Deep inside the Swiss Alps, about 2,000 feet underground, an engineering project 14 years in the making quietly came online in 2022 C.E. — a pumped-storage power plant so large it can store the equivalent energy of 400,000 electric car batteries. Built in the Canton of Valais, the Nant de Drance facility is now the biggest pumped-storage plant on Earth, and it plays a direct role in keeping electricity grids stable across Switzerland and much of Europe.
At a glance
- Pumped-storage power plant: Nant de Drance stores up to 20 million kWh of energy by moving water between two reservoirs at different altitudes — pumping uphill when electricity is surplus, releasing downhill through turbines when demand spikes.
- Grid stabilization: Six pump turbines give the plant a total output of 900 megawatts, enough to supply roughly 900,000 homes and to act as a rapid-response buffer for the broader European grid.
- Alpine construction: Workers drilled and blasted 11 miles of tunnels through the Alps over 14 years to move materials and machinery into the subterranean cavern where the plant now operates.
How a water battery works
The concept behind pumped-storage hydro is elegantly simple. Two large reservoirs sit at different elevations. When solar panels and wind turbines produce more electricity than the grid needs, that surplus power runs pumps that push water uphill into the higher reservoir. The water sits there, holding the energy in potential form — like a charged battery.
When demand rises — during a summer heatwave, a cold night, or an unexpected drop in wind — operators release that water back down through tunnels. It drives turbines, generating hydroelectric power on demand. The system can go from idle to full output in minutes, which no conventional power plant can match.
At Nant de Drance, the upper reservoir is the Lac du Vieux Emosson and the lower is the Lac des Toules. The height difference between them is what gives the plant its storage muscle. That 20 million kWh capacity makes it the largest facility of its kind in the world.
Why it matters now
Renewable energy’s great challenge has always been timing. The sun doesn’t shine at peak demand. Wind doesn’t blow on schedule. Without large-scale storage, grids have to keep fossil fuel plants on standby — wasting money and emitting carbon just to be ready.
Pumped storage is the most proven large-scale solution to that problem. According to the International Energy Agency, pumped-storage plants currently account for over 90 percent of all grid-scale energy storage capacity worldwide. As wind and solar continue their rapid expansion — now making up a growing share of global power generation — facilities like Nant de Drance become increasingly critical infrastructure.
Switzerland sits at the center of Europe’s interconnected electricity grid, making it a natural hub for cross-border balancing. When Germany or France produces excess solar power on a bright afternoon, that energy can flow into Switzerland, pump water uphill at Nant de Drance, and flow back out as electricity during the evening peak. The Alps, it turns out, are not just scenery — they are a battery pack for the continent.
What it took to build
The project broke ground in 2008 C.E. and didn’t reach full commercial operation until 2022 C.E. The core challenge was geological: everything had to happen inside a mountain. Engineers carved out a machine hall cavern 600 meters (about 2,000 feet) below the surface, large enough to house six turbine units, each the size of a small building.
Getting materials in required boring 18 kilometers (about 11 miles) of access and transport tunnels through alpine rock. The Nant de Drance consortium — which includes major Swiss utilities Alpiq and IWB among others — managed the project through years of blasting, concrete pouring, and precision mechanical installation in conditions that ruled out most conventional construction logistics.
The finished plant connects to the Swiss national grid and can begin generating power within minutes of a dispatch signal. That speed matters: grid operators managing sudden drops in renewable output need a response measured in seconds and minutes, not hours.
Still imperfect, still essential
Pumped storage isn’t a complete answer to the storage challenge. Research published in Nature Energy points out that geography limits where these plants can be built — you need two reservoirs, significant elevation change, and proximity to both grid infrastructure and a water supply. Most of the world’s best pumped-storage sites are already developed or protected. The technology also loses roughly 20 to 30 percent of the energy it stores through the pump-and-release cycle, a real efficiency cost.
And construction timelines like Nant de Drance’s — 14 years from start to full operation — underline a hard truth: the clean energy transition needs storage solutions that can be built faster, and in more places, than alpine megaprojects allow. Battery storage, compressed air, and hydrogen are all advancing to fill those gaps.
None of that diminishes what Nant de Drance represents. It is a working proof of concept at the largest scale ever attempted, operating at the heart of one of the world’s most complex electricity grids. Swiss grid operator Swissgrid has called large-scale storage a cornerstone of the country’s energy future, and this plant is its most powerful example yet.
Read more
For more on this story, see: The Optimist Daily
For more from Good News for Humankind, see:
- Renewables now make up at least 49% of global power capacity
- Indigenous land rights at COP30: 160 million hectares
- The Good News for Humankind archive on renewable energy
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