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

Abstract: 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.

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