Experimental study of thermal performance of a solar water heating system using Al2O3/water nanofluid

Abstract

This study presents a systematic experimental investigation of the thermal performance of a flat-plate solar water heating (SWH) system employing Al₂O₃/water nanofluid as the working fluid across three nanoparticle mass concentrations (1, 2, and 3 wt%) and four mass flow rates (5, 10, 15, and 20 kg/h) under real outdoor climatic conditions. The nanofluids were prepared via the two-step method, combining mechanical stirring and ultrasonication to ensure colloidal homogeneity and suspension stability. Thermal performance was evaluated in terms of collector outlet temperature, useful energy gain, and instantaneous thermal efficiency, benchmarked against a pure water baseline under identical operating conditions. The results demonstrate that increasing nanoparticle concentration from 1 wt% to 3 wt% enhanced thermal conductivity by up to 4.4%, yielding a corresponding improvement in collector thermal efficiency of approximately 23.6% relative to pure water. The maximum thermal efficiency of 80.3% was recorded at 3 wt% and 20 kg/h during peak solar irradiance hours (14:00–15:00 h), compared to 65.1% for pure water under equivalent conditions representing an absolute gain of 15.2 percentage points. Useful energy gain increased by 31.7% at the optimal concentration relative to the water baseline. Notably, the rate of efficiency improvement diminishes beyond 2 wt%, suggesting that this concentration represents the practical optimum at which thermal enhancement is maximised without incurring disproportionate viscosity penalties and pumping power demand. The experimental results show good agreement with established correlations reported in the open literature, validating the reliability of the measurement methodology. These findings provide quantitative engineering guidance for the design and optimisation of nanofluid-based solar water heating systems, demonstrating that Al₂O₃/water nanofluid at 2–3 wt% offers a technically viable and practically deployable route to significantly improving solar thermal energy conversion efficiency.