Thermal behavior of phase change plates using tunnel lining GHEs for cool storage: Effect of pipe design parameters and PCM latent heat

Abstract

An innovative energy storage technique of phase change plates (PCPs) using tunnel lining ground heat exchangers (GHEs) for cool storage facilitates the geothermal energy extraction and usage, which can serve tunnels and buildings cooling. In this study, numerical simulation was employed to analyze and assess this new energy storage technique. A coupled computational model of PCPs and tunnel lining GHEs was established to explore and analyze the thermal behavior of PCPs under varying PCP pipe design parameters and phase change material (PCM) latent heat values. The results show that at the same PCP pipe length, the effect of PCP pipe configuration on the drop rate of the PCM liquid fraction and the ratio of solid PCM is relatively evident after the cold energy charging operation mode of PCPs for a certain time. The energy storage time of the PCP under Shape 5 PCP pipe configuration reduces by 11.1 %, 26.7 %, 7.1 %, and 28.8 % compared to Shape 1, Shape 2, Shape 3, and Shape 4 PCP pipe configurations. Increasing the PCP pipe length can remarkably accelerate the drop rate of PCM liquid fraction and increase the ratio of solid PCM inside the PCPs. The energy storage time of the PCP saves by 75.0 % and 40.9 % as the PCP pipe length increases from 0.9 and 1.7 m to 2.5 m. Specifically, the energy storage time of PCPs decreases exponentially with an increase in the PCP pipe length. Moreover, PCM latent heat is a key material property to affect the PCM liquid fraction variation and the ratio of solid PCM inside the PCPs. The energy storage time of the PCP reduces by 70.7 % and 53.0 % when the PCM latent heat decreases by 200 and 100 kJ/kg, and the energy storage time shows a linear increase relationship with increasing PCM latent heat.