Page - 156 - in Water, Energy, and Environment - A Primer

braking. As a result, the number of market applications and manufacturers is growing steadily. 9.2.2 Flowbatteries Flow batteries are large-scale rechargeable energy storage systems where rechargeability is provided by chemical compounds dissolved in liquids which, when mixed together, generate electricity. A major advantage of flow batteries is that they can be recharged quickly by replacing the electrolyte liquid while allowing recovery of the active chemical components in the used electrolyte. By storing energy in the electrolyte fluid they differ from conventional batteries, in which energy is stored as electrodematerial. Redox (reduction/oxidation) flow batteries are particularly well suited to storing large amounts of energy – for example, the surplus energy created by solar orwind power generation – and are on the verge of wide application in the electric utility industry. The energy storage materials are liquids that are stored in separate tanks, and when energy is needed the liquids are pumped through a ‘stack’ where they interact to generate electricity. Many different chemical liquids have been tested for flow battery operation, with most current attention being focused on vanadium compounds. Disadvantages are that flow batteries have relatively low round-trip efficiencies, long response times, and the ratio of power to energy is fixed at the design stage. Because of vanadium cost concerns other chemical possibilities are being examined, for example, zinc– bromine, zinc–chlorine, and iron–chromium. An important flexibility in the design of flowbatteries is that the energy storage capability, that is, the size of the storage tanks, can be tailored to the need of the particular application. They are well suited for a broad range of applications, with power requirements ranging from tens of kilowatts to tens of Water,Energy, andEnvironment–APrimer156