2012|13 Annual Report Fraunhofer IGB

116 ROBUST AUTOMATION CONCEPT FOR THE OUTDOOR PRODUCTION OF ALGAL BIOMASS Dipl.-Ing. Ronja Münkel, Dr. rer. nat. Ulrike Schmid-Staiger Microalgae as a sustainable energy source The production of biofuels on the basis of food crops or feed crops (e.g. biodiesel from rapeseed oil or palm oil) is in direct competition with food and feedstuff production. Produc- ing second-generation biofuels with plants not used as food or feedstuffs, for example Jatropha, results in competition in terms of water consumption and cropland. Oil from microal- gae is a potential alternative to plant biofuels and belongs to the third generation of biofuels. Compared with the cultiva- tion of higher plants, the cultivation of microalgae offers nu- merous advantages. These include a higher yield per area, a reduced requirement for water and the possibility of growing microalgae on land that can not be used for agriculture. Oils produced by algae can be used as a biofuel, resulting waste gases fed back into the process and the residual biomass that remains is fermented to produce biogas. In order to convert the process to an industrial scale, we have developed a stand- ardized process automation concept for the cultivation of mi- croalgae. Production process requirements For the commercial production of microalgae and their use as a sustainable energy source, outdoor cultivation using so- lar energy is essential. Here, special challenges for the process control are the changeable weather conditions and the inher- ent day-night-rhythm. To deal with these circumstances, it is important to establish as robust a biomass production process as possible, comprising a high degree of automation and sim- ple measurement technique. The process control should there- fore be based exclusively on the measurement of the pH value and the reactor temperature. Key parameters The starting point for all experiments was the biomass pro- duction process with the microalga Chlorella vulgaris (Fig. 1) in a 30-liter ﬂat panel airlift (FPA) reactor. In order to achieve a stable production process, it is vital to supply the microalgae culture continually with carbon dioxide, to make required nu- trients such as ammonium available and to maintain the pH value and temperature within an optimum range. The higher the CO2 concentration in the supply air, the more becomes dissolved as carbon dioxide in the culture medium. This lowers the pH value. This is counteracted by the ammonium dissolved in the medium: the higher the ammonium concentration, the higher the pH value in the culture medium. In addition, the solubility of CO2 in the medium is inﬂuenced by its compo- sition and by the temperature. If, in such a system, the pH value is constantly regulated by means of the carbon dioxide concentration in the supply air, this allows conclusions to be drawn about the ammonium concentration in the reactor. This link was used to determine the consumption of nutrients in the reactor. On the basis of these calculations, we were able to successfully control feeding cycles and exclude nutrient and carbon dioxide limitation. Programmable logic controller The automation concept was achieved – in line with the cur- rent industry standard – with the aid of a programmable logic controller (SIMATIC S7-1200, Siemens) and set up outdoors on a test rig with 30-liter ﬂat panel airlift reactors (Figs. 2 and 3). When setting up the control software, it was ensured that it was very user and operator-friendly. The overall process was visualized on a display screen (Fig. 4) and all online data con- tinuously recorded. The control software is constructed in a ENERGY NH4 PO4 Fe Scales CO2 pH Medium Outlet HarvestAir Cooling water inlet ﬂow Cooling water inlet ﬂow TIR FIC QIR 1 2 3

2012|13 Annual Report Fraunhofer IGB

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