On the power coefficient overshoot of an offshore floating wind turbine in surge oscillations

Abstract

Abstract The Betz limit is widely accepted to be the upper limit for the power coefficient of open wind turbines. However, when investigating the power performance of an offshore floating wind turbine in surge oscillations, it is found that the Betz limit can be exceeded by the instantaneous power coefficient if the tip speed ratio is near the optimal tip speed ratio and the platform surge motion is severe enough. This exceeding phenomenon is defined as the power coefficient overshoot. Numerical simulations with the free vortex method show that the power coefficient overshoot is caused by the time lag between the power output and the wind farm power. The time lag and the resultant power coefficient overshoot intensify with the increasing platform surge frequency. Equivalent dynamic modeling analysis shows that the time lag is mainly caused by the added mass effect, which increases with the surge frequency. The time lag and the resultant power coefficient overshoot are attributed to the unsteady profile dynamics and the blade‐wake interaction. The former tends to induce the time lag to appear as a time advance in the power output when compared with the wind farm power. The latter, however, appears to induce the time lag to appear as a time delay in the power output. As the tip speed ratio increases, a shift from time advance to time delay is detected due to the decrease of the unsteady profile aerodynamics and the enhancement of the blade‐wake interaction.