Experimental assessment of thermal performance limits in a thermosyphon-based PV/T retrofit of a commercial photovoltaic panel

Photovoltaic/thermal (PV/T) systems are widely investigated to enhance solar energy utilization; however, most studies focus on custom-designed collectors, while the feasibility of retrofitting commercial photovoltaic modules remains insufficiently explored. This study presents an experimental assessment of a thermosyphon-based PV/T retrofit applied to a standard commercial photovoltaic panel under real outdoor conditions.The system consists of aluminum absorber bars coupled to the rear surface of the panel and connected to four copper–water thermosyphons, with heat removal achieved through a water-cooled manifold. Experiments were conducted under both sunny and cloudy conditions, with different coolant flow rates, while a reference PV panel operated simultaneously under identical conditions for direct comparison. Thermal and electrical performances were evaluated based on energy and exergy efficiencies, and a thermodynamic sizing approach was applied to quantify the heat extraction requirements. The results show that the retrofit system achieved an overall efficiency of 45.8%; however, due to thermal resistance at the interface between the panel and the absorber, the PV/T module operated at higher temperatures than the reference panel, leading to an average electrical efficiency reduction (14.09% vs. 14.65%). The analysis revealed an apparent upper limit in heat extraction, indicating that increasing coolant flow alone is insufficient to overcome retrofit-induced thermal constraints. The findings demonstrate that the performance of retrofit-based PV/T systems is strongly governed by heat extraction density and thermal interface quality. A minimum heat removal requirement was identified to avoid thermal penalties, providing practical design guidelines for the development of efficient and cost-effective PV/T retrofit solutions.