Experimental Techniques for Flywheel Energy Storage System Self-discharge Characterisation

In this paper, an experimental characterisation technique for Flywheel Energy Storage Systems (FESS) behaviour in self-discharge phase is presented. The self-discharge phase characterisation is crucial in order to design performing and sustainable FESS. Unlike other energy storage systems, FESS presents several peculiar characteristics as strict high vacuum environment, high angular speed and the presence of low friction rotor supports such as active bearings. The experimental measurement of rotor characteristics as angular speed is not feasible with traditional techniques as encoders or contact tachometers. Therefore, two different techniques to measure angular speed are proposed and compared. The first one, more traditional, is based on the usage of a laser tachometer, not applicable in vacuum environment. The second one, compliant with high vacuum, adopts a single high speed laser displacement sensor to indirectly measure the rotor angular speed. The proposed technique is validated on a dedicated experimental test-rig with different power loss conditions and used at different vacuum levels.