Flow Batteries: Definition, Pros + Cons, Market Analysis & Outlook

While you may be familiar with traditional battery types such as lead-acid, Ni-Cd and lithium-ion, flow batteries are a lesser-known but increasingly important technology in the energy storage sector.

In this article, we'll explore what flow batteries are, their advantages and disadvantages, and the current state and future development of the market.

What Are Flow Batteries?

You might believe that flow batteries are a new technology merely invented over the past few years.

Actually, the development of flow batteries can be traced back to the 1970s when Lawrence Thaller at NASA created the first prototype of this battery type. Now flow batteries haev evolved into a promising technology for certain solar energy storage applications.

The Composition of Flow Batteries

Flow batteries typically include three major components: the cell stack (CS), electrolyte storage (ES) and auxiliary parts.

A flow battery's cell stack (CS) consists of electrodes and a membrane. It is where electrochemical reactions occur between two electrolytes, converting chemical energy into electrical energy.

Unlike traditional rechargeable batteries, the electrolytes in a flow battery are not stored in the cell stack around the electrodes; rather, they are stored in exterior tanks separately.

Apart from the tanks for storing electrolytes, other auxiliary parts of a flow battery generally include pipes and valves for electrolyte flow control, pumps for circulating electrolytes, sensors for monitoring temperature, pressure and flow rate, and a control system.

The Classification of Flow Batteries

Flow batteries are primarily classified based on the electrochemical reactions and materials used in the electrolytes. The main types of flow batteries are:

• Redox flow batteries (RFBs)
• Hybrid flow batteries (HFBs)
• Organic flow batteries (OFBs)

Among the various types, some well-known variants include vanadium redox flow batteries (VRFBs) and zinc-based flow batteries.

How a Flow Battery Works

Flow batteries work by storing energy in chemical form in separate tanks and utilizing electrochemical reactions to generate electricity.

Specifically, each tank of a flow battery contains one of the electrolyte solutions. The electrolytes are pumped through a cell stack, where they flow past electrodes immersed in the solutions.  These electrodes are separated by a membrane that allows the passage of ions but prevents the mixing of the electrolytes.

During charging, an external power source such as solar power drives the oxidation-reduction reactions (one electrolyte loses electrons while the other gains electrons), storing energy in the electrolytes. During discharging, the reverse reactions occur, releasing the stored energy as electricity.

What Are the Pros & Cons of Flow Batteries Over Traditional Ones?

As a newer battery energy storage technology, flow batteries hold some distinct strengths over traditional batteries. But without question, there are some downsides that hinder their wide-scale commercial applications.

Pros of Flow Batteries

Deep Discharge Capability

Flow batteries exhibit superior discharge capability compared to traditional batteries, as they can be almost fully discharged without causing damage to the battery or reducing its lifespan. Traditional batteries like lead-acid and lithium-ion ones, on the other hand, can experience a decreased lifespan and reduced performance if they are frequently deeply discharged.

For lithium-ion batteries, you can expect them to last no more than 10 years. But for flow batteries, some can last up to 30 years.

Longevity

Talking about lifespan from a chemical standpoint, flow batteries store energy in electrolytes and involve reversible chemical reactions, allowing for decoupling of power and energy capacity—being charged and discharged repeatedly without significant degradation.

In contrast, most prevalent lithium-ion batteries are known to suffer from degradation mechanisms such as lithium plating, growth of passivated surface film layers on electrodes, and loss of recyclable lithium ions and electrode material, all of which adversely affect their longevity.

Additionally, certain amount of heat will be generated during the operation of lithium-ion batteries and the capacity of them decreases during cycling, leading to a reduction in their overall lifespan.

Scalability

Compared to lithium-ion batteries, flow batteries offer superior scalability due to their ability to easily increase energy capacity by adding more electrolytes to the tanks. Lithium-ion batteries, on the other hand, have limited scalability, as their capacity is primarily determined by the number of cells in the battery pack. As a result, lithium-ion batteries may require complete replacement or more modifications to scale up.

Long Duration Storage

Thanks to their deep discharge capability and excellent scalability, flow batteries excel at storing energy for longer durations, from hours to even days. Conversely, lithium-ion batteries have a typical duration of several hours.

Safety

Unlike some other types of batteries, flow batteries don't contain flammable electrolytes, which reduces the risk of fire or explosion. The design of flow battery storage systems allows for the storage tanks to be installed separately from the conducting cell membrane and power stack, further enhancing safety.

Cons of Flow Batteries

Lower Energy Density

Flow batteries, while offering advantages in terms of decoupled power and energy capacity, suffer from lower energy density due to limitations in the solubility of active materials and electrode capacity. The broad voltage windows of non-aqueous electrolytes in flow batteries can also impact their energy density. 

Traditional batteries like lithium-ion batteries have been optimized over the years, approaching their theoretical energy density limits based on the materials used in their cathodes and anodes.

Higher Initial Cost

Flow batteries have a higher initial cost compared to other battery types due to their complex design, which includes separate tanks for storing electrolytes, pumps, plumbing, and control systems. Moreover, their relatively low charge and discharge rates necessitate the use of substantial quantities of materials. The requirement for various ancillary components coupled with the employment of costly materials, including membranes and electrodes, further amplifies the overall upfront investment needed to deploy flow battery technology.

However, the cost may be offset by their longer lifespan and lower replacement costs over time.

So, What Are the Ideal Solar Applications for Flow Batteries?

Flow batteries exhibit significant advantages over alternative battery technologies in several aspects, including storage duration, scalability and longevity, making them particularly well-suited for large-scale solar energy storage projects.

These projects are developed to cater to a vast consumer base over an extended period, meanwhile with the objectives of bolstering the resilience and flexibility of the utility grid. To supply reliable, clean energy at scale at a competitive cost while effectively addressing the intermittent nature of solar power, these projects call for an energy storage technology like the flow battery that offers large capacity, stable performance, a longer lifespan, and exceptional flexibility.

Along with an increasing number of large-scale solar projects being rolled out and under construction globally, the arena for this battery technology is progressively growing. A CAGR of 11.7% is forecast to propel the global flow battery market from a value of USD 0.73 billion in 2023 to an impressive USD 1.59 billion by the end of 2030.

Key players like RedFlow, ESS Inc, UniEnergy Technologies and VRB Energy are dedicated to developing and manufacturing innovative and efficient flow battery systems. They have made significant contributions to the global adoption and growth of this battery technology in the renewable energy sector.

Are They Ideal for Commercial and Residential Projects?

The challenges of lower energy density and high initial investment constrain the adoption of this technology in commercial and residential projects to some extent.

As it will require a large space for the installation of flow batteries, most residential solar projects naturally go for the use of the lithium-ion storage technology with higher energy density and compact design. The elevated initial cost in contrast to the relatively small financial gains of a residential photovoltaic system is also a crucial factor that hinders their residential adoption.

Nevertheless, thanks to their modular design, scalability and extended lifespan, some commercial and industrial (C&I) projects are starting to favor flow batteries.

Final Words

So far, the predominant electrolyte material in commercially-available flow batteries has been vanadium. While vanadium shows excellent durability through numerous cycles of electron addition and removal without significant degradation, its rarity, high cost and complex processing procedure pose challenges to the deployment of these batteries.

Beyond seeking for alternative materials with closer performance to vanadium, researchers are also concentrating on improving the energy density, efficiency and overall cost-effectiveness of flow batteries to enhance their competitiveness with traditional battery technologies.

Additional measures, such as MIT’s modeling framework, have also been developed and implemented to better understand and assess the economic viability of flow batteries for projects, as well as speed up deployment.