Thermodynamic Analyses of Fuel Production via Solar-Driven Non-stoichiometric Metal Oxide Redox Cycling. Part 1. Revisiting Flow and Equilibrium Assumptions

We present a thermodynamic model describing the operation of solar thermochemical reduction and oxidation chambers utilizing a non-stoichiometric metal oxide. The system under consideration is a generic reactor implementing an ideal counter-current flow (CF) configuration with prescribed inlet conditions of reactant flow rates and thermodynamic states. Conservation of species and mass as well as Gibbs's criterion are used to determine the maximum and minimum limits of oxygen non-stoichiometry for reduction and oxidation, respectively, under a CF configuration. The methodology presented here is first used to analyze the previous models appearing in literature. It is found that existing efforts to model the CF configuration can violate Gibbs's criterion. Motivated by this, a revised CF model is formulated and ensures the criterion is met thereby ensuring process spontaneity of the desired reaction for all conditions existing within the reaction chambers. The model identifies the highest reduction (oxidation) extent possible for a given inlet condition of the reduction (oxidation) chamber. This work offers an enhanced understanding of the CF flow configuration that will lead to more realistic estimates of the upper limit on solar-to-fuel efficiency for a reactor system.