Off the coast of California, a gray ball 9 meters in diameter is about to disappear beneath the surface. It is neither a film set nor a giant sonar, but a prototype underwater battery imagined by German researchers. Their challenge: transform simple concrete spheres into a real energy storage system to smooth wind and solar production.
Behind this project called StEnSea is the German institute Fraunhofer IEEwho has been working on this idea of underwater “pumped storage” for more than ten years. After a first conclusive experience in Germany, the team is scaling up with a 9-meter demonstrator submerged off the coast of Long Beach. And this is where the technology starts to get very serious.
StEnSea concrete spheres: how these underwater “batteries” really work
The principle is similar to that of classic WWTPs, but reversed and moved to the bottom of the sea. A large hollow concrete sphere is placed several hundred meters underwater. When there is too much electricity on the network, motor pumps expel the water out of the sphere: we then “charge” the system by creating a partial vacuum inside.
When demand rises, a valve opens, water compressed by ocean pressure rushes into the cavity, drives a turbine then a generator, and sends electricity back to the network. The deeper we go, the higher the pressure and the more efficient the storage. Fraunhofer targets the 600 to 800 meter zone, an ideal compromise between pressure, wall thickness and material cost.
A first 3 meter sphere has already been tested in Lake Constance at a depth of around a hundred meters. Result: validated concept, cycle efficiency of around 75 to 80% and no unpleasant mechanical surprises. “The development potential of traditional pumping stations is limited and the ecological constraints are strong. Underwater, the situation is reversed” explains Dr. Bernhard Ernst of the Fraunhofer IEE.
What does the 9-meter prototype submerged in California promise for energy storage
The Californian demonstrator is moving to a completely different size: 9 meters in diameter, around 400 tonnes, a power of 0.5 MW for a capacity of 0.4 MWh, the equivalent of several days of consumption for a household. Immersed between 500 and 600 meters off the coast of Long Beach, it must validate the behavior of the system in the real sea, connected to the network and potentially to offshore wind power.
The sphere is manufactured by the American startup Sperra via 3D printing of concrete. An interesting feature is that the surface is not smooth: it is deliberately rough and “bio-receptive” to be colonized by algae, corals and micro-organisms. Each ball thus becomes an artificial reef, supposed to limit the ecological impact and even locally promote biodiversity.
In terms of lifespan, Fraunhofer is aiming for 50 to 60 years for the concrete structure, with replacement of the pump units and generators approximately every twenty years, directly underwater. Engineers announce an average storage cost of around 4.6 euro cents per kilowatt hour, in the same area as a land-based WWTP, but without flooding valleys or disfiguring landscapes.
When will these giant concrete spheres grow to 30 meters and on a global scale?
The Long Beach prototype, supported by the German Federal Ministry of Economic Affairs and Climate Action and the US Department of Energy, must enter service by the end of 2026. Objective: to test the entire chain, from construction to maintenance, before daring to take the next step.
This next stage involves spheres 30 meters in diameter, capable of storing around 20 MWh each. In a field of several units, this would result in storage “farms” placed under offshore wind farms, with an efficiency still close to 80%. Researchers estimate the global potential at around 817 TWh, a large part of which is on the European coasts, from Norway to the Mediterranean.
It remains to prove the industrial feasibility, to manage the carbon footprint of concrete and to supervise these installations in seas that are already very busy. But if the Californian test keeps its promises, these concrete spheres could quickly find their way into the energy transition plans of the major coastal powers.