A startup seeks to store electricity in the ocean

When Manuel Oviero was a child, his parents would take him for a walk along a reservoir in northern Italy. However, it was not a typical tank. Water is constantly drained and refilled, with pumps raising the water level when electricity is cheap. When nearby towns needed electricity, the pumps reversed and turned into generators as water drained from the reservoir.

This technology, known as pumped storage hydropower, or pumped hydropower for short, has been around for more than a century. These facilities are among the largest “batteries” ever built by humans. 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 power plants have increased in importance with the proliferation of intermittent power sources such as wind and solar. But there are only very few places on Earth with suitable terrain to host a pumped water reservoir.

“I love hydropumping,” Oviero told TechCrunch. “It’s not enough to keep up with renewables.”

So Oviero decided to solve this problem by transferring 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 like an hourglass. The Sizable concept specifies two closed, flexible tanks, one floating on top and the other located below on the seafloor. It is connected by a plastic pipe and some turbines.

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

“From an energy balance point of view, what we do is lift a block of salt,” Oviero said. “But instead of using levers, we melt it and pump it just because it’s easier and simpler.” “Otherwise, we’re just raising too much salt.”

By taking hydropumping to the ocean, Sizable hopes to mass-produce this technology, something that’s not really possible on land.

“Every time you build an onshore pumpout, you have to design a concrete embankment for that specific location, and you have to adapt the technology there,” Oviero said. “Building offshore allows us to streamline production, and everything we do is identical, regardless of the final deployment location.”

Sisable tested a small model of the tanks in wave tanks and off the coast of Reggio Calabria, Italy. It is now deploying a trial version of the floating components before setting up a full pilot factory. By 2026, it hopes to deploy several commercial projects in locations around the world.

At full size, the turbines will generate about 6 to 7 megawatts of electricity each, and there will be one for every 100 meters of pipe. Deeper sites will have greater storage capabilities, and each commercial site will host multiple tanks. Sisable hopes to offer energy storage for 20 euros per kilowatt-hour (about $23), about a tenth of the cost of a grid-scale battery.

This technology would pair well with offshore wind projects because sharing the electrical connection to shore would reduce costs. But Oviero said Sisabel’s tanks can connect to any nearby network with water at least 500 meters (1,640 feet) deep.

“We believe that long-term energy storage is needed not only for renewable integration, but also to make the grid resilient,” he said. “There’s no way we can keep up with traditional hydropumping or batteries. We need something new.”