Free-piston Stirling Machine Commercialization

Transcription

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Free-piston Stirling Machine Commercialization Status
at Sunpower
Lane, N.W., Wood, J.G., Unger, R.Z.
Sunpower Inc
Athens,
Ohio 45701,
USA
Background
The free-piston Stirling engine ( FPSE) was invented by William Beale in 1964 and
patented in 1971. i Sunpower Inc. was founded in 1974 with the express purpose of
commercializing free-piston engines. It is now approaching thirty years later, as well as
the 187th anniversary of Robert Stirling’s original patent, and is thus timely to present a
summary of the technical performance and commercialization status of free-piston
Stirling machines at Sunpower.
The original basic FPSE technology evolved over the last thirty years to include a variety
of related technologies. Among these free-piston machines are Stirling engines, Stirling
cryocoolers and pulse tubes, the development of which is detailed in this paper. Not
detailed in this paper are other related free-piston technologies whose development has
also been key to the success of free-piston machines in general.
Immense barriers stand before widespread, cost-effective commercial introduction of any
new technology. Widely adopted old technologies are entrenched on the basis of years of
development, capital investment in manufacturing, and known and predictable markets.
An illustrative example is the fractional horsepower refrigeration compressor, typically
manufactured in purpose-built greenfield factories in volumes of between 1 and 15
million units per annum for a single model. These units, without electronic controls, cost
the purchaser between $17 and $30, while providing handy profits to the manufacturer.
When competing at these volumes and prices it is not possible to bootstrap the success of
a new “replacement” technology. Instead, the demand for the new product must justify
the investment in manufacturing. Furthermore, the first units off the assembly line have
to sell for the same price as the millionth unit.
Commercialization Approach
Sunpower’s core corporate objective has always been widespread adoption of its freepiston technology (whether engines, coolers or linear compressors) in all possible fields.
Given the barriers to rapid, widespread, low-cost adoption of the technology, Sunpower’s
parallel-path commercialization approach is incremental. This approach includes
identification of the following market opportunities:
Niche commercial markets where higher costs are acceptable and existing
technology is weak or not entrenched
1
Copyright © 2003 by Sunpower Inc. International Stirling Engine Conference, 19-21 November 2003, Rome, Italy

Potential high-volume markets where free-piston technology is sufficiently
greater in performance or other improvements (e.g. environmental acceptability),
to justify the time and cost to market
Potential markets where free-piston Stirling technology enables a previously
unimagined product and business
Sunpower employes several exploitation strategies for these markets, including:

Licensing the basic intellectual property, typically by field-of-use.
o LG Electronics has completed successful commercial introduction of
Sunpower linear compressors ii . Refrigerators using these linear
compressors have been on sale in LGE’s Korean home market since 2002,
and will be available in the US and European Union in 2004. These
refrigerators will offer energy savings of up to 50% iii .
o MicroGen iv is a part of the BG Group, which specializes in natural gas and
is one of the top 30 FTSE companies. BG is an exclusive worldwide
licensee to use Sunpower’s free-piston engine technology in domestic and
small commercial heating and power generation systems. Development of
the engine and appliance are well advanced. The commercial pilot test is
scheduled to take place in the United Kingdom this winter (2003/2004).
Partnering with entities that have the resources and knowledge of the market to
ensure a high likelihood of success.
o Sunpower and LG Electronics formed a Strategic Alliance to develop freepiston Stirling cryocoolers for wireless superconducting filter applications.
This partnership has developed the highest performance small cryocoolers
in the world. Units are in the hands of customers in Asia, Europe and the
USA. The performance of the machines is described below.
Creating spin-offs in which where the risk/return ratio is highest.
o Sunpower spin-off Global Cooling successfully led development of higher
temperature free-piston Stirling coolers. v , vi These machines are in use
commercially vii and are on the verge of widespread adoption viii .
Manufacturing when the volumes and margins are appropriate to a start-up
endeavor within an R&D environment.
o Sunpower developed its own free piston Stirling cryocooler manufacturing
business. This business currently serves some low volume commercial and
government research users.
General features of machines
The free-piston Stirling machines developed at Sunpower are Beta type machines with
integral permanent magnet linear alternators or motors ix,x . Much of the linear motor,
bearings, topology, fabrication techniques and control aspects of FPSE’s have also been
successfully applied to linear compressors. These machines have intrinsically low
side-loads on the moving piston and displacer. This permits employment of low power
consumption gas bearings xi . The subsequent non-contact operation and oil-free
hermetically sealed designs offer zero maintenance and the promise of very long life.
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Sunpower has ongoing tests of over 65,000 hours on a single free piston Stirling cooler
(FPSC) and similar machines are described with mean time before failure (MTBF)’s of
over 250,000 hours xii . The key attributes of these designs that lead to high efficiency and
low mass are:
The alternator design uses an efficient coil winding arrangement with no unused
copper end turns.
The alternator magnets never leave the poles, thus minimizing fringing field
losses.
There is zero mean lamination flux, minimizing lamination iron requirements and
reducing weight.
The alternator moving magnet design reduces side loads compared with moving
iron designs.
Gas bearings rather than flexures are employed to maintain the required
clearances. The removal of flexure design constraints allows a higher power
density.
The displacers are sprung externally from the working space. This decouples the
spring design from the heat exchanger design, allowing a smaller and more
efficient machine.
Stirling Engines
Sunpower’s current engine development portfolio includes the EG-1000 engine which is
nominally 1000We but produces up to 1300We. Including the alternator, this engine is
29% efficient (electrical power out to heat into the head) when operating at a temperature
ratio of 2.6 (550 C hot end and 50 C reject). Although the machine is designed for low
cost, it has the highest efficiency (as a percent of Carnot) of any free-piston engine to
date. The engine-only efficiency of the unit is 58 % of Carnot (Figure 1). Originally
designed for cogeneration applications, this machine operates at 50 Hz. In production
form, the machine is anticipated to have a specific power of around 45 W/kg
Sunpower is also currently developing the EG-35-SRG, a 35We convertor funded through
a NASA Phase II SBIR. Because of the potential space power application, goals for this
machine include both high efficiency (>30 % overall) and a specific power exceeding 90
W/kg. The machine is designed to operate at 100 Hz and at a temperature ratio of 2.6.
As of this writing, the machine has just entered into hardware testing and is producing
over 40We at over 28% efficiency in initial tests. The performance of this machine versus
the EG-1000 and other free-piston machines is shown in Figure 3. The Curzon-Ahlborn
Efficiency xiii represents a likely realizable practical efficiency such as that achieved in a
modern central power generating facility. It is anticipated that this engine will exceed the
stated performance targets
This machine has exciting terrestrial applications as a fuel-fired battery replacement. This
is because of the poor energy density of batteries such as the lithium-ion, with an energy
density of less than 1% (abysmal compared to fossil fuels). Thus, even when considering
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0.65
0.60
Fraction of Carnot
EG-1000
Curzon-Ahlborn Efficiency
0.55
EE-35-SRG
0.50
STC 55W
0.45
0.40
1.5
2
2.5
3
Temperature Ratio
Figure 1. Engine efficiency comparison. STC data from xiv
engine and burner efficiency in a final product, small fuel-fired Stirling engines will have
significant mass savings compared to batteries for portable power.
Figures 2 and 3 show the exterior and moving components respectively of the EG-1000
and EG-35-SRG engines.
Figure 2. EG-1000 and EE-35-SRG Engines
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Figure 3. EG-1000 and EE-35-SRG pistons and displacers
Stirling Cryocoolers
Sunpower started the development of small scale Stirling coolers in the early 1990’s with
the introduction of the M77. The M77 was designed to operate at cryogenic temperatures
with a cooling capacity of 4W @ 77K. The M77 is a linear, free piston, integral Stirling
cooler where the piston and displacer are arranged in an in-line configuration. The M77
has been in limited production at Sunpower for the last ten years, and has been delivered
to various customers including NASA, which qualified it to fly on the RHESSI solar
orbiter mission. The M77 has been operating flawlessly on RHESSI since February of
2002. The M77 is shown in Figure 4 and performance summarized in Figure 5.
Figure 1. M77 Cryocooler with active dynamic balance unit
5
20
16
15
12
10
100 W
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Heat Lift (watts)
Fraction of Carnot COP (%)
20
80 W
5
60 W
4
40 W input power
0
40
50
60
70
80
0
100
90
Cold End Temperature (K)
Figure 5. M77 Cryocooler performance
25
25
20
20
160 W
15
15
120 W
80 W
10
10
40W input power
5
5
0
20
40
60
80
100
120
140
Figure 6. Cryotel CT Performance
6
160
0
180
Heat Lift (watts)
While the M77 has enjoyed great technical success, it is intrinsically high-cost and
therefore has limited applications. Sunpower recognized the need for a family of small,
highly efficient, low-cost cryocoolers. Starting in 1998, Sunpower developed a number
of low-cost coolers for particular customer applications. The most recent of these
developments is the two-model Cryotel family . The CryoTel CT has a mass of 2.7kg and
is capable of lifting 10W from 77K to 308K with 160 watts of input power. The smaller
CryoTel MT is designed for 5 W of cooling @ 77K with an input of 80We and a mass of
1.65kg. The performance of the CT and MT are shown in Figures 6 and 7 respectively.
20
20
16
15
12
100 W
80 W
60 W
45 W
10
8
Heat Lift (watts)
25
30 W input power
4
5
0
20
40
60
80
100
120
140
160
0
180
Figure 7. Cryotel MT Performance
Pulse Tubes
Sunpower and Gedeon Associates have partnered to develop single and multi-stage pulse
tube cryocoolers. The strength of the partnership lies in the melding of the two firms’
strengths: Sunpower’s high-efficiency, low-cost linear compressor technology (already
taken to the manufacturing level with Stirling cryocoolers), and Gedeon Associates’
experience in the design and analysis of pulse tube cryocoolers. Under SBIR funding
from NASA Goddard Space Flight Center (GSFC), Sunpower has completed a program
to build both a single and a two-stage pulse tube cryocooler and is currently involved in a
program to develop a three-stage pulse tube cryocooler.
The first development stage (Nov 1999 – Nov 2001) involved construction of both single
and two-stage pulse tube cold heads. All designs implemented inertance tubes as
acoustical tuning devices. High efficiency was demonstrated with the single-stage pulse
tube cooler achieving 6 W of heat lift at 77K with 100We input power, and the two-stage
pulse tube cooler lifting 260mW at 30K, again with 100We input.
Currently under development through additional NASA GSFC SBIR funding is a threestage, high frequency, linear-compressor driven pulse tube cryocooler for cooling below
10K, a temperature regime traditionally dominated by large, high-power GiffordMcMahon cryocoolers. The goal is to produce a cryocooler that can provide small
amounts of cooling down to 4K at a fraction of the mass and electrical input of GM
machines. Testing of the three-stage pulse tube cryocooler is scheduled to begin in the
fourth quarter of 2003.
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Conclusions
Sunpower’s nearly three decades’ experience in the design, development and
commercialization of free piston and linear machines enables the firm to summarize
certain conclusions regarding commercialization of these technologies, which is well
advanced at Sunpower. Linear compressors, high temperatures free piston Stirling
coolers, and cryocoolers are currently available commercially. Engines are poised to
enter the market. Thermodynamic performance has not been compromised in the process
of successful commercialization.
Once the technical hurdles had been overcome, Sunpower’s commercial success , in large
part, comes from the use of numerous commercialization approaches. Among these are
identifying a range of markets from high value low volume niche markets to high volume
markets and a range of exploitation strategies ranging from in-house manufacturing to
licensing. Commercial success requires both the market and the exploitation approach to
be correct.
New products based upon free piston Stirling and linear machine innovations are under
continuing development.
Acknowledgements
Sunpower has benefited from the significant and unique contributions of major
innovators in the field of Stirling and linear devices, notable among them William Beale,
David Berchowitz, David Gedeon, and Robert Redlich.
References
i
U.S. Patent 3,552,120, “Stirling Cycle Type Thermal Device,” Beale, William T.
LG Electronics Press Release:http://www.lge.be/pages/FrontEnd/nieuws/globaal/2003/refrigerators.html
iii
Heo, J.T., Hyun, S.Y., Kim, J.W., Jung, W.H. and Kim, J.D., “Linear Compressor for Natural
Refrigerant,” LG Electronics at the IMechE International Conference on Compressors and their
Systems, 7-10 Sept. 2003 London, UK.
iv
MicroGen web site http://www.microgen.com/main2.swf
v Kwon, Y.-R., Berchowitz, D. M., “Operational Characteristics of Stirling Machinery,”, International
Congress of Refrigeration 2003, Washington, D.C.
vi Global Cooling: www.globalcooling.com .
vii Instrumentation Scientifique de Laboratoire (ISP) http://www.paclp.com/
viii Twinbird: http://www.twinbird.jp/sc/sc_top_en.html
ix US patent 4,602,172. Electromechanical Transducer Particularly suitable for a Linear Alternator Driven
by a Free-Piston Stirling Engine. Issued 7-22-1986.
x “A Summary of Twenty Years Experience with Linear Motors and Alternators,” Robert Redlich,
Sunpower, Linear Drives for Industry Applications, May 31-June2 1995 Nagasaki, Japan.
xi US patent 5,525,845. Fluid Bearing with Compliant Linkage for Centering Reciprocating Bodies. Issued
6-11-1996.
xii Superconductor Technologies Inc presentation: http://www.suptech.com/pdf/ISIS.pdf
xiii
Curzon, F.L. and Ahlborn, B. “Efficiency of a Carnot Engine at Maximum Power Output,” AM. J. Phys.
43 22-24 (1975).
ii
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xiv
Qiu, S., Augenblick, J.E., White, M., Peterson, A.A., Redigner, D.L., and Petersen, S.L., “Developing a
Free-Piston Stirling Convertor for Advanced Radioisotope Space Power Systems,” in Proceedings
of STAIF 2002 Space Technology and Applications International Forum, edited by M.S. El-Genk,
CP608, American Institute of Physics 2002, pp. 912-917.
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