What if the key to storing solar power isn’t on rooftops or in batteries—but hidden deep beneath the waves?

That’s exactly what researchers at Germany’s Fraunhofer Institute are exploring, with plans underway to submerge massive concrete spheres in the ocean, offering a sea-based alternative to land-hungry energy storage solutions.

As part of the StEnSea (Stored Energy in the Sea) project, the renowned institute has been investigating how deep-sea pressure can be harnessed to store energy in the short to medium term.

Since 2011, the team has focused on deploying giant hollow concrete spheres sunk hundreds of feet beneath the ocean surface to test the waters on this new frontier, according to a report on New Atlas.

Spheres store solar surge

An empty sphere functions as a fully charged storage unit. When its valve opens, seawater flows inside, driving a turbine connected to a generator that feeds electricity into the grid. To recharge, water is pumped back out against the surrounding pressure using energy from the grid.

The institute has conducted successful tests in Europe’s Lake Constance. Fraunhofer

Each sphere measures about 30 feet (9 meters) in diameter, weighs 400 tons, and is anchored at depths between 1,970 and 2,625 feet (600–800 meters) for optimal efficiency.

After successful testing of a smaller model in Europe’s Lake Constance near the Rhine River, Fraunhofer plans to deploy a full-scale 3D-printed prototype off the coast of Long Beach, California, by the end of 2026, under a $4 million US Department of Energy project.

This unit is expected to generate 0.5 megawatts and store 0.4 megawatt-hours—enough to power an average U.S. household for around two weeks.

The institute has set an ambitious goal to further scale this technology to handle much larger spheres, approaching 100 feet (30 meters) in diameter.

Energy storage beast?

Fraunhofer researchers estimate that the StEnSea system could offer a colossal global energy storage capacity of about 817,000 gigawatt-hours, enough to power nearly 75 million homes across Germany, France, and the UK for an entire year.

The institute projects storage costs at roughly 5.1 cents (4.6 euro cents) per kilowatt-hour, with initial investment costs near $177 (158 euros) per kilowatt-hour of capacity. These estimates are based on a model storage park featuring six spheres, delivering a combined power output of 30 megawatts and a total storage capacity of 120 megawatt-hours.

The institute plans to deploy a full-scale 3D-printed prototype off the coast of Long Beach, California, by the end of 2026. Fraunhofer

According to Fraunhofer, StEnSea’s spherical storage is particularly well-suited for stabilizing power grids by providing frequency regulation and operating reserves. It also supports arbitrage — the process of purchasing electricity when prices are low and selling it when prices peak, a strategy commonly used by grid operators, utility companies, and energy traders.

In the long run, StEnSea could compete with traditional pumped hydro storage as a means to store excess grid electricity, with no use of valuable land area playing to its advantage.

Unlike pumped storage, which depends on having two reservoirs at different elevations to move water through turbines, StEnSea’s underwater spheres can be deployed in multiple locations around the globe, enabling vast storage potential.

While pumped storage remains cheaper to operate and slightly more efficient over a full cycle, StEnSea’s flexibility and scale could make it a vital component of future energy grids.