Comparative Thermal Performance of Double Helical Coil and Finned Tube Heat Exchangers in PCM-Based Solar Water Heating

<title>Abstract</title> <p>The intermittent nature of solar radiation limits the continuous availability of hot water in solar water heating systems, highlighting the need for efficient thermal energy storage technologies. This study experimentally and numerically investigates the thermal performance of a latent heat thermal energy storage system utilizing paraffin wax as the phase change material (PCM). Two heat transfer enhancement techniques, namely a double helical copper coil and an aluminum finned tube, were integrated into a PCM storage tank and compared with a conventional storage configuration. A laboratory-scale prototype was developed and tested under identical operating conditions, while three-dimensional computational fluid dynamics (CFD) simulations and steady-state thermal finite element analyses were performed using ANSYS Workbench. The numerical model was validated against experimental measurements, with a maximum prediction error below 10%. The experimental results demonstrated that the double helical copper coil provided the best thermal performance, achieving a maximum outlet water temperature of 71.2°C, extending hot water availability above 45°C to 6.8 h, and increasing the thermal retention time above 40°C to 8.1 h. In comparison, the aluminum finned tube also improved the thermal storage performance, producing a maximum outlet water temperature of 66.3°C and extending the thermal retention time to 7.2 h, outperforming the conventional storage tank. The CFD simulations accurately predicted the temperature evolution, PCM melting behavior, and heat transfer characteristics, confirming that the double helical copper coil produced a more uniform temperature distribution and higher heat flux within the PCM. The proposed integrated experimental–numerical approach demonstrates that enhancing the internal heat transfer area significantly improves latent heat storage performance and provides an effective solution for increasing the efficiency and reliability of solar water heating systems operating under Gulf climatic conditions.</p>