|
|
| Fig. 1: 2030 Estimated Fuel Consumption, after Aiken et al. [11] |
Algal biofuel production is a rapidly evolving technology providing an alternative source of hydrocarbons to fossil fuels. The dramatic appreciation in value of crude oil over the past fifteen years has prompted significant interest in alternative fuel sources. There has been significant Federal interest in developing algal biofuel as a long term alternative to fossil fuels. [1] The Biotechnologies Office of the Department of Energy is engaged in long-term research aimed at increasing yields and lowering costs of algal biofuels. [1] Algal fuels are particularly appealing as they can be grown on land poorly suited to other forms of agriculture, can be grown in saline solutions not drawing off fresh water reserves, are biodegradable and are considerably less harmful than fossil fuels should spillage occur. [2] Algae hold considerable potential as a renewable fuel source due to the quantity of biomass that can be farmed on a given area. [1] According to DOE studies algae yields thirty times more energy per acre than soy beans. [3] Placed in another light, if all the diesel fuel in the US were replaced with algal biofuel, a piece of land slightly larger than the state of Maryland would be required to farm all the algae necessary for replacement fuel. [3] Another primary motivation for algae based fuels is in their ability to reduce global warming through carbon sequestration. Acting as a "scrubber," there is optimism that factory emissions can be passed over algae, fixing the carbon dioxide.
Algae cultivation for biofuel has taken place using two prevailing techniques, open-ponds and photobioreactors. A photobioreactor pumps water through plastic or borosilicate glass tubes which are in turn exposed to sunlight. [4] Photobioreactors are more costly than the open-pond technique, but are more easily managed and more agriculturally productive. [4]
 |
| Fig. 2: Photobioreactor (Source: Wikimedia Commons) |
Open-pond cultivation is cheaper than the photobioreactor method, however, it is highly reliant on the hardiness and competitiveness of the algal strain chosen for cultivation. The algae must survive the wider temperature and pH ranges associated with open systems as well as competition for nutrients from any other species that may contaminate an open system. [5]
Clear demand exists for an alternative biofuel that is sustainable. Such a fuel must also meet economic demands for cost efficiency. As such, any fuel produced that requires more energy input relative to output is not a viable solution. Dark grown algae require net energy input through the use of corn sugars and short government subsidies will likely never be a viable solution. To date technology is still attempting to close the energy gap. [5] A 2007 report derived an equation projecting the cost needed for algal fuel to become a suitable alternative to fossil fuel: [5]
C(algal oil) = the price of microalgal oil in $/gallon and C(petroleum) the price of crude oil in $/barrel. This equation assumes that algal oil has roughly 80% of the caloric energy value of crude petroleum. [5] As shown Algal oil needs to be produced at 1/40th the cost of crude oil to become an economically viable alternative. As of 26 October 2014, with crude oil priced at $81/barrel algal oil must cost no more than $88 per barrel ($2.09/gal.) in order to be competitive with petroleum diesel. (1 barrel = 42 US gal.)[5]
 |
| Fig. 3: Open Raceway Pond (Source: Wikimedia Commons) |
In early 2013 Exxon-Mobil chairman/CEO Rex Tillerson, after investing $600 million in R&D over 3.5 years in a partnership with J. Craig Venter's Synthetic Genomics, announced that he deemed algae based fuels will not be economically viable for another 25 years. [6] The issues surrounding this announcement center on the algae strains being used for cultivation - to date, an algae strain with high lipid content and a rapid growth rate has not been developed by scientists. [6] An optimal strain, one exhibiting: high growth rate, high lipid content, and ease of harvest has yet to be developed. [7] Much of the funding has now been channeled into R&D for Synthetic Genomics - targeted at its namesake, synthetic genomics.
Numerous other challenges exist in processing algae into biofuel. The high polyunsaturated fatty acil chains of lipids found in algae are prone to oxidation and in turn deterioration of their fuel qualities. [7] Existing methods of converting plant oils to biodiesel occur at 95% triglyceride content - research points to an inverse relationship between triglyceride production and growth rate. [7] This inverse relationship poses a significant hurdle in optimizing the production of high lipid content algae.
New technologies offer optimism, some more viable than others, that algal biofuels will eventually become economically feasible. Researchers at the DOE's labs in the Pacific Northwest have made advances in hydrothermal liquefaction that make it possible to process algae at a rapid rate. Through exposure to high temperatures, in a manner similar to a pressure cooker, they can recreate a type of light crude in a matter of hours. [8] Solazyme, a privately held biotech company, has explored the potential for algae grown in complete darkness, feeding it sugars to promote growth, and going so far as to provide jet fuel derived from this process to the US Navy. [9] The future of algal biofuel hinges on technological advances that can lower the per-unit cost of farmed algae and increase oil yields. At present biodiesel yields 13.26 MJ/Kg, gasoline 13.64-14.64 MJ/Kg, and ethanol 12.25-14.03 MJ/Kg. [10] This high energy yield, coupled with a projected maximized yield of 95,000 L/hectare for algae, versus corn at 172 L/hectare, is what makes biodiesel so enticing. [10]
Considerable work remains in full grasping the biology of algae, enhancing lipid synthesis through genetic manipulation, and in optimizing the farming process. [7] Natural limits of the light to chemical energy conversion process create a ceiling on the potential of algae farmed via open-ponds and photobioreactors. [7] It is unclear when algal biofuels will become economically viable, but it is clear that it will take the coordination of researchers across multiple disciplines to make it a reality.
© Michael Anderson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Demirbas and M. F. Demirbas. "Importance of Algae Oil as a Source of Biodiesel," Energy Convers. Manage. 52, 163 (2011).
[2] A. Demirbas, "Biodiesel from Oilgae, Biofixation of Carbon Dioxide by Microalgae: A Solution to Pollution Problems," Appl. Energ. 88, 3541 (2011).
[3] E. Hartman, "A Promising Oil Alternative: Algae Energy," Washington Post, 6 Jan 08.
[4] T. M. Mata, A. A. Martins, and N. S. Caetano, "Microalgae for Biodiesel Production and Other Applications: A Review," Renew. Sustain. Energ. Rev. 14 217 (2010).
[5] Y. Chisti, "Biodiesel From Microalgae," Biotechnol. Adv. 25, 294 (2007).
[6] J. Carroll, "Exxon at Least 25 Years Away From Making Fuel From Algae," Bloomberg, 3 Aug 13.
[7] H.C. Greenwell et al., "Placing Microalgae on the Biofuels Priority List: A Review of the Technological Challenges," J. R. Soc. Interface 7, 703 (2010).
[8] T. C. Nguyen, "Scientists Turn Algae Into Crude Oil In Less Than An Hour," Smithsonian Magazine, 31 Dec 13.
[9] D. Biello, "How to Survive as a Biofuel-Maker: Sell Algae to Bakers," Scientific American, 25 Jul 13.
[10] B. Patel et al., "Algal Biofuels: A Credible Prospective?," ISRN Renewable Energy 2012, 631574 (2012).
[11] M. Hannon et al., "Biofuels From Algae: Challenges and Potential," Biofuels 1, 763 (2010).
