Environmental Engineering Reference
In-Depth Information
Biological productivity has been identified as the single largest influence on fuel cost (DOE,
2010; Yang et al. , 2011). This includes such parameters as growth rate, metabolite production,
tolerance to environmental variables, nutrient requirements, resistance to predators, and the cul-
turing system. As with jatropha, the reported yields for algal biomass have been wildly disparate
because the technology for mass production is not mature.
The two major categories of growth facilities for photosynthetic production are: (i) open ponds,
typically in a racetrack configuration with flow driven by a paddlewheel; and (ii) closed, transpar-
ent photobioreactor systems that consist of an array of cylindrical tubes or an enclosure formed
by two flat plates. Open ponds are the simplest design, and they are by far the least expensive to
set up, operate, and clean after a growth cycle (about 2 weeks). However, they must contend with
environmental conditions, such as large temperature swings, evaporative losses, and the threat
of contamination by opportunistic wild species and predators. The algae selected for this type
of operation must be hardy and fast-growing to outperform any competitors that are present in
the environment. There are many examples of commercial success with this type of aquaculture
for higher-value products, such as
3 fatty acids. Photobioreactors are, in com-
parison, expensive to build, run, and maintain. On the other hand, they can operate at higher
algal concentrations, so that the water that must be extracted at the end of the process is reduced.
Depending on the design, the algae may have better access to light. They are also better equipped
to handle less robust organisms, such as algae that have been genetically modified for improved
oil production. Scale-up remains an issue; there is a limit as to the length of the cylindrical tubes
if it is desirable to maintain a uniform temperature, pH, and dissolved gas content in the growth
medium. Other design challenges are that heat and oxygen must be removed from the reactor, and
a carbon source such as carbon dioxide must be replenished. The design must also have a protocol
for preventing or removing biofilm buildup on the transparent surfaces. Both types of systems
have strong advocates. For further discussion, see Brennan and Owende (2010), Carvalho et al .
(2006), Cheng and Ogden (2011), and Jorquera et al. (2010). Within a few years, production
facilities that are under construction or in development should provide more quantitative data,
and there may well be a complementary role for open and closed systems.
In open-pond raceways that are supplied with nutrients from the flue gas of power plants,
many strains of algae in an autotrophic growth regime can consistently increase its biomass
on average by 20 grams (dry weight) per square meter of pond surface area per day (g/m 2 /d),
(Ben-Amotz, A., personal commun. , 2009). Overall biomass production from the Aquatic
Species Program averaged 10 g/m 2 /d, but at times achieved up to a maximum of 50 g/m 2 /d
(Sheehan et al ., 1998). Theoretically, the average maximum yield could be substantially higher
than that, but its attainment is elusive in a commercial-scale setting.
For algae that generate enough oil to be considered for biodiesel production, they are usually
about 15-45% lipids when grown in the lab, depending on the strain and growth conditions,
although some studies report lipid contents of up to 70%. Weyer and co-workers (Weyer et al. ,
2010) examined the theoretical maxima for oil production in many geographical locations
with an optimistic algal lipid content of 50% and with efficiencies for photon transmission,
photon conversion, and biomass conversion of 95, 50 and 50%, respectively. With those assump-
tions, the annual oil yield ranged from the worst case in Kuala Lumpur of 40,700 L/ha/y
(4350 gal/acre/y) to the best case in Phoenix, AZ of 53,200 L/ha/y (5700 gal/acre/y)
This work is consistent with another study that examined theoretical maxima. Cooney and
co-workers (Cooney et al. , 2011) assumed that the maximum solar irradiance normal to the
earth's surface at ground level is 1000 watts per square meter [W/m 2 ], multiplied by correction
factors that correspond to a sunny location with occasional clouds and account for the changing
angle of the sun during the day. Since algae can only use a specific spectrum of the incoming
sunlight (about 45% of it), that number is also multiplied by a correction factor of 0.45. Then,
they used the highest photosynthetic conversion efficiencies to represent the maximum biomass
that the algae could produce from that incoming energy. By relating the specific energy of the
biomass constituents (protein and carbohydrates at 16.7MJ/kg, lipids at 37.4MJ/kg, and ash with
zero energy content), they could derive an expression for biomass production as a function of lipid
β
-carotene and
ω
Search WWH ::




Custom Search