A novel approach for modelling thermal energy storage with phase change materials and immersed coil heat exchangers

A novel modelling approach is presented for a thermal energy storage system with immersed coil heat exchangers. The energy store consists of a water tank in which rectangular phase change material (PCM) modules are submerged. The immersed coil heat exchangers are placed at the bottom and top sections of the tank for charging and discharging, respectively. This design promotes thermal mixing inside the tank when charged from the bottom coil and discharged from the top coil. In addition, the location of the coil heat exchangers favorably limits the heat transfer when charged through the top coil or discharged through the bottom coil giving rise to the thermal diode effect. PCMs can offer large storage capacity since they store large amounts of thermal energy in the form of latent heat of phase change (solid-liquid). This paper proposes a simplified physics-based numerical model of the heat transfer and phase change in the thermal storage tank. The model discretizes the storage tank one-dimensionally into three fully-mixed control volumes; two control volumes surrounding the immersed coil heat exchangers and one control volume in the middle of the tank including the PCM modules. All the heat transfer dynamics between the control volumes, the heat exchangers and the PCM modules are considered. Experimental validations, using a full-scale test facility, are carried out for systems with and without PCMs under various operation scenarios and the results are found to agree well within the experimental uncertainties. Such model potentially provides high computational efficiency that is desirable in simulating the performance of storage systems under long-term operations.