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.

A prototype 4 kW solar thermochemical reactor for the continuous splitting of carbon dioxide via the isothermal ceria redox cycle is demonstrated. These first tests of the new reactor showcase both the innovation of continuous on-sun fuel production in a single reactor and remarkably effective heat recovery of the sensible heat of the reactant and product gases. The impact of selection of gas flow rates is explored with respect to reactor fuel productivity and external energy costs of gas separation and pumping. Thermal impacts of gas flow selection are explored by coupling measured temperatures with a computational fluid dynamics (CFD) model to calculate internal temperature distributions and estimate heat recovery. Optimized gas flows selected for operation provide a 75% increase in fuel productivity and reduction in parasitic energy costs by 10% with respect to the design case.

A model to predict the elastic material properties of reticulated porous ceramics (RPCs) based on the microstructural geometry is presented. The RPC is represented by a repeating unit structure of truncated octahedrons (tetrakaidecahedrons) with the ligaments represented by the cell edges. The deformations of the ligaments in the cellular structure under applied loads are used to determine the effective moduli and Poisson's ratio of the bulk material. The ligament cross section is represented as having a Plateau border exterior surface with a cusp half-angle that is varied between 0 and 90 deg, and a Plateau border interior void with a cusp half-angle of zero, representative of the ranges seen in RPCs. The ligament cross-sectional area is permitted to vary along its length and the distance between internal and external cusps is assumed constant. The relative density of the foam, corresponding to the length, cusp distances, and external-cusp half-angle of the ligaments, is determined using solid geometry. The relative density has the dominant effect on the moduli, while normalized ligament length varies the moduli by 11–49% at a specified relative density. The impact of the external shape of a ligament on the relative moduli is insignificant. The model is validated through comparisons with the measured elastic properties of RPCs in the literature and new data. The model is the first to consider the effect of the microstructural features of ligaments of RPCs on the elastic moduli of the bulk material.

This work reports the elastic modulus and four-point flexural strength of a gelcast ceramic, cerium dioxide (ceria), with a microporosity of nominally 20% and a grain size of 11 μm from 23 to 1500 °C. The data augment the sparse data published for ceria and extend previous results by 150 °C. The ceria tested is representative of that constituting the ligaments of a reticulated porous ceramic. The elastic modulus decreases from 90 GPa at 23 °C to 16 GPa at 1500 °C. The flexural strength is 78 MPa below 900 °C and then decreases rapidly to 5 MPa at 1500 °C. These trends are consistent with data reported for other ceramics. Comparing the measured elastic modulus to prior data obtained for lower porosity shows the minimum solid area (MSA) model can be used to extend the modulus data to other porosities. Similarly, the flexural strength data agree with prior data when the effects of specimen size, porosity, and grain size are taken into account.