Thermodynamic assessment of solar photon-enhanced thermionic conversion

Photon-enhanced thermionic conversion, an innovative solar power technology, combines photovoltaic and thermionic effects into a single process, and has the potential to surpass the Shockley-Queisser limit and conventional photo-thermal limit. However, there is little understanding about the energy conversion process from a thermodynamic point of view. A detailed thermodynamic model is proposed, encompassing energy and exergy balance, and entropy analysis to evaluate a process for solar photon-enhanced thermionic conversion. The correlation of photons, phonons and electrons is presented, as well as the energy transfer pathway in solar thermionic conversion. The total solar-to-electricity efficiency of energy and exergy are 54.32% and 58.42%, respectively, for a photon-enhanced thermionic converter combined with a Carnot engine, at a 1.20 eV bandgap with an electron affinity of 1.20 eV when the concentrated solar flux is 500 kW/m2. The combination of photoexcitation and thermalization facilitates the overall thermionic emission exergy ratio up to 62.36%, higher than that of conventional thermionic conversion by 10.92%. Temperature-entropy diagrams with quantitative analysis are proposed for the thermodynamic processes of thermionic and photon-enhanced thermionic conversion. The electron fluid cycles from the Fermi level of the anode back to the valance band of the cathode with a reduced entropy, while being thermalized from the conduction band in photon-enhanced thermionic conversion, contributing to the entire conversion of photoexcited energy to electricity.