Investigations of ZnSO4 Hydrate for Solar Heat Storage Applications
Abstract
The reversible hydration of ZnSO4∙H2O to ZnSO4∙7H2O is a promising way to store solar heat, due to the high enthalpy of hydration from ZnSO4∙6H2O to ZnSO4∙7H2O of ΔhydH6/7 = –58.3 kJ/mol. The hydration behavior was investigated by using isothermal calorimetry at different constant humidities. A lower limit of 70 % RH was chosen which is slightly below the equilibrium humidity for the hydration from ZnSO4∙6H2O to ZnSO4∙7H2O, thus, ZnSO4∙6H2O was expected as the hydration product. The upper limit of 85 % RH was chosen below the deliquescence humidity of ZnSO4∙7H2O, therefore, the heptahydrate was the expected hydration product in this case. It turned out that neither the hydration to ZnSO4∙6H2O at 70 % RH nor the formation of ZnSO4∙7H2O at 75 % RH was achieved whilst a water content above 7 mol H2O per mol ZnSO4 indicated the formation of a solution at 80–85 % RH. The heat of reaction was found to be significantly smaller than the enthalpy of hydration ΔhydH6/7. The heat of reaction for the dehydration was found to be significantly higher compared to the heat of reaction evolved by the hydration and agrees with the theoretical expected value. In addition the hydration of ZnSO4∙H2O was investigated by using Raman microscopy. It turns out that the hydration to the thermodynamic stable products ZnSO4∙6H2O or ZnSO4∙7H2O does not take place instead a solution of ZnSO4∙H2O was formed. Hence the lower heat or reaction evolving during the hydration is caused by the formation of a solution due to a kinetic hindrance of the hydration to the thermodynamically stable products ZnSO4∙6H2O or ZnSO4∙7H2O.
Keywords
Zinc sulfate hydrate, Thermochemical heat storage, Reversible salt hydration