Application of nanofluids as a potential working fluid in solar-to-thermal energy conversion systems has shown remarkable improvements in solar power systems. Herein, a combined numerical and experimental study has been conducted on a nanofluid direct absorption collector utilizing three types of nanoparticles (i.e., graphite, magnetite, and silver) dispersed in deionized water as the absorbing medium. To increase the dispersion stability, surface modification was performed on the nanoparticles prior to the preparation of nanofluids via the two-step method and the optical characteristics of nanofluids were experimentally evaluated prior to their use in the DASC. A two-dimensional computational fluid dynamics simulation model was developed to solve the radiative transfer in particulate media and heat transfer equations. Considering the absorption and scattering within the nanofluid medium, the nanofluid temperature distribution within the collector was evaluated. Simultaneously, experiments were performed on a direct absorption collector to validate the numerical model and to investigate the effect of solar flux intensity, nanoparticle concentration and flow rate on the collector thermal performance. According to the results, nanofluids promoted the thermal and exergy efficiencies by 33-57% and 13-20%, respectively than the base fluid. (C) 2016 Elsevier Ltd. All rights reserved.