Novel bioreactor and sponges that thin out light
Microalgae are veritable treasure troves. The cosmetics, food and chemical industries already use algal metabolic products for various applications. In future, the green unicellular organisms might also be grown on a large scale in photobioreactors installed on fallow land where they will be used as regenerative sources of energy. Mark Fresewinkel from the Karlsruhe Institute of Technology (KIT) is involved in a cooperative project aimed at developing an effective photobioreactor that integrates a biogas plant. The reactor’s light-guiding structures will also bring more light to the shady side of the algal world.
Microalgae are unicellular organisms that are able to convert the sun’s energy into chemical energy. Driven by photosynthesis, they can produce polysaccharides, proteins and fatty acids that are already used by the food and cosmetics industries. It is expected that algae will also be used for the production of biogas. They produce biomass that can be turned into energy-rich methane in a biogas plant in a process that withdraws the same quantity of CO2 from the air as will later be released by combustion. The vision of the future is to install bioreactors on large fallow areas where large quantities of algae will be grown. “The processes of currently used bioreactors are still not efficient enough,” said Mark Fresewinkel, PhD student at the Institute of Bio- and Food Technology (director: Prof. Dr. Clemens Posten) at the Karlsruhe Institute of Technology (KIT). “Biomass is produced in a bioreactor, which subsequently needs to be fermented into methane in a biogas plant, involving a rather costly detour.”
A shorter distance between algae and bacteria
Fresewinkel is working on the development of a bioreactor in which the relevant methane production processes are spatially and chemically coupled with each other. The BMBF-funded project is being coordinated by Prof. Dr. Christian Wilhelm, an inventor and scientist from the Department of Plant Physiology at the Institute of Biology at the University of Leipzig. The key idea behind the novel bioreactor is to integrate different process steps into one reactor, thereby doing away with the one step that normally requires huge amounts of energy. This is the step where algae produce biomass; biomass also consists of proteins, carbohydrates and lipids that are actually far too valuable to be used for the production of biogas. The production of such building blocks consumes sunlight that could be used for other more important processes. Fresewinkel and his cooperation partners have therefore looked into developing a two-chamber bioreactor, one chamber for the cultivation of algae and the second one containing methane-producing bacteria. It works on a very similar principle to a biogas production plant. “The algae in the first chamber do not provide the bacteria with biomass. They only provide them with glycolate which they can produce using far less energy,” said Fresewinkel pointing out that the bacteria then use the glycolate to produce methane.
Another major advantage of the technology is that the algal biomass does not need to be separated from the culture medium. Rather than growing in an aqueous solution, the algae form solid biofilms. This leads to major cost savings in energy otherwise required for separation and pumping. The researchers believe that the technology has huge potential to be used, for example, in biogas farms next to motorways. Fresewinkel is currently testing the immobilisation of algal biofilms and their supply with carbon dioxide gas and other nutrients.
Fresewinkel’s partners from Leipzig along with other biologists from the Saxon Institute of Applied Biotechnology are focused on the general conditions under which the algae produce the highest glycolate yield. Glycolate is only produced when oxygen concentrations are low, so great care needs to be taken to keep the concentrations low. Fresewinkel’s partners from Leipzig are also testing the performance of the bacteria that produce methane from glycolate while researchers from the Bremen-based Institute for Environmental Process Engineering (director: Prof. Dr.-Ing. Norbert Räbiger) are focused on developing optimal materials for the membranes that separate the algal and bacterial chambers from each other. These membranes need to retain oxygen but must be permeable for glycolate.
Light on the shady side
Reducing the energy loss associated with biogas production is only one of the parameters the scientists and process engineers can alter as they seek ways to optimise existing technology. Another option is to optimise existing photobioreactors where getting sunlight to the algae is a crucial issue. In traditional bioreactors, the algae float in a liquid solution; sunlight does not penetrate the entire tank and the rear part is left in darkness. In addition, algae are unable to use high intensities of sunlight (i.e. photon flow densities). “Dark areas reduce the yield of photobioreactors, as bacteria that are deprived of sunlight are unable to produce biomass,” said Fresewinkel. “However, high light intensities lead to light saturation as the molecular machinery is unable to keep up with photosynthesis.”
One way of solving the problem is by “thinning out” the intruding light and guiding it into the depths of the reactor. In a previous project, Fresewinkel and his colleagues produced sponge-like structures from glass and tested whether these structures are able to transport some of the light into deeper layers of the bioreactor. This enabled the researchers to increase the biomass by up to 25 percent. In his second project, Fresewinkel is looking for suitable materials that would also be more flexible in terms of sponge pore geometry. Glass is extremely fragile and therefore not very variable in form.
The sponge project is still in its infancy. However, the idea behind the two-chamber bioreactor that Fresewinkel is working on has already been patented and he is also thinking of establishing contact with industry in order to translate the principle onto a larger scale. If everything goes to plan, algae might soon be making a major contribution to the cocktail of regenerative energy sources. Algae have enormous advantages over plant energy sources as they are able to convert up to five percent of the sunlight they take up into chemical energy. Rapeseed, maize and other crops are only able to convert around one percent of the sunlight into chemical energy; only a small proportion of the plant material can actually carry out photosynthesis; the stems and shoots also need to be supplied with chemical energy and therefore reduce the energy balance. In addition, algae do not need to be grown on agricultural land; bioreactor tanks can be placed in the sea or on infertile areas. With all these advantages, algal biotechnology provides a way out of the so-called “food or fuel” dilemma, offering a way of producing energy that does not compete for agricultural land with the food industry. Process engineers like Fresewinkel are on the right track towards providing the technology needed to do this.
Dipl. Ing. Mark Fresewinkel
Karlsruhe Institute of Technology (KIT)
Institute of Bio- and Food Technology
Area III: Bioprocess Engineering
Tel.: +49 (0)721/ 608 - 45 205
Fax: +49 (0)721/ 608 - 47 553