Free-Piston Stirling power conversion has been identified as a viable option for potential fission power systems on planetary surfaces. Studies examined the use of Stirling convertors coupled to a low-temperature (< 900K), uranium-dioxide fueled, liquid-metal-cooled reactor for potential surface application. The system is considered a low development risk based on the use of terrestrial-derived reactor technology and conventional materials. All required materials and components will be compatible with expected planetary surface environments.
NASA has begun long lead technology development on multi-kilowatt Stirling power conversion under the Nuclear Systems Project in the Space Technology Mission Directorate Game Changing Development Program.
Before beginning development of full-scale technology demonstration Stirling Engines, a pair of 1-kWe free-piston Stirling power convertors was modified into a thermodynamically coupled configuration. Performance map testing was conducted over a range of conditions using a pair of commercially available AC power supplies to regulate the convertors via bus voltage and frequency control. This was done 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. As follow-on work, a GRC designed Power Management and Control (PMAD) system was built and tested with the engines. Tests confirmed that these engines could be operated in the same manner as previously demonstrated with the AC power supplies. The results of these tests are guiding controller development and instrumentation selection for the higher power Stirling engine Test Demonstration Unit (TDU).
Sunpower, Inc., has subsequently developed and tested a full scale thermodynamically coupled Power Conversion Unit (PCU) for the TDU. The PCU consists of two 6 kWe Stirling engines thermodynamically coupled at their hot ends. The PCU has been operated at 12 kWe with hit supplied by an electrically-heated test heater head at Sunpower The engines will be decoupled and reassembled to liquid metal NaK heater head prior to integration into the TDU.
Four main activities related to Stirling are:
- Technology Demonstration Unit (TDU)
- Liquid Metal Loop Integration
- Alternator/Power Management and Distribution (PMAD) Integration
- Convertor Scale-Up