Output and capacity can be scaled separately. Capacity is determined by the size of the tanks, while output is determined by the quantity of energy converters.
During the reduction process the electrolytes gain electrons, while during oxidation they lose electrons. Both chemical reactions have to occur separately from one another in the anode and cathode spaces of the energy converter. The stored chemical energy is converted into electricity as a result of a redox reaction on the surfaces of both electrodes. Conversely, electricity is added and the redox reaction is reversed during the charging process.
During the charging and discharging processes, the electrolytes – posolyte and negolyte – are pumped continuously through the stack’s cells, hence the term “flow”. The term “redox” is derived from the chemical reactions taking place in the energy converter stack.
Energy converter stacks
The stacks consist of serially connected half cells. Anode and cathode half cells are linked in pairs by a semipermeable membrane, which selectively allows ion migration to enable the necessary charge equalization to take place, but prevents the two electrolytes from commingling.
Both electrochemical reactions, oxidation and reduction, take place in these stacks, physically separated from one another. The output of the flow battery depends on the material and surface area of the electrodes, as well as on the kinetics of the redox process on the electrode surface, i.e. an intrinsic property of the electrolytes.
Two aqueous organic active materials are used as electrolytes. An organic compound based on a specially developed enhanced aromatic ring system serves as the negolyte. The posolyte is a common organic complex of iron.
The organic active material can be obtained medium- to long-term from renewable raw materials. One possible source is lignin, approximately 20 billion tonnes of which are an annual byproduct of global pulp production and for which there have previously been no alternative uses. Specifically breaking the lignin down provides access to precursors, from which the organic active materials can be produced.
Scalable power electronics
System-synchronized, scalable inverters for AC-side grid connection purposes also form part of the system. Solutions involving DC/DC conversion are also available for the purposes of integration into existing PV and storage applications.
Both electrolyte solutions are stored in separate tanks. They circulate through the anode and cathode spaces of the stacks via pipe systems.
A monitoring system monitors these processes and a battery management system electrically and hydraulically controls them. The BMS can also be integrated into a higher-level energy management system.