Climate modify 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 becaapply they limit the economic benefits brought by renewables” states Adrian Gonzalez, Programme Officer in Innovation and End-apply Sectors at the International Renewable Energy Agency (IRENA).
Flexibility in the electricity grid will be necessaryed and can be achieved both on the offer and demand side, by utilizing 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.” states Gonzalez.
The necessary 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 necessary to parallel the rising share of renewables. IRENA’s global tarobtain 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 necessary to be considered for energy storage: volume and duration. Depfinishing on the type of storage, hourly consumption displacements for personal necessarys or inter-seasonal grid necessarys can be covered.
Lithium-ion batteries tarobtain the compact-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 depfinishencies on critical materials. Gonzalez states: “short-term storage will be necessaryed everywhere, but for longer terms you should assess on case by case”.
For inter-season storage and hugeger 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.
Energy storage technologies classified by discharge duration and power capacity, revealing the complementary roles of batteries (short-term), CAES (medium-term), and hydrogen/STEP (long-term). P2G means power-to-gas. The percentages refer to energy efficiency. CC BY 4.0 – Cantisani, Nicola & Ritschel, Tobias & Thilker, Christian & Madsen, Henrik & Jørgensen, John. (2022). Modeling, scientific computing and optimal control for renewable energy systems with storage. 10.48550/arXiv.2212.08842.
Compressed air energy systems (CAES) could be applyd for mid-range necessarys. These systems have been in apply since the second half of the 20th century: electricity is applyd to compress air, which produces electricity again when air is decompressed. Traditional CAES have low efficiency (40%) and necessary 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 necessaryed 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 exmodify, 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 utilizing 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.”
© Air4NRG – Test bench for the liquid piston compressor developed by Air4NRG at IMT Atlantique in Nantes
Overall, “the idea is to do without critical materials,” contrary to batteries, explains Guyomarc’h. The project’s prototypes apply 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 finish 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 applyd at full capacity.
© Air4NRG – Prototype of the liquid piston compressor developed by Air4NRG, currently in Eibar, Spain, before its transport for testing in Portugal
Guymoarc’h states: “At that scale, the advantage of having mid-range storage that is centralised at the neighbourhood or industest level is that it is more efficient and supports 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 apply 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 reapplyd as fuel. Power-to-gas creates it possible to dispense with natural gas and reduce greenhoapply gas emissions. NegaWatt’s spokesperson, Marc Jedliczka, states the infrastructure already exists for natural gas and could be reapplyd. 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 necessary to multiply the number of batteries to obtain more energy capacity. In the case of hydrogen, you don’t necessary more turbines or fuel cells. What you necessary is more tanks that are way cheaper.”
Mixed storage for mixed energy and context
Gonzalez states that IRENA’s global tarobtain is a general roadmap, but “it’s more important to adapt the tarobtain to each national context”. Every European countest 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 becaapply 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 rapider storage solutions to cover all necessarys.”
Storage innovation must scale up rapid enough to meet 2030 tarobtains 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 necessaryed 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.
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