Understanding the Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRB) are large stationary electricity storage systems with many potential applications in a deregulated and decentralized network. Flow batteries (FB) store chemical energy and generate electricity by a redox reaction between vanadium ions dissolved in the electrolytes. The most significant feature of the FB is maybe the modularity of their power (kW) and energy (kWh) ratings which are independent of each other. In fact, the power is defined by the size and number of cells whereas the energetic capacity is set by the amount of electrolyte stored in the reservoirs. Hence, FB can be optimized for either energy and/or power delivery. Over the past 30 years, several redox couples have been investigated (Bartolozzi, 1989): zinc bromine, polysulfide bromide, cerium zinc, all vanadium, etc. Among them, VRB has the best chance to be widely adopted, thanks to its very competitive cost, its simplicity and because it contains no toxic materials. In order to enhance the VRB performance, the system behaviour along with its interactions with the different subsystems, typically between the stack and its auxiliaries (i.e. electrolyte circulation and electrolyte state of charge), and the electrical system it is being connected to, have to be understood and appropriately modeled. Obviously, modeling a VRB is a strongly multidisciplinary task based on electrochemistry and fluid mechanics. New control strategies, based on the knowledge of the VRB operating principles provided by the model, are proposed to enhance the overall performance of the battery.