Performance and Stability of CaCl2·6H2O-Based Phase Change Materials

Thermal energy storage is receiving increased attention as the world transitions away from fossil fuels and seeks to improve the efficiency of existing energy production systems. While higher-temperature thermal energy storage has received the majority of attention in the field, thermal energy storage at temperatures close to ambient conditions may also have some practical applications. Hydrated salt phase change materials (PCMs) store thermal energy in this temperature range and can have high gravimetric and volumetric energy storage capacities while having a very low cost. Despite the benefits of these salts, they generally suffer from supercooling, phase segregation during cycling, and a high corrosion potential. In the current work, these inorganic salts were characterized and tested by a variety of laboratory methods; including: differential scanning calorimetry, drop calorimetry, rapid and slow thermal cycling, thermogravimetric analysis, and a large dehydration cell. Through these methods, the following parameters or conditions could be measured or observed: the melt temperature, the heat of fusion of melting, the material's water content, supercooling, and phase separation during cycling.After initial characterization of the thermal performance of several hydrated salt candidate materials, three calcium chloride hexahydrate (CaCl2·6H2O)-based salts were selected for additional testing. For the first of these salts, CaCl2·6H2O + magnesium chloride hexahydrate (MgCl2·6H2O) a new eutectic composition was found along with a modeled phase diagram, which agrees well with the experimental findings. This salt was cycled for over two thousand seven hundred cycles with minimal changes in the heat of fusion or melt temperature as measured by drop calorimetry. A second material, CaCl2·6H2O + potassium nitrate (KNO3), was also fully characterized and cycled with good long-term performance during cycling. When CaCl2·6H2O was similarly characterized and cycled, it was found to experience phase segregation during cycling. Potassium chloride (KCl) was found to stabilize CaCl2·6H2O, although this stabilizing effect was only realized after a supernatant liquid was removed from the frozen PCM. The ternary phase diagram of the CaCl2-H2O-KCl system suggests that several stable mixtures are possible at different H2O weight percentages, which was verified through experimentation. It was also found that CaCl2·6H2O, which has separated, can be restored to a homogenous state through heating to temperatures above 45°C. In addition, a clear relationship between the PCM cooling rate and the rate at which separation progresses was found; with faster cooling resulting in less separation.Several common metals were tested for corrosion resistance when in direct contact with the three CaCl2·6H2O-based PCMs. Test were conducted under isothermal (molten) and cycling conditions, where metal coupons were immersed in the PCM test samples for test periods of up to a year. For the CaCl2·6H2O and CaCl2·6H2O + MgCl2·6H2O PCMs, an aluminum alloy and carbon steel were found to have good corrosion resistance under both isothermal and cyclic testing. An anodized aluminum was found to provide superior corrosion resistance. If the CaCl2·6H2O + KNO3 PCM is used, aluminum samples were found to deteriorate very quickly and either carbon steel or stainless steel are recommended for containment.Medium-scale (100-1000 kg) batches of industrial-grade CaCl2·6H2O with additives were prepared and cycled in a prototype cold storage system. When tested by drop calorimetry, these materials showed good thermal performance and stability, although there appears to be a slight decrease in latent energy as the number of cycles was increased. Additional testing of industrial-grade materials is required in order to determine if they maintain their stability and performance at greater than a hundred cycles. This work has resulted in a demonstrably stable, low-temperature PCM with good thermal performance, low cost, and good material compatibility with several common, inexpensive metals.