When Manuele Aufiero was a child, his parents would take him hiking along a reservoir in northern Italy. It was not a typical reservoir. This one drained and refilled constantly, using pumps to raise the water level when electricity was cheap. When nearby cities needed power, the pumps would reverse, turning into generators as the water drained out.
This technology, known as pumped-storage hydropower or pumped hydro, has existed for over a century. Such facilities are some of the largest batteries humans have ever constructed. Globally, pumped hydro reservoirs store 8,500 gigawatt-hours of electricity according to the International Energy Agency.
Pumped hydro can generate electricity for hours at a time, and its importance has grown as intermittent energy sources like wind and solar have become more widespread. However, there are only so many places on Earth with the right topography to build a pumped hydro reservoir.
Aufiero expressed his fondness for pumped hydro but acknowledged its limitations in keeping up with renewable energy growth. He decided to solve this problem by moving the technology to the sea and co-founded a startup called Sizable Energy to turn his idea into reality.
Sizable recently raised eight million dollars in a funding round led by Playground Global, with participation from EDEN/IAG, Exa Ventures, Satgana, Unruly Capital, and Verve Ventures.
The startup’s power plant resembles an hourglass. The concept uses two sealed, flexible reservoirs, one floating at the surface and another resting on the seabed. They are connected by a plastic tube and turbines.
When power is cheap, the turbines pump super salty water from the bottom reservoir to the top. When the grid needs energy, a valve opens. Because the water in the reservoir contains more salt than the surrounding seawater, it is heavier and falls down to the lower reservoir. As it flows through the pipe, it spins the turbines, which act as generators.
Aufiero explained that from an energy balance perspective, they are essentially lifting a block of salt. Instead of using cranes, they dissolve it and pump it because that method is easier and simpler. The core concept is lifting a heavy amount of salt.
By moving pumped hydro to the ocean, Sizable aims to mass-produce the technology, which is not really feasible on land. Aufiero noted that every onshore project requires designing a concrete dam for a specific site and adapting the technology there. Building offshore allows them to streamline production, with every component being identical regardless of the final deployment location.
Sizable has tested a small model of the reservoirs in wave tanks and off the coast of Reggio Calabria, Italy. The company is now deploying a pilot of the floating components ahead of a full demonstration plant. By 2026, it hopes to deploy several commercial projects at sites around the world.
At full size, each turbine would generate around six to seven megawatts of electricity, with one turbine for every one hundred meters of pipe. Deeper sites offer more storage potential, and each commercial site would host multiple reservoirs. Sizable hopes to deliver energy storage for twenty euros per kilowatt-hour, which is about one-tenth the cost of a grid-scale battery.
This technology would pair well with offshore wind projects, as sharing an electrical connection to the shore would reduce costs. However, Aufiero stated that Sizable’s reservoirs could connect to any grid near waters that are at least five hundred meters deep.
He believes that long-duration energy storage is essential not only for integrating renewables but also for making the grid resilient. He concluded that there is no way to keep up with demand using traditional pumped hydro or batteries alone, and that something new is needed.