Advanced Radiative Emitters for Radioisotope Thermophotovoltaic Power Systems

Radioisotope Power Systems (RPS) are critical for future space and planetary exploration missions. Small improvements in the RPS performance, weight, size, and/or reliability can have a dramatic effect on the scientific capability of the vehicle and the overall mission costs. Radioisotope thermophotovoltaic (RTPV) energy converters are a particular type of RPS that directly convert the heat produced by a general purpose heat source to electrical power using a specialized photovoltaic (PV) cell. A key element in an RTPV system is the radiative emitter that converts thermal energy to radiative energy that illuminates the PV cell. In this project, Creare and the Massachusetts Institute of Technology (MIT) propose further development of an advanced, 2-D, photonic crystal radiative emitter optimized for RTPV systems that provides high emittance matched to the bandgap of the PV cell with low emittance elsewhere that will provide high system efficiency. In Phase I, we designed, fabricated, and tested prototype emitters. In Phase II, we will improve and scale up the fabrication processes, and fabricate larger, improved test samples, which will be fully characterized for high-temperature emittance and durability. We will also assess the impact of this new emitter on the overall RTPV system design and performance.

RTPV and the associated technology proposed on this project have a number of potential non-NASA government uses. Radioisotope power sources based on thermoelectrics have been used for terrestrial military applications for many years. For example, they are used to provide power for deep sea monitoring instruments deployed by the Navy. They have also been used in remote locations for monitoring stations. RTPV could be a credible alternative in all these applications. Small radioisotope batteries based on TPV have been proposed and are being developed for a number of military sensing applications. The technology being developed on this project has the potential to benefit these ongoing efforts.

NASA has a critical, long-term need for deep space power systems based on the decay of radioisotopes and the resulting thermal energy produced. These systems are critical for flagship missions to explore the outer solar system for both spacecraft and rovers. The current radioisotope converters based on thermoelectric technology are inefficient and have low mass specific power. Radioisotope thermophotovoltaic (RTPV) energy converters have the potential to be an attractive alternative to competing radioisotope energy converter technology. RTPV has the potential to provide comparable or higher mass specific power, at similar (probably lower) conversion efficiency.

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