Experimental Characterization of Low-Speed Passive Discharge Losses of a Flywheel Energy Storage System

Flywheel energy storage has a wide range of applications in energy grids and transportation. The adoption of high-performance components has made this technology a viable alternative for substituting or complementing other storage devices. Flywheel energy storage systems are subject to passive discharge attributed primarily to electrical machine losses, bearing rolling friction, and aerodynamic drag of the flywheel rotor. In the present study, measurements are presented for complete discharge experiments using a flywheel system featuring a vacuum enclosure. Best-fit equations were applied to the test data and compared to analytical models. Analysis of the best-fit equations indicates that they may serve as empirical models for approximating passive discharge under given conditions. Bearing losses, which varied linearly with velocity but were otherwise unaffected throughout the experiments, were larger than aerodynamic drag at low air pressures and low velocities. Aerodynamic drag became significant as velocity exceeded approximately 3400 rpm. The electrical machine was found to be the most significant source of passive discharge at all velocities and pressures. Based on these findings, it is recommended to maintain a low-pressure environment in the flywheel enclosure and to decouple the electrical machine from the rotor whenever possible to eliminate associated losses.