Microalgal Biofuel: Is it the Right Dream?
09 August 2011 |
Sugar Land, Texas, USA — Algae are the simplest and oldest photosynthetic organisms on Earth. Their simple structure allows them to convert solar energy into chemical energy efficiently. Earth’s first form of life now holds the potential to become the next major energy source and a vital part of the solution to climate change, food security and fossil fuel dependence.
It took nature millions of years to produce the fossil fuel we are using to support our daily energy needs and keep the wheel of civilization turning. Fossil fuels like petroleum, methane and coal are non-renewable sources of energy. Some of these are made up of pure carbon and others are a low ratio of carbon and hydrogen. Unfortunately, what the Earth produced in millions of years we are consuming in few hundred years. The current rate of consumption is so high that with the most optimistic proven reserve estimates, we will totally exhaust coal in 417 years, oil in 43 years and natural gas in 167 years. This requires international joint efforts to, at least, extend these years so that the future will not be too gloomy.
The uncontrolled use of these fuels released great amounts of CO2 to the Earth’s atmosphere. With over 800 billion metric tons of carbon in the atmosphere and an annual exchange with the biosphere and oceans around 200 billion metric tons, an average carbon atom spends only about four years in the atmosphere before it goes into the oceans or the terrestrial biosphere.
The terrestrial biosphere and oceans not only take up carbon from the atmosphere (absorption by the photosynthesis by plants oceans) but they also give it back (emission from oceans and respiration by animals). That is, most of these carbon atoms are “recycled” so the atmosphere is not entirely rid of them. It takes about 100 years for a carbon atom to make it out of this recycling system and to get into the deep ocean. This is another side of the problem that desperately needs to be addressed so that global warming is reduced and the Earth’s atmosphere is back to near normal.
Microalgae are a superior biofuel feedstock as opposed to plants such as corn and soy used today. They need substantially less water and fertilizer than other plant life and they are extremely resilient. Algae doubles its mass approximately every four hours or more depending on the type, and can be cultivated in a controlled environment. There are over 300,000 different algae strains and approximately 10 are used in commercial production today. Microalgae are veritable miniature biochemical factories, appear more photosynthetically efficient than terrestrial plants and are efficient CO2 fixers. The ability of algae to fix CO2 has been proposed as a method of removing CO2 from flue gases from power plants, and thus can be used to reduce emission of GHG.
Large-scale biofuel production is facing a major criticism, often leveled against biomass in general, is that it will consume vast swaths of farmland and native habitats, drive up food prices, and result in little reduction in greenhouse gases (GHG) emissions. However, this so-called “food versus fuel” controversy appears to have been exaggerated in many cases. Credible studies show that with plausible technology developments, biofuels could supply some 30 percent of global demand in an environmentally responsible manner without affecting food production.
Many algae are exceedingly rich in oil, which can be converted to biodiesel. The oil content of some microalgae exceeds 80 percent of the dry weight of algae biomass. The net annual harvest of algal biomass cultivated in subtropical areas can be as high as 40 tons per hectare (dry matter), and even higher if CO2 is supplied. It is possible to produce about 100 grams per square meter per day of algal dry matter in simple cultivation systems. In theory, high oil content algae could produce almost 100 times the amount of soybean per unit area of land.
Microalgal biomass placed in a well designed production system and located in a tropical zone can be in the region of 1.5 kilograms per cubic meter per day or a lipid productivity of four grams per kilogram of culture per day and 2.8 grams per square meter per day (1,200 gallons per acre per year). The use of algae as energy crops has potential due to their easy adaptability to growth conditions, the possibility of growing either in fresh or marine waters and avoiding the use of land. Furthermore, two thirds of earth’s surface is covered with water, thus algae would truly be renewable option of great potential for global energy needs.
Reported microalgal efficiencies vary according to microalgal species and experimental conditions. Values of 2.3 percent, 4.7 percent, 5.6 percent, 6.9 percent, 8.1 percent, 9.4 percent, 9.6 percent and 15 percent have been reported. At 15 percent efficiency, productivity of about 60,000 gallons per acre per year is theoretically achievable. If this theoretical productivity is translated to reality, we can easily replace a great part of our daily energy requirement without sacrificing huge areas of fertile land that are intended to grow oil rich crops.
These efficiencies are bound by a number of limitations, such as light intensity, contamination, temperature, water loss and many other technical issues that require a lot of effort, capital and time to resolve. Nevertheless, the number is large enough and can be even extended further if we consider the tremendous daily amount of photosynthetically active radiation (PAR), the visible portion is 43 percent of sunlight, reaching the earth’s surface per acre; such is equivalent to about 28,000 MW (10.2 million MW per acre per year).
This amount of photosynthetically active radiation is equivalent to the energy content of 275,675 kg or 93,133 gallons of biodiesel (estimation based of the value of average energy content of biodiesel of 37 MJ per kg – the caloric value of algal biodiesel ranges between 33 to 41 MJ per kg). This value is more than four times of that possibility achieved with 15 percent efficiency. Therefore, using microalgae for the production of biofuel should be further investigated and solutions to solve the existing problems will get us closer to achieving much higher efficiencies.
The question is: where are we now? We are struggling to find the right means to convert solar energy to a readily useable form of energy and to lower atmospheric CO2 concentration. However, our achievement is still in the infancy stage and a lot of time, money and patience need to be employed in order to get close to our dreams. This sounds very disappointing but could be true when considering the little progress we see in the fields of microalgal biofuel production.
To fulfill our dream, productivity should be improved by better utilization of solar energy through improving photobioreactor designs, choosing the right species, minimizing water loss, contamination, controlling growth environment and optimizing all other chemical and mechanical processes.