Design and loss analysis of a high speed flywheel energy storage system based on axial-flux flywheel-rotor electric machines

A novel high speed flywheel energy storage system is presented in this paper. The rated power, maximum speed and energy stored are 4 kW, 60,000 rpm and 300 Whr respectively. High power density, energy density and efficiency can be obtained in this system with the compact design. In this design, the rotor with composite rim acts as the flywheel of the system and is sandwiched between two disk type stators. Concepts of active magnetic bearings and axial flux PM synchronous machine are adopted in the design to assist the rotor-flywheel to spin and remain in magnetic levitation in the vertical orientation. The other four degrees of freedom in radial directions are constrained by mechanical bearings for simple operation, and the touchdown bearings are not required at the meantime. Three-dimensional FEM analyses are implemented, and the analysis results support the feasibility of the proposed system.