by the water splitting and remaining 20–30% comes from the catabolism of carbohydrate through fermentation pathway. In this way, sulphur deprivation mechanism employs both direct as well as indirect biophotolysis for H 2 generation
(Fig. 1). Re-addition of sulphur in limiting amounts during the H 2 production phase
helps in regenerating the depleted algal cells for another round of H 2 generation
without re-establishment of aerobic condition (Kosourov et al. 2005). However, the
cycling of algal suspension cultures between the sulphur deplete and sulphur replete
conditions is challenging and might become simpler by using the immobilized,
sulphur-deprived algal cells for sustained H 2 evolution (Laurinavichene et al. 2006).
Direct photolysis is an interesting process due to the fact that this process utilizes
the most abundant natural resources, solar energy and water for the production of
efficient fuel “hydrogen”. However, this process suffers from the limitation of low
yield and hydrogen production rate. The energy productivity via this process ranges
from 0.02 to 0.12 kJ/L/h (Yu and Takahashi 2007).
2.1.2 Indirect Biophotolysis
In this process, the reductive equivalents or electrons are directly derived by the
endogenously stored carbohydrates such as starch in green algae and glycogen in
cyanobacteria (Fig. 2). In this method firstly, during the photosynthesis, CO 2 fixation and accumulation of carbohydrate take place. Secondly, fermentation of the
carbon reserves occurs which leads to hydrogen production by the following
reaction:
Fig. 1 Biohydrogen production via direct and indirect biophotolysis carried out by green
microalgae
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205
(Fig. 1). Re-addition of sulphur in limiting amounts during the H 2 production phase
helps in regenerating the depleted algal cells for another round of H 2 generation
without re-establishment of aerobic condition (Kosourov et al. 2005). However, the
cycling of algal suspension cultures between the sulphur deplete and sulphur replete
conditions is challenging and might become simpler by using the immobilized,
sulphur-deprived algal cells for sustained H 2 evolution (Laurinavichene et al. 2006).
Direct photolysis is an interesting process due to the fact that this process utilizes
the most abundant natural resources, solar energy and water for the production of
efficient fuel “hydrogen”. However, this process suffers from the limitation of low
yield and hydrogen production rate. The energy productivity via this process ranges
from 0.02 to 0.12 kJ/L/h (Yu and Takahashi 2007).
2.1.2 Indirect Biophotolysis
In this process, the reductive equivalents or electrons are directly derived by the
endogenously stored carbohydrates such as starch in green algae and glycogen in
cyanobacteria (Fig. 2). In this method firstly, during the photosynthesis, CO 2 fixation and accumulation of carbohydrate take place. Secondly, fermentation of the
carbon reserves occurs which leads to hydrogen production by the following
reaction:
Fig. 1 Biohydrogen production via direct and indirect biophotolysis carried out by green
microalgae
10 Biofuels from Microalgae: Biohydrogen
205