The team earlier published LCA results finding that terrestrial bioenergy feedstock crops such as switchgrass, canola and corn have lower environmental life cycle impacts than algae in energy use, greenhouse gas emissions, and water regardless of cultivation location. (Earlier post.) In this new study, reported in the ACS journal Environmental Science & Technology, Clarens et al. assess four algae conversion pathways resulting in combinations of bioelectricity and biodiesel:

1. Anaerobic digestion of bulk algae biomass to produce methane-derived bioelectricity;

2. Production of biodiesel from algae lipids with anaerobic digestion of residual algae biomass to produce methane-derived bioelectricity;

3. Production of biodiesel from algae lipids with direct combustion of residual algae biomass to produce bioelectricity; and

4. Direct combustion of bulk algae biomass to produce bioelectricity.

The cultivation results from our previous work identified several intriguing opportunities for improving large-scale algae production; however, these results cannot tell us whether algae is more or less suitable for production of usable transportation energy than the benchmark crops. This is because some crops are more easily converted into energy carriers than others, and this was not included in the previous paper’s system boundaries.

The present study addresses this shortcoming by adopting “well-to-wheel” system boundaries; i.e., by expanding our original analysis to include conversion of each biomass into transportation energy sources. The array of selected conversion technologies reflects several prominent strategies being promoted by the algae community, in order to assess environmental performance of algae-to-energy systems as they are currently envisioned.

—Clarens et al.

The authors reported the results on the basis of two complementary functional units to illuminate expected trade-offs between land use and other environmental impacts:

• Usable energy production per unit land area, as expressed using “vehicle kilometers traveled” (VKT per ha); and

• environmental burdens (net energy use, water use, and GHG) per VKT.

Schematic of a generic algae-to-energy production system (top), as evaluated. Arrow widths are proportional to mass flows. Energy flows are indicated using dashed lines. Unit operations pertinent to assessment of four specific conversion pathways are shaded (gray) in panels A-D at bottom. These pathways include A) anaerobic digestion to produce bioelectricity; B) biodiesel production from algae lipids with anaerobic digestion of residuals to make bioelectricity; C) biodiesel production from algae lipids with combustion of residuals to make bioelectricity; and D) combustion of bulk algae biomass to make bioelectricity. Credit: ACS, Clarens et al. Click to enlarge.

They also assessed energy return on investment (EROI) (i.e., the amount of energy produced per energy consumed to deliver one functional unit) for each system, “because this study evaluates many algae conversion options and it was desirable to make direct comparisons among dissimilar conversion systems.” They then compared the most promising algae systems (as identified using EROI) with benchmark terrestrial biofuels.

Broadly, their results suggested that conversion pathways involving direct combustion for bioelectricity production generally outperformed systems involving anaerobic digestion and biodiesel production, and they were found to generate four and fifteen times as many vehicle kilometers traveled (VKT) per hectare as switchgrass or canola, respectively.

However, algae systems exhibited mixed performance for environmental impacts (energy use, water use, and greenhouse gas emissions) on a “per km” basis relative to the benchmark crops.

Among the many findings:

• Algae EROI values computed in the study ranged from 0.65 to 4.10. Previously reported EROI for corn ethanol has been on the order of 1.25. It has been suggested, the authors noted, that the minimum sustainable EROI is roughly 3 but that values from 5 to 10 will be required to maintain quality of life in the absence of readily abundant fossil energy.

• Direct combustion of algae to produce bioelectricity is seemingly more efficient than anaerobic digestion regardless of whether or not algae lipids are extracted to make biodiesel.

• Selected algae systems dramatically outperform the terrestrial crop systems in terms of VKT production per hectare. Algae generates, on average, 4.2 times and 15.7 times more VKT than the switchgrass and canola systems, respectively.

  Misalignment of system boundaries precludes direct comparison with corn ethanol, the authors note, but they estimate that the average algae VKT is roughly nineteen times greater than could be derived from corn ethanol (27,000 km/ha-yr) even when accounting for ethanol coproducts. 29

• In terms of VKT, algae bioelectricity systems outperform algae combined biodiesel/bioelectricity systems.

• Algae biodiesel and bioelectricity systems exhibit higher net energy use but lower water use and GHG emissions per km than their respective terrestrial benchmarks.

...these results emphasize a key trade-off, namely: Algae are capable of producing many more VKT per hectare than the selected terrestrial crops, but in doing so they create larger environmental burdens on a per-km basis.

...Unfortunately, even the most rigorous LCA analyses fall short when it comes to making normative judgments. Thus, our results cannot tell us whether the US and other countries should pursue algae-derived transportation fuels. Economic considerations, which we have not addressed, will undoubtedly have a large impact on whether algae-derived biodiesel and bioelectricity become widely commercialized. On one hand, direct biomass energy technologies have a favorable levelized cost of energy vs other alternative energy technologies and are cost-competitive with fossil power plants.

On the other hand, establishing and maintaining the infrastructure for algae cultivation and conversion will be quite expensive. Regardless, the tremendous demand for transportation energy, increasing fuel prices, and a lack of mechanisms for monetizing environmental performance in the US make it reasonable to expect that algae’s excellent land use efficiency could render it financially attractive over the next several decades. For this reason, environmental and economic LCA studies will be key tools for improving the overall sustainability of algae-derived transportation energy systems.

—Clarens et al.

Resources

• Andres F. Clarens, Hagai Nassau, Eleazer P. Resurreccion, Mark A. White, Lisa M. Colosi (2011) Environmental Impacts of Algae-Derived Biodiesel and Bioelectricity for Transportation. Environmental Science & Technology Article ASAP doi: /10.1021/es200760