Algae as a source of renewable energy

Algae have a great biotechnological potential with applications in human and animal food industry, cosmetics and medicine. They can be considered as a promising source of energy thanks to their ability for CO₂ absorption and photosynthesis, their rapid biomass production, and their high content of vegetable oils. The extraction of such oils from bulk algae biomass and their chemical transformation (called transesterification) will yield in biodiesel. 

This table shows the biochemical composition of different algae and reveals which algaes are of particular interest for biodiesel production thanks to their high lipid concentration.

To obtain rapid proliferation rates, algae can be cultivated in ponds or bioreactor particularly designed for their culture. When the biomass reaches a certain quantity, it can be harvested and serve for bioenergy production.

The fact that algae are able to produce hydrogen makes them even more interesting for energy productions via fuel cells. Nowadays, we know that algae have three different ways to produce hydrogen: direct and indirect photolysis, and  ATP based synthesis. Direct photolysis produces both hydrogen and oxygen simultaneously with subsequent explosion risk and high separation costs. Moreover, hydrogenases, the enzymes responsible of hydrogen synthesis, loose their activity in the presence of oxygen.
The indirect process is more favorable, since in anaerobic conditions and with sulfur deprivation in the environment, part of the stocked carbohydrates is converted into hydrogen. 

Positive aspects

- Algae culture present several advantages:

• Their growth is rapid and the culture period can be short. Several yearly harvests are possible in comparison with one yearly harvest for rapeseed, maize etc. 
• Their culture is relatively simple, needing essentially light and H₂O to enable the photosynthesis reactions to occur.
• Algae have a high environmental adaptability: they can proliferate in salt water at high and low temperatures. 80000 varieties have been identified but their potential in poorly known. Today, about 160 varieties are commercially used (source: Planet Wissen). These varieties probably include interesting candidates concerning their exploitation for bioenergy.

- The productivity of algae (in terms of litres of biodiesel produced per hectare and year) can be much higher than that of maize, soya or rape (factor of 100 to 400, source: Kiplinger report, SolarBioFuels).

- In terms of greenhouse gas production, it is interesting to use algae culture as an energy source the overall CO₂ emission from the conversion of biomass is null. The amount of CO₂ emitted is equal to the amount initially absorbed by algaes during photosynthesis. Due to their CO₂ absorption capacity, algae could also be used to mitigate CO₂ emissions from industrial installations.

- Under a number of environmental conditions, several varieties of algae such as, for example, the green alga Chlamydomonas reinhardtii, can produce hydrogen. A consortium of Australian and German scientists was created to promote research and development on this subject - in 2007, SolarBiofuels received as a subsidy 225.000 USD from the Australian government. Ben Hankamer, one of the consortium’s associates and professor at the University of Queensland, is optimistic concerning commercial exploitation in a near future. The importance of atmospheric CO₂ capture and the increasing price of petrol are in favour of the commercial exploitation of algae as energy source in the form of biodiesel and hydrogen (source: energyitv.com).

- Contrarily to other renewable resources for biomass production, algae production is not in conflict with food production or forest management. Algae do not require arable land for their culture, and their culture does not require much water compared to the plants classically cultivated for biofuel production.

- It can be expected that the culture of genetically modified algae (enhanced for their oil content and biomass or hydrogen production) in closed reactors has less negative public acceptance in Europe than the cultivation of genetically modified plants on land as the uncontrolled propagation into the environment can be estimated as negligible.

- Multiple uses of algae other than as energy sources are possible but still in an early stage (examples: food, cosmetics). Furthermore, the residues left after extraction of the oils present can be used for construction or in agriculture, thus reducing waste production..

Negative aspects

- The best values of biomass production have only been observed during short term cultivation and under highly controlled conditions (shaking of the algae solution, low cell density, optimal temperature and illumination). These conditions are difficult to maintain for a longer culture duration. The scale-up is still difficult and costly. Until today, no large scale industrial installation for the production of algae derived energy has been developed.

- In 1998, it was supposed that biodiesel production from algae was too costly compared to the production of fossil diesel. With the increase in the oil barrel price, the balance could change. However, today, only 0,2% of the photons acquiered by algae are transformed into hydrogen. However, 7 - 10% would be needed for a satisfactory yield. This implies that a high light entry is required and therefore a higher cost of production (source: SolarBioFuels).

Constraints

- Hydrogen production requires the assessment of methods for stocking and distribution of this gas which is highly explosive when it comes in contact with oxygen. Future installations with minimal risk will most probably be costly.

Lines of reflection

- The cultivation of algae for biomass production is more productive in a closed system (a transparent bioreactor that lets the light pass through) than in an open pond. Indeed, a closed system uses less water (about 1000 times less than for cultivated plants), produces more biomass (about 5 times that of an open system) and reduces the contamination risks (source: SolarBioFuels).
How far are we away from a economically viable model for the production of biomass, biodiesel or hydrogen from algae? Who will be the first to propose one? Who are the top players in terms of research and development? Today, several start-ups (particularly in the United States) and research laboratories invest in the development and improvement of photobioreactors to reduce the production cost to 10 €/m².

Perspectives

- According to a number of experts, using algae is highly advantagous compared to other vegetable sources for the production of biodieseland will become the best solution in about 5 years through mass marketing (source: Kiplinger report on biofuels). Several needs have to be addressed before:

• - Identify the algae the most suited to the production of biodiesel and biohydrogen
• - Improve the technologies (photobioreactor) to reach an industrial scale and start marketing.
• - Innovate on the secondary products after extraction of the oils: proteins, sugars and other residues can be used as animal food or fertilizer.

- What is the potential of the use of algae in developing countries? A technology where algae are cultured in open ponds could be applied in a number of countries where the warm climate is favorable to a rapid proliferation. Some varieties of algae are adapted to difficult environmental conditions. Offshore algae cultivation or in sewage water can also be considered.

References

Research:

SolarBioFuels

South Australian Research and Development Institute SARDI

Realising the Economic Potential of Sustainable Ressources - Bioproducts from Non-food Crops EPOBIO

Algae Biotech Group, University of Bielefeld

Photobiotechnology Group, University of Bochum

 Information about Algae (Biology, Applications)

Planet Wissen (in German)

energyitv

FAO Agricultural Services Bulletin

Kiplinger report on biofuels