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When Manuele Aufiero was a child, his parents would take him hiking along a reservoir in northern Italy. It wasn’t a typical reservoir, though. This one drained and refilled constantly, with pumps raising the water level when electricity was cheap. When nearby cities needed electricity, the pumps would reverse, turning into generators as the water drained out of the reservoir.

The technology, known as pumped-storage hydropower, or pumped hydro for short, has been around for over a century. Such facilities are some of the biggest “batteries” humans have ever built. Globally, pumped hydro reservoirs store 8,500 gigawatt-hours of electricity, according to the International Energy Agency.

Pumped hydro can generate electricity for hours on end, and the power plants have grown in importance as intermittent energy sources like wind and solar have become more widespread. But there are only so many places on Earth with suitable topography to host a pumped hydro reservoir.

“I’m in love with pumped hydro,” Aufiero told TechCrunch. “It’s just not enough to keep up with renewables.”

So Aufiero decided to solve that problem by moving the technology to the sea. He co-founded a startup, Sizable Energy, to turn his idea into reality.

Sizable recently raised $8 million in a funding round led by Playground Global with participation from EDEN/IAG, Exa Ventures, Satgana, Unruly Capital, and Verve Ventures, the company exclusively told TechCrunch.

The startup’s power plant looks something like an hourglass. Sizable’s concept specifies two sealed, flexible reservoirs, one that floats at the top and another that sits at the bottom on the seabed. They’re connected by a plastic tube and some turbines.

When power is cheap, the turbines will pump super salty water from the bottom reservoir to the top. When the grid needs energy, Sizable will open a valve, and because the water in the reservoir contains more salt than the surrounding seawater, it’s heavier and will fall down to the lower reservoir. As it flows through the pipe, it spins the turbines, which act as generators.

“From the energy balance point of view, what we are doing is lifting block of salt. But instead of using cranes, we dissolve it and pump it just because it’s easier, simpler,” Aufiero said. “Other than that, we are just lifting a heavy amount of salt.”

By moving pumped hydro to the ocean, Sizable is hoping to mass produce the technology, something that isn’t really possible on land.

“Every time you build pumped hydro on shore, you have to design a concrete dam for that specific site, and you have to adapt the technology there,” Aufiero said. “Building offshore allows us to streamline the production, and everything we do is identical, regardless of the final deployment site.”

Sizable has tested a small model of the reservoirs in wave tanks and off the coast of Reggio Calabria, Italy. It’s now deploying a pilot of the floating components in advance of a full demonstration plant. By 2026, it’s hoping to deploy several commercial projects at sites around the world. 

At full size, the turbines would generate around 6 to 7 megawatts of electricity each, and there will be one for every 100 meters of pipe. Deeper sites would have more storage potential, and each commercial site would host multiple reservoirs. Sizable hopes to deliver energy storage for €20 per kilowatt-hour (about $23), about one-tenth what a grid-scale battery costs.

The technology would pair well with offshore wind projects since sharing an electrical connection to the shore would reduce costs. But Aufiero said that Sizable’s reservoirs could connect to any grid that’s near waters that are at least 500 meters (1,640 feet) deep.

“We believe that long duration energy storage is required not only for renewable integration, but also for just making the grid resilient,” he said. “There is no way we can keep up with that with traditional pumped hydro or batteries. We need something new.”