(NAS-NAE-NRC, 2010, p.292). Scenarios do not simply extrapolate historical data but try to develop internally consistent sets of conditions that are needed to occur to attain a given set of outcomes. For example, scenarios for algal biofuels might look at potential system design, resource requirements, and infrastructure needs required to reach a given percentage of the liquid fluids market. Scenarios can help define the environmental and resource sustainability issues that might accompany a greatly expanded algal biofuel production system.
Scenarios are a part of a more general future analysis. Elements of future analysis include trend projections, systems modeling, and scenarios; the analysis can combine these elements in different ways. Trend projection involves extrapolation of retrospective information to the future. A central element of the trend projection process is simply deciding on a functional form for the trend, such as linear, exponential, or some other relationship (Craig et al., 2002). Further, trend analysis is best for known systems where there is a large quantity of historical data, which is not true for algal biofuels. Systems modeling consists of identifying relationships between variables of interest (Ibid). For example, an econometric model finds the optimal statistical fit between variables that are assumed to be related by a predefined functional form. A systems dynamics model develops causal relationships between quantities of interest and evolves the future from some initial condition using these relationships. Future issues relevant to the sustainability of algal biofuels include: how individual technology elements will develop (for example, algae cultivation), how technology elements will combine to yield a fuel production system, and how the production system will link to natural systems (for example, salt versus fresh water).
Algal biofuels can be produced from a variety of feedstocks (autotrophic microalgae and cyanobacteria, heterotrophic microalgae, and macroalgae) using different processing technologies (for example, transesterification of algal oil, thermochemical conversion of algal biomass such as gasification and pyrolysis, or direct synthesis of alcohol). Examining the promise of different combinations of feedstocks and processing technologies to sustainably develop algal biofuels within the timeframe of this study was not feasible. Therefore, the committee limited the scope of the report in three ways following the guidance of the study sponsor and the committee’s expert judgment.
First, this study focuses on biofuel production systems that use autotrophic microalgae as a feedstock in the United States. Heterotrophic approaches for algae cultivation are excluded because DOE-EERE considers production of biofuel using heterotrophic algae as a biochemical pathway to convert another feedstock (a sugar source such as cellulosic biomass) rather than a pathway that directly produces fuels from algae (Pate, 2011). The exclusion of the heterotrophic pathways is not a judgment on the validity of these approaches. Second, the study sponsor indicated macroalgae as a feedstock was of lower priority for this study than microalgae and cyanobacteria and suggested that the committee could consider macroalgae if time and budget allowed. The committee could not fully address the sustainability of using macroalgae as a feedstock not only because of time and budget constraints, but also because of the sparse literature on this topic. The focus on microalgae also is consistent with the research and investment patterns in algal biofuels. Third, the study relies on published literature so that the well-studied topics are emphasized more often than less well-studied topics relative to others in the report.
The committee developed its report based on members’ expertise and information gathered from the public record. In its examination of publicly available information, the