On the importance of accurate wind profile for wind power applications

The vertical profile of the wind in the layer between the surface and around 100m usually deviates from the logarithmic approximation. Furthermore, this approximation is only valid in the constant flux layer whose actual existence is often disputed. On the other hand the accurate knowledge of the wind profile throughout this layer at any time is extremely important for wind power applications. The energy yield and the turbine load stress estimates mainly depend on the wind speed and the wind shear through the rotor plane. Given the fact that the available wind measurements are usually available at limited number of heights, and moreover typically far below the height of the rotor upper tip, numerical models must be used to provide additional needed information.

Due to the above listed issues the majority of wind power applications nowadays operates on the assumption of neutral atmosphere and pure logarithmic (or in some cases power-law) wind profiles.

We are presenting some concrete examples of the amplitude of the problem just described. Meteorological mast data has been first checked for consistency, where the sector-wise surface roughness of the fetch area was used to calculate the logarithmic profile. Then it was compared with the profiles obtained with a numerical model (WRF). Different ways of extrapolating the wind profile above the highest mast measurement have been applied, based on a constant wind shear estimated from the mast measurements themselves, and from the model wind profiles. Because the typical resolution of the numerical models used does not allow for the accurate terrain representation end therefore the modelled wind speed is usually less accurate than the measurements, it is favourable to only use the model's information about the temperature profile to derive the appropriate wind profile approximation.