Pumped-storage hydroelectricity
Based on Wikipedia: Pumped-storage hydroelectricity
Imagine a battery the size of a mountain. That's essentially what pumped-storage hydroelectricity is—and it accounts for ninety-five percent of all grid-scale energy storage on Earth. Not lithium-ion. Not compressed air. Not flywheels. Water, lifted uphill.
The concept is almost embarrassingly simple.
When electricity is cheap and plentiful—say, at three in the morning when everyone's asleep, or on a brilliantly sunny afternoon when solar panels are flooding the grid—you use that surplus power to pump water from a lower reservoir up to a higher one. You're essentially converting electrical energy into gravitational potential energy. The water just sits there, patient and heavy, waiting at the top of a hill.
Then, when electricity demand spikes—during the evening dinner rush, or a blistering summer afternoon when every air conditioner in the city is straining—you open the gates. Water rushes downhill through turbines, spinning generators, and that stored gravitational energy becomes electricity again. The whole cycle can respond to grid demands within seconds. Coal plants and nuclear reactors, by contrast, take hours to ramp up or down.
The Efficiency Paradox
Here's the thing that confuses people at first: pumped-storage plants are net consumers of energy. You put in one hundred units of electricity to pump the water up, and you only get seventy to eighty units back when it flows down. The rest is lost to friction, turbulence, evaporation, and the general thermodynamic stubbornness of the universe.
So why bother?
Because electricity isn't like grain in a silo. You can't just pile it up. The grid must balance supply and demand in real-time, every second of every day. When supply exceeds demand, frequency rises dangerously. When demand exceeds supply, the grid can collapse. And crucially, electricity prices aren't constant—they swing wildly based on this supply-demand dance.
Pumped storage plants buy low and sell high. They consume electricity when it's so abundant that prices approach zero—or even go negative (yes, sometimes power companies will pay you to use their electricity, just to keep the grid stable). Then they sell that stored energy back during peak hours when prices soar. The twenty to thirty percent energy loss is more than offset by the price differential.
Think of it this way: you're not storing energy so much as arbitraging time.
Why Water Beats Batteries (For Now)
Lithium-ion batteries dominate headlines. They're in your phone, your laptop, your electric car, and increasingly, in utility-scale installations. As of May 2025, China alone has installed nearly 107 gigawatts of battery storage capacity.
But here's the scale that pumped storage operates at: globally, it provides 200 gigawatts of power capacity and 9,000 gigawatt-hours of energy storage. That second number is the crucial one. A gigawatt-hour is the amount of energy needed to power roughly 700,000 homes for an hour. Batteries are catching up fast on power capacity—the ability to deliver a burst of electricity—but energy storage capacity, the ability to sustain that output over time, is where pumped hydro remains unmatched.
There's another advantage that rarely makes the brochures: longevity.
Pumped-storage plants last for decades. Some have operated for over a century. Lithium-ion batteries, by contrast, degrade with every charge cycle. They last perhaps fifteen to twenty years before needing replacement. When you're building infrastructure that costs billions, that lifespan difference matters enormously.
The Geography Problem
If pumped storage is so wonderful, why isn't it everywhere?
Because you need hills. Preferably mountains.
The energy you can store is directly proportional to the mass of water and the height difference between your two reservoirs. Physics offers no shortcuts here. To store meaningful amounts of energy, you need either enormous volumes of water or significant elevation changes—ideally both.
This geographical constraint explains why countries like Switzerland, Austria, and Norway—blessed with Alps and fjords—punch far above their weight in pumped storage. Meanwhile, pancake-flat Netherlands or Bangladesh can only dream. Jordan, the subject of the related article on renewable energy, has some hilly terrain in its western regions, but much of the country is relatively flat desert—limiting its pumped storage options just when it needs grid flexibility most.
But here's where the story gets interesting. A global survey identified over 800,000 potential pumped hydro sites worldwide, with a combined theoretical storage capacity of 86 million gigawatt-hours. That's roughly a hundred times more storage than we'd need for a completely renewable global electricity system. Most of these sites are "closed-loop"—meaning they don't involve damming rivers or flooding valleys. They're just two artificial reservoirs on a hillside, connected by tunnels and turbines.
Underground and Underwater: The Next Frontier
What if you don't have hills but you do have holes?
Abandoned mines are emerging as surprisingly promising pumped storage sites. In Bendigo, Australia, engineers are exploring the remnants of nineteenth-century gold mining—over 5,000 shafts honeycomb the ground beneath the city, some plunging nearly 1,500 meters straight down. A recent feasibility study suggested a 30-megawatt plant could operate using just one of these old mine shafts, with water falling over 750 meters through the earth itself.
The appeal is obvious: the excavation work is already done. You're not carving reservoirs out of mountainsides; you're repurposing industrial ghosts.
Similar proposals exist for abandoned coal mines, limestone quarries, and—in a delightfully ironic twist—defunct oil and gas wells. A startup called Quidnet Energy is exploring whether the roughly three million abandoned petroleum wells scattered across the United States could serve as pressure vessels for a modified form of pumped storage.
Then there's the ocean itself.
A German research project called StEnSea (Storing Energy at Sea) successfully tested hollow concrete spheres anchored to the seafloor. When electricity is cheap, pumps empty the spheres, pushing water out against the crushing pressure of the deep ocean. When electricity is needed, valves open, and the ocean rushes back in through turbines, generating power.
The deeper you go, the better it works. At great depths, the pressure difference is enormous, meaning you can store far more energy in a much smaller volume. The seafloor, it turns out, is one giant natural battery—if you can build structures tough enough to harness it.
Seawater: The Corrosion Challenge
Most pumped storage uses fresh water. But fresh water is increasingly precious, and coastal areas often lack the inland geography for traditional installations. Why not just use the ocean?
Japan tried. In 1999, a 30-megawatt demonstration plant in Okinawa became the first seawater pumped-storage facility. It worked. But it also revealed the brutal realities of marine engineering: saltwater corrodes turbines, barnacles colonize every available surface, and maintenance costs spiral upward. The plant was eventually decommissioned.
Still, the idea refuses to die. In Chile's Atacama Desert—one of the sunniest places on Earth—developers have proposed pairing a 600-megawatt solar farm with a pumped-storage system that would lift seawater 600 meters up the sheer coastal cliffs. The combination would provide both daytime solar generation and nighttime stored power, all without requiring a single drop of the region's scarce fresh water.
France's Rance Tidal Power Station, operating since 1966, demonstrates a hybrid approach. It harnesses tidal flows for direct generation but can also pump extra seawater into its reservoir during off-peak hours, effectively combining tidal and pumped-storage technology. After nearly sixty years, it remains the only large-scale plant of its kind—a testament to both the concept's viability and the difficulty of replicating it.
The Renewable Revolution's Hidden Partner
Wind turbines spin when the wind blows, not when you need electricity. Solar panels produce power when the sun shines, which inconveniently peaks hours before evening demand does. This intermittency is the Achilles' heel of renewable energy.
Pumped storage may be its cure.
Traditional power grids were designed around "baseload" plants—big coal and nuclear stations that run continuously at maximum efficiency, supplemented by flexible "peaking" plants that fire up to meet demand spikes. This model works, but it's optimized for dispatchable power sources that you can turn on whenever you want.
Renewables flip this model upside down. Generation happens when nature permits, not when accountants schedule it. Grid operators must now match demand to variable supply, rather than the other way around. This requires either curtailment—essentially throwing away excess renewable power when no one needs it—or storage.
Some regions have already pushed renewable penetration to forty percent of annual electricity generation without massive storage buildouts. Studies suggest sixty percent might be achievable through grid interconnections and smart demand management alone. But beyond that, storage becomes essential. And for truly enormous storage capacity—measured in terawatt-hours rather than gigawatt-hours—pumped hydro remains the only proven technology.
The Small-Scale Revolution
Not every pumped-storage plant needs to be a megaproject. A 13-megawatt facility recently came online in Germany. Shell Energy has proposed a 5-megawatt installation in Washington State. Engineers are experimenting with integrating micro-pumped-storage into existing infrastructure—drinking water networks, stormwater basins, even artificial snow-making systems at ski resorts.
The economics are challenging at small scales. Big plants achieve economies of scale that tiny ones cannot. But distributed storage has its own advantages: resilience, reduced transmission losses, and the ability to provide local grid services without long-distance infrastructure.
One Swiss study suggested that the country's small pumped-storage capacity could increase by three to nine times with appropriate policy support. The sites exist. The technology is proven. What's missing is often just the regulatory framework and financial incentives.
At the smallest extreme, some enthusiasts have proposed "pico hydro" systems using cisterns and micro-generators for home energy storage. These remain more proof-of-concept than practical reality—but they illustrate how adaptable the basic physics can be.
Making Water Heavier
Here's a question that sounds absurd until you think about it: what if water were denser?
Energy storage capacity scales with mass. Double the density of your working fluid, and you can store twice as much energy in the same volume—or achieve the same storage in half the space. This matters enormously when you're trying to build pumped storage in places without dramatic elevation changes.
A company called RheEnergise is pursuing exactly this approach, using a slurry of finely-milled mineral particles suspended in water. Their fluid is two and a half times denser than pure water, meaning their installations can be two and a half times smaller for equivalent storage capacity. Hills that would be too modest for conventional pumped hydro suddenly become viable sites.
It's a reminder that century-old technology still has room for innovation.
A Technology Hiding in Plain Sight
Pumped-storage hydroelectricity has been operating since 1907, when the first plant came online in Switzerland near Schaffhausen. More than a century later, it remains the dominant form of grid-scale energy storage by an overwhelming margin.
Yet it rarely captures public imagination the way batteries or hydrogen do. Perhaps because it's too simple—just water and gravity, dressed up with turbines. Perhaps because the facilities themselves are often invisible, buried in mountains or hidden in valleys. Perhaps because the technology peaked in public consciousness during the great dam-building era of the mid-twentieth century and has felt old-fashioned ever since.
But as the world grapples with storing intermittent renewable energy at massive scales, this venerable technology deserves a fresh look. The physics haven't changed. The need has only grown.
Somewhere right now, billions of tons of water are sitting patiently at the tops of hills, waiting for the evening rush. When you flip on your lights tonight, some of that electricity may have spent the afternoon as pure gravitational potential, suspended between two reservoirs, ready to fall.
It's not glamorous. But it works.