Investigation on ettringite as a low-cost high-density thermochemical heat storage material: Thermodynamics and kinetics

Ettringite-based thermochemical energy storage has been recommended to be used in buildings for thermal energy supply because of its low material cost (<1000 €/m ³ ), low working temperature and high energy storage capacity. However, several physicochemical properties of ettringite were still not clear, such as reversible reaction enthalpies, thermodynamic equilibrium, and kinetics of the dehydration/hydration reactions. In this study, the dehydration of ettringite was experimentally confirmed as stepwise in TGA-DSC, Ett30.6 → Ett30 → Met 17.8 → Met11.7 → Met7.5, which is consistent with the inference according to the crystal structure. The thermodynamic equilibrium of the reversible reactions presented the impact of water activity on the stability of ettringite at controlled temperatures and water vapor partial pressures. The thermodynamic modeling based on experiment results is consistent with the extrapolated model from the literature. The rehydration enthalpy of ettringite has been confirmed about 1370 kJ/mol, which equals an energy density of 547 kWh/m ³ under the operating conditions of 20 °C and 90% RH.