Thermionic Energy Conversion Efficiencies Using Low Emissivity Materials

In thermionic power systems, thermal radiation heat transfer between the hot emitter and cold collector constitutes a parasitic heat loss which reduces conversion and system efficiencies. After observing that the radiant heat depends linearly on the effective emissivity of the electrode pair, a method of reducing this parasitic heat loss was developed. For the typical case of tungsten emitter and niobium collector, this effective emissivity is 0.17 to 0.20. Some materials such as copper and sodium have total emissivities less than 0.10 for the typical range of collector temperatures used in space power applications. Coating the collector with such a material could potentially reduce the effective apir emissivity significantly. Phase 1 will: identify low emissivity materials which would be compatible with a converter environment; establish requirements to maintain low-emissivity surfaces by controlling emitter evaporation; and measure, in a vacuum, the radiant heat transfer in a thermionic convertor structure incorporating a low-emissivity collector surface. Estimates will be made of the impact on system performance in proposed system concepts. Phase 2, will address issues raised in fabrication and performance stability and will build and test a converter prototype.