Indirect biophotolysis

In indirect biophotolysis problems of the sensitivity of the hydrogen evolving process are potentially circumvented by separating temporally and/or spatially oxygen evolution and hydrogen evolution. Thus indirect biophotolysis processes involve the separation of the H2 and 02 evolution reactions in different stages. 

In conclusion, the rates and efficiencies of hydrogen production by systems directly using photosynthesis fall short of even plausible economic feasibility. Since organic substrates are the ultimate source of hydrogen in photofermentations or indirect biophotolysis processes, it can be argued that it should be simpler and more efficient to extract the hydrogen from substrates using a dark fermentation process, an approach which is discussed next. 

Fig. 2.11 Nitrogenase mediated indirect biophotolysis process in cyanobacteria.

Both direct and indirect biophotolysis routes suffer from a number of challenges that currently prohibit them from becoming commercially viable processes [180] these include, but are not limited to, solar energy conversion efficiency and the difficulty of bioreactor design. In particular the low solar energy to hydrogen conversion efficiency is a matter of major concern in the field of biophotolysis. The efficiency is calculated as [180,172] ... 

A preliminary assessment of a conceptualized two stage indirect biophotolysis system was reported by John Benemann (1997) to the International Energy Agency (IEA) which in... 

The five most important technologies for biological hydrogen production are direct biophotolysis, indirect biophotolysis, biological water-gas shift reaction, photo-fermentation and dark fermentation (Ni et ah, 2006). 

In indirect biophotolysis, the problem of sensitivity of the H2 evolving process to O2 is usually circumvented by separating O2 and H2 (Figure 13.9b). 

Biophotolysis of water through (a) direct biophotolysis and (b) indirect biophotolysis. 

Under indirect biophotolysis, carbohydrates are firstly produced and accumulated through photosynthesis, and are then converted for hydrogen production via anaerobic fermentation (Noth et al., 2013). There are two types of indirect biophotolysis depending on how photosynthesis and fermentation processes are separated, ie, temporal separation and spatial separation. 

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