Growing heterotrophic algae in fermenters with sugar as the energy source rather than sunlight is an approach being commercialized in the private sector. While this strategy might be more technically and economically viable in the near term, the full environmental impacts of such an approach as a pathway for fuel production have not been rigorously explored. The objective of this analysis is to compare the life-cycle impacts of algal biodiesel produced heterotrophically to a phototrophic pathway featuring algae grown in open ponds. A third, hybrid approach utilizing algae capable of both phototrophy and heterotrophy is also explored. Both beet and cane were examined as sources of sugar for the heterotrophic process. The results indicate that a reduction in global warming impact (GWP) and an improvement in the fossil energy ratio (FER) could be possible for the heterotrophic and hybrid pathways, but only if efficient fermentation technologies are used, cane is used as the sugar source, and energy is recovered from combustion of the bagasse. Carbon emissions from indirect land use change (ILUC) could offset these GWP benefits, however, particularly if the production of the sugar leads to the clearing of rainforest. The water stress results were regionally variable with no pathway presenting a clear advantage. The yields of crops used to produce sugars for the heterotrophic and hybrid pathways requires greater land occupation and present concerns about competition with food markets and scalability that are not an issue for the phototrophic pathway. Use of cellulosic sugars or waste feedstocks as a fermentable substrate could reduce these concerns.
CSS Publication Number:
Life Cycle Assessment
Proceedings of the 7th Annual International Society for Industrial Ecology (ISIE) Conference
June 25, 2013
Orfield, Nolan, Robert Levine, Gregory Keoleian, Shellie Miller and Phillip Savage. (2013) “Growing Algae for Biodiesel on Sunlight or Sugars: A Comparative Life Cycle Assessment.” Proceedings of the 7th Annual International Society for Industrial Ecology (ISIE) Conference. Ulsan, South Korea. June 25-28, 2013. Abstract No. 092