Salvatore M. Oriti and Nicholas A. Schifer

Glenn Research Center, Cleveland, Ohio

Recent Stirling Conversion Technology

Developments and Operational Measurements

at NASA Glenn Research Center

NASA/TM—2010-216245

August 2010

AIAA–2009–4556
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Salvatore M. Oriti and Nicholas A. Schifer

Glenn Research Center, Cleveland, Ohio

Recent Stirling Conversion Technology

Developments and Operational Measurements

at NASA Glenn Research Center

NASA/TM—2010-216245

August 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Seventh International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Denver, Colorado, August 2–5, 2009

AIAA–2009–4556
Acknowledgments

Some work described in this paper was performed for the Science Mission Directorate (SMD) and the

Radioisotope Power System (RPS) Program, which provided funding for these projects.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216245 1 

Recent Stirling Conversion Technology Developments and 

Operational Measurements at NASA Glenn Research Center 

Salvatore M. Oriti and Nicholas A. Schifer

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

In support of the Advanced Stirling Radioisotope Generator (ASRG) project and other potential 

applications, NASA Glenn Research Center (GRC) has initiated convertor technology development 

efforts in the areas of acoustic emission, electromagnetic field mitigation, thermoacoustic Stirling 

conversion, and multiple-cylinder alpha arrangements of Stirling machines. The acoustic emission 

measurement effort was developed as a health monitoring metric for several Stirling convertors 

undergoing life testing. While accelerometers have been used in the past to monitor dynamic signature, 

the acoustic sensors were chosen to monitor cycle events, such gas bearing operation. Several 

electromagnetic interference (EMI) experiments were performed on a pair of Advanced Stirling 

Convertors (ASC). These tests demonstrated that a simple bucking coil was capable of reducing the 

alternating current (ac) magnetic field below the ASRG system specification. The thermoacoustic Stirling 

concept eliminates the displacer typically found in Stirling machines by making use of the pressure 

oscillations of a traveling acoustic wave. A 100 W-class thermoacoustic Stirling prototype manufactured 

by Northrop Grumman Space and Technology was received and tested. Another thermoacoustic prototype 

designed and fabricated by Sunpower, Inc., will be tested in the near future. A four-cylinder free piston 

alpha prototype convertor was received from Sunpower, Inc. and has been tested at GRC. This hardware 

was used as a proof of concept to validate thermodynamic models and demonstrate stable operation of 

multiple-cylinder free-piston Stirling conversion. This paper will discuss each of these activities and the 

results they produced. 

Introduction 

The NASA Glenn Research Center (GRC) has been supporting the joint NASA, Department of 

Energy (DOE), and Lockheed Martin (LM) development of the Advanced Stirling Radioisotope 

Generator (ASRG) for several years (Ref. 1). The ASRG design comprises two Advanced Stirling 

Convertors (ASCs) designed and manufactured by Sunpower, Inc. The use of Stirling power conversion 

offers a four-fold increase in conversion efficiency when compared to thermoelectric conversion. The 

ASC began as a technology development contract under a NASA Research Announcement in 2003 

(Ref. 2). In 2008, LM successfully completed an engineering unit (EU) generator, with two ASC-Es 

(engineering) (Ref. 3). The ASRG-EU was subsequently transferred to GRC and began continuous 

extended operation in late 2008. 

In addition to operation of the ASRG-EU, eight other ASC prototypes have been put on extended 

operation to provide life and reliability data (Refs. 1 and 4). The performance of each convertor is 

monitored by measurements such as alternator power, current and voltage output, in addition to 

temperatures, piston position, and thermal energy input. From these data, performance metrics, such as 

efficiency and West number, can be calculated. Since the aforementioned measurements are external to 

the convertor (with the exception of piston position), it was desired to augment these with a technique that 

could reveal information about the internal components. In 2008, an effort was initiated to utilize acoustic 

emission data to monitor health of the convertors and detect changes in operation. Acoustic sensors were 

chosen because they can be easily situated on the external surfaces of the convertors. 
NASA/TM—2010-216245 2 

Also supporting the ASRG project, an activity was initiated to develop and test methods for 

mitigating convertor electromagnetic interference (EMI) emissions. For the ASRG flight unit there exists 

a system specification for the alternating current (ac) magnetic field emissions. Some missions may have 

specialized instruments that require even lower field emissions. Initial measurements on a pair of ASC–1s 

suggested further ac magnetic field mitigation would be required to meet the ASRG system specification. 

Two techniques have been employed to accomplish this. The first is a GRC-developed bucking coil that is 

positioned around the outer surface of the alternator housing and wired in series with the alternator 

output. This coil passively taps a portion of the convertor power output to generate a magnetic field that 

nullifies that emitted by the alternator. The second technique is the implementation of an alternator 

housing fabricated from magnetic-shielding material. This material has a large magnetic permeability, and 

when placed around the alternator, it will provide greater magnetic field attenuation than traditional vessel 

materials. 

Small Business Innovative Research (SBIR) contracts have also focused on development of relatively 

low technology readiness level concepts like thermoacoustic and multiple-cylinder alpha free-piston 

Stirling arrangements. These concepts offer the potential for further increases in specific power and 

conversion efficiency over current state of the art designs. These concepts also expand the environments 

that may utilize Stirling conversion. Most notably, thermoacoustic convertors may prove capable of 

higher hot-end operating temperatures, making them viable for power generation on Venus. As is true for 

any technology, research is required during the initial stages so that it may develop to a level sufficient for 

application. Effort is being put into these two concepts with the intention that they will in the future offer 

enhanced options to missions requiring power conversion. 

The ASC-Es incorporated into the ASRG-EU are of the typical beta style, meaning the piston and 

displacer share a common axis of motion. Arrangement in the alpha, or multiple-cylinder configuration 

has been shown to offer potential improvements in specific power and efficiency over the beta 

arrangement (Ref. 5). Only recently has free-piston technology been demonstrated to operate in the alpha 

arrangement (Ref. 6). In 2008, a Phase II SBIR contract was completed by Sunpower, Inc. with Global 

Cooling Manufacturing, Inc. as a subcontractor. This SBIR effort investigated various alpha arrangements 

of convertors in the 5 kWe output range. The effort also produced a four-cylinder free-piston proof of 

concept convertor in the 100 We output range. This device was subsequently tested at GRC to gather 

baseline performance data. 

Some activity has also focused on the thermoacoustic arrangement of Stirling conversion. Traditional 

free-piston Stirling machines, such as those described Reference 2, make use of a mechanical displacer to 

generate the Stirling cycle’s pressure wave that drives the piston. The thermoacoustic arrangement 

eliminates the displacer and instead utilizes a traveling or standing wave to move the working gas between 

the hot and cold thermal reservoirs. In this configuration, the portion of the machine that converts heat into a 

pressure wave has no moving parts. This configuration simplifies the design in some ways since moving 

components in the hot-end are eliminated. The conversion from mechanical to electrical energy is still done 

by a piston-driven linear alternator, and thus moving parts are present there, but these are exposed to only 

relatively mild temperatures. In 2006, a Northrop Grumman Space and Technology thermoacoustic Stirling 

convertor (Ref. 8) was tested at GRC. In 2008, Sunpower, Inc. delivered their version of a thermoacoustic 

Stirling convertor, developed under a Phase II SBIR contract. A test station is being built at GRC for 

operation of this unit and is expected to commence in the summer of 2009. 

Nomenclature 

ASC Advanced Stirling Convertor 

ASC-E Advanced Stirling Convertor, Engineering Unit 

ASRG Advanced Stirling Radioisotope Generator 

CTPC Component Test Power Convertor 
NASA/TM—2010-216245 3 

dBpT Decibel picoTesla 

DOE Department of Energy 

EMI Electromagnetic Interference 

EU Engineering Unit 

GRC Glenn Research Center 

LM Lockheed Martin 

pT picoTesla 

RPS Radioisotope Power System 

SBIR Small Business Innovative Research 

TASHE Thermoacoustic Stirling Heat Engine 

Acoustic Emission Characterization 

As of publication, GRC is capable of operating 14 100 We-class Stirling convertors in a continuous 

unattended mode to gather performance data over a time period of thousands of hours. The purpose of 

these tests is to record and analyze convertor performance on many units over a sufficiently long period 

of time to produce life and reliability data for the ASRG project. An in depth description of the test setup 

and status of extended operation can be found in Reference 4. Several data are recorded to track 

performance trends, including input and rejection temperatures, power output, alternator power, working 

gas pressure, and piston amplitude of oscillation. From the measured data performance parameters such as 

efficiency and West number are calculated. These parameters are then plotted as a function of time to 

observe performance trends. With the exception of piston motion measurement, all measured parameters 

are external to the convertor. In 2008 an effort was initiated to develop a method for monitoring internal 

component status by observing acoustic emissions through the convertor outer surfaces. In mind were 

events such as changes in gas bearing operation, piston displacer contact, or changes in operation of the 

displacer planar spring. The goal of this effort was to develop another convertor performance metric 

based on the signature of acoustic emissions over time. Acoustic sensors were chosen for this purpose 

because of their high frequency response which is required for detection of sound waves through metals. 

The sensors chosen in particular operate in the range of 100 to 600 kHz. In contrast, accelerometers have 

been used to track dynamic balance of the test article as a whole near the convertor operating frequency in 

the 100 Hz range. 

Acoustic emissions characterization focused mainly on the Sunpower, Inc. ASC–0s and ASC–1HSs, 

but some measurements were taken on Infinia, Corporation TDCs. Plans are in place to begin acoustic 

characterization of the ASRG-EU. The sensor chosen for this activity was the Micro30S model from 

Physical Acoustics Corporation which operates in the frequency range of 100 to 600 kHz. The sensors 

were loaded onto the outer surfaces of the convertors by custom-designed clamps that permitted precise 

control of the clamping force (Fig. 1). 

A Yokogawa DL750 Scopecorder was found to be most useful for signal acquisition. This device was 

capable of accepting and storing the large amount of data generated via sampling the acoustic sensor and 

piston position sensor signals. Capturing of the acoustic and piston position signals simultaneously 

permitted investigation of events that were related to piston motion. The signals were sampled at a rate of 

1 MHz over a time period of 100 ms, which captured up to ten cycles of convertor operation. 

Furthermore, the device was found capable of storing data at this rate for a window up to 100 sec, 

permitting relatively long-term observation of the acoustic information. 
NASA/TM—2010-216245 4 

At the time the acoustic emission characterization activity began, the ASC–0s #3 and #4 and 

ASC–1HSs #1 and #2 were undergoing extended operation, so baseline measurements were first taken on 

these convertors. The acoustic measurements were then taken on several occasions thereafter on a weekly 

basis to generate baseline data for trending analysis. The emission spectrum of ASC–0 #3 was consistent 

throughout the observation period. A waveform from the acoustic sensor on ASC–0 #3 is shown in 

Figure 2. The signature consists of a primary and secondary pulse. The larger primary pulse coincides 

with the midpoint of the piston stroke moving towards the hot end of the convertor. The smaller 

secondary pulse coincides with the midpoint of the stroke in the other direction. The acoustic signatures 

of the other convertors were similar to that of ASC–0 #3, but differences were observed in the 

nature of the secondary pulses. On ASC–0 #4 for example, the timing and amplitude of the secondary 

pulse did not remain consistent. Rather than lag the primary pulse by 190°, it was observed to lag by 

various amounts ranging from 66° to 123°. The amplitude of the secondary pulse from ASC–0 #4 also 

varied. On certain occasions it was almost absent, while on others it was as large as the primary. 
NASA/TM—2010-216245 5 

Similar acoustic emission behavior was observed from ASC–1HS #1 and #2. Several acoustic sensor 

waveforms samples from ASC–1HS #1 are shown in Figure 3. The signature possesses a primary pulse 

but ASC–1HS #1 sometimes showed more than one secondary pulse. The arrival and departure of the 

secondary pulses was a repeatable pattern, and followed the sequence shown in Figure 3. The acoustic 

emission of ASC–1HS #2 was consentient and resembled that of ASC–0 #3. 

Few conclusions could be drawn from the activity just described as little was known of the status of 

the internal components while acoustic signatures were observed. To gather more insight, further testing 

was performed on a convertor during which the internal components were deliberately disrupted and the 

effect on acoustic emission observed. These disruptions included disabling of the gas bearings, operating 

with piston to displacer contact, and loose planar spring fasteners. The intent of this task was to determine 

if the acoustic emission signature could be used to detect these types of operational changes. While piston 

to displacer contact and planar spring loosening are catastrophic changes that are not anticipated, slight 

changes in gas bearing performance could potentially occur, and the ability to detect this offers a useful 

tool for extended operation analysis. 

This testing showed that acoustic signatures were indistinguishable whether or not the gas bearings 

were functioning. This unfortunately suggested that acoustic emissions were not a viable method for 

monitoring gas bearing performance. However, the sensors were able to detect piston to displacer contact, 

as the acoustic emissions were large enough to saturate the sensor. It was found during this investigation 

that piston to planar spring contact produced a similar effect, and could thus be detected by acoustic 

sensors (Fig. 4). The acoustic signature during contact is shown in brown, and that of normal, noncontacting operation is shown in yellow. Two of the planar spring fasteners were loosened to observe the 

effect of a loose planar spring. Similarly, the acoustic emissions were large in this case relative to the case 

where the spring was properly seated. These tests showed that while the desired insight into gas bearing 

operation may not be observable with acoustic sensors, other more severe convertor disruptions could be 

detected. 
NASA/TM—2010-216245 6 

Electromagnetic Interference Mitigation 

As a power system, the ASRG must meet certain electromagnetic emission requirements so as to not 

interfere with spacecraft hardware. Not all missions have the same magnetic field emission limits. In 

particular, those using magnetometers are subject to specifications with significantly lower limits for the 

ac and dc magnetic field emissions from the power system. Typically, the dc magnetic field is measured 

in nanoTesla, while the ac magnetic field is measured using the decibel scale. The decibel scale is 

logarithmic and allows convenient representation of a large range of values that span many orders of 

magnitude. Expressing a parameter using the decibel scale must be done relative to some value. In the 

case of ac magnetic field, measurements are typically made relative to 1 picoTesla (pT). With this, the ac 

magnetic field can be expressed using Equation (1). 















  1pT dBpT 20 log M (1) 

where M is the ac magnetic field strength in pT and the output of the equation is the magnetic field 

strength in decibel pT. A magnetic field strength of 1 pT corresponds to 0 dBpT, since the ratio inside the 

log operation would be 1. Also, a change of 1 dB corresponds to a 12 percent change in the field strength. 

Emitted field strengths are typically measured at distances ranging from 0.25 to 3.0 m, with the field 

strength measurement at 1.0 m being most commonly used. For spacecraft specifications, a maximum 

field strength at 1.0 m is usually specified. The dc magnetic fields can be nullified by arrangement of 

compensating permanent magnets at appropriate positions around the source of the field. The ac magnetic 

fields can be shielded with materials commonly referred to as “mu-metal” and “permalloy” which are 

alloys of iron and nickel. Mu-metals consist of an alloy with a large percentage nickel, and a 

comparatively small percentage iron. The opposite is true for permalloy materials, where the alloy is 

comprised mostly of iron. The term mu-metal originates from the desire for high magnetic permeability, 
NASA/TM—2010-216245 7 

which is represented by the Greek letter mu (μ). Magnetic field emissions from the ASRG originate from 

the linear alternator in the Stirling convertors. The permanent magnets contribute to both the dc and ac 

magnetic fields. The alternating electrical current flowing through the alternator windings also contribute 

to the ac magnetic field. 

The first test to characterize Sunpower convertor dc and ac magnetic field emissions was performed 

by Rocketdyne at their Space Power Electronics Laboratory, in Canoga Park, California. In March 2006, 

a single ASC prototype (ASC–1 #2) was tested in both the shielded and unshielded configuration. The dc 

and ac magnetic field emissions were measured in four different configurations: unshielded, mu-metal 

shield, permalloy shield, and mu-metal plus permalloy shield. The dc magnetic field was mapped 360° 

around the convertor’s horizontal plane while in the non-operating state at distances of 1.0 and 1.5 m. The 

maximum dc magnetic field was found to be located along the axis of the convertor when facing the 

pressure vessel end. The mu-metal shield alone reduced the dc magnetic field at 1.0 m by 17 percent. The 

permalloy shield was slightly more effective, and reduced the dc magnetic field at 1.0 m by 23 percent. 

Both shields together reduced the magnetic field by 68 percent. The ac magnetic field was also measured 

in each of the four shielding configurations, but with the convertor operating. The measurements were 

taken along the radial direction away from the convertor alternator at distances ranging from 0.5 to 2.0 m 

in 0.25 m increments. Similarly, the shields were effective at attenuating the ac magnetic field. The mumetal shield alone reduced the ac magnetic field at 1.0 m by 70 percent. The permalloy shield reduced the 

field by 77 percent. Together, both shields reduced the field by 83 percent. This test demonstrated that 

commercially available magnetic shielding materials were capable of significantly attenuating both the dc 

and ac magnetic field emissions. The test also revealed that the unshielded dc and ac magnetic field 

strengths at 1.0 m were the same. This suggested that the majority of the ac magnetic field was due to the 

motion of the permanent magnets themselves, rather than the current flowing through the alternator 

windings. 

The test performed at Rocketdyne suggested further investigation was warranted to find methods of 

reducing magnetic field emissions. It was also desired to learn more about which component of the 

alternator was most responsible for the ac magnetic field emissions. In August 2006, an experiment was 

conducted for this purpose at GRC. Instead of an operating convertor, an ASC–1 research alternator was 

driven by a linear motor of a different design than the ASC. This configuration isolated the source of the 

magnetic fields (the alternator) so that measurements could be made on the alternator alone. Since in this 

configuration the linear motor was coupled to the research alternator, the motor had to be shielded to 

allow accurate measurements of the research alternator. The dc magnetic field measurements were taken 

at distances from flush with the alternator up to 10 cm away, and at various locations along the axis. 

Because of the available instrumentation, the dc magnetic field could not be measured at 1.0 m. The ac 

magnetic field measurements were taken at various distances (including 1.0 m) and axial locations. 

Despite the use of mu-metal shielding around the linear motor, it was found that the motor’s ac magnetic 

field still dominated the emissions. The mu-metal shield was capable of attenuating the linear motor 

emissions by 15 dB, or 82 percent. This is a typical shield effectiveness for a simple cylindrical shield, 

but further attenuation was desired to isolate research alternator emissions. However, because of apertures 

necessary for wiring and mechanical connections, a large enough portion of the motor’s ac magnetic field 

was able to escape the shield to interfere with the desired measurements. After measuring the ac magnetic 

field of the shielded linear motor and research alternator pair, the research alternator was then 

mechanically disconnected from the linear motor. Measurements of the ac magnetic field were then taken 

while exciting the shielded motor alone at the same frequency and amplitude. The ac magnetic field 

strength at 1.0 m was indistinguishable from the field measured while coupled to the alternator. This 

result suggested that efforts to isolate magnetic field emissions from the alternator would not be possible 

in this configuration due to the interference introduced by the drive motor. With this, the only conclusion 

that could be drawn was that the ac magnetic field emission from the alternator was lower than that 

measured while driving it with the linear motor. While driving the research alternator with a shielded 

linear motor, the measured ac magnetic field at 1.0 m was approximately 10 dBpT higher than the limit 

imposed by the ASRG system specification. However, the system specification applies to a dual-opposed 
NASA/TM—2010-216245 8 

pair of convertors, and references the geometric center of the two alternators. Since this test was 

conducted on a single alternator, the results may not correlate directly to the system specification. 

After finding that motoring an alternator to measure ac magnetic field emissions was inconclusive, 

the decision was made to repeat measurements on a dual-opposed pair of ASC–1s. In June 2007 

ASC–1 #3 and #4 were operated in the EMI laboratory at NASA GRC. In this configuration, a more 

insightful measurement could be made, since there now existed a geometric center, as referenced in the 

ASRG system specification for EMI. The dc magnetic field measurements were again taken, but only in 

close proximity to one of the alternators. The dc magnetic field was measured on the surface of 

ASC–1 #3’s alternator housing at 5 axial locations and 2 radial locations. As was true during the prior 

test, the dc magnetic field at 1.0 m could not be measured due to available instrumentation. The ac 

magnetic field was measured at distances ranging from 1.0 to 3.0 m in both radial and axial directions 

from the center of the convertor pair. The ac magnetic field strengths at 1.0 m in the radial and axial 

directions differed by only 1.3 dB, suggesting either location was suitable for this particular measurement. 

The ac magnetic field measurements suggested that the emission from this particular test article was up to 

11 dBpT higher than the ASRG system specification. This result was in close agreement with that of the 

research alternator test described previously. Since the measured field emissions were higher than the 

ASRG system specification, efforts focused on exploring mitigation techniques. 

In November 2007, GRC engineers developed a novel EMI mitigating technique referred to as a 

bucking coil, for which a patent is pending. This coil consists of several loops of magnet wire wound 

around the alternator housing and wired in series with the alternator output. The coil thus uses a portion of 

the convertor output to generate a magnetic field. The coil axis is oriented parallel to the convertor axis 

and in a direction such that the field produced by the current flowing through the coil opposes and 

nullifies that generated by the alternator magnets and windings. Before EMI testing, the effect of the 

bucking coil on convertor performance was quantified in the Stirling research laboratory at GRC. 

ASC–1 #3 and #4 were operated with and without the coils inserted into the convertor power output 

pathway. The net effect was a 3 to 5 percent drop in power, which was in agreement with predictions due 

to the added resistance introduced by the bucking coil. After demonstrating that the bucking coil did not 

adversely affect convertor performance, the units were then transported to the EMI laboratory (Fig. 5). 

The convertors were first operated at the same baseline conditions as the last EMI test, with the bucking 

coil bypassed. Magnetic field measurements were then taken with the bucking coil in the circuit. The 
NASA/TM—2010-216245 9 

number of turns in the bucking coil was also varied to determine the optimum configuration. The net 

effect of the optimum bucking coil configuration was a reduction in the ac magnetic field of 17.7 dB, or 

87 percent. This attenuated the ac magnetic field emissions to a level well below the limit imposed by the 

ASRG system specification. 

In addition to EMI tests on ASC–1 prototypes, an EMI test was also conducted on the ASRG-EU, 

which comprises two ASC-E units. In March 2008, the EU generator underwent EMI testing at LM’s 

facility in Valley Forge, Pennsylvania (Ref. 3). Both dc and ac magnetic fields were measured at 1.0 m. 

The dc magnetic field was measured to be less than that of a GPHS-RTG. Specifications for certain 

missions requiring lower dc magnetic emissions can be met by the use of compensating permanent 

magnets. This technique has been used in the past to meet specifications for low frequency 

magnetometers, such as those on the Ulysses mission. The ac magnetic field measurements were similar 

in magnitude to those measured on the ASC–1 #3 and #4 convertors at NASA GRC. 

The alternator housing of the ASCs is typically made from stainless steel or Inconel alloys. The idea 

was presented to further reduce magnetic field emissions by fabricating custom housings from a 

magnetic-shielding material. This is an attractive solution, since the housing would also serve the function 

of a magnetic shield, and avoid the extra mass associated with retrofitting an external shield. Housings 

made of Carpenter Technology’s Perm-49 high magnetic permeability alloy were fabricated and installed 

on ASC–1 #3 and #4. EMI tests of these convertors with the Perm-49 housings began in June 2009, and 

results will be reported in future publications. 

Thermoacoustic Stirling Conversion 

The traditional method of free-piston Stirling conversion utilizes a displacer to shuttle working gas 

between the hot and cold ends of the machine. Since the displacer is exposed to the hot-end temperature, 

it must be made of a high temperature capable material, much like that of the heater head, which 

undergoes no motion but is exposed to the stress from working gas pressurization and thermal gradients. 

It is possible to eliminate the displacer, and thus eliminate all hot-end moving components, by 

implementing the thermoacoustic arrangement of Stirling conversion. Such an arrangement simplifies the 

design, since the only moving parts would be located in the relatively cool alternator. The thermoacoustic 

method accomplishes this by using high amplitude standing or traveling waves to send the working gas 

through the same pressure and volume oscillations as the displacer in a traditional machine. 

Thermoacoustic machines still make use of regenerators and hot and cold heat exchangers, but the gas is 

shuttled through these components by the acoustic wave. The temperature gradient across the hot and cold 

thermal reservoirs acts to supply energy to the acoustic wave, which may then act on a piston connected 

to a linear alternator to generate electricity. 

In 2003, Northrop Grumman completed a contract during which a 100 We-class thermoacoustic 

convertor was designed and fabricated (Fig. 6). The convertor comprises a thermoacoustic Stirling heat 

engine (TASHE) coupled to a pair of linear alternators. The TASHE converts the heat into acoustic 

power, while the linear alternators accept this acoustic power and convert it to electricity. This unit 

demonstrated a peak conversion efficiency of 17.9 percent and a peak power output of 57 We. The 

Northrop Grumman thermoacoustic convertor was delivered to GRC in 2006, where it underwent 

checkout testing. The convertor was operated at GRC up to a hot-end temperature of 450 °C, and 

demonstrated the ability to self start without the need for external excitation. 

In 2008, Sunpower, Inc. produced a 100 We-class thermoacoustic Stirling convertor during a Phase II 

SBIR (Fig. 7). This design was similar in size, shape, and arrangement to the ASC that has been 

integrated into the ASRG-EU, except that it was modified to use an acoustic feedback circuit instead of a 

displacer. As such, the Sunpower design is coaxial, and thus the heat exchangers surround the thermal 

buffer tube. In contrast, the Northrop Grumman design was a loop arrangement, where the heat 

exchangers did not share a common axis. The Sunpower thermoacoustic convertor was designed for a 

radioisotope application and to accept heat from one GPHS module. The unit successfully operated as 
NASA/TM—2010-216245 10 

desired and produced 50 We. This activity demonstrated that an existing design could be modified to 

achieve thermoacoustic operation. The hardware was subsequently delivered to GRC in late 2008. A test 

station is being built at GRC that will be used to conduct operational research of this device. 

Thermoacoustic Stirling technology offers an attractive option for power systems for long-lived 

Venus surface missions (Ref. 9). The Venus environment offers a significant challenge for the power 

system, as the conversion device must operate at a rejection temperature of up to 480 °C. To maintain 

desired efficiency, the hot-end operating temperature must increase accordingly, perhaps up to 1200 °C, 

which is a sizeable increase over the current state-of-the-art ASC operating at 850 °C. Reference 9 

describes in detail trade studies that were performed to investigate the use of a radioisotope-fueled, 

thermoacoustic Stirling power system that would provide power and cooling to a Venus lander. To 

advance this effort further, testing is planned to take place at NASA GRC on both the Sunpower and 

Northrop Grumman hardware. 
NASA/TM—2010-216245 11 

Multiple-Cylinder Free Piston Stirling Conversion 

The majority of free-piston Stirling conversion technology development has been in the area of the 

“beta” configuration. The beta style comprises a lightweight displacer that oscillates between the hot and 

cold ends of the machine to generate the pressure wave (Fig. 8). This pressure wave then acts on a piston 

coupled to a linear alternator, which is coaxial with the displacer. This configuration utilizes two moving 

parts. The displacer does not have to maintain a high pressure seal across its body. Instead, it must only 

seal the pressure generated while shuttling the gas from one end to the other. The piston however must 

seal the working gas pressure from the aft end of the convertor, usually referred to as the bounce space. 

Since the piston is situated in the cold end of the convertor, there is little to no temperature gradient that 

must be accommodated. 

Recently, new technology development efforts have focused on the alpha arrangement of pistons 

(Fig. 9). In this configuration, the double-acting pistons shuttle the gas between the hot and cold ends, in 

addition to absorbing mechanical energy from the pressure wave. The arrangement of cylinders is such 

that the expansion space above one piston is connected to the compression space of the adjacent piston 

through an interconnect passage that contains the regenerator. This arrangement has the advantage of 

eliminating one of the moving components in a given cylinder when compared to the beta arrangement. 

Alpha arrangements also offer a higher power density than that of the beta style, since the pistons are 

double-acting. One particular disadvantage to the alpha arrangement is the need for the piston seal to 

isolate across both a pressure gradient and a temperature gradient, which is not true for the beta 

arrangement. Because of this, the seal is usually placed at the bottom of the piston, near the cold end of 

the device. The hot-end would then have a relatively large clearance within the cylinder, introducing 

some additional dead volume. 
NASA/TM—2010-216245 12 

The alpha arrangement has a long history of demonstrated feasibility in kinematic engines, meaning 

the motions of the pistons are connected via mechanical linkages. To enable long life however, the 

machine may make use of the free-piston configuration, meaning the output of each piston is isolated and 

connected to a linear alternator. This poses a significant technical challenge since the phasing of the 

piston motions cannot be controlled by mechanical means. Instead, the phasing must be controlled by the 

working gas dynamics, and electrical connection of the alternator outputs. For example, a 180° piston 

phase difference can be realized by connecting the respective alternator outputs in anti-phase. Only 

recently has a free-piston Stirling alpha device been demonstrated (Ref. 6). The activity described by 

Reference 6 showed that even without physical connectivity between pistons, it was possible to maintain 

the desired phase angle between piston motions. 

In 2005, an SBIR contract was awarded to Sunpower, Inc. to explore the design space for high power 

(5 to 25 kWe) Stirling convertors (Ref. 5). This power level is representative of a fission surface power 

system, such as that which may be used for a permanent lunar base. Phase I of the SBIR evaluated several 

configurations for this power level, including single cylinder beta and multiple-cylinder alpha designs. 

The study suggested that specific power and efficiency could be improved over the baseline Component 

Test Power Convertor (CTPC), which was a 12.5 kWe convertor designed during the SP-100 project 

(Ref. 7). One of the configurations, a novel three-cylinder arrangement, was shown to be the best option. 

It was estimated that this design could improve specific power by 40 percent, and efficiency by 34 percent 

over that of the CTPC. 

The SBIR then progressed to Phase II, which encompassed two goals. First a 30 kWe convertor 

assembly was designed, using two dual-opposed, three-cylinder alpha convertors, for a total of six 

cylinders each producing 5 kWe. The Phase II effort also included a task to convert an existing fourcylinder alpha free-piston cooler into a convertor. This was undertaken to provide a demonstration device 

that could be used for research and thermodynamic code validation in support of the 30 kWe design. The 

use of multiple cylinders introduces other effects in cycle operation that are not present in beta convertors, 

such as seal loss through temperature gradient, cylinder interconnect dead volumes, and off-resonance 

pistons. The cooler was converted to operate as an engine by making small modifications, such as 

changing the hot-end material and installing an electric heat source. No other major modifications were 

required to permit operation of the device as an engine. The four-cylinder proof of concept convertor 

comprised four pistons each attached to a linear alternator. The inter-cycle connections were made via 

passages machined into the cap on the hot-end. This convertor successfully operated at Sunpower with no 

instability. It produced over 135 W at a hot-end temperature of 290 °C and rejection temperature of 

50 °C, for a temperature ratio of approximately 1.74. The results of this operation agreed closely with the 

predictions made by the thermodynamic models. Information gathered during the design and operation of 

this unit was used to guide the design of the 30 kWe convertor. 

The four-cylinder demonstration convertor was then delivered to GRC (Fig. 10), where it underwent 

further testing. During the summer of 2008, the convertor completed both low and full temperature 

checkout, and performance mapping. Low temperature checkout was performed as the first step, during 

which the hot-end temperature was limited to 180 °C. Full temperature operation was performed at the 

design hot-end temperature of 300 °C. Performance mapping was performed at various hot-end 

temperatures ranging from 180 to 300 °C and various piston amplitudes ranging from 75 percent design 

amplitude up to full design amplitude. The convertor produced 20 We in the low temperature condition, 

and 115 We in the full temperature condition (Fig. 11). Some test points could not be achieved due to offcenter oscillation of two pistons that was present when approaching the full design amplitude. This can be 

attributed to the fact that no centering mechanism was integrated into the design of the demonstration 

unit. The alternator outputs were connected to persuade the desired phasing between piston motions. Two 

of the alternators were connected in anti-phase, forcing those associated pistons to oscillate with a 180° 

phase difference. The same was done with the other two alternators. The output of the two alternator pairs 

was then connected in parallel across a simple load resistor. No other methods were required to achieve 
NASA/TM—2010-216245 13 

the desired phase angle between piston motions. This is an accomplishment in the area of alpha freepiston Stirling conversion, since typically a more sophisticated control method is required that dithers the 

load to maintain stable operation. Furthermore, it demonstrated that the desired piston motions could be 

attained without physical linkages. When observed on an oscilloscope, the piston motions appeared 90° 

apart, as desired, with no visible fluctuations. 

Conclusions 

Several recent Stirling conversion operational measurements and technology developments have been 

discussed. Activities related to operational measurements included acoustic emission characterization for 

health monitoring of several convertor life tests. Baseline acoustic signatures were characterized for 

several ASCs, and this information will be used throughout extended operation to track long term 

convertor performance. It was also found that while acoustic sensors could not detect the difference between 

operating and non-operating gas bearings, they could detect major changes such as piston to displacer 
NASA/TM—2010-216245 14 

contact or loosening of the planar spring fasteners. Also discussed were measurements of EMI emissions 

and the investigation of two mitigation techniques. During EMI tests on two ASC–1s the ac magnetic field 

could be attenuated to below the ASRG system specification with a simple bucking coil situated around the 

alternator. Custom alternator housings fabricated from high magnetic permeability material were also tested, 

and the results of this test are expected in the near future. Recent technology developments in the areas of 

thermoacoustic Stirling, and multiple-cylinder alpha arrangements were also discussed. Thermoacoustic 

Stirling conversion has been demonstrated in hardware on two separate occasions. Northrop Grumman and 

Sunpower Inc. each designed, fabricated, and tested their own 100 We-class devices, demonstrating Stirling 

conversion with no hot-end moving components. These devices were both delivered to GRC, where testing 

and research is scheduled to continue in the summer of 2009. Sunpower, Inc. and Global Cooling produced 

a four-cylinder alpha free-piston Stirling convertor prototype that was subsequently tested at GRC. The 

convertor produced over 100 We at a hot-end temperature of 300 °C. This effort demonstrated that freepiston Stirling conversion could also be accomplished in the multiple-cylinder arrangement, which offers 

higher power density and specific power. 

References 

1. Schreiber, J.G., “Supporting Technology at GRC to Mitigate Risk as Stirling Power Conversion 

Transitions to Flight,” 6th International Energy Conversion Engineering Conference (IECEC), 

Cleveland, OH, 2008. 

2. Wilson, K., “Sunpower Delivery of ASC-E Convertors and Progress Toward Flight,” 6th International 

Energy Conversion Engineering Conference (IECEC), Cleveland, OH, 2008. 

3. Chan, J., “Development of Advanced Stirling Radioisotope Generator for Planetary Surface and Deep 

Space Missions,” 6th International Energy Conversion Engineering Conference (IECEC), Cleveland, 

OH, 2008. 

4. Lewandowski, E.J., “Extended Operation Testing of Stirling Convertors in Support of Stirling 

Radioisotope Power System Development,” 6th International Energy Conversion Engineering 

Conference (IECEC), Cleveland, OH, 2008. 

5. Kim, S.Y., “Specific Power Estimations for Free-Piston Stirling Engines,” 4th International Energy 

Conversion Engineering Conference (IECEC), San Diego, CA, 2006. 

6. White, M.A., “The Multi-Cylinder Free-Piston Stirling Engine – Taking Performance to a New 

Level,” 3rd International Energy Conversion Engineering Conference (IECEC), San Francisco, CA, 

2005. 

7. Jones, D., “Space Power Free-Piston Stirling Engine Scaling Study,” NASA/CR-182218, 1989.

8. Petach, M., “Design and Testing of a thermal to Electric Power Convertor Based on Thermoacoustic 

Technology,” 2nd International Energy Conversion Engineering Conference (IECEC), Providence, RI, 

2004. 

9. Dyson, R., “Long-lived Venus Lander Conceptual Design: How to Keep it Cool,” 7th International 

Energy Conversion Engineering Conference (IECEC), Denver, Co, 2009.
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Recent Stirling Conversion Technology Developments and Operational Measurements at 

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Oriti, Salvatore, M.; Schifer, Nicholas, A. 

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14. ABSTRACT

In support of the Advanced Stirling Radioisotope Generator (ASRG) project and other potential applications, NASA Glenn Research Center 

(GRC) has initiated convertor technology development efforts in the areas of acoustic emission, electromagnetic field mitigation, 

thermoacoustic Stirling conversion, and multiple-cylinder alpha arrangements of Stirling machines. The acoustic emission measurement 

effort was developed as a health monitoring metric for several Stirling convertors undergoing life testing. While accelerometers have been 

used in the past to monitor dynamic signature, the acoustic sensors were chosen to monitor cycle events, such gas bearing operation. Several 

electromagnetic interference (EMI) experiments were performed on a pair of Advanced Stirling Convertors (ASC). These tests demonstrated 

that a simple bucking coil was capable of reducing the alternating current (ac) magnetic field below the ASRG system specification. The 

thermoacoustic Stirling concept eliminates the displacer typically found in Stirling machines by making use of the pressure oscillations of a 

traveling acoustic wave. A 100 W-class thermoacoustic Stirling prototype manufactured by Northrop Grumman Space and Technology was 

received and tested. Another thermoacoustic prototype designed and fabricated by Sunpower, Inc., will be tested in the near future. A fourcylinder free piston alpha prototype convertor was received from Sunpower, Inc. and has been tested at GRC. This hardware was used as a 

proof of concept to validate thermodynamic models and demonstrate stable operation of multiple-cylinder free-piston Stirling conversion. 

This paper will discuss each of these activities and the results they produced.

15. SUBJECT TERMS

Advanced Stirling radioisotope generator; Acoustic emission measurement; Stirling convertors 

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