A flow battery is a type of rechargeable battery that stores energy in liquid electrolytes held in external tanks, rather than in solid materials packed inside the battery cell itself. This design means you can increase how much energy the battery holds simply by using bigger tanks, making flow batteries especially well suited for large-scale energy storage on the electrical grid. They’re one of the leading technologies for storing hours or even days’ worth of electricity from solar and wind farms.
How a Flow Battery Works
A conventional battery like the lithium-ion cell in your phone keeps all its energy-storing chemicals sealed inside a compact package. A flow battery takes a fundamentally different approach. Two tanks hold liquid electrolyte solutions, one positive and one negative. Pumps circulate these liquids through a central unit called a cell stack, where the actual chemistry happens. Inside the cell stack, the two liquids are separated by a thin membrane that allows charged particles (ions) to pass through while keeping the two solutions from mixing.
During discharge, chemical reactions at the electrodes on either side of the membrane release electrons, generating electricity. During charging, electricity from an external source (like a solar farm) drives those reactions in reverse, restoring the electrolytes to their original charged state. The electrodes are typically made of carbon felt, and the membrane is an ion-exchange material that selectively lets ions cross. Multiple cells are stacked together using carbon-plastic plates to produce higher voltages, forming what’s called a battery cell stack.
The pumps and piping used to circulate the electrolytes are standard chemical-plant equipment, which keeps the mechanical side of the system straightforward. But those pumps do consume energy continuously while the battery operates, which is one of the system’s trade-offs.
Why Decoupling Power and Energy Matters
The most distinctive feature of a flow battery is that its power output and its energy capacity are independent of each other. Power (how fast electricity flows in or out) depends on the size of the cell stack. Energy capacity (how long the battery can keep delivering power) depends on the volume of electrolyte in the tanks. If you need a battery that can discharge for 4 hours instead of 2, you don’t redesign the whole system. You just install larger tanks with more electrolyte. This is a major advantage for grid-scale storage, where projects might need to store hundreds of megawatt-hours of energy.
In a lithium-ion battery, power and energy are tied together in the same cell. Scaling up means adding more battery packs, which gets expensive quickly at very large sizes. Flow batteries become more cost-competitive the longer the storage duration needs to be.
Common Flow Battery Chemistries
Several different chemical combinations can power a flow battery. Each has its own strengths.
All-vanadium is the most commercially mature type. Both tanks contain vanadium dissolved in sulfuric acid, just in different oxidation states. Because the same element is on both sides, accidental mixing of the electrolytes doesn’t cause permanent contamination. You can simply rebalance the system. Vanadium concentrations typically range from 1 to 3 molar.
Iron-chromium systems use iron on one side and chromium on the other, both in acidic water-based solutions. This chemistry relies on abundant, inexpensive materials, which makes it attractive from a cost standpoint.
All-iron batteries use iron compounds on both sides, sometimes with sophisticated chemical complexes to boost voltage and stability. Iron is cheap, widely available, and nontoxic, making this chemistry appealing for very large installations.
Zinc-bromine batteries pair zinc metal with bromine. Special chemical agents are added to keep the bromine stable and prevent zinc from forming problematic growths on the electrode surface. This chemistry offers higher energy density than most other flow batteries, but managing the bromine vapor adds complexity.
Efficiency and Energy Losses
Flow batteries are less efficient per charge-discharge cycle than lithium-ion batteries. A vanadium flow battery operating at full load achieves a round-trip efficiency of roughly 60%, meaning that for every 100 units of electricity you put in, you get about 60 back out. The remaining 40% is lost to heat and other inefficiencies.
These losses come from three main sources: the electrochemical reactions in the cell stack, the power electronics that convert between AC and DC electricity, and the auxiliary power consumed by the pumps and control systems. Pumping alone accounts for a significant fraction of losses. Even when the battery is on but not actively charging or discharging, the pumps draw between 1.1 and 1.6 kilowatts. The system also loses a small amount of stored energy over time through water diffusion across the membrane, roughly 1.5% of its state of charge per measurement period in one study at Denmark’s Technical University.
Lithium-ion batteries, by comparison, typically achieve round-trip efficiencies of 85% to 95%. So flow batteries waste more energy per cycle. The question is whether their other advantages, particularly long duration and long lifespan, make up for that gap.
Lifespan and Durability
Flow batteries generally outlast lithium-ion systems in terms of cycle life. Because the energy-storing chemicals are liquids flowing through the cell rather than solid materials that physically expand and contract with each cycle, the degradation mechanisms are different and often slower. The electrolyte itself doesn’t wear out in the same way a lithium-ion electrode does.
Flow batteries also tolerate being fully charged and fully discharged without the same penalties that shorten lithium-ion lifespan. This makes them well suited for applications where the battery cycles deeply every day for years. Commercial vanadium flow battery installations are designed for 20-year lifespans, and the vanadium electrolyte can be recycled or reused at end of life, retaining much of its value.
Grid Storage and Renewable Energy
The primary use case for flow batteries is long-duration energy storage on the electrical grid. Solar panels only generate electricity during daylight, and wind turbines only produce when the wind blows. There can be hours or even days of low generation. A reliable grid needs storage that can bridge those gaps.
Flow batteries can store hundreds of megawatt-hours of energy, enough to keep thousands of homes running for many hours on a single charge. Their sweet spot is storage durations of 4 to 12 hours or longer, where the ability to add cheap tank capacity gives them an edge over lithium-ion systems. Specific applications include peak shaving (reducing demand spikes on the grid), renewable energy firming (smoothing out the variable output of wind and solar), and providing backup power during extended low-generation periods.
As grids worldwide add more renewable generation, the need for long-duration storage grows. Lithium-ion dominates the 1 to 4 hour storage market today, but flow batteries are increasingly competitive for longer durations.
Cost Outlook
The economics of flow batteries depend heavily on how long they need to store energy and how often they cycle. A Department of Energy analysis estimated the levelized cost of storage for various flow battery chemistries at different use cases. For a 4-hour system cycling 300 times per year, iron-chromium systems came in around $73 per megawatt-hour, while all-vanadium systems were closer to $98 per megawatt-hour. Some newer chemistries using organic or earth-abundant materials showed costs as low as $60 per megawatt-hour.
For very long duration storage (100 hours), target costs for installed energy capacity drop to around $3 per kilowatt-hour, which is aggressive but necessary for flow batteries to compete with other grid flexibility options. Various analyses suggest that renewable energy paired with storage costing under $100 per kilowatt-hour would be the cheapest way to produce electricity in many scenarios. Several flow battery chemistries are approaching or meeting that threshold.
How Flow Batteries Compare to Lithium-Ion
The two technologies serve different needs. Lithium-ion batteries pack far more energy into a smaller, lighter package, which is why they dominate phones, laptops, and electric vehicles. Flow batteries are bulky by comparison. Their energy density is low because the active materials are dissolved in large volumes of water-based solution. You would never put a flow battery in a car.
But for stationary grid storage, weight and size matter less. What matters is cost per stored kilowatt-hour over the system’s lifetime, cycle durability, safety, and the ability to scale. Flow batteries use water-based electrolytes that don’t catch fire, a meaningful safety advantage over lithium-ion systems. They can cycle deeply for decades without significant capacity loss. And adding storage duration is cheap because tanks of liquid are far less expensive than additional lithium-ion cells.
The practical split is emerging along duration lines. Lithium-ion handles short-duration, high-power applications. Flow batteries handle long-duration, high-capacity applications. Both will likely play essential roles in a grid powered predominantly by renewables.