Gryning, Sven-Erik

Hasager, Charlotte Bay

Kelly, Mark C.

Mikkelsen, Torben

Pena Diaz, Alfredo

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Downloaded from orbit.dtu.dk on: Dec 20, 2017The modeling and observation of the long-term offshore vertical wind profile and windshearPena Diaz, Alfredo; Gryning, Sven-Erik; Mikkelsen, Torben Krogh; Hasager, Charlotte Bay; Kelly, Mark C.Published in:Proceedings of EWEA 2012 - European Wind Energy Conference &amp; ExhibitionPublication date:2012Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Pena Diaz, A., Gryning, S-E., Mikkelsen, T., Hasager, C. B., & Kelly, M. C. (2012). The modeling andobservation of the long-term offshore vertical wind profile and wind shear. In Proceedings of EWEA 2012 -European Wind Energy Conference & Exhibition European Wind Energy Association (EWEA).ABSTRACT ID:   16Theme: RESOURCE ASSESSMENT  Topic:  Resources offshoreTHE MODELING AND OBSERVATION OF THE LONG-TERM OFFSHORE VERTICAL WIND PROFILE AND WIND SHEAR  Alfredo Peña(1)  (F)  (P) Sven-Erik Gryning(1) Torben Mikkelsen(1) Charlotte  Hasager(1) Mark Kelly(1) (1) DTU Wind Energy, Roskilde, DenmarkIntroductionThe North Sea is the most promising offshore area for continuing with the development and harvesting of wind energy in Europe and also the regionwhere most of the offshore wind farms are planned to be installed [1]. With the purpose to assess wind resources and validate results from numericalweather prediction models (NWP) over the North Sea, vertical profiles of wind speed and direction have been observed from a network of wind lidarsinstalled at fixed offshore platforms and oil rigs as part of the EU FP7 NORSEWInD project. ApproachThe wind lidars are velocity-azimuth-display scanning units able to observe winds at 10 simultaneous levels in the range between 40 and 300 m abovethe instrument (a.i.), in the case of the pulsed wind lidars, and at 5 non-simultaneous levels between 20 and 160 m a.i., in the case of the continuouswave wind lidars. This allows us to study the temporal and vertical variation of the wind speed, direction and at some degree of turbulence (the windlidars measure wind characteristics within a much longer volume compared to that of the cup and sonic anemometers and thus they filter theturbulence). These observations lead to more accurate wind power estimates, since we do not only measure the wind speed at hub height over a longperiod (the so-called long-term), but also its vertical variation (wind shear) and the vertical variation in wind direction (wind veer) [2]. In this fashion weare able to estimate the long-term wind speed and direction profiles.Main body of abstractWe find that the wind profile shape, and therefore the wind shear, observed over heights’ ranges were large offshore wind turbines operate can varyconsiderably. This mainly depends on atmospheric stability and on the measuring height among others. From our results it is very clear that thecurrent wind engineering commonly-used wind shear norms and standards seem to be far too optimistic and not very conservative.ConclusionWe derive the long-term wind profile and wind shear at each specific offshore location. Since we do not have atmospheric stability measures at manyof the wind lidar locations, we make use of the advanced weather research and forecast (WRF) NWP model to estimate the surface turbulent fluxesand thus atmospheric stability. WRF, which is proven to estimate well the long-term characteristics of atmospheric stability at the Horns Rev offshorewind farm [3], is used to compute the long-term stability correction of the long-term wind profile based on similarity theory [4]. When the analysis isperformed based on the traditional diabatic wind profile models, there is generally an overestimation of the long-term wind resource. Suchoverestimation is avoided by using wind profile parameterizations, like those in [5], that include the boundary-layer height as a scale parameter. References:[1] Global offshore wind farms database: www.4coffshore.com/offshorewind[2] Wagner R., Courtney M., Gottschall J. and Lindelöw-Marsden P (2011) Accounting for the speed shear in wind turbine power performancemeasurement. Wind Energy, in press[3] Peña A. and Hahmann A. (2011) Atmospheric stability and turbulence fluxes at Horns Rev—and intercomparison of sonic, bulk and WRF modeldata. Wind Energy, in press[4] Kelly M. and Gryning S.-E. (2010) Long-term wind profiles based on similarity theory. Bound.-Layer Meteorol. 136:377—390[5] Peña A., Gryning S.-E and Hasager C. B. (2008) Measurements and modelling of the wind speed profile in the maribe atmospheric boundary layer.Bound.-Layer Meteorol. 129:479—495

The modeling and observation of the long-term offshore vertical wind profile and wind shear

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The modeling and observation of the long-term offshore vertical wind profile and wind shear

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