Status of Free-Piston Stirling Technology at Sunpower, Inc

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STATUS OF FREE-PISTON STIRLING TECHNOLOGY AT SUNPOWER, INC.
J. Gary Wood
Sunpower, Inc.
Athens, OH
Sunpower’s overriding goal is the commercialization of
these efficient and mechanically simple machines.
Commercialization efforts have resulted in several freepiston machines now in production under license to
Sunpower. LG Electronics is currently marketing its
free-piston based (Rankine cycle) refrigerator DIOS
model in Korea, to be introduced worldwide in the near
future. CryoGen, a California based company, is
selling a free-piston compressor-based JT cryosurgical
device. ISL, a French company, markets an aircraft
fuel gel-point tester based on a free-piston Stirling
cooler. Superconductor Technologies Inc (STI) has
1000s of telecommunication cryocoolers in the field.
All of these machines are built under license to
Sunpower.
ABSTRACT
This paper includes an overview of the status of freepiston Stirling machine technology at Sunpower, Inc.
Sunpower has achieved record-setting performance and
specific power for a wide range of machines primarily
designed for commercial applications. These machines
range in power from approximately 100 We input
Stirling and pulse-tubes cryocoolers to approximately
1300 We output engines designed for European domestic cogeneration. An overview of the performance and
specific powers of these current machines is presented.
Also included is a description of Sunpower’s design
topology that uses gas bearings to ensure long life
through the use of a compliant connection with
mechanical springs. All Sunpower machines also
include low cost and high specific power linear
alternator/motors that make maximum use of alternator
materials. The major portion of the paper describes the
design and current status of an advanced 35 We
convertor (engine) that promises even higher projected
performance. This convertor is currently under
development with NASA Phase II SBIR funding.
Because of its extremely high conversion efficiency,
this small convertor is attractive for use in a Stirling
radioisotope space power system as a possible
replacement for Radioisotope Thermoelectric
Generators (RTGs). Achieving the projected high
efficiency would allow a reduction in the amount of
radioisotope fuel by approximately a factor of four.
The small convertor is projected to have an efficiency
of greater than 50 percent of Carnot efficiency when
operating at a temperature ratio of 2.6. Projected
specific power of the convertor, which is designed to
run at 100 Hertz, is approximately 90 W/kg. This
engine also has significant terrestrial applications as a
fuel-fired battery replacement. Liquid fuels have
approximately 300 times the energy density as Ni-Cad
batteries and 150 times that of Lithium-ion batteries.
Thus such an engine coupled with a liquid fossil fuel
burner is an attractive product.
Several other machines are currently in field trials or in
specialized applications. Among these is a nominally
1000 We engine (based on the Sunpower EG-1000
engine) which is in field trials in Europe as part of a
domestic cogeneration system. NASA-Goddard has
also used Sunpower cryocoolers in several applications.
Most notable of these is the RHESSI satellite which
was launched in February 2002 to study solar flares.
RHESSI relies on a Sunpower M77 cryocooler to cool
its sensors, and minimum mission science objectives
have already been achieved. Sunpower has also built a
cryocooler manufacturing facility with the capability of
manufacturing tens of thousands of cryocoolers per
year.
In addition to the commercial effort, several
government-funded programs on advanced free-piston
concepts as well as basic research are currently being
investigated. Current government programs include our
second Phase II SBIR pulse-tube cryocooler for NASAGoddard. The objective of this project is a 3-stage
pulse-tube cryocooler achieving <10 K with an input
power of 200 We.
Another current Phase II SBIR is funded by NASAGlenn for a highly efficient and lightweight 35 We
convertor (engine) for possible use in an advanced
Stirling radioisotope space power system. Stirling
radioisotope power systems are being developed by
DOE and NASA as a possible replacement for
Radioisotope Thermoelectric Generators (RTGs),
promising to reduce the amount of required plutonium
CURRENT AREAS OF DEVELOPMENT
Sunpower was founded in 1974 by William Beale, the
inventor of the free-piston Stirling engine. Today,
nearly 30 years later, Sunpower works on a wide
variety of free-piston machines ranging from Stirling
cycle engines and cryocoolers, to linear compressors for
pulse tube, Joule-Thomson, and Rankine cycle cooling.
1
Copyright © 2003 by Sunpower Inc. 1st International Energy Conversion Engineering Conference, 17-21 August 2003, Portsumouth, Virginia
by a factor of four. This 35 We convertor is described
later in this paper.
to frequency. Frequency in many cases is specified by
the intended end use. For example the Sunpower EG1000 engine is designed for European grid-connected
cogeneration which requires operation at 50 hertz.
Sunpower also operates a unique in-house oscillating
flow regenerator heat transfer test rig. This rig was
designed by Sunpower for NASA-Lewis (now Glenn)
in the 1980’s and currently is on loan to Sunpower.
It is thus desirable to compare machines on a
“frequency adjusted specific power” basis where the
units are of the form W/(kg*Hz). Figure 3 presents
representative frequency adjusted specific powers of
Sunpower’s current machines.
PERFORMANCE AND SPECIFIC POWER
While the majority of Sunpower machines are designed
for low manufacturing cost, the efficiency and specific
power of these machines typically exceed that of
machines where low production cost was not a major
design constraint. The high thermodynamic
performance of Sunpower’s cryocoolers and engines is
presented in the following two figures.
It must be noted that Figure 3 is intended only to be an
approximate representation of specific power. For a
given alternator/motor configuration, electrical
conversion efficiency can be increased (with
diminishing returns) by increasing alternator mass.
This is the primary reason for the difference in the two
points labeled “SBIR I Engine” and “SBIR II Engine”
in Figure 3, which is further explained in reference 15.
Also the rejection temperature of the machine
influences alternator efficiency, primarily because the
coil resistance increases with temperature.
Sunpower machines also have exceptionally high
specific power. This is largely due to an efficient and
mechanically simple linear alternator/motor design.
Typically the alternator/motor and the surrounding
vessel make up the major mass of the machine.
Figure 3 includes machines with largely different
design constraints and operating temperatures. As such
it only presents an approximate representation of
achievable specific powers.
It is important to consider operating frequency when
comparing specific power of machines. Alternator mass
at a given electrical efficiency is inversely proportional
25
Sunpower MiniTel
20
Sunpower
Fraction of Carnot (%)
CryoTel
5
2
Sunpower M77
4
3
Sunpower
Sunpower M87
1
Pulse Tube
15
Hymatic Stirling
7
Lockheed
Martin Pulse
10
Ricor K535
Tube
1-Stage Stirling
10
13
Ball 1-Stage
Stirling
Leybold Coolpower
150
1-Stage Gifford-
11
SB160
McMahon
14
5
TRW 1-Stage
Miniature
Ball 2-Stage
Stirling
SB230
0
Creare Brayton
0
Stirling
12
20
15
Mixed Gas
APD Joule-Thomson
6
Joule-Thomson
9
Cryotiger
8
40
60
Cold-End Temperature (K)
80
100
Figure 1: Comparative Performance of Cryocoolers with Motor (from references 1-14)
2
120
Projected EG-1000 with improvements
0.7
Curzon-Ahlborn Efficiency
(for reference only)
MTI Mod 2
kinematic hydrogen (1987)
Predicted 35 W SBIR engine
0.6
MTI CPTC (1993)
0.5
Sunpower EG-1000 (2000)
MTI SPRE (1990)
0.4
TDC 55 W
(2000)
Sunpower RE-1000
(1979)
0.3
0.2
1.0
1.5
2.0
2.5
3.0
3.5
Temperature Ratio (Theater / Trejector)
Figure 2: Stirling Engine-Only Efficiencies (PV power/ Heat into Head)
From reference 15
Frequency Adjusted Specific Power (W/(kg*Hz)
Engine-only Efficiency (fraction of Carnot)
0.8
2.0
1.8
MTI CPTC (projected
17
light weight)
1.6
1.4
18
MTI SPRE
1.2
SBIR I Engine
1.0
M-87 Cryocooler
EG-1000 without
flanges
SBIR II
Engine
0.8
0.6
19
Lightweight TDC
(55 W, 80 Hz, 1.7 kg)
0.4
EG-1000 Engine with
Flanges
19
TDC
(55 W, 80Hz 4.0 kg)
0.2
0.0
10
100
1000
10000
Electrical Power watts (scaled to 100 Hz)
100000
Figure 3: Representative Frequency Adjusted Specific Power of Free-Piston Machines
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SUNPOWER MACHINE DESIGN TOPOLOGY
Figure 4 is from US patent number 5,525,845 of a freepiston compressor. In this figure, the labels 35-38
identify the gas bearings that provide non-contact
operation between the piston (31) and the cylinder (32).
CURRENT ENGINE RESEARCH
EG-1000 Engine
Among Sunpower current engine development work is
the EG-1000 engine which is nominally 1000 We but
produces up to 1300 We. The research form of this
machine, which includes flanges, is shown in Figure 5.
This engine is currently in field trials in England as part
of a residential cogeneration package. This engine
including alternator is 29% efficient (electrical out to
heat into the head) when operating at a temperature
ratio of 2.6 (550 C hot end and 50 C reject). Although
this machine is designed for low cost, it has the highest
efficiency (as a percent of Carnot) of any free-piston
engine to date as is illustrated in Figure 2. Meanwhile
the low cost constraint has pushed this design to have a
relatively high specific power as shown in Figure 3.
This machine has a mass of 35 kg and is approximately
230mm in diameter and 440mm in length.
The planar springs (46) provide mechanical springing
only. The compliant link (47) is designed for flexibility
without buckling under the dynamic loads. This
compliant link largely reduces costs because high
precision is not required of the planar springs nor of the
area of the mounting to the casing (48). This
compliance allows greatly reduced cost of the
manufactured parts, simple assembly because of
reduced alignment constraints, and a very compact
overall assembly.
Nearly all Sunpower machines incorporate the
following design features, which are illustrated in
Figure 4.:
1.
2.
3.
4.
5.
Self-pumped gas bearings for contact free
operation
Low cost planar mechanical springs used for
springing only
Patented compliant (flexible) method of interfacing
1 and 2
Hermetic sealing
Patented low mass, high efficiency and
mechanically simple alternator, which makes
maximum use of materials
The figure also illustrates the basic configuration of
Sunpower’s linear alternators and motors. A single
annular ring of magnets (42) oscillates within the stator
(40-41) which produces AC power in the coil (44).
This alternator is both highly efficient and has low
specific mass. Some of the advantages are that the
entire copper coil is within the reversing flux, and that
the magnets never leave the stator (therefore reducing
eddy current losses in the surrounding structure).
Figure 5. EG-1000 Engine
35 Watt Convertor
Another active engine project at Sunpower is a 35 We
convertor being developed under NASA Phase II SBIR
funding. Figure 6 shows both the current research form
and a model of the final configuration with a
lightweight vessel. More specific details of this
machine can be found in reference 15.
The first run of this machine is scheduled for midyear
2003. Projections for this machine, which operates at
100 Hz, is a power output between 35 and 40 watts with
Figure 4
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a final configuration mass of 434 gm. The projected
efficiency of this machine is expected to fall between
30.5 to 35% when operating at a temperature ratio of
2.6. The projected specific power of this machine is
included in Fig. 3, labeled SBIR II engine.
Figure 7: Size Comparison. M87 Cryocooler (left),
Cryotel (center), and NASA 35 watt convertor (right)
machines are also demonstrating their long life in many
circumstances. Both Stirling Technology Company and
Sunpower have single unit life demonstrations that have
achieved more than 70,000 hours of operation and are
still accumulating hours. Thus, it is becoming evident
that Stirling convertors can meet the life requirements.
Confidence in reliability is also greatly increased as
more and more free-piston devices are placed in the
field as described earlier under Sunpower’s
commercialization efforts.
Figure 6: NASA-funded 35 We Convertor (in test
vessel at left, model of final configuration on right)
shown with Ohio state quarter for size scale
This convertor is designed to be the smallest highefficiency Stirling engine developed to date, and as
such represents a significant technical challenge.
However, the machine is similar in physical size and
internal construction to Sunpower’s line of cryocoolers.
Figure 7 compares the size of this convertor to
Sunpower’s M87 (150 We @ 60 Hz) and Cryotel (100
We @ 60 Hz) cryocoolers.
Terrestrial Applications for the 35 We Convertor
There is also significant potential for the 35 We
convertor in commercial terrestrial applications as a
fuel-fired battery replacement. Batteries have
extremely low energy density when compared to liquid
fuels. Roughly, liquid fuels have 300 times the energy
density of NiCad batteries and 150 times that of
Lithium-ion batteries. Even when considering the
conversion efficiency of a small engine plus burner
(roughly between 25 and 33%), a small fuel-fired
Stirling engine would have extremely high energy
density compared to battery packs, approaching 50
times that of Lithium-ion batteries.
This 35 We convertor is being developed for possible
use in an advanced Stirling radioisotope space power
system. Stirling radioisotope power systems are of
interest to NASA as a possible replacement for much
lower efficiency Radioisotope Thermoelectric
Generators (RTGs). RTGs have been used as the power
source on many NASA missions (Voyager, Cassini,
etc.). Stirling radioisotope power systems offer a
significant reduction (~4) in the required amount of
Plutonium 238, as RTGs have a very low conversion
efficiency (~5-7%)
SUMMARY
This paper presents an overview of current free-piston
Stirling machine status at Sunpower. In accordance
with the most recent data available, we compare the
performance of Sunpower machines to other current
and past machines. Through a combination of many
years of government-funded research and
commercialization efforts, free-piston machines are
demonstrating their long promised high efficiency and
reliability.
RTGs are very reliable devices, with proven long
lifetime. Typical life requirements set by NASA are
100,000+ hours of operation. However, Stirling
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16. Sunpower Inc. test data, using electrically heated
head. P-V power / heat into head. P-V power
calculated from electric output assuming the
alternator is 85% efficient.
17. Dhar, M., Stirling Space Power Program, Volumes
1 & 2, Final Report, 1997, NASA/CR-1999209164/ Vol. 1. Point shown is average of heat-towater and heat-to-head means of calculating
efficiency (from fig. 79). Heat-to-head efficiency is
45% of Carnot efficiency and heat-to-water
efficiency is about 54 % of Carnot efficiency
18. Dochat, G., SPDE / SPRE Final Summary Report,
1993, NASA/CR-187086, data based on heat-towater from plot on pg. 92, max efficiency (upper
point) occurs at 35 % of design power (lower point6
Data from NASA web site
19. White, M. Technology Development of a FreePiston Stirling Advanced Radioisotope Space Power
System, STAIF February 1999 Albuquerque New
Mexico
REFERENCES
1. Sunpower Pulse Tube, 4.8 W @ 77 K,
Internal Sunpower Test 2001.
2. Sunpower M77, 5 W @ 77 K, 313 K Reject,
Internal Sunpower Test 1997.
3. Sunpower M87, 7.5 W @ 77 K, 323 K Reject,
Internal Sunpower Test 1999.
4. Mixed Gas Joule-Thomson, 100 W @ 100 K, 245 K
Reject, “Mixed Gas J-T Cryocooler with Precooling
Stage,” 10th International Cryocooler Conference,
1998, Monterey CA.
5. Hymatic Miniature Stirling, 0.3 W @ 80 K, 293 K
Reject, “Miniature Long Life Tactical Stirling
Cryocoolers,” 9th International Cryocooler
Conference, 1996, Waterville Valley NH.
6. Creare Brayton 1-Stage, 5 W @ 65 K, 310 K
Reject, “A Single Stage Reverse Brayton
Cryocooler: Performance and Endurance Tests on
the Engineering Model,” 9th International
Cryocooler Conference, 1996, Waterville Valley
NH.
7. APD Joule-Thomson Cryotiger, 3 W @ 80 K, 308
K Reject, “A Throttle Cycle Cryocooler Operating
with Mixed Gas Refrigerants in 70 K to 120 K
Temperature Range,” 9th International Cryocooler
Conference, 1996, Waterville Valley NH.
8. Lockheed-Martin Pulse Tube, 2 W @ 60 K, 295 K
Reject, “Development of a 2 W at 60 K Pulse
Tube Cryocooler for Spaceborne Operation,” 10th
International Cryocooler Conference, 1998,
Monterey CA.
9. Sunpower HTSC Filter Cooler, 12 W @ 87K
Internal Sunpower Test 2001.
10. Ball 1-Stage Stirling SB160, 1.6 W @ 60 K, 298 K
Reject, Ball Aerospace test,
www.ball.com/aerospace/crysb160.html, last
updated 1999.
11. Ball 2-Stage Stirling SB230, 0.45 W @ 30 K, 298 K
Reject, Ball Aerospace test,
www.ball.com/aerospace/crysb230.html, last
updated 1999.
12. Ricor K535 1-Stage Stirling, 4 W @ 65 K, 318 K
Reject, Ricor test, www.ricor.com/k535.htm.
13. Leybold Coolpower 150 1-Stage Gifford McMahon,
150 W @ 77 K, 311 K Reject, Leybold test,
www.leyboldcryogenics.com/coolpower150.html.
14. TRW 1-Stage Miniature Stirling, 0.25 W @ 65 K,
290 K Reject, “Miniature Long-Life SpaceQualified Pulse Tube and Stirling Cryocoolers,” 8th
International Cryocooler Conference, 1994, Vail
CO.
15. Wood, J.G. and Lane, NW, “Advanced 35 Watt
Free-Piston Stirling Engine for Space Power
Applications,” STAIF February 2003 Albuquerque
New Mexico.
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