Screening double salt sulfate hydrates for application in thermochemical heat storage

Abstract

In this paper a relatively new class of thermochemical compounds is tested for their heat storage potential. 24 different double salt sulfate hydrates with the general formula AI2BII(SO4)2·nH2O were synthesized and screened for the desirable properties of a thermochemical heat battery material. The materials were tested on the following set of criteria: energy density ≥ 1.3 GJ/m3, dehydration temperature ≤ 120 °C, 10-fold cyclability at PH2O ≤ 12 mbar. The 24 salts have dehydration temperatures between 55 and 198 °C and energy densities between 1.1 and 2.0 GJ/m3. (NH4)2Zn(SO4)2·6H2O is the only material that passes all the set criteria and is therefore suitable for further investigation. This material has an energy density of 1.78 GJ/m3, a dehydration temperature of 84 °C after one dehydration-hydration cycle and can perform at least 10 cycles without loss of performance. There are five other salts of interest that meet two out of the three criteria. (NH4)2Ni(SO4)2·6H2O has a 1.8 GJ/m3 energy density and is cyclable 10 times, but has a dehydration temperature of 132 °C. (NH4)2Fe(SO4)2·6H2O, (NH4)2Mg(SO4)2·6H2O, Cs2Mg(SO4)2·6H2O and Cs2Ni(SO4)2·6H2O have energy densities of 1.6 to 1.76 GJ/m3 and dehydration temperatures below 120 °C, but they require 22.7 mbar to achieve cyclability.