Algae for economical hydrogen production
Energy experts have been dreaming about using hydrogen as an inexhaustible energy source for a long time. However, this vision has not yet become reality. The search for intelligent, affordable methods that can be used to convert sun energy, wind or water power into hydrogen has so far led to dead ends. However, small algae might now pave the way into the hydrogen era.
Chlamydomonas reinhardtii seems to be the hope for the future. These unicellular algae are able to produce hydrogen under somewhat difficult living conditions. Something that nature originally conceived as an emergency mechanism will now be further developed in an economically interesting hydrogen production method by Florian Lehr from the Bioprocess Engineering Department at the University of Karlsruhe.Sulphur diet triggers hydrogen production
Lehr plays dirty with the Chlamydomonas algae. At first, he takes great care of them to ensure that they proliferate. Then he withdraws sulphur and leaves them to starve. However, the lack of sulphur inhibits photosynthesis, which Chlamydomonas algae use to produce sugar from carbon dioxide, sunlight and water. “The key enzyme that governs photosynthesis can no longer be repaired,” explained Lehr. However, the algae have to somehow cope with the energy to which they are still exposed. After all, they are unable to simply switch off the sunlight. In this miserable situation, the algae activate hydrogenases that then produce hydrogen. However, the algae are unable to survive for long under such conditions; the algae need a regeneration phase after 12 days.
Theoretically, the sun’s energy that reaches earth amounts to 178,000 Terrawatt (TW) and is thus able to cover the global energy requirements of 13 TW (1TW = 1,000,000,000,000 Watt) several times over. However, to date it has proved difficult to convert and store the sun’s energy. Energy producers are obviously highly interested in the possibility of turning sunlight into hydrogen. The energy stored in hydrogen can be made available and used in fuel cells.
Lehr uses a special Chlamydomonas reinhardtii mutant, which produces ten times more hydrogen than the wild-type species. Prof. Dr. Olaf Kruse, algae biotechnologist at the University of Bielefeld, has produced this variant (stm6) by selecting and reproducing effective hydrogen producers. These algae have a degree of efficiency of two to three per cent. However, this is still not enough for Lehr and his boss, Prof. Clemens Posten, who would like to have algae that are able to use ten per cent of sunlight and convert it into hydrogen.
First important task has been solved – construction of a laboratory reactor
At present, the most limiting factor is the very oxygen sensitive hydrogenase enzymes. Large quantities of oxygen are produced during photosynthesis, and can be kept at bay through the deprivation of sulphur. The conditions that are currently in place eventually lead to an oxygen demand that exceeds the oxygen produced and hydrogen production therefore starts. If Chlamydomonas were able to continue photosynthesis at full capacity, then more electrons could be produced and the energy yield increased. However, this requires oxygen to be captured somewhere in the system before it is able to inhibit the hydrogenases. Another possibility is to modify the oxygen sensitivity of the enzyme; but this has to be done by the biologists that are involved in the project.
The bioprocess engineers from Karlsruhe, who are also involved in the European biotechnology course taught at the universities on the upper Rhine, are concentrating on their strengths, i.e. the development of bioreactors and the optimisation of process control. They are testing the conditions for the economic hydrogen production with stm6 and are developing concepts for the construction of a commercial plant. Lehr started work on this project about one and a half years ago. Initially, he constructed a three-litre laboratory reactor, which he used to determine the conditions under which Chlamydomonas produced which quantities of hydrogen. “No useful data were available,” said the engineer. The small research reactor is not made of glass or plastic, but stainless steel; the light does not come from the sun but from a special LED illumination apparatus. This laboratory reactor offers the scientists excellent measurement conditions, even though an outdoor reactor will certainly look different.
Something that sounds simple is hard scientific work
Lehr uses the test reactor to evaluate a broad range of kinetics: At what light intensity do the algae produce what quantity of hydrogen? At what point is the light intensity strong enough to entail photooxidative processes and lead to stm6 damage? What effect does temperature have on the storage of energy? What dynamics are triggered by light- and darkness phases? Since the continuous flow of energy is a prerequisite for reliable energy supply, the engineer has to test whether the production plant will require an intermediate store. However, this would lead to higher reactor costs.
“A measurement series with preparation and post-processing takes about four weeks; such a cycle is also very complex in technical terms,” said Lehr. However, the results obtained from the pilot plant will provide the engineers with the data they require to determine suitable parameters for effective hydrogen production and plan bigger plants. There are plans to build a 30-litre bioreactor in 2008. “This will be very close to a proper production reactor,” said Lehr highlighting their plans to set up a 250-litre reactor in 2010 as a prototype for a large-scale plant. And who knows, maybe this will be an important step towards hydrogen energy and away from the continuously decreasing fossil fuels. The dead algae can also be used for energy production. The fermentation of biomass generates methane.
kb - 25.03.2008
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