  Thanks to the unique advantages such as long life cycles, high power density,
minimal environmental impact, and high power quality such as fast response and
voltage stability, the flywheel/kinetic energy storage system (FESS) is gaining
attention recently. There is noticeable progress made in FESS, especially in
utility, large-scale deployment for the electrical grid, and renewable energy
applications. This paper gives a review of the recent developments in FESS
technologies. Due to the highly interdisciplinary nature of FESSs, we survey
different design approaches, choices of subsystems, and the effects on
performance, cost, and applications. This review focuses on the state of the
art of FESS technologies, especially for those who have been commissioned or
prototyped. We also highlighted the opportunities and potential directions for
the future development of FESS technologies.
