Improvement on simultaneous thermal energy storage and recovery with a novel layout consisting of two separate phase change materials

This study reveals how a unique layout of a thermal energy storage system filled with two different phase change materials (PCMs) can efficiently improve the simultaneous melting and freezing processes of the system. A horizontal shell-and-tube energy storage system is adopted where the hot and cold fluids pass through two pipes inside the shell at constant boundary temperatures of 345 K and 317 K, respectively. A validated transient numerical model is used to predict the melting and solidification behaviours of PCMs. The thermal performance of the baseline case (with a single PCM) is analysed and compared with the performance of three different dual-PCM configurations. For comparative purposes, PCMs are selected such that the average property of the dual-PCMs matches the baseline single PCM case. Numerical simulations are carried out for two different initial conditions, representing PCMS being completely melted and completely solidified. The results show that the arrangement of two PCMs with different melting-solidus points affect the charging and discharging rates dramatically when the fast-melting PCM is placed in the upper half of the unit, and the fast-solidifying PCM is placed in the lower half. The proposed layout utilises 58% of the storage capacity for the case with the initial condition that defined the PCMs as completely solidified, while the baseline case utilises 54% of its capacity. For the cases with the initial condition that defined the PCMs as completely melted, the proposed layout retrieves 24% of the stored heat, while the baseline case retrieves only 12% of the capacity, highlighting the benefit of the proposed duo PCM arrangement demonstrated in this study. Subsequently, the storage and recovery enhancement ratios during 5 h simultaneous charging and discharging are 17% and 67%, respectively.