Alternative Energy NOW Conference Abstract

Overall Energy Considerations for Algae Species Comparison and Selection in Algae-to-Fuels Processes

Dirk Link1, Amalie Tuerk2, Lisa Grady2, Brian Kail1, Wayne R. Curtis2. (1) US DOE / NETL, 626 Cochrans Mill Road, Pittsburgh, PA 15236. (2) The Department of Chemical Engineering, Penn State University, 232 Fenske Laboratory, University Park, PA.

The controlled growth of microalgae as a feedstock for alternative transportation fuel continues to receive much attention. Microalgae have the characteristics of rapid growth rate, high oil (lipid) content, and the ability to be grown in unconventional scenarios. Algae have also been touted as beneficial for CO2 reuse, as algae can be grown using CO2 emissions from fossil-fuel-based energy generation. Moreover, algae does not compete in the food chain, lessening the “food versus fuel” debate. Most often, it is assumed that either rapid growth rate or high oil content should be the primary factor in algal strain selection for algae-to-fuels production systems. However, many important, interrelated characteristics of algae growth and lipid production must be considered for species selection, growth conditions, and scale-up. Under light limited, high density, photoautotrophic conditions in a semi-continuous reactor system, the inherent growth rate of an organism does not affect biomass productivity and carbon fixation rate. However, the oil productivity and energy fixation rate is organism dependent, due to physiological differences in how the organisms allocate captured photons for growth and oil production and due to the differing conditions under which organisms accumulate oils. Therefore, many different factors must be considered when assessing the overall energy efficiency of fuel production for a given algae species.

Two species, Chlorella vulgaris and Botryococcus braunii, are popular choices when discussing algae-to-fuels systems. Chlorella is a very robust, fast-growing species, often outcompeting other species in mixed-culture systems, and produces a lipid that is composed primarily of free fatty acids and glycerides. Botryococcus is regarded as a slower growing species, and the lipid that it produces is characterized by high hydrocarbon content, primarily C28 – C34 botryococcenes. While the difference in growth rates between these two species might suggest superior productivity of Chlorella, the biomass produced and energy captured is limited instead by total light flux to the system. Furthermore, the species physiology affects whether the photon captured are allocated into high energy content species (oils and lipids) or shuttled into lower energy oxidized products. Data will be presented that shows that Botryococcus invests greater energy in oil production than Chlorella under these growth conditions. In essence, the Chlorella can grow “fast and lean” or can be slowed to grow “slow and fat”. The overall energy potential between the Chlorella and Botryococcus, then, becomes much more equivalent on a per-photon basis.

This work will indicate an interesting relationship between two very different algae species, in terms of growth rate, lipid content and composition, and energy efficiency of the overall process. The presentation will indicate that in light-limited growth, it cannot be assumed that either rapid growth rate or lipid production rate can be used as stand-alone indicators of which species-lipid relationships will truly be more effective in algae-to-fuels scenarios.