Redox Materials and Related Processes for Solar Thermal Energy Conversion

Abstract

Coupling concentrated sunlight to suitable sequences of thermochemical reactions enables the production of fuels by water and CO2 splitting and the thermochemical separation of air. One of the major barriers of those processes is the identification of suitable redox materials exhibiting satisfactory durability, reactivity and efficiencies. Solar thermochemical redox processes utilize metal oxides like ceria or perovskites as reactive intermediates. An analytical thermodynamic model is developed to relate material properties to process performance. Perovskites like SrFe0.95Cu0.05O 3-δ and Ca0.8Sr0.2MnO 3-δ show large gravimetric oxygen storage capacities in air. The experimentally measured re-oxidation reaction of those materials is one order of magnitude faster than with state-of-the-art materials. The use of particulate redox materials enables to decouple the reduction from the oxidation reaction and the absorption of solar radiation from the reaction and a counter current heat exchanger design. A numerical model was developed to analyze the heat losses of a particle conveyor using an insulated container in a cycled operation. The model calculated that temperature losses reach values of about 2-4K per lifting. A detailed model of the heat exchanger calculates the process performance considering temperature dependent material data and other factors. The system reaches a heat recovery rate over 70% in case of six stages, connected in a quasi-counter-current principle.