Photobioreactor design and illumination systems for H2 production with anoxygenic photosynthetic bacteria: A review

•Advances in photobioreactor design for high light conversion efficiency are discussed.•Outdoor studies on sunlight utilization for H2 production are reported and discussed.•The way of calculating light conversion efficiency is discussed.•Light distribution and light/dark cycles are the main issues to be addressed.

H2 is a clean, renewable and energy-efficient fuel. However, in order for it to be a fuel effectively utilizable at an industrial level, key issues about its economically and environmentally sustainable production have still to be solved. Microbial hydrogen production is a process with a low environmental impact and, among microbial processes, photofermentation is considered a promising and sustainable solution. However, the energy input for the biological processes is still higher than the energy output in the form of H2 gas. One possibility for improving this ratio is to increase the efficiency of the process while at the same time reducing electricity consumption, both of which relate to the issue of an optimal photobioreactor design.This review focuses on recent advances made in photobioreactor design towards higher light conversion efficiency, a greater hydrogen production rate and substrate conversion in hydrogen production processes carried out with purple non sulfur bacteria, giving particular attention to the light source and to illumination protocols. Recent achievements in outdoor hydrogen production in large scale photobioreactors are also reviewed.