Linda Benedict, Theegala, Chandra
Despite successful production and use of vegetable oil-based biodiesels, the contribution of these alternative fuels (including virgin oil, used cooking oil and animal fat) to the overall transportation fuel scenario is fractional at best. Biodiesel production accounts for about 1 percent of the 50-60 billion gallons of diesel needed annually in the United States. This amount is small primarily because of limitations in the production and availability of oil feedstocks, which are linked to the low energy density of traditional oilseed crops. Microalgae, on the other hand, produces lipids (or oils) at an energy density several orders of magnitude higher than traditional oilseed crops such as corn and soybeans. Microalgae are microscopic cells as contrasted with algae, which include larger plants like kelp and seaweed.
Like most vegetable oils, the microalgal oils can be easily converted (transesterified) to biodiesels by reacting with about 10 percent alcohol in the presence of a catalyst such as lye. According to one estimate, growing microalgae as a fuel-producing crop would require only 5 percent of existing U.S. crop acreage to meet 100 percent of present transportation fuel needs. This is compared to 1,700 percent of existing U.S. crop area under corn and 650 percent of existing U.S. crop area in soybeans.
With no commercial facility producing microalgal lipids on a continuous basis, estimates and educated projections from smaller cultures are the only means to assess the potential of microalgae as a fuel source. These estimates vary significantly. They range from 600 to 15,000 gallons per acre per year. The disparity can arise from variations in species, culture conditions, nutrients, environmental conditions, mode of operation (batch or continuous) or type of culture (open or sealed photobioreactor). Based on a U.S. Department of Energy’s Aquatic Species Program report, one could assume conservatively that microalgae with a 20 percent lipid content could produce about 2,000 gallons of oil per acre per year. Certain algal species have been reported to have lipid contents as high as 60 percent, which could yield 6,000 gallons per acre per year.
With the lowest microalgal lipid yields beating every known yield from land-based oilseed crops, the potential of microalgae is definitely worth further exploration. Several barriers to successful mass production must be overcome, however. These include:
Assuming the same conservative annual lipid productivity figure of 2,000 gallons per acre generated from 100 harvests per year, calculations reveal that the entire volume of 1 acre of culture water, at about 2 feet deep, has to be harvested to yield 20 gallons of oil. Revenue from these 20 gallons of oil must account for pond/photobioreactor capital costs, nutrients, maintenance costs, harvesting costs and lipid extraction costs. Cost-effective harvesting and lipid extraction are perhaps the most critical needs in the area of microalgal lipids. Without advances in these areas, it will be difficult for microalgae to compete with fossil diesel or soybean oil, which sell for about $2.50 to $3 a gallon. Until the technological advances are made or until the economic scenario changes, the huge potential of microalgae may remain commercially untapped.
The solution to the high microalgal oil production costs may also come from other or nontraditional approaches. For example, genetic research may one day permit accumulation of high concentration of lipids in filamentous microalgae, which traditionally have very low lipid contents but are fairly easy to harvest. A biorefinery model, where high value nutraceuticals or co-products are produced along with microalgal oils, may offer promise by tilting the economic balance.
Chandra Theegala, Associate Professor, Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.
(This article was published in the fall 2009 issue of Louisiana Agriculture.)