Environmental Engineering Reference
In-Depth Information
2.3. Direct Photolysis
Direct photolysis is the process in which bidirectional hydrogenases use
photosynthetically-activated electrons, via reduced Fd and/or NADPH, to reduce the
hydrogen ion (H+)to H
2
[28] (Figure 16). This process is carried out by green algae such as
Chlamydomonas reinhardtii, Scenedesmus, and Chlorella [22, 33, 28], as well as by
cyanobacteria such as Synechocystis [13]. Light-dependent H2 production by cyanobacteria
utilizing nitrogenases and photosynthetically-generated ATP is known as indirect photolysis,
and light-dependent H2 production by anoxygenic phototrophs utilizing organic electron
donors, also making use of nitrogenases, is known instead as photofermentation.
2.3.1. Photosynthetic efficiency
. Energy efficiency, defined as the ratio of energy
produced as H2 to the resources consumed by the microorganism (including its requirements
for space as well as nutrients), is a central consideration guiding research directions and
assessment of commercial applicability of biohydrogen systems. The rate of H2 production is
an equally important concern, however, and systems of lower efficiency but higher H2
production rate may be competitive with those of higher efficiency in cases where substrate
and space costs are low [34, 5].
For photochemical processes, Einstein's law of photochemistry states that a primary
photochemical process is caused by the action of one absorbed photon acting on another
molecule, emphasizing the important fact that a photon must first be absorbed before it can
carry out photochemistry. The next important parameter is the photochemical quantum yield,
Φ, defined as the ratio of the number of photochemical products to the number of absorbed
protons. The quantum yield is a measure of the efficiency of the photochemical proces: the
primary quantum yield ranges from one for a process in which every absorbed photon leads to
products, to zero when no products are formed. In photosynthetic systems, the primary
quantum yields are often close to one under optimal conditions, indicating that almost all
absorbed photons are effective in forming initial products. However, more than one photon is
usually needed to produce a stable final product such as H
2
, so overall quantum yields are
usually much less than one [7].
As a consequence in part of the high primary quantum yield, direct photolysis has a very
high potential energetic efficiency compared to other known systems [35-39]. This efficiency
is estimated to be as high as 10 percent for microalgal photosynthesis resulting in CO
2
fixation [34] and as high as 24 percent for H2 production, under ideal conditions [35]. For
comparison, a typical commercial steam turbine generator is about 30 percent efficient,
photovoltaic cells also approach 30 percent efficiency, automobiles are ~20 percent efficient,
and a well-tuned bicycle rates about 75 percent [40].
The important limitation to photosynthetic conversion efficiency lies in the difficulty of
providing so-called ideal conditions to photosynthetic cultures, resulting in reported
conversion efficiencies that are usually less than 1 percent [34].
2.3.2. Light saturation effect.
A salient difficulty in providing ideal conditions for
phototroph culture involves provision of the light itself. The microbial aspect of this problem
lies in the ability of both algae and cyanobacteria to absorb many more photons with the light-
harvesting antennae, and therefore to generate many more excited electrons, than the
photosynthetic electron transport chain can accommodate. Although these microbes have
evolved sensitive regulatory systems to optimize the size of their photon-gathering antennae
(chlorophylls a and b, xanthophylls, phycobilins, etc.), diminishing them considerably under
high light and increasing them under low light, phototrophs nevertheless typically absorb
