Various types of flow batteries

Over the past 20 years, four designs of flow batteries have been demonstrated: vanadium redox (VRB), zinc bromine (ZnBr), polysulphide bromide (PSB) and cerium zinc (CeZn). Major installations, in Japan and North American, use the vanadium redox and zinc bromine designs. Energy efficiency is about 85% for VRB system and 75% for ZnBr system.

Flow batteries are a type of electrochemical ES, which consists of two chemical components dissolved in liquid separated by a membrane. Charging and discharging of batteries occur by ion transferring from one component to another component through the membrane. The biggest advantages of flow batteries are the capability of pack in large volumes.

In contrast with conventional batteries, flow batteries store energy in the electrolyte solutions. Therefore, the power and energy ratings are independent, the storage capacity being determined by the quantity of electrolyte used and the power rating determined by the active area of the cell stack.

True flow batteries have all the reactants and products of the electro-active chemicals stored external to the power conversion device. Systems in which all the electro-active materials are dissolved in a liquid electrolyte are called redox (for reduction/oxidation) flow batteries.

While flow batteries ought to be able to operate at relatively high current densities, as convection can be employed to deliver reactants to the electrode surface, flow batteries have typically been operated at ~50 mA / cm 2, a current density consistent with conventional batteries without convection.

Flow batteries can release energy continuously at a high rate of discharge for up to 10 h. Three different electrolytes form the basis of existing designs of flow batteries currently in demonstration or in large-scale project development.