Photobioreactors: Design and performance with respect to light energy input

Abstract

Photosynthesis is the most important natural process for the neogenesis of biological material. More than 40 000 species of oligocellular algae constitute a unique biochemical fund in the plant world. As the energy supply to the phototrophic microorganisms is transported via photons and as the physics of light distribution is a technical challenge, the technique of biomass production of single cell algae as against heterotrophic organisms is in a stage of intensive development. In contrast to area intensive open pond technology, space saving closed cultivation systems could be gradually established in the market of industrial photobioreactors. The present review surveys outdoor cultivation systems and indoor photobioreactors with respect to their light utilization and productivity. To evaluation reactor performance independently of their design and location, growth yields were recalculated from published data. It became obvious that closed systems may out-perform raceway ponds by about 300%, which represents productivities over 100 g dry algal biomass per m2 per day. Thus, economic aspects for the cultivation of phototrophs, even in moderate climates, are better satisfied, but microalgal biotechnology is still of minor importance because of high biomass production costs. Considering indoor systems with internal illumination arrangements, the growth potential can be increased by a further 100%. Due to the high technical expenses, this technology will be confined to the production of high value products, e.g. pharmaceuticals. Consequently, very defined axenic cultivitation has to be guaranteed by all means available.