What the redox flow battery technology does is to store energy in a liquid.
A redox flow battery features a converter, two tanks and ancillary equipment (pumps, valves, power conversion, etc.). The liquids are pumped through the converter where, under the action of the electric current, they undergo a chemical reaction. The electricity is thus stored under a chemical form. When the process is reversed, the chemicals regain their initial form and return the electricity.
One of the key advantages of the technology is that it separates power from capacity. The battery can be sized at 1 MW, for instance, to match an energy source, and this will be the size of the converter. Then the capacity , i.e. the size of the tanks, can be selected independently for instance at 4 or 8 MWh, so that the system can store 4 or 8 hours of production.
If the chemicals used in the redox flow battery are not expensive, then extra capacity will cost far less than its equivalent in conventional batteries: instead of adding batteries, you just add extra electrolytes (liquids) in the tanks. Redox batteries are therefore very interesting when large capacities are required, for instance over 4 hours. This is why they are particularly suitable for instance for the storage of renewable energy, like solar energy produced for hours between peak times.
Another advantage of the redox flow battery is that the electrodes are not chemically involved in the process. In conventional batteries, the electrolytes and the electrodes react with each other during the charge and discharge. This often causes a progressive degradation of the system, cycle after cycle, until the performance is no longer acceptable. By contrast, electrodes in redox flow batteries just convey the current and are not chemically involved. The chemical reaction takes place only in the electrolyte, turning one chemical species into another. This property makes it easier to design a long-lasting system, since the chemical reaction at stake will be reversible for thousands of cycles.
If the electrolytes are stable, the energy can be stored for a long time with no self discharge, because the system keeps it under a chemical form. It is possible to get the energy back months after the charge with the rated efficiency.