Redox Flow Batteries
Redox flow batteries (RFB) represent one class of electrochemical energy storage devices. The name “redox” refers to chemical reduction and oxidation reactions employed in the RFB to store energy in liquid electrolyte solutions which flow through a battery of electrochemical cells during charge and discharge.
During discharge, an electron is released via an oxidation reaction from a high chemical potential state on the negative or anode side of the battery. The electron moves through an external circuit to do useful work. Finally, the electron is accepted via a reduction reaction at a lower chemical potential state on the positive or cathode side of the battery. The direction of the current and the chemical reactions are reversed during charging.
The total difference in chemical potential between the chemical states of the active elements on the two sides of the battery determines the electromotive force (emf or voltage) generated in each cell of the battery. The voltage developed by the RFB is specific to the chemical species involved in the reactions and the number of cells that are connected in series. The current developed by the battery is determined by the number of atoms or molecules of the active chemical species that are reacted within the cells as a function of time. The power delivered by the RFB is the product of the total current and total voltage developed in the electrochemical cells. The amount of energy stored in the RFB is determined by the total amount of active chemical species available in the volume of electrolyte solution present in the system.
The separation of power and energy is a key distinction of RFB’s, 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 10’s 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 RFB’s 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 RFB’s. 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, RFB’s 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.
RFB’s 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 RFB’s include the vanadium-vanadium and iron-chromium systems. Examples of hybrid RFB’s include the zinc-bromine and zinc-chlorine systems. See additional ESA Technical Notes for more details on these particular systems.
True RFB’s achieve the complete separation of power and energy, along with the full advantages. In hybrid RFB’s, complete separation of power and energy is not achieved, because energy is stored in the metal which is plated in the electrochemical stack during charge. Larger energy storage capacity requires a larger stack, so the distinction of the hybrid RFB from integrated cell architectures is only partly achieved.
Finally, RFB’s are well suited for applications with power requirements in the range of 10’s of kilowatts to 10’s of megawatts, and energy storage requirements in the range of 500 kilowatt-hours to 100’s of megawatt-hours. RFB’s can be the most economic choice in this range because storage tanks and flow controls are easy and economical to scale, and electrochemical stacks can have repeat units with power ratings in the 10’s to 100’sof kilowatts.
Redox flow batteries have one main architectural disadvantage compared with integrated cell architectures of electrochemical storage. RFB’s tend to have lower volumetric energy densities than integrated cell architectures, especially in the high power, short duration applications. This is due to the volume of electrolyte flow delivery and control components of the system, which is not used to store energy. In spite of this, RFB’s are available with system footprint below the EPRI substation target of <500 ft2 / MWh.
Conclusions and Observations
Redox flow batteries offer an economical, low vulnerability means to store electrical energy at grid scale. Redox flow batteries also offer greater flexibility to independently tailor power rating and energy rating for a given application than other electrochemical means for storing electrical energy. Redox flow batteries are suitable for energy storage applications with power ratings from 10’s of kW to 10’s of MW and storage durations of 2 to 10 hours.