Researchers at Razi University in Iran have investigated how the geometry of copper cooling tubes affects the electrical and thermal performance of photovoltaic-thermal (PVT) collectors. They aimed to determine whether wavy tubes could improve heat extraction and boost overall system efficiency compared with conventional straight tubes. To this end, they experimentally assessed three tube configurations at different water flow rates.
“This study presents an experimental investigation into the performance of a water-cooled PVT system integrated with wavy copper channels, aiming to quantify the impact of channel geometry and flow rate on thermal and electrical efficiencies,” the researchers said. “Notably, unlike many small-scale studies, this research is conducted on a commercially representative PV module.”
The researchers built a water-cooled PVT collector using a 180 W PV module measuring 68 cm × 148 cm, with an aluminum absorber plate attached to its rear side. They fabricated three copper-tube cooling assemblies: one with straight tubes and two with mechanically bent wavy tubes featuring pitches of 200 mm and 300 mm, respectively, and a wave height of 50 mm. They used silicone thermal paste to improve thermal contact between the tubes and the absorber plate, while copper strips and thermally conductive adhesive secured the assemblies.
The team conducted outdoor tests in Kermanshah, western Iran, in May 2025, under an average ambient temperature of 27 C and solar irradiance of 1,038 W/m2. Each configuration was tested at water flow rates ranging from 200 mL/min to 800 mL/min, with the inlet water temperature maintained at 20 C. The researchers also tested an uncooled PV module for comparison. Each test lasted approximately 30 minutes, until operating conditions stabilized.
“The implementation of the liquid cooling effectively reduced the panel temperature,” the academics said. “In the optimal configuration tested of W-200 tube at 800mL⋅min 1, the absorber plate temperature was reduced to approximately 49 C, compared to 64 C in the uncooled system, a reduction of ∼15 C. Similarly, the uncooled panel surface temperature, which reached a maximum of nearly 67 C, was significantly mitigated.”
The researchers also found that the W-200 configuration, featuring a 200 mm pitch, achieved the highest electrical efficiency of 13.65% at a water flow rate of 800 mL/min, compared with 12.63% for the uncooled module. This represented an improvement of 1.02 percentage points.
The W-200 configuration also recorded the highest pressure drop, reaching approximately 1,200 Pa at a flow rate of 400 mL/min, compared with around 750 Pa for the straight-tube configuration at the same flow rate.
“The W-200 tube exhibited the most effective heat extraction among the tested cases, achieving a maximum thermal efficiency of approximately 24% at 800 mL · min-1, compared to just 18%for the straight tube,” the scientists said. “Consequently, the optimal W-200 configuration delivered a maximum overall efficiency exceeding 37%, substantially outperforming the straight tube with ∼31%.”
The researchers presented their findings in “Experimental analysis of thermal performance of a photovoltaic – thermal hybrid solar collector (PVT) with wavy channel,” published in Energy Nexus.
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