Experimental investigation of Y ₂ O ₃ /water nanofluid on the performance of evacuated tube solar collector

Evacuated tube solar collectors (ETSCs) are widely used solar thermal devices due to their ability to operate efficiently under low and diffuse solar radiation conditions. However, their thermal performance strongly depends on the heat transfer characteristics of the working fluid. In this study, the experimental performance of an ETSC using Y 2 O 3 /water nanofluid was investigated in terms of energy and exergy efficiencies. The thermophysical properties of the nanofluid, including thermal conductivity, viscosity, density and specific heat capacity, were experimentally measured for nanoparticle concentrations ranging from 0.25 vol% to 1.5 vol% and temperatures between 55°C and 80°C. Nanofluid stability was confirmed through zeta potential analysis, indicating strong dispersion stability with the use of CTAB surfactant. The experimental results show that the incorporation of Y 2 O 3 nanoparticles significantly enhances the collector performance. The maximum thermal efficiency of 73.14% was obtained at 1.0 vol% nanoparticle concentration and a mass flow rate of 0.041 kg/s, while the maximum exergy efficiency reached 33.1% at 1.0 vol% concentration and 0.033 kg/s flow rate. The results further indicate that increasing nanoparticle concentration improves thermal conductivity and heat transfer capability, although excessive concentration leads to higher viscosity and reduced efficiency. Overall, the study demonstrates that Y 2 O 3 -based nanofluids are a promising alternative heat transfer medium for enhancing the thermal performance of ETSC systems, thereby contributing to improved solar energy utilization.