Comparative thermal analysis of heat transfer fluid injection orientation in phase change material based latent heat thermal energy storage systems

Abstract This study aims to experimentally investigate the influence of heat exchanger orientation and heat transfer fluid (HTF) injection direction on the thermal performance of a latent heat thermal energy storage (LHTES) system using a phase change material (PCM). A comparative analysis is conducted between horizontal and vertical double-pipe heat exchanger configurations, considering both top and bottom injection of the HTF. The experimental methodology involves systematic variations of inlet temperature and volumetric flow rate while monitoring temperature evolution, charging and discharging durations, heat transfer coefficients, and Nusselt numbers. The results demonstrate that the direction of HTF injection plays a critical role in governing heat transfer behavior. Top injection promotes faster charging by enabling the PCM to reach higher average temperatures, whereas bottom injection enhances convective heat transfer, yielding the highest Nusselt numbers and heat transfer coefficients, albeit with longer charging and discharging times. In the horizontal configuration, increasing the flow rate from 60 to 120 L h −1 reduces the charging time from 70 to 60 min, corresponding to an improvement of 14.2%. Discharge tests performed at inlet temperatures of 70 °C and 80 °C reveal performance reductions of 8.9% for the horizontal configuration, 43.3% for bottom injection, and 42.6% for top injection at a constant flow rate. Overall, the horizontal configuration exhibits the fastest thermal response during both charging and discharging processes, while bottom injection achieves the highest thermal efficiency. These findings highlight the importance of heat exchanger orientation and flow direction optimization for improving the performance of LHTES systems.