An investigation of serial connected U-pipe evacuated tube solar collector performance using TiO2/Water nanofluid

•Thermal efficiency of serial connected U-pipe evacuated tube collector performed.•Maximum thermal efficiency was 80% with using nanofluid 2% concentration.•Efficiency increased by 14.3% to 22.1% compared to water with nanofluid 2%.•Pressure drop of working fluid increased 20.2% with nanofluid 2% concentration.•The lowest efficiency was almost 60% with water using as working fluid.