Storage characteristics of solar thermal collector integrated with latent heat material: A comparative experimental study of two types of paraffin wax under varying incident radiation and different water flow rates

• Thermal-efficient design and construction of PCM-based collectors is introduced. • The interplay between PCM behavior and water flow conditions is addressed. • Flow rate of 0.3 L/min is optimal for hot water availability and discharging time. • Higher PCM melting temperature (50°C) is superior for nighttime performance. • Sizing of PCM-based collector should be optimized with PCM type for water heating. Due to its high energy density and near-isothermal storage capability, phase change materials (PCMs) can play a pivotal role in mitigating the temporal mismatch between energy supply and demand in solar thermal systems. The present research addresses the interplay between PCM behavior and fluid flow conditions through comprehensive experimental investigation of two systems of PCM-based thermal collectors throughout charging, melting, and discharging eras. Both systems use paraffin wax as PCM, one with a melting temperature of 38°C, and the other with 50°C. The two systems were designed, manufactured, and tested simultaneously in same real outdoor conditions under four water flow rates covering laminar and turbulent regimes. It is found that during daytime period, the second system (PCM50) exhibited the highest results of average outlet water temperature above ambient, thermal gain, and efficiency of 17.3°C, 550 W, and 79.8% respectively at the turbulent flow of 1 L/min, while the first system (PCM38) gave somewhat lower corresponding values. Also, during nighttime period, the second system displayed the best outputs, however at laminar flow, of average outlet water temperature above ambient 9.3°C at 0.1 L/min and thermal gain 255 W at 0.3 L/min, while the first system showed much smaller comparable results. Moreover, it is observed that the PCM melting duration in both systems shortened with water flow rate, and it is much longer for the first system. The research concluded that sizing of PCM-based collector needs to be optimized with PCM type for improving outlet temperature profile during nighttime in cold-weather environments.