Wind Gusts at Heights of Wind Turbines

A wind gust or peak gust is defined by WMO as a maximum wind speed of short duration during a longer period of time. A common practice at synoptic weather stations is to record gust as a maximum of 3 s averages of wind speed during a 10 minutes sampling period. There has recently been an increasing trend in the size of wind turbines. The hub heights can reach up to 100 meters or even more and simultaneously the blade diameters have increased to more than 100 meters. Hence, to estimate the load on turbines, it is not enough to study gusts at a single height, but information on the vertical profile is also needed. In meteorological applications, gusts are usually considered together with the mean wind speed using the concept of a gust factor, which is the ratio of gust wind speed and the mean wind speed. In engineering applications, gusts are more often related to the standard deviation rather than to the mean wind speed. In this study we combined these two approaches to derive a new gust parametrization, and applied it to the heights relevant for wind energy.

We analysed long time series of high-resolution wind data from a 100-m-high meteorological tower located on flat grassland in Høvsøre, close to the western coast of Denmark. We excluded cases of weak winds (less than 5 m/s) since they are often very turbulent and, on the other hand, not so relevant from the point of view of wind turbines. The data cover various stability conditions. We further compare the Høvsøre results with those from two coastal/archipelago sites in the Baltic Sea with different surface conditions, and evaluate which features of gusts are related to observation practices (gust duration / averaging period length), surface roughness, height above the surface and, above all, meteorological conditions.

The new parametrization method for gusts is based on the standard deviation of the wind speed, which is parametrized on the basis of the surface friction velocity, Obukhov length, height above the surface, and the boundary-layer height. We compared the method with two earlier methods originally developed for the surface layer with reference height of 10 meters. It was found that the new method outperformed the two older methods: the effects of surface roughness, stability and the height above surface were well represented by the new method.