If you want to visit the Fraunhofer Institute for Chemical Technology ICT in Pfinztal, Germany, you should not rely on your GPS. It has been in existence for a few years, but the comfortable new access road is still unknown to most devices. Since March 2017, though, you can already see where the institute is located from some distance away, because there is a large wind turbine spinning on the summit of the Hummelberg.
The success of the German Renewable Energy Act made Germany an early pioneer in an energy transition that is driving the transition from use of fossil fuels and nuclear energy to a sustainable energy supply derived from renewable sources. The government decision on a final phase-out of nuclear energy by 2022 lends even greater urgency to the expansion of renewable energies. The German federal government’s expansion targets are ambitious, with the share of renewable energy in gross electricity consumption in Germany increasing to 80 percent by the year 2050. Renewable energies already provided 191 terawatt hours, or 32 percent, in 2016.
The problem: Winds don't blow all the time
Often, though, the times for peak wind and solar energy generation do not coincide with the main demand times of electricity consumers. Besides, energy production from renewable sources can only be planned and managed up to a point. To stabilize theelectricity mix, other power plants have to be switched off or on, or electricity has to be exported or imported. So the main problem with renewable energies is not their volume, but rather the wide fluctuations in their production. And the problem is getting bigger and bigger as the share of renewables increases.
The solution: From wind turbine to power storage
For around five years, the effort to solve this problem has been the aim of the battery research project “Redox Wind” by Fraunhofer ICT in Pfinztal near Karlsruhe. Fraunhofer is active in all fields of battery-technology research. 19 million € are devoted to the Redox Wind project alone; 16 million € of this are provided by German federal and state governments, but Fraunhofer itself has also invested 3 million €. Speaking at the dedication of the application center, the Minister of Economic Affairs, Labour and Housing for the German state of Baden-Württemberg, Dr. Nicole Hoffmeister-Kraut, summarized the center's mission: “Decentralized, grid-integrated storage of fluctuating renewable energy is a key technology for the sustainable energy supply of the future. The storage of electrical energy has become one of the major challenges of energy technology, particularly where large-scale applications are concerned.” And this requirement is becoming increasingly urgent. “The goal of our research project is to provide an efficient way to use as much decentrally generated energy as possible for local applications,” Prof. Dr. Jens Tübke, Head of the Applied Electrochemistry Department at Fraunhofer ICT, explained. “Buffers like these will bring relief to power grids and help curb the necessary expansion.”
The research team has developed a redox flow battery that will gradually be expanded to the largest in Germany. The battery stores the current electrochemically in a liquid medium. The system is of particular interest for the stationary storage of energy.
With energy stored in an external tank, battery power can be scaled independently of the capacity: The amount of electrolyte determines battery capacity, while the area and number of stacks – six interconnected cell stacks in each case – determine battery power. Redox flow batteries have a slightly lower energy density than lead batteries, but their service life is almost ten times as high, and because energy quantity and system performance are separate, the cost is significantly lower than for comparable lithium-ion systems. “Another beautiful aspect: with the exception of vanadium – the basis of the storage solution – all of the necessary raw materials are available in Germany,” Tübke pointed out. The technology has existed since 1948. “Until now, however, there has simply been little demand for storage on this order of magnitude,” Tübke explained. “The situation has changed in the meantime.”
“Roughly speaking, the redox-wind installation can be reduced to three main components,” explained Dr. Peter Fischer, Group Manager Redox Flow Batteries at ICT. With a total height of 141 meters and a rated performance of 2 MW, the wind turbine installed at this particular location can deliver just under 2.5 GWh of green electricity annually. “The wind turbine was custom-designed for us as a prototype by turbine manufacturer Qreon and enables direct coupling of the battery to the DC link,” Fischer explained. “That way, the mains inverter on the wind turbine can be used as a battery charger and discharger at the same time.”
Huge storage tanks with vanadium solution
The real heart of the project, the redox flow battery, consists of two components. One component is made up of the interconnected stacks in which the chemical energy of the electrolytes can be converted to electrical energy using the electrodes installed in the stack. The battery is planned to contain a total of 540 stacks. The maximum power of a stack to date is around 5 kW. The second component consists of the storage tanks on the lower level of the application center. People look small alongside the huge green vessels with a total capacity of 660,000 liters of vanadium solution for use in storing the energy in electrolytes. There are eight large tanks, eight medium-sized tanks and one reserve tank. “With the system in idle, virtually no self-discharge takes place,” Fischer pointed out. The redox flow battery will be able to store 20 MWh of power upon completion of the expansion in late 2018. This is enough to cover the power needs of a small town for ten hours. During the research phase, the ICT first wants to generate enough power to run its own campus off-the-grid. “We’ve created a nucleus, so to speak, to which we can now link additional projects,” Fischer remarked. “Because plannable and constant volumes of electricity, generated from renewable sources, is exactly what the energy transition needs.”