1. As renewable generation continues to grow, why is long-duration energy storage becoming increasingly important?

The needs of the electricity system are changing. As we integrate more variable renewable generation, storage will have to do much more than respond to short-term fluctuations. In many situations, it will also be necessary to move significant amounts of energy from periods of high generation to periods when demand is higher or renewable production is lower.

This makes storage duration increasingly relevant. Technologies capable of operating over many hours can contribute to greater flexibility, resilience and better integration of renewable energy.

However, there will not be one storage technology for every application. Future grids will require a combination of solutions designed for different response times, durations and operating conditions.

2. What makes a technology suitable for long-duration storage?

There is no single parameter that determines whether a technology will succeed.

Cost is obviously fundamental, but it has to be considered over the complete lifetime of the system. Efficiency, maintenance requirements, cycling behaviour and durability all influence the real cost of storing energy.

Safety and reliability are equally important, particularly when we are talking about large stationary installations operating for many years. And the availability of the materials required to manufacture these systems is becoming an increasingly strategic consideration.

The challenge is therefore to develop technologies that can deliver competitive lifetime economics under real-world operating conditions, while ensuring safety, reliability and scalability.

3. Within this landscape, what makes redox flow batteries particularly interesting?

Redox flow batteries have a very different architecture from conventional batteries.

One of their most interesting characteristics is that the amount of energy stored and the power delivered by the system can, to a large extent, be designed independently. This provides considerable flexibility when adapting the technology to applications that require longer discharge times.

They can also offer advantages in terms of safety and lifetime, particularly when aqueous electrolytes are used.

These characteristics make flow batteries an interesting platform for stationary applications where energy needs to be stored and delivered repeatedly over long periods.

4. Does that mean flow batteries are competing directly with lithium-ion batteries?

I think it is more useful to talk about complementarity than direct competition.

Lithium-ion batteries have achieved enormous technological and industrial progress and are very effective in many applications. But an electricity system has very different storage requirements depending on whether we are talking about rapid grid response, several hours of energy shifting or much longer periods of storage.

Different technologies have different strengths. The important question is not which battery will dominate everything, but which technology provides the greatest value for a specific application.

Long-duration storage is one of the areas where alternative battery architectures such as redox flow batteries could find an increasingly important role. 

5. Vanadium flow batteries are currently the most established technology in this field. What have they demonstrated?

Vanadium flow batteries have been extremely important for the development of this technology.

They have demonstrated that flow batteries can operate reliably for stationary storage applications and have helped validate many of the fundamental advantages associated with the concept, particularly long lifetime and operational safety.

They have also created an industrial reference point for the sector. When we develop new flow battery technologies today, vanadium systems provide a very valuable benchmark for understanding what already works and what still needs to be improved.

6. What are the main barriers preventing flow batteries from achieving much wider deployment?

One of the biggest challenges is still economics.

The key challenge is therefore not simply to reduce the cost of one component, but to reduce LCOS systematically at the complete system level.

Material availability is another important factor. Some existing technologies depend on relatively expensive or strategically sensitive raw materials, which can introduce cost uncertainty and supply-chain risks.

The next generation of flow batteries therefore needs to reduce cost and material dependence while preserving the characteristics that make this technology attractive in the first place.

7. Researchers are investigating many new chemistries. What should we expect from the next generation of flow batteries?

There is considerable research activity around new active materials and new system architectures.

A major objective is to make greater use of abundant, inexpensive and widely available materials. But replacing one material with a cheaper one is not enough.

A new chemistry also has to demonstrate efficiency, stability, safety and a sufficiently long operating life. Sometimes improving one parameter creates new challenges elsewhere in the system.

This is why the development of next-generation flow batteries requires a very integrated approach, combining electrochemistry, materials science and engineering.

8. Where is CIC energiGUNE focusing its research in this area?

Our work is focused on understanding how flow battery technologies can become more competitive for long-duration stationary storage.

That involves exploring new electrochemical concepts based on abundant materials, but also improving the different components that determine the performance and cost of the complete battery.

We are particularly interested in the relationship between chemistry and engineering. A promising reaction in the laboratory is only the starting point. The real challenge is to transform that chemistry into a robust, efficient and scalable battery architecture.

That transition from scientific concept to functional technology is a major part of our research.

9. What is usually the most difficult step when moving from a promising laboratory concept towards a real storage technology?

Scale changes many things.

At laboratory level, we can study individual materials or electrochemical reactions under highly controlled conditions. As the technology becomes larger, issues related to transport, current distribution, fluid management, component integration and manufacturing become increasingly important.

At the same time, the economic perspective becomes much more demanding. A solution must not only work technically; it must also be manufacturable, reliable and cost-effective.

For this reason, development requires continuous interaction between fundamental research, component design, modelling, prototyping and system engineering.

10. Looking ahead, what role do you expect flow batteries to play in the future energy system?

I believe their opportunity will grow as electricity systems require larger amounts of flexible, long-duration storage.

But success will depend on our ability to continue improving cost, efficiency, materials availability and scalability while maintaining the advantages of safety and long lifetime.

The broader picture is that we are moving towards a much more diversified storage landscape. Lithium-ion, sodium-ion, solid-state batteries, flow batteries and other technologies will all evolve, and each may address different needs.

The goal should not be to find a single winner, but to develop a portfolio of technologies capable of supporting a more renewable, resilient and flexible energy system.