The separation of power and energy is a key distinction of RFBs, compared to other electrochemical storage systems. As described above, the system energy is stored in the volume of electrolyte, which can easily and economically be in the range of kilowatt-hours to tens of megawatt-hours, depending on the size of the storage tanks. The power capability of the system is determined by the size of the stack of electrochemical cells. The amount of electrolyte flowing in the electrochemical stack at any moment is rarely more than a few percent of the total amount of electrolyte present (for energy ratings corresponding to discharge at rated power for two to eight hours). Flow can easily be stopped during a fault condition. As a result, system vulnerability to uncontrolled energy release in the case of RFBs is limited by system architecture to a few percent of the total energy stored. This feature is in contrast with packaged, integrated cell storage architectures (lead-acid, NAS, Li Ion), where the full energy of the system is connected at all times and available for discharge.
The separation of power and energy also provides design flexibility in the application of RFBs. The power capability (stack size) can be directly tailored to the associated load or generating asset. The storage capability (size of storage tanks) can be independently tailored to the energy storage need of the specific application. In this way, RFBs can economically provide an optimized storage system for each application. In contrast, the ratio of power to energy is fixed for integrated cells at the time of design and manufacture of the cells. Economies of scale in cell production limit the practical number of different cell designs that are available. Hence, storage applications with integrated cells will usually have an excess of power or energy capability.
RFBs can be divided into two categories: 1) true redox flow batteries, where all of the chemical species active in storing energy are fully dissolved in solution at all times; and 2) hybrid redox flow batteries, where at least one chemical specie is plated as a solid in the electrochemical cells during charge. Examples of true RFBs include the vanadium-vanadium and iron-chromium systems. Examples of hybrid RFBs include zinc-bromine and zinc-chlorine systems.