Thermal performance analysis of compact thermal energy storage unit - An experimental study

In this study, an experimental setup is developed to assess the thermal performance of a compact Latent Heat Thermal Energy Storage System (LHTESS) prototype during the charging/discharging stages. The LHTESS consists of a shell and horizontally oriented multi-tube heat exchanger and a commercially available paraffin wax RT44HC, which has a phase change temperature between 41°C and 43 °C as the energy storage medium. The testing campaign evaluated the influence of several operating conditions including the heat transfer fluid (HTF) volume flow rate and inlet temperature on the LHTESS power input and output, melting and solidification time and the energy stored and released. From the experimental results, it was observed that increasing the HTF inlet temperature has a significant effect on charging time compared to changing the HTF volume flow rate. When the LHTESS was charged using a fixed HTF inlet temperature of 60 °C, the charging process period took 296.3 min, 233.5 min, 204.8 min and 197.8 min when the HTF volume flow rate is 3.0, 4.5, 6.0 and 7.5 L/min. However, when the LHTESS was charged at HTF volume flow rate of 4.5 L/min, the results show that the charging completion time for HTF inlet temperatures of 55°C, 60 °C and 65°C are 316.6, 233.5 and 209.67 min, respectively. The results from the experimental analysis showed that the discharge time was significantly longer than the charging time due to an ever-growing layer of solid PCM around the external surface of heat exchanger throughout the discharging process which reduces the heat transfer coefficient between the PCM and HTF. This did not change substantially with the changing HTF volume flow rate.