First and second laws analysis of a minichannel-based solar collector using boehmite alumina nanofluids: Effects of nanoparticle shape and tube materials

This paper evaluates the first and second laws for a flat plate minichannel-based solar collector where the working fluid is a suspension of boehmite alumina nanoparticles in a mixture of water and ethylene glycol. Four different shapes of nanoparticles, including platelets, blades, cylinders, and bricks are selected for this study. The results of Nusselt number, heat transfer coefficient, outlet temperature, and entropy generation are presented for different shapes of nanoparticles and volume concentrations up to 4% in turbulent flow. Two different solar collectors are investigated so that in one of them the tubes and absorber plate are made of copper and in another the material is of steel. It is found that the platelet shaped nanoparticles show the lowest heat transfer coefficient while bricks, except at the highest volume fraction, i.e. 4%, display the highest heat transfer coefficient. The heat transfer coefficient for copper tubes is lower than that of steel tubes. The findings indicate that, independent of nanoparticle shape, the outlet temperature increases by adding the nanoparticles but the rate of increase for platelets is lowest. The results of entropy generation analysis for copper tubes elucidate that the entropy generation rate is minimized by using brick shaped particles with volume fraction of 2%, while for steel tubes the minimum entropy generation is achieved by using blade shaped particles with volume fraction of 4%. In mass flow rate of 0.5 kg/s, the entropy generation rate for steel tubes is, on average, about 11% higher than copper tubes and this value attains 18% when the mass flow rate increases from 0.5 to 0.75 kg/s. (C) 2014 Elsevier Ltd. All rights reserved.