Exceptionally high energy storage density for seasonal thermochemical energy storage by encapsulation of calcium chloride into hydrophobic nanosilica capsules

Thermochemical energy storage (TCES) in salt hydrates is a promising method for seasonal thermal energy storage. However, salt hydrates suffer from agglomeration and deliquescence drastically degrading their performance after just a few cycles. Here, modified dry water-style preparation methods are developed for leakage-free microencapsulation of CaCl₂ with hydrophobic fumed silica nanoparticles. Using a novel phase inversion method, as little as 2 wt% of silica is needed for complete encapsulation, ensuring exceptionally high gravimetric energy storage densities up to 98% of pure CaCl₂. With temperature lifts of 30 °C, volumetric energy storage densities up to 1.4 GJ/m³ are shown to be achievable. These values are unprecedented for cycle-stable sorption TCES materials. Optical microscopy, scanning electron microscopy and laser diffraction analysis confirm the encapsulation of CaCl₂ into capsules with volume median diameters ranging from 90 to 210 μm depending on silica content and preparation method. The encapsulated CaCl₂ deliquesces with lower water vapor pressures than pure CaCl₂ and no formation of tetrahydrate and hexahydrate is observed. Despite deliquescence, the encapsulated CaCl₂ is completely stable for at least 30 cycles of charge-discharge. Thus, the new materials show excellent potential for seasonal thermal energy storage.