The main objective of this work is to investigate the utilization of nanofluids in the solar field in order to achieve higher system performance. An Organic Rankine Cycle (ORC) driven by solar parabolic trough collectors (PTCs) is the examined system. Four different nanoparticles are examined (Al2O3, CuO, TiO2 and Cu) in the base fluid (Syltherm 800), as well as the pure thermal oil is examined as working fluid. The examined ORC is a regenerative cycle and four organic fluids are also tested (toluene, MDM, cyclohexane and n-pentane). In every combination between the organic fluid in ORC and working fluid (nanofluid) in the solar field, an optimization procedure is followed. The concentration of every nanoparticle and the pressure ratio (pressure in the turbine inlet to critical pressure) are the optimization parameters. According to the final results, toluene is the best organic fluid and CuO is the most suitable nanoparticle. The combination of these two working fluids leads to 167.05 kW electricity production and to 20.11% system efficiency with concentration 4.16%. The enhancement by the use of nanofluids is found up to 1.75% compared to the respective case with pure thermal oil and this result indicates that the use of them is able to improve the performance of solar driven ORCs. For the other nanoparticles and Toluene in the ORC, Cu, Al2O3 and TiO2 lead to 166.18 kW, 165.72 kW and 165.60 kW electricity productions respectively with optimum concentrations 3.98%, 2.51% and 2.57% respectively. (C) 2017 Elsevier Ltd. All rights reserved.