Evaluation of the calcium chloride-calcium fluoride system for high temperature thermal energy storage

Abstract

The storage of excess or unutilised energy generated by renewables remains a key unsolved piece in the decarbonisation of the energy sector. One potential way to solve this is through the use of thermal energy storage which can use the heat as is (e.g., in concentrated solar power systems) or through the conversion of electricity to heat (e.g., through resistive heating) at very low costs. To achieve these low costs, high storage densities and low-cost materials should be utilised. A potential way to achieve this is by harnessing the energy released during phase change in appropriate materials such as molten salts. Therefore, the current study investigates calcium chloride and mixtures of calcium chloride and calcium fluoride as storage options. Aside from the pure salt, a eutectic was found at 647 ± 1.7 °C and 18.3 mol% calcium fluoride, while a peritectic was also found at 730 ± 2.3 °C and 41 mol% calcium fluoride. Determination of the storage density of the materials was undertaken using differential thermal analysis and differential scanning calorimetry to measure the enthalpy of transition and specific heat capacity. These results showed high heats of transition (298–310 ± 15.0 J/g) and heat capacities (0.72–1.02 ± 0.1 J/g∙K) making them potentially a cost-effective high temperature storage medium.