A numerical simulation of transport phenomena in the photo-electrochemical (PEC) reactor is performed. The transport phenomena equations include the NaviereStokes, the respective energy equation for electrolyte, and the radiative transfer equation (RTE). Two different designs, design A, and design B of photoelectrochemical reactors are suggested. The hydrogen production rate and solar-to-hydrogen efficiency are estimated for each design at different solar incident flux ranged from 500 to 2000 W/m2 in increments of 500 W/m2. Results have shown that the solar-to-hydrogen efficiency increases as solar flux increases for both designs. Its predicted values could reach 12.8% for design A, and 13.1% for design B. Moreover, by increasing the solar incident flux, the hydrogen volume production rate is increased as well. It is found to be 79 L/m2.h for design A, and 85.4 L/m2.h for design B. Comparison between currently predicted results and previous data indicates an enhancement of solar-to-hydrogen efficiency and hydrogen production that can be achieved with the suggested design.