In all LC studies, a reference flow is needed to which all other modeling flows of the system will be related [13]. This flow must be a quantitative measure and for some industries, e. g., steel, the choice is usually obvious like X kg steel at the foundry. In other cases, including algae-to-energy systems, this decision can be more complicated. Recent studies have selected a wide variety of functional units (FU) including volume of biodiesel, dry mass of algae produced, kilometers of truck transport, and total energy embedded in the algae assuming the biomass is burned (see Table 2). All of these FUs are valid bases from which to evaluate algae LC, but this diversity in FUs does not make for straightforward comparison between studies. The lack of consensus on a standard FU reflects the lack of indus­try agreement on what the best products to make with the algae will be. Some of the assumptions about goal and scope setting carry over into the functional unit since a FU of liters of biodiesel will inherently exclude the value that could come from a by-product such as ethanol.

LEmEMD

5% RMEE Boundary

Fig. 2 Many studies assume that the upstream impacts of delivering fertilizers and carbon dioxide should not be included. A cut off of 5% was assigned to LC contributions that would be neglected in the analysis (from Sander and Murthy [24])

In LCA more broadly, FUs sometimes require that a performance constraint be applied in order to normalize between dissimilar systems. A carpet, for example, is quieter than a wood floor, even if the latter is more durable. Using a square meter of flooring as the functional unit may overlook performance characteristics (noise buffering and durability) that will ultimately impact the analysis [1]. In the case of algae, performance constraints are certainly limiting in a few important ways. When benchmarking algae to other terrestrial crops, it is useful to apply an FU that is com­monly accepted by the biofuels industry. Though bushels of corn or liters of ethanol do not apply directly, analogs are possible. For example, algae might be compared in terms of dried biomass generated per unit area or liters of biodiesel produced per unit area per time. Energy content can be used as an FU, though it can overlook important differences between biomass. Algae may have a high heating value com­parable to switchgrass though in practice, converting algae to usable fuel is quite a bit more straightforward.