Reactive Phase‐Change Materials for Enhanced Thermal Energy Storage

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 thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) analysis and shows a total energy density of approximately 1170±94 kJ kg−1 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−1—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 the stored volumetric energy density. Reactive phase‐change material: Seasonal energy storage has vast potential for decreasing energy consumption. Salt hydrates have been investigated previously for thermal storage through either latent (heat of fusion) heat storage or thermochemical (heat of hydration) storage. Demonstrated here is a concept in which a salt hydrate (MgNO3) takes advantage of both latent heat and heat of hydration to increase the energy density of the system by approximately 10 % over using thermochemical storage alone.