Redox flow batteries as the means for energy storage

•Energy generated by renewable sources must be stored during times of excess generation.•Vast arrays of means to store energy are available.•The most efficient energy storage is through hydroelectric and compressed air which is not available on small scale.•Arguably the best means of small scale energy storage is electrochemical.•Electrochemical storage using redox flow cells is one of the promising energy storage methods.

As the deployment of solar and wind electrical energy increases, the intermittency of these power plants necessitates some means of energy storage for rebalancing the load and the supply. Storage of excess produced energy in electrochemical cells (batteries) is an obvious choice, and perhaps even the most practical method. However, the challenge is to find batteries that would be reliable, widely available, effective and cost-attractive. Within electrochemical storage the redox flow batteries constitute an important subgroup of storage and both the concepts and a number of redox systems is discussed here. The possible chemistries cover the more traditional vanadium redox flow cell. The modern trends are covered in the bromine-hydrogen, flow-through lead, zinc-bromine, cells with organic electrolytes and chemically regenerated redox fuel cells. A table comparing these systems is included.