Flow batteries

A flow battery is a type of battery that can be designed very flexibly. It can be designed for high power applications as well for high-capacity electricity storage. Other types of electricity storage, like conventional batteries or flywheels, do not show this flexibility and therefore have some limitations to their applications. Flywheels are mostly used for short durations (<5 minutes) and high power storage (> 500 kW), while batteries are used for lower power (<500 kW) and long durations (> 1 hour). Flow batteries are used for large-scale projects that require high-capacity storage and also high power storage, for instance for grid-connected electricity storage at wind farms.

In a flow battery the battery is charged and discharged by a (reversible) chemical reaction between the two liquid electrolytes of the battery. These electrolytes are not stored in the power cell of the battery as in a conventional battery, but in separated storage tanks. During operation these electrolytes are pumped through the electro-chemical reactor, in which a chemical redox reaction takes place and electricity is produced. Due to this storage of the electrolytes outside the reactor, the specifications of the battery are flexible; the power and the energy content of the system can be specified separately. It is very easy to increase the amount of electrolytes or to replace the electrolytes. Moreover, the design of the power cell can be optimized for the power rating needed, as this is independent of the amount of electrolyte used.

The development of flow batteries has reached the stage of demonstration projects. Small- scale products are already available on a commercial basis, while for the larger-scale projects demonstrations have been started. These demonstration projects prove the technology and show that it can be applied on a large scale. The costs of the technology will decrease as soon as the technology becomes available as a commercial product. Based on current feasibility studies, the life cycle costs will be lower than those of the alternatives, based on the capital costs and the expected life time. Flow batteries can be very attractive for future applications, especially for large-scale applications, like peak power support at wind farms or distribution level balancing.