How can the enormous amounts of electricity generated through offshore wind power be temporarily stored on site?

Until now, there was no answer to this question. But after several years’ research, the Stored Energy in the Sea project is entering the testing phase, with the deployment of an underwater pumped storage unit in Lake Constance, on the Rhine River at the foot of the Alps.

The system could potentially provide temporary storage of electricity generated through offshore wind power. Image credit: Pixabay.A one-tenth-scale model of what its inventors—Professor Horst Schmidt-Böcking, of Goethe University, in Frankfurt, and Dr. Gerhard Luther, of Saarland University, Saarbrücken—ultimately hope to install has been lowered to a depth of 100 meters. The three-meter-diameter "marine egg" will be tested for four weeks.

“Pumped storage power plants installed on the seabed can use the high water pressure in very deep water to store electrical energy with the aid of hollow spheres,” explains Horst Schmidt-Böcking. "To store energy, water is pumped out of the sphere using an electric pump; to generate power, water flows through a turbine into the empty sphere and produces electrical energy via a generator."

According to Matthias Puchta, project manager from the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES), the scheme has undergone a detailed systems analysis comprising a design, construction and logistics concept for the pressure tank; development of a turbine-pump unit; examination of how to connect the sphere to the electricity grid; calculated profitability; and creation of a roadmap for the system’s technical implementation. The four-week test will check details concerning design, installation, configuration of the drivetrain and the electrical system, operation and control, condition monitoring and dynamic modeling and simulation of the system as a whole.

“With the results from the model trials, we want first of all to look more closely at suitable sites for a [full-scale] demonstration project in Europe," says IWES Head of Division Jochen Bard. "We are aiming at a sphere diameter of 30 meters for the demonstration-scale system. At the moment, that’s the most practical size in terms of engineering. What’s already certain is that the system can only be used economically in the sea at depths of about 600-800 meters upwards. Storage capacity with the same volume increases linearly with the depth of the water and at 700 meters is about 20 megawatt hours (MWh) for a 30-meter sphere.”

Bard says there is great potential for the use of marine pumped storage systems in coastal areas—in particular, near the coast in highly populated regions such as Norway, Spain, Japan and the United States. With a storage capacity of 20 MWh per sphere and standard technology available today, he envisions a total electricity storage capacity of 893,000 MWh worldwide.