Flow battery
An electrochemical battery storing energy in liquid electrolytes held in external tanks, so power (stack size) and energy (tank volume) scale independently.
In a flow battery, two electrolytes are pumped from tanks through a cell stack where they are separated by an ion-exchange membrane. Redox reactions in the stack charge or discharge the liquids. Because energy is stored in the electrolyte volume and power is set by the stack area, a flow battery can be sized for 4–12 hours of discharge without adding stacks, which suits long-duration storage.
(n electrons, F = 96,485 C/mol, c concentration, E_cell cell voltage)
All-vanadium: E_cell ≈ 1.26 V, energy density about 15–35 Wh/L
The most mature type is the vanadium redox flow battery (VRFB), which uses vanadium in four oxidation states on both sides, so cross-contamination does not permanently degrade capacity. Other chemistries include zinc-bromine, iron-chromium, all-iron and organic electrolytes.
Typical characteristics: round-trip efficiency of 65–80% (pumping and shunt losses), more than 15,000–20,000 cycles with negligible capacity fade, non-flammable aqueous electrolytes, and 100% usable depth of discharge. Low energy density and higher upfront cost per kWh than LFP have limited deployment; the largest systems, in China, exceed 100 MW / 400 MWh.
Related terms
All terms →- Long-duration energy storage (LDES)
Storage able to discharge at rated power for roughly 8 hours or more, covering multi-day lulls and seasonal imbalances that 2–4 hour batteries cannot.
- BESS (Battery Energy Storage System)
A complete grid- or site-connected battery installation, including cells, battery management, power conversion, thermal management and controls.
- Round-trip efficiency
The share of energy put into a storage system that can be recovered on discharge, measured at a defined boundary (cell, DC, or AC grid connection).
- Lithium-ion battery
A rechargeable battery in which lithium ions shuttle between a graphite anode and a metal-oxide or phosphate cathode; the dominant chemistry for EVs and grid storage.
- LCOS (Levelized Cost of Storage)
The average cost per MWh discharged over a storage asset’s lifetime, including investment, operation, charging energy and efficiency losses.