How concrete spheres on the ocean floor can store clean energy

As countries shift to renewable power, attention is turning to energy storage. Wind and solar generate power on nature’s schedule, not the grid’s. To bridge this gap, energy must be stored in ways that are reliable, scalable and low-impact. While pumped hydro and lithium-ion batteries dominate the market, both face limitations. These include geographic constraints, material use or long-term sustainability. A new approach from Germany is emerging using underwater energy storage with concrete spheres.

A closer look at the StEnSea system and how it mimics pumped hydro under the sea

The system, known as StEnSea (Stored Energy in the Sea), operates like pumped hydro but under the ocean. Instead of two reservoirs on land, it uses hollow concrete spheres anchored to the sea floor. When surplus power is available, water is pumped out of the sphere to create a vacuum. When energy is needed, water flows back in and drives a turbine to generate electricity.

Fraunhofer IEE, in partnership with industry and academic institutions, developed the concept to use ocean depth and pressure without relying on land-based elevation. The goal is to store renewable energy near offshore wind and tidal sources while minimizing environmental and land use conflicts.

Inside the pilot tests and the early promise of submerged storage spheres

Initial testing took place in Lake Constance, Germany, using a small 3-meter prototype. This demonstration showed that the storage and generation cycle worked under controlled underwater conditions.

More recently, a larger pilot off the California coast deployed a 9-meter concrete sphere weighing 400 tons at a depth of 500 to 600 meters. The test aimed to validate the system’s mechanical durability, operational performance and long-term reliability in marine environments. Results so far point to technical feasibility and safe operation under real-world conditions.

Efficiency metrics, scalability and the engineering behind the concrete spheres

The StEnSea system achieves 75 to 80 percent efficiency, which is competitive with land-based pumped storage. The system is designed for modular deployment. Multiple spheres can be placed on the ocean floor in arrays to create large-scale storage fields. Each unit includes built-in turbines and control systems. The reinforced concrete shell is engineered to withstand external pressure and ensure watertight performance during long-term use.

Studies suggest that many coastal regions are suitable for StEnSea deployment. These include the North Sea near Norway, the Atlantic coast of Portugal and parts of Brazil, Japan and the United States. Ideal sites offer deep water near energy infrastructure and stable seabed conditions.

Sources:
New Atlas – StEnSea concrete spheres
Fraunhofer IEE official release