Experimental evaluation of CuO and Al2O3 nanofluids for enhanced heat transfer in evacuated tube heat pipe solar collectors

• CuO-H 2 O nanofluid at 4% vol. achieved the highest thermal efficiency of 56.95%. • Nanofluids significantly outperformed distilled water and R134a in heat transfer. • Thermal conductivity improved with increased nanoparticle concentration (3% to 4%). • CuO nanofluid exhibited superior performance due to enhanced convective heat transfer. • Higher nanoparticle loading reduced thermal resistance and boosted phase change efficiency. This study investigates the enhancement of thermal performance in evacuated tube heat pipe solar collectors (ETHPSCs) using the metal oxide nanofluids. The novelty lies in the comparative performance evaluation of CuO-H 2 O and Al 2 O 3 -H 2 O nanofluids at 3% and 4% concentrations under tropical solar conditions, using systematically synthesized nanoparticles via a controlled hydrothermal method. The Nanoparticles are characterized by using XRD, SEM, and FTIR and dispersed to prepare nanofluids. ETHPSC was experimentally tested, and thermal efficiency was evaluated. CuO-H 2 O nanofluid at 4% achieved the highest outlet temperature (78 °C) and peak thermal efficiency (56.95%). The study further demonstrates the effectiveness of nanofluids over conventional fluids due to enhanced thermal conductivity, Brownian motion, and improved wettability. Comparative results show that the metal oxide nanofluids, especially at higher concentrations, are promising substitutes for conventional fluids in solar thermal systems, enhancing the sustainable energy applications.