Design, analysis, and optimization of a radioisotope thermophotovoltaic (RTPV) generator, and its applicability to an illustrative space mission

Abstract

The paper describes the results of a DOE-sponsored design study of a radioisotope thermophotovoltaic generator (RTPV), to complement similar studies of Radioisotope Thermoelectric Generators (RTGs) and Stirling Generators (RSGs) previously published by the author. To focus the design effort, it was decided to direct it at a specific illustrative space mission, Pluto Fast Flyby (PFF). That mission, under study by the Jet Propulsion Laboratory (JPL), envisages a direct eight to nine-year flight to Pluto (the only unexplored planet in the solar system), followed by comprehensive mapping, surface composition, and atmospheric structure measurements during a brief flyby of the planet and its moon Charon, and transmission of the recorded science data to Earth during a six-week post-encounter cruise. Because of Pluto's long distance from the sun (30–50 A.U.) and the mission's large energy demand, JPL has baselined the use of a radioisotope power system for the PFF spacecraft. RTGs have been tentatively selected, because they have been successfully flown on many space missions, and have demonstrated exceptional reliability and durability. The only reason for exploring the applicability of the far less mature RTPV systems is their potential for much higher conversion efficiencies, which would greatly reduce the mass and cost of the required radioisotope heat source. Those attributes are particularly important for the PFF mission, which — like all NASA missions under current consideration — is severely mass- and cost-limited. The paper describes the design of an RTPV system consisting of a radioisotope heat source, a thermophotovoltaic converter, and an optimized heat rejection system; and depicts its integration with the PFF spacecraft. It then describes the optical, thermal, electrical, and structural analyses which led to that optimized design, and compares the computed performance of an RTPV system to that of an RTG designed for the same mission. Our analytical results indicated that — when fully developed — it could result in a 67% reduction of the heat source's mass, cost, and fuel loading, a 50% reduction of generator mass, a tripling of the power system's specific power, and a quadrupling of its efficiency. And that even greater improvements may be achievable by combining the RTPV radiator with the spacecraft's antenna. The paper concludes by briefly summarizing the RTPV's current technology status and assessing its potential applicability to the PFF mission. For other power systems (e.g. RTGs), demonstrating their flight readiness for a long-term mission is a very time-consuming process, because of the need for long-duration performance degradation tests. But for the case of the described RTPV design, the paper lists a number of factors, primarily its cold (0 to 10 °C) converter temperature, that may greatly reduce the need for long-term tests to demonstrate generator lifetime. In any event, our analytical results suggest that the RTPV generator, when developed by DOE and/or NASA, would be quite valuable not only for the Pluto mission but also for other future missions requiring small, long-lived, low-mass generators.