The performance of a 4.4 kW solar receiver/reactor to split carbon dioxide via the isothermal cerium dioxide thermochemical redox cycle is characterized during steady-state operation in a high-flux solar simulator. The solar reactor is the first to implement the isothermal redox cycle. Design innovations for continuous fuel production and gas-phase heat recuperation distinguish it from prior works. During steady-periodic operation at 1750 K, 360 mL min −1 of CO is produced continuously over 45 redox cycles, and up to 95% of the sensible heat of the process gases is recovered. The solar-to-fuel efficiency is 1.64% without consideration of the energy costs of producing nitrogen used as a sweep gas during reduction. With inclusion of the solar energy required to produce N 2 via cryogenic separation, the efficiency is 0.72%. On the basis of the thermodynamic limitations of the cycle and the limited opportunity for increasing reactor efficiency beyond 2%, we conclude that the isothermal approach to split CO 2 or water via a thermochemical metal oxide redox cycle is not attractive for future development. Future research should leverage the demonstrated advances in reactor design that permit continuous fuel production and recovery of the sensible heat of process gases for alternative cycles such as hybrid isothermal reforming/redox cycles or two-temperature metal redox cycles capable of solid-phase heat recovery.

Simulations of coupled light absorption, transport and kinetics processes in a tandem, vertically-stacked particle-suspension reactor design for solar-water splitting.

The design procedure for a 3 kWth prototype solar thermochemical reactor to implement isothermal redox cycling of ceria for CO2 splitting is presented. The reactor uses beds of mm-sized porous ceria particles contained in the annulus of concentric alumina tube assemblies that line the cylindrical wall of a solar cavity receiver. The porous particle beds provide high surface area for the heterogeneous reactions, rapid heat and mass transfer, and low pressure drop. Redox cycling is accomplished by alternating flows of inert sweep gas and CO2 through the bed. The gas flow rates and cycle step durations are selected by scaling the results from small-scale experiments. Thermal and thermo-mechanical models of the reactor and reactive element tubes are developed to predict the steady-state temperature and stress distributions for nominal operating conditions. The simulation results indicate that the target temperature of 1773 K will be reached in the prototype reactor and that the Mohr–Coulomb static factor of safety is above two everywhere in the tubes, indicating that thermo-mechanical stresses in the tubes remain acceptably low.