Experimental investigation of solar collector efficiency enhancement using surfactant stabilized MWCNT and Al₂O₃ hybrid nanofluid

This work presents a novel outdoor experimental evaluation of a thermal performance of solar flat plate collector employing water-based alumina (Al₂O₃) nanofluids and a multi-walled carbon nanotube (MWCNTs)–Al₂O₃ (50:50) hybrid nanofluid as working fluids. Experiments were conducted under real solar conditions (9:00–15:00 h) at nanoparticle concentrations of 0.1, 0.3, and 0.5 vol% to examine heat transfer coefficient, thermal efficiency, and exergy performance. Results reveal that the hybrid nanofluid exhibited consistent superiority over mono-nanofluids and water, achieving a maximum heat transfer coefficient of 357.4 W/m²K at 0.5 vol%. At a peak solar irradiance of 1065 W/m 2 , the 0.5 vol% hybrid nanofluid delivered a thermal efficiency of 83.9%. Exergy efficiency reached 3.0% at 1065 W/m² for 0.5 vol% hybrid nanofluid which is more than double that of water. These results demonstrate the synergistic advantage of hybrid nanofluids under real operating conditions, establishing their potential as high-performance working fluids for advanced solar thermal collectors.