Reactive phase change materials for enhanced thermal energy storage

Abstract

Effective storage and release of low-to-moderate temperature thermal energy (e.g. solar thermal or geothermal) could be transformational for applications such as space heating/cooling, domestic hot water, or off-grid cooking. Good candidates for thermal energy storage in this temperature range include latent heat storage (LHS) systems and thermochemical energy storage (TCES) systems using reversible salt-hydrate dehydration reactions. Here we propose that an energy storage system by use of magnesium nitrate hexahydrate can potentially improve upon independent TCES or LHS systems by utilizing both the thermochemical hydration reaction and the latent heat available through the solid-liquid phase change of one magnesium nitrate hydrate eutectic. This chemistry is investigated through TGA/DSC analysis and shows a total energy density of approximately 1170±94 kJ/kg when dehydrating the material up to 145°C. Reversible latent heat cycling at a eutectic melting temperature of 130°C is shown by the DSC signal and estimated to be on the order of 115±9.2 kJ/kg—a 10% increase over the thermochemical energy storage alone. Although the latent energy release was found to decrease slightly over several cycles, the mass was found to stabilize near an asymptotic value corresponding to the published eutectic composition. These results suggest the concept of reactive phase change materials could be a promising solution to increasing volumetric stored energy density