Energy characteristics of a fixed-speed flywheel energy storage system with direct grid-connection

Flywheel energy storage systems (FESSs) store kinetic energy in the form of Jω2⁄2, where J is the moment of inertia and ω is the angular frequency. Although conventional FESSs vary ω to charge and discharge the stored energy, in this study a fixed-speed FESS, in which J is changed actively while maintaining ω, was demonstrated. A fixed-speed FESS has the advantage of being capable of direct grid-connection without requiring a power electronic interface. A prototype with an output power of several hundred watts and a charge/discharge period of several seconds has been developed and discharge/charge operations have been conducted while mechanically measuring the output energy Em from the flywheel and mechanical work Ec, to vary J. Theoretical analysis suggests a ratio of 2.2 for Em/Ec, and experimental values of Em/Ec show high reproducibility, approximately 1.8 and 2.7 in discharge and charge operations, respectively. The reason for the errors can be explained by the loss incurred in varying J.