High Speed Drop Down in a Planetary Touch-Down Bearing of an Outer Rotor Flywheel System

Flywheel energy storage systems are well-suited to stabilize the electricity grid, due to their high power, low ageing and low maintenance. To increase the energy density, the rotor is designed as an outer rotor system. To reduce drag losses magnetic bearings are used. For safety reasons two planetary touch-down bearings (TDB) are installed. Due to the unconventional TDB design in combination with the outer rotor geometry, high uncertainties exist regarding the loads and the resulting durability of the TDB. This paper focuses on a drop-down test that was performed using a full-scale outer rotor prototype. The goal is to analyze the rotodynamic behavior of an outer rotor system at high speeds resulting from a drop-down and quantify the effect on the durability of the TDBs as well as on the environment. The rotational speed during drop-down was 27,000 rpm resulting in a rotor surface speed of 250 m/s at the TDB. One second after the drop down the TDBs were destroyed and the system decelerated to standstill in three seconds, due to the uncontrolled rotor-stator-contact. Despite the fast dissipation of kinetic energy, the containment kept its integrity and the environment was not affected.