This study presents a comprehensive theoretical and experimental evaluation of Photovoltaic–Thermal (PVT.) systems, focusing on the performance trade-offs between water and high-viscosity heat transfer fluids. Two identical custom-designed PVT. collectors were constructed to compare the thermodynamic behavior of water (SET-1) and Mobiltherm 605 thermal oil (SET-2) against a conventional uncooled PV panel. A steady-state mathematical model was developed to predict system performance, and its accuracy was rigorously validated against experimental data, achieving Root Mean Square Error (RMSE) values of 0.93°C for water and 2.27°C for thermal oil. Experimental results obtained under clear-sky conditions revealed that the conventional PV panel exhibited approximately 6% higher electrical efficiency compared to the PVT. systems. This disparity is attributed to optical losses caused by the additional glazing layer and heat accumulation due to rear thermal insulation in the PVT. design. However, among the PVT. configurations, the water-based system demonstrated superior performance. At a mass flow rate of 0.007 kg/s, the water-based system achieved an electrical efficiency 4% higher and a thermal efficiency nearly double that of the oil-based system approximately 8.6%. This performance gap is governed by the lower thermal conductivity and higher viscosity of the thermal oil, which suppresses turbulent flow and heat transfer coefficients at low velocities. Conversely, the oil-based system provided higher outlet temperatures, validating its potential for specific high-temperature applications where freeze protection is critical. The study concludes that while water is thermodynamically superior for maximizing instantaneous efficiency, thermal oils offer operational stability for harsh climates, provided that flow rates are optimized to compensate for their viscous nature.
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