One startup’s quest to store electricity in the ocean

As a child, Manuele Aufiero often explored the scenic hiking trails alongside a unique reservoir in northern Italy with his parents. Unlike conventional reservoirs, this one was dynamic, constantly draining and refilling. During off-peak electricity hours, pumps would raise the water level, and when demand surged, the pumps would act as generators, harnessing the falling water’s energy.

This fascinating technology is known as pumped-storage hydropower, or simply pumped hydro. Having existed for over a century, these facilities serve as some of the largest “batteries” engineered by humanity. According to the International Energy Agency, pumped hydro reservoirs currently store a staggering 8,500 gigawatt-hours of electricity globally, as per their reports.

Pumped hydro systems can generate power for extended periods, making them increasingly vital as renewable energy sources like wind and solar gain prevalence. However, suitable locations for these reservoirs on land are limited.

“I’m truly passionate about pumped hydro,” Aufiero expressed to TechCrunch. “Yet it’s insufficient to meet the demands of renewable energy.”

To address this challenge, Aufiero has taken innovative steps by proposing to extend this technology into marine settings. He co-founded a startup named Sizable Energy to bring his ambitious vision to life.

Recently, Sizable secured $8 million in funding, led by Playground Global, along with contributions from EDEN/IAG, Exa Ventures, Satgana, Unruly Capital, and Verve Ventures, as reported exclusively by TechCrunch.

The design of Sizable’s power plant resembles an hourglass. The concept features two sealed, flexible reservoirs—one buoyant at the surface and the other resting on the seabed. These are interconnected via a plastic tube and turbines.

When electricity prices are low, the turbines pump highly saline water from the seabed reservoir up to the floating reservoir. When the grid demands energy, a valve is opened, allowing the denser saltwater to flow back down. As it descends through the pipe, it powers the turbines, generating electricity.

“In essence, we are lifting a massive block of salt,” Aufiero explained. “Instead of using cranes, we dissolve it and pump it for simplicity and efficiency.”

By relocating pumped hydro systems to the ocean, Sizable aims to mass-produce this technology, a feat not feasible on land.

“Every time we establish a pumped hydro facility on land, we must construct a customized concrete dam for that precise location,” Aufiero noted. “Offshore development allows us to standardize production, ensuring all installations are identical, regardless of the deployment site.”

So far, Sizable has conducted tests of its small-scale reservoir models in wave tanks and off the coast of Reggio Calabria, Italy. They are now in the process of deploying pilot floating components ahead of a full demonstration plant. The company aims to launch several commercial projects worldwide by 2026.

When fully operational, each turbine is projected to generate approximately 6 to 7 megawatts of electricity, with one turbine for every 100 meters of piping. Deeper reservoirs will offer enhanced storage capabilities, and multiple reservoirs will be hosted at each commercial site. Sizable aspires to provide energy storage at €20 per kilowatt-hour (around $23), which is about one-tenth the cost of grid-scale batteries.

This innovative technology is particularly advantageous for offshore wind projects, as sharing an electrical connection with the shore can lower costs. Aufiero emphasized that Sizable’s reservoirs can link to any grid situated near waters at least 500 meters (1,640 feet) deep.

“We believe that long-duration energy storage is essential not just for integrating renewables but also for enhancing grid resilience,” Aufiero stated. “Traditional pumped hydro or batteries won’t suffice; we need pioneering solutions.”