Algae have been studied for many years as a potential renewable energy feedstock to produce motor fuels. Several aspects make algae an attractive fuel in the future, but there are many technological and economic challenges in algae cultivation, harvesting, and oil extraction that must be addressed before algae-based fuels can be commercially produced on a large scale.

Algae are plant-like organisms that convert light, carbon dioxide, water, nitro­gen, and phosphorus into oxygen and biomass. This includes lipids — the generic name for the primary storage form of natural oils. Single-cell algae (“microalgae”) are a compelling case of clean energy generators, because of the speed and effi­ciency with which they produce these lipids.

However, this green slime we know as algae can be very delicate, because algae are sometimes contaminated by bacteria, viruses, and even other undesirable algal species. These negative influences can reduce the quality and yield of the lipids.

Each Cell is a
Tiny Ethanol
Factory

Algae are like "free radicals” and they are not domesticated. Research today is concentrated on developing algal species efficient at lipid production and resistant to contamination.

Figure 4.2 provides a schematic overview of how algae can produce ethanol using sunlight through the process of photosynthesis.

The big attraction of algae is energy storage. Algae can produce more lipids per acre or hectare of harvested land than terrestrial plants because of this high lipid content and extremely rapid growth rates. In the United States, the National Renewable Energy Laboratory (NREL) estimates that the oil yield for a moderately productive algal species could be about 1200 gallons per acre (compared to 48 gallons per acre for soybeans) (www. nrel. gov/docs/fy08osti/42414.pdf).

The high productivity of algae could significantly reduce the land use associated with the production of biofuels. For example, it would take 62.5 million acres of soybeans (an area approximately the size of Wyoming) to produce the same 3 billion gallons of oil that could be produced from only 2.5 million acres of algae (an area approximately 70% the size of Connecticut). Three billion gallons of biodiesel represent about 8% of all the diesel fuel used for on-road transportation in the United States in 2008.

Algae have other desirable properties. Some can be grown on non-arable or non­productive land. They grow in brackish, saline, and fresh water, and can thrive in wastewater. Although algae can also produce valuable products such as vitamins and dietary supplements, they are not themselves a human food source so there is no direct competition between food and fuel. They do, however, compete with some of the nutrients required for growing food. Since they require carbon dioxide for growth, algae can also sequester carbon dioxide from power plants or other carbon dioxide sources.

Currently, there are “open” and “closed” approaches to cultivating algae. Open cultivation essentially grows algae much like it grows in nature. Open systems usually consist of one or more ponds exposed to the atmosphere, or protected in greenhouses. Although open systems are the cheapest of the current cultivation approaches, they create more risk for contamination. Other disadvantages include lack of temperature and light control, requiring that open systems must be located where the climate is warm and sunlight is abundant, such as in California.

Closed systems, called photobioreactors, typically comprise enclosed translucent containers that allow photosynthesis to occur. The plastic or glass containers are arranged to maximize algae exposure to light. Indoor systems require artificial light, while outdoor systems can use natural sunlight or a combination of sunlight and artificial illumination. In closed systems, temperature, evaporation loss, light intensity, and contamination by other algal species can be controlled better. However, elements needed for algal growth, such as water, carbon dioxide, and other minerals, must be artificially introduced. Scaling these input requirements for commercial production is difficult and expensive. Capital costs for closed systems are generally substantially higher than for open systems. Algae can be grown in closed systems anywhere in the world.

Scalability remains a major obstacle. Harvesting and oil extraction are relatively costly. Large volumes of water are needed to be managed and recycled in the processing of algae. In addition, the use of chemical solvents for extracting the oil and energy requirements for each phase of the harvesting and oil extraction process add cost to the process. Once the oil has been extracted, various conversion pathways exist for transforming the oil into a liquid fuel. Just like with Jatropha crude oil, “transesterification” is the pathway from algae oil to biodiesel. Alter­natively, you can refine crude oil of algae oil into jet fuel, very similar to fuels produced from petroleum.

Currently, most estimates of the production cost of algal oil range from $4 to $40 per gallon depending on the type of cultivation system used. Despite the many challenges, however, the US government, large energy companies, and venture capitalists are continuing to fund demonstration projects and research to develop large-scale algae-based biofuels for commercial application.

According to the German newspaper Der Spiegel (15 April 2009), Billy Glover, managing director of Environmental Strategy for Boeing Commercial Airplanes, said that Jatropha and Camelina represented the strongest near-term options; algae were described as technically acceptable, but “not quite ready for prime time” in

terms of developing a means of delivering large quantities of algae-based fuels on a commercial scale at the present time. Boeing has also commented that they believe algae-derived jet fuel will be the mainstay in the 2030-2050 time period.

Among the attractive characteristics of algal fuels are that they do not affect freshwater resources, can be produced anywhere in the world using ocean and wastewater, and are biodegradable and relatively harmless to the environment if spilled. Algae cost more per kilogram, yet can yield over 30 times more energy per hectare than other, second-generation biofuel crops. During photosynthesis, algae and other photosynthetic organisms capture carbon dioxide and sunlight, and convert them into oxygen and biomass. Up to 99% of the carbon dioxide in solution can be converted. The production of biofuels from algae does not reduce atmospheric carbon dioxide, because any carbon dioxide taken out of the atmosphere by the algae is returned when the biofuels are burned. They do, however, eliminate the introduction of new carbon dioxide by displacing fossil hydrocarbon fuels.

4.2.2