Cascaded Thermoelectric Converters for Advanced Radioisotope Power Systems

Abstract

Three Cascaded Thermoelectric Converters (CTCs) are optimized for potential use in Multi-Mission Advanced Radioisotope Power Systems (MM-ARPS) for electrical powers up to 1 kW e , or even higher, in support of 7-10 year missions. The peak efficiencies of these CTCs of 9.43% to 14.32% are 40% to 110% higher than that of SiGe in State-of-the-Art (SOA) Radioisotope Thermoelectric Generators (RTGs). Such high efficiencies would significantly reduce the amount of 238 PuO 2 fuel and the total system mass for a lower mission cost. Each CTC is comprised of a SiGe top unicouple that is thermally, but not electrically, coupled to a bottom unicouple with one of the following three choices of thermoelectric materials: (a) p-leg of TAGS-85 and n-leg of 2N-PbTe; (b) p-leg of CeFe 3.5 Co 0.5 Sb 12 and nleg of CoSb 3 ; and (c) segmented p-leg of CeFe 3.5 Co 0.5 Sb 12 and Zn 4 Sb 3 and n-leg of CoSb 3 . The length of the top and bottom unicouples is 10 mm, but the cross-sectional areas of the n-and p-legs of the unicouples are optimized for maximum efficiency operation. They vary with the thermal power inputs of 1, 2, and 3 W th per SiGe unicouple, and the heat rejection temperature of 375 K, 475 K, and 575 K, from the bottom unicouple. Such geometrical optimization is at nominal hot shoe temperature of 1273 K for the SiGe unicouple and cold shoe temperature of either 780 K or 980 K, depending on the materials of the bottom unicouples. The hot shoe temperature of the bottom unicouples is 20 K lower than the cold shoe of the top SiGe unicouple, but the rate of heat input is the same as the rate of heat rejection from the top unicouple. The present results are conservative as they assume a contact resistance of 150 µΩ-cm 2 per leg for the top and the bottom unicouples in the CTCs; however, decreasing this resistance to 50 µΩ-cm 2 per leg could increase the current efficiency estimates by an additional 1 -2 percentage points.