MS13-P14 Structural Studies of Crystalline Phases Occurring when Temperature Cycling a Stabilised System of Sodium Sulfate Decahydrate Emily Goddard (EaStChem School of Chemistry, University of Edinburgh, Edinburgh, United Kingdom) David Oliver (Sunamp, Macmerry, United Kingdom) Colin Pulham (EaStChem School of Chemistry, University of Edinburgh, Edinburgh, United Kingdom)email: hydrates represent a class of compound with a high latent heat of energy; they absorb a large amount of heat upon melting and release it upon re-crystallisation. They form part of a group of materials known as phase-change materials (PCMs), which are of great interest in the field of heat storage. Each PCM has a defined melting point and energy density and can therefore be tailored for different applications. The materials should show a stable performance over thousands of charge–store–discharge cycles, with no degradation in performance. They should also nucleate reliably and crystallise at a suitable rate. However, few systems fit this profile.

Sodium sulfate decahydrate (Na2SO4.10H2O) is a good candidate for storing low-grade solar energy. However, it melts incongruently, decomposing into a mixture of the anhydrous Na2SO4 and a solution of salt in water. Upon formation, the dense anhydrous salt precipitates out to the bottom of the container, decreasing the concentration of the active PCM in the system, so the amount of heat stored decreases with each cycle. Various approaches have been trialled to supress the incongruent melting and allow the system to be used as a PCM.

This poster describes the development of a method that supresses the formation of the anhydrous salt, allowing the system to be cycled reliably, reversibly and reproducibly. Crystallographic techniques were used to identify and characterise the various crystalline phases that appeared during temperature cycling of the system.

Keywords: heat storage, phase-change materials, incongruent melting