Vanadium Flow batteries: a path to long duration energy storage

Energy storage is a key technology for phasing out fossil fuels in the transition to renewable energy sources and smart grids, since it can provide different services, ranging from energy management, with long discharges, to power quality, with fast responses. Recently, the interest for long duration energy storage (LDES) is also emerging, that can include multi-hour discharges as well as storage over long periods of time (seasonal storage). Different technologies are available to store energy, among which closed batteries (lithium, sodium, …) are strong enablers, thanks to features such as low environmental impact, flexible location, scalability, stillness, high efficiency. However, closed batteries present issues e.g. life duration, safety and self-discharge. Hydrogen storage promises to cope with long term storage, but it is prone to efficiency issues. In this framework, redox flow batteries (RFBs) are emerging as a competitive option for several services. Storing energy in liquid electrolytes held in tanks outside the reactor, they provide independent sizing of energy and power, thus allowing for long discharge times at full power which are inaccessible for closed batteries. Their most developed version, the vanadium RFB, exhibits very long life, virtually no self-discharge, operation at room temperature and pressure and absence of hazard risks such as fires and explosions. While VRFBs are gaining increasing commercial importance, more research is ongoing to improve their performance and competitiveness.