Optimizing supercooling and phase stability by additives in calcium chloride hexahydrate for cyclical latent heat storage

Abstract

Calcium chloride hexahydrate (CaCl2·6H2O) based phase change material (PCM) is promising for latent heat storage due to its favorable thermal properties, but supercooling and thermal cyclic instability limits its wider application. We examine the role of nucleation agents, in particular strontium chloride hexahydrate (SrCl2·6H2O), as an additive to the PCM, on the degree of supercooling and the latent heat. Utilizing a water bath and a cooling chamber, we systematically evaluate the degree of supercooling in various PCM samples, while X-Ray diffraction and microscopic analysis are employed to characterize the role of the additive. The results suggest that particle shapes of the additives and their crystallographic phase contribute significantly to supercooling suppression. Optimizing a tradeoff between supercooling and latent heat reduction for long-term nucleation performance, we identify designer PCM composition with an optimal 3 wt% of SrCl2·6H2O for an automated thermal cycling system by differential scanning as well as drop calorimetry. Incorporating 2 wt% potassium chloride (KCl) shifts the composition at the peritectic point away from the unfavorable CaCl2·4H2O phase, enhancing the freeze-thaw cycle stability of CaCl2·6H2O. Notably, the latent heat variation is ∼0.08% over 1000 freeze-thaw cycles.