Are algae really feasible as fuel?
Great effort-and lots of money-is being expended to develop algae as a feedstock for transportation fuel. There doesn't seem to be a clear answer yet, however, as to whether algae will ever be economically viable for this purpose.
Research out of the US Pacific Northwest National Laboratory (PNNL; Richland, Washington). One of the biggest questions relating to growing algae for fuel is the amount of water needed for these microscopic plants. Researchers at the US Department of Energy's PNNL found that water use for this purpose will be much less if algae are grown in the US regions having the sunniest and most humid climates, that is, the coast of the Gulf of Mexico, the southeastern seaboard, and the Great Lakes.
Mark Wigmosta, lead author and a PNNL hydrologist, analyzed previously published data to determine how much algae can be grown in open, outdoor ponds of fresh water using current technologies. (For the purpose of the study, algae grown in salt water and covered ponds were not considered.)
First, the scientists developed a comprehensive national geographic information system database that evaluated topography, population, land use, and other information about the contiguous United States. This information allowed them to identify available areas that are suited for algal growth. Next, the scientists gathered 30 years of meteorological information to help them determine the amount of sunlight that algae could realistically use to photosynthesize, and how warm the ponds would become.
The researchers found that 21 billion gallons (79.5 thousand million liters) of algal oil, equal to the 2022 advanced biofuels goal set out by the Energy Independence and Security Act of 2007, can be produced with American-grown algae, or 17% of the petroleum that the United States imported in 2008 for transportation fuels. Furthermore, it could be grown on land roughly the size of South Carolina.
The authors also indicated that algae's water use isn't very different from most other biofuel sources. For a standard light-utility vehicle, they estimated growing algae uses 8.6-50.2 gallons of water per mile driven on algal biofuel. Previous research indicated that corn ethanol requires 0.6-61.9 gallons of water per mile driven. For comparison, conventional petroleum gasoline, which doesn't need to be grown-as do algae and corn-uses 0.09-0.3 gallons of water per mile.
Other advantages of algae are that they are productive (algae produce more than 80 times more oil than corn per hectare per year), that algae are not a widespread food source for humans, that they are considered a carbon-neutral energy source, and that they can grow in and clean municipal wastewater of pollutants such as nitrogen and phosphorus.
In a statement from PNNL, Wigmosta said, "Water is an important consideration when choosing a biofuel source. . . . Algae could be part of the solution to the nation's energy puzzle if we're smart about where we place growth ponds and the technical challenges to achieving commercial-scale algal biofuel production are met."
Research out of Kansas State University (K-State; Manhattan, Kansas, USA). Peter Pfromm led an interdisciplinary team at K-State that analyzed oil produced by algae as a source of biodiesel. The team applied engineering fundamentals-mainly a carbon mass balance-to evaluate the sustainability of algae-derived biodiesel.
The first part of the study focused on the science and technology of algae biodiesel. It showed that from a technical standpoint, producing algae-based biodiesel in a sustainable way works-but not to the extent needed to eliminate dependence on petroleum diesel. From the standpoint of sustainability, they found that the amount of algae diesel produced per day was drastically lower than the projected ideal quantities from many algae production concepts.
Pfromm commented, "We found that phycologists-algae scientists-maintain that some popular estimates of producing 200 to 500 grams of algae per square meter of open pond per day weren't feasible because there's simply not enough sunlight coming through the atmosphere to do so. Unless we can change the sun, such production is physically impossible."
Using a more realistic production number-50 grams per square meter per day-they determined it would take 11 square miles of open ponds making 14,000 tons of algae a day to replace 50 million gallons of petroleum diesel per year, or about 0.1% of the US annual diesel consumption.
The team is now analyzing data on the economics of algae production. Pfromm said, "Once money is involved, technological sustainability becomes theoretical because nobody is going to use the technology or science unless there's an incentive . . . [I]f it takes 20 years before anyone starts making a buck in profit, no one's going to back it."
Open ponds are the cheapest containment unit in which to grow algae. But as a production facility increases in size, so do the number of ponds it operates-and a facility close to 11 square miles in size is a steep investment. These ponds are also problematic because they are prone to invasions by algae-eating organisms or microorganisms that can be spread by the wind.
Growing algae in photobioreactors will stop algal predators and other contamination, but they are much more costly. Cooling becomes a necessity, because sunlight warms the containers and can overheat the algae. A refrigeration unit is too costly, but cooling half-a-million containers with water spraying is also costly. Additionally, the dirty containers have to be cleaned periodically to avoid sunlight-blocking buildup that would limit production.
Pfromm concluded, "Right now, the fundamentals are the problem. . . . The best option right now is to invest in fundamental research and design so that the yield can hopefully reach beyond the 50 grams per square meter per day on our most optimistic assumption."
Further information is available at www.eurekalert.org/pub_releases/2011-04/ksu-epa040511.php. The original report appeared in Bioresource Technology 102:1185-1193 (2011).