Most scientists realize that conventional sources of carbon-based fuels are problematic from multiple viewpoints. A notable exception are the scientists-from-Mars who always inundate the "letters to the editor" section of Chemical and Engineering News whenever the editor writes on global warming.
Prominent issues of concern with respect to carbon-based fuels are those related to the environment, such as pollution of the atmosphere, and land and water degradation. Security issues, such as being held hostage by unpleasant governments that happen to control needed fuel reserves, can bring into the fold those who don't seem to buy into environmental concerns.
Biofuels to the (qualified) rescue.
Motivated by such issues, there has been a big push for biofuels, i.e., carbon-based fuels extracted from plants and other biomatter. It is important to note that since biofuels are comprised of carbon-based molecules, they are not a long-term solution to environmental problems caused by the combustion of carbon-based fuels.
They do have the advantage of utilizing energy distribution networks already in place, and therefore may help ease the transition to sources of energy that are truly environmentally-sustainable (such as solar and hydrogen-based power). Many sources of biofuels have been proposed; unfortunately, not all of them are environmentally-friendly.
Clear examples of unsustainable biofuels are those derived from United States corn. Although the United States produces loads of corn, production of this crop in the United States is not environmentally-sustainable, as currently practiced, meaning that corn-based biofuels from the United States won't help the environment.
Are microalgae a solution?
In general, biofuels produced from crops have been widely criticized, most prominently based on arguments related to the environment, land-use, and public nutrition. Biofuels from microalgae (unicellular organisms grown in water, and which do not divert crops from consumption to energy usage) may help to alleviate such concerns.
However, it isn't clear whether harvesting biofuels from microalgae is a realistic route to fuels of the future. If it isn't, as currently practiced, can anything be done to improve the prospects of microalgae?
Laurent Lardon (INRA, the French National Institute for Agricultural Research) and coworkers have found that microalgae can be efficient (and environmentally-friendly) sources of biodiesel. However, fertilizers and energy inputs need to be minimized for microalgae to reach its full potential.
Unique advantages of microalgae.
What motivated the scientists to study the prospects of microalgae for biodiesel production? Microalgae have advantages over land-based organisms for harvesting energy.
One big advantage is that the former utilize up to 8% of the light they absorb into producing carbon-based molecules (fuel). Land-based plants are much less efficient.
Typically, fuel synthesis in plants is only approximately 0.5% efficient. Compare both of these efficiencies to the roughly 20% maximum efficiency of solar cells (which by themselves produce energy, not carbon-based fuel).
Another advantage microalgae have over land-based plants is that the former accumulate fat (oil) molecules more readily than the latter. These molecules can be more readily (and efficiently) converted into energy than sugar molecules (more heavily produced by land-based plants).
The future of microalgae should be tested now.
What this means is that microalgae have the potential to generate far higher oil yields (per acre, per year) than other sources of oil, such as sunflower and rapeseed. However, industrial-scale energy harvesting from microalgae has not yet been implemented.
This means that it has not yet been conclusively demonstrated whether or not microalgae can serve as efficient (and environmentally-friendly) biofuel production factories. Now is a good time to test this question, before huge amounts of money are invested in the technology, and essentially wasted if it doesn't work out after all.
Lardon and coworkers have undertaken such a test, on a reasonable hypothetical basis. Their goal was to identify limitations and challenges in current microalgae biodiesel technology such that its large-scale prospects can be improved.
Designing the hypothetical biodiesel factory.
The scientists based their hypothetical study on known data from laboratory-scale experiments, and extrapolated them to an industrial-scale. Their goal was to evaluate the environmental impact and fuel extraction efficiency of microalgae, not to evaluate a specific technology.
Reasonable general assumptions were made that should be general to any microalgae factory. For example, it was assumed that electrical engines in the factory will be replaced every ten years, and that buildings will be constructed, used for thirty years, and then dismantled.
The scientists also assumed that Chlorella vulgaris will be the microalgae used in the hypothetical industrial facility; much laboratory-scale data is available for this microalgae. Growth under two different nutrient conditions and two different methods of fat (oil) extraction were considered, because there is much debate as to which protocol will yield superior results.
Note that the scientists' assumptions were far more extensive than is noted here. However, all assumptions are consistent with currently proposed factory design and microalgae growth conditions.
Bottlenecks for the future of microalgae.
The scientists found that an overwhelming proportion (70-90%) of the energy put into harvesting microalgae for fuel will get dumped into extracting the lipids (oil) they produce, under current proposed factory designs. It is obvious that new technologies need to be developed to improve upon this huge energy sink.
Nitrogen-deprived growth seems to be necessary to induce the microalgae to produce a significant amount of fat (oil). Given that depriving microalgae of nitrogen slows their growth and fat (oil)-production rate, this is another challenge that must be addressed.
The scientists also emphasize the point that microalgae may reduce the need for pesticides, and reduce land use conflicts, in relation to land-based crops. However, fertilizer requirements are not reduced, at least not according to current proposed growth protocols.
The scientists propose that these challenges must be addressed before we start to produce biofuels from microalgae on a large scale. One place to start may be to grow the microalgae in wastewater.
Another may be to utilize Nannochloropsis microalgae. This species is thought to be a more efficient "fuel factory" under nitrogen-deprived conditions than Chlorella vulgaris microalgae.
It is hard to overemphasize that biofuels are a realistic short-term, but definitely not a long-term and large-scale, solution to the planet's energy and environmental challenges. They do have a place as temporary sources of energy, and with the appropriate growth protocols microalgae may address many of the concerns put forth by strident opponents of biofuels.
for more information:
Lardon, L., Hélias, A., Sialve, B., Steyer, J. P., & Bernard, O. (2009). Life-Cycle Assessment of Biodiesel Production from Microalgae Environmental Science & Technology, 43 (17), 6475-6481 DOI: 10.1021/es900705j