Utilization of MMRTG's "Waste Heat" to Increase Overall Thermal to Electrical Conversion Efficiency

Since the launch of the first radioisotope power system ( RPS) on Transit 4A in 1961, numerous research and development activities have been performed centered on increasing the overall thermal to electrical conversion efficiency of the selected nuclear fueled power system. The latest U.S. fielded RPS ( MMRTG - Multi-Mission Radioisotope Thermoelectric Generator) contains eight GPHS ( General Purpose Heat Source) modules which nominally yield similar to 2000W(Th) from the thirty-two (PuO2)-Pu-238 ceramic fuel pellets. MMRTG's PbTe/TAGS-85 based thermoelectric couples have a thermal to electrical conversion efficiency of similar to 5.5% thus yielding similar to 110W(e) at launch. This paper centers on the discussion of a conceptual idea that entails employing a second set of thermoelectrics on an MMRTG. These would be employed for possibly converting a portion of the excess "waste heat" (similar to 1750 W-th) into additional electrical mission power. First-order experiments and calculations employing bismuth telluride ( Bi2Te3) based thermoelectric modules being considered for a European RPS by the University of Leicester indicate that an improvement in the efficiency of an MMRTG could be achieved by integrating them with the PbTe/TAGS-85 thermoelectrics being utilized in the MMRTG in a "dual" or "cascaded" arrangement. This arrangement of two different integral thermoelectric materials suggests the intriguing possibility of the additional harvesting of a portion of the MMRTG's currently unutilized "waste heat". This appears to be feasible since the cold side temperature of the PbTe/TAGS85 in an MMRTG is similar to 200 degrees C which corresponds to typical hot-side operating temperatures of the Bi2Te3 thermoelectric modules. It is recognized that extensive thermo-mechanical-electrical design, modeling, and analysis are required to fully investigate the cascaded thermoelectric concept. In addition, materials compatibility and assembly aspects will need to be fully addressed in the future. The present work indicates that system-level performance gains could be achieved via a "cascaded" or cMMRTG which could result in electrical power increases at BOM ( Beginning of Mission) of up to similar to 25%, and perhaps more significantly, gains in EODL ( End of Design Life) power of up to similar to 40% which could be utilized on a future space mission.