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.

The use of concentrated solar energy as a heat source for pyroiysis and gasification of biomass is an efficient means for production of hydrogen rich synthesis gas. Utilizing molten alkali carbonate salts as a reaction and heat transfer media promises enhanced stability to solar transients and faster reaction rates. The present study establishes and compares the reaction kinetics of pyroiysis and gasification of cellulose from 1124 K to 1235 K in an electric furnace. Data are presented in an inert environment and in a bath of a ternary eutectic blend of lithium, potassium, and sodium carbonate salts. Arrhenius rate expressions are derived frotn the data supported by a numerical model of heat and mass transfer. The molten salt increases the rate of pyroiysis by 74% and increases gasification rates by more than an order of magnitude while promoting a product gas composition nearer to thermodynamic equilibrium predictions. These results justify using molten carbonate salts as a combined catalyst and heat transfer media for solar gasification.

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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.

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