Performance Enhancement of a Flat Plate Solar Collector Using Nanofluid-Based Heat Transfer Fluids

The low thermal conductivity of conventional water and water–glycol working fluids causes the thermal efficiency of flat plate solar collectors (FPCs) to be limited by the insufficient cooling of the absorber plate that increases the absorber plate temperature and consequently the radiative and convective losses of the collector, especially at high inlet temperatures and low irradiance levels encountered in real duty applications. This study compares the performance improvement of using a metal-oxide Nano fluid as the base fluid of an instrumented 2 m² FPC to experimental measurements, three dimensional conjugate CFD, and data-driven optimization. Nanoparticles of aluminum oxide (Al₂O₃), copper oxide (CuO) and titanium dioxide (TiO₂) were dispersed in a base fluid of 50:50 water–ethylene-glycol at 0.1–1.0 vol% by the two-step method involving surfactant stabilization and ultrasonication; 30-day sedimentation observations and zeta-potential measurements were used to confirm dispersion stability. The collector was tested outdoors, according to ASHRAE 93 steady state protocol, and the conjugate model developed in ANSYS-Fluent is able to reproduce the measured efficiency curves within 5.9%, which indicates that the model of single phase with temperature dependent effective properties is adequate at these dilute loadings. The 150 cases of the expanded Box–Behnken design were used to train a 4-14-10-2 artificial neural network (ANN) with test coefficient of determination of 0.9934 to predict the thermal efficiency and pumping power. The surrogate was coupled with a non-dominated sorting genetic algorithm (NSGA-II) to obtain the efficiency–pumping Pareto front and the optimal configuration was determined using TOPSIS. The optimum conditions: CuO at 0.6 vol% and 0.035kg s-1 m-2, increased the thermal efficiency by 13.4 percentage points, from 61.4% to 74.8%, with a rise of 18.6% in pumping power, and an increase of 11.2% in heat-removal factor and an increase of 1.8% in energy efficiency from 4.9% to 6.7%. The enhancement level reaches saturation at an approximate value of 0.6 vol%, and CuO is better than Al₂O₃ and TiO₂ because it has better conductivity-to-viscosity ratio.