Thermal performance enhancement of a flat-plate solar water heater using closed-loop pulsating heat pipes as fins

Flat-plate solar water heaters (FPSHs) offer a compact and cost-effective solution for domestic water heating; however, their limited thermal efficiency restricts their energy utilization. To address this limitation, this study proposes a retrofit approach using mathematically optimized closed-loop pulsating heat pipes (CLPHPs) as extended heat-transfer fins in a household-scale flat-plate solar collector—an application that has received limited attention in previous studies. A mathematical model was developed to determine the optimal CLPHP-fin configuration for integration into the existing flat-plate solar collector. The resulting design consisted of 16 turns, a 2 mm inner diameter, two heat-source sections, and six fin sets, with ethanol used as the working fluid at a 50% filling ratio. The optimized CLPHP fins were subsequently fabricated and installed in a 2.28 m 2 flat-plate solar collector coupled with a 100 L water storage tank. Outdoor experiments were conducted to compare the modified solar collector with an otherwise identical conventional solar collector. The CLPHP fins increased the maximum water temperature by 8.6 °C, average solar collector efficiency by 12.8%, and zero-loss efficiency by 11.2%, while reducing the heat loss coefficient by 17.1%. The retrofit system also shortened the simple payback period by 7 months and increased the net present value, internal rate of return, and net savings by 362.2 USD, 8.9%, and 461 USD, respectively. These findings demonstrate that optimized CLPHP fins provide an effective retrofit solution for improving the thermal and economic performance of household-scale flat-plate solar water heaters, potentially extending the usefulness of existing collectors without requiring complete system replacement.