Salt hydrate-based gas-solid thermochemical energy storage: Current progress, challenges, and perspectives

Due to the prominent advantages of high energy density and long-term energy conservation ability, salt hydrate based gas-solid thermochemical energy storage (TCES) is a promising technology for effectively employing lowgrade energy such as industrial waste heat and minimising fossil fuel-based sources depletion. As an innovative thermal energy storage technology that has drawn great attention from scholars in recent years, it still remains in the stage of a laboratory-scale investigation. This study establishes a reasonable classification of salt hydrates based TCES systems, discusses the properties and performance regulation strategies of materials, types of reactors, applications, heat and mass transfer process, reaction mechanisms, and also provides critical comments and outlooks on this adsorption TCES technology. It is comprehensively elaborated and evaluated in the following steps. (i) The development of various thermochemical materials (TCMs), including pure salts, mixtures of salt hydrates, and composite TCMs of salt/matrix, is summarised and assessed in detail from the perspectives of thermochemical performances such as ESD and cyclability. (ii) The progress of the conceptual design of the reactor and prototype used for vapour/salt gas-solid reaction are presented and analysed. (iii) The existing theoretical models ranging from the views of microcosmic molecular dynamics to macrocosmic reaction kinetics are discussed. (iv) Additionally, the existing challenges regarding salt hydrate-based TCES technology are identified, and the prospects are also provided. This review enables researchers to timely grasp the latest advancements and thus may provide some rewarding insights for future investigations of salt hydrate-based gas-solid TCES and facilitate scholars to achieve better improvements.