Design and experimental analysis of a helical coil phase change heat exchanger for thermal energy storage

Latent heat energy storage systems have superior features over conventional sensible storage systems. With a large latent heat of fusion, a phase change material (PCM) can absorb and release a great amount of thermal energy at nearly a constant temperature. This improves the capacity and efficiency of the energy storage unit while extending the service time. In this study, a prototype PCM heat exchanger with a helical coil tube is designed, fabricated, and experimentally analyzed for its thermal storage performance under different operational conditions. Paraffin wax is used as PCM and Ethylene glycol (EG)-water mixture is used as heat transfer fluid (HTF). Different HTF inlet temperatures, flow directions, and flow rates were tested to find out the effects of these parameters on the performance, including charging and discharging time, of the thermal storage unit. It was found that the most significant factor during charging is the HTF inlet temperature. The experimental results showed that increasing the HTF inlet temperature from 70 degrees C to 75 degrees C shortened the charging time by 35% while charging time was reduced up to 21% with increasing flow rate from 0.5 to 4 L/min. The discharging time, however, did not change substantially with flow rates. It was also found that higher flow rate leads to higher recovery efficiency. The flow direction of the HTF was found to have an insignificant effect on the total charging and discharging time but showed effects on the temperature variations of PCM in the energy storage unit.