A computational analysis for maximizing efficiency in triplex tube latent thermal energy storage

Abstract

This study presents a comprehensive numerical study on a triplex tube latent thermal energy storage (LTES) system under three modes of operation to maximize the utilization of available thermal energy for applications in built environments, including charging-only, simultaneous charge/discharge (SCD), and discharge-only pro- cesses. Charging-only process ensures the onset of SCD with a fully liquified phase change material (PCM) for better thermal performance of LTES in steady-state SCD. Eventually, the discharge-only process is examined during periods of unavailability of a thermal energy source. PCM solidification/melting is modelled using the Boussinesq approximation while various configurations are explored by altering the orientation (horizontal and vertical), the placement of heat transfer fluid (HTF) tubes (inner or outer), the HTF flow direction (counter-current and co-current), and the HTF flow rate. Initially, a constant HTF mass flow rate of 0.062 kg/s is used to identify an optimum configuration. Based on the heat transfer rates between PCM and HTFs, along with steady-state SCD liquid fraction, the horizontal LTES configuration with outer tube charge/inner tube discharge is concluded to be the most effective. Further, variations in charge and discharge HTF mass flow rates reveal that a higher charge fluid mass flow rate offers better performance. Computational results demonstrate that this optimal configuration reduces charging time by 41 % compared to the configuration with the longest charging period. In addition, at steady-state SCD, the optimal case exhibits a heat transfer rate of 84.19 W and a PCM liquid fraction of 0.88. These findings contribute to the optimal design and more efficient operation of thermal storage systems in built environments, thus improving overall system performance.