Loads on a horizontal axis wind turbine operating in wake

Abstract

Most investigations of wake effects of wind turbines have concentrated on the wind speeds. In this paper the emphasis is put on the loads, which are determined by the wind pattern. The paper covers investigations of experimental as well as calculated loads on a turbine operating in the near-wake two rotor-diameters behind a single turbine. A large increase in the flapwise blade load fluctuations is found. Compared to wake-free operation the standard deviation of the flapwise blade bending moment is increased by more than a factor of two, when operating half in the wake and half in wake-free wind. Relative to this the loads decrease, when the rotor is operating fully in wake. The increase in standard deviation of flap moment is caused by two effects. Firstly the turbulence is higher in wake than in the surroundings, and this causes stochastic load fluctuations. Secondly the blade once per revolution enters and leaves the low speed area of the wake. The second effect causes large deterministic blade loads and is reponsible for local maxima on each side of the line of the turbines. The increase in the level of the load fluctuations is found not to be symmetric around the line of the turbines, indicating an asymmetry in the wind speed profile. The electrical power production power in wake is also presented as function of the wind direction. At 7.5 m/s a roughly linear drop from the level during wake-free operation to 0 kW at operation fully in wake was found over a 60 ° wind direction range. The measurements are compared with results from two models. One is a rather simple model based on the mass conservation law for an expanding wake. The other is a rather complex model based on solution of the Euler equations, with a term to model the Reynolds stresses. The agreement between the measured and the calculated loads is rather good. The results from the complex model is in better agreement with the measured values than the results from the simple model.