About 2,000 feet beneath the ocean’s surface, in the dark stillness off the coast of California, a concrete sphere the size of a small house will soon have its moment. Its walls must resist pressures 77 times greater than what we feel at sea level. Yet it has no living inhabitants. Its cargo is not oil or gas. It’s electricity (sort of).

This is StEnSea — Stored Energy in the Sea — a bold new venture that seeks to solve one of the greatest challenges of the clean energy age: how to store renewable energy when the wind stops blowing and the sun disappears.

A Giant Battery Made of Concrete and Water

The concept is disarmingly simple. Picture a hollow concrete sphere on the seafloor. When excess power is available — say, from a nearby offshore wind farm — it’s used to pump seawater out of the sphere, creating a vacuum-like condition inside. Then, when energy is needed, a valve opens. Seawater rushes back in, driven by the crushing pressure of the ocean. As it re-enters, it spins a turbine, which generates electricity. The process is reversible and can be repeated hundreds of times per year.

In 2017, the Fraunhofer Institute for Energy Economics and Energy Systems Technology (IEE) tested this system with a 10-foot-diameter sphere in Germany’s Lake Constance. That small, freshwater trial worked. Now, the team is preparing to test a larger, far more ambitious version in the deep Pacific waters off Long Beach, California.

The new prototype — about 29.5 feet in diameter and weighing 400 tons — will be anchored between 500 and 600 meters (roughly 2,000 feet) underwater. It’s expected to begin operating by the end of 2026, storing up to 0.4 megawatt-hours of electricity, enough to power a typical home for two to three weeks.

“Test run is a big step towards scaling the technology,” said Dr. Bernhard Ernst, Senior Project Manager at Fraunhofer IEE. “With the global energy transition, the demand for storage will increase enormously in the next few years.”

The eventual vision is ambitious: imagine fields of 98-foot spheres blanketing the seafloor, each capable of storing far more energy. Fraunhofer IEE estimates a global storage potential of 817,000 gigawatt-hours — enough to power roughly 75 million homes annually.

Turning the Ocean Floor into an Energy Bank

At its core, StEnSea is a variation of a century-old concept: pumped-storage hydroelectricity. Traditional versions involve pumping water uphill into a reservoir, then releasing it downhill to generate electricity when needed from its massive potential energy. But these require two water bodies at different elevations and vast amounts of land.

StEnSea cleverly swaps mountain slopes for ocean depths. “We are transferring their functional principle to the seabed — the natural and ecological restrictions are far lower there,” explained Ernst. “In addition, the acceptance of the citizens is likely to be significantly higher.”

There’s a practical edge to the idea as well. Offshore locations are often close to where renewable energy is produced, like wind farms. Underwater spheres can be deployed nearby without consuming land or drawing public opposition. The deep ocean becomes the upper reservoir, and the sphere the lower one.

The prototype’s construction reflects this blend of engineering and innovation. Sperra, a U.S. startup specializing in 3D concrete printing, is building the massive orb in Long Beach. Pleuger Industries, based in Miami but with German roots, provides the underwater motor pumps critical to the system. A valve at the top of the sphere lets seawater rush in or be pumped out. The technology’s elegance lies in its mechanical simplicity and the immense pressure the ocean itself provides.

“Pumped storage power plants are particularly suitable for storing electricity for several hours to a few days,” said Ernst. “However, their expansion potential is severely limited worldwide.”

GIS analyses conducted by Fraunhofer IEE suggest otherwise — for ocean-based storage, that is. From the fjords of Norway to the coastlines of Japan, from the U.S. East Coast to the Portuguese shelf, the team has mapped numerous ideal sites: locations between 600 and 800 meters deep, where pressure, concrete strength, and existing pump designs strike an economical balance.

The efficiency of the system — around 75 to 80 percent — is slightly lower than traditional pumped storage. But the lifespan of the concrete spheres is estimated at 50 to 60 years, with the turbines and generators needing replacement only every two decades.

The Depth of Tomorrow’s Power Grid

Each individual sphere in the current design stores a modest amount of energy. But the technology scales well. A park of six large spheres, for instance, could deliver a capacity of 120 megawatt-hours and 30 megawatts of power output, cycling 520 times a year. These installations could play a key role in energy arbitrage — buying electricity when it’s cheap and storing it to sell when prices rise — or in providing ancillary services to stabilize an increasingly complex grid.

The economic case is competitive. Fraunhofer IEE pegs the cost at 4.6 euro cents per kilowatt-hour stored, with capital expenses estimated at €1,354 per kilowatt of power and €158 per kilowatt-hour of storage capacity. That’s cheaper than many battery technologies on the market today, and potentially less disruptive than large-scale hydroelectric dams.

But perhaps StEnSea’s biggest advantage is its potential scale. Compared to the 40 gigawatt-hours of pumped storage available across Germany, even a fraction of the 817,000 gigawatt-hour global potential could reshape how we manage renewable energy.

That kind of capacity might seem futuristic. But it started with an idea in 2011, dreamed up by physicist Prof. Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther. Today, it’s taking concrete form — literally.

“We have developed a cost-effective technology that is particularly suitable for short to medium-term storage,” said Ernst. “With the test run off the US coast, we are making a big step towards scaling and commercializing this storage concept.”

As nations race to decarbonize, the challenge is no longer just how to generate clean power — but how to store it. Maybe the solution doesn’t lie on land, or in lithium, or in clouds of hydrogen. Maybe it’s waiting on the ocean floor.