▎ 摘　　要

In the present article, we report the results of several experiments conducted on the thermal performance and efficiency of an evacuated tube solar collector (ETSC). The heat pipes were charged with graphene-methanol nanofluid and the effect of various operating conditions including the installation tilt angle of the collector, the value of the filling ratio, the mass fraction of graphene nanoplatelets and the flow rate of the collector's loop on the thermal efficiency of the collector was experimentally investigated. Results showed that the presence of graphene nanoplatelets improved the thermal conductivity of methanol (enhanced by 19% at wt. % = 0.1), while the heat capacity of the nanofluid decreased by a decrease in the mass fraction of the graphene (decreased by 4% at wt. % = 0.1). Also, it was identified that the thermal efficiency of the solar thermal collector reached 95% with nanofluid at wt. %= 0.1 and at flow rate of 3 lit/min. The largest temperature difference between the inlet and the outlet of the collector belonged to the graphene-methanol nanofluid at wt. % = 0.1, which represented the lowest heat capacity. Also, the daily thermal energy absorption of the collector was the highest at the tilt angle and the filling ratio values of 35 and 60%, respectively. (C) 2019 Elsevier Ltd. All rights reserved.