Experimental and thermal performance evaluation of a PCM-enhanced solar water heater with a spiral coil heat exchanger

Latent heat thermal energy storage is an effective approach for enhancing the performance of solar water heating (SWH) systems by storing surplus thermal energy and delivering it during periods of low solar availability. In this study, the thermal performance of a solar water heater integrated with a phase change material (PCM) storage unit is experimentally investigated under real outdoor operating conditions. Paraffin wax is employed as the PCM, while water serves as the heat transfer fluid (HTF). Thermal energy is transferred from the solar collector to a spiral copper coil heat exchanger embedded within the PCM tank, enabling indirect heat exchange between the HTF and the storage medium. The charging and discharging processes were examined on clear-sky days by monitoring the temperature evolution of the PCM and HTF using a distributed thermocouple array. The effects of HTF inlet temperature and flow rate on the thermal response, energy storage capacity, and system efficiency were systematically analyzed. The results demonstrate that the PCM-based storage system significantly enhances the thermal performance of the solar water heater. At a representative HTF inlet temperature of 80°C, the charging and discharging efficiencies reached 78.2% and 61.1%, respectively, while higher inlet temperatures led to further efficiency enhancement. During the charging process, the PCM stored 2.8–3.0 MJ of thermal energy, depending on the operating conditions, confirming its substantial latent heat storage capacity. During discharge, up to 1.65 MJ of useful thermal energy was recovered over 45–50 min at an inlet water temperature of 25°C and a flow rate of 48 L/h. Overall, integrating a PCM-based storage unit with a spiral coil heat exchanger improves the energy storage capacity and thermal efficiency of SWH systems, thereby extending hot water availability beyond periods of solar radiation.