Gas–solid thermochemical heat storage reactors for high-temperature applications

Reversible reactions exhibit considerable potential for thermal energy storage because of their high energy density and capability for long-term storage at ambient temperature. This paper presents the research progress on gas–solid thermochemical heat storage reactors and their corresponding systems. The comprehensive state-of-the-art knowledge on gas–solid thermochemical reactors, namely, packed bed, continuous, and direct-type reactors, for high-temperature heat storage applications is reviewed. Up till now, the performance of packed bed reactors has been extensively investigated. However, the intrinsic drawbacks of packed bed reactors limit their applications. Continuous and direct-type reactors can efficiently store heat, but studies on these reactors are still on the stage of material characterization and prototype designing. Various numerical studies have successfully predicted the reaction trends in the three reactors to elucidate their performances and features. In these studies, porous thermochemical materials are studied on the scale of representative element volume. So far, numerical or experimental approaches have been rarely used to investigate physical and chemical processes at the particle scale. Energy and exergy analyses on conceptual thermochemical heat storage systems came into existence recently. In the future, more efficiency analyses based on practical experimental results are required.