Calcium sulfate hemihydrate and calcium sulfate [$CaSO_4$·(1/2)$H_2O$/$CaSO_4$] reaction cycle having both the endothermic (dehydration) and exothermic (hydration) reactions has been used for thermal energy storage system at relatively low temperature around 100℃. The energy storage characteristics has been studied in a closed system having two interconnected vessels - a cylindrical reaction chamber (0.16 m i.d. X 0.17 m high.) and a condenser/evaporator (0.07 m-i.d. X 0.38 m-high).
In the heat storage process (dehydration reaction), the dehydration rate of $CaSO_4ㆍ(1/2)H_2O$ is strongly influenced by the driving temperature potential and the bed depth. Reaction temperature decreases with increasing the driving temperature potential. The flow of vapor in the reactant bed decreases with increasing bed depth and consequent decrease in the reaction rate with longer reaction time. The optimum bed depth and dehydration temperature are found to be 114 mm and 90~105℃, respectively.
In the heat release process (hydration reaction), as the steam pressure in the evaporator and the initial temperature in the reactant bed are increased, the bed temperature increases with the progression of hydration reaction. The hydration rate of $CaSO_4$ is controlled by diffusion of vapor and exhibits a maximum value at the temperature range 70~80℃. The optimum bed depth for the heat release process is also 114 mm. The overall heat transfer coefficient has been found to be in the range of 800~1200W/㎡.K.
Deactivation of $CaSo_4$ㆍ(1/2)$H_2O$/$CaSO_4$ in the thermal cycles has not been observed in the present system. Therefore, this system can be utilized for thermochemical driven heat pump as a thermal storage system for off-peak electric power.
