In the present study, in addition to using nanofluid, the flow direction in a flat plate solar collector is changed to increase the convective heat transfer coefficient. To this end, U-shaped, wavy and spiral pipes with identical pipe lengths on a flat plate collector are simulated. Three-dimensional and steady state equations of continuity, momentum, SST k-omega turbulence model, and energy are solved. Al2O3/water and CuO/water nanofluids are used in volume fractions of 1% and 4%. Results show that using wavy and spiral pipes can significantly increase the heat transfer coefficient and Nusselt number. Also, it is observed that the pressure drop has its highest value for the wavy pipes. In all cases, the heat transfer coefficient increases by using nanofluid instead of water. In all cases except for the CuO 4%, the Nusselt number has decreased due to a remarkable increase in thermal conductivity by adding nanoparticles to water. Results reveal that by using wavy pipes and CuO/water nanofluid with a volume fraction of 4%, the heat transfer coefficient can increase up to 78.25%. (C) 2019 Elsevier Ltd. All rights reserved.