Structural and filling ratio optimization of flexible separate heat pipe developed for tracking PV/T systems

While separated heat pipes enhance PV/T thermal regulation, tracking-induced inclination variations disrupt gravitational fluid circulation, precipitating severe performance degradation under negative angles. To address the instability and heat transfer deterioration observed in separated heat pipes under the variable inclinations characteristic of tracking systems, a novel flexible separate heat pipe configuration optimized for tracking PV/T applications is proposed and experimentally validated. Through a comprehensive experimental investigation encompassing evaporator geometry, adiabatic section connectivity, and filling ratio optimization, the thermal performance under diverse side-altitude orientations were was systematically elucidated. The experimental results demonstrate that a refined configuration employing a center inlet/outlet arrangement with a 50% filling ratio substantially mitigates localized dry-out phenomena and enhances flow uniformity. Consequently, this optimized design yields a 28.9% increase in thermal efficiency and a 27.2% enhancement in heat transfer capacity compared to the baseline lateral inlet/outlet configuration. Furthermore, the system attains a peak thermal efficiency of 71.6% under a simulated solar irradiance of 636 W/m2, with the optimal operating orientation identified as a side inclination angle of 15° and an altitude angle of 50°. These findings establish a practical strategy for addressing heat transfer degradation in heat pipes operating under complex orientations and offer original insights for advancing heat pipe applications in integrated solar energy systems.