In this pilot project, the group led by project manager Dr. Matthias Puchta from the Fraunhofer IWES-Institute for Wind Energy and Energy System Technology lowered a concrete sphere, three metres in diameter, to a depth of about 100 metres into Lake Constance. When there was a simulated surplus of power, an electric pump pumped the water out of the hollow concrete structure; when there was a simulated lack of power, the researchers allowed water to flow back into the sphere through a turbine – driving a generator that turned the accumulated energy back into electricity.

During the four weeks the concrete sphere lay on the seabed, the scientists tested several scenarios and documented the results of the measurements they took. The next step will be to set up a demonstration project in Europe. In the follow-up project, the concrete sphere will be built with a diameter of 30 metres, considered by engineers to be the ideal dimensions.

The aim behind the project funded by the Federal Ministry for Economic Affairs and Energy (BMWi) is to find solutions for storing surplus electrical power generated by offshore wind farms. This requires high pressure, as can be found at underwater depths of 600 to 800 metres. Under these conditions, a sphere with a diameter of 30 metres can store 20 MWh of energy. The scientists believe the ideal areas of application for this new technology will be countries with long coasts, such as Norway, Spain, the USA or Japan. They estimate the world’s overall electrical capacity to be around 893,000 MWh.