Thermodynamics and Transport Phenomena of Thermochemical Systems for Solar Fuel Production

Abstract

Two-step solar thermochemical water splitting is a promising pathway for renewable fuel production due to its potential for high solar-to-fuel efficiency via full-spectrum sunlight utilization. However, such a promise critically relies on simultaneous innovation in the redox materials and the reactor systems that utilize them. The present research aims to gain a fundamental understanding of the process thermodynamics and transport phenomena of the solar thermochemical systems. This will help guide material development and reactor design towards achieving an unprecedentedly high solar-to-fuel thermal efficiency. With more materials and reactors being developed, thermodynamic analysis serves as a critical starting point to explore the maximum efficiencies of their various combinations. Materials under study are the state-of-the-art metal oxides including pure ceria, Zr-doped ceria and doped lanthanum manganite perovskites, while reactors of interest are the conventional and membrane counterflow types. Previous studies typically employed a simplified equilibrium approach that could overpredict the fuel output as well as the thermal efficiency. Herein, a revised model is developed to offer more accurate performance upper limits based on the first and second laws of thermodynamics. It is found that the conventional counterflow reactor system is far more efficient than the membrane type, while pure and Zr-doped ceria outperform the perovskites under most scenarios. In addition, the effects of hypothetical materials are investigated in order to guide future material design. A global efficiency map is presented for all redox materials, revealing important tradeoffs due to competing effects such as thermodynamic favorability, heat losses, sweep gas and oxidizer supply, as well as metal oxide preheating. An optimal material regime is thus identified for a set of system conditions, leading to peak efficiency that could reach 46%. The conventional counterflow reactor is further studied from the transport phenomena viewpoint to offer more realistic performance. A single tube reactor composed of a downward particle flow against an upward inert gas flow is employed as the model system with ceria reduction being the model reaction. Coupled phenomena of mass and momentum transfer as well as chemical kinetics are simulated based on Euler-Lagrange approach in the dilute particle flow regime under isothermal operation. The model predicts the reduction extent under a variety of design and operational conditions, with the critical conversion-limiting factors also being identified. This numerical work corroborates the first-stage thermodynamic counterflow model and paves the way for the development of a two-phase heat transfer model in the near future. Show more