Flow batteries allow storage of the active materials external to the battery and these reactants are circulated through the cell stack as required. The first such battery was Zinc/chlorine battery in which the chlorine was stored in a separate cylinder. It was first used in 1884 by Charles Renard to power his airship La France which contained its own on board chlorine generator.
The technology was revived in the mid 1970s.
Modern flow batteries are generally two electrolyte systems in which the two electrolytes, acting as liquid energy carriers, are pumped simultaneously through the two half-cells of the reaction cell separated by a membrane. On charging, the electrical energy supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other. The thin ion exchange membrane between the half-cells prevents the electrolytes from mixing but allows selected ions to pass through to complete the redox reaction. On discharge the chemical energy contained in the electrolyte is released in the reverse reaction and electrical energy can be drawn from the electrodes. When in use the electrolytes are continuously pumped in a circuit between reactor and storage tanks.
High power batteries are constructed using a multiple stack of cells in a bipolar arrangement. The power rating of the system is fixed and determined by the size and number of electrodes in the cell stacks, however the great advantage of this system is that it provides almost unlimited electrical storage capacity, the limitation being only the capacity of the electrolyte storage reservoirs. Opportunities for thermal management are also facilitated by using the electrolytes as the thermal working fluids as they are pumped through the cells.
The above facts provide a very seductive argument in favour of flow batteries in preference to conventional secondary cells. - For the same power, flow batteries are typically dimensioned to store five times the energy stored in conventional cells.
But the same facts presented in a slightly different way can lead to exactly the opposite conclusion.
The following is also true for the same batteries. For the same storage capacity, conventional cells will provide five times the power of flow batteries and in addition they have no moving parts or energy consuming pumps.
The Zinc-Bromine battery is a modern example of a flow battery. It is based on the reaction between two commonly available chemicals, Zinc and Bromine. The battery consists of a Zinc negative electrode and a Bromine positive electrode separated by a microporous separator. An aqueous solution of Zinc Bromide is circulated through the two compartments of the cell from two separate reservoirs. The other electrolyte stream in contact with the positive electrode contains Bromine. The Bromine storage medium is immiscible with the aqueous solution containing Zinc Bromide.
The battery uses electrodes that cannot and do not take part in the reactions but merely serve as substrates for the reactions. There is therefore no loss of performance, as in most rechargeable batteries, from repeated cycling causing electrode material deterioration. When the Zinc-Bromine battery is completely discharged, all the metal Zinc plated on the negative electrodes is dissolved in the electrolyte. The Zinc is deposited again when the battery is charged. In the fully discharged state the Zinc-Bromine battery can be left indefinitely.
Energy densities three times better than Lead Acid batteries are claimed however the Coulombic (round trip) efficiency is typically only between 60% and 75%.
The flow battery technologies provide very high power and very high capacity batteries for load levelling applications on the national electricity grid system.
The so called Redox Battery is an example of a two electrode flow system.
The Regenesys Sodium Polysulfide Bromine battery is another example.
These are very high cost systems and so far there are very few successful installations.