Nowadays energy is an essential factor in economic growth of human societies. Among all kinds of energy sources available, solar energy is the most abundant. Both nature and human kind invented different ways for using energy of sun. Converting solar energy to heat is the simplest way to use it. In the last decades different types of solar water heaters have been developed and used. Heart of every solar water heater is its solar collector. Mainly two types of solar collectors are used: flat plate solar collectors and evacuated tube solar collectors. In comparison to evacuated tube solar collectors, flat plate solar collectors have more heat losses, so it has less thermal efficiency, especially in higher temperatures. In this research filled-type U-tube evacuated tube solar collector has been chosen for numerical simulations. Effect of length, diameter, configurations of U-tube in the collector, thermal conductivity of filled material and CuO, Al 2 O 3 and TiO 2 aqueous nanofluids as working fluid on heat transfer and thermal efficiency of solar collector have been studied. It was been found that increasing length, diameter and thermal conductivity of filled material leads to an increase in thermal efficiency of solar collector. Among all five configurations of U-tube in collector studied, configuration in which the U-tube is mounted vertically is the efficient. Also when U-tube was mounted horizontally, the collector has less efficiency compared to other configurations. It has been revealed that using nanofluid as heat transfer medium in the evacuated tube solar collector leads to an increase in thermal efficiency. CuO nanofluid leads to an increase of about 13.8 percent in efficiency of solar collector. Efficiency of CuO nanofluid is about 1.3 and 1.5 times for that of TiO 2 and Al 2 O 3 nanofluids respectively. Due to reduction in heat capacities of nanofluids with increasing nanoparticle concentration, it can be deduced that heat capacity is a key factor in evacuated tube solar collectors when using nanofluids. Key works : solar energy, evacuated tube solar collectors, thermal efficiency, and nanofluid
