Surface driven thermal dynamics of Casson CNT nanofluids under nonlinear radiation and convective-radiative interface conditions: Framework for photovoltaic cooling

Enhancing the thermal performance of photovoltaic (PV) systems is critical for improving their electrical efficiency, particularly in high-irradiance environments where panel overheating leads to significant energy losses. This study presents a novel mathematical framework for optimizing heat transfer in PV-integrated cooling systems using a non-Newtonian carbon nanotube (CNT)-based nanofluid. A thin-film Casson nanofluid flow and heat transfer characteristics are studied over a vertical stretching surface under combined convective and radiative boundary conditions. The model incorporates linear, quadratic, and nonlinear Rosseland approximations to capture varying intensities of radiative heat transfer, and the Casson fluid formulation captures shear-thinning behavior. The governing equations are transformed using similarity variables and solved numerically via MATLAB’s BVP4C solver, with validation against benchmark studies. The analysis shows that increasing the Casson parameter suppresses fluid velocity but enhances thermal energy retention, with up to 12% improvement in heat transfer performance. Nanoparticle loading improves buoyancy-driven flow but introduces a trade-off with thermal boundary layer thickness. Stronger radiation coupling improves heat transfer near the boundary. The conduction–radiation coupling boosts the Nusselt number by 5%–11%. The nonlinear radiation modeling results in the most significant gains in thermal performance. The velocity rises approximately 14%–16% and temperature surges 62% under high irradiance. These insights provide actionable strategies for designing next-generation systems, bridging the gap between theoretical fluid dynamics and sustainable energy engineering.