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 free-piston 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

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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 MicroGeniv 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 free-piston 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 commerciallyvii and are on the verge of widespread adoptionviii.

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 motorsix,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 hoursxii. 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 Efficiencyxiii 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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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

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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.

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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 three-stage, high frequency, linear-compressor driven pulse tube cryocooler for cooling below 10K, a temperature regime traditionally dominated by large, high-power Gifford-McMahon 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. ii 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.pdfxiii Curzon, F.L. and Ahlborn, B. “Efficiency of a Carnot Engine at Maximum Power Output,” AM. J. Phys.

43 22-24 (1975).

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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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