Experimental modifications for optimizing supercooling and phase separation in phase change materials: Calcium chloride hexahydrate and barium hydroxide octahydrate eutectic hydrate salts

Low-cost composite phase change materials (PCMs) are promising candidates for large-scale thermal energy storage and industrial waste heat utilization. In this study, two eutectic hydrated salt PCMs, CaCl₂·6H₂O and Ba(OH)₂·8H₂O, are prepared, modified, and characterized to improve thermal performance and cycling stability. The CaCl₂·6H₂O-based composite PCM is formulated using industrial-grade CaCl₂·6H₂O, MgCl₂·6H₂O, SrCl₂·6H₂O, and hydroxyethyl cellulose (HEC). The influence of SrCl₂·6H₂O at 1 wt%, 2 wt%, and 3 wt% on supercooling was evaluated, with 2 wt% SrCl₂·6H₂O exhibiting the most effective suppression of supercooling. A low content of MgCl₂·6H₂O improves cycling stability, while 0.5 wt% HEC significantly reduces phase separation. The optimized composite shows a melting temperature of 29.2 °C, with latent heats of melting and solidification of 170.6 J/g and 183.9 J/g, reflecting changes of −6.8 %, 2.1 %, and 1.3 % compared to the unmodified sample. The supercooling and phase separation behavior of Ba(OH)₂·8H₂O is also experimentally optimized. Experimental results demonstrate that CaF₂ and BaCO₃ effectively suppress supercooling, while a combination of 1 wt% gelatin and 1 wt% HEC achieves optimal phase separation control, outperforming either additive alone. The final formulation consists of 97 wt% Ba(OH)₂·8H₂O, 1 wt% BaCO₃, 1 wt% gelatin, and 1 wt% HEC. The resulting composite exhibits a melting temperature of 80.0 °C, and latent heats of melting and solidification of 244.4 J/g and 223.6 J/g. Compared to the ones before modification, these properties change slightly by −1.5 %, 4.5 %, and 3.1 %, respectively.