After the goal and scope of a study have been specified, a life cycle inventory (LCI) is typically carried out. The LCI is the accounting stage in which all the physical flows are reconciled with known emissions data to quantify the environmental bur­dens and resource requirements over the entire life cycle [1]. The outcome from this process is typically an exhaustive list of emissions factors; many more than can be reasonably expected or necessary in a report. Therefore, an important step in devel­oping an LCA is the process of simplifying raw LCI data into specific metrics. Table 2 lists the impact metrics used in a few recent LCA papers of algae-to-energy systems. The differences in study endpoints contribute to the difficulties in compar­ing the results. The decision to include some metrics and exclude others can have important implications for the results and interpretation of the study. Most LCA guidebooks divide impact categories into three principle categories: resource use, ecological consequences, and human health [1]. Each category is discussed briefly here in the context of algae-to-energy systems.

1.1 Resource Use

Resource use is the most straightforward of the impact factor categories because the metrics involved are typically simple sums of flows from the environment. For example, total nonrenewable energy use, normalized by energy content, is a com­monly encountered metric. Total land use is an important resource metric that has been hotly debated by the life cycle community because of the important upstream or indirect land use that is required to maintain the productivity of the agricultural region (e. g., land associated with production of fertilizer) or because of land could be used for alternative uses if not for agriculture (e. g., primary growth forest). Similarly, total water use is a resource that is relevant for most biofuel life cycle studies as shown in recent work [9] . An important distinction when it comes to water use is that of consumptive vs. nonconsumptive use. Most energy generation facilities use a large amount of water, primarily for cooling, so even though the amount of water needed for these systems is large, a comparatively small amount of the water is actually consumed [16].

Most models of biofuels systems include, at a minimum, total net energy use as a metric. This is an obvious and important metric because many biofuels such as ethanol consume a considerable amount of fossil fuels to generate a certain amount of ethanol. Recognizing that biofuels are not worth pursuing if there is no energetic gain, many studies have explored the net energy balance associated with alternative energy options. Algae-derived energy is no exception, and several studies report on the energy that is required to produce energy carriers from algae. Whether these estimates are net positive or net negative depends on the modeling assumptions selected in the study. In addition to energy use, there are at least two other impact factors that should be considered when evaluating algae-to-energy systems. The first is land use. Algae grow more efficiently than terrestrial crops, and so quantify­ing this parameter is important as a means to highlight one of algae’s most pro­nounced advantages. Similarly, water use is an important parameter since large-scale algae cultivation is likely to require large volumes of water. How much, and how this relates to the water use of terrestrial crops is likely to be an important factor in water-limited growing regions. Including water as a key metric is important.