Abstract
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.
Highlights
Due to the intermittent behavior of wind and solar power generation, significant energy storage capacity is required to satisfy demand
The baseline losses are characterized by treatment combinations #1 to #4 in Table 2, which has the electrical machine decoupled such that PEM = 0
It is important to under‐ stand the effects that design parameters have on energy losses and passive discharge, which can primarily be attributed to bearing rolling friction, aerodynamic drag on the flywheel rotor, and the electrical machine employed for transferring energy to and from the rotor
Summary
Due to the intermittent behavior of wind and solar power generation, significant energy storage capacity is required to satisfy demand. Flywheel energy storage systems (FESSs) have been implemented in electric grids to reduce power spikes, provide frequency regulation, improve power quality, and serve as uninterrupted power supply (UPS) systems due to several advantageous characteristics of FESS technology. This includes high charge and discharge rates, lifetimes ranging from 10 to 20 years, and high specific energy [3]. They do not experience depth of discharge effects and have a relatively high cycle efficiency—up to 95% depending on the electrical components [2]
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