Experimental screening of salt hydrates for thermochemical energy storage for building heating application

The selection of a suitable salt hydrate for use in a thermochemical energy storage system is challenging. In this work, the most promising salts to store intermediate heat energy were selected and tested. The criteria set are; volumetric energy density of >500 kWh m⁻³ with a dehydration temperature of <100 °C, material cost of <3.5 USD kg⁻¹ (<15 USD kWh⁻¹), melting does not occur during dehydration and safety. Based on that, the salt hydrates SrCl2, MgSO4, Na3PO4, MgCl2 and SrBr2 were selected and tested experimentally. A constant temperature and humidity chamber was used to measure moisture sorption of different salts. The hydration is investigated under the conditions of 20 °C and 20, 30, 40, 60 and 80% RH, while dehydration is investigated under the conditions of 70, 100 and 140 °C using an oven. These dehydration temperatures of <100 °C are suitable for solar application whereas the dehydration temperature of 140 °C is suitable for waste heat. Furthermore, the salts are cycled over 30 times under the hydration conditions of 20 °C, 60% RH with a dehydration temperature of 100 °C. From these results, it is concluded that SrCl2 and SrBr2 are the most promising salts. This study outlines both the advantages and disadvantages of each salt and states the conditions they are most suitable for.