On the Efficiency of Solar H2 and CO Production via the Thermochemical Cerium Oxide Redox Cycle: The Option of Inert-Swept Reduction

A thermodynamic model of the ceria-based solar thermochemical redox cycle is presented with the objective of resolving the widely varying predictions of the solar-to-fuel efficiency possible with reduction carried out in a flow of inert sweep gas. The implications of the treatment of the gassolid interaction are explored through comparison of mixed flow and countercurrent flow configurations of reactants. The mixed flow model is applied for the first time to both reduction and oxidation reactions. The mechanical work to produce sweep gas of varying purity, separate the products, and pump gases is included. The results identify the conditions necessary for efficient operation. The two models lead to substantially different predictions of the usage of sweep gas and oxidizer and process efficiency. Efficiencies predicted with the conservative mixed flow model reach a maximum of 11% for water splitting at 1073 K, assuming reduction at 1773 K, heat recovery of 80% of the sensible heat of the gases, and an optimistic 75% heat recovery of the sensible heat of the solid ceria. Without solid phase heat recovery, the maximum efficiency is 4%. With the countercurrent model, the predicted solar-to-fuel efficiency reaches 41% for water splitting at 1563 K without solid phase heat recovery. Though theoretically attractive, the countercurrent flow model predicts unattainable efficiencies due to the assumption of chemical equilibrium at both the inlet and outlet of the reduction and oxidation reactors and yields nonphysical results for oxidation temperatures less than 1563 K. Thus, we urge caution in drawing conclusions about the promise of metal oxide cycles on the basis of the countercurrent flow model. Nonetheless, it is reasonable to anticipate that the ceria cycle with inert-swept reduction and a temperature swing between reduction and oxidation may achieve commercially viable solar-to-fuel efficiencies, in between the predictions of the mixed and countercurrent models, through the careful design of reactors to provide gas and solid phase heat recovery and favorable gassolid flow configurations.