Redox-Flow-Battery – Flexible Energy Storage Technology

Nowadays, we rely mostly on one particular type of battery – the lithium-ion battery – but it is not the only one that exists and certainly not the only one being further researched. Every type of battery has strengths and weaknesses and therefore different use cases, especially now that we need to investigate longer-duration energy storage solutions to deal with so-called “Dunkelflauten”. Dunkelflauten – literally “dark doldrums” – is a German word describing periods with little sunlight and little wind during which there is less renewable energy production. This is a problem especially in winter. So how can we bridge periods of higher vs. lower energy production from renewable resources? 

One promising solution lies within redox-flow batteries. Unlike conventional lithium-ion batteries, they allow for the independent scaling of energy capacity and power output, making them particularly well-suited for seasonal storage, grid stabilization, and smoothing renewable energy fluctuations. From pilot projects to large-scale deployments across Germany and other countries world-wide, these systems are already proving their worth in real-world applications. Let’s take a closer look at how they work and where they’re making a difference! 

How Do Redox Flow Batteries Work? 

Basic Principles  

Redox-flow batteries are made up of two half-cells, each with an electrode immersed in an electrolyte solution and separated from each other by a semi-permeable membrane. The membrane allows for an exchange of ions to keep the overall charge in the cell balanced. The solutions are made up of ions in different oxidation states, which change their oxidation states either through giving or taking up an electron. Usually vanadium (V) is used for this as it readily takes on varying oxidation states, which can be identified by their distinct colour. 

As the vanadium ions are oxidized or reduced at the electrodes, electrons flow through an external circuit, which creates the electric current of the battery. This reduction-oxidation reaction furthermore gives the battery the first half of its name. 

The second half of the redox-flow battery comes from the electrolyte tanks. Since the current is based on the changing oxidation states of vanadium – which remain in solution in both forms – a continuing flow of the electrolyte solutions means a steady flow of electrons (electricity). When charging the battery, the electrical current is reversed, causing the redox reactions to run backwards: previously oxidized ions are reduced and vice versa.  

The cells can be placed together to build stacks, which influences the power of the battery (basically the speed of the energy delivered), while the amount of energy is influenced by the size of the electrolyte tanks out of which the solution is pumped through the cell. Since they are separate components of the battery, they can be scaled independently, which is useful, as it can be customized without changing the baseline structure.  

What Are The Advantages & Challenges? 

Now that we understand how the technology works, let’s examine what makes redox-flow batteries particularly attractive and where they still face hurdles: 

Advantages: 

• Very long shelf life – pumps can be turned off; only minimal electrolyte remains in the cell 
• High cycle life – over 10,000 cycles possible because no material is deposited on electrodes 
• Deep discharge tolerance – can discharge to 0% without damage as electrodes remain unchanged and don’t degrade 

Disadvantages:
However, vanadium-based systems do face some challenges. Vanadium is relatively expensive and not abundantly available everywhere, which can drive up costs and create supply chain dependencies. The electrolyte solutions also require careful temperature management, as vanadium can precipitate out of solution at extreme temperatures. For these reasons, researchers are actively exploring alternative chemistries – using elements like iron, zinc, or organic molecules – that could offer lower costs and improved performance while maintaining the core advantages of flow battery technology. 

Despite these challenges, the fundamental advantages of flow batteries – especially their longevity and flexibility – make them increasingly attractive for specific applications where conventional batteries fall short. We’ll explore this more in the following sections. 

Use Cases and Application Fields  

As explained above, redox-flow batteries have virtually no self-discharge, so their long shelf-life means that they show great potential as grid stabilizers. They are suitable for backup power and uninterruptable power supply (UPS) systems since even after months of standby, they can deliver almost at full capacity. They can help maintain grid stability by quickly responding to fluctuations in supply and demand. During expensive, high-demand periods, they can enable peak shaving or load shifting – shifting energy use from expensive peak hours to cheaper off-peak times – thereby avoiding further stress to the grid while providing reserve capacity during unexpected outages. 

Redox-flow batteries are also quite suitable for integrating renewable energy sources, both in the private sector to increase self-sufficiency and also on a broader scale. They can be used as intermediate storage for surplus electricity from solar and wind farms during high-production periods and release the energy when generation drops, thereby bridging the gap. This is especially valuable during Dunkelflauten. 

Additional Applications include:  

EV Charging Support:

• Serve as energy buffers at charging stations 
• Smooth out demand spikes when multiple vehicles charge simultaneously 
• Prevent strain on the local grid 

Off-Grid & Remote Power:

• Power remote locations too far from power lines (isolated villages, island communities) 
• Provide reliable electricity when paired with local solar or wind generation 

Current Deployment and Market Opportunities 

So where do we stand today with redox-flow battery technology? Europe is already taking concrete steps beyond research. In 2025, the Fraunhofer Institute for Chemical Technology (ICT) commissioned Europe’s largest vanadium redox-flow battery in Pfinztal, Germany. The system delivers 2 MW of power and stores 20 MWh of energy, which is enough to supply several hundred households for many hours. Coupled directly to a wind turbine, it demonstrates exactly what we discussed earlier: bridging those problematic Dunkelflauten by storing wind energy when it’s available and releasing it when the wind dies down. This isn’t an isolated project and redox-flow batteries are being deployed globally. China’s Dalian facility (100 MW/400 MWh, operational since 2022) demonstrates utility-scale viability, while projects in Japan, Australia, and the United States show growing commercial adoption.

But beyond these implementations, flow batteries represent a particularly interesting market opportunity for Europe. As mentioned above, redox-flow batteries show great potential for long-term storage in the context of seasonal changes (such as solar abundance in summer vs. scarcity in winter). China, by contrast, relies heavily on massive pumped hydro infrastructure and faces less severe seasonal solar variations – particularly in its southern regions where day-night cycles are more consistent year-round. This makes pumped storage more practical for their energy system, and consequently, flow battery technology for seasonal storage appears to be less of a priority in their industrial strategy.  

Since China is otherwise rapidly deploying new technologies across everyday life, this may be the space that European innovation can step into. However, ramping up production remains challenging because flow batteries require higher upfront investment than lithium-ion systems, and scaling manufacturing to competitive volumes demands significant industrial commitment. Still, combined with Europe’s existing strengths in precision engineering and process technology, flow batteries could become a genuine competitive advantage in the global energy storage market – if we act quickly enough to establish ourselves in this space. 

Conclusion and Outlook 

It is important to remember that no single battery can cover all our needs. As we move towards more renewable energy and compete in global technological development, flow batteries offer valuable solutions, valuable particularly for long-term storage and grid stabilization. Their independent scaling of power and energy means they can fill gaps that others cannot. Their ability to store energy for longer periods, combined with minimal degradation over thousands of cycles, makes them particularly well-suited for smoothing out the fluctuations inherent in renewable energy generation – especially during Dunkelflauten. 

While challenges remain, particularly around upfront costs and manufacturing scale, projects worldwide are already demonstrating real-world viability. As Europe seeks to secure its energy independence and manage seasonal renewable fluctuations, flow batteries represent not just a technical solution but a strategic opportunity. The question is no longer whether this technology works, but how quickly we can deploy it where it’s needed most. 

Image Sources: 

• Header: Fraunhofer ICT
• Image 1: Wikimedia Commons – W. Oelen
• Image 2: Adobe Stock
• Image 3: Fraunhofer ICT

Further Reading: 

• Fraunhofer ICT research area on the redox-flow battery 
• PDF: Redox-Flow-Battery – Fraunhofer ICT 
• Press Article: Renewable energy is introduced into the power grid from a large-scale battery 
• Focus Article (German): Deutsche Forscher starten die größte Flüssigbatterie Europas 
• Press Article: Redox Flow Batteries – Large Energy Storage Systems of the Future? 
• Geladen Batteriepodcast: In China lachen sie über unsere Energiewende – Tim Meyer & Jan Hegenberg