Environmental Engineering Reference
In-Depth Information
irradiation. Photoautotrophic green algae and cyanobacteria use sunlight and
carbon dioxide as the sole sources for energy and carbon [1]. Based on a
preliminary engineering and economic analysis, biophotolysis processes
must achieve close to an overall 10% solar energy conversion efficiency to
be competitive with alternatives sources of renewable hydrogen [69].
4.4.1.1 Direct Biophotolysis
Direct biophotolysis of hydrogen production is a biological process that
utilizes light energy and photosynthetic systems of microalgae to convert
water into chemical energy.
2
H O Light energy
+
→
2
H
+ .
O
(4.12)
2
2
2
The solar energy is absorbed by the pigments at photosystem I (PSI), or
photosystem II (PSII) or both, which raises the energy level of electrons from
water oxidation when they are transferred from PSI via PSII to ferredoxin.
The concept of ''direct biophotolysis'' envisions light-driven simultaneous
O
2
evolution on the oxidizing side of PSII and H
2
production on the reducing
side of PSI, with a maximum H
2
: O
2
(mol : mol) ratio of 2 : 1 [70].
Since hydrogenase is sensitive to oxygen, it is necessary to maintain the
oxygen content at a low level under 0.1% so that hydrogen production can
be sustained. This condition can be obtained by the use of green algae
Chlamydomonas reinhardtii that can deplete oxygen during oxidative respi-
ration [23, 67]. Reported hydrogen production rates using this method are
approximately 0.07 mmol·h
-1
per liter [71, 72].
4.4.1.2 Indirect Biophotolysis
In indirect biophotolysis, the problem of sensitivity of the hydrogen evolving
process to oxygen is usually circumvented by separating oxygen and hydro-
gen [72-74]. The concept of indirect biophotolysis involves the following
four steps: [23] (1) biomass production by photosynthesis, (2) biomass con-
centration, (3) aerobic dark fermentation yielding 4 mol hydrogen/mol
glucose in the algae cell, along with 2 mol of acetates, and (4) conversion
of 2 mol of acetates into hydrogen. In a typical indirect biophotolysis, cya-
nobacteria can synthesize and evolve hydrogen through photosynthesis by
following reactions:
12
H O
+
6
CO
+
Light energy
→
C H O
+
6
O
(4.13)
2
2
6
12
6
2
C H O
+
12
H O Light energy
+
→
12
H
+
6
CO
.
(4.14)
6
12
6
2
2
2
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