Energy storage

An energy storage, comprising

• an electrical machine with a rotor and a stator, wherein
• the stator
• is separated from the rotor by an air gap, and comprises
• at least one stator coil which during operation interacts with the rotor via a rotating field, and wherein
• the rotor
• surrounds the stator and is adapted to rotate about the stator, or is surrounded by the stator and is adapted to rotate in the stator about an axis of rotation (R),
• is provided with a rotating mass with which is forms a cylindrical body with two end faces and one lateral surface, and the rotor
• comprising at least one permanent magnet in the area of at least one of the end faces, which corresponds with at least one stationary permanent magnet of identical polarity in order to keep the cylindrical body at a distance from the stationary permanent magnet, and/or
• a main axis of inertia (H) which at least approximately coincides with the axis of rotation (R) and having a non-rotation symmetrical shape in a sectional plane which extends transversely to the main axis of inertia (H).