Researchers in Brazil found that retrofitting commercial PV panels into PVT systems can boost total efficiency to around 46–50%, but added thermal resistance limits heat extraction and slightly reduces electrical performance. Their experiments showed performance is capped by interface quality and system design, requiring improved heat extraction capacity.
The PV module retrofitted with thermosyphons
Image: Federal University of Technology-Parana, Energy Conversion and Management, CC BY 4.0
Researchers at the Federal University of Paraná (UFPR) in Brazil have assessed the potential for retrofitting commercial photovoltaic modules into photovoltaic-thermal (PVT) panels, identifying practical constraints that currently limit the approach’s technical and economic viability.
The scientists explained that previous research focused on custom-designed PV/T collectors with optimized integration, rather than standard commercial PV modules, with only a few studies having explored retrofitting existing panels. The literature indicates that effective performance depends not only on component optimization but also on overcoming inherent thermal limitations of commercial modules.
“Our work provides a design-oriented, experimental assessment of PVT retrofitting, identifying the key structural and operational parameters that govern system performance and establishing minimum requirements for heat extraction capacity to achieve effective thermal regulation under real operating conditions,” the academics stressed, noting that their analysis focused on PVT modules using heat pipes and thermosyphons, which stand out for their high efficiency, passive operation, and ability to transfer heat via phase-change mechanisms with minimal temperature gradients.
Thermosyphons, in particular, rely on gravity-driven circulation between evaporator and condenser sections, enabling effective heat removal without capillary structures. Their performance depends on factors such as filling ratio, inclination angle, working fluid, and system geometry.
The experimental setup consisted of a standard 60 W polycrystalline photovoltaic panel with four rear-mounted thermosyphons under real outdoor conditions. The thermosyphons, made of copper and filled with distilled water, were designed with distinct evaporator, adiabatic, and condenser sections to enable passive heat transfer.
Aluminum absorber bars were used to ensure thermal contact between the panel and the thermosyphons, while the condenser sections were integrated into a water-cooled manifold acting as a heat sink. A closed-loop circulation system, including a thermal reservoir, pump, expansion vessel, and flow meter, was used to manage water flow and heat recovery. The setup allowed continuous recirculation and storage of heated water for further use, according to the research team.
The PVT system was installed alongside a reference PV panel to enable direct performance comparison under identical environmental conditions. Both panels were mounted at a 25° tilt and oriented north to maximize solar exposure. Temperature measurements were taken using thermocouples placed on the panel surface and within the water circuit, while solar irradiance was recorded with a pyranometer. An Arduino-based I–V curve tracer was developed to measure electrical performance, including voltage, current, and power output.
Image: Federal University of Technology-Parana, Energy Conversion and Management, CC BY 4.0
The two systems were experimentally evaluated under real outdoor conditions, using different water flow rates and weather scenarios. Tests were conducted on sunny and cloudy days at 6.5 L/min and on a sunny day at 1.5 L/min, with controlled inlet water temperatures to ensure consistency.
The results showed that the PVT module consistently operated at higher temperatures than the reference panel due to added thermal resistance and reduced natural convection at the rear, which resulted in a slight electrical efficiency drop, highlighting a thermal penalty associated with the retrofit design. However, the PVT system achieved significantly higher total energy efficiency, reaching about 45.75% under sunny conditions, driven by effective heat recovery. The system also exhibited strong thermal inertia, smoothing temperature fluctuations and enabling continued heat transfer even when solar irradiance decreased.
On cloudy days, this thermal inertia improved performance further, increasing total efficiency to over 50% due to delayed heat release from stored energy. However, heat extraction rates showed a plateau, indicating an upper limit imposed by thermal resistances and thermosyphon capacity. Flow rate, meanwhile proved to be a critical parameter. At 6.5 L/min, efficient cooling maintained better electrical and thermal performance, while at 1.5 L/min, reduced convection led to overheating, significantly lowering electrical efficiency, down to 10.93%, and overall efficiency, down to 19.02%.
Further findings confirmed that increasing flow rate alone cannot fully overcome the limitations, as heat extraction is ultimately capped by interface quality and system design.
Overall, the results demonstrate that PVT retrofit performance depends on balancing heat extraction capacity with inherent thermal resistances. They also highlight the existence of a maximum heat removal threshold and the need for optimized design parameters, such as flow rate and thermosyphon configuration.
“The current four-thermosyphon configuration is undersized for achieving thermal parity with a standard PV module,” the academics emphasized. “An increase of approximately 60% in heat extraction capacity is required, which can be achieved by increasing the number of thermosyphons (to six or seven) or enhancing the effective thermal contact area. Despite these limitations, the system demonstrated stable operation and consistent heat recovery under variable environmental conditions, supporting its applicability for low-grade thermal energy utilization.”
Looking forward, the scientists are planning to prioritize enhancing heat extraction capacity by optimizing thermosyphon design, spatial configuration, and thermal interface quality, as well as exploring alternative working fluids and geometries tailored to retrofit constraints. Moreover, they want to address long-term performance, system integration in real applications such as building-integrated PV, and comprehensive economic assessments to evaluate cost-effectiveness and scalability.
Their findings were presented in the paper “Experimental assessment of thermal performance limits in a thermosyphon-based PV/T retrofit of a commercial photovoltaic panel,” published in Energy Conversion and Management. 
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More articles from Emiliano Bellini
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