Flow Batteries Explained: How They Work and Where They Fit

How a flow battery works

In a conventional battery the active materials are sealed inside the cell. In a flow battery they are dissolved in liquid electrolytes stored outside the cell. Pumps circulate the two electrolytes past either side of an ion-exchange membrane. Charging drives a reduction-oxidation (redox) reaction that stores energy in the liquids; discharging reverses it.

Chemistries

• Vanadium redox: the most mature design; uses vanadium in different oxidation states on both sides, which limits cross-contamination.
• Zinc-bromine: a hybrid design with zinc plated during charge.
• Iron and organic chemistries: aim to cut material costs using abundant elements.

Strengths

Flow batteries can cycle many thousands of times with little capacity fade, use non-flammable water-based electrolytes, and add hours of storage simply by adding electrolyte. The electrolyte, especially vanadium, can often be recovered at end of life.

Weaknesses

Energy density is low, so systems are large. Round-trip efficiency is typically lower than lithium-ion, roughly 65–80% depending on the system. Pumps and balance-of-plant add complexity, and upfront costs have been higher than lithium-ion for short durations.

Where they fit

Flow batteries target durations of roughly 4 to 12 hours or more: shifting solar into the night, backing up microgrids and providing long-duration capacity where daily deep cycling would wear out other batteries.