Environmental Engineering Reference
In-Depth Information
typically at between 1 and 2%. The reasons for such a difference generally revolve
around rate limitations due to factors other than light (H
2
O and nutrient availabil-
ity, for example), photosaturation (some plants, or portions of plants receive more
sunlight than they can process while others receive less than they could process),
and photorespiration due to Rubisco (the protein that serves ultimately as a catalyst
for photosynthesis) also accepting atmospheric O
2
(rather than CO
2
), resulting in
photorespiration.
In the US, the average daily incident solar energy (across the entire spectrum)
reaching the earth's surface ranges from 12,000 to 22,000 kJ/m
2
(varying primarily
with latitude). If the maximum photosynthetic efficiency is 11.6%, then the max-
imum conversion to chemical energy is around 1,400-2,550 kJ/m
2
/day, or 3.8
×
10
12
J/acre-year in the sunniest parts of the country. Assuming the heating value of
biodiesel to be 0.137 GJ/gal, the maximum possible biodiesel production in the sun-
niest part of the US works out to be approximately 28,000 gal/acre-year, assuming
100% conversion of algae biomass to biodiesel, which is infeasible.
It is important to keep in mind that this is strictly a theoretical “upper limit” based
on the quantum limits to photosynthetic efficiency, and does not account for factors
that decrease efficiency and conversion. Based on this simple analysis though, it is
clear that claims of algal biodiesel production yields in excess of 40,000 gal/acre-
year or higher should be viewed with considerable skepticism. While such yields
may be possible with artificial lighting, this approach would be very ill-advised, as
at best only about 1% of the energy of the energy used to power the lights would
ultimately be turned into a liquid fuel (clearly, one needs to look at the overall
efficiency).
This upper limit also allows us to assess how truly inefficient many crops are
when viewed strictly as biofuel producers. With soybeans yielding on average 60
gal of oil (and hence biodiesel) per acre-year, the actual fuel production is stag-
geringly small in comparison to the amount of solar energy available. This should
further make it clear that using typical biofuels for the purpose of electricity gener-
ation (as opposed to the transportation sector) is an inefficient means of harnessing
solar energy. Considering that photovoltaic panels currently on the market achieve
net efficiencies (for solar energy to electrical energy) on the order of 15-20%, with
multi-layer photovoltaics and solar thermal-electric systems achieving efficiencies
of twice that in trial runs, biomass to electricity production falls far behind (con-
sidering typical plant photosynthetic efficiencies of 1-2%), with conversion of that
biomass energy to electrical energy dropping the net efficiency to well under 1%.
Currently, the research for algae growth for fuel production is being done using
photobioreactors. Unfortunately, current designs demand a high capital cost, which
makes large-scale production uneconomical until a low cost design or new method
of production is discovered. Storing energy as oil rather than as carbohydrates slows
the reproduction rate of any algae, so higher oil strains generally grow slower than
low oil strains. The result is that an open system (such as open raceway ponds) is
readily taken over by lower oil strains, despite efforts to maintain a culture of higher
oil algae. Attempts to grow higher oil extremophiles, which can survive in extreme
conditions (such as high salinity or alkalinity) that most other strains cannot tolerate,
have yielded poor results, in terms of the net productivity of the system. While an
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