Building Integrated Photovoltaics - Heriot

Building Integrated Photovoltaics:
New trends and Challenges’
Prof. Hari M. Upadhyaya
Head, Energy Engineering Group
Institute of Mechanical Process and Energy Engineering
School of Engineering & Physical Sciences, Heriot Watt University,
Edinburgh- EH14 4AS
India-UK Collaboration
in Solar Energy
Contents
1. Introduction: Global PV and BIPV scenario
2. PV Technologies
3. New Developments
4. Future Trends
5. Summary and Conclusion
Global PV Market
• PV remains as the most popular and
favoured of all Renewable for Venture
Capital and Private Equity.
• The global R&D investment $86Bn was
also highest for Solar, worldwide in 2010
rooftop solar projects surged to $60
billion-worth of investment.
• The global cumulative production
surpassed 70GW in 2011
•Electricity from PV is set to compete
with grid electricity price by 2013, in
some market segments and estimated to
cover majority of segments by 2020 in
EU2.
1. Global trends in renewable energy investment 2011 New Energy Finance, UNEP, October, 2011
2. A strategic research agenda for photovoltaics solar energy technology Edition
September 2011, by EUPV platform
Building Integrated Photovoltaics
The built environment, although representing almost half of all EU
carbon emissions, presents the ideal least impact environment, to both
generate renewable energy and save energy…
[HM government, Definition of zero carbon homes
and non-domestic building, consultation Dec 2008]
• This glazing market is some 6.6bn m2 per year globally worth €51bn
annually when processed
(Float glass for building 5.4 Billion m2 + 80 Million m2 for PV )
• BIPV market was1201MWp in 2011, is expected to grow annually over
the next couple of years by 56% p.a. to 11GWp by 2015 (BCC
Research).
• Combining with façade glazing and all BIPV applications together
this is around a £100bn, which compares very well with the current
terrestrial solar PV market.
Solar Cell Glass Market
The solar cell glass market showed 6.8% degrowth in 2012 over last year. While
the entire PV module production increased by about 5GW, the selling price of
glass for solar cells declined by about 20%, causing market shrinkage.
it is estimated that the solar cell glass market will enter a recovery phase
this year, reaching 10% increase over 2012. This upward trend will
continue until 2015 with an about more than 10% growth
PV Technologies
1st Generation
crystalline silicon (mono & multi)
Courtesy:
This will be the PV-backbone technology
and leader of the BIPV sector.
module efficiency:
13%
20%
2nd Generation
thin film: a-Si, CdTe, CIGS
Viable competitor in BIPV and roll-toroll process for flexible substrates.
9%
module efficiency:
15%
3rdGeneration
Dye cell and organic
Multijunction or Tandem solar cells
front
contact
Initially niche market oriented, but breakthroughs
could push field towards mass power generation
AR coat
AlInP window
InGaP p-n cell
GaAs tunnel junction
AlGaAs window
module efficiency:
4%
10+%
GaAs p-n cell
new concepts
metal
2000
$0.30/kWh
2010
2020
2030
$0.05/kWh
Crystalline Silicon (c-Si)
AR coating
textured surface
front contact
emitter
base
n-type Si
p-type Si
rear contact
Polycrystalline wafer solar cells
Monocrystalline Silicon solar cell
Amorphous Silicon (a-Si:H)
• Thin a-Si layers (< 1 micron)
would not absorb sufficient Light,
leading to photon losses
• To enhance the optical absorption
in thin a-Si layers, texturing of
TCO and reflecting mirror are
applied
Some examples of a-Si flexible modules on mpolymer and metal foils
CIGS Solar Cells
Ligh
t
TCO contact
ZnO
p-CIGS
1-2 µm
Metal contact
Substrate
Key Material’s properties
n- CdS (50 nm) Chemical Bath
CdS window
CuInGaSe2 absorber
Sputtering
Mo
• Direct Bandgap
Co-evaporation • High abs. coefficient >105 cm-1
• Variable Eg with Ga:In for
Electronic property control
Sputtering •An optimum Na is essential for
high efficiency cells
Substrate( Metal Foil/
Schematics of layer architecturePolymer)
Flexible CIGS solar module
Key processing steps
• Substrate temperature (550 – 600 C) for
high efficiency
Current State of Art
• Efficiency on rigid substrates
Glass using CdS, CBD for buffer layer: >20.3%
(NREL,ZSW)
• Highest efficiency on flexible substrates
S. Steel with barrier layer: > 17.2/17.5%
(Daystar/ NREL)
Polyimide: 20.4 % (EMPA,Zurich)
New World record
Flisom, Switzerland
CdTe solar cell
Leading technology demonstrated first time at commercial
Level the production cost of modules <$1/W
+
Back Contact
Metal
Buffer Layer
Front
Contact
p - CdTe
n - CdS
~ 50 nm
~ 10 nm
p+ - Te-rich Layer
window
µm
0.5 µm
0.05 - 0.5
TCO
(FTO)
Glass
3-5 µm
absorber
substrate
1-3 mm
© M. Terheggen, IAP ETHZ
-
Irradiation
Current State of Art
On glass substrates:
Highest efficiency achieved ~18.2 % (GE Solar)
On polymer substrates:
Highest efficinecy ahieved ~ 13.8% (EMPA)
Largest commercial production > 1GW
achieved by First Solar in 2009
Key properties
Direct bandgap~ 1.45 eV
Bandgap matching with solar spectrum
Controlled and easy formation of juntion
Key issues
Difficulty in stable ohmic back contact
Ion mixing at interface
Essential CdCl2 treatment
Dye-sensitised Solar Cells (DSCs)
Derived from nature’s way of energy conversion i.e. Photosynthesis
I- / I2 redox
TCO
coated
glass
Nanocrystalline
electrolyte
TiO2 film
Sensitizer
Platinised TCO
dye
coated glass
Light
II3-
External circuit
• Semiconductor/ Electrolyte junction.
• Impure starting material
• Environment friendly
• Cheaper processing
Quasi Solid State Flexible DSC:
(Room temperature processing)
ITO-PET
Nanocomposite film
ITO-PET / Pt
(In Collaboration with Johnson Matthey
and Imperial College, London)
1.2
(B)
1.0
0.8
0.6
0.4
0.2
(A)
Current density / mAcm-2
Chemical Communication, 24 (2003) 3008
(Ten best cited publications for year 2003)
+ Al2O3 coating
No Al2O3 coating
Photocurrent density / mAcm-2
TiO2
Al2O3
Dye
Polymer
electrolyte
Efficiency ~ 5.3 %
at low light (0.1 Sun)
0.0
-0.2
0.0
(B)
-0.4
0.0
0.0
(A)
0.2
0.4
0.6
0.8
Voltage / Volts
0.2
0.4
Voltage / Volts
0.6
0.8
Organic Solar Cell Structures
10 nm
ITO
ITO
Hole transporter
h+
Electron transporter
e
-
Metal contact (Al, Ca, Mg)
Metal contact (Al, Ca, Mg)
Bilayer structure
Ideal bulk-heterojunction
Charge separation at interface
Charge separation at bulk heterointerface (dissociation of excitons)
Limitation:
Optical length for light absorption (200
nm) typically greater than exciton
diffusion length (< 10 nm)
Limitation:
High surface area for higher
interfacial recombination
Blends of materials used:
• Polymer/polymer,
• Polymer/dye,
• Polymer/fullerene
13
Wafer vs Thin Films
Mono/Polycrystalline -Si
or III-V compounds
• Thickness: > 200 µm
Thin Film Solar Cells
• Amorphous Silicon (a-Si), Cadmium
Telluride (CdTe) and CIGS
• Area limited by wafer size
• Thickness: 2 - 10 µm
• Rigid
• Large area deposition
• Complex Modul Integration
⇒ Expensive
• Flexible Substrates
• Monolithical Module Integration
⇒ Low cost potential
New BIPV Applications: Thin film Bifacial PV
Double glazed PV windows using bifacial solar cells
(Innovative research with leading industries)
Illumination
(secondary)
TCO
Modified surface of CdTe
Industrial Partners:
• Ove Arup
• Pilkington Group
CdTe
CVD process for ultra-thin Layer
development with Prof. Stuart Irvine
Glyndwr University
CdS
TCO
Substrate (glass )
CdS
CdTe
Illumination (primary)
Outer
Illumination glass
(primary) panel
Inner
glass
panel
Illumination
(secondary)
TCO Ar
Schematic of double glazed PV window
Sputtering System
Ultra-thin CdTe solar cells
(a) The photoresponse of ultrathin CdTe(400nm)/ CdS(60nm) solar cell , out
of joint work between HW and Glyndwr
(b) The external quantum efficiency of CdTe solar cells with different thickness
Stationary 3-D solar concentrators
Ellipse
Entry aperture
•Three dimensional solar concentrator
•Low concentration (4x to 10x)
•Static (non-tracking system)
•Elliptical entry aperture
•Square or rectangular exit aperture
•Hyperbolic side profile on cross section
•Convex and concave hyperbolic profile
Hyperbola
Square or
Rectangular
Exit



compound parabolic concentrator (CPC)
Can accept wide range of incidence
angles with a higher concentration ratio
Optimum design for collection of
maximum diffuse radiation
Suitable design for building integration
in higher latitude along with stand alone
application
Future Trends:
 The dis-advantages of c-Si viz. high materials cost, low
optical absorption, brittle, rigid, heavy, opaqueness will have
to be overcome by flexible PV design
LARGE AREA, THIN,
LIGHT WEIGHT AND
FLEXIBE PRODUCTS
Flexible, Conformal Monocrystalline Si PV
2 mm
Yoon et al. Nature Materials, 7, 907 (2008)
Summary and Conclusions
 The BIPV sector which is backed by an established glazing
industry is set to grow over 50% rate in the future.
 The dis-advantages of c-Si viz. high materials cost, low optical
absorption, brittle, rigid, heavy, opaqueness will have to be
overcome by flexible thin-film bifacial PV.
 Thin-film modules allow greater freedom to select size and colour
than c-Si module, e.g. module efficiency decreases as the
temperature increases for mono and poly silicon cells but not for
amorphous silicon cells.
 The challenge lies with achieving the colour neutral glazing which
is limited by a coloured tinge of the glass.
 New concentrator PV designs combined with thermal integration
will make it a viable industry in the future in a large number of
countries.
Acknowledgements:
M. Graetzel, EPFL, Switzerland
Ayodhya Tiwari, ETH Zurich, Switzerland
Marc Kaelin, ETH Zurich, Switzerland
Prof. Tapas K. Mallick, Exeter University
Prof. Stuart Irvine, Glyndwr Univeristy
Thanks for your attention!