Photovoltaic–thermal (PVT) systems provide a promising approach for simultaneous electrical and thermal energy generation under high-irradiance tropical conditions. However, most existing studies primarily focus on first-law energy performance or global exergy indicators without detailed thermodynamic irreversibility diagnostics. This study presents a second-law thermodynamic analysis of a closed-loop water-cooled PVT system coupled to a well-mixed thermal storage tank under tropical climatic conditions. A coupled transient thermal–electrical numerical framework incorporating explicit Gibbs \(\:Tds\) thermodynamic relations is developed to evaluate entropy generation, exergy destruction, and subsystem-level thermodynamic irreversibilities within the PVT system. The model was applied to a representative tropical day using Typical Meteorological Year data for Kampala, Uganda. Under baseline operating conditions for a 1.66 m^2 collector integrated with a 100 L storage tank, the system maintained PV temperatures below approximately 43 °C while delivering about 1.9 \(\:{\text{k}\text{W}\text{h}}_{th}\) of useful thermal energy and 1.95 \(\:{\text{k}\text{W}\text{h}}_{el}\) of electrical energy. The second-law analysis further revealed that ambient heat rejection accounted for nearly 95% of the total exergy destruction, identifying it as the dominant thermodynamic irreversibility mechanism. Parametric analysis showed that increasing mass flow rate, solar irradiance, and ambient temperature reduced exergy efficiency despite relatively stable first-law energy efficiencies, confirming that energy-optimal and exergy-optimal operating conditions do not necessarily coincide. The thermodynamically favorable operating region was identified at low-to-moderate flow rates near 0.01 kg s^{-1} for the investigated configuration. Unlike many conventional PVT studies that primarily report overall energy or exergy efficiencies, the present work provides a mechanism-resolved second-law interpretation by explicitly linking entropy generation and exergy destruction to the governing thermodynamic processes within the collector and storage subsystems under tropical operating conditions. The results further demonstrate that the principal advantage of the proposed configuration lies in enhanced overall solar-energy utilization through simultaneous electrical and useful low-temperature thermal energy recovery rather than electrical enhancement alone. The study therefore provides second-law-based thermodynamic design guidance for optimization of tropical PVT systems, particularly regarding reduction of ambient thermal losses and improvement of PV–absorber thermal coupling.
