Experimental analysis of exergy efficiency and entropy generation of diamond/water nanofluids flow in a thermosyphon flat plate solar collector

The performance of flat plate solar collector was experimentally investigated using nanodiamond nanofluid that circulates in it under thermosyphon conditions. The nanofluids were prepared by adding nanodiamond (ND) nanoparticles at 0.2, 0.4, 0.6, 0.8 and 1.0% volume concentrations into the distilled water. The study was performed to analyze the collector thermal efficiency, exergy efficiency, entropy generation, heat transfer coefficient, friction factor and power consumption requirement in the test time interval between 10:00 a.m. to 16:30 p.m. The experimental results show that the trend of these parameters is strongly influenced by the changes of solar irradiation. The highest collector thermal efficiency of 69.85% is achieved by 1.0 vol% ND/water nanofluid with 12.7% increase over that when pure water is used. Water as the working fluid generates a maximum entropy of 5.725 W/K, and this value drops to 5.541 W/K when 1.0 vol% ND/water nanofluid is used. Heat transfer irreversibility is shown by Bejan number to be the dominant contributor to the total entropy, and the entropy generation number shows that the ND/water nanofluid is more sensitive to entropy generation than water as the working fluid.