Simulation of the transient behaviour of encapsulated organic and inorganic phase change materials for low-temperature energy storage

In this paper, a detailed mathematical model was presented for the transient behaviour of rectangular macro-encapsulated phase change material (PCM) in both melting and freezing phases and was validated using published experimental data. A second order fully implicit finite difference scheme was employed to solve for the storage material solid–liquid moving boundary problem. Two PCMs of melting temperature in the range of 30 °C, calcium chloride hexahydrate and paraffin wax C18 have been studied and investigated under various operational parameters to compare their thermal performance and storage characteristics. Simulations have been carried out to assess the effect of working fluid temperature, convective heat transfer coefficient and storage material thickness employed on the transient variation of the PCM temperature, the thermal energy storage capacity and the material melting time. It was shown that a paraffin C18 storage mass needs double the container volume, about 3 times the melting time and has 1.2 times the thermal storage capacity of a similar mass of inorganic CaCl2.6H2O salt hydrate. However, CaCl2.6H2O PCM has higher volumetric storage capacity providing an opportunity to design compact energy storage systems. In addition, a relationship between the PCM storage mass and the corresponding melting time and storage capacity has been established.