▎ 摘　　要

Solar collectors are essential to enable the direct use of solar energy. Graphene nanoparticles stand out among other nanoparticles because it presents high thermal conductivity. The aim of this study is to evaluate the thermal efficiency increase achieved applying multilayer graphene (MLG) in low volumetric fractions dispersed in water. An evacuated tube solar collector equipped with parabolic concentrator was herein used in a closed circuit. Thermal efficiency was evaluated considering different recirculating flow rates (6, 24, 42 and 60 L h(-1)), two volumetric fractions of MLG (0.00045% and 0.00068%) with and without concentrator. A semi-empirical equation was developed to estimate thermal efficiency based on hydraulic- and thermal-flow parameters. In relation to the flow rate the best performance was obtained at 42 Lh(-1). Furthermore, the use of the parabolic concentrator increased by approximately 298% the thermal efficiency in comparison to the collector without concentrator. An excellent fit (R-2 = 0.970) was obtained between efficiencies estimated through the semi-empirical equation and those estimated through measurements performed in the current study. MLG nanofluid in concentrations of 0.00045 vol% and 0.00068 vol% increased the thermal efficiency of the solar collector by 31% and 76%, respectively in comparison to the base fluid. (C) 2019 Elsevier Ltd. All rights reserved.