Energy Conversion Options for Advanced Radioisotope Power Systems Mohamed S. El-Genk Institute for Space and Nuclear Power Studies and Chemical and Nuclear Engineering Department The University of New Mexico, Albuquerque, NM, 8713 (505) 277 - 5442, mgenk@unm.edu Abstract. Static and dynamic energy conversion technologies for Advanced Radioisotope Power Systems (ARPSs) are reviewed and their impact on the system's total mass and specific electrical power and the amount of 238 PuO 2 fuel needed for the heat source are assessed and compared. Conversion technologies considered are Segmented and cascaded Thermoelectric, Alkali-Metal Thermal-to-Electric Conversion, and Free Piston Stirling Engines (FPSEs) and, for comparison, SiGe thermoelectric. Estimates for a 100 We ARPS indicate that when using SiGe thermoelectric, operating between 1273 K and 573 K, 8 General Purpose Heat Source (GPHS) modules would be required and the system's specific power is ~ 4.6 We/kg. Using STE converters, operating between 973 K and 373 K, 5 GPHS modules are required and the ARPS's specific power is ~ 7.28 We/kg. The next generation STE converters that could operate between 1273 K and 573 K, for a projected system efficiency of 13.8%, decrease the number of GPHS modules needed to 4 and increase the system's specific power to ~ 9.9 We/kg. With cascaded SiGe-STE converters, operating between 1273 K and 373 K, the system's efficiency could be as much as 16%, requiring only 3 GPHS modules, for an estimated specific power of 10.7 We/kg. This specific power is more than twice that for SOA RTG. With the current version 1.0 of FPSEs, the 100 We ARPS needs only two GPHS modules, but its specific power (4.1 we/kg) is slightly lower than that of SOA RTG (4.6 We/kg). Future introduction of versions 1.1 and 2.0 engines, with slightly higher conversion efficiency and significantly lower mass, could increase the system's specific power to ~ 7.5 We/kg, using the same number of GPHS modules as version 1.0 engines. With Na-AMTEC and K-AMTEC, the 100 We ARPS needs 3 and 4 GPHS modules, respectively, for an estimated specific power of 5.3 and 5.8 We/kg, respectively. INTRODUCTION The recent NASA Nuclear Space Initiative (NSI) aims at advancing the conversion technologies for future Advanced Radioisotope Power Systems (ARPSs) in the 100 - 200 We power levels. These ARPSs are enabling for deep space missions and long-duration surface and subsurface exploration of Mars and other planets in the solar system. Going beyond Mars, solar illumination is nil, making the mass and size of a solar power system prohibitively large to be practical. For some surface and subsurface missions, ARPSs could also offer the option of full retention of the helium gas generated by radioactive decay of the 238 Pu isotope fuel (El-Genk and Tournier, 2002 and 2003). The current State-Of-The-Art (SOA) Radioisotope Thermoelectric Generators (RTGs) with SiGe thermoelectric converters have served the U.S. space exploration program very well during more than three decades (Carpenter, 1970; Schock, 1980; and Bennet, Lombardo, and Rick, 1987). However, in order to reduce the cost of future deep space missions and the amount of 238 PuO2 fuel needed, ARPSs need to be lighter and more efficient than SOA RTGs. In order to realize the objectives of the NASA NSI, successful conversion technologies for future ARPSs are those able to demonstrate the system's efficiencies > 10%, and at the same time result in the system's specific powers > 8 We/kg. During the next 5 years, NASA's NSI will provide for the advanced development of promising conversion technologies capable of: (a) increasing the specific powers of ARPRs to twice that of SOA RTGs (~4.5 We/kg, Fig 1.); and (b) demonstrating system's conversion efficiencies that are 2 to 4 that of SOA RTG (~5.7%). Energy conversion technologies currently being considered include Segmented Thermoelectric (STE) (Caillat et al., 1999 and 2000; El-Genk and Saber, 2002; and El-Genk, Saber, and Caillat, 2002) (Figs. 2 and 3), Alkali Metal Thermal- To-Electric Conversion (AMTEC) (Cole, 1983; El-Genk and Tournier, 1998, 2002; Tournier and El-Genk, 1999; Hendricks, Huang, and Huang, 1999; and El-Genk and King, 2001), and Free Piston Stirling Engines (FPSEs) (Dochat, 1992; Wong et al., 1992; Schreiber, 2001; and Thieme, Schreiber, and Mason, 2002). These energy CP654, Space Technology and Applications International Forum-STAIF 2003, edited by M.S. El-Genk © 2003 American Institute of Physics 0-7354-0114-4/03/$20.00 368