Micro-gap thermo-photo-thermionics: An alternative approach to harvesting thermo-photons and its comparison with thermophotovoltaics

Abstract

This work investigates an alternative to thermophotovoltaics for harvesting thermal and optical energy via photon coupling and thermionic energy conversion. In this device, a heat source is radiatively coupled to a thermionic electron emitter through a nanoscale gap and the electron emitter is coupled to the collector through a microscale gap. The analysis using fluctuational electrodynamics and finite-time thermodynamics shows that for identical thermal radiator and photon-to-electron conversion materials, the output power density in the thermionic device can be more than twice that of the thermophotovoltaic device; the thermionic mechanism can also provide more than 30% improvement in the energy conversion efficiency compared to the thermophotovoltaic device. Moreover, the maximum-power-point voltage in the thermionic device is shown to significantly exceed the conversion material’s band gap, which determines the theoretical upper limit of the open-circuit voltage in a thermophotovoltaic cell. Therefore, the results of this study indicate that thermionic energy harvesting via thermo-photon coupling can be a promising alternative to thermophotovoltaics.