Calcium chloride hexahydrate (CaCl2·6H2O) is an attractive candidate as a phase change material for supplemental cooling in air-cooled thermal power-plant since it has a low-phase transition temperature of 29.3 °C and a relatively large volumetric energy storage density of 289 MJ/m3. The volumetric energy storage density is approximately double the energy densities of comparable paraffins with similar melting points. However, calcium chloride hexahydrate often requires high degree of supercooling to initiate solidification and long-term thermal stability has impeded the adoption of calcium chloride hexahydrate in the latent heat thermal energy storage system. There are only a few literatures which report on long-term stability of pure calcium chloride hexahydrate accurately. In this paper, the effects of sodium chloride and strontium chloride in mitigating supercooling in calcium chloride hexahydrate over 1000 melt–freeze cycles and thermal stability at elevated temperatures were studied in large sample size. Since there is not much data available on calcium chloride hexahydrate with nucleating additives, the current data available do not provide an insight into the effects of thermal cycling on supercooling. Therefore, this study also aims to measure the reliability of calcium chloride hexahydrate and report it, in terms of variations in melting temperature, supercooling, energy storage density, and change in mass over 1000 melt–freeze cycles. The results have shown that strontium chloride as heterogenous nucleators reduces supercooling by 2.5 °C and survived up to 1000 melt–freeze cycles (i.e., 2.7 years).