Abstract: To select suitable nanofluids used as recycle fluids for direct absorption solar collector,three nanofluids(copper,carbon black,carbon-coated copper) of different concentrations were prepared by the "two step method".The spectral absorption properties and photo-thermal conversion properties were compared among the three nanofluids through measuring the transmissivity and insolation experiment.The results show that the spectral absorption properties of copper nanofluids and carbon-coated copper n...