Environmental Engineering Reference
In-Depth Information
many more photons than they are able to use productively. Wild-type algae exposed to full
sunlight absorb up to nine times more photons than they can accommodate, wasting the
remainder in the form of heat or fluorescence and shading cells below them quite effectively.
This phenomenon is known as the light-saturation effect and represents one of the prominent
challenges facing commercialization of direct photolysis [34].
One promising approach to this problem is the creation of algal mutants with diminished
light-harvesting antennae, with the logic that a photosynthetic apparatus with less light-
harvesting capacity absorbs fewer photons at high light intensities and therefore wastes fewer
photons. Japanese researchers working with microalgal mutants with reduced antenna sizes
found increases in productivity of up to 50 percent under high light intensity, compared to the
wild-type [41, 42]; antenna mutants isolated in the United States also showed efficiency
improvements, further supporting the potential of this approach [43, 44]. Extensive cost
analysis by modelers at the NREL confirmed the value of increased light transmission:
increasing incident light transmission by a factor of 10, thought to be well within the
capability of modern genetic techniques, reduced the cost of H2 production 57 percent [45].
2.3.3. Land use
. The land-use aspect of the light provision problem is the relatively low
density of solar energy, estimated at a maximum of 5 kilowatt hours per square meter per day
or 6.6 gigajoules per square meter per year in the most favorable locations [34, 46]. Assuming
a conversion of 10 percent of the solar energy into H2 and a price for H2 of $15 per gigajoule
(the benchmark price set by DOE for H
2
), this computes to ~0.66 gigajoules or ~$10 H
2
per
square meter per year [34]. For comparison, an average electrically-heated house in North
America consumes approximately 55-70 gigajoules of energy per year [5, 47, 48]. While this
discrepancy seems large, it is worth noting that energy-efficient construction and practices,
including higher-density housing, can reduce residential energy requirements substantially.
Even without such measures, it is instructive to realize that, given a 10 percent solar
conversion efficiency, the annual energy needs of the United States could be met by a square
~100 miles on a side located in a virtually unoccupied area of southern Nevada [49].
To address the problem presented by the limits of solar irradiance, potentially requiring
large land areas for photobioreactors, efforts have been made to optimize vertical
photobioreactor arrays [34], to investigate thin-layer bioreactors (0.5-5 centimeters in depth)
with corresponding low masses that could be installed on rooftops [45], and most
futuristically, to design solar collectors to transmit solar energy to bioreactors through optical
fibers [50].
2.3.4. Oxygen sensitivity
. The second primary difficulty in providing ideal conditions for
direct photolysis lies in the extreme sensitivity of hydrogenases to the molecular oxygen
produced by photosynthesis. As a result, cultures maintained under dark, anoxic conditions to
induce hydrogenase synthesis are able to sustain H
2
production for only a few minutes
following exposure to light and consequent photosynthetic O2 generation [51]. In response to
this problem, two primary approaches are being taken: the first is the development of
specialized bioreactors to separate O
2
generation from H2 generation temporally, and the
second is the effort to generate O
2
-tolerant hydrogenases through various mutagenic methods.
The most successful approach to date, to minimize the problem of O
2
inhibition of the
algal Fe
hydrogenase,
has been the development of the two-stage photobioreactor. In this
system, algal cultures experience two alternating growth conditions: the first supports
photoautotrophic growth, in the presence of all essential nutrients as well as O
2
, while the
second deprives the cultures of both O2 and sulfur. Reduced sulfur is essential to synthesis of
