"Pumped storage power plants are particularly suitable for storing electricity for several hours to a few days. However, their expansion potential is severely limited worldwide. Therefore, we are transferring their functional principle to the seabed – the natural and ecological restrictions are far lower there. In addition, the acceptance of the citizens is likely to be significantly higher," explains Dr. Bernhard Ernst, Senior Project Manager at Fraunhofer IEE. Fraunhofer IEE is working on this project with the US start-up Sperra, which specializes in 3D concrete printing for applications in the field of renewable energies. The second partner is Pleuger Industries. The German-origin company, headquartered in Miami, is one of the world's leading manufacturers of underwater motor pumps, a key component of the StEnSea spherical storage. The partners have selected a coastal area off Long Beach near Los Angeles as the storage site. They plan to put it into operation by the end of 2026 at the latest. The German Federal Ministry for Economic Affairs and Climate Action is funding the project with nearly 3.4 million euros, the US Department of Energy with around four million US dollars.

Successful Field Test in Lake Constance

Sperra will manufacture the concrete sphere in Long Beach using a 3D printing process, possibly in combination with traditional concrete construction. It will have an opening at the top, into which an underwater motor pump, also called a pump turbine, will be integrated in a pipe. When a valve is opened, water flows through the pipe into the sphere. The integrated pump runs in reverse and works as a turbine. The water drives the motor, generating electricity. Thus, the storage is discharged. The idea dates back to 2011 and was developed by Prof. Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther. An underwater cable connects to the power grid on land or to a floating transformer station of an offshore wind farm. To store energy, the motor pump pumps the water out of the sphere against the pressure of the surrounding water column. The cycle can then begin again. In a field trial with a three-meter sphere in Lake Constance, Fraunhofer IEE researchers, together with partners, have already proven that this concept works well.

Water Depths of 600 to 800 Meters are ideal

The capacity and performance of the spherical storage depend primarily on two factors: the volume of the spheres and the water column pressing on them. Fraunhofer IEE experts have calculated that water depths of 600 to 800 meters are ideal locations from an economic perspective. There, parameters such as pressure, the required sphere weight, and the necessary wall thickness are in optimal proportion to each other. Moreover, conventional underwater motor pumps can still be used at this depth. High-strength special concrete is also not necessary here. There are more than enough potential locations for StEnSea spherical storage at this depth, as a GIS analysis of coastal marine areas shows. Fraunhofer IEE experts have taken parameters like bottom slope, currents, sediment displacement, or distance to land into account. For example, off the coasts of Norway, Portugal, the US East and West Coasts, Brazil, or Japan, the spherical storage could be installed in large numbers. The technology is also suitable for deep natural or artificial lakes, such as flooded open-pit mines.

Huge Global Potential

According to Fraunhofer researchers, the global storage potential of this technology is 817,000 gigawatt-hours in total. At the ten best European locations, it is still 166,000 gigawatt-hours. In comparison, the capacity of existing pumped storage power plants on land in Germany is just under 40 gigawatt-hours. Fraunhofer IEE researchers estimate storage costs at around 4.6 cents per kilowatt-hour, investment costs at 1,354 euros per kilowatt of power, and 158 euros per kilowatt-hour of capacity. The lifespan of the concrete sphere is 50 to 60 years. Pump turbines and generators would need to be replaced every 20 years. The efficiency over an entire storage cycle is slightly lower than that of a conventional pumped storage power plant, at 75 to 80 percent. This calculation is based on a storage park with six spheres, a total power capacity of 30 megawatts, and a capacity of 120 megawatt-hours, with 520 storage cycles per year. StEnSea spherical storage is particularly suitable for two business models: for arbitrage, i.e., buying electricity at low and selling at high market prices, and for providing ancillary services to stabilize power grids.

"Test Run is a Big Step Towards Scaling the Technology"

After the successful test in Lake Constance, the experts now want to test the application in deep water under offshore conditions with the new project. The goal is to investigate and evaluate all steps along the manufacture, installation, operation, and maintenance regarding the intended size of the sphere – a diameter of 30 meters. They want to check whether and how the solutions found in this project can be applied to a 30-meter sphere. "With the global energy transition, the demand for storage will increase enormously in the next few years," says Bernhard Ernst of Fraunhofer IEE. "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."