An experimental study of the effect of using Ag-water nanofluid in different concentrations on the performance of circular collectors

The utilization of nanofluids as heat transfer fluid is an effective way to increase the efficiency of solar collectors. In this research, the performance of a circular flat-panel solar collector with spiral coolant passage projected from the periphery to the center of the circle, which is like a sunflower, has been studied experimentally according to the ASHRAE standard in Aghajari (latitude 300 69/ 8// N, longitude 490 82/ 40// E) in southern Iran. Pure water and silver-water nanofluid in three mass concentrations of 0.1%, 0.2% and 0.3% have been used for evaluating collector thermal efficiency in different conditions. The results show that compared to pure water, silver-water nanofluid has a better performance and increasing silver nanoparticles from 0.1% to 0.3% improves the collector thermal efficiency by around 5.7%. Also, the efficiency of the collector becomes higher after increasing solar irradiation. In addition, it is observed that there is no relationship between air humidity and efficiency. The highest efficiency obtained from this collector at a volume fraction of 0.3% and coolant flow rate of 2.268 lit/min was about 80.3%, which is14.8% more than that of pure water.