Advanced Stirling Radioisotope Generator (ASRG) Thermal Power Model in MATLAB

Abstract

This paper presents a one-dimensional steady-state mathematical thermal power model of the ASRG. It aims to provide a guideline of understanding how the ASRG works and what can change its performance. The thermal dynamics and energy balance of the generator is explained using the thermal circuit of the ASRG. The Stirling convertor performance map is used to represent the convertor. How the convertor performance map is coupled in the thermal circuit is explained. The ASRG performance characteristics under i) different sink temperatures and ii) over the years of mission (YOM) are predicted using the one-dimensional model. Two Stirling converter control strategies, i) fixing the hot-end of temperature of the convertor by adjusting piston amplitude and ii) fixing the piston amplitude, were tested in the model. Numerical results show that the first control strategy can result in a higher system efficiency than the second control strategy when the ambient gets warmer or the general-purpose heat source (GPHS) fuel load decays over the YOM. The ASRG performance data presented in this paper doesn't pertain to the ASRG flight unit. Some data of the ASRG engineering unit (EU) and flight unit that are available in public domain are used in this paper for the purpose of numerical studies. IntroductionThe advanced Stirling radioisotope generator (ASRG) (Refs. 1 to 3) is being developed for multimission applications to provide a high-efficiency power source alternative to radioisotope thermoelectric generators (RTGs). The ASRG efficiency could reach 28 to 32 percent, which results in reducing the required amount of radioisotope by roughly a factor of 4 compared to RTGs. Thus, because of the limited supply of Pu-238, utilization of the ASRG can extend radioisotope power available for future space science missions, such as deep-space missions, large planetary surface rovers, and systems in support of human exploration activities.An overview of the ASRG is shown in Figure 1. The ASRG consists of two advanced Stirling convertors (ASCs) enclosed in the housing; each has a general purpose heat source (GPHS) attached at the hot end to provide the heat. A gas management valve (GMV) and pressure relief device (PRD) are located at the top of the housing. The housing with attached fins radiates the heat to the environment. The GMV is used to maintain a near-atmosphere pressure of inert gas inside the housing during ground operations. This gas is permanently vented to vacuum by the PRD when the ambient becomes vacuum. The ASC control unit (ACU) is separate from the ASRG housing. It converts the AC signals from both ASC to 28 to 34 VDC for a typical spacecraft electrical bus. The controller is used to maintain synchronized displacer/piston movement of the two directionally opposed Stirling convertors to minimize induced disturbance to the spacecraft and its precision instrumentation.An ASC consists of a free-piston Stirling converter and an integral linear alternator that converts the piston reciprocating motion to electrical power...