Reactor modeling for two-step solar thermochemical fuel production: a concise review

Solar thermochemical fuel production based on the use of non-volatile, non-stoichiometric redox materials to drive water and carbon dioxide splitting in two-step redox cycles represents a promising approach for efficient solar energy conversion and storage. High theoretical solar-to-fuel efficiencies can be achieved, as reduction and oxidation of redox materials are separated, unlocking the possibility to optimize the thermodynamic conditions of each half-reaction. However, the scale-up of the technology is still hindered by low efficiencies reached by reactor prototypes. In this context, the multiphysics modeling of reactor concepts represents a key tool in the engineering process to find optimal designs and suitable operating conditions. This work presents a comprehensive literature review on reactor modeling, by identifying and discussing the most diffused modeling techniques. Our review unveiled that, in terms of heat and mass transfer modeling, most of the studies are aligned in considering laminar flow conditions and local thermal non-equilibrium between the solid and fluid phases in the reactive porous medium. On the other hand, diverse approaches were proposed to include the chemical reaction and radiative heat transfer in the reactor model. Specifically, the redox reaction is modelled either by assuming thermodynamic equilibrium or by including kinetics, with several expressions proposed up to date. The main literature gap identified in the present study is represented by the limited number of multiphysics models that integrate redox materials other than state-of-the-art ceria, as well as innovative reactor designs aiming at overcoming the main limitations of state-of-the-art systems.