One of the routes to a renewable society is the usage of biomass to produce electricity and fuels. Examples of biomass that can be used for fuel and electricity production are crops, wood and garbage. A type of biomass with a lot of possibilities for the future is algae. Algae are photosynthetic organisms that use solar energy to combine water and carbon dioxide to produce organic materials, i.e. biomass.


Pond with algae from www.ecofriend.org.

In general there are two types of algae: the macroalgae and microalgae. Macroalgae are also known as seaweed and can become very large, up to like 60 meter in length. There are also microalgae, which are far smaller than macroalgae and are practically small photosynthetic factories. Both types of algae grow in marine and fresh waters and can be found over the whole earth.

Algae production

To make algae a viable source for biomass they have to be produced in large quantities. There are some important factors that determine the growth rate of algae

  • Algae type: different types of algae have different growth rates
  • Medium/nutrients: composition of the water
  • Light: light is needed for the photosynthesis process
  • pH: algae need a pH between 7 and 9 to have an optimum growth rate
  • Aeration: the algae need to have contact with air (CO2)
  • Mixing: mixing prevents sedimentation of algae and makes sure all cells are equally exposed to light
  • Temperature: there is an ideal temperature for algae to grow

Ideal circumstances to grow algae can be created in a laboratory. However, the costs to create these circumstances are very high and it is very difficult to scale up the laboratory environment efficiently.

At the moment there are two distinct methods that are being researched for growing algae. The first method is the so called 'raceway pond'. The 'raceway pond' is a large open water raceway track where algae and nutrients are pumped around by a motorized paddle. Carbon dioxide also has to be added to the pond. The algae culture will grow continuously and part of the algae will be removed during the growing process. The biggest advantage of these open ponds is their simplicity, yielding low production costs and low operating costs. However, not all algae species can be grown in these ponds, due to contamination of other algae and bacteria. Also the process conditions, like temperature and the amount of light, are hard or impossible to control.


Raceway pond from John Sheehan et al.

The second method to grow an alga culture is the photo bioreactor. This is, opposed to the open pools, a closed system with a light source to grow the algae. Conditions in the photo bioreactor are much easier to control compared to the open pools. However, operation and production costs of there bioreactors are much higher than the open pool systems due to more complicated technology.


Photo bioreactor from www.bio-pro.de.

Fuel or energy production

Algae can be turned into a fuel or electrical energy as follows:

  • Transesterification to biodiesel
  • Fermentation to ethanol or methane
  • Gasification to methane or hydrogen
  • Pyrolysis to gas/liquid fuels
  • Burning to generate heat or electricity

Many sources indicate that the creation of biodiesel from algae is the most promising path to follow. The subsidies on biodiesel are very high worldwide and biodiesel is in general seen as a clean fuel. Also the process of producing diesel from biomass with lipids by transesterification is becoming well established. Methane production from algae is possible by fermentation, pyrolysis or gasification. At this moment gasification is the most efficient process, i.e. most of the biomass is converted into methane although fermentation is cheaper to perform.

Advantages

One of the biggest advantages of using algae as a source for methane is the fact that the total process is neutral with regard to carbon dioxide. With an increasing focus on environmental friendliness this could point out to be the most important advantage. Also the fact that algae can use carbon dioxide and other flue gasses to grow makes them a very interesting subject of research. Furthermore, algae are a sustainable source of energy. Furthermore all conditions for algae to grow fast are present in abundance: feedstock in the form of carbon dioxide and water and energy source in the form of sunlight.

A second advantage is the fact that algae are highly efficient converters of solar energy to biomass, compared to crops or trees. It is possible for algae to use almost 10% of the incoming sunlight for the photosynthesis process. This makes it possible to get a high biomass output per square meter per day. Currently the maximum output found in literature is about 50 gr/m2/d for open pond systems and about 150 gr/m2/d for photo bioreactors. These values are peak values so not an average.

A third advantage is that some algae grow in water and not on land. If grown in traditional lakes, or even salty waters, they don't compete with other sources for biomass.

Disadvantages

The biggest disadvantage at this moment is the fact that it is still hard to grow mass cultures of algae for a competitive price. To earn maximum growth per square meter a photo bioreactor is necessary. These photo bioreactors however are extremely expensive compared to the traditional open pond systems. The disadvantage of the open pool systems however is the fact that it is hard to control the growth process of the algae. Only a few algae species can be grown in open pool systems at this moment, because there are only a few species that are not very sensitive to contamination. These species are often not the most efficient converters of sunlight and carbon dioxide to biomass.

Besides the fact that growing algae is not economically viable at the moment, the conversion is not optimal as well. Because algae are wet they cannot be gasified without drying, which consumes a lot of energy and therefore lowers the overall efficiency. Supercritical water gasification is a solution for this problem because wet biomass can be a feedstock for this process. However, this technology is still very immature and not applied on a large scale.

For conversion with fermentation five decades of research has been done by Oswald. He concluded that 1.5 kWh electrical energy per kg of algae can be produced by algae anaerobic fermentation and burning of the produced methane. With already established open pool growth rates of 10 g/m2/h this would yield 15 W/m2. This means that the total electricity production of the Netherlands can be produced with 18.000 km2 of algae farms. This is about 45% of the total surface area of the Netherlands, which is of course not viable. These calculations have only taken into account the area needed to grow the algae, not the processing of the algae to methane.

Greenfuel, a company focused on producing bio diesel from algae, recently developed a technique to increase algae production in a bioreactor. With this technique they are able to produce an average of 100 g/m2/h, which would decrease the area needed for growing algae to 4.5% of the total area of the Netherlands. Greenfuel claims this process is economically viable. However, a case study done by Ph. D. K. Dimitrov shows this is absolutely not the case.

Weissman and Goebbel developed an open pool system, which produces biomass at a cost of about 200 $/mt. With the previous information this would yield an electricity cost of 0.18 $/kWh. This is actually far less expensive than solar power but still twice as expensive as regular fossil fuel based electricity. However, in the calculation of the price of the electricity from algae the costs of the creation of the methane from the algae is not included. So actually only the costs of creating the biomass will give an addition of 0.18 $/kWh to the electricity price.

Another disadvantage of the production of algae is the fact that the farms have to be built near a power plant or something equivalent to have enough flue gasses with carbon dioxide to produce the algae. This reduces the possibilities for placement of algae farms.

Future prospects

At this moment it is commercially not viable to produce methane from algae. Cheap algae can be produced by open pool systems, but they need a too large surface to replace fossil fuels. Bioreactors need about ten times less surface to produce the algae, but the costs for these systems are far higher. A lot of technological progression has to be made in order to make power generation from algae feasible. Initial focus of research should be on growing mass cultures of algae efficiently. By genetic engineering more productive species and species that are more suitable for open pond systems should be created.

A big question at the moment is how to use algae as efficient as possible as a source of energy. There are many paths to take, but at the moment major research is focused on the production of biodiesel. The process of making biodiesel from biomass is well known and already used on a large scale. Another path, which is easy to follow, is the fermentation of biomass to ethanol. This process is also applied on a large scale. If there are not any major breakthroughs in the conversion of algae to methane (supercritical water gasification for example) methane from algae is not expected to be the source of energy for the future. However, with increasing demands of gas and more subsidies on biogas (subsidies on biodiesel are very high globally) it is possible that more research will be done on the production of methane from algae.

Sources

'A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae', John Sheehan, Terri Dunahay, John Benemann, Paul Roessler, the National Renewable Energy Laboratory, U.S. Department of Energy's Office of Fuels Development (1998)

'Manual on the production and use of live food for aquaculture', Patrick Lavens and Patrick Sorgeloos, Laboratory of Aquaculture and Artemia Reference Center, University of Gent (1996)

'Technologies: a case study', Greenfuel (2007)

'Power from Solar Energy via Algae-Produced Methane', Golueke, C., W. Oswald. Solar Energy 7(3), 86-92. (1968)

'Evaluation of Greenfuel's 3D Matrix Algae Growth Engeneering Scale Unit', O. Pulz, Germany (2007)