Limit of efficiency for photon-enhanced thermionic emission vs. photovoltaic and thermal conversion

Conversion of sunlight by photon-enhanced thermionic emission (PETE) combines a photonic process similar to photovoltaic cells, and a thermal process similar to conventional thermionic converters. As a result, the upper limit on the conversion efficiency of PETE devices is not the same as the Shockley– Queisser (SQ) limit that corresponds to the bandgap of the absorbing material, nor to the Carnot efficiency corresponding to its temperature. Here we analyze the upper limit on efficiency of ideal PETE devices in several possible configurations, in comparison to ideal photovoltaic cells and ideal solar thermal converters. Isothermal PETE converters are shown to be restricted to less than the SQ limit, but non-isothermal devices can exceed this limit. The limit of efficiency increases with the flux concentration reaching for example 52% at concentration of 1000 suns. Spectral splitting leads to a modest increase in conversion efficiency to 56% at 1000 suns. Addition of a secondary thermal cycle increases the efficiency limit for all PETE configurations, up to 69.8% and 70.4% for the cases of isothermal PETE and a dual bandgap PETE system at 1000 suns. & 2015 Published by Elsevier B.V.