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. 1. SITE AND MEASUREMENTS The measurements were carried out at the Danish National Test Station of Wind Turbines at Hovsore, which is located at the western coast of Jutland, Fig. 1. Except for the presence of the North Sea to the west, the terrain is flat and homogeneous consisting of grass, various agricultural crops and a few shrubs. The intensively instrumented 116.5 m high reference meteorological mast is located about 1.8 km east of the coastline and south of the closest wind turbine stands. Wind speed is measured at 10, 40, 60, 80, 100, 116.5 m with Riso cup anemometers and the wind direction at 10, 60 and 100 m. with wind vanes. Observations from the 160 m top level at the nearby light tower are also used. Both the light tower and the meteorological mast are instrumented with METEK Scientific USA-1 sonic anemometers installed at heights: 10, 20, 40, 60, 80, 100 and 160 m. The three-dimensional wind speeds are measured with a frequency of 20 Hz and then reduced to 10-min statistics of linearly de-trended mean values, variances and co-variances. Figure 1 Geographical location of the Hovsore site. In addition a pulsed wind lidar (WLS70) has been operating near the meteorological mast between April 2010 and March 2011. The wind lidar is equipped with a rotating silicon prism providing an optical scanning cone of 15 degrees to zenith. The lidar scans the atmosphere at four azimuth angles separated by 90°. One 360 degree full scan (rotation) is performed about every 30 s. The wind lidars Doppler shift based measurements of the wind are available measures from 100 m above the ground and every 50 m up till 1 to 2 km height dependent on the attainable 10-min averaged Carrier to Noise (CNR) ratio. The upper measuring height is often determined by the cloud base where the lidar signal (1.55 nm) largely weaken. 2. NUMERICAL MODELLING Wind profiles were predicted using a research real-time forecast system based on the WRF ARW model version 3.2. 1, developed by the National Centre for Atmospheric Research (NCAR), [1]. It is a numerical weather prediction and atmospheric simulation system 16th International Symposium for the Advancement of Boundary-Layer Remote Sensing