In 2007, nearly 90% of the world's energy budget was derived from combusting fossil fuels.  Since these fuels are finite and sources of greenhouse gases when burned, significant emphasis has been placed on the discovery and implementation of sustainable, carbon neutral fuels. Biomass and other biofuels have emerged as popular alternative energy sources, primarily for their carbon neutrality and, in the case of biomass, their widespread accessibility. Microalgae have been studied as a biofuel and as a vehicle for capturing and storing atmospheric carbon due to their relatively large photosynthetic efficiency and fast growing rates.  However, despite these attributes, algae based fuels and carbon-sequestering techniques suffer from a variety of drawbacks.
Algae are attractive biofuel sources for several reasons. They typically exhibit photosynthetic efficiencies near 6% which is 2-3 times larger than higher order plants.  Here photosynthetic efficiency is defined as the ratio between chemical energy stored in a plant and solar energy incident upon the plant. Algae also exhibit superior growing rates with growth cycles on the order of several days compared to weeks, months or even years for higher order plants.  Lastly, algae contain energy dense oil at up to 50% by weight. Approximately 75% of this oil can be extracted with mechanical pressing techniques which requires considerably less infrastructure than the elaborate thermal reformation processes needed to convert conventional biomass to more widely applicable fuel (e.g. syngas). 
Despite algae's exceptional ability to rapidly convert photons and carbon dioxide into easily extractable chemical energy, the large scale applicability of algae based fuels and carbon capture techniques is currently impractical.
From a biofuel perspective, the ultimate constraint on algae is the simple fact that they rely on the quantity of solar energy they're exposed to. This essentially boils down to a problem of land availability since the incident solar energy flux at a given location on the earth is fixed based on the time of year and cloud cover. From here it's immediately clear that algae fuels are not a sensible option for stationary power generation because the efficiency of solar thermal power plants is currently 3-4 times larger than algae's photosynthetic efficiency which doesn't account for energy losses incurred while transforming algae to usable fuels. 
Algae based fuels remain an option for the transportation sector because of their relatively large energy density (20-25 MJ/kg) and transportability, however, significant obstacles must be considered. From an energy perspective, the biggest cost lies in the harvesting and preparation required to transform algae into usable fuel. This ultimately results from the fact that algae are heavily diluted in water (about 0.1% H2O by weight at harvest time). Even after passing algae through a centrifuge they remain about 90% H2O by weight.  Since most reformation processes require drying the algae beforehand, including mechanical pressing, an enormous energy penalty is incurred in drying the algae. For example, for every 1 kg of algae about 3 MJ is required for the centrifuge to get the algae to around 90% H2O by weight and another 20 MJ of energy is required to dry the algae to 10% H2O by weight.  With heating values around 25 MJ/kg, a 92% energy penalty is incurred in the harvesting and drying process! However, if dry air, sunlight, and plenty of land is available to allow the algae to dry naturally, this massive evaporation penalty can be neglected, however, this would represent a unique advantage specific only to certain geographic regions (e.g. desert climates). Given the volume of algae required to fuel modern energy systems, a significant amount of energy would most likely need to be supplied to accelerate the drying process.
From a land investment perspective, the large-scale implementation of algae fuels is simply impractical. Demirbas estimates that algae could produce 12,000 tons of oil per square mile each year. This is consistent with findings from an elementary energy balance using appropriate values of algae photosynthetic efficiencies, growth rates, and incident solar flux. To put things in better perspective, let's draw an analogy with the amount of fuel a 1 GW power plant requires. A 1 GW power plant operating 15 hrs per day with a thermal efficiency of 40% consumes just under 5 × 1016 Joules per year. If all of this energy was supplied by algae based oil with a heating value of 25 MJ/kg, 164 square miles would be required to grow the required amount of algae. With a typical American farm standing in at under 1 square mile, the amount of land required to fuel an algae powered 1 GW power plant is truly enormous. To make matters worse, the amount of fossil fuels consumed by the world in 2007 is equivalent to the operation of roughly 30,000 1 GW power plants. 
From a carbon capture perspective, the problem again reduces to the enormous amout of land required for algae farms. Neglecting differences in the carbon content of algae and coal, it intuitively follows that the amount of algae required to capture the CO2 emissions of a 1 GW coal plant is also 164 sq. miles. Furthermore, algae are not a stable medium for trapping carbon. Unless the algae is transformed into more stable forms of carbon (e.g. biochar), the algae would biodegrade and release their carbon into the atmosphere as mostly carbon-dioxide and methane. 
In the world of biomass and biofuels, algae are elite contenders in the business of transforming photons and CO2 into hydrocarbon based fuels. Their rapid growing rates also make them an attractive tool for capturing carbon-dioxide. However, growing algae requires a tremendous land investment and several energy intensive processes are required in transforming them from organisms to fuel. These costs and investments currently make the large scale implementation of algae based fuel or carbon capture facilities impractical, however, under unique circumstances where their obstacles are mitigated, algae remain a viable option as a carbon neutral fuel for small scale implementation.
© 2011 Christopher Goldenstein. 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.
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