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. Instead of conducting a generic study, it was decided to focus the design effort by directing it at a specific illustrative space mission, Pluto Fast Flyby (PFF). That mission, under study by JPL, envisages a direct eight-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 post-encounter cruise lasting up to one year.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 thermal, electrical, and structural analysis 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 thatwhen fully developed -it could result in a 60% 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.The paper concludes by briefly summarizing the RTPV's current technology status and assessing its potential applicability for 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. 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.

Advanced Thermophotovoltaic Devices for Space Nuclear Power Systems

“…The use of TPV modules for radioisotope powered space systems has been previously invested (Chubb, 1993, Schock, 1994, and Schock, 1996. However, the performance of the TPV modules used in these reports was inferior to that given in Figs.…”

“…Radioisotope Nuclear (Fission) Effective System Radiator Emissivity 1 0.9 0.9 System Radiator Mass/Area (kg/m 2 ) 1.8 (Chubb, 1993 andSchock, 1994) 1.8 (Chubb, 1993 andSchock, 1994) Module to Radiator ∆T (K) 12 50 Converter Efficiency Engineering Losses (%) 17 17 Converter Power Density Engineering Losses (%) 7 7 GPHS Unit Mass 2 (kg) 2.08 --GPHS Unit Thermal Power (W t ) 250 (Schock, 1996) --As shown in earlier TPV radioisotope studies, the system radiator can be a large portion of the size and mass of the entire system. Figure 4 shows the system radiator and mass for a system using 2 and 3 GPHS units as a function of the system radiator and environment temperature.…”

“…Previously, TPV has been investigated for use in radioisotope powered systems (Chubb, 1993, Schock, 1994, and Schock, 1996. However, the radiant heat conversion efficiency of the converter radiator/modules considered was relatively low (< 20%).…”

Thermophotovoltaic Energy Conversion for Space Applications

“…The combination of nuclear power and TPV will give rise to an innovative electric power system that would enable many space missions and applications that were previously thought of as impossible or impractical. This has been studied recently and reported in 2003 by Lockheed Martin-KAPL (Brown, et al, 2003), which extended the 1994 Teledyne study of a radioisotope TPV generation (Schock, et al, 1994) by the use of a tandem spectral filter and up-to-date measured efficiency data for GaSb diodes. The result was a design value of 12.12 We/kg for space applications.…”

Analysis and optimization of In_1-xGaxAsyBb_1-y thermophotovoltaic cells under low radiator temperatures

“…For efficient RTPV conversion, the radioisotope source need a number of important requirements such as: high temperature of the heat source, decays without too much gamma or neutron emission, a long half-life of several years or decades so that extended missions are supported and high decay energy per isotope mass. Some current RTPV systems use 238 Pu isotope as a heat source since, in a pure form, 238 Pu can reach surface temperature of 1300°k [7], others uses 238 PuO2 with molybdenum multi-foil insulations which can withstand hot side temperature up to 1500°k [5,8]. However, recently published works show that the 90 Sr isotope meets all the above mentioned requirements [9].…”