goals for developing algal biofuels are to contribute to energy security by providing domestically sourced fuels, to maintain and enhance the natural resource base and environmental quality, to produce fuel that is economically viable, and to enhance the quality of life for society as a whole. Although economics is an important aspect of sustainability, this report does not assess the economics or costs of algal biofuels, as specified in the statement of task. Heterotrophic approaches3 for algae cultivation are not considered in this report because DOE-EERE considers the production of biofuel using heterotrophic algae a biochemical pathway to convert another feedstock (a sugar source such as cellulosic biomass) rather than a pathway that uses algae as a feedstock for fuels.
The intent of this report is to help anticipate the major sustainability concerns associated with resource use and the potential environmental and societal consequences if commercial-scale algal biofuel production is widely deployed and to explore the opportunities for mitigating the concerns. However, the ultimate productivity of algal biofuels, some of their resource use and environmental concerns, and some strategies for mitigating the concerns might affect the economic viability of algal biofuels. This report makes reference to economics if there are synergies or trade-offs among economics, productivity, resource use, and environmental effects. This report also discusses tools for assessing the multi-attribute nature of sustainability of algal biofuels.
POTENTIAL SUSTAINABILITY CONCERNS
Assessing the sustainability of an algal biofuel requires an understanding of the individual components that make up an algal biofuel production system. An algal biofuel production system involves cultivating selected strain(s) of algae; collecting the biomass and dewatering it, if necessary; and processing the algal lipid, biomass, or secreted products into fuels and possibly other co-products. The production of fuels and energy from algae is not an established industry and a variety of production systems have been proposed. Figure S-1 is a simplified diagram that attempts to limit and group the potential steps in the algal biofuel production pathway. Each row of the diagram details a processing step or process option. Different combinations of cultivation and processing options have resulted in more than 60 different proposed pathways for producing algal biofuels.
Based on a review of literature published until the authoring of this report, the committee concluded that the scale-up of algal biofuel production sufficient to meet at least 5 percent of U.S. demand for transportation fuels4 would place unsustainable demands on energy, water, and nutrients with current technologies and knowledge. However, the potential to shift this dynamic through improvements in biological and engineering variables exists.
For some system designs analyzed, the energy outputs of algal biofuels (and co-products if they are produced) are less than the energy inputs for producing the fuel. Estimated values for energy return on investment range from 0.13 to 3.33. The estimated
3 Some algae can grow heterotrophically in the absence of light by taking up organic molecules (such as glucose) as a source of carbon.
4 U.S. consumption of fuels for transportation was about 784 billion liters in 2010. Five percent of the annual U.S. consumption of transportation fuels, which would be about 39 billion liters, is mentioned to provide a quantitative illustration of the water and nutrients required to produce algal biofuels to meet a small portion of the U.S. fuel demand.