The use of flow batteries in storing electricity for national grids

Redox flow batteries (RFBs) are-in contrast to conventional batteries, such as lithium-ion batteries-independently scalable in power and capacity. This flexibility results from their unique design, which is based on a power conversion stack (comparable to fuel cells) and a separated electrolyte tanks for storing the energy. As a result, flow batteries can be adapted to store electricity from national grids. In this case, a power-to-capacity ratio of between 1: 4 and 1: 10 is often planned, since the increase in capacity can be achieved simply and cost-effectively by using larger amounts of electrolyte. Other important advantages of the RFBs include long lasting operation time; long cycle life time; good stability; ease of regeneration or recycling of the electrolyte; and the battery units are hardly flammable. One of the few disadvantages is the low energy density (volumetric, gravimetric), making RFBs suitable for stationary applications. At the same time, a scale-up, in contrast to conventional, modular batteries, allows for cost reduction, so that RFBs in particular qualify for large-scale applications. These mostly grid-connected applications are the focus of this chapter. In addition, different technologies of flow batteries are discussed. These include flow batteries which differ in design (flow vs. hybrid flow) and in the electrolyte used (e.g., all-vanadium vs. zinc-bromine). Furthermore, various applications and market segments for flow batteries are discussed. The focus will be on real projects which have been implemented or are currently being planned worldwide. Finally, current suppliers and their products are briefly presented.