Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage

Salt hydrate is one kind of the most promising materials used for both latent heat and thermochemical heat storage systems. Currently, there is a lack of fundamental understanding of the correlation mechanism of salt hydrate as phase change material (PCM) and thermochemical material (TCM). The dehydrating and melting processes of salt hydrates (Na2SO4 center dot 10H(2)O, CH3COONa center dot 3H(2)O, MgSO4 center dot 7H(2)O, and SrBry(6)center dot H2O) were carefully analyzed at different heating rates. The experimental results show that the melt of salt hydrate occurs during its dehydrating process at high heating rates, thus leads to the formation of molten hydrate. Moreover, the molten hydrate can directly dehydrate to lower hydrate with a slower dehydrating rate and higher activation energy. Furthermore, the temperature-pressure equilibrium of salt hydrate is higher than its dehydrating equilibrium line before phase change, but it is closer to the dissolving line. An equilibrium equation is developed for describing the transition process from molten hydrate salt to directly dehydrating into lower hydrate, and also used to explain the equilibrium curve after phase change. These findings of mutual effects between phase change and thermochemical dehydration can provide a deeper understanding on the transition process of salt hydrate for thermal energy storage.