Heat transfer in a photoelectrochemical (PEC) hydrogen generation reactor system is studied numerically. Four different reactor designs are investigated in this work. Solar irradiation is spectrally separated into short and long wavelength parts depending on the energy band gap of the photoelectrode. The short wave energy is directed to the anode to generate electron and hole pairs, and the long wave energy is used for heating the reactor. Results indicate that the use of the excess long wave energy by careful reactor design can effectively increase the system efficiency. Comparing designs 1 and 4 under 6000 W/m(2) irradiation and a quantum efficiency of 30%, the enhancement of the solar-to-hydrogen efficiency is respectively 12.7% and 18.2% for electrodes with E-g = 2.1 eV and 3.2 eV, such as Fe2O3 and TiO2. Effects of several parameters on the PEG hydrogen reactor system are discussed in details. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.