How Do Flow Batteries Work? A Practical Guide to Flow Battery Technology

Flow batteries store electrical energy in liquid electrolytes held in external tanks rather than storing all of the active energy-storage material inside conventional battery cells. During operation, the electrolytes are pumped through an electrochemical cell stack, where chemical reactions convert stored energy into electricity or absorb electricity for later use.

This architecture separates the system's power capacity from its energy capacity to a greater degree than many conventional battery technologies. The stack primarily determines how much power the system can deliver, while the amount of electrolyte and tank capacity influences how much energy can be stored.

That makes flow battery technology particularly relevant to stationary energy storage applications where storage duration, repeated cycling, and scalable energy capacity are important considerations.

In this guide, we'll look at how flow batteries work, their main components, how energy is stored, the major types of flow batteries, and where the technology can be useful.

How Do Flow Batteries Work?

A flow battery stores energy in electrolyte solutions that are kept in external tanks. When the system charges or discharges, pumps circulate these electrolytes through an electrochemical cell stack, where the chemical reactions responsible for storing and releasing energy take place.

During charging, electricity from the grid or another power source drives an electrochemical reaction that changes the chemical state of the electrolytes. The resulting chemical energy remains stored in the electrolyte until the battery needs to discharge.

During discharge, the electrolytes flow through the stack in the opposite electrochemical process. The reactions generate an electrical current that can then be delivered to the connected load or grid.

The basic operating process can be simplified as:

Charging: Electricity → Electrochemical reaction → Chemical energy stored in electrolyte

Discharging: Chemical energy in electrolyte → Electrochemical reaction → Electricity

Unlike conventional batteries where the active materials are largely contained within individual cells, flow batteries use pumps, tanks, piping, and an electrochemical stack as part of the energy-storage system. This architecture is what allows the amount of stored energy and the system's power output to be designed with greater separation.

For example, increasing the amount of electrolyte and tank capacity can increase the system's energy capacity, while increasing the size or number of cell stacks can increase its power capability.

What Are the Main Components of a Flow Battery?

A flow battery combines electrochemical, fluid-handling, and electrical components to store and deliver energy as part of a stationary Battery Energy Storage System. Unlike a conventional battery pack, the energy-storage medium is held outside the electrochemical cell in tanks and circulated through the system during operation.

The main components include:

• Electrolyte tanks: Store the liquid electrolytes that contain the active chemical species used for energy storage.

• Electrochemical cell stack: Contains the cells where the oxidation and reduction reactions take place during charging and discharging.

• Pumps: Circulate the electrolytes between the storage tanks and cell stack.

• Piping and valves: Direct and control electrolyte flow throughout the system.

• Membrane or separator: Separates the two electrolyte sides while allowing the necessary ionic transport between them.

• Power conversion system: Converts the electrical output of the battery into a form suitable for the connected load or grid and manages electricity flowing into the system during charging.

• Control and monitoring equipment: Tracks operating conditions and manages system performance.

These components work together as one integrated system. The cell stack primarily determines power capability, while the quantity of electrolyte and the size of the storage tanks are more closely associated with energy capacity.

This separation between power-producing components and energy-storage components is one of the defining characteristics of flow battery technology.

How Is Energy Stored in a Flow Battery?

A flow battery stores energy through reversible chemical reactions in its liquid electrolytes. The electrolytes contain dissolved chemical species that can change between different oxidation states during charging and discharging.

During charging, electricity drives these chemical reactions and increases the chemical energy stored in the electrolytes. The charged electrolytes are then held in their respective tanks until the stored energy is needed.

When the system discharges, the electrolytes are pumped through the cell stack. The chemical reactions reverse, releasing electrons through an external electrical circuit and producing electricity for the connected application.

One important characteristic of flow battery technology is that the electrolyte itself acts as the energy-storage medium. This means that increasing the quantity of electrolyte can increase the amount of energy the system can store, while adding or enlarging cell stacks can increase its power output.

This separation gives flow batteries a different design approach from conventional battery systems, where power and energy capacity are more closely tied to the number and configuration of battery cells.

What Is the Difference Between Power and Energy Capacity?

Power capacity and energy capacity describe two different characteristics of a flow battery. Understanding the distinction is important when determining whether a system can meet a project's requirements.

Power capacity, measured in megawatts (MW), describes how quickly the system can deliver or absorb electricity. It is primarily influenced by the size and number of electrochemical cell stacks.

Energy capacity, measured in megawatt-hours (MWh), describes how much electricity the system can store. In a flow battery, it is more closely associated with the quantity of electrolyte and the capacity of the storage tanks.

This separation allows the two characteristics to be configured with greater flexibility. For example, a project could increase its energy capacity by adding more electrolyte storage while maintaining a similar power rating.

The relationship can be expressed simply as:

Energy capacity (MWh) = Power capacity (MW) × Storage duration (hours)

A 10 MW flow battery designed for 4 hours of storage would therefore require approximately 40 MWh of energy capacity. Increasing the duration to 8 hours would require approximately 80 MWh, while the power rating could remain at 10 MW.

This architecture is one reason flow batteries are considered for applications where longer storage durations are required.

What Are the Different Types of Flow Batteries?

Flow batteries are not a single technology. They use different electrolyte chemistries, and each chemistry has its own operating characteristics, materials, performance considerations, and potential applications.

Some of the main flow battery chemistries include:

• Vanadium redox flow batteries (VRFBs): Use vanadium ions in different oxidation states on both sides of the electrochemical cell. Using the same element in both electrolytes helps avoid certain cross-contamination issues associated with different-element chemistries.

• Iron-based flow batteries: Use iron-based electrolyte chemistry and are being developed for stationary energy storage applications where cost and material availability are important considerations.

• Zinc-bromine flow batteries: Use zinc and bromine-based chemistry and differ from fully dissolved redox systems because zinc can be deposited during charging and removed during discharge.

• Organic flow batteries: Use organic molecules as the active electrochemical species. These chemistries are being researched as alternatives that could potentially reduce reliance on certain inorganic materials.

The chemistry affects important characteristics such as energy density, efficiency, operating conditions, material requirements, cost, and system design.

For this reason, the term flow battery describes an architectural approach to energy storage rather than a single battery chemistry. Vanadium redox flow batteries are one important example, but the broader flow battery category includes several different approaches.

What Are Flow Batteries Used For?

Flow batteries are primarily suited to stationary energy storage applications where the system needs to store and deliver electricity over a defined period. They can be particularly relevant to Long-Duration Energy Storage applications where several hours of discharge may be required. Their architecture can be particularly useful when a project requires longer storage duration or frequent charge-discharge operation.

Common applications include renewable energy integration, Grid-Scale Energy Storage, peak demand management, energy arbitrage, microgrids, and backup or resilience applications. 

• Renewable energy integration: Store excess electricity generated by solar or wind systems and release it when generation falls or demand increases.

• Grid-scale energy storage: Shift electricity between different periods and support grid balancing and other grid services.

• Peak demand management: Discharge stored electricity during periods of high demand to reduce the amount of electricity drawn from the grid.

• Energy arbitrage: Store electricity when it is less expensive or readily available and discharge it when electricity has greater value.

• Microgrids: Provide flexible energy storage alongside local generation and other power resources.

• Backup and resilience: Maintain an energy reserve that can be used when the primary electricity supply is interrupted.

The appropriate application depends on factors such as power requirements, storage duration, cycling frequency, electricity prices, renewable generation patterns, and site conditions.

Flow batteries are therefore not intended to replace every type of battery system. Their strongest potential is in applications where their particular combination of scalable energy capacity, long-duration operation, and stationary system architecture aligns with the project's requirements.

What Are the Benefits of Flow Battery Technology?

Flow battery technology offers several characteristics that can make it suitable for stationary energy storage, particularly when a project requires longer-duration operation and repeated cycling. These characteristics are among the key Redox Flow Battery Benefits considered for stationary applications.

One of its key characteristics is the separation of power and energy capacity. The power output is primarily determined by the electrochemical stack, while energy capacity can be increased by adding electrolyte and storage-tank capacity. This can provide greater flexibility when configuring systems for different storage durations.

Flow batteries can also support repeated charge-discharge operation. Because the energy is stored in liquid electrolyte rather than primarily within solid electrode materials, their operating characteristics differ from conventional battery systems.

Other potential advantages include:

• Scalable energy capacity: Additional electrolyte and tank capacity can increase stored energy.

• Long-duration capability: The architecture can be configured for applications requiring several hours of storage.

• Stationary operation: The technology is designed primarily for fixed energy storage installations rather than portable applications.

• Flexible system sizing: Power and energy requirements can be considered separately during system design.

• Potentially long operating life: Lifecycle performance depends on the specific chemistry, components, operating conditions, and maintenance practices.

These characteristics do not make flow batteries the best choice for every application. Energy density, efficiency, footprint, capital cost, and system complexity also need to be considered when comparing flow batteries with other energy storage technologies.

What Are the Limitations of Flow Batteries?

Flow batteries have characteristics that can be valuable for stationary storage, but they also involve trade-offs that should be considered before selecting the technology.

One of the main considerations is energy density. Flow batteries generally require more physical space than many lithium-ion systems because energy is stored in external electrolyte tanks and the system includes pumps, piping, and other supporting equipment.

The system also has greater mechanical complexity. Pumps, valves, piping, tanks, and control systems all need to operate reliably, adding components that are not present in the same form in a conventional battery pack.

Other limitations can include:

• Larger physical footprint: More space may be required for tanks and supporting equipment.

• Lower energy density: More installation area may be needed for a given amount of stored energy.

• Higher system complexity: Fluid-handling equipment adds mechanical components and maintenance requirements.

• Capital cost: Project economics depend on electrolyte, stacks, tanks, power electronics, installation, and other system components.

• Efficiency: Round-trip efficiency varies by chemistry and system design and should be evaluated against the application's requirements.

These limitations do not necessarily prevent flow batteries from being competitive. Their suitability depends on whether the project's requirements align with the technology's strengths. For applications where long-duration storage and repeated cycling are more important than compactness, the trade-offs may be acceptable.

How Do Flow Batteries Compare With Lithium-Ion Batteries?

Flow batteries and lithium-ion batteries can both provide stationary energy storage, but their architectures lead to different strengths and trade-offs.

A lithium-ion battery stores energy within electrochemical cells assembled into modules and battery packs. A flow battery stores energy in liquid electrolytes held in external tanks and circulates them through cell stacks during charging and discharging.

This difference becomes particularly relevant when storage duration increases. Flow battery systems can increase energy capacity by adding electrolyte and tank capacity, while lithium-ion systems generally require additional battery cells and modules.

Factor	Flow battery	Lithium-ion battery
Energy storage	Liquid electrolyte in external tanks	Electrochemical cells
Power capacity	Primarily determined by cell stacks	Influenced by cell and module configuration
Energy capacity	Increased through electrolyte and tank capacity	Increased through additional cells/modules
Energy density	Generally lower	Generally higher
Long-duration applications	Well suited to some longer-duration configurations	Widely used across short- and medium-duration applications
Physical footprint	Generally larger	Generally more compact
System complexity	Includes tanks, pumps and piping	Primarily cell, module and thermal-management systems

The comparison should not be reduced to a single metric such as efficiency, energy density, or upfront cost. The better choice depends on the project's power requirements, storage duration, cycling profile, available space, operating environment, and lifecycle economics.

For a project that prioritizes compactness and high energy density, lithium-ion may have an advantage. Where longer-duration stationary storage and scalable energy capacity are more important, a flow battery may be worth evaluating alongside other technologies.

When Does a Flow Battery Make Sense?

A flow battery is worth considering when the application's requirements align with the technology's particular architecture rather than simply because it offers a certain technical specification.

It can be a strong candidate when a project requires:

• Several hours of energy storage

• Frequent charge-discharge cycles

• Scalable energy capacity

• Stationary, long-term operation

• Integration with variable renewable generation

• Flexible separation between power and energy capacity

The technology may be less suitable when space is severely constrained, very high energy density is required, or the application's economics strongly favor a more compact battery system.

The decision should ultimately come down to the project's operating requirements. Define the required MW, MWh, storage duration, cycling profile, site constraints, efficiency, and lifecycle economics, then compare flow batteries with other available storage technologies.

A flow battery should therefore be evaluated as part of a broader technology selection process not as a universally better alternative to conventional battery systems.

How Do Flow Batteries Support Renewable Energy Integration?

Flow batteries can help manage the variable output of Renewable Energy generation by storing electricity when production exceeds immediate demand and releasing it when generation declines. 

This can be particularly useful with solar and wind projects, where electricity generation does not always align with the timing of electricity demand. For example, excess solar generation during the middle of the day can be stored and discharged later when solar output falls.

The storage system can be configured according to the project's generation and demand profiles. Important considerations include:

• Generation pattern: When renewable electricity is available.

• Demand pattern: When electricity is required.

• Storage duration: How long stored energy needs to be delivered.

• Power requirement: How quickly the system needs to discharge.

• Cycling frequency: How often the storage system will operate.

Flow batteries can therefore provide a way to shift renewable electricity across time rather than requiring generation and consumption to occur simultaneously.

However, the suitability of a flow battery depends on the project's specific requirements. Storage duration, available space, efficiency, capital cost, and operating profile should all be evaluated before selecting the technology.

How Do Flow Batteries Perform Over Long Operating Periods?

Flow batteries are designed for repeated charge-discharge operation, but their long-term performance depends on more than the electrolyte chemistry alone. The cell stack, membrane, pumps, piping, controls, electrolyte condition, and operating environment can all influence system performance over time.

Because the electrolyte is stored separately from the electrochemical stack, the energy-storage medium can be managed independently from some of the components responsible for power conversion. This is one of the architectural differences between flow batteries and conventional battery systems.

Long-term performance should be evaluated using several factors:

• Cycle requirements: How frequently the system will charge and discharge.

• Electrolyte condition: Whether the electrolyte maintains the required chemical characteristics.

• Stack performance: How the electrochemical components perform over repeated operation.

• Maintenance: Requirements for pumps, valves, piping, controls, and other balance-of-system components.

• Operating conditions: Temperature and other environmental factors that can affect system performance.

The expected operating life should therefore be assessed at the complete system level, rather than assuming that the electrolyte or any individual component determines the lifespan of the entire installation.

What Should You Consider Before Choosing a Flow Battery?

A flow battery should be evaluated against the specific requirements of the Energy Storage System, rather than selected solely because it offers longer-duration storage or scalable energy capacity.

Before making a decision, consider:

• Power requirement: How much electricity must the system deliver or absorb?

• Energy capacity: How much electricity needs to be stored?

• Storage duration: How many hours must the system operate at its required power?

• Cycling profile: How frequently will the system charge and discharge?

• Available space: Can the site accommodate electrolyte tanks and supporting equipment?

• Efficiency: How much of the stored electricity can be recovered?

• Operating conditions: What environmental and site conditions will affect the system?

• Lifecycle economics: What are the capital, maintenance, and long-term operating costs?

The final decision should compare the flow battery with other suitable technologies using the same project requirements and operating assumptions. This provides a more useful basis for determining whether flow battery technology is technically and economically appropriate for the application.

What Is the Future of Flow Battery Technology?

Flow battery technology continues to be evaluated for stationary energy storage applications where longer storage durations, repeated cycling, and flexible energy capacity are important.

The technology is not intended to replace lithium-ion batteries in every application. Instead, flow batteries represent another approach to storing electricity, with an architecture that can be particularly relevant when a project values longer-duration operation and the ability to scale energy capacity separately from power capacity.

Future development is focused on improving areas such as cost, efficiency, energy density, materials, system components, and overall project economics. Progress in these areas could influence where flow batteries become commercially attractive as electricity systems incorporate more variable renewable generation.

For project developers and energy users, the important question is not whether flow batteries will replace other storage technologies, but where their technical characteristics provide the best fit for the storage requirement. That application-specific approach is likely to remain central as the energy storage market develops.

What Is the Future of Flow Battery Technology?

Flow battery technology continues to be evaluated for stationary energy storage applications where longer storage durations, repeated cycling, and flexible energy capacity are important.

The technology is not intended to replace lithium-ion batteries in every application. Instead, flow batteries represent another approach to storing electricity, with an architecture that can be particularly relevant when a project values longer-duration operation and the ability to scale energy capacity separately from power capacity.

Future development is focused on improving areas such as cost, efficiency, energy density, materials, system components, and overall project economics. Progress in these areas could influence where flow batteries become commercially attractive as electricity systems incorporate more variable renewable generation.

For project developers and energy users, the important question is not whether flow batteries will replace other storage technologies, but where their technical characteristics provide the best fit for the storage requirement. That application-specific approach is likely to remain central as the energy storage market develops.

Conclusion

Flow batteries use a different approach to energy storage by keeping liquid electrolytes in external tanks and circulating them through electrochemical cell stacks during charging and discharging. This architecture allows power capacity and energy capacity to be configured with greater separation than in many conventional battery systems.

The technology can be particularly relevant for stationary applications that require longer-duration storage, repeated cycling, and scalable energy capacity. However, lower energy density, larger physical footprint, system complexity, efficiency, and project economics also need to be considered.

Ultimately, the right storage technology depends on the application. Evaluating power requirements, energy capacity, storage duration, cycling profile, site conditions, and lifecycle economics provides a more useful basis for deciding whether flow battery technology is appropriate for a project.

Frequently Asked Questions

1. What is a flow battery?

A flow battery is an energy storage system that stores energy in liquid electrolytes held in external tanks. During charging and discharging, pumps circulate the electrolytes through an electrochemical cell stack where reversible chemical reactions store or release energy.

2. How does a flow battery store energy?

Electrical energy is converted into chemical energy through electrochemical reactions in the electrolytes during charging. When the battery discharges, the reactions reverse and generate electricity that can be supplied to a load or the grid.

3. What is the difference between a flow battery and a lithium-ion battery?

The main difference is how energy is stored. Flow batteries use liquid electrolytes in external tanks, while lithium-ion batteries store energy within electrochemical cells. Flow batteries can also separate power and energy capacity more independently, whereas lithium-ion systems generally increase energy capacity by adding cells or modules.

4. Are flow batteries suitable for long-duration energy storage?

Yes. Flow batteries can be configured for applications requiring several hours of storage. Their ability to increase energy capacity through additional electrolyte and tank capacity makes them one technology considered for long-duration energy storage applications.

5. What are the main advantages of flow batteries?

Key characteristics include scalable energy capacity, suitability for repeated cycling, long-duration storage capability, and greater separation between power and energy capacity. However, these benefits need to be evaluated alongside factors such as footprint, efficiency, system complexity, and cost.

6. What are the limitations of flow batteries?

Flow batteries generally have a larger physical footprint and lower energy density than lithium-ion systems. They also include pumps, tanks, piping, and other balance-of-system components, which can increase system complexity and maintenance requirements.

7. What types of flow batteries are available?

Several flow battery chemistries are being developed or deployed, including vanadium redox flow batteries, iron-based flow batteries, zinc-bromine flow batteries, and organic flow batteries. Each chemistry has different performance, material, cost, and operating characteristics.

8. Where are flow batteries used?

Flow batteries are primarily considered for stationary applications such as renewable energy integration, grid-scale energy storage, peak demand management, energy arbitrage, microgrids, and backup or resilience applications.

Sources

1. U.S. Department of Energy — Long-Duration Energy Storage Provides background on long-duration storage technologies and their role in supporting grid reliability and renewable energy integration. DOE — Long-Duration Energy Storage

2. U.S. Department of Energy — 2022 Grid Energy Storage Technology Cost and Performance Assessment Useful for technology characteristics, performance, cost, efficiency, and lifecycle considerations across different energy storage technologies. DOE — Grid Energy Storage Technology Cost and Performance Assessment

3. National Renewable Energy Laboratory — Storage Futures Study Provides research and analysis on energy storage technologies, applications, and the evolving role of storage in future electricity systems. NREL — Storage Futures Study

4. U.S. Department of Energy — Energy Storage Provides broader technical context on energy storage technologies and their applications across the electricity system. DOE — Energy Storage