Experimental study on performance improvement of U-tube solar collector depending on nanoparticle size and concentration of Al2O3 nanofluid

In this study, the efficiency of a U-tube solar collector as a function of the concentration of Al2O3 nanofluid and the size of nanoparticles was investigated experimentally. Further, the efficiency of a Utube solar collector with Al2O3 nanofluid as the working fluid was compared to that with water. The thermal conductivity of the Al2O3 nanofluid was found to increase with increase in its concentration and decrease in its nanoparticle size. When the inlet fluid and ambient environment are the same temperature, the solar collector with 1.0 vol% Al2O3 nanofluid of 20 nm-nanoparticles and a mass flow rate of 0.047 kg/s showed the highest efficiency, 24.1% higher than that of the solar collector with water. In addition, the efficiency of the solar collector with 1.0 vol% Al2O3 nanofluid was 5.6% and 9.7% higher than those with 1.5 vol% and 0.5 vol% nanofluid, respectively. For equal nanofluid concentrations, the maximum efficiency of the solar collector was 72.4% at the nanoparticle size of 20 nm-3.05% and 5.32% higher than those with nanoparticle sizes of 50 and 100 nm, respectively. Therefore, Al2O3 nanofluid was concluded to be effective in increasing the efficiency of the U-tube solar collector. (C) 2016 Elsevier Ltd. All rights reserved.