Design of RTPV generators integrated with new millennium spacecraft for outer solar system

Abstract

The National Aeronautics and Space Administration’s recently inaugurated New Millennium program, with its emphasis on miniaturized spacecraft, has generated interest in a low‐power (10‐ to 30‐watt), low‐mass, high‐efficiency RTPV (Radioisotope Thermophotovoltaic) power system. This led to a Department of Energy (DOE)‐sponsored design study of such a system, which was assigned to OSC (formerly Fairchild) personnel, who have been conducting similar studies of a 75‐watt RTPV system with very encouraging results. The 75‐watt design employed two 250‐watt General Purpose Heat Source (GPHS) modules that DOE had previously developed and safety‐qualified for various space missions. These modules were too large for the small RTPVs described in this paper. To minimize the need for new development and safety verification studies, OSC generated derivative designs for 125‐watt and 62.5‐watt heat source modules containing identical fuel pellets, clads, impact shell, and thermal insulation. OSC also generated a novel heat source support scheme to reduce the heat losses through the structural supports, and a new and much simpler radiator structure, employing no honeycombs or heat pipes. OSC’s previous RTPV study had been based on the use of GaSb PV cells and spectrally selective IR filters that had been partially developed and characterized by Boeing (now EDTEK) personnel, who had supplied us with spectral data on filter reflectivities and cell quantum efficiencies. Because of the very encouraging results of our system design studies, OSC in the fall of 1994 initiated an experimental program at EDTEK to develop improved filters and cells, to demonstrate how much improvement can actually be achieved. OSC requested that first priority be given to filter improvements, because our system studies indicated that improved filters would have a much greater effect on system performance than cell improvements. By September 1995 EDTEK had achieved about 94% of the filter performance improvement projected in 1993. The improved filter performance data have been applied to the design of low‐power (10–30 watt) RTPV power systems, for possible application to New Millennium spacecraft for missions to the outer solar system, where solar power generation is impractical. The results reported in this paper indicate that such systems can yield very attractive performance. The option of integrating the RTPV generator with the miniaturized New Millennium spacecraft, by using the spacecraft’s large antenna as a radiator for rejecting the generator’s waste heat to space, is also described and analyzed. Work on further filter and cell improvements is continuing at EDTEK, as part of a joint effort with OSC and with DOE’s Mound Laboratory to develop and test a prototypic RTPV generator, with an electrical heater which will subsequently be replaced with a radioisotope heat source.