Response surface methodology-based optimization of zinc oxide-ethylene glycol-water nanofluids for hemispherical cavity receiver solar thermal systems

Abstract This study presents an experimental investigation of zinc oxide (ZnO) nanofluids in a parabolic dish collector with a hemispherical cavity receiver under real outdoor solar conditions. ZnO concentrations of 0.1, 0.2 and 0.3 vol% were tested in water and in ethylene glycol-water (EG: W) base fluids with volume ratios of 0:100, 10:90, 20:80 and 30:70, at flow rates of 1 to 5 LPM, giving 60 experimental conditions. Performance parameters (thermal efficiency, exergy efficiency, useful heat gain, heat loss, Nusselt number and pumping power) were evaluated from the measured data at each condition. They were then modeled by response surface methodology (RSM) as functions of ZnO concentration, EG ratio and flow rate, and optimized using the desirability approach with equal importance for all six responses. For the water-based fluid at 0.1 vol% ZnO, increasing the flow rate from 1 to 5 LPM raised the thermal efficiency from 25.5% to 39.6% and the Nusselt number about 3.2 times, while the heat loss fell by 28%. Increasing ZnO from 0.1 to 0.3 vol% raised the useful heat gain and thermal efficiency by about 7% at all flow rates and EG ratios. EG addition reduced the thermal and exergy efficiencies, by up to 17% and 25% (30:70 at 5 LPM), and increased pumping power by up to about 97%, whereas the 10:90 mixture performed close to water at low flow. Heat loss varied non-monotonically with EG content. Response surface models for six responses fitted well (R 2 = 96.07–98.39%, predicted R 2 ≥ 96.92%). Desirability optimization with equal importance gave the best operating condition within the tested range: water base fluid, 0.27 vol% ZnO and 5 LPM (D = 0.93), with predicted thermal efficiency 40.0%, exergy efficiency 15.9% and useful heat gain 778 W. The findings establish experimentally supported performance trends and confirm the RSM-derived operating condition within the investigated experimental range.