Extending the diabatic surface layer wind shear profile for offshore wind energy

Highlights

Surface layer wind profiles are extended offshore based on work of Gryning.

The wind profile is parametrized as a continuous function of atmospheric stability.

The wind profile is validated up to a maximum of 315 m height far offshore.

For wind energy, blade root flapwise loads decrease for stable conditions.

The rotor disc kinetic energy flux also decreases for stable conditions.

Abstract

In this research the diabatic surface layer wind shear model is extended for offshore wind energy purposes to higher altitudes based on Gryning’s wind profile and the resistance functions proposed by Byun. The wind profile is in theory applicable up to the boundary layer height, which is parametrized with the Rossby-Montgommery equation. The coefficient c of the Rossby-Montgommery equation is found to be stability dependent with decreasing values up to 0.04 for stable conditions and increasing values up to 0.17 for unstable conditions. The proposed shear profile has been validated with 1 year of offshore observation data, and a significant improvement in accuracy is found compared to traditional surface layer shear profiles or power laws. The influence of adopting this extended shear profile for wind energy is analysed in terms of the kinetic energy flux and blade root fatigue loads experienced by a wind turbine. It is found that, especially for stable conditions, results deviate significantly compared to using the traditional surface layer shear profile. The kinetic energy flux decreases by up to 15%.

Keywords

  • Wind profile;
  • Boundary-layer meteorology;
  • Monin-Obukhov similarity theory;
  • Offshore atmosphere;
  • Atmospheric stability;
  • Wind energy

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