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| Fig. 1: This image shows microalgae being grown in a photobioreactor. (Source: Wikimedia Commons) |
With the increasingly energy crisis, there needs to be an effort to find alternatives, one of which is algae biofuel. Algae are morphologically simple, chlorophyll-containing organisms that are autotrophic, driving their energy from their surrounding in the form of sunlight. [1] What makes the algae valuable is the various byproducts, such as lipids, carbohydrates, and proteins that can be converted into biofuels and other useful material. [2] Focusing on algae biofuel, the lipids that are extracted from the walls of the algae cells can be processed into a biofuel. [2]
The most productive method to produce algae in the fastest and most efficient way is with photobioreactors. [2] Photobioreactors are closed, controlled environments that creates an ideal environment to maximize algae growth (See Fig. 1). In the controlled environment, optimal growth requirements are able to be met while managing amounts of carbon dioxide and water, temperature, exposure to light, mixing, culture density, pH levels, and gas supply and exchange rate. [2] Algae is also more productive compared to other potential biofuels. Algae yields about 2,500 gallons of biofuel per acre per year, while soybeans yield approximately 48 gallons and corn about 18 gallons. [3]
Another method is open-pond growing, but this comes with various drawbacks. Weather can stunt algae growth and there can be potential contamination from strains of bacteria or other outside organisms. [2] Additionally, the water that the algae grows in has to be kept at a certain temperature, which can potentially be difficult to maintain. The advantages of open ponds are that they are easy to construct, or can be naturally occurring. An additional drawback is that this method is simply less productive. The cost of production for a gallon of crude bio-oil is $109.12 for open-pond growing, while the cost is $76.98 for using photobioreactors. [4] Open-pond growing is much less cost efficient and a poor method of harvesting. [5]
Algae converts sunlight into energy, and some algae store energy as natural oils. With proper conditions, algae can make large amount of their natural oils that can be converted into biofuels. Once the algae is harvested, an oil press is used to extract the oils from the walls of the algae cells. [2] An organic solvent, such as hexane, is mixed with the oil from the algae, which is filtered and cleaned and have to chemical removed from the oil. Next in the process, carbon dioxide is then used as a supercritical fluid to extract the all of the oil from the algae. [2] A supercritical fluid is any substance a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It has the low viscosity of a gas and the high density of a liquid, making it impossible to liquefy the matter using any amount of pressure. [6] With the extracted oil, fatty acid chains are used during a process called transesterification, where a catalyst is mixed in with an alcohol, creating a biodiesel fuel combined with a glycerol. [2] This new mixture is refined to remove and glycerol and leaves the final product of algae biodiesel fuel.
As diesel begins to run out in the world, there needs to be an alternative fuel to replace it. Algae presents a compelling case, but there must be more advances in order to reach production levels that would be substantial enough to replace diesel. [7] It is important to begin researching these options now because when diesel runs out we want to have options of where to go next.
© Sarah Klass. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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] T. J. Lundquist et al., "A Realistic Technology and Engineering Assessment of Algae Biofuel Production," University of California, Berkeley, October 2010.
[2] M. Y. Menetrez, "An Overview of Algae Biofuel Production and Potential Environmental Impact," Environ. Sci. Technol. 46, 7073 (2012).
[3] A. A. Adenle, G. E. Haslam, and L. Lee, "Global Assessment of Research and Development for Algae Biofuel Production and Its Potential Role for Sustainable Development in Developing Countries," Energy Policy 61, 182 (2013).
[4] J. W. Richardson et al., "A Financial Assessment of Two Alternative Cultivation Systems and Their Contributions to Algae Biofuel Economic Viability," Algal Res. 4, 96 (2014).
[5] A. Summerville, "Algae Biofuels," Physics 240, Stanford University, Fall 2016.
[6] E. Kiran and J. M. H. Levelt Sengers, Supercritical Fluids: Fundamentals for Application (Springer, 1994).
[7] A. Noll, "The Future of Algae Biofuel," Physics 240, Stanford University, Fall 2015.
