Atmospheric stability and its influence on wind turbine loads

Simulations of wind turbine loads for the NREL 5 MW reference wind turbine under diabatic wind conditions are performed for mean wind speeds between 3 { 16 m/s at the turbine hub height. The loads are quantified as the cumulative sum of the damage equivalent load for different wind speeds that are weighted according to the wind speed and stability distribution.

It is observed that atmospheric stability influences the tower and rotor loads. The difference in the calculated tower loads using diabatic wind conditions and those obtained assuming neutral conditions only is approximately 16%, whereas the difference for the rotor loads is up to 11%. The blade loads are hardly influenced by atmospheric stability, where the difference between the calculated loads using diabatic and neutral input wind conditions is less than 1%.

The wind profiles and turbulence under diabatic conditions have contrasting influences on the loads, e.g. under stable conditions, loads induced by the wind profile are larger due to increased wind shear, whereas those induced by turbulence are lower due to less turbulent energy. The tower base loads are mainly influenced by diabatic turbulence, whereas the rotor loads are influenced by diabatic wind proles.

The blade loads are influenced by both, diabatic wind profile and turbulence, that leads to nullifying the contrasting influences on the loads. The importance of using a detailed boundary-layer wind profile model is also demonstrated. The difference in the calculated blade and rotor loads is up to 6% and 8% respectively, when only the surface-layer wind profile model is used in comparison to those obtained using a boundary-layer wind profile model.

Finally, a comparison of the calculated loads obtained using site-specific and IEC wind conditions is carried out. It is observed that the IEC loads are up to 75% larger than those obtained using site-specific wind conditions.