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Wind Energy Science The interactive open-access journal of the European Academy of Wind Energy
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 30 Jul 2019

Submitted as: research article | 30 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Wind Energy Science (WES).

Comparing Abnormalities in Onshore and Offshore Vertical Wind Profiles

Mathias Møller1, Piotr Domagalski2,3, and Lars Roar Sætran1 Mathias Møller et al.
  • 1Department of Energy and Process Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway
  • 2Institute of Turbomachinery, Lodz University of Technology, Wolczanska 219/223, 90-924, Lodz, Poland
  • 3WindTak LLC, Wroblewskiego street 38a, 93-578 Lodz, Poland

Abstract. Understanding the vertical wind profile is paramount for design & operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues. As wind turbines grow in size and development transitions offshore, issues such as shallow surface layers, low-level jets and internal boundary layers are raising questions to the applicability of the commonly used Monin-Obukhov similarity theory to accurately describe the vertical wind development to modern wind turbine hub heights. In this study the 10-minute averaged vertical wind profile up to a minimum elevation of 100 m is analyzed through measurements collected from seven sites which represent a span of conditions. Three sites are located offshore in the North/Baltic Sea with varying fetch, two onshore by the Norwegian coast, one further onshore by the Danish coast, and one is an inland forested site in Sweden. Through analysis of data series ranging from 8 months to several years depending on the site, the wind profile has been quantitatively categorized according to the number of exhibited local maxima which are not possible within Monin-Obukhov similarity theory. The results reveal that the occurrence of local maxima scales inversely to the roughness length, causing 65–75 % abnormal profiles offshore which decreases as the location transitions from offshore to coastal to further inland, and is lowest at the forested site. The results indicate that issues in predicting the vertical wind profile are most prevalent offshore, where very stable inflections cause severe deviations which may be related to an offshore internal boundary layer. These findings suggest that there is evident need of an improved vertical wind profile description in order to improve the accuracy of power predictions and load calculations, especially at offshore and coastal sites.

Mathias Møller et al.
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Status: open (until 23 Oct 2019)
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Mathias Møller et al.
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Frøta wind data P. Domagalski and L. R. Sætran

Mathias Møller et al.
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Short summary
This work has analyzed historical data of 10-minute averaged wind speed measurements to investigate the accuracy of the commonly used equations for describing the wind velocity as a function of the height above ground. The results of analyzing data from a wide range of sites shows that the common equations do not sufficiently describe certain physical phenomenon, especially at offshore and coastal locations. The results imply that there is need for more advanced models.
This work has analyzed historical data of 10-minute averaged wind speed measurements to...