FIGURE 1-1 Schematic representation of a production system, including system inputs and outputs.

same time, this diversity creates challenges for documenting critical material, energy, and monetary flows needed to assess performance.

Understanding the performance of alternative designs for producing liquid fuels from algae requires the adoption of a systems framework for assessing alternative designs. The systems framework illustrates the interdependent nature of the individual supply chain components and the system inputs and outputs. The understanding developed from such a representation is fundamental for applying a wide array of sustainability tools such as LCA, engineering process modeling, and cost-benefit analysis.

1.3.2 Indicators

Biofuel sustainability indicators are metrics of defined aspects of sustainability that represent system status or progress toward sustainability goals. Some researchers and institutions distinguish between definitions of indicators and metrics, while others see substantial overlap in the concepts. The definition of an indicator used in this report is “a measure that is somehow indicative of some unmeasurable environmental goal such as environmental integrity, ecosystem health, or sustainable resources” (Suter, 2001). Indication of sustainable development of algal biofuels is indirect, through the union of metrics of resource use, other environmental impacts, social acceptance (all considered in this report), and economics and energy security (not considered in this report). Specific metrics of water quality or quantity or GHG emissions, for example, are viewed as indicators of sustainability or sustainable development.

Because sustainability includes environmental, economic, and social dimensions (in addition to energy and energy security, which may be classified separately), indicators also typically are divided among these categories. Categories of resource requirement indicators that have been discussed for biofuels include total and consumptive water use,

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