Photobiological Systems. Photosynthetic organisms carry out an energy-storing fuel production reaction, which stores solar energy in vast quantities all over the world. Normally, photosynthetic systems do not evolve hydrogen, but rather reduce CO2 to carbohydrates. However, it is possible to modify conditions such that the reducing end of the photosynthetic process (Photosystem I) is coupled to a hydrogen-evolving enzyme, such as hydrogenase or nitrogenase. Three recent reviews (Xxxxxxxxx, 1988b; Benemann, 1994a,b; Markov et al., 1995) cover this field quite well. The most effective photobiological systems for H2/O2 evolution are those based on microalgae, such as green algae and cyanobacteria. Green algae produce hydrogen after incubation under anaerobic conditions, during which hydrogenase is synthesized and activated. When these dark-adapted algae are returned to the light, they evolve H2 and O2 initially at efficiencies approaching 12% (Xxxxxxxxx, 1988a), but yields fall off as normal photosynthesis is reestablished. Nitrogen-fixing cyanobacteria can produce hydrogen, catalyzed by the nitrogenase enzyme, if the bacteria are starved of N2.
Photobiological Systems. Photobiological systems have the distinct advantage that the collector system self assembles. Thus as long as the cells can be kept alive and viable for extended periods of time, the capital costs of such a systems are potentially quite low. However, there may be considerable costs involved in the fabrication of reactor systems necessary to maintain the organisms under optimum conditions. As noted in Section 4.3, these systems saturate at unacceptably low irradiances. Nevertheless, there is a reasonable possibility that this deficiency can be removed by genetic engineering. Another major disadvantage is that hydrogen and oxygen are generated together and must be separated rapidly to avoid an severe explosion hazard. Benemann (1994) has proposed a two-stage process in which green algae would be cultivated in large open ponds to produce a high carbohydrate algal biomass, which then becomes the substrate for the production of hydrogen in closed tubular photobioreactors.
