Power fluctuations of wind generators may affect power quality especially in weak or isolated grids. This paper proposes an energy management strategy for a flywheel-based energy storage device. The aim of the flywheel is to smooth the net power flow injected to the grid by a variable speed wind turbine. The design of the energy management strategy is conducted through several phases. First, a definition and determination of the optimal operation of the storage device is carried out through the formulation and deterministic solution of an optimization problem in GAMS. Based on this solution, an online energy management algorithm is proposed to achieve a close to optimal operation of the flywheel. This algorithm determines the set points of the torque control of the flywheel electrical machine. The proposed methodology is illustrated by simulations. (C) 2013 Elsevier Ltd. All rights reserved.