DESIGN AND INTEGRATION OF SMALL RTPV GENERATORS WITH NEW MILLENNIUM SPACECRAFT FOR OUTER SOLAR-SYSTEM

The National Aeronautics and Space Administration's recently inaugurat ed New Millennium program, with its emphasis on miniaturized spacecraf t, has generated interest in a low-power (10-30 W), low-mass, high-eff iciency RTPV (Radioisotope Thermophotovoltaic) power system. This led to a Department of Energy (DOE)-sponsored design study of such a syste m, which was assigned to OSC (formerly Fairchild) personnel, who have been conducting similar studies of a 75 W RTPV system for the Pluto Ex press Mission, with very encouraging results. The 75 W design employed two 250 W general purpose heat source (GPHS) modules that DOE had pre viously developed and safety-qualified for various space missions. The se modules were too large for the small RTPVs described in this paper. To minimize the need for new development and safety verification stud ies, OSC generated derivative designs for 125 W and 62.5 W heat source modules containing identical fuel pellets, clads, impact shell and th ermal insulation. OSC also generated a novel heat source support schem e to reduce the heat losses through the structural supports, and a new and much simpler radiator structure, eliminating the need for honeyco mbs and heat pipes. OSCs previous RTPV study had been based on the use of GaSb PV cells and spectrally selective IR filters that had been pa rtially developed and characterized by Boeing (now EDTEK) personnel. T hey had supplied us with spectral data on filter reflectivities and ce ll quantum efficiencies. Two sets of data were furnished: one based on actual measurements made in 1993, and a more optimistic set based on projected performance improvements. Even the measured data set yielded significantly better system performance than present thermoelectric s ystems, but the projected data yielded much better system performance. Because of these encouraging results, OSC in the fall of 1994 initiat ed an experimental program at EDTEK to develop improved filters and ce lls, to demonstrate how much improvement can actually be achieved. OSC requested that first priority be given to filter improvements, becaus e our system studies indicated that improved filters would have a much greater effect on system performance than cell improvements. By July 1995 EDTEK had achieved about 90% of the filter performance improvemen t projected in 1993. Work on further filter and cell improvements is c ontinuing 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 both an electrical heater and a radioisotope heat source. The improve d filter performance data have been applied to the design of low-power (10-30 W) RTPV power systems, for possible application to new millenn ium spacecraft for missions to the outer solar system, where solar pow er generation is impractical. The results reported in this paper indic ate that such systems can yield very attractive performance with the R TPV generator integrated with the miniaturized new millennium spacecra ft. (C) 1998 Elsevier Science Ltd. All rights reserved.