This paper presents a short time storage based on a flywheel with a squirrel cage induction machine (IM) without speed sensor. For the sensorless control of this IM-based flywheel energy storage system (FESS) a speed estimation method based on an extended complex Kalman filter (ECKF) is proposed, Simulation tests on the behavior of the system of linked flywheel storage and wind generator connected to the grid are presented. Results show the efficiency of the proposed sensorless control technique along with the FESS contribution to smooth the active power fluctuations of wind generation.