NASA has studied a variety of mission concepts that would use low power conversion technologies able to convert heat from radioisotope heat sources. These studies proposed using fractional versions of the General Purpose Heat Source (GPHS) or multiple Light Weight Radioisotope Heater Units (LWRHU) to heat power conversion technologies for science instruments and communication on miniaturized probes, landers, rovers, and signal repeaters.
The power conversion technologies of interest would ideally be small enough to support a particular space vehicle concept and efficient enough to supply the desired amount of power for the available heat input. Thermoelectrics have been demonstrated at power levels below 50 mW at 5% system efficiency for a design that would utilize heat from a single LWRHU.
Building on that approach, low power Stirling convertors are being developed at NASA Glenn Research Center (GRC) to convert heat from multiple LWRHU to 1-2 watts of usable electrical power for small applications. Development of the concept includes maturation of the major subassemblies, including the Stirling convertor, electronic controller, and multi-layer insulation.
The Stirling convertor design contains a gap regenerator, flexure bearings to constrain the piston and displacer, and a moving coil alternator. Design goals include demonstration of the 1 watt Stirling convertor at system efficiency of 12% using heat that would come from eight LWRHUs.