Some challenges of wind modelling for modern wind turbines: The Weibull distribution

Wind power assessments, as well as forecast of wind energy production, are key issues in wind energy and grid related studies. However the hub height of today’s wind turbines is well above the surface layer. Wind profiles studies based on mast data show that the wind profile above the surface layer depends on the planetary boundary layer (PBL) structure and height, thus parameters that are not accounted for in today’s traditional applied flow simulation models and parameterizations. Here we report on one year of measurements of the wind profile performed by use of a long range wind
lidar (WSL 70) up to a height of 600 meters with 50 meters resolution. The lidar is located at a flat coastal site.
The applicability of the WRF model to predict some of the important parameters for wind energy has been investigated. In this presentation, some general results on the ability of WRF to predict the wind profile and the turning of the wind direction with height will be touched upon, but we mainly will discuss the long term distribution of the wind speed, which is often represented by a Weibull distribution. It was found that above 100 meters both the measured scale (A) and shape (k) parameter are larger than predicted by WRF. The under prediction of scale parameter is in accordance with the general underestimation of the wind speed by WRF. The consequence for wind energy is discussed and a simple parameterization for the shape parameter is put forward.