TABLE 4-7 Energy Use in Cultivation Stage in Closed Photobioreactors


Source

Bioreactor Type

Cultivation Energy
(MJ Input/MJ Biodiesel)


Stephenson et al. (2010)

Air lift tubular

  6

Jorquera et al. (2010)

Air lift tubular

14

Jorquera et al. (2010)

Flat plate

  0.61

Brentner et al. (2011)

Annular

19

Brentner et al. (2011)

Tubular

57

Brentner et al. (2011)

Flat plate

  1.4


Coproducts significantly affect the energy analysis. The typical scenario analyzed is anaerobic digestion of algae residuals to produce electricity and recover nutrients. One can see from Brenter et al. (2011) that changing from landfilling of algae residuals to anaerobic digestion nearly doubles EROI in their calculations. In Sander and Murthy (2010), the energy credit for using algae residuals is 10 times larger than the energy content of the produced biodiesel. This result can be interpreted as an assertion that the algal biorefinery becomes primarily a source of fermentation inputs and biodiesel is a secondary output.

There is considerable variability within similar technology or supply pathways—for example, Sander and Murphy’s EROI of 1.77 versus Brentner et al.’s EROI of 0.28. This variability is related to the use of different data or assumptions for the same processes and different boundary choices for supply chains. Variability in data has two sources. One source of variability is generic in LCA. Different analysts choose different data sources, and there is not a systematic way to realize convergence (Williams et al., 2009). The second source of variability is tied to the emerging nature of the technology. LCA is a method designed to assess existing technologies through chaining process input-output tables. Many processes in algal biofuel production systems are still in the laboratory or pilot phase. There is much uncertainty in how these technologies will evolve and scale up in the future; actual energy use could be much higher or much lower than suggested by the current suite of initial LCA studies.

The results in Table 4-6 address open-pond cultivation. Given the potential for closed photobioreactors to mitigate other resource and environmental issues such as water consumption (Harto et al., 2010), the energy use of closed systems is important to consider. A few studies estimated the direct energy use for the feedstock cultivation step in algal biofuel production systems that use photobioreactors (Table 4-7; Jorquera et al., 2010; Stephenson et al., 2010; Brentner et al., 2011). Tubular and annular reactors are thought to require far more energy to operate than is contained in the biodiesel product. Flat-plate reactors are thought to require far less energy, though Brentner et al. (2011) reported a net energy input higher than contained in biodiesel output (1.4 megajoules per megajoule of biodiesel). While the caveats for other results apply here as well, these studies suggest that the energy use of photobioreactors could fundamentally affect the net energy balance of algal biofuels (Jorquera et al., 2010).

4.4.2 Energy Requirements in the Supply Chain and Credits for Coproducts

Analyses of prior studies provide insight into the current understanding of what production stages are important contributors to energy requirements despite the large uncertainties and variability associated with energy requirements of algal biofuel production. Table 4-8 abstracts results from a meta-analysis of LCA studies to summarize the range in energy requirements of different stages (Liu et al., 2011). This meta-analysis included data



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