Selective emitters for thermophotovoltaic power systems for use in aerospace applications

Thermophotovoltaics is the term applied to the technique for energy conversion whereby the energy emitted by an incandescent source is converted to electrical energy by a photovoltaic cell. There are three approaches currently being investigated for practical devices. These are 1) selective filtering, 2) multibandgap conversion, and 3) selective emission. This article will concentrate on selective emission. The selective emitters are made from oxides of the rare Earths such as erbia, holmia, ytterbia, and neodemia. Large area emitters are made through a specialized series of processes that begin with nitrates of the rare Earth and end with rare-Earth oxide filaments. A special requirement for these applications is the need for a robust large area emitter that could take the shock and vibration of space launch or those associated with Army applications. Conventional paper-making techniques have been used to combine materials suitable as binders with the radiating material. As a result, this technique allows the fabrication of large area robust emitters that were heretofore unobtainable. These emitters will be described in some detail, both for fabrication and performance. Nomenclature c{ = first radiation constant, 3.742 x 10 8 W /x,m 4/m2 c2 = second radiation constant, 1.439 x 10 4 //,m K P = power density, W/m2 Px = spectral emissive power/m2/m wavelength interval T = temperature, K e = emissivity A = wavelength cr = Stefan-Boltzmann constant