Engineers are testing a massive underwater battery in California

Off the coast of California, engineers are preparing to sink a hollow concrete sphere that works like an underwater battery. They’re calling it StEnSea style storage.

The prototype will rest on the seabed near Long Beach and store electricity by using the pressure of deep ocean water.

This first full scale test comes from the Fraunhofer Institute for Energy Economics and Energy System Technology (FIEEEST) and its partners.

Instead of building new dams in mountain valleys, the team wants to tuck large energy stores thousands of feet below the sea surface.

How an underwater battery works

At the heart of the project is pumped storage, a method that uses spare electricity to move water uphill and then recover power.

In the sea version, the upper reservoir is the surrounding ocean and the lower reservoir is an empty concrete sphere on the seabed.

The system runs in a simple cycle with two main steps. When there is extra wind or solar power, an electric pump pushes water out of the sphere against the surrounding pressure.

The work is led by Dr. Bernhard Ernst, Senior Project Manager at the FIEEST. His research focuses on large scale ways to store renewable electricity reliably for grids that depend heavily on wind and solar.

When electricity is needed again, a valve opens and water rushes back into the empty sphere through the same machine. The turbine then spins the generator, turning the stored pressure into power that can be sent to the grid.

California prototype on the seabed

In the new pilot program, engineers plan to place a hollow concrete sphere 30 feet across and 400 tons on the seabed off Long Beach. A New Atlas report notes that it will sit 2,000 feet deep and is designed to deliver about half a megawatt of power.

The sphere will be built in the harbor by Sperra using concrete printing techniques and fitted with an underwater pump turbine from Pleuger Industries.

The pump empties the sphere when renewable power is abundant, then runs as a turbine when water flows back in to generate electricity.

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

Learning from a lake test

Engineers first tested the idea in Lake Constance by lowering a 3-meter-wide concrete sphere to about 330 feet. They ran repeated cycles through winter 2016, and the results showed that the system could handle the pressure difference, according to a test of the deployment.

The lake study refined practical details, including how to lower and recover the sphere, control water flow, and run the system safely without divers.

Those lessons now feed into the offshore project near Long Beach, where technicians must work at greater depths in saltwater.

Underwater battery in deep seas

One concept explains how underwater pumped storage uses static water pressure, which rises with depth, to pack energy into hollow spheres.

When water is pumped out of a sphere on the seabed, the stored energy depends on the outside pressure and the empty volume inside.

For the StEnSea system, Fraunhofer scientists identified water depths around 2,000 to 2,500 feet as a sweet spot. These depths give strong pressure while still allowing standard underwater pumps and ordinary concrete.

Global mapping using a geographic information system, a digital tool that links data to locations, suggests huge room for expansion of this technology.

One Fraunhofer analysis potential estimates that suitable coastal seabeds could hold 817 terawatt hours of storage spread across many countries.

From one sphere to underwater parks

In the long run, the vision is to group many spheres into underwater storage parks connected to offshore wind farms or coastal grids.

Spheres can be added in modules and wired together, making it easier to match storage size to local demand and expand projects over time.

Planned spheres are about 100 feet in diameter and can store 20 megawatt hours of energy while delivering around 6 megawatts of power.

An engineering design examines pairing these seabed batteries with offshore wind turbines off California, using cables and anchors to move power to shore.

What this means for clean energy

As more wind and solar power flow into electricity systems, grid operators must handle larger swings between high and low output.

Short duration batteries are useful for fluctuations over minutes or hours, while StEnSea style storage targets gaps from few hours up to two days.

StEnSea style storage is suited to two main business models that help keep power systems stable. One is price arbitrage, a trading strategy that earns money by buying electricity when prices are low and selling it when prices rise again.

The other key market is ancillary services, specialized grid support tasks that keep voltage and frequency within tight limits.

Because underwater spheres can both absorb and release power quickly, they can provide these balancing services while helping offshore wind farms maintain steadier output.

One assessment of the StEnSea concept found that storage costs could match those of conventional pumped hydropower when many units are installed together.

That study also concluded that, in suitable locations, underwater spheres could compete with other long duration storage options for balancing variable renewables.

Challenges still on the horizon

Big engineering questions remain before concrete spheres on the seabed can play a major role in real power systems. Each unit must survive decades in saltwater while withstanding pressure, and the pumps, valves, and generators need designs that allow maintenance from the surface.

Environmental and social questions are just as important. Work around Lake Constance suggested that careful siting and low intake speeds keep impacts small, but future ocean projects must check habitats and noise.

If the California prototype performs well, underwater storage spheres could join hydropower dams and battery farms as another grid scale option. That would give countries another flexible tool for running power systems dominated by wind and solar without turning back to fossil fuels.

Image credits: Fraunhofer IEE.

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