Solar Thermochemical Hydrogen Production via Terbium Oxide Based Redox Reactions

The computational thermodynamic modeling of the terbium oxide based two-step solar thermochemical water splitting (Tb-WS) cycle is reported. The 1st step of the Tb-WS cycle involves thermal reduction of TbO2 into Tb and O-2, whereas the 2nd step corresponds to the production of H-2 through Tb oxidation by water splitting reaction. Equilibrium compositions associated with the thermal reduction and water splitting steps were determined via HSC simulations. Influence of oxygen partial pressure in the inert gas on thermal reduction of TbO2 and effect of water splitting temperature (T-L) on Gibbs free energy related to the H-2 production step were examined in detail. The cycle (eta(cycle)) and solar-to-fuel energy conversion (eta(solar-to-fuel)) efficiency of the Tb-WS cycle were determined by performing the second-law thermodynamic analysis. Results obtained indicate that eta(cycle) cycle and eta(solar-to-fuel) increase with the decrease in oxygen partial pressure in the inert flushing gas and thermal reduction temperature (T-H). It was also realized that the recuperation of the heat released by the water splitting reactor and quench unit further enhances the solar reactor efficiency. At T-H = 2280 K, by applying 60% heat recuperation, maximum eta(cycle) of 39.0% and eta(solar-to-fuel) of 47.1% for the Tb-WS cycle can be attained.