An optimization study on the performance of hot water tank integrated phase change material

Abstract

In this paper, a new structure of a phase change thermal storage tank is proposed with the aim of enhancing the thermal storage capacity of the domestic hot water tank without occupying the water storage volume. The structure of the proposed tank is that the phase change material is filled in the outer side wall of the tank and the cavity formed by an insulation layer, which does not occupy the water storage space inside the tank. The cavity is equipped with metal spacers in parallel with each other as a method to strengthen heat transfer. The effects of three structural design parameters, namely the thickness of phase change material, the number and thickness of spacers on the charging and discharging process were investigated by the means of numerical calculations. A numerical model was developed by using COMSOL Multiphysics, and the accuracy of the model was verified with experimental data from the literature. The average temperature of phase change material, average water temperature, and liquid/solid phase fraction were used to evaluate the thermal performance of the phase change thermal storage tank.It was found that the low thermal conductivity of the phase change material affected the thermal performance. When the thickness of the phase change material was 30 mm, the discharging time was prolonged by 10 %, which was 5 h longer than the common water tank. The installation of metal spacers was an effective method to enhance heat transfer. During the charging process, the phase change material had a stronger liquefaction trend, and the lowest value of the solid fraction was 0.25. During the discharging process, when the number of spacers was 17 and the thickness was 5 mm, the discharging time was extended to 80 h, which was 1.5 times of the common water tank. The results help to understand the optimized design parameters of the phase change thermal storage tank from the perspective of the structural design.