It weighs 20 tons and has a diameter of almost three metres. What looks like a big concrete ball from the outside is in fact a mini pumped-storage power station. In future, dozens of hollow bodies like these will store excess wind energy from offshore parks in the depths of the oceans. They work on a simple principle. The green electricity is initially used to pump all of the water out of the concrete sphere. When energy is needed on land, water flows from the sea into the sphere; its flow drives a generator that produces electricity. This is the first time that the concept of pumped storage used onshore has been successfully transferred to the sea – an important building block on the road to a successful energy transition.