An Advanced Stirling Radioisotope Generator using a pair of free-piston Stirling convertors is being developed to convert heat from a radioisotope source into electric power as a potential electric power source for NASA science or exploration missions. The Stirling convertor System Dynamic Model (SDM), developed at the NASA Glenn Research Center, provided a model that includes the Stirling cycle thermodynamics, heat flow, gas dynamics, mechanical dynamics, mounting dynamics, linear alternator, and controller. SDM has been used to answer many engineering questions during development of the power system.
The specification for the Stirling convertor power system being developed for NASA by the U.S. Department of Energy requires a pair of convertors to operate in synchronization at a power factor near unity. SDM simulations were performed to examine 10 candidate architectures and to determine which of the 10 could meet the operating requirements. All the configurations involved convertors in the dual-opposed configuration, controlled with active power factor correction (APFC) controller technology. The differences among the candidates included the number of controllers required (one or two), the electrical connections between the convertors (series, parallel, or isolated), and the thermodynamic configuration (isolated or combined working spaces). For the configurations involving isolated electrical output, different methods for synchronizing the two convertors were developed. Of the 10 configurations studied, stable operating modes were found for four. Three of the four had a common expansion space. One stable configuration was found for the dual-opposed convertors with separate working spaces. That connection architecture is being pursued in Glenn’s controller development.
Possible architectures for a dual-opposed Stirling convertor power system using APFC control technology.
SDM was used to evaluate two methods of implementing this architecture. One method uses a frequency-control loop to synchronize the two isolated convertors. The other uses a phase control to accomplish the same result. The phase-control method is being implemented in Glenn’s advanced controller. In August 2006, the phase-control loop implementation was used in the first successful parallel operation of electrically isolated, free-piston Stirling convertors. Two of Glenn’s advanced controllers provided power factor correction of the convertors through the use of power electronics. The phase control kept the convertors synchronized, allowing the dynamic forces to be cancelled. SDM was an important tool in developing the controller systems being implemented in the advanced Stirling Radioisotope Generator and operating in Glenn’s Stirling convertor test cells today. The Glenn SDM uses the Ansoft Simplorer 7.0 platform (Ansoft Corp.).
Find out more about the research of Glenn’s Thermal Energy Conversion Branch: http://www.grc.nasa.gov/WWW/TECB/
Sest, Inc., contacts: Dr. Edward J. Lewandowski, 216–433–5525, Edward.J.Lewandowski@nasa.govLast updated: December 28, 2007
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Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
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