This paper presents the design optimization of a photovoltaic/thermal (PV/T) system using both non-concentrated and concentrated solar radiation. The system consists of a photovoltaic (PV) module using silicon solar cell and a thermal unit based on the direct absorption collector (DAC) concept. First, the working fluid of the thermal unit absorbs the solar infrared radiation. Then, the remaining visible light is transmitted and converted into electricity by the solar cell. This arrangement prevents excessive heating of the solar cell which would otherwise negatively affects its electrical efficiency. The optical properties of the working fluid were modeled based on the damped oscillator Lorentz-Drude model satisfying the Kramers-Kronig relations. The coefficients of the model were retrieved by inverse method based on genetic algorithm, in order to (i) maximize transmission of solar radiation between 200 nm and 800 nm and (ii) maximize absorption in the infrared part of the spectrum from 800 nm to 2000 nm. The results indicate that the optimum system can effectively and separately use the visible and infrared part of solar radiation. The thermal unit absorbs 89% of the infrared radiation for photothermal conversion and transmits 84% of visible light to the solar cell for photoelectric conversion. When reducing the mass flow rate, the outflow temperature of the working fluid reaches 74 degrees C, the temperature of the PV module remains around 31 degrees C at a constant electrical efficiency about 9.6%. Furthermore, when the incident solar irradiance increases from 800 W/m(2) to 8000 W/m(2), the system generates 196 degrees C working fluid with constant thermal efficiency around 40%, and the exergetic efficiency increases from 12% to 22%. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.