Sizable Energy just closed an $8 million funding round to bring pumped hydropower storage to the ocean. The startup's innovative approach uses flexible underwater reservoirs connected by tubes and turbines to store excess renewable energy where traditional land-based systems can't reach. With offshore wind expanding globally and grid storage becoming critical, this ocean-based solution could unlock massive new capacity for renewable energy storage.
Sizable Energy is taking the century-old concept of pumped hydropower and dropping it into the ocean. The startup just raised $8 million in funding led by Playground Global to commercialize what could be the next breakthrough in renewable energy storage.
The inspiration came from childhood hikes in northern Italy, where co-founder Manuele Aufiero watched reservoirs constantly drain and refill as they stored electricity. Traditional pumped hydro facilities are among the world's largest 'batteries,' storing 8,500 gigawatt-hours globally according to International Energy Agency data. But there's a problem - suitable topography limits where these systems can be built.
"I'm in love with pumped hydro," Aufiero told TechCrunch. "It's just not enough to keep up with renewables."
Sizable's solution looks like an underwater hourglass. Two flexible reservoirs - one floating at the surface, another anchored to the seabed - connect via plastic tubes lined with turbines. When electricity is cheap, the system pumps super-salty water from bottom to top. When the grid needs power, that dense brine flows back down, spinning turbines to generate electricity.
"From the energy balance point of view, what we are doing is lifting a block of salt," Aufiero explained to TechCrunch. "But instead of using cranes, we dissolve it and pump it just because it's easier, simpler."
The ocean location enables something impossible on land - mass production. Traditional pumped hydro requires custom concrete dams designed for each specific site. Sizable's standardized underwater systems can be manufactured identically and deployed anywhere with waters deeper than 500 meters.
The startup has already tested small-scale prototypes in wave tanks and off Italy's coast near Reggio Calabria. A pilot deployment of floating components is underway, with full demonstration plants planned before commercial rollout by 2026.
The economics look compelling. Each turbine generates 6-7 megawatts, with one turbine per 100 meters of connecting pipe. Deeper waters offer more storage capacity, and multiple reservoir pairs can operate at each site. Sizable targets energy storage costs of €20 per kilowatt-hour - roughly $23, or about .
