Glenn Research Center (GRC) is supporting life and reliability database for free-piston Stirilng conversion via extended convertor operation Ongoing convertor operation: 18 convertors (4 TDCs from Infinia, 14 ASCs from Sunpower). 350,000 total convertor hours of operation. 218,000 on Infinia units and 132,000 on Sunpower units. Demonstrating steady convertor performance requires precise maintenance of operating conditions. Sources of disruption : Investigative tests: Varying operating frequency, hot-end temp, cold-end temp. Hot end control method: Constant heat input mode requires more user-adjustment than constant temperature mode. Long-term transients in hot end insulation were observed. Support facility: Open-bath circulator fluid concentration drifting. Nuisance shutdowns (instrumentation failure, EMI, power outages). Ambient temperature fluctuations due to room HVAC

Oriti, Salvatore

NASA Technical Reports Server

Glenn Research Center  at Lewis Field 
Extended Operation of Stirling Convertors at 
NASA Glenn Research Center 
International Energy Conversion Engineering Conference 
August 2, 2011 
 
 
Salvatore Oriti 
NASA Glenn Research Center 
RPT – Thermal Energy Conversion Branch 
 
https://ntrs.nasa.gov/search.jsp?R=20120016970 2019-08-30T23:18:22+00:00Z
Glenn Research Center  at Lewis Field 
Introduction 
Advanced Stirling Radioisotope Generator (ASRG) being 
developed by Lockheed Martin, DOE, Sunpower, NASA GRC 
• 4 times more efficient than thermoelectric 
conversion 
• Requires ¼ amount of Pu-238 for same electrical 
power output 
• Two Advanced Stirling Convertors (ASCs) operating 
up to 850 °C hot-end temperature 
• 130 We from 2 heat source modules 
(beginning-of-mission, current best estimate) 
 
ASRG Flight Unit Design 
Image Courtesy of Lockheed Martin Space Systems 
GRC Provides Technical Support for ASC Life and 
Reliability: 
• Structural benchmark testing 
• Vibration testing 
• High-temperature materials 
• Magnet life testing 
• Convertor extended operation 
38 free-piston Stirling convertors, 18 ongoing  
 
Piston Position 
Telemetry
Shunt
PRD
GMV
GPHS
ASC
ACU
Heat Source 
Support
SDU
AC Power
Analog 
Telemetry 
HarnessDC Power 
Connector 
to SV
TLM/Command 
to SV
Glenn Research Center  at Lewis Field 
Ongoing Stirling Convertor Testing 
Purpose: 
• Generate performance data over tens of thousands of hours to observe long-term trends 
• Support reliability database 
Discovery 12 proposed missions : 7 years + 3 years max storage (87,000 hours) 
Outer planet missions : 17 years (150,000 hours) 
Convertors 
Nominal Operating 
Temperatures 
(Hot/Cold, °C) 
Nominal 
Per-Convertor 
Power Output 
(We) 
Convertor 
Output 
Voltage 
(Vrms) 
Supplier Date Initiated 
Per-Convertor 
Runtime 
(Hrs) 
As of July 1, 2011 
TDC #13 & #14 650/80 65 85 Infinia Jun 2003 60,000 TDC #15 & #16 Mar 2005 49,000 
ASC-0 #3 & #4 650/90 75 25 
Sunpower 
Aug 2007 25,000 
ASC-E #2 &# 3 
(ASRG-EU) 625/70 65 11 Nov 2008 19,000 
ASC-E #1 & #4 650/70 65 11 Dec 2009 10,000 
ASC-E2 #1* 
850/50 80 15 
Mar 2010 6,200 
ASC-E2 #2 Feb 2010 2,700 
ASC-E2 #3 & #4 Aug 2010 800 
ASC-E2 #5 & #6 Aug 2010 4,800 
ASC-E2 #7 Nov 2010 2,100 
ASC-E2 #8 Jun 2011 20 
*ASC-E2 #1 delivery delayed due to heater head manufacturing flaw 
Glenn Research Center  at Lewis Field 
Convertor Test Station 
ASC-E2 
ASC-E2 
Cold end  
fluid plumbing 
Circulators 
Cold-end and alternator  
housingtemp control 
Test Rack 
• Operator controls 
• Data acquisition 
• Software protection 
• Hard-wired protection 
• Automated error notification 
via email and text messaging 
• UPS and generator backup 
Example ASC-E2 Test Station 
ASC-E2 
Heat Collector 
Heat Input 
Cold-Side Adapter 
Flange (CSAF) 
Heat Rejection 
Alternator 
Electricity Output 
Glenn Research Center  at Lewis Field 
Test Methodology 
• 24/7 Operation 
• Data acquisition: 
 2-second – All parameters recorded once every two seconds, for transient or 24-hr period 
evaluation 
 5-min – Each parameter’s 2-second data averaged over 5-minute period, recorded once per 
hour, for long-term performance data evaluation 
• Maintain constant operating conditions (during extended operation): 
 
 
 
 
 
 
 
 
 
• Off-nominal operation included: 
 Performance mapping 
 Operating frequency variation 
 Heat input variation 
 Controller variation 
 Individual temperature variation  
  
Parameter Control Methods
Hot-end temperature
PID control, thermocouple feedback
Constant heat input, heater power feedback
Cold-end temperature Circulator with fluid temperature PID control
Alternator housing temperature 
Auxiliary surface heaters
Fluid heat exchanger
Piston amplitude
AC Bus power supply voltage setpoint
Zener-diode controller DC output setpoint
ASC Controller Unit (ACU, flight method)
Glenn Research Center  at Lewis Field 
0
10
20
30
40
50
60
70
80
90
100
TD
C
 #
14
 P
ow
er
 O
ut
pu
t (
W
e)
0    10000 20000 30000 40000 50000 60000
  0
 10
 20
 30
 40
 50
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 70
 80
 90
100
Hours of Operation
TD
C
 #
13
 P
ow
er
 O
ut
pu
t (
W
e)
TDC #13 & #14 Performance Data
11
35
9 
: C
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18
42
4 
: L
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32
88
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45
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51
63
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54
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R
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56
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Technology Demonstration Convertors (TDCs) #13, #14 
Regenerator oxidation 
• Longest-running convertor pair (60,000 hours each) 
• Pressure joints welded at 19,000 hours, but helium fill tube remains 
• Periodic charge pressure adjustments required, manifests as “saw-tooth” output 
• Zener diode controller hardware drift required adjustment to maintain piston amplitude 
 
Pr
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Hours of Operation
ASRG EU (ASC-E #2, #3) 
• Fully hermetically sealed before delivery (pressure joints and pinched fill tube) 
• 19,000 hours each (13,000 on Lockheed Martin controller) 
• Test rack improvements required during initial operation 
• Tests conducted for Lockheed Martin in support of controller development and flight system development 
• Good repeatability on ASC controller unit (ACU) with consistent operating conditions 
Rack improvements 
required 
Steady 
Controller development tests 
Repeatable 
Performance 
ASC-E #2 & #3 Performance Data 
Glenn Research Center  at Lewis Field 
0
5
10
15
20
25
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0
5
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0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
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 #
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 #
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Hours of OperationEff iciency = (gross heat input)/(alternator power output)
ASRG EU (ASC-E #2, #3) 
• 25% conversion efficiency demonstrated at the system level on flight-like controller 
• Repeatable and constant conversion efficiency over 19,000 hours of operation 
Rack improvements 
required 
Steady 
Controller development tests 
Repeatable 
Performance 
Efficiency =  
Alternator output power/Heater power  
ASC-E #2 & #3 Performance Data 
  
  
Glenn Research Center  at Lewis Field 
  0 1000 2000 3000 4000 5000 6000
  0
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Hours of Operation
AS
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#1
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ASC-E2 #1 Performance Data
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ASC-E2 #1 
• Fully hermetically sealed before delivery (pressure joints and pinched fill tube) 
• 6,200 hours – majority at 850 °C 
• Known heater head flaw and helium leakage 
• AC bus voltage requires adjustment to negate helium leakage 
Other 
tests 
Adjustments due to helium leakage 
Other tests 
Ambient temp 
disturbance BOM = Beginning of Mission 
EOM = End of Mission 
LR      = Low Rejection 
HR     = High Rejection 
Glenn Research Center  at Lewis Field 
0
10
20
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AS
C
-E
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Hours of Operation
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ASC-E2 #5, #6 
• Fully hermetically sealed before delivery (pressure joints and pinched fill tube) 
• 4,800 hours – all at 850 °C 
• Steady when maintaining constant conditions 
Perf mapping 
Checkout testing 
Heat source 
failure 
Circulator 
fluid changes 
Repeatable 
Performance 
Circulator fluid 
changes 
Ambient temp 
disturbance 
BOM = Beginning of Mission 
EOM = End of Mission 
LR      = Low Rejection 
HR     = High Rejection 
 
 
 
 
  
  
 
 
 
 
 
 
 
 
 
  
 
 
 
 
     
  
 
 
 
 
  
 
 
 
 
 
  
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
  
 
 
 
  
 
 
 
  
 
 
  
 
 
  
 
 
  
 
 
 
 
 
 
 
 
 
  
  
 
 
  
 
 
 
 
     
  
 
 
 
 
  
 
 
 
 
 
  
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
  
 
 
 
  
 
 
 
  
 
 
  
 
 
  
 
 
  
 
 
 
 
 
 
 
 
 
  
  
 
  
 
 
 
 
     
  
 
 
 
 
  
 
 
 
 
 
  
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
  
 
 
 
  
 
 
 
  
 
 
  
 
 
  
 
 
  
 
 
 
 
 
Glenn Research Center  at Lewis Field 
ASC-E2 #7 & #8 
•  Slated for durability testing 
Stress components to above-nominal levels  
•  Removable alternator housings for inspection  
 
ASC-E2 #7& #8 with removable alternator housings 
 
 
Heat Collector 
Cold-side Adapter 
Flange (CSAF) 
Removable 
Alternator 
Housings 
Test Description Purpose 
Start/Stop Cycling 
August 2011 
Cycle the convertor repeatedly through start/stop 
cycle to exacerbate any possible wear induced 
before gas bearings become fully functional 
Centrifugal 
Acceleration 
September 2011 
Expose operating convertor to 30 g static load using 
a centrifuge facility to observe response in moving 
components 
Contact Events During 
Launch 
Simulate a limited number of contact events during 
off-nominal launch by adjusting piston amplitude 
Piston Overstroke 
Simulate a limited number of contact events with 
desired  relative velocities between the piston and 
displacer with short-term controller disconnection 
Glenn Research Center  at Lewis Field 
Conclusion 
GRC is supporting life and reliability database for free-piston Stirilng conversion via 
extended convertor operation 
Ongoing convertor operation: 
•   18 convertors (4 TDCs from Infinia, 14 ASCs from Sunpower) 
•   350,000 total convertor hours of operation  
•   218,000 on Infinia units and 132,000 on Sunpower units 
 
Demonstrating steady convertor performance requires precise maintenance of operating 
conditions 
Sources of disruption : 
•  Investigative tests 
Varying operating frequency, hot-end temp, cold-end temp 
•  Hot end control method 
Constant heat input mode requires more user-adjustment than constant temperature mode 
Long-term transients in hot end insulation were observed 
•  Support facility  
Open-bath circulator fluid concentration drifting 
Nuisance shutdowns (instrumentation failure, EMI, power outages) 
Ambient temperature fluctuations due to room HVAC 
Glenn Research Center  at Lewis Field 
Acknowledgements 
This work was funded through the NASA Science Mission Directorate and the 
Radioisotope Power Systems Program Office. Any opinions, findings, and conclusions or 
recommendations expressed in this article are those of the authors and do not necessarily 
reflect the views of the National Aeronautics and Space Administration. 


Extended Operation of Stirling Convertors at NASA Glenn Research Center

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