Climate change threatens the very solutions our societies have to fight against it. Wind and solar energy are intermittent and vulnerable to climate extremes, which will become more frequent. Heatwaves, droughts, cloudy days and prolonged periods without wind: these “dunkelflautes” or "energy droughts" impact hydropower, solar panels and wind turbines. Energy storage thus becomes inevitable as the share of renewables grows.

Intermittent renewable energy has grown from 5% of the final consumption mix of the European Union in 1990 to 21% in 2024. The aim is to reach 42.5% by 2030.

“It's not just production troughs. It's also the production peaks when excess of renewable generation can result in curtailments to prevent system overflow, which are equally concerning because they limit the economic benefits brought by renewables” says Adrian Gonzalez, Programme Officer in Innovation and End-use Sectors at the International Renewable Energy Agency (IRENA).

Flexibility in the electricity grid will be needed and can be achieved both on the offer and demand side, by using curtailment (voluntary reduction of production to prevent grid overload), smart grids and consumption reduction. As ADEME, the French Environment and Energy Management Agency, sums it up: “It is about consuming less and consuming better, i.e., at a time when there is no strain on the electricity system.”

Full capacity storage will be indispensable when renewables exceed 74% of total installed capacity, according to the EU. “Storage is a key element that is lagging, and the effort must be more intense.” says Gonzalez.

The need to increase storage capacity for the renewable transition has been recognised internationally. At COP29 in 2024, 65 countries, including 27 from Europe, signed a pledge for energy storage, committing to “deploying 1,500 GW of energy storage in the power sector globally by 2030, more than six times the level of 2022” of 230 GW, requiring an average deployment of over 200 GW annually.

Experts agree that energy storage does not need to parallel the rising share of renewables. IRENA’s global target is 1-2 MWh of storage for each 10 MWh of renewable power capacity added.
Finding solutions beyond batteries: revisiting decades-old technologies
Two main variables need to be considered for energy storage: volume and duration. Depending on the type of storage, hourly consumption displacements for personal needs or inter-seasonal grid needs can be covered.

Lithium-ion batteries target the small-volume, short-duration (2-4 hours) market. They have so far led the storage market and are adequate for daily peaks but not for multi-day weather events. They also carry high environmental costs and create new dependencies on critical materials. Gonzalez says: “short-term storage will be needed everywhere, but for longer terms you should assess on case by case”.

For inter-season storage and bigger volumes, dams or STorage of Energy & Power Systems (STEPs), for instance, is a mature solution with good efficiency levels around 80%. However, the potential of new developments shrinks due to ecological and social acceptance issues. They also remain vulnerable to droughts or floods.

Compressed air energy systems (CAES) could be used for mid-range needs. These systems have been in use since the second half of the 20^th century: electricity is used to compress air, which produces electricity again when air is decompressed. Traditional CAES have low efficiency (40%) and need large cavities for storage. Compression generates heat that is lost in older CAES. Some others rely on fossil fuels to cool the air when it is decompressed. But they are not influenced by “dunkelflautes”.

Air4NRG, an EU-funded transnational project, is exploring ways to overcome the limitations of compressed air. This isothermal innovation tries to solve the space needed for storage and the low efficiency rates. “We work on liquid piston compression [...]. The advantage of compressing with liquid is to be able to do heat exchange, which captures excess heat or, conversely, releases heat during expansion,” David Guyomarc’h, Head of Research & Development and technical coordinator of the project, explains.

Guyomarc’h adds: “Another advantage of using liquid is that we can imagine compressor systems that are not large empty chambers but complex geometries into which the liquid will simply rise through.”

Overall, “the idea is to do without critical materials,” contrary to batteries, explains Guyomarc’h. The project’s prototypes use the ambient air and tap water, working with little electronics. So far, the European team has raised the system efficiency to 70% overall. The compressor and the container end up being the size of a standard lorry container.

After a year of fundamental research and months of laboratory testing in France, Air4NRG will soon operate a second prototype in Portugal in real conditions so as to prepare for the next step: the commercial scale. It aims at offering an alternative mid-term solution of 5 hours to 30 hours at full power and of 200 KWh, which could work for sites such as industrial complexes, shopping centres, renewable production sites or eco-neighbourhoods. It could even cover up to days if not used at full capacity.

Guymoarc’h says: “At that scale, the advantage of having mid-range storage that is centralised at the neighbourhood or industry level is that it is more efficient and helps to smooth out production and consumption.”
For longer-duration seasonal storage, the much-discussed hydrogen remains a key solution in expert scenarios, even though it has been overshadowed by batteries lately. NegaWatt, a French NGO pushing for an energy policy based on energy conservation and efficiency and for greater use of renewable energy, favours power-to-gas in a 100% renewable scenario for France. Using electrolysers, a physical principle discovered almost 200 years ago, surplus electricity is transformed into hydrogen that can be stored or reused as fuel. Power-to-gas makes it possible to dispense with natural gas and reduce greenhouse gas emissions. NegaWatt’s spokesperson, Marc Jedliczka, says the infrastructure already exists for natural gas and could be reused. The barrier remains high costs at an industrial scale.

Gonzalez also considers hydrogen as a “relevant solution for the last mile of decarbonising power systems”, which has a specific advantage for large-scale, long-duration storage: "In the case of batteries, you need to multiply the number of batteries to get more energy capacity. In the case of hydrogen, you don't need more turbines or fuel cells. What you need is more tanks that are way cheaper."
Mixed storage for mixed energy and context
Gonzalez says that IRENA’s global target is a general roadmap, but “it's more important to adapt the target to each national context”. Every European country has different parameters to take into account: the existing energy mix to shift from, natural potential for STEPs or wind or solar energy for instance.

Jedliczka explains: “There is not a single solution; it will be a combination of efforts. In what proportions? That remains a matter for discussion. It will be adapted to the territory and the network configurations. Industrial scales are not yet sufficient, and costs are still too high because we are in an initial phase, but the technologies are available today.”

Guymoarc’h also considers mixing storage levels will be the way forward: “What we see more and more clearly is that we will combine with other faster storage solutions to cover all needs.”

Storage innovation must scale up fast enough to meet 2030 targets while climate variability compounds the very problem it aims to solve. Yet as IRENA’s Gonzalez warns: “a lot of storage innovations are not receiving the needed attention.” The challenge ahead is as much about orchestrating this evolving mix of old and new storage technologies to allow network flexibility as it is about reducing overall demand to reduce pressure on our planet's resources. Project coordinator:
Álvaro Sánchez, ZABALA Innovation, ALVAROSANCHEZ@zabala.eu

Scientific coordinator:
David Guyomarch, Segula Technologies, david.guyomarch@segula.fr

Communication officer:
Valentina Ferrara, ICONS, valentina.ferrara@icons.it

Comunication material

Leaflet (web)
Leaflet (print)
Brand book

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https://air4nrg.eu/

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