SrBr₂.6H₂O-Based Composites for Efficient and Sustainable Thermal Energy Storage

Scalable, affordable, and sustainable energy storage is essential for advancing renewable electricity adoption, particularly in buildings, which account for 40% of global energy consumption and 60% of electricity use, significantly contributing to greenhouse gas emissions. Thermal energy storage (TES) is efficient for large-scale, long-duration storage, utilizing abundant, low-cost materials like salts. In addition, TES offers a longer lifespan, excels in high-temperature environments by storing heat directly, and has a lower environmental impact, reducing the need for rare materials, making it a more sustainable option for grid-level energy storage. In typical TES processes with salt hydrates, heat causes the salt to release water molecules while absorbing energy, which is stored in chemical bonds. This energy can be retained for extended periods if the environment prevents rehydration. Energy is released when the anhydrous salt is exposed to humid air, allowing water molecules to bond and generate heat. Thermochemical materials (TCMs), particularly inorganic salts like SrCl₂6.H₂O, MgSO₄.7H₂O, and K₂CO₃.1.5H₂O, offer higher energy densities (around 500 kWh/m³) compared to adsorption-based materials (~200 kWh/m³), making them ideal for applications with low charge-discharge temperature requirements. However, the efficiency of salt hydrates as TCMs is limited by poor multi-cyclic performance, with energy density declining after 20-50 cycles due to instabilities like pulverization and agglomeration. Incorporating salts into composites further reduces energy density due to low salt loading, while high salt loading can lead to instability and leakage. Compacting pure salts into specific shapes may improve energy density, but this compromises stability.

This study focuses on SrBr₂.6H₂O composites where we explored the structural integrity, long-term stability, and hydration kinetics at various relative humidity levels using various host matrices such as activated carbon, expanded graphite, and silica gel. SrBr₂.6H₂O composites with activated carbon exhibited faster hydration kinetics, even at low relative humidity (RH) levels. The results show that these composites can be hydrated at RH levels as low as 35% and maintain stable performance for up to 40 cycles. The favorable performance of SrBr₂.6H₂O at low relative humidity (RH) suggests that it is also a suitable candidate for a cascade system. In such a setup, SrBr₂.6H₂O could be used downstream to take advantage of the low-humidity air, which previously had higher RH and was expected to hydrate salts requiring higher moisture levels. This approach can further enhance the overall energy density of the system. Converting the composites into millimeter-sized particles further improved stability, extending cyclic performance to nearly 100 cycles. The energy density achieved by SrBr₂.6H₂O composites was approximately 150 kWh/m³, nearly double that of conventional phase change materials (PCMs) like ice melting (~85 kWh/m³).

In conclusion, SrBr₂.6H₂O composites demonstrate stable performance over multiple cycles and can operate at low RH levels, making them a promising option for thermal energy storage systems. With an energy density of around 150 kWh/m³ and good cyclic stability, these composites offer significant potential for enhancing the energy efficiency of thermal storage systems.