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Response to Comment on “Environmental Life Cycle Comparison of Algae to Other Bioenergy Feedstocks” In his recent comment on our life cycle study of algae cultivation (1), Subhadra (2) raises several interesting points that echo the sentiment of many in the biofuels industry. His principal criticisms focus on our system boundary assignments for algae cultivation, particularly with respect to our assumptions about reactor configuration, nutrient requirements, and conversion of biomass into an appropriate energy carrier. We appreciate that questions of this nature will arise in any life cycle assessment, wherein normative assumptions can impact the results of an analysis. However, after carefully reviewing his comments and the feedback from a number of other interested readers, we assert that these modifications would not substantively change the overall conclusions of our study. First-generation algae-based production would still exhibit larger environmental impacts than conventional terrestrial crops. We expect that this is evident in the dramatic differences between algae and each other crops’ energy use, water use, and greenhouse gas emissions as noted in our paper. Nevertheless, we disagree with those that would construe this work as an indictment of algae-based fuels, since the intent of this study was to deliver a tool to direct engineering research and inform practice such that the algaefuel life cycle can be improved. The most thought-provoking criticism offered by Subhadra and others is related to our choice of functional unit. It has been suggested that we could have used alternative comparison between crops in place of high heating value multiplied by total productivity or that we could have modeled the creation of valuable byproducts. The argument is that 1 MJ of switchgrass biomass is not equivalent to 1 MJ of algae biomass because the latter can be more readily converted into useable fuels and byproducts. Many researchers and companies have advocated that coproduction of biodiesel plus high value nutraceuticals, pigments, or other products would greatly enhance the economic viability of algae-based fuels. Undoubtedly, this would influence the life cycle assessment of algae production since offset credits associated with coproducts could reduce the overall impacts of the cultivation process; however, we stopped short of including these products in our system boundaries for two key reasons. First, it would have been necessary to also include the byproducts derived from production of terrestrial crops. Second, and perhaps more important, the large number of possible products that could be coproduced with algae make it impossible to describe a generic process. Since our goal was to model bioenergy capacity of crops, a thermodynamic basis seemed like the most representative way to structure our analysis. Several readers have asked us about our use of “old data”. To this, we respond that our study was based on the only three pilot-scale studies published to date (3–5) and supplemented with first principles calculations. The former were our sources for estimating nutrient demands; the latter are exemplified by our use of radiation-use-efficiency to estimate bioproductivity as a function of geographic location. Our modeling efforts were intentionally focused on existing rather than emerging technologies. To our knowledge there is no satisfactory way to model changing technologies from a life cycle standpoint because the rate of innovation varies greatly between industrial sectors and because this innovation is
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typically nonlinear. We find Subhadra’s and others’ assertion that recent unpublished advances make algae cultivation much more sustainable than our model suggests encouraging, because algae has clear potential to become an important source of renewable energy, but also frustrating, because lack of transparency makes for very slow progress toward this goal. More to the point, it is impossible to analyze data that has not been published. Thus we heartily endorse publication of these recent advances in the peer-reviewed scientific literature. A final criticism raised by Subhadra is that we chose to model open ponds rather than bioreactors. The motivation for focusing on open ponds was economic since our calculations reveal that photobioreactors are not economically viable in the same way growing corn in greenhouses is not viable. The academic literature is largely silent on this issue, however, and a life cycle costing study would be very interesting in this regard. Subhadra mentions the prevalence of state and federal loan guarantees for alternative energy development and suggests that these could tip the balance in favor of photobioreactors. These programs are generally intended to catalyze research and development but cannot ultimately supplant economic realities that apply to all technologies. It is reasonable to expect that photobioreactor production will be improved in coming decades, but agricultural practices are also likely to improve. Thus, with so many moving targets, we chose to present a wellcharacterized comparison: open ponds as published in the existing literature versus terrestrial agriculture. In summary, the concerns raised by Subhadra provide for a valid discussion, but we do not believe that the issues he raised affect the overall conclusions of our study. As mentioned in the paper, our model was largely insensitive to individual model inputs and large improvements are needed to reduce the impact of delivering nutrients or carbon dioxide. We did not set out to undermine further research in algae, and we remain as committed as ever to advancing the knowledge needed to produce fuels from algae. Our hope is that this paper will help in some way to guide the focus of future work. We thank Subhadra and many others for an opportunity to continue this important discussion.
Literature Cited (1) Clarens, A. F.; Resurreccion, E. P.; White, M. A.; Colosi, L. M. Environmental life cycle comparison of algae to other bioenergy feedstocks. Environ. Sci. Technol. 2010, 44 (5), 1813–1819. (2) Subhadra, B., Comment on “environmental life cycle comparison of algae to other bioenergy feedstocks”. Environ. Sci. Technol. 2010 (doi:10.1021/es1000389s). (3) Benemann, J.; Oswald, W. Systems and Economic Analysis of Microalgae Ponds for Conversion of CO2 to Biomass - Final Report; Department of Energy: Washington, DC, 1996. (4) Kadam, K. L. Environmental implications of power generation via coal-microalgae cofiring. Energy 2002, 27 (10), 905–22. (5) Weissman, J. C.; Tillett, D. M. Design and Operation of an Outdoor Microalgae test Facility: Large-Scale System Results; National Renewable Energy Laboratory: Golden, CO, 1990.
Andres F. Clarens, Eleazer P. Resurreccion, Mark A. White, and Lisa M. Colosi Civil and Environmental Engineering, and McIntire School of Commerce, University of Virginia, Charlottesville, Virginia 22904-4742 ES1007438
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