Quantifying the algae used to make biofuels is a difficult prospect for two reasons. First, there are thousands of known species of algae with more being discovered every day. The second problem has to do with intellectual property rights. A number of algae are genetically modified and fall under patent protections. So, this article makes do by talking about algae in generalities.
Algae can be used to produce biodiesel, biobutanol, biogasoline, methanol, methane, ethanol, and even jet fuel. In many cases, the algae must be harvested (killed), the oil extracted, and then the oil refined to produce a fuel. This is usually the case with traditional fuels like jet fuel, biogasoline, or biodiesel. The problem with such an approach is that growing algae from scratch requires more nutrients than if they are simply maintained. To that end, several companies and institutions have developed ways to harvest ethanol, butanol, and even biodiesel from algae that excrete the fuel into the water they grown in.
Growth and Cultivation
There are three basic ways to cultivate algae: open pond, closed-loop, and photobioreactor. Each has advantages and disadvantages.
Open Pond Growth Systems
Open pond growth is the most common system used to produce algae that will be harvested for their high oil content. It is also the most similar to how algae grown naturally. The benefits to this system are it requires little capital investment and the components of the system require less maintenance. These benefits, however, are well offset by the vast drawbacks to the open growth system.
The most important drawback to the system is that it leaves algae open to contamination by bacteria, viruses and fungi. Beyond that, the pH, temperature, and light conditions of the system are much harder to control, which means it is more difficult to obtain maximum algae growth at all times. Finally, the only algae that do well in this system are those that are high in oil, so it limits the diversity of species that can be used and thus the flexibility of the facility.
Closed-loop Growth Systems
Closed-loop systems work much like open ponds except that the water is not exposed to the environment. These systems inject nutrients into closed a space where algae grow, thus avoiding the problems associated with contamination. Closed-loop systems work very well in cogeneration settings where waste carbon dioxide from an industrial process is harvested to produce energy.
The problems with closed-loop systems are that they are expensive to maintain and have more working parts. Beyond that, the CO2 injected into the system for use by the algae must be sterile or contamination will still occur. When contamination does occur in these systems, the clean-up process can be expensive. Closed-loop systems are often used to grow algae with high oil contents.
Photobioreactor Growth Systems
The final system for growing algae is often called a photobioreactor (PBR), which is a type of a closed-loop system. PBRs are nothing more than glass or plastic tubes that are exposed to sunlight. Nutrient-rich water and algae flow through these tubes and are exposed to sunlight for energy. These systems allow for the growth of algae with lower lipid content than the previous two systems. This is important because low-oil algae can grow up to 30 fold faster than other species, potentially allowing PBR systems to harvest algae up to 30 times more often. Additionally, low-oil algae are less prone to disease and so the system need not be perfectly sterile. This is of major benefit in cogeneration settings where sterilizing CO2 may be difficult or even impossible. The system also allows algae to be grown in less than pure water, such as water from wasted treatment facilities. All of these factors help to increase the efficiency and sustainability of the system. Finally, PBR systems, because they are closed, can be situated in deserts and other undesirable locations. This helps to reduce their impact on food supply chains and limit their impact in terms of land use changes.
The drawback to PBR systems is that they require a high upfront investment. This investment pays for itself over time through increased productivity and increased control over growth, but can be a barrier to entry. PBR systems are also technically more difficult to operate than open ponds.
PBR systems are popular among institutions that are developing genetically modified algae for several reasons. First, they allow for separation of the colony from the environment, ensuring that the GM alga remains pure and making it easier to test the efficacy of the species. The second reason PBRs are popular with GM algae is that it protects the environment from accidental cross-contamination with algae that may harm existing ecosystems. Finally, PBRs are effective for GM algae because they help to maximize growth by maximizing exposure of algae to nutrients and sunlight. In other words, PBR systems allows GO algae to grow to their full potential.
One of the major benefits of algae is their limited impact on land use, both direct and indirect. In the direct land use category, algae are the most efficient of all major systems explored to date. To produce the same amount of biofuel with algae as compared to Jatropha requires around 400 times less land. To supply the aviation industry entirely would require 2.7 million square kilometres of Jatropha and only 68,000 square kilometres of algae.
In terms of indirect land use change, using algae allows for growth on marginal land. For instance, using PBRs, algae can easily be grown in the desert or other marginal settings with the need to clear existing land and thus invoke a massive carbon debt before any fuel is even produced. Further, algae can be grown in cogeneration settings, which can actually mean it has a positive land use impact by reducing industrial waste.
Bad News from the National Research Council (NRC)
The NRC has stated that large-scale production of biofuels from algae is untenable with existing technology and would require too much water, energy, and fertilizer. According to the NRC, using algae to produce one litre of gasoline requires 3.15 litres of water as a minimum (maximums range up to 3650 litres). Furthermore, meeting the demand for only 5% of the fuel used in the United States would require 6-15 million metric tons of nitrogen and up to 2 million metric tons of phosphorus. That is up to twice the amount of nitrogen fertilizers used to grow all the food the U.S. currently produces.
The NRC has stated that none of its concerns is a definitive barrier, but that research and development is needed to determine if the process can be made sustainable. Some solutions include the recycling of used water and nutrients and many strategies are currently under development with help from government grants.