Flow Battery
A flow battery stores energy in liquid electrolytes held in external tanks and pumped through a reaction cell stack. Its defining advantage is that energy capacity (tank size) is decoupled from power (stack size), so storage scales by enlarging the tanks. Vanadium redox is the common type, with 10,000-20,000+ cycle life but low energy density (roughly 20-50 Wh/L), suiting multi-hour grid storage.
A flow battery is a rechargeable battery that stores energy in two liquid electrolytes held in external tanks, rather than in solid electrode materials. The charged liquids are pumped through a reaction cell stack where ion transfer occurs across a membrane while an external circuit carries the current. Its defining feature is that energy and power are decoupled: storage capacity is set by tank volume while power output is set by the size of the cell stack. To store more energy you simply build bigger tanks, leaving the stack unchanged, which is unusual among storage technologies and well suited to scaling up for Grid Energy Storage. The most common commercial type is the vanadium redox flow battery, which uses vanadium at both electrodes and achieves long durability with 15,000 to 20,000 cycle lives; iron-based flow chemistries are cheaper but lower performance. Trade-offs: round-trip efficiency is roughly 50 to 80 percent, below lithium-ion but above fuel cells, and energy density is low at about 20 to 50 Wh/L, compared with several hundred Wh/L for lithium iron phosphate (see Energy Density). That makes flow batteries bulky but acceptable for stationary applications from about 1 kWh to 10 MWh with multi-hour charge and discharge cycles, where durability and long discharge times matter more than compactness. Flow batteries are one of the electrochemical options alongside the Lithium-Ion Battery and the Iron-Air Battery.