Microalgae are a real treasure trove. The cosmetics, food and chemical industries already use many algal metabolic products and it is envisaged that algae will become an important regenerative source of energy in the future. However, the methods required to harvest the raw materials for biodiesel production are still far too uneconomical for large-scale industrial application. How can bioreactors and harvesting methods be optimised? Prof. Dr. Clemens Posten’s Bioprocess Engineering team at the Karlsruhe Institute of Technology (KIT) focuses on bioprocess development and is investigating the effect of different diets and the dilution of light on algal product yields.
Microalgae are unicellular organisms that are able to convert the sun’s energy into chemical energy. Driven by photosynthesis, microalgae produce polysaccharides, proteins and fatty acids, substances that are already being used in large quantities by the food industry as food supplements or in aquacultures to feed fish. The cosmetics industry also makes substantial use of the broad range of algal metabolic products. It is envisaged that in future algae will also be used for the production of biodiesel as they produce energy-rich oils under certain conditions and in quantities of up to 50 per cent of their own cell mass. Algae are sustainable sources of energy as they absorb the same quantity of CO2 from the air as is released when they are processed. In future, cultures of algae could be grown in vast bioreactors. “This vision could become reality,” said Prof. Dr. Clemens Posten, head of the Department of Bioprocess Engineering at the Institute of Bio- and Food Technology of the Karlsruhe Institute of Technology (KIT). “But there are quite a number of obstacles that need to be overcome first.”
Algae have huge advantages over plant-derived sources of energy. They can convert up to five per cent of the energy of the sunlight they take up into chemical energy. Rape, maize and similar crops can only convert around one per cent of the energy of sunlight into chemical energy. One reason for this is that only a small proportion of a plant can actually carry out photosynthesis, and this does not include structures like stalks and shoots, which need to be supplied with energy that is produced in the leaves, thus reducing the energy balance. Another great advantage of algae is the fact that they do not need to be cultivated on agricultural areas. The tanks in which they are cultivated can be positioned in the sea or on infertile ground. Algal biotechnology is therefore a potential way out of the tank-plate dilemma: biodiesel can be produced from algae without occupying areas needed to grow food. Algae would appear to be the perfect energy suppliers. However, while cosmetics and food companies grow large quantities of algae in tanks, the energy industry is still lagging behind. Why? “Methods for producing biodiesel from algae are still too expensive,” said Posten.
The researchers in Posten’s team are process engineers who are focusing more on technical processing issues than on the biological aspects of algal biotechnology. Under which conditions do algae grow best? How can the yield of oils, polysaccharides and other interesting substances be increased? How can the energy requirements be reduced and what costs are involved? To find answers to these questions, Posten’s team has constructed different types of bioreactors in which they are able to grow and harvest algae under controlled conditions. Bioreactors are no more than tanks filled with medium and gassed with CO2. Four rather simple tanks of between ten to thirty litres in volume are used for pilot tests, six smaller tanks are equipped with sensors and LED lamps that enable the researchers to measure and model different physiological processes in greater detail.
Where in the overall process can money be saved? Posten and his team initially test the growth rates and product yields of a number of natural and genetically modified algal strains provided by cooperation partners such as the algal biotechnologist Prof. Dr. Olaf Kruse from Bielefeld. “Which strain grows best under which light conditions and produces the largest quantities of oils and other molecules?” asks doctoral student Robert Dillschneider who puts his green experimental subjects on a strict nitrogen or phosphorus diet. He does this in order to boost the production of oils as algae are known to use the energy of sunlight for the generation of energy-rich substances in situations when protein production is hindered by the lack of protein building blocks. Which light intensities and brightness and darkness patterns are best? Dillschneider determines different physiological parameters in order to gain a better understanding of an algae’s performance. He then uses these findings to develop mathematical models of physiological processes that enable him to make predictions on the behaviour of algae under different conditions.In principle, the objective of such experiments is to increase the output of the technology. The cost balance of currently used methods is also due to the relatively high energy requirements, i.e. the input. In order to optimise the balance on the input side, the scientists need to improve the light yield of the algae. “Under conditions of high light intensity or photon flow densities, algae are unable to use a large proportion of the light that enters a bioreactor,” said Dillschneider. “Algal cells are quickly saturated with light because the molecular photosynthesis machinery cannot keep up with the high light intensities available. High light intensities might even impede photosynthesis.” The researchers need to find out which light intensities and temporal light patterns are optimal for the algae. In addition, the researchers from Karlsruhe are attempting to dilute the light that enters the bioreactors by shaping the reactor’s surface so that the light that illuminates a specific surface area is diverted onto a larger area of algal suspension inside the reactor. Another approach involves using light conductor structures inside the bioreactors that transport some of the light deeper into the bioreactor.
There are numerous parameters that the researchers can change in order to reduce the investment costs of the technology. For example, Dillschneider is also investigating different harvesting processes. “Can we achieve a cost reduction by introducing a semi-continuous harvesting method that enables some of the algal cells to be removed and replaced by fresh medium?” asks Dillschneider. Posten and his team are also testing inexpensive materials for the production of bioreactor surfaces as well as methods to efficiently feed CO2 into the bioreactors. The gassing of bioreactors can eat up huge amounts of money when the direct introduction of CO2 leads to bubbles, which results in a huge loss of pneumatic energy. “We are currently testing whether CO2 can be fed into the bioreactors by way of membranes,” said Posten who cannot say any more about this project as the researchers have just filed a patent for the method.All of Posten’s team’s projects are part of national and international research networks. In cooperation with biologists, material researchers and industry partners from Germany and other EU countries, the Karlsruhe researchers are slowly but surely approaching their goal of turning algae into one of several resources that are used for the sustainable production of energy. What is the time scale? “I think that algal biotechnology will be able to cover a significant percentage of energy in around six to eight years’ time,” said Posten. It is worth mentioning that the cost balance for energy produced from bioethanol is not that positive either; this type of energy production is generally subsidised. However, it appears that algae will become a major source of energy even without external financing.
Prof. Dr.-Ing. Clemens PostenHead of Bioprocess EngineeringKarlsruhe Institute of Technology (KIT) - Institute of Bio- and Food Technology Bioprocess Engineering Haid-und-Neu-Straße 976131 KarlsruheTel.: +49 721 608-45200Fax: +49 721 608-45202E-mail: clemens.posten(at)kit.eduRobert DillschneiderKarlsruhe Institute of Technology (KIT) - Institute of Bio- and Food Technology Bioprocess Engineering Haid-und-Neu-Straße 976131 KarlsruheTel.: +49 721 608-48311Fax: +49 721 608-7553E-mail: Robert.Dillschneider(at)kit.edu