Thermodynamic study on solar thermochemical fuel production with oxygen permeation membrane reactors

Thermodynamic analysis is performed on a conceptual oxygen permeation membrane reactor driven by concentrated solar energy (heat) for isothermal H₂O splitting for the purpose of solar fuel derivation. By way of a plug-flow reactor model, kinetic and thermodynamic factors responsible for conversion rate, reactor dimension, and solar-to-fuel efficiency are analyzed for the case of pump-assisted and methane-assisted scenarios. The pump-assisted case achieves the same solar-to-fuel efficiency (2.9% at 1500°C) as isothermal solar thermochemical cycling, while the methane-assisted case attains much higher efficiencies at much lower temperatures, whose net solar-to-fuel efficiency reaches 63% at around 900°C. The theoretical framework developed in this study can be applied to the solar thermochemical splitting of other gases such as CO₂ and can be further extended to the co-splitting of H₂O and CO₂ for syngas production driven by solar energy only (i.e., without the participation of hydrocarbon fuels).