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Figure 1. FTB Early Development Unit. Soft drink can for scale.

Advanced Stirling Convertor Program Update

J. Gary Wood Sunpower, Inc., Athens, Ohio 45701

Cliff Carroll The Boeing Company, Canoga Park, California, 91309-7922

Exploration of the outer planets and deep space requires the use of high specific power and high efficiency electrical power supplies. Diminished solar fluxes at these distant locations preclude the use of solar power. Stirling convertors interfaced to radioisotope heat sources enable exceptionally high specific power and efficiency. The NASA NRA-funded Advanced Stirling Convertor (ASC) program, described here, is striving to increase the performance of Stirling systems through the use of high temperature heater heads, refined thermodynamic understanding of cycle processes, dimensionally stable gas bearings, and high efficiency electrical controls. The goal of the ASC program is to provide an engine plus alternator specific power on the order of 100W/kg, and to produce more than 80 W electric power from the heat supplied by a single GPHS and to have a life of greater than 14 years.

I. Introduction HE project described here is to develop an Advanced Stirling Convertor (ASC). The ASC is intended for use in radioisotope power conversion. This program is funded by a NASA Research Announcement (NRA) project

that began on July 29, 2003. The convertor is sized for the heat input from a single GPHS of nominally 250-watt thermal output, and is expected to have an electrical output exceeding 80 watts.

The ASC effort is being performed by a highly qualified

team led by Sunpower Inc. Sunpower has over 30 years experience in free-piston Stirling machines. Several derivations of Sunpower machines are now entering the commercial market. Also Sunpower’s free-piston coolers have a space heritage; several coolers having already been used or currently employed in space applications. More information on Sunpower’s free piston machines can be found in Reference 1.

The ASC team is further strengthened by Boeing/ Rocketdyne. Boeing/Rocketdyne brings extensive experience in high temperature materials and joining, as well and experience in space electronics.

Cleveland State University and the University of Minnesota add to the ASC team expertise in regenerator research and the study of other thermodynamic and flow losses within the machine. Several respected consultants round out the ASC development team: Dr. David Berchowitz. James Cairelli, David Gedeon, and Barry Penswick.

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2nd International Energy Conversion Engineering Conference16 - 19 August 2004, Providence, Rhode Island

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Copyright © 2004 by Sunpower, Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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The first year of this NRA project focuses on the design of the ASC-1. The ASC-1 will have a test pressure vessel with flanges to facilitate the development program. Year 1 of the project also includes a quickly built developmental unit, termed the “Frequency Test Bed” (FTB) convertor shown in Figure 1. The FTB was originally intended to test the effects of frequency on efficiency, in the range of 90-110 Hz, but is now being used to test for several other effects, in support of the ASC design.

The ASC-1 will be fabricated and tested during year 2 of the project. Additionally during year 2, the final hermetically sealed and lightweight ASC-2 will be designed. Finally, year 3 involves the fabrication and testing of four hermetically sealed ASC-2 convertors.

Sunpower is also just completing a NASA SBIR funded project on a smaller, 35 We convertor. Success on that program, which is based on conventional materials and understandings of the cycle processes, adds high credence that the goals of the ASC program will be achieved. The 35 We convertor has demonstrated over 31% engine plus alternator efficiency at a temperature ratio of 2.6 . Information on the performance of the 35 We convertor can be found in Reference 2.

II. The FTB Early Development Unit Year 1 of the project includes a quickly fabricated approximately 80 We FTB convertor for developmental use.

Two FTB units have been built to date, one for use in controller development at Boeing/Rocketdyne, and the second unit for developmental testing at Sunpower Inc. These units are fabricated of conventional materials, such as stainless steel for the heater head.

Early performance testing on the FTB is very encouraging. At frequencies near 105 Hz, the unit in early development has already produced 85 We at 32.6% efficiency at a temperature ratio typical of current state of the art machines. This temperature ratio is 2.6 (572C hot end, 50C reject). The FTB has also produced 97 We at 34.7% efficiency at a temperature ratio of 3.0 (600C hot, end 20 reject). Efficiencies quoted here are for the engine plus alternator only and do not include a controller.

At present, it is anticipated that the temperature ratio of the final ASC design will be on the order of 3.1 leading to even higher performance. Additionally, the FTB is not a fully optimized machine at the now anticipated temperature levels of the ASC. Also some design aspects affecting performance of the FTB were compromised to facilitate developmental testing. Thus we now have high confidence that the ASC machines will have an engine plus alternator efficiency well exceeding 35%.

III. Heater head Design Drivers The heater head is being designed for a 14-year life with a hot end temperature of 850C. To achieve this

requirement, Boeing will apply its material expertise in high temperature rocket engines. Sophisticated high temperature joints are common place in the rocket industry. Boeing’s fabrication facilities augment current practices in Stirling manufacturing and will increase convertor performance by the use of advanced materials and joining techniques.

The process of designing the heater head begins with the examination of the hot end and identifying its requirements. The high temperature heater head is required to have high creep strength and must maintain the helium charge pressure of the system during the life of the convertor. Selection of the heater head material must also include consideration of the grain structure of this thin walled member. Alloys that exhibit elongated grain boundaries may provide the required creep strength; however, such grain boundaries may yield unacceptable helium permeability rates.

The heater head is also required to contain an internal acceptor heat exchanger. The use of an integral heater head/acceptor heat exchanger would eliminate the need for a joining process between these pieces. Studies in this area have led us to select a separate acceptor that is joined to the head during the fabrication process.

Also the regenerator matrix fits within the heater head. All manufacturing details associated with the acceptor heat exchanger must be completed prior to installation of the regenerator matrix.

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Figure 2. Heater Head Performance Drivers.

IV. High Temperature Heater Head Performance Drivers High temperature heater head materials will be used to

increase convertor performance. Current Stirling convertors use heater heads constructed of stainless steel or Inconel 718 to achieve temperatures of 550 C to 650 C. The goal of the ASC is to operate the heater head at temperatures near 850 C. High creep strength materials are required to achieve this goal; this is considered a primary performance driver for the heater head. Additional heater head performance drivers are listed in Figure 2. These include axial conduction losses, radial thermal conduction, acceptor heat exchanger and cold end interface.

The heater head may be considered as a long life, thin wall, high temperature pressure vessel. Maintaining the integrity of this thin wall member over its 14-year life requires the use of high creep strength materials. Nickel based super alloys are well suited for this application.

The second performance driver is the axial thermal conduction loss of the heater head. The heater head is required to provide a thermal impedance between the hot

end (850 C) and the cold end (~100 C); the resulting geometry is a tapered thin wall cylinder. The wall of the heater head is thicker at the hot end and tapers to a thinner wall at the cold end. This approach allows for lower stress and strives to minimize the conduction losses with a thinner wall at the cold end.

A thin wall heater head at the cold end also assists the third performance driver: radial thermal conduction. High radial thermal conduction is required at both the cold end and hot ends; this allows the heater head to effectively reject and accept heat.

The fourth performance driver is the acceptor heater exchanger. This heat exchanger is required to transfer heat to the working gas of the convertor. High temperature material compatibility between the acceptor heat exchanger and heater head is critical to the success of the ASC.

The fifth performance driver of the heater head is the cold end radial heat transfer in the area of the rejector. Reduction of rejector wall temperature drops results in a significant performance gain. The influence of through- wall temperature drops on efficiency is more significant at the rejector than for the acceptor. This ratio of influence is equal to the temperature ratio of the machine.

V. Electronics Controller An electronics controller is being developed as part of the ASC project. The goal of the controller is to control

piston stroke and to provide a high electrical power factor to maximize alternator efficiency. Power output of the controller is 28 V DC, which is well suited for a variety of charging applications. The stroke control is used to maintain heater head at a given temperature. As heat input varies, the system maintains a constant heater head temperature. .

VI. Regenerator and Displacer Loss Studies Regenerator wall effects are an area of concern for small convertors of the power level of the ASC. Lower power

machines have much more wall perimeter for given area of the regenerator than do larger machines. The University of Minnesota is performing testing of regenerator wall effects in support of the ASC design on their scaled up regenerator test rig. Also in support of better understanding regenerator wall effects, temperature profiles have been measured for the regenerator wall of the FTB.

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Figure 3. Test Section of the Displacer Loss Rig

Cleveland State University is performing CFD studies of losses both in the appendix gap as well as in the interior of the displacer. Both of these areas are not well understood with regards to the thermal losses that occur.

In support of the Cleveland State University modeling of displacer internal losses, Sunpower has built a Displacer Loss Rig (DLR). The test section of the DLR is shown in Figure 3. The test section consists of a short section of displacer across which a temperature difference is maintained. The test section can be charged to different levels of pressure. This section is then driven by a linear motor and the thermal losses are measured.

VII. Conclusions This paper describes a NASA-NRA funded program to

develop the Advanced Stirling Convertor. The convertor is designed for the heat supplied by a single GPHS and the anticipated power of the final convertor is expected to exceed 80 W electrical, with a specific power for the engine plus alternator exceeding 100 W/Kg. Various efforts toward achieving this high performance are described in the paper.

VIII. Acknowledgments Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration.

IX. References

1. Wood J.G. “Status of Free Piston Stirling Technology at Sunpower, Inc.,” Proceedings of the 1st IECEC, Virginia, 2003.

2. Wood J.G and Lane N. “Progress Update on the Sunpower 35We Stirling Convertor”. Proc of 2nd IECEC, Rhode Island, 2004