About Flow Batteries

Flow batteries are notable for their scalability and long-duration energy storage capabilities, making them ideal for stationary applications that demand consistent and reliable power. Their unique design, which separates energy storage from power generation, provides flexibility and durability. Ongoing advancements are enhancing their efficiency, cost-effectiveness, and environmental sustainability

Flow Battery Snapshot

Flow batteries offer energy storage solutions for various customers and applications, including utilities, as well as industrial, commercial, and residential uses. Their growth in grid-scale applications and microgrids are primary drivers of market expansion.

There is a variety of designs and chemistries for flow batteries, and in general they offer several advantages over traditional energy storage solutions (ESS), including:

  • Scalable capacity
  • Fast (dis)charging times
  • Intrinsic safety
  • Long cycle life
  • Competitive footprint energy density

Flow battery innovations are an increasingly important part of a diverse energy storage industry. To support the commercialization of flow batteries and continued research and improvement, Battery Council International established the Flow Battery Industry Group in 2023 as well as the annual Flow Batteries North America conference.

Flow Batteries North America BCI conference logo.

Learn More About FBNA

Diagram with callouts on the components of a flow battery.

What Are Flow Batteries?

Flow batteries are rechargeable electrochemical energy storage systems that consist of two tanks containing liquid electrolytes (a negolyte and a posolyte) that are pumped through one or more electrochemical cells. These cells can be connected in series or parallel to achieve the desired power capacity, while the tanks can be scaled to achieve the desired energy capacity.

This design allows for flexible scaling of both energy and power independently. For all of these reasons, especially their ability to attain 10+ hours (dis)charge, flow batteries are a strong contender for stationary long-duration energy storage (LDES) solutions.

Redox flow battery (RFB) system installed in California as part of a microgrid serving residential and commercial customers.
Photo credit Sumitomo Electric

Flow Battery Applications

With so many special qualities, including as scalability, high cycle life, and long-duration energy storage capacity, flow batteries are especially well-suited for a number of applications. Some particularly attractive real-word applications categories include:

  • Renewable Energy Source Integration: Flow batteries help the grid during periods of low generation, making it easier to integrate intermittent renewable energy sources like wind and solar. For example, flow batteries are used at the Sempra Energy and SDG&E plant to store excess solar energy, which is then released during times of high demand.
  • Grid and Long-Duration Storage:  Flow batteries are widely used for grid storage, helping to manage energy during peak demand and ensuring grid stability. Flow batteries are also ideal for long-duration storage, particularly in renewable energy projects, where they store excess energy for later use.
  • Microgrid Systems: In microgrid applications, flow batteries offer dependable backup and effective energy management, often working alongside renewable sources to maintain system independence.
  • Power Quality and Backup: Flow Batteries are unique in their ability to service both long term and very short timelines (10s of milliseconds). Often referred to as stacked services, Flow Batteries can provide quick burst grid support services such as frequency regulation, stabilizing grid voltage, and maintaining a high power factor while still preserving capacity for energy shifting with a single solution. These properties can be particularly attractive for Flow Batteries located at the “edge” of the grid,” i.e., far away from a central distribution station. For instance, flow batteries can plug gaps in renewable energy generators (solar and wind) to provide uninterrupted load support  during fluctuations in generating capacity with passing clouds during wind lulls/gusts. Flow batteries can store a lot of energy for a long time, so they are also excellent at handling long-term/inter-day demand fluctuations and load levelling. Flow batteries can store a lot of energy for a long time, so they are also excellent at handling long-term / inter-day demand fluctuations and load levelling. Flow batteries supplement resources such as pumped hydro energy storage (PHES) by giving grid operators dependable energy storage to balance supply and demand over several hours or days, taking strain away from already overloaded transmission lines/avoiding the high cost of rapidly upgrading these systems. Flow batteries can also serve as a backup power source, guaranteeing a steady supply of electricity during blackouts.
Electric towers and power lines supported by flow battery energy storage solutions.

Key Benefits of Flow Batteries

Flow batteries offer a unique advantage for large-scale applications because they have expandable storage capacity and longer life cycles than conventional batteries. Some of the performance benefits of flow batteries include:

  • An extended battery life lasting 20 years or more
  • Cost-effective, constructed of low cost and readily available materials
  • Recyclable, many components can be recycled, and electrolytes can be recovered and reused
  • Modular and scalable to fit a wide range of stationary applications
  • A safe, non-flammable energy storage alternative, electrolytes can be used for heat management
  • Extreme durability and stability
  • Long-duration energy storage of 4+ hours (4-12 hours or even days)

Growth and Opportunities for Flow Batteries

The demand for dependable long duration energy storage to facilitate grid stability, energy independence, and renewable integration is propelling the market for flow batteries. Flow batteries are perfect for large-scale applications because they have expandable storage capacity and longer life cycles than conventional batteries. Strong, long-duration storage systems like flow batteries are anticipated to become increasingly in demand as the world moves more toward renewable energy, especially in the industrial and utility-scale sectors.

According to some estimates, the global flow battery market is projected to grow to a valuation of more than $1.18 billion by 2030, and is expected to record a compound annual growth rate of 23% during that forecast period.

Growth prospects are particularly robust in places vulnerable to grid instability and those with ambitious renewable energy targets. Flow batteries are ideal for balancing intermittent renewables, rural electrification, and microgrid applications because of their long-term, dependable energy supply.

Recent Successful Installations of Flow Batteries

Sumitomo Electric, Bonia, California: In 2017, a 2MW/8MWh vanadium redox flow battery system was installed in at an SDG&E facility near San Diego. The system, which was monitored through 2021 achieved a remarkable 99% operating rate in its final year. The battery is projected to maintain a 90% or higher capacity rate for 20 years and was deemed highly successful from external reviewers. The energy storage system functions as part of a microgrid serving 66 residential and commercial customers and can provide power for approximately 5 hours. This installation is part of Sumitomo’s broader deployment of redox flow battery systems, with 37 systems installed globally totalling 47 MW/162 MWh of capacity.

Energy Superhub Oxford (ESO), UK: As part of the Energy Superhub Oxford project, Invinity Energy Systems and Pivot Power have successfully turned on a 5MWh vanadium flow battery. The biggest hybrid energy storage system in the UK to date is created by integrating this technology with a 50MWh lithium-ion battery. The ESO project is essential to Oxford’s zero-carbon objectives, which include balancing the supply of renewable energy, stabilizing the local system, and drastically lowering emissions. For urban energy resilience, the hybrid configuration is a prime example of the creative application of mixed battery technologies.

Motherwell, Scotland: With a new assembly factory in Motherwell and upgraded facilities in Bathgate, Invinity Energy Systems has increased its manufacturing capacity in Scotland with the goal of producing more than 500MWh of flow batteries annually. This development significantly increases the capacity to manufacture vanadium flow batteries and meets the increasing demand for large-scale energy storage projects in Canada, Australia, and other countries. Through local production and supply chain expansion, the facility also demonstrates Scotland’s strategic role in promoting renewable energy technology.

Graphic showing the three types of flow batteries, Vanadium, Iron Flow, and Zinc-based.

Types of Flow Batteries

There are several variations of flow batteries based on electrolyte chemistry. The electroactive materials are redox pairs, i.e. chemical compounds that can reversibly undergo reduction and oxidation. More than 60 redox pairs are known currently, and there are efforts going on to find more options, each with distinct characteristics suited to different applications. Some of the types of flow batteries include:

  • Vanadium redox flow battery (VRFB) – is currently the most commercialized and technologically mature flow battery technology.
  • All iron flow battery – All-iron flow batteries are divided into acidic and alkaline systems, and acidic all-iron flow batteries are relatively mature in commercial development. Compared with vanadium, iron has higher utility and lower cost.
  • High-performance zinc-based flow batteries – The discharge capacity of the improved zinc-iodine flow battery has been significantly increased and it can cycle stably for 600 cycles at 70% energy efficiency, which provides a model for the development of high-performance zinc-iodine flow battery.

Working Phenomenon of a Flow Battery

Flow batteries operate distinctively from “solid” batteries (e.g., lead and lithium) in that a flow battery’s energy is stored in the liquid electrolytes that are pumped through the battery system (see image above) while a solid-state battery stores its energy in solid electrodes.

There are several components that make up a flow battery system:

Illustration of an electrolyte tank for flow batteries.

Electrolyte Reservoirs: Storage of Reactive Active Species

Two sizable reservoirs containing liquid electrolytes—one positive (posolyte / catholyte) and one negative (negolyte / anolyte)—are used in flow batteries. Dissolved active species, which are reactive molecules or ions capable of undergoing oxidation-reduction (redox) processes, are present in these electrolytes. The catholyte species undergo oxidation (loss of electrons) while the anolyte species undergo reduction (gain of electrons) during charging. The battery’s energy storage and release mechanism is based on this redox cycle.

Illustration of a cell stack for a flow battery.

Electrochemical Cell Stack: Reaction Chambers with Porous Electrode Surfaces

Pumped electrolytes move via porous electrodes that optimize reactive surface area in the cell stack, which is the location of electrochemical reactions. For the redox processes, these electrodes help with electron transport. Effective electron exchange is made possible by interactions between the electrode surfaces and the electrolyte’s active species. In order to achieve the power density and reactivity necessary for efficient large-scale energy storage, this high surface-area architecture is essential.

Ion Exchange Membrane: Controlled Ionic Conduction for Charge Balancing

“Supporting” ions—charged species that aid in preserving electrical neutrality throughout the cell—can flow through the ion-selective membrane that separates the electrodes. The two electrolytes cannot combine directly thanks to this barrier, which might cause capacity loss and parasitic reactions. The membrane’s selective ionic conductivity allows just specific ions to flow through, maintaining the charge balance and optimizing energy efficiency during charge-discharge cycles.

Electron Flow and Circuit Powering: External Pathway for Grid-Compatible Energy

During discharging process, active species present in anolyte oxidize. This reaction releases electrons that would flow through an external circuit powering load(s) or energizing grid. These electrons, ultimately, travel towards catholyte and reduce active species present there. To maintain neutrality of battery system and complete the reaction, “supporting” ions travel through the membrane within cell.

Reversible Cycling: Restoring Electrolytes to Original State

Once all the active species in electrolytes have reacted and the energy stored in battery is utilized; it is needed to reverse the redox processes using an external energy source, such as solar or wind, so the flow battery could be recharged. The active species’ initial oxidation states are restored by this reverse reaction, preparing the battery for a further discharge cycle. Flow batteries are a durable option for intermittent renewable energy storage with low deterioration and almost consistent performance for years.

Formulations Used in Flow Batteries

Flow batteries can use a variety of chemistries/electrolytes for energy storage, each resulting in unique redox reactions that occur between active species or molecules in the electrolyte. Due to these redox reactions, flow batteries are also called as redox flow batteries.

While conducting research on any technology, it is important to know how different components within a system are affecting this specific technology. Categorizing different technology solutions helps determine appropriate type according to application needs. This also helps researchers in evolving technology over time while focusing on each component separately.

In the case of flow batteries, the chemistry of electrolytes, materials of electrodes and membrane, size of electrolyte storage tank, flow control, and environmental conditions introduce a range of technology considerations. These factors affect cost, reliability, output voltage, and the supply chain. Here, a few important factors and resulting technology solutions are briefed.

Chart showing the formulates used in flow batteries.

Flow Battery Electrolytes

There are several variations of flow batteries based on electrolyte chemistry.

The energy density and efficiency for each redox pair, such as vanadium or zinc-bromine, are influenced by their chemical characteristics. Based on application requirements and environmental conditions, appropriate redox couples can be chosen. Vanadium flow batteries, for example, simplify electrolyte management because the same formulation on both sides prevents crossover-based capacity loss.   There are however, a variety of electrolytes that can be used in these ESS’ (see chart).

A key consideration for electrolyte choice is reduction of costs through:

  • Efficient and environmentally friendly production process;
  • Design around scaling for domestic manufacture;
  • Increased energy density (reduces property and capex costs);
  • On-site constitution (don’t ship liquid, reduces factory build and transport cost);
  • End-of-life process / recycling; and
  • Accelerating the discovery loop for innovative chemistry.

Membranes

For flow batteries, membranes must let ions of supporting electrolyte move while keeping electrodes and electrolytes separate from each other.

Specifically for Vanadium flow batteries specifically, which are established for grid-sized use, both CEMs (cation-exchange membranes) and AEMs (anion-exchange membranes) can be utilized. CEMs have low ionic resistance because of the high mobility of acid protons, but they also have higher crossover rates for vanadium ions. On the contrary, AEMs have better vanadium barrier properties, but their conductivity is also lower for selective ions. Similar specific membrane considerations exist for any flow battery chemistry, but we will not detail those here.

Electrode Design

Overall design of electrodes plays an important role in achieving desired performance of flow batteries.  Electrode’s surface area, material, shape of pores, and size of rods are among most common factors that affect power output and battery performance. Electrode designs are also related to selection of electrolytes. For example, because of its enormous surface area, porous carbon is frequently employed in vanadium batteries, whereas zinc systems may require corrosion-resistant metals to control dendritic development.

Safety Features

For safe operation, highly reactive chemicals, such as bromine, require strong containment and ventilation. Safer chemistries, like vanadium or organic electrolytes, have less complicated safety criteria, so they therefore may be used in a wider range of situations. Safer chemistries also reduce the engineering controls needed for safe operation as well as the cost of chemical-resistant materials, and this lowers the operational expense and capital expense associated with those battery systems. This is a big advantage of flow batteries over other chemistries, including lithium ion energy storage solutions.

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