The project aimed to implement and test flywheel energy storage systems for smoothing power fluctuations from wind turbines and other renewable energy systems. A small-scale energy storage system has other potential applications in electrical power systems, such as the support of weak grids, regenerative power-saving systems, and uninterruptible power supplies. The requirements for flywheel energy storage systems (FESS) to be used with wind energy systems, in both mains grid-connected, and autonomous (diesel genset) applications, were defined by Rutherford Appleton Laboratory (RAL) and University of Leicester. It was anticipated that the requirements could be met by conventional steel flywheels operating at speeds up to 6000 rev/min. Two FESS configurations were considered, using a mechanical Continuously Variable Transmission (CVT) drive, and a power electronic (PE) drive, which were developed and factory tested by their respective manufacturers P.I.V. Antrieb Werner Reimers GmbH & Co. KG, and Cegelec Industrial Controls. These drives were integrated with electrical machines and steel flywheels on the RAL Wind Test Site, resulting in two prototype FESS with characteristics suitable for the target applications. Controllers for the CVT drive and PE drive were developed by RAL and University of Leicester respectively, and the two FESS were tested for specified performance under test conditions, and in the wind-power smoothing application. The overall performance of both drives has been shown to be excellent, with fast response times being achieved; and the capability of the FESS and controllers to smooth wind power fluctuations, leaving only relatively low amplitude power fluctuations at frequencies above several Hertz, was successfully demonstrated in both mains grid-connected, and autonomous (diesel genset) applications. Finally, an evaluation of the application test results of the two systems enabled a comparison to be made of their relative merits. This revealed that the system utilizing a power electronic drive is suitable for power smoothing and reactive power compensation in all types of power system, and particularly in grid-connected systems with rated powers in the range 10kW to 2MW. The system utilizing a mechanical CVT is applicable to small autonomous (diesel-genset) power systems, with rated powers up to 300kW, where the synchronous machine together with the high effective inertia provided by the CVT and flywheel offers reinforcement to power systems, which are weak in terms of frequency and voltage. In the light of operational results, recommendations were made for possible improvements to both systems, including configuration and controller design. A total of seven technical papers have been written and presented by the investigators at six national and international conferences and colloquia. The experimental systems on the RAL Wind Test Site have been viewed by visitors during Open Days. Both industrial collaborators have demonstrated improved drives resulting from the project developments.