Overview of Flywheel Systems for Renewable Energy Storage with a Design Study for High-speed Axial-flux Permanent-magnet Machines

Flywheel energy storage is considered in this paper for grid integration of renewable energy sources due to its inherent advantages of fast response, long cycle life and flexibility in providing ancillary services to the grid, such as frequency regulation, voltage support, etc. The fundamentals of the technology and recent developments are reviewed, firstly with an emphasis on the design considerations and performance metrics. Then the progress and development trends in electric motor/generators employed in flywheel energy storage systems (FESS) are summarized, showing the potential of axial-flux permanent-magnet (AFPM) machines in such applications. Design examples of high-speed AFPM machines are provided and evaluated in terms of specific power, efficiency, and open-circuit losses in order to validate their suitability in FESS.