Solar hydrogen production via thermochemical iron oxide-iron sulfate water splitting cycle

Abstract

This paper reports the thermodynamic analysis of solar H2 production via two-step thermochemical iron oxide–iron sulfate (IO–IS) water splitting cycle. The first step belongs to the exothermic oxidation of FeO via SO2 and H2O producing FeSO4 and H2 and second step corresponds to the endothermic reduction of FeSO4 into FeO, SO2, and O2. The products, FeO and SO2 can be recycled to step 1 and hence, reutilized for the production of H2 via water splitting reaction. Thermodynamic equilibrium compositions and variations in enthalpy, entropy and Gibbs free energy of the thermal reduction and water splitting reactions were computed as a function of reaction temperatures. Furthermore, the effect of molar flow rate of inert Ar (n˙Ar) on thermal reduction temperature (TR) and equilibrium compositions during the thermal reduction of FeSO4 was also examined. Second law thermodynamic analysis was performed to determine the cycle efficiency (ηcycle) and solar to fuel energy conversion efficiency (ηsolar−to−fuel) attainable with and without heat recuperation for varying n˙Ar (0–30 mol/s) and TR (1280–1510 K). Results obtained indicate ηcycle = 39.56% and ηsolar−to−fuel = 47.74% (without heat recuperation) and ηcycle = 51.77% and ηsolar−to−fuel = 62.43% (by applying 50% heat recuperation) at TR = 1510 K.