The Effects of Nucleating Agents on Phase Transition of a Salt Hydrate Phase-Change Material for Thermal Energy Storage Heat Exchangers

Salt hydrate based Phase-Change Materials (PCM) typically store and discharge large quantities of heat during their phase transition process. However, the repeated use of these PCM, especially when they are completely dehydrated and superheated in a cyclic process, is impeded by certain irreversible behavior. These include (1) phase segregation: where the PCM degrades to forms a lower hydrate during phase transition while losing its storage capacity, and (2) subcooling: when crystallization and rehydration from the liquid PCM state occurs at a temperature lower than their phase transition point. Higher degrees of subcooling (ΔTs), which is how much the PCM has to be super cooled below its phase transition temperature for crystallization to begin, adversely affects its performance and in some cases renders it ineffective. Lithium Nitrate Trihydrate [(LNT) LiNO3.3H2O] is one of the more promising candidate among these inorganic salt hydrate PCMs because of its rather stable phase change behavior and relatively very high latent heat capacity (282kJ/kg). Nevertheless, when completely in superheated liquid phase, it requires a high degree of subcooling (ΔTs > 20°C), which limits its utility. This study reports a novel nucleating agent — Zinc Nitrate Hexahydrate (ZNH) that reduces the subcooling to below 5°C (1000 Cycles). However, the latent heat capacity reduced to half the initial value after the cycling.