Flow batteries are among the technologies best suited to long-duration energy storage. Because the energy is held as electrolyte in external tanks rather than within the cell, the storage capacity of a flow battery can be increased independently of its power rating. The incremental cost of additional hours of storage therefore reflects the cost of tanks and electrolyte rather than the cost of additional electrochemical cells.
The underlying principle is described on the page What is a flow battery? This page sets out what long-duration energy storage requires, how flow battery systems meet those requirements, and how both have changed over the past decade.
Defining long-duration energy storage
Long-duration energy storage, commonly abbreviated to LDES, refers to systems capable of discharging at rated power for extended periods. No single threshold is universally applied. Some support programmes set the boundary at four hours, others at eight or ten, and a growing body of grid planning work considers storage measured in days rather than hours.
The boundary has moved. Papers presented at the International Flow Battery Forum in 2017 discussed durations of four to six hours as the requirement for supporting wind and solar generation. By 2024 the requirement was being described as four to twelve hours of discharge for the net zero transition. Commercial platforms announced in 2025 offer durations of four to eighteen hours, and papers presented in 2026 place the daily shifting requirement at one to eight hours, identifying the step to ten hours and beyond as the point at which long-duration storage becomes a distinct market rather than an extension of the existing one.
For comparison, lithium-ion systems are generally described in the same papers as addressing durations of one to four hours, the segment in which lithium iron phosphate is currently dominant.
Duration and system design
In a flow battery the electrochemical stack determines the power rating and the volume of electrolyte determines the energy rating. Extending the discharge duration of a system is therefore a matter of increasing electrolyte volume and tank capacity, while the stack remains unchanged.
This has two consequences for long-duration applications. First, the marginal cost per additional hour of storage is lower than for technologies in which energy and power scale together. Second, the duration of an installed system can in principle be extended after commissioning by increasing the electrolyte inventory, without replacing the electrochemical hardware.
The practical constraints are the footprint required for the tanks, the cost and availability of the electroactive materials, and the design of the ancillary systems that circulate and manage the electrolyte. Projects in Singapore have addressed the first of these by repurposing existing oil and underground tanks for electrolyte storage.
Cost
Reported capital costs for vanadium flow battery systems have fallen substantially over the past decade. Papers presented at the Forum quoted a target of below US$400 per kilowatt-hour in 2016, an actual figure of approximately US$500 per kilowatt-hour in 2019, below US$200 per kilowatt-hour for demonstration projects in 2022, and below US$100 per kilowatt-hour including EPC at 100 MWh production scale by 2024, falling below US$70 per kilowatt-hour at gigawatt-hour scale.
Electrolyte represents a substantial share of the cost of a long-duration system, and leasing arrangements have been introduced to separate it from the initial capital outlay. One model reported in 2025 places the initial electrolyte cost at or below US$85 per kilowatt-hour with an annual lease payment below US$10 per kilowatt-hour.
These are figures reported by individual manufacturers and researchers rather than sector averages, and the relevant comparison is always at a stated duration. Containerised lithium-ion systems were quoted at approximately US$200 per kilowatt-hour outside China in 2025.
Comparison with lithium-ion batteries
Lithium-ion and flow battery systems address different parts of the storage requirement, and the point at which one becomes preferable to the other is largely determined by duration.
| Lithium-ion | Flow batteries | |
|---|---|---|
| Energy and power | Scale together | Scale independently |
| Increasing storage capacity | Additional complete cells | Additional electrolyte and tank volume |
| Typical duration addressed | 1 to 4 hours | 4 to 18 hours and beyond |
| Energy density | High; compact footprint | Lower; larger footprint required |
| Capacity fade | Declines with cycling | Limited fade over service life |
| Electrolyte | Flammable organic solvents | Predominantly aqueous; low flammability |
| End of life | Cell recycling | Electrolyte recovery and reuse |
The comparison on service life has narrowed. Flow battery systems have demonstrated more than 20,000 cycles over five years of continuous operation, and cycle lives above 15,000 are cited as a general characteristic of the technology; manufacturers of lithium iron phosphate cells now claim 12,000 to 15,000 cycles. The distinguishing characteristics of flow batteries in long-duration service are therefore the way cost scales with duration, the limited capacity fade, the tolerance of deep discharge and overcharge, and the low flammability of aqueous electrolytes in installations where fire risk governs siting.
Chemistries used in long-duration applications
The redox pairs described on What is a flow battery? are deployed at different stages of maturity, and the balance among them has shifted over the past decade.
- Vanadium / vanadium systems account for all of the largest installations to date. The use of a single element on both sides of the cell avoids irreversible cross-contamination of the electrolytes, which supports long service life.
- Aqueous organic systems, using redox pairs such as quinones and TEMPO, have grown from a marginal topic to a consistent presence in the research and early commercial literature, with material cost and cycle stability as the principal variables.
- All-iron systems have attracted renewed attention on the grounds of material abundance and cost.
- Iron / chromium systems, first demonstrated at scale over a decade ago, returned to the programme in the early 2020s.
- Zinc / bromine and other zinc-based hybrid systems deposit one active species on the electrode, which constrains how far duration can be extended by electrolyte volume alone.
Applications
Long-duration flow battery installations are found in a number of roles:
- Firming of wind and solar generation, shifting output from periods of surplus to periods of demand, including direct DC coupling to a photovoltaic plant.
- Deferral of transmission and distribution network reinforcement.
- Industrial and commercial installations requiring peak shaving and time-shifting.
- Data centres, including facilities supporting artificial intelligence workloads, where continuity of supply over extended periods is combined with a safety profile suited to siting close to critical infrastructure.
- Isolated and island grids, and support for mining operations, where the alternative is extended operation of fuel-based generation.
Information on installed projects and regional market reports is available here.
Policy and market context
Public policy increasingly distinguishes long-duration storage from storage in general. Flow Batteries Europe has set a target of 20 GW and 200 GWh of deployment in Europe by 2030. In the United Kingdom, NESO and the National Infrastructure Commission have estimated a requirement for more than 25 TWh of long-duration energy storage by 2050, with 20 GW of capacity estimated to deliver £24 billion in long-term consumer savings. In the United States, storage with durations above four hours was projected to approach 35 GW in 2023 and to exceed 200 GW by 2050. The China Energy Storage Alliance has indicated plans for up to 177 GW of new types of energy storage by 2030.
Standards
Long-duration storage systems are permanent infrastructure, and standards governing their specification, installation and operation are correspondingly important. The formal definitions of a flow cell and a flow battery are given in IEC 62932-1. Information regarding flow battery standards is available here.
Common questions
How many hours can a flow battery store?
Discharge duration is determined by electrolyte volume rather than by the chemistry itself. Commercial platforms are offered with durations of four to eighteen hours, and papers presented in 2026 identify durations of ten hours and beyond as the emerging long-duration market.
Are flow batteries less expensive than lithium-ion batteries?
The comparison depends on duration. At short durations lithium-ion systems generally have the lower installed cost per kilowatt-hour. Because a flow battery increases its energy capacity by adding electrolyte rather than cells, the cost of each additional hour is lower, and the comparison moves in favour of flow batteries as duration increases.
What is the service life of a flow battery?
Systems reported at the Forum have completed more than 20,000 cycles over five years of continuous operation, equivalent to some 28 years of daily cycling, and cycle lives above 15,000 are cited as a general characteristic. Overcharging and full discharge do not normally cause permanent damage, and electrolytes can in many cases be recovered and reused at the end of the system’s life.
Are flow batteries safe?
Most flow battery systems use aqueous electrolytes with low flammability and operate at ambient temperature and pressure. The electrolytes are nonetheless chemicals requiring appropriate handling, containment and management, and the relevant standards address these requirements.
What is the difference between a flow battery and a fuel cell?
Both circulate a fluid through an electrochemical stack. A fuel cell consumes its fuel, whereas the electrolyte of a flow battery is recharged and cycled repeatedly.
Further reading
Redox flow batteries for energy storage, Jens Noack, Nataliya Roznyatovskaya, Chris Menictas and Maria Skyllas-Kazacos
Redox flow batteries – fundamentals and applications, CRC Press – Edited by Huamin Zhang, Xianfeng Li and Jiujun Zhang, ISBN 978-1-498-75394-4
IFBF conference papers
The figures on this page are drawn from papers presented at the International Flow Battery Forum between 2016 and 2026. Presenters are asked each year to write a short, standalone paper supporting their presentation, reporting on progress in research programmes and on the technical and commercial operation of flow batteries, materials and components.
A complete list of previous conference papers is available here. The next edition of the International Flow Battery Forum takes place in Barcelona on 22-24 June 2027; details are available here.