A thermophotovoltaic (TPV) system consists of a narrow-bandgap photovoltaic cell (about 0.7 eV) that is illuminated by black – or greybody radiation from a hot source but at a lower temperature than the sun [Couts, 2001]. In order to give an advantage, thermal emission incident on the cell must be filtered by a selective emitter that only passes light just above the cell bandgap with photons at other energies above or below the bandgap, reflected back to the emitter. Hence, each photon’s energy or excess energy respectively, is still utilised in reheating or maintaining the temperature of the emitter. This approach would normally use waste heat from an industrial process or similar and hence not be a solar cell, but it can be coupled to an emitter heated by solar thermal energy. In practice, the large number of different elements with their multiplying inefficiencies, and the need to thermally insulate some elements from others, makes the approach very difficult to optimise.