Experimental evaluation of shell geometry impact on thermal and exergy performance in helical coiled tube heat exchanger with phase change material

In this paper, the melting (charging) and solidification (discharging) processes of a Phase Change Material (PCM) in a helical coiled tube heat exchanger as a thermal energy storage unit are investigated experimentally. During the charging (discharging) process, hot (cold) water flows through the coil. Paraffin wax is used as a PCM in the shell. The thermal and exergy performances for three different geometries of the shell (cubic, spherical, and trapezoidal) and different flow rates (0.6, 3, and 6 L/min) are investigated. Additionally, numerical simulation is conducted for the spherical thermal energy storage. The obtained results indicate that, at a volumetric flow rate of 6 L/min, the trapezoidal shell reduces the total melting time by 23.38 % and 10.1 % compared to the cubic and spherical shells, respectively. In the solidification process, the spherical shell exhibits the highest efficiency, reducing the total solidification time by 13.79 % and 21.21 % compared to cubic and trapezoidal shells, respectively. Moreover, in a total cycle (both melting and solidification processes) of the thermal energy storage, the spherical shell outperforms, reducing the complete cycle time by 13.4 % and 1.19 % compared to the cubic and trapezoidal shells, respectively. In the all investigated shells, energy, and exergy efficiencies increase with the higher flow rates, with the most significant improvement observed when increasing the flow rate from 0.6 to 3 L/min.