Experimental investigation on low concentration nanofluids with Fresnel lens and evacuated tubes for solar applications

Optical and thermo-physical properties of the nanofluids play a major role in the absorption of solar energy. In the present study, photo-thermal energy conversion of low concentration Al₂O₃/Deionised water (DI) water and CuO/DI water nanofluids in solar thermal collector is experimentally investigated. Properties of 50,100,150, and 200 ppm concentrations of nanofluids are reported. The absorbance results in the visible range indicate that CuO nanofluid of 200 ppm concentration is nearly three times higher compared to Al₂O₃ nanofluid. The extinction coefficient, optical energy band gap, and photoluminescence obtained from the absorbance data are also reported. Surfactant free nanofluids are used, and the thermal conductivity measurements show a negligible enhancement for both the nanofluids. Maximum receiver temperatures of 89 and 72°C are found with CuO and Al₂O₃ nanofluids, respectively, for the maximum concentration. A maximum receiver efficiency of 34.89% is obtained for CuO nanofluids.