Currently, there is a lack of large, stationary energy storage systems that can be flexibly installed in any location and that are scalable as needed to actual energy demands. Hydropower, the current standard technology for stationary storage, cannot accomplish this. More responsive storage systems and flexibility are needed: whether to store energy from traditional energy production to ensure coverage during peak demand or for network stabilization—but also, especially, because of the continual increase in renewable energy sources.
Studies, for instance one from Navigant Research on behalf of the World Bank, show that in industrialized nations the use of large stationary energy storage solutions will be crucial for the successful expansion of renewable energies. The report also predicts a forty-fold increase in stationary energy storage capacities in developing nations by 2025. However, the report goes on to state that these countries will have to double their electricity generation by 2020 to meet rising demand. The largest energy storage markets in the coming decades will be large and heavily-populated nations such as China and India, and furthermore Latin America and South Africa.
It is no wonder that a lot of hopes are resting on the further development and broad availability of stationary energy storage. Especially in comparison with fossil fuels or nuclear energy, energy obtained from the sun and wind has a serious drawback: it is only produced when the sun shines or the wind blows. And this isn’t necessarily when the demand is greatest. The solution for the future is thus an energy storage system that can supply energy at all times.
Near Karlsruhe, in southwest Germany, just such an energy storage system is being created.
In technical terms, a redox-flow battery consists of electrochemical cells, so-called stacks, in which the energy conversion takes place and which determine the installation’s power output, and the electrolyte tanks, in which the energy is absorbed and thus define the energy storage capacity. Consequently, the power and the capacity can be adjusted independently from one another and adapted to the respective individual requirements—an advantage that no other energy storage technology offers. Furthermore, the storage tanks that contain the electrolyte solutions can be of any size. In Pfinztal, there is a whole series of tanks, each holding 45,000 liters.
It takes up the space of a mid-sized gym and stores enough energy to supply thousands of households with electricity for twenty-four hours. The Fraunhofer Institute for Chemical Technology (ICT) has based this battery on what is known as redox-flow technology and began operating it last year in Pfinztal, just outside of Karlsruhe. Developed by ICT scientists, the battery is part of the RedoxWind project, which is being supported by the German federal state of Baden–Wurttemberg, the Federal Ministry of Education and Research as well as the Fraunhofer Institute to the tune of €19 million. The project includes a 2-megawatt wind power system that delivers electrical energy, which can be temporarily stored in the battery. The battery’s storage capacity will be a total of 20 megawatt-hours. In theory, this means that the complete output of the wind power system can be stored for up to ten hours.
The stacks are then composed of a multitude of galvanic cells. Each cell consists of two half cells which are separated by an ion-exchange membrane. The half cells are flooded by the electrolyte solution which contain different metal ions.
All the conductive parts of the cells are made from carbon, for instance the current collectors, the so-called bipolar plates. In order to have the largest possible reaction surface for the electrochemical reactions, electrically conductive carbon felts are used in the cells as the electrode material.
Peter Fischer, Fraunhofer-Institut Pfinztal
Besides a large surface area, the carbon felt for redox-flow batteries must additionally offer as little resistance as possible. The felts are therefore additionally activated by a special surface treatment.
In order to optimize the cells of the battery, the project scientists rely among others on SGL Carbon’s expertise. Thus, the company has already delivered to IC a total of 3,500 square meters of SIGRACELL® felt electrodes for the redox-flow battery – which could cover half of a soccer field – plus 1,750 square meters of bipolar plates.
Yet redox-flow batteries, as large stationary storage units, are conceived not only as a means for modifying the energy supply system to use renewable energy sources. The combination of wind power systems and batteries also offers the possibility of creating a self-sufficient energy supply. One conceivable application is electrification in remote or poorly served locations using isolated solutions, as well as the temporary provision of electricity, for instance at large construction sites or in times of crisis after natural disasters.
What’s certain is that the demand for temporarily stored energy will increase in most countries around the world. Conventional solutions alone won’t be enough. And so, in the future, along with a progressive and flexible technology, apparently size really does matter.