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

摘要

Flywheel energy storage systems (FESSs) store kinetic energy in the form ofJω2⁄2, whereJis the moment of inertia andωis the angular frequency. Although conventional FESSs varyωto charge and discharge the stored energy, in this study a fixed-speed FESS, in whichJis changed actively while maintainingω, 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 energyEmfrom the flywheel and mechanical workEc, to varyJ. Theoretical analysis suggests a ratio of 2.2 forEm/Ec, and experimental values ofEm/Ecshow 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 varyingJ.