Steven M. Geng and Maxwell H. Briggs

Glenn Research Center, Cleveland, Ohio

David S. Hervol

QinetiQ North America, Cleveland, Ohio

Performance of a Kilowatt-Class Stirling Power 

Conversion System in a Thermodynamically

Coupled Confi guration

NASA/TM—2011-217098

September 2011

NETS–2011–3269
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Steven M. Geng and Maxwell H. Briggs

Glenn Research Center, Cleveland, Ohio

David S. Hervol

QinetiQ North America, Cleveland, Ohio

Performance of a Kilowatt-Class Stirling Power 

Conversion System in a Thermodynamically

Coupled Confi guration

NASA/TM—2011-217098

September 2011

NETS–2011–3269

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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Acknowledgments

This work was performed for the NASA Enabling Technology Development and Demonstration and the Fission Power 

Systems Projects. The authors wish to acknowledge the technical contributions of Gary Wood and Dave Shade of

Sunpower Inc., for the design and fabrication of the shared expansion-space heater head, and the assembly of

the 1-kWe Stirling power convertors into the thermodynamically coupled confi guration.

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NASA/TM—2011-217098 1 

Performance of a Kilowatt-Class Stirling Power Conversion System 

in a Thermodynamically Coupled Configuration 

Steven M. Geng and Maxwell H. Briggs 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

David S. Hervol 

QinetiQ North America 

Cleveland, Ohio 44135 

Abstract

A pair of 1-kWe free-piston Stirling power convertors has been modified into a thermodynamically 

coupled configuration, and performance map testing has been completed. This is the same configuration 

planned for the full-scale 12-kWe power conversion unit (PCU) that will be used in the Fission Power 

System Technology Demonstration Unit (TDU). The 1-kWe convertors were operated over a range of 

conditions to evaluate the effects of thermodynamic coupling on convertor performance and to identify 

any possible control challenges. The thermodynamically coupled convertor showed no measureable 

difference in performance from the baseline data collected when the engines were separate, and no major 

control issues were encountered during operation. The results of this test are guiding controller 

development and instrumentation selection for the TDU. 

Introduction 

Free-piston Stirling power conversion has been identified as a viable option for potential Fission 

Power Systems (FPSs) on the Moon and Mars (Mason 2006a; 2006b). Recent studies have examined the 

use of Stirling convertors coupled to a low temperature (<900 K), uranium-dioxide fueled, liquid-metalcooled reactor for potential lunar application in the year 2020. The system is considered a low 

development risk based on the use of terrestrial-derived reactor technology and conventional materials 

(Fission Surface Power Team, 2010). In addition, all materials and components are compatible with the 

lunar and martian environments. Therefore, Mars-based power conversion system designs are expected to 

be very similar to lunar designs in configuration, set-up, and operations (Geng 2008). 

NASA Glenn Research Center (GRC) has been involved in Stirling convertor research in support of 

FPS development over the past several years. In 2007, two P2A (formerly known as EG–1000) 1-kW freepiston Stirling power convertors were procured from Sunpower Inc., of Athens, Ohio. The P2As are 

designed to produce 1.1 kWe at their design operating conditions of 550 C hot-end temperature, a 50 C 

cold-end temperature, and a mean working space pressure of 3.0 MPa. The P2As are typically configured 

with a gas burner for thermal energy input; however, the P2As as initially delivered to GRC were 

configured with electric heater heads to simplify testing. In 2008, a performance map test (over various hotand cold-end temperatures and piston amplitudes) was conducted at GRC to establish the baseline Stirling 

convertor performance (Briggs 2010a). The P2As were operated for about 48 hr during this baseline testing. 

After the performance map test was completed, the convertors were reconfigured; the electric heater heads 

were replaced with GRC designed and developed NaK heat exchangers. The NaK heat exchangers enabled 

the Stirling convertors to be operated with a NaK liquid metal pumped loop for thermal energy input. In 

addition to the NaK heat exchangers, the Stirling cold-end heat exchangers were also replaced. The original 

P2A coolers utilized o-rings to contain the cooling water. To minimize the chance of a water leak, new 

coolers were built and installed that utilized braze joints to contain the cooling water. The convertors were 
NASA/TM—2011-217098 2 

then tested in the Marshall Space Flight Center (MSFC) pumped NaK loop test facility during the summer 

of 2009 (Briggs 2010b; Polzin 2010). This was the first-ever attempt at powering a free-piston Stirling 

engine with a pumped liquid metal heat source. The tests included a) performance mapping the convertors 

over various hot- and cold-end temperatures, piston amplitudes, and NaK flow rates, b) transient test 

conditions to simulate various startup and fault scenarios, and c) simulated nuclear reactivity feedback 

control tests. The P2As were operated for about 66 hr during testing at MSFC. 

After the successful test at MSFC, the convertors were reconfigured a second time. The NaK heat 

exchangers were removed from the convertors and replaced with a modified electrically heated head. The 

new heater head is basically the two original heater heads joined at their domes. A short stainless steel 

joining ring was fabricated and welded to the heater head domes of each engine. The joining ring contains 

a passage along its axis of symmetry that connects the expansion spaces of the two convertors. There are 

three advantages associated with this type of arrangement: 1) the common expansion space helps to 

synchronize the frequencies of the two convertors, thereby reducing vibration, 2) the footprint of the dual 

power convertors is reduced (the engine pair is about 18 in. shorter in the new arrangement), and 3) 

electrical control of the convertors is potentially simplified. This new configuration is referred to as the 

“thermodynamically coupled arrangement” since the expansion spaces of each convertor are now in direct 

communication. 

In late 2010, the performance map test (over various hot- and cold-end temperatures and piston 

amplitudes) was repeated for the thermodynamically coupled configuration. The P2As were operated for 

about 26 hr in this configuration. The data from this test were then compared with the baseline data so 

that any changes in performance due to the new configuration could be evaluated. After correcting the 

data for slight differences in the experimental test conditions, it was determined that performance data 

from the thermodynamically coupled configuration was essentially the same as the baseline data. 

Nomenclature 

FLDT Fast Linear Displacement Transducer 

FPS Fission Power System 

GRC Glenn Research Center 

MSFC Marshall Space Flight Center 

MTI Mechanical Technology Incorporated 

RTD Resistance Temperature Detector 

SPDE Space Power Demonstrator Engine 

TDU Technology Demonstration Unit 

Thermodynamically Coupled Engine Configuration 

Joining a pair of free-piston convertors at their expansion spaces is not a new concept. Mechanical 

Technology Inc. (MTI), of Latham, New York, designed, built, and tested the Space Power Demonstrator 

Engine (SPDE) in the 1980s, which was configured in a thermodynamically coupled arrangement (Brown 

1987; Dhar 1999). The SPDE was a 25-kWe free-piston Stirling power convertor that consisted of two 

12.5-kWe engine submodules that operated with a linearly opposed piston/displacer motion. The SPDE 

was designed for steady-state operation at its design point (Thot = 630 K, Tcold = 315 K, and Frequency 

= 105 Hz), and as such, had no control system. MTI learned that for the two engines to operate at a 180

phase relative to each other, a stable power flow path needed to exist between the two engines. MTI 

identified two potential flow paths: 1) electrical––by connecting the two alternators in series or in parallel 

with each other, or 2) mechanical––by connecting the two engines thermodynamically at their expansion 

spaces. The SPDE utilized both power flow paths to ensure stable operation with the alternators 

connected in parallel. 
NASA/TM—2011-217098 3 

Sunpower also explored the thermodynamically coupled configuration in the past. Sunpower joined a 

pair of 2.5-kWe free-piston Stirling engines at their expansion spaces (Lane 1996). The coupled engine 

pair ran smoothly; no instability was observed over the full power range tested. 

Many Stirling engine designers believed that the thermodynamically coupled configuration would not 

have a negative impact on engine performance. The work described in this paper was performed in an 

attempt to verify this belief experimentally. 

A photograph and sketch of the joined P2A heater heads are shown in Figure 1. The heater head is 

electrically heated using 12 cartridge heaters per engine. The cartridge heaters are mounted in a nickel ring 

that is brazed to the expansion space dome, adjacent to the Stirling heat exchanger. Also shown in the 

picture are the thermocouple locations for measuring the heater head temperatures. The passage connecting 

the two expansion spaces is tapered from a diameter of about 14 mm down to 6 mm. The helium gas flow 

through this passage is extremely low since the pressure waves in each engine are in phase. 

Figure 2 shows the P2A engines mounted on the test stand at GRC. In this figure, the heater head is 

covered with several layers of Kaowool insulation and contained within a stainless steel can. The Fast 

Linear Displacement Transducers (FLDTs) that are used to measure the piston and displacer positions 

protrude from the alternator housings. Two of the four cooling water temperature measurement locations 

are also shown. The convertor pair is supported on a pair of springs that allow movement in the axial 

direction, but are firm in the radial direction. 

Figure 1.—P2A Stirling heater heads joined at expansion space. 

Figure 2.—Thermodynamically coupled 1-kWe Stirling convertors. 
NASA/TM—2011-217098 4 

Performance Map Testing 

Test Setup 

Testing was conducted on the 1-kWe thermodynamically coupled Stirling engine pair in the Stirling 

Research Facility at GRC. The test rig, shown in Figure 3 and represented schematically in Figure 4, 

consists of two Variacs (one for providing power to each engine heater head), two free-piston Stirling 

convertors, a chiller for heat rejection, a helium fill station, two alternating current (AC) power supplies 

(used for piston control), and a data acquisition system. 

Power is provided to the cartridge heating elements through two Variacs that receive 208 VAC from 

the facility power grid. Piston frequency and amplitude, as well as convertor synchronization were 

controlled using two AC power supplies that apply an AC voltage across the alternator of each convertor. 

Electric power produced by the alternators was dissipated using 39.7 Ω load resistors. Waste heat was 

rejected to a circulating water loop that is cooled by a Neslab HX–300 recirculating chiller. Hot-end, 

cold-end, and alternator housing temperatures, as well as all coolant inlet and exit temperatures, were 

monitored using type-K thermocouples. Cooling water flow rates were measured using turbine flow 

meters on the cooling supply line. Piston and displacer amplitudes were calculated from FLDT position 

sensor measurements. 

A key improvement to the test setup (from when the baseline data was obtained) was the addition of 

several Resistance Temperature Detectors (RTDs) on both the cooling water inlet and outlet lines. The 

RTD measurements were averaged for each the inlet and outlet to obtain the cooling water temperatures. 

These RTDs have made a dramatic improvement to the calculation of the thermal energy removed from 

the convertors and in the overall energy balance. During the baseline testing, single thermocouples were 

used to measure each of the cooling water inlet and outlet temperatures. 

Figure 3.—A 1-kWe dual-opposed Stirling engine test rig. 

Convertor #1 Convertor #2

Heater Power 

Supply (Variac)

Cooling Water

Chiller

(Neslab)

Helium

Supply

Vacuum

Pump

AC Power

Supplies

Load Resistors

Figure 4.—Schematic of test rig. 
NASA/TM—2011-217098 5 

Method 

Performance map testing involved measuring power output and gross thermal efficiency (Crowley 

1983) of each convertor over a matrix of hot- and cold-end temperatures and a range of piston amplitudes. 

These convertors were designed to operate at a charge pressure of 3.0 MPa, a hot-end temperature of 

550 C, and a cold-end temperature of 50 C, at a piston amplitude of 10 mm. During performance map 

testing, the convertor hot-end temperature was varied between 400 and 550 C, in increments of 50 C, at 

cooling water inlet temperatures of 30, 50, and 70 C. At the design hot- and cold-end temperatures, the 

power piston amplitude was varied from 6 to 11 mm in increments of 1 mm. At all other hot- and coldend temperatures, the power piston amplitude was varied from 9 to 11 mm in increments of 1 mm. 

Due to a constraint on the AC power supplies used to control the pistons, the power factor was 

maintained above 0.93 for all test conditions. Some of the planned data points were omitted from the test 

matrix to prevent this constraint from being violated. This constraint came into play at high piston 

amplitudes, low hot-end temperatures, and low cold-end temperatures during both baseline testing and 

thermodynamically coupled testing. However, the power factors observed during thermodynamically 

coupled testing were higher than those observed during baseline testing at nearly every operating 

condition, particularly for engine 1; as a result, more data points were obtained during the 

thermodynamically coupled testing. The reason for the higher power factors measured for engine 1 might 

be due to a change in its motor constant. This change is discussed in more detail later in the paper. 

Results 

The performance maps of the thermodynamically coupled configuration are shown in Figures 5 and 6. 

Each data point shown in these maps is the average of 150 measurements taken at 2-sec intervals over a 

5-min. period. Each 5-min. period began when the alternator gas temperature (the last temperature to 

reach steady state) changed at a rate of no more than 1 °C over a 5-min. period. The legends shown in 

Figures 5 and 6 indicate the hot- and cold-end temperatures of the convertors in Celsius. As expected, 

power increased approximately linearly with piston amplitude especially close to the design operating 

condition. Efficiency reached a maximum at about 8 mm of piston amplitude. Efficiency plots for the 

various hot- and cold-end temperatures, but at similar temperature ratios lie in good agreement with each 

other. For example, the 550/70 (temperature ratio = 2.40) and 500/50 (temperature ratio = 2.39) efficiency 

curves are almost identical. This is all consistent with results observed previously for the baseline testing. 

Comparison With Baseline Configuration 

The performance of the convertors in the thermodynamically coupled configuration is approximately 

the same as the baseline measurements. Although the data recorded during this test are slightly below the 

baseline data in terms of performance, the major reasons for these differences are not related to the 

thermodynamically coupled configuration. Differences in the experimental conditions are the biggest 

reason for the differences in the data. First of all, there were slight differences in the Stirling convertor 

cold-end temperatures between the baseline and thermodynamically coupled data. A single thermocouple 

was used to measure the cooling water inlet temperatures during the baseline testing. A set of three RTDs 

were averaged to measure the cooling water inlet temperatures during the recent test. In addition, the 

cooling water inlet temperatures during baseline testing were adjusted using a crude correlation to obtain 

the desired cooler metal temperatures. During the thermodynamically coupled configuration test, this 

correlation was not used to adjust the cooling water inlet temperature. This implies that the 

thermodynamically coupled Stirling convertors were tested at slightly higher cold-end temperatures 

(i.e., lower engine temperature ratios) than the baseline configuration. The approximate loss in engine 

power due to the reduced temperature ratios is about 1 percent per engine. As a result, the loss in 

convertor power output is approximately 10 W per convertor at the design operating conditions. 
NASA/TM—2011-217098 6 

Figure 5.—Total power output of thermodynamically coupled convertors versus Piston amplitude. 

Figure 6.—Gross thermal efficiency of thermodynamically coupled convertors versus Piston amplitude. 
NASA/TM—2011-217098 7 

A second reason for the slightly lower performance of the thermodynamically coupled configuration is 

that the motor constant of one of the two alternators was about 1.9 percent lower than what it was during the 

baseline testing at design operating conditions. The motor constant of the second alternator had not changed. 

The impact of the lower motor constant is that the alternator voltage has dropped by about 1.9 percent while 

the current has increased by about 1.9 percent at a given piston amplitude and frequency. The additional 

alternator current increases the I2

R loss by about 3.5 percent, which in turn reduces the measured alternator 

output power by about 0.4 percent (assuming a 90 percent efficient alternator). The reason for the change in 

the motor constant is most likely due to a reduction in the flux density of the magnets in one of the 

alternators. Since both alternators have experienced similar temperatures and operating conditions 

throughout the entire test program, differences in the intrinsic coercivities between the sets of magnets could 

be the cause. The intrinsic coercivity of a magnet has a direct impact on its demagnetization temperature in a 

given application. At the design operating conditions, the loss in convertor power output due to the motor 

constant degradation is approximately 3 to 4 W. 

In Figure 7, the total power output of the thermodynamically coupled engines is compared with that 

of the baseline configuration. The data shown in this figure were acquired for the convertors operating at 

a hot-end temperature of 550 C and cold-end temperature of 50 C. The thermodynamically coupled data 

are shown both as recorded and as corrected for the differences in the experimental conditions as 

described previously. The actual and corrected data are in good agreement with the baseline data over the 

range of piston amplitudes tested. At design operating conditions (Thot = 550 C, Tcold = 50 C, and 

Piston Amplitude = 10 mm), the thermodynamically coupled convertors produced approximately 2 kWe 

as expected. 

Figure 7.—Comparison of total power output of thermodynamically coupled configuration 

(Corrected for differences in experimental conditions) with baseline data. 
NASA/TM—2011-217098 8 

The minor differences in performance between the two configurations, as shown in Figure 7, can be 

attributed to the small increase in dead volume associated with the thermodynamically coupled 

configuration. The joining spool that is used to connect the expansion spaces of the two convertors adds 

approximately 5.6 cc of dead volume to each convertor. With the baseline P2A engine working space 

volume of 537 cc, the joining spool adds only 1 percent of additional volume. Sunpower computer 

simulation of the P2A convertors predicted about a 0.5 percent power degradation (per engine) with the 

additional 5.6 cc dead volume in the expansion space (Wood 2011). 

Conclusions 

Significant progress has been made toward reducing the risk associated with the development of 

Stirling power conversion technology for potential FPS applications. A pair of proven 1-kWe free-piston 

Stirling power convertors was used to demonstrate that free-piston Stirling convertors can be joined at 

their expansion-spaces without dramatically affecting engine performance. The advantage of this type of 

configuration is that it can reduce the size and weight of the convertor pair, and also simplify thermal 

energy input for Stirling convertors interfaced with a pumped-NaK loop, as planned for the FPS 

Technology Demonstration Unit (TDU). Stirling technology is becoming a viable option for FPS 

applications. 

References 

Mason, L.S., “A Comparison of Fission Power System Options for Lunar and Mars Surface 

Applications,” NASA/TM—2006-214120, 2006a. 

Mason, L.S., “A Practical Approach to Starting Fission Surface Power Development,” in proceedings of 

International Congress on Advances in Nuclear Power Plants (ICAPP’06), American Nuclear 

Society, La Grange Park, IL, paper 6297, NASA/TM—2006-214366, NASA Glenn Research Center, 

Cleveland, OH, 2006b. 

Fission Surface Power Team, “Fission Surface Power System Initial Concept Definition,” 

NASA/TM—2010-216772, 2010. 

Geng, S.M., Mason, L.S., Dyson, R.W., Penswick, L.B., “Overview of Multi-Kilowatt Free-Piston 

Stirling Conversion Research at GRC,” NASA/TM—2008-215061, 2008. 

Briggs, M.H., Geng, S.M., Robbie M.G., “Status of Kilowatt Class Stirling Power Conversion Using a 

Pumped NaK Loop for Thermal Input,” NASA/TM—2010-216265, 2010a. 

Briggs, M.H., Geng, S.M., Pearson, J.B., Godfroy, T.J., “Summary of Test Results from a 1 kWe-Class 

Free-Piston Stirling Power Convertor Integrated with a Pumped NaK Loop,” NASA/TM—2010-

216934, 2010b. 

Polzin, K.A., Pearson, J.B., Godfroy, T.J., et al., “Testing of Liquid Metal Components for Nuclear 

Surface Power Systems,” in proceedings of the 57th Joint Army-Navy-NASA-Air Force (JANNAF) 

Propulsion Meeting, paper 1394, 2010. 

Brown, A.T., “Space Power Demonstrator Engine––Phase I Final Report,” NASA CR–179555, 1987. 

Dhar, M., “Stirling Space Engine Program,” NASA/CR—1999-209164/VOL1, 1999. 

Lane, N.W., Beale, W.T., “A Free-Piston Stirling Engine-Alternator for Solar Electric Power,” in 

Proceedings of the 8th International Symposium on Solar Thermal Concentrating Technologies, 

Köln Germany, October 6–11, 1996. 

Crowley, J.L., “Efficiency Terms for Stirling Engine Systems,” ORNL/CON–131, 1983. 

Wood, J.G., personal communication, 2011. 
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Performance of a Kilowatt-Class Stirling Power Conversion System in a Thermodynamically 

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14. ABSTRACT

A pair of 1-kWe free-piston Stirling power convertors has been modified into a thermodynamically coupled configuration, and performance 

map testing has been completed. This is the same configuration planned for the full-scale 12-kWe power conversion unit (PCU) that will be 

used in the Fission Power System Technology Demonstration Unit (TDU). The 1-kWe convertors were operated over a range of conditions 

to evaluate the effects of thermodynamic coupling on convertor performance and to identify any possible control challenges. The

thermodynamically coupled convertor showed no measureable difference in performance from the baseline data collected when the engines 

were separate, and no major control issues were encountered during operation. The results of this test are guiding controller development 

and instrumentation selection for the TDU. 

15. SUBJECT TERMS

Stirling, Free-piston, Fission Power System (FPS) 

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