Scientists Are Sinking Concrete Batteries Underwater

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Scientists Are Sinking Concrete Batteries Underwater. An Energy Revolution Is Coming.

May 30, 2025

Experiments with these big hollow spheres are proving an innovative source of energy storage that could power millions of homes.

About 330 feet below the surface, a spherical shell of concrete rested on the floor of Lake Constance in Bodensee, Germany. Nearly 10 feet in diameter, the sphere was full of fresh lake water.

Then, a pump-turbine and valve system attached to the sphere forced water out, creating a vacuum inside it. When engineers cut energy to the system, the pumping stopped, and the surrounding seawater rushed back in, spinning the turbine, and generating electricity.

The German-based Fraunhofer Institute for Energy Economics and Energy Systems Technology (IEE) designed this novel ocean battery, which can store energy and then release it as needed.

The prototype’s lake test in 2017 allowed engineers to envision sprawling undersea fields of such batteries, which will use deep ocean water’s natural pressure to store energy from, say, solar or wind power systems.

When such systems generate more energy than the power grid demands, the excess energy is put to work pumping water out of the spheres. When energy needs spike, the pumps stop working, and sea water gushes back into the spheres.

A cable returns electricity back to the grid. The undersea technology is called StEnSea (Stored Energy in the Sea).

Giant concrete spheres anchored to the ocean floor are an innovative approach to the world’s increasing energy needs.

They don’t take up valuable land space, and they are simpler than traditional pump-storage solutions, which must use two reservoirs at different elevations to move water through turbines.

In this design however, the water column above the spheres creates an upper reservoir, while the spheres themselves act as the lower reservoir.

This makes them vastly adaptable to many more offshore locations around the world, according to the company. StEnSea “enables us to provide huge storage capacities offshore, near the shore, near the coast, and also in deep water—even in areas where we have high population densities—and in that context, offers a new option to store large amounts of energy in the future energy systems,” said Fraunhofer IEE Division Director Jochen Bard in a 2020 YouTube video about the company’s energy projects.

Fraunhofer IEE’s simulations with concrete proved that it was strong enough to do the job in far deeper waters than Lake Constance—about 600 to 800 meters below the surface (or 1,970 to 2,620 feet).

There, the pressure can reach up to 77 atmospheres, or 77 times the pressure we feel at sea level.

Now, the company is spearheading a more ambitious project off the coast of California with an international team, including U.S.-based Sperra for its 3D concrete printing technology, and Germany-based Pleuger Industries, which designs underwater pumps.

This larger test will use a 29.5-foot hollow sphere, weighing around 400 tons, which will be anchored at depths of roughly 2,000 feet.

Expected to be active by the end of 2026, this project aims to demonstrate the feasibility of concrete sphere technology in real ocean conditions.

The 3D-printed concrete spheres need to stand up to the massive water pressure at these depths. The single sphere should have a storage capacity of 0.4 megawatt hours.

While that’s only enough energy to power one average household for two weeks, plans are already underway for even larger spheres—up to 98 feet in diameter—which could bank significantly more energy.

They could play a crucial role in stabilizing power grids, especially those that rely on variable renewable energy sources like solar and wind. Eventually, the Fraunhofer Institute’s plan is to deploy 330-foot spheres.

The scientists and engineers hope to reach a global storage potential of 817,000 gigawatt-hours, according to Fraunhofer IEE estimates.

That means [i[StEnSea systems[/i] could one day power nearly 75 million homes per year, enabling a vastly expanded energy storage solution.

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