Impact of Wind Turbulences on the Energy Yield of a Wind Turbine with a Hydraulic-Pneumatic Flywheel System in its Rotor

Wind turbulence introduces variations and oscillations in the power output of wind turbines and vibrations of wind turbine components. Higher turbulence intensities lead to significant wind speed fluctuations, necessitating frequent aerodynamic regulation of the rotor in near-rated operating points. Reducing these fluctuations stabilizes the operation of the wind turbine, which reduces power fluctuations in the grid, and which reduces wear and tear in the wind turbine. This study investigates a hydraulic-pneumatic flywheel system designed to leverage these fluctuations by absorbing excess energy during gusts (charging) and releasing it during lulls (discharging), effectively smoothing the power output. The charging phase of this system introduces an additional electrical load by driving hydraulic pumps with electric motors, thereby increasing the generator torque. Simultaneously, fluid movement along the rotor blades induces Coriolis forces, generating a negative torque that slows down the rotor. This flywheel system application reduces rotor speed and pitch angle excursions during peak wind events by storing surplus energy. When the wind speeds drop below rated wind speed due to turbulence, the stored energy is released by transferring the fluid from the tip accumulators to storage tanks in the blade roots, reducing the flywheel inertia, and hence, releasing energy. This bidirectional functionality allows mitigating the downsides of turbulence. The objective of this study is to assess the performance of the flywheel system in stabilizing power output and to quantify the energy yield improvement, with a particular focus on the influence of turbulence intensity.