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.