Inorganic salt hydrates-hydrogel composites as phase change materials for energy storage in buildings

There are few drawbacks to implementing hydrogel phase change materials (PCMs) for building applications requiring long duration stability of over 20 years. Both inorganic salts and functional groups of hydrogel polymers compete for interaction with water molecules. In the highly saline solutions, a competition for hydration water occurs between salt ions and polymer chains, resulting in a dramatic effect of salt on the polymer solubility and gelation. The three-dimensional network of hydrogels is damaged, and polymer solubility and gelation properties are influenced by the addition of large amounts of inorganic salt salts. On the other hand, functional groups such as hydroxyl and carboxyl also electrostatically bond strongly with metal cations affecting their crystallization as hydrates. For instance, calcium salts formed hydrophobic precipitates with sodium acrylate hydrogels resulting in a phase separation (stratification) of water and Ca-hydrogel precipitate. Thus, traditional hydrogel materials based on sodium acrylate and acrylamide are unsuitable for PCM applications. Therefore, a new hydrogel PCM system based on polyvinyl pyrrolidone (PVP) was investigated to avoid incompatibility issues with hydrated salts and polymer hydrogel systems.