FIGURE 4-8 Comparison of conventional esterified biodiesel production with green diesel (GD) production based on soybean oil (SBO) and rapeseed oil (RSO).
NOTE: Each of the three completed studies shows little difference on energy use or carbon emissions between green diesel and biodiesel. By analogy, green diesel production from algal lipids relative to conventional biodiesel processing is likely to have similar life-cycle impacts.
SOURCE: Adapted from Kalnes et al. (2009).

embodied energy of the process change needs to be assessed and compared to direct energy savings. Water management is clearly an important factor in energy use. Efficient pumps and gravity-driven designs, for example, could mitigate high energy use for water management. The embodied energy in providing nutrients, including CO2, can be substantial. The extent to which waste products can be used as nutrients has the potential to substantially reduce energy use. Separating algal products from water is a major factor driving energy use in separation and extraction. Drying processes in particular are energy intensive. Brentner et al. (2011) called attention to the potential of supercritical processes to reduce energy use for processing algae. Given the importance of coproducts in the net energy balance, developing higher “energy value” coproducts could be an important mitigation strategy.

4.4.5 Sustainability Indicators

A number of different metrics are already in use to assess energy systems (Farrell et al., 2006; GBEP, 2011; Baral et al., 2012), including:

•  EROI.

•  Net Energy Value (NEV).

•  Fossil Inputs.

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