Thermochemical Energy Storage Using Salt Hydrates

We investigate the capability of salt hydrates, using magnesium sulfate heptahydrate as a model salt, to store thermo-chemical energy as they dissociate into anhydrous salts or lower hydrates and water vapor upon heating. When salt hydrates are heated to the temperature required to activate the dehydration reaction, water desorption occurs from the compound. While thermal diffusion governs thermal transport below this reaction temperature, the heat transfer during the dehydration process is influenced by thermochemical kinetics. An anhydrous salt that has relatively higher energy content than its hydrated counterpart can be stably stored over long durations and transported at ambient temperatures. Thus, thermal energy can be released by allowing water vapor to flow across the anhydrous salt, which transforms its chemically stored heat into a sensible form. We model the thermochemical process based on the conservation of mass and energy and a relation describing the chemical kinetics, and employ finite difference technique to solve them. Different cases are considered to provide suggestions to improve the process performance. This storage application has potential for long-term thermal applications, e.g., for storing solar heat during summer months and releasing it in the winter to warm buildings.