions in the electrolyte and available electrons released from the anode.1
To put it more simply, the anode reaction releases electrons, and the cathode absorbs them.1 In the process,
the movement of electrons through the circuit generates
work on the connected load. In a rechargeable battery, or
secondary cell, the anode and cathode switch while the
battery is being recharged. This switch occurs because,
by definition, the anode always releases electrons and the
cathode always uses them.1
Italian physicist Count Alessandro Volta is credited for
discovering the process in 1799 when he created a simple battery from metal plates which acted as the electrodes and brine-soaked cardboard which acted as the
electrolyte. The resulting Voltaic Pile was able to generate a sustained current.1 Some archeological evidence
suggests that crude batteries may have been developed as
early as 200 B.C.1 In the last two centuries, the basic battery concept has been greatly improved upon. A variety
of components and elements for the anode, cathode, and
electrolyte have been explored and investigated, yielding a range of batteries with different characteristics and
properties and a wide range of applications. This paper
will focus primarily on the three most common forms of
batteries currently used in modern vehicles. Those batteries are the lead-acid battery, the nickel-metal hydride
battery, or NiMH, and the lithium-ion battery, or Li-ion.
battery only provided modest propulsion, it was still able
to increase the miles per gallon (mpg) metric, to 42 mpg.
By employing a hybrid drive system, Toyota was able to
reduce the Prius carbon dioxide (CO2 ) emissions by up
to 37.4% The NiMH battery is still being deployed today
in HEVs and plug-in hybrid electric vehicles (PHEVs).
PHEVs operate in the same manner as HEVs, however,
they have the additional ability to plug into the electric
grid to charge the battery. The NiMH batterys limited
specific energy does constrain the electric-only range of
HEVs and PHEVs.
Battery Use in Modern Vehicles
The most common battery in current vehicles is the
lead-acid battery. Lead-acid batteries have their benefits, and as a result, have been widely adopted for todays current internal combustion engine (ICE) vehicles,
for a specific purpose. Lead-acid batteries have a long
shelf life, are inexpensive, reliable, easily recyclable, and
are safe when properly handled and maintained.2 The
lead-acid battery provides the functions of starting, lighting, and igniting the vehicles ICE, cabin, and lighting
systems.2 The lead-acid battery, developed in 1859 by the
French physicist Gaston Plante, was the first rechargeable battery.1 It is able to produce a large amount of
power for a short period of time to the starter to turn
the engine over and begin the combustion process. The
deployment of the lead-acid battery allowed vehicle manufactures to forgo the hand crank that was originally used
to start ICE vehicles and implement modern computer
processing, sensing applications, and lighting. Once the
vehicle has started, the engine generates current, via the
alternator, and sends electricity back to the battery to
be recharged.
The performance requirements of the lead-acid battery
are limited, and therefore, it need not possess a high
energy density compared to newer battery technologies.
One of the greatest limitations of the lead-acid battery is
its considerably poor specific energy compared to modern
technologies. Specific energy related to energy storage
is a measure of the amount of energy (watt-hours per
kilogram) Whkg−1 , the energy source can store.2 Leadacid batteries have a specific energy of 30-40 Whkg−1 .2
By comparison, gasoline has a specific energy of 13,000
Whkg−1 .3 Lead-acid batteries would add a tremendous
amount of weight to the vehicle if they were used for
other functions such as propulsion. As a result, it is
not realistically viable to propel vehicles exclusively with
lead-acid batteries.
The nickel-metal hydride (NiMH) battery was the next
rechargeable battery widely produced for commercial applications in hybrid electric vehicles (HEV). Toyota released its Prius HEV in 1997 to the Japanese market and
to the rest of the world in 2000.4 The Prius used both
an electric motor and a small ICE for propulsion. At low
speeds, the electric motor drove the vehicle exclusively,
and when more power or speed was needed, a small ICE
turned on automatically to provide additional power for
propulsion. NiMH batteries were used to supply energy
to the electric motor because they offered a higher specific
energy than lead-acid batteries, at 60 Whkg−1 and had a
much better energy density (watt-hours per liter) of 140
Whl−1 compared to a lead-acid batterys 70 Whl−1 (see
Figure 1).5 NiMH batteries are also highly reliable, and
safe similar to lead-acid batteries. In contrast to leadacid batteries, NiMH batteries are composed of noncorrosive substances resulting in safer handling and recycling.
Therefore, NiMH batteries were well suited for the development of hybrid vehicle technology.5 Although the Prius
FIG. 1: A comparison of electricity storage
characteristics of Lead-Acid, Nickel-Metal Hydride and
Lithium-Ion batteries
The next step in the progression of battery technology and its implementation with relation to HEVs, and
battery electric vehicles (BEVs or EVs) was the lithiumion battery. The limiting factor for vehicles is size and
weight, and as a result, the automotive industry constantly seeks a battery that has a greater specific energy
and energy density to increase the range of electric vehicles, one of consumers biggest concerns regarding EVs.
The Li-ion battery is a step in that direction. The Li-ion
battery has a specific energy of over 200 Whkg−1 , and
an energy density of 250 Whl−1 (see Figure 1).6 In evaluating the performance of EVs, range becomes a realistic
concern and engineering challenge. This challenge is often referred to as range anxiety and will be discussed later
in this paper. The implementation of the Li-ion batteries
and their improved battery-only range has allowed some
automobile manufacturers to realistically produce EVs
for the consumer market. The most notable vehicles today are the Nissan Leaf and Tesla Model S.7 The Nissan
Leaf is an all electric vehicle with an estimated range of
84 miles per charge costing $29,000 and the Tesla Model
S has an estimated range of 265 miles per charge and
costs $71,000. Both vehicles rely on Li-ion batteries.8
Similar to NiMH batteries, Li-ion batteries are reliable, require low maintenance, and have a long life-cycle
of about 8-10 years or 100,000 miles. The two greatest
concerns for Li-ion batteries are safety and cost.9 While
Li-ion batteries are considered safe, there are concerns
with the batteries concerning thermal runaway.9 Thermal runaway can be caused by defects in the internal
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that contribute to acid rain and ocean acidification. In
addition to global warming, governments and automobile manufacturers are trying to reduce these noxious
pollutants from vehicle emissions. One action some
governments have taken in this direction is increasing
the mandated corporate average fuel economy standards
(CAFE). The CAFE standard requires manufacturers to
design and manufacture more fuel-efficient vehicles over
time, and creates incentives for low emission or zero
emission vehicles to be produced to offset the lower fuel
economies of heavier duty vehicles. When vehicles use
less fuel they also produce fewer emissions. In addition
to CAFE standards, regulations also exist to specifically
limit individual pollutants from tailpipe emissions. Some
argue that using battery powered vehicles only transfers
greenhouse gas emissions from gasoline to dirtier coal
generated electricity. This statement should be considered when assessing the effects of EVs and HEVs as they
gain market share. The overall percentage of coal generated electricity in America is declining. Furthermore,
large, stationary power plants are able to deploy more
enhanced pollution reducing technologies than vehicles
realistically can. As a result, even though power plants
produce a tremendous amount of emissions, EVs powered
exclusively from coal generated electricity are ultimately
responsible for fewer emissions than the average ICE vehicle of a comparable size.11 EVs will be responsible for
even fewer emissions as more renewable resources generate electricity, and older, notoriously dirty power plants
are decommissioned.
Noise pollution seems like a mild inconvenience. However, it can contribute to hearing loss, increased blood
pressure, higher rates of coronary heart disease, higher
levels of stress and a lower quality of life.12 Additionally,
it can harm animals by disrupting their reproductive capabilities, disrupting their navigation abilities, contributing to their hearing loss, and interfering with their prey
detection or predatory abilities.12 EVs and HEVs are exceptionally quiet. They are so quiet that regulators are
considering requiring such vehicles to produce an artificial noise at low speeds for the safety of pedestrians.
Greater deployment of EVs and HEVs will dramatically
reduce the noise levels in cities in particular, and contribute to a higher quality of life.
Regarding Americas dependence on petroleum, America uses approximately 19 million barrels a day of
petroleum mostly for vehicle, boat, and plane fuel.13 Of
the 19 million barrels of petroleum used each day, America imports approximately 9 million barrels (see Figure
2).13 A portion of these imports come from nations that
are hostile to the U.S., or from nations that do not share
Americas values, some of which are political allies. While
this paper will not go into depth about the geopolitical
challenges associated with fossil fuel extraction and its
use, it can be said that it is in the interest of national security, and the economy to be more energy independent.
Achieving that goal requires generating energy from a
higher percentage of domestic resources. EVs are able to
cells of the battery that short the cell between the anode and cathode. Such a short results in current flowing
freely from one electrode to the other building up heat
and eventually starting a fire or exploding.9 Electronic
circuitry and monitoring components are integrated into
the battery packs of EVs to prevent such events from occurring. Li-ion batteries have been used for quite some
time in portable electronics and laptops with minimal defects. However, when dealing with vehicles safety standards are much higher. Battery manufacturers are constantly working to improve manufacturing processes and
safety. It is common, however, for new technologies to be
held under greater scrutiny by the public until they are
better understood and become more commonplace.?
Positive and Negative Aspects of HEVs, PHEVs,
and EVs
Benefits of HEVs, PHEVs, EVs
Electric drive technology is not new. The last decade
has seen a dramatic increase in research and implementation of electric powered vehicles as well as HEVs. There
are a number of challenges that the widespread implementation of these technologies can help address, such as
climate change, air pollution, noise pollution, and dependence on petroleum, both domestic and foreign.7 While
EVs and HEVs are able to mitigate some of the issues
mentioned above, they also provide additional benefits
that were not the primary focus of the technologies deployment, including increased energy security and independence, instant torque at low speeds provided by electric motors creating more fun for the driver, the benefit
of not having to use refueling stations, reduced maintenance costs, and increased fuel energy savings and price
stability for the consumer.10
It is widely accepted by the scientific community that
the earth is undergoing global warming caused by increased levels of greenhouse gases in the atmosphere,
primarily CO2 , and that humans are the cause. Global
warming and the resultant climate change will cause a
host of problems for humans and the environment. As
a result, governments globally have implemented policies to reduce greenhouse emissions including further implementation of HEVs and EVs. Some studies regarding greenhouse emissions and PHEVs have shown that
PHEVs emit as much as 50% fewer greenhouse gas emissions compared to ICE vehicles, even when coal is the
primary source of electricity.7 Furthermore, EVs have
zero tailpipe emissions, and can have no emissions at the
source of electricity if the electricity is generated with
renewable forms of energy.7
Air pollution is also a concern for governments. Air
pollution consist of airborne particles from vehicle exhaust and industrial smokestacks which contribute to
smog, mercury that contributes to cancer and brain deficiencies, as well as nitrous oxides and sulfur dioxides
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gasoline prices have fluctuated between approximately
$1.30 per gallon to approximately $4.20 per gallon.15
This price constancy better allows consumers to budget
their expenses without unexpected and dramatic price
swings affecting spending habits.
Challenges of HEVs, PHEVs, EVs
The greatest challenges facing widespread adoption of
EVs and HEVs are related to the limitations of current battery technology and consumers negative or uninformed perceptions. The restrictions of current batteries
include the high upfront cost of the battery packs, the low
energy densities of these batteries, and the long amount
of time required to charge them.
One of the challenges EVs are currently facing is their
higher retail price compared to equivalent gasoline powered vehicles. Vehicle battery packs are expensive. The
approximate cost of such batteries are between $750 per
kWh to $1,500 per kWh for EVs, PHEVs, and HEVs.16
As a result, battery packs can add $3,000 to $30,000 dollars to the cost of electric vehicles, depending on the size
of the battery.16 Even after accounting for fuel savings
and maintenance savings, these high additional battery
costs sometimes do not result in cheaper ownership of
the vehicle when factoring in a fixed, projected gasoline
equivalent cost.
Because the battery packs currently have a low specific energy and energy density relative to gasoline, the
batteries used in EVs have to be very large to get an acceptable range. Such battery packs can weigh over 600
lbs. for the Nissan leaf and 1300 lbs. for the Tesla Model
S.17,18 Therefore, adding additional battery cells would
have a diminishing return as they would also increase the
weight of the vehicles and decrease their performance.
Additionally, the time required to charge the massive
battery packs is another engineering challenge. Currently, to charge a dead Leafs battery on a standard 120
V wall outlet would require up to 20 hrs, or 8 hrs on a 240
V circuit.18 However, an 80% charge could be achieved
in about 30 minutes with a fast charger.19 Consumers are
used to being able to refill there gasoline vehicles in about
5-10 minutes and get another 300 miles of range. As a
result, consumers would have to dramatically alter their
expectations, and habits to accommodate this technical
hurdle. However, reducing the impact on the drivers inconvenience can be achieved by the drivers if they plan
for their needs. In particular, charging at night while the
driver is home would save additional time and effort for
most drivers. One concern that arises from EVs charging
at night is their effect on the electric grid and its ability
to satisfy the increased load. One study tracking 484 Atlanta, GA commuters driving and parking habits found
over 10 minute intervals, the greatest average increase in
cars parked at the same time was 1% at 7:00 p.m. local
time.20 As such, the current grid would be able to handle
modest increases in EV and PHEV deployment without
FIG. 2: A representation of the proportion of petroleum
used each day in America that is produced domestically
or imported
eliminate reliance on petroleum, particularly imported
petroleum by using domestic energy from the grid. Even
if that energy is generated from coal or gas, America is
able to extract those fossil fuel resources domestically.
Similarly, HEVs reduce dependence on petroleum, and
petroleum imports by requiring less gasoline as fuel and,
therefore, reducing overall demand.
Electric motors are notorious for having a great
amount of torque especially at low speeds, meaning electric cars have tremendous acceleration and a sense of
power and control while driving. One reporter stated
that the quick and powerful yet quiet acceleration is simply awesome and believes the fun of driving an EV is its
greatest selling point.14
Furthermore, because EVs use electricity as the fuel,
the bulk of refueling can be done while the driver is asleep
at home.10 In the convenience culture of America, this
ability to recharge at home could be a very attractive
feature. One GM salesperson stated that he regretted
trading in his Volt, (a PHEV), the first time he had to
go back to the gas station.10
One study stated that 87% of respondents traveled
fewer than 40 miles per day.7 Driving that limited distance means that EVs can obtain all the energy needed
for each day for most drivers from the their own electricity while he or she is at home.
Additionally, electric drive systems are much simpler
than ICE drive systems for a comparable car. EVs do not
have a transmission, alternator, starter, large engine or
lead-acid battery, all gasoline or diesel ICE components
stated require maintenance. The most common maintenance costs related to EVs are changing tires, brakes,
shocks, and windshield wiper fluid. EVs also have the potential to save approximately two thirds in fuel expenses
or more, while at the same time, maintaining lower price
volatility in the electricity market than gasoline.15
Electricity prices have been consistently in the dollara-gallon-equivalent mark for comparable energy to gasoline over the last 15 years. During that same time period
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U.S. and Canada.21 The company also plans to build
200 stations over the next two years and place 98% of
the American and Canadian populations within range of
a Tesla supercharger.21 While the infrastructure is developing, it is still far behind the 160,000 gas stations that
are currently available across the U.S. for ICE vehicles.
Therefore, EV drivers will need to continue to plan and
better anticipate their driving needs.
Concerning reliability and safety, while EVs and HEVs
are a relatively new technology, they still have to adhere
to the same safety regulations as ICE vehicles. There
have been very few incidents where the technology specifically related to EVs or HEVs has failed in a manner as to
cause harm or injury. Furthermore, most companies producing EVs and HEVs are established automobile manufacturers with proven safety and reliability records, and
they offer the same warrantees to their alternative vehicles as they do to their ICE vehicles. Tesla would be one
of the exceptions. Even though Tesla is a start-up automobile manufacturer, it has proven itself to be a leader
in EV innovation, development, and quality with many
accomplishments established car companies have yet to
achieve. Tesla also warrantees its products.
any modification to the existing grid.20 The concept that
all drivers leave work at the same time and would, therefore, arrive at home and begin charging their vehicles at
the same time is not true. As EVs and PHEVs capture
greater portions of the auto market utilities may need
to improve their distribution capabilities, however, this
paper will not further explore the topic.
Customers perceptions of battery powered vehicles are
the greatest hurdle from a marketing and sales perspective. EVs and HEVs are still a relatively new technology and people have a hard time adapting to such
new technologies.7 Consumers primary concerns regarding EVs are range, cost, charging infrastructure, reliability, and safety.7 Because EVs are not perfect substitutes
for ICE vehicles they require some changes to the drivers
normal habits such as the added hassle of charging and
planning trips. Some populations, like apartment or dormitory residents, would have the additional challenge of
charging their EVs exclusively at public charging stations. In the face of these substantial changes in habits,
consumers have to take time to better understand and
learn more about the technology, its applications, and its
possible effects and benefits on their lives. Furthermore,
when many people think of EVs they do not have a familiar perspective from which to evaluate the technology.
As a result of misperceptions, some people underestimate
the performance of such vehicles.10
Worry about the vehicles actual range is a real concern.
When an ICE vehicle runs out of fuel, the driver has to
get a can of gas, bring it back to his or her car, dump it
in, and the problem is solved. While it is inconvenient
to run out of gas, it is not a major problem. However,
if an EV ran out of energy the driver could not easily
top off the battery with more electricity. He or she could
find an outlet and start charging, but the more realistic
solution would be to get towed home, or to a charging
station. Such a situation would be highly inconvenient
and troublesome contributing to range anxiety for EV
drivers.
Charging infrastructure is another concern. As mentioned earlier, most drivers, for a majority of driving
trips, would be able to satisfy range requirements by
charging the EV at home. If the driver needed to charge
the EV while on the go, options are continually increasing as infrastructure continues to develop. Many public
parking lots or garages offer free charging. Furthermore,
chargers are frequently being installed both by EV manufactures and by third-party companies entering the infrastructure market.21 In particular, Tesla, the innovator
behind the supercharger, has vowed to install superchargers every 80 to 100 miles on major routes throughout the
1
2
Conclusion
In summary, advancements in battery development, in
particular over the past few decades, has enabled the implementation of modern EV, HEV, and PHEV technology to augment the existing auto market. While these
technologies still have a ways to go before they are on
par with ICE vehicles, they are beginning to impact the
market and consumers perceptions. As EVs, HEVs, and
PHEVs become more widely accepted, consumers will be
able to save money, be energy independent, have a lower
impact on the environment, pollution, and greenhouse
gases and have an enjoyable driving experience. Future
research and development is needed to continue improving the specific energy and energy density of batteries
being used by vehicles, while at the same time reducing the cost of the technology. Infrastructure to support
widespread adoption of electric vehicles will also need further development and implementation. Efforts, such as
those noted above, can add to alleviating range anxiety
for consumers and potentially change overall perceptions
of electric vehicles, and as a result, HEVs, PHEVs and
EVs will have better market penetration leading to a dramatic change in the automobile and petroleum industries.
3
A. Brain, B. Marshall, Charles W. Bryant, and,
Clint Pumphrey, How Batteries Work (2000).
Automotive Battery, http://en.wikipedia.org/wiki/
Automotive_battery (2014).
74
Farhad Manjoo, Better Batteries Will Save the World,
Slate,
http://www.slate.com/articles/technology/
technology/2011/06/better_batteries_will_save_
the_world.single.html (2011).
c
2015
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Toyota Prius, http://en.wikipedia.org/wiki/Toyota_
Prius (2014).
Nickel-Metal Hydride Battery, http://en.wikipedia.
org/wiki/Nickel\%E2\%80\%93metal_hydride_battery
(2014).
Lithium-Ion Battery, http://en.wikipedia.org/wiki/
Lithium-ion_battery (2014).
O. Egbue and S. Long, Energy Policy, Barriers to
Widespread Adoption of Electric Vehicles: An Analysis
of Consumer Attitudes and Perceptions, 729 (2012).
E. Sachaal, Top 10 Electric Vehicles With the Longest
Driving Range (2014).
Batteries
for
Electric
Cars,
BU-1204,
http:
//batteryuniversity.com/learn/article/batteries_
for_electric_cars (2011).
Z. Shahan, The Other 1 Reason Why Electric Cars Will
Domination the Car Market (2014).
D. Anair and A. Mahmassani, State of Charge (2012).
Noise Pollution, http://en.wikipedia.org/wiki/Noise_
pollution (2014).
Petroleum, http://en.wikipedia.org/wiki/Petroleum#
Petroleum_by_country (2014).
Z. Shahan, Missed Messaging, Electric Cars Are Totally
Bloody Awesome (2013).
M. Baumhefner, Counterpoint, Good News: We Can Drive
on Clean Buck-a-Gallon Fuel (2012).
Hybrid
Electric
Vehicle,
BU-1201,
http://
batteryuniversity.com/learn/article/hybrid_
electric_vehicle (2011).
Electric Vehicle, BU-1203, http://batteryuniversity.
com/learn/article/electric_vehicle (2011).
Teslarati Network, Tesla Model S Weight Distribution
(2013).
Plug In America, How Long Does it Take to Charge
a Plug-in Car, http://www.pluginamerica.org/faq/
how-long-does-it-take-charge-plug-car (2014).
N.S. Pearre, W. Kempton, R.L. Guensler, and
V.V. Elango, Transportation Research Part C, Electric Vehicles: How Much Range is Required for a Day’s
Driving, 1184 (2011).
E. Wesoff, Tesla Supercharger Network Operable During
Zombie Apocalypse (2013).
75
c
2015

Journal of Undergraduate Research - The Journal of Undergraduate

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