Following biomass harvesting it is then necessary to extract the maximum energy possible from the biomass to provide the best return. The three main fuel types that have received the majority of research related to algal biomass are biodiesel, biogas and bioethanol. Due to the potentially high oil content of certain strains of algae, the ease of extraction and value of the end product biodiesel has received the most attention. Algal strains such as Chlorella are noted for being able to produce up to 70% oil content within their cell walls [14]. This scenario however requires very specific conditions (low nitrogen and no contamination) and would be very diffi­cult to obtain in practice. It has been suggested by Sialve et al. [85] that it would not be economically viable to extract lipids from algae containing an oil yield any less than 40%, and therefore for the majority of algal spe­cies anaerobic digestion would provide the highest positive energy bal­ance due to low input requirements. Similarly in their environmental study Clarens et al. [97] found that direct combustion of biomass to produce electricity provided the highest energy return on investment when com­pared to anaerobic digestion, biodiesel production plus anaerobic diges­tion and biodiesel production plus direct combustion.

Given the high energy consumption required to produce biodiesel from algae, the process does not seem beneficial to be used for a flexible system where the algae cultivated are likely to be a mix of species with low lipid content. Bioethanol has potential and in such a system the algae is likely to contain a high proportion of convertible carbohydrates but the energy balance of such a process is untested and is unlikely to yield great efficien­cies in the near future and is likely to require high inputs (enzymes and yeast). It seems far more likely that anaerobic digestion or combustion of the biomass will provide the maximum energy recovery. Another benefit of such a concept is that facilities to carry out the digestion or combus­tion are likely to be already operating on site with no requirement for new development.