© (2014) by the Committee of WHEC2014.In applications of industrial biotechnology, maintaining an optimal temperature range is crucial for growth and proper functioning of microorganisms. For outdoor photobiological hydrogen production many parameters are beyond manipulation, hence effective control of temperature in photobioreactors is a challenge. In this work, an internal cooling system was designed and built, and its performance in outdoor tubular photobioreactors tested during summer months in Ankara, Turkey. Media with and without bacteria (Rhodobacter capsulatus YO3) were used. Countercurrent and co-current cooling modes were implemented to stabilize the reactor temperature. The temperatures were found to be strongly influenced by solar irradiation and ambient air temperature during daytime, and the surface temperature was found to be approximately constant along the reactor length. Heat effects on the external pumping and piping units were found to significantly increase the cooling duty. Counter-current cooling was found to be more effective compared to co-current cooling in controlling temperatures inside the reactor. High biomass growth rate (0.10 1/h) and hydrogen production rate (maximum 1.28 mmol/L/h) was achieved in the outdoor operations.