Impacts of the low-level jet's negative wind shear on the wind turbine

Gutierrez, Walter; Ruiz-Columbie, Arquimedes; Tutkun, Murat; Castillo, Luciano

doi:10.5194/wes-2-533-2017

Cited by 36 publications

(32 citation statements)

References 22 publications

(26 reference statements)

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“…Wind shear and turbulence intensity associated with low-level jets also differ substantially from that assumed under 'standard' conditions. Low-level jets modify wind power performance and loading by impacting wake recovery rates and vertical profiles of wind 5 speed, direction and turbulence (Wharton and Lundquist, 2012;Bhaganagar and Debnath, 2014;Park et al, 2014;Gutierrez et al, 2017). Thus, for a complete assessment of loads and power, it is important to have a broad understanding of the sitespecific low-level jet characteristics: how often do they occur, under which circumstances, at what height and with what strength, and what mechanisms are responsible for their formation?…”

Section: Introductionmentioning

confidence: 99%

Low-level jets over the North Sea based on ERA5 and observations: together they do better

Kalverla¹,

Steeneveld²,

Holtslag³

2019

Preprint

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Abstract. Ten years of ERA5 reanalysis data are combined with met-mast and LiDAR observations from ten offshore platforms to investigate low-level jet characteristics over the Dutch North Sea. The objective of this study is to combine the best of two worlds: (1) ERA5 data with large spatiotemporal extent but inherent accuracy limitations due to a relatively coarse grid and an incomplete representation of physical processes, and (2) observations that provide more reliable estimates of the measured quantity, but are limited in both space and time. We demonstrate the effect of time and range limitations on the reconstructed wind climate, with special attention paid to the impact on low-level jets. For both measurement and model data, the representation of wind speed is biased. The limited temporal extent of observations leads to a wind speed bias on the order of 1 m s−1. In part due to data-assimilation strategies that cause abrupt discontinuities in the diurnal cycle, ERA5 also exhibits a wind speed bias of approximately 0.5 m s−1. Representation of low-level jets in ERA5 is poor in terms of a one-to-one correspondence, and the jets appear vertically displaced (`smeared out'). However, climatological characteristics such as the shape of the seasonal cycle and the affinity with certain circulation patterns are represented quite well, albeit with different magnitudes. We therefore experiment with various methods to adjust modelled low-level jet rate to the observations or, vice versa, to correct for the erratic nature of the short observation periods using long-term ERA5 information. While quantitative uncertainty is still quite large, the presented results provide valuable insight into North Sea low-level jet characteristics. These jets occur predominantly for circulation types with an easterly component, with a clear peak in spring, and concentrate along the coasts at heights between 50–200 m. Further, it is demonstrated that these characteristics can be used as predictors to infer the observed low-level jet rate from ERA5 data with reasonable accuracy.

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Section: Introductionmentioning

confidence: 99%

Low-level jets over the North Sea based on ERA5 and observations: together they do better

Kalverla¹,

Steeneveld²,

Holtslag³

2019

Preprint

View full text Add to dashboard Cite

show abstract

“…Local maxima at wind turbine hub heights were shown by Wagner et al (2009) to significantly impact the power output due to the impact the negative shear has on the available energy across the rotor area. The inflections can however also have positive consequences as Gutierrez et al (2017) found the negative shear in the top half of a low-level jet to dampen motion, forces and moments acting on the turbine tower and nacelle.…”

Section: Height Of Local Maximum 15mentioning

confidence: 99%

“…The increased occurrence of local maxima at higher elevations should be a concern as these elevations are within common rotor swept areas of modern turbines and can have a direct impact on the available energy in the wind. The impact on the turbine loads may however not be only negative, 15 as Gutierrez et al (2017) showed that a negative shear dampened motion, moments and forces acting on the turbine tower and nacelle.…”

Section: Offshore Sitesmentioning

confidence: 99%

Comparing Abnormalities in Onshore and Offshore Vertical Wind Profiles

Møller¹,

Domagalski²,

Sætran³

2019

Preprint

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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 5 vertical wind profile up to a minimum elevation of 100m 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 10 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 15 calculations, especially at offshore and coastal sites.

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“…Local maxima at wind turbine hub heights were by Wagner et al (2009) shown to significantly impact the power output due to the impact the negative shear has on the available energy across the rotor area. The inflections can however also have positive consequences as Gutierrez et al (2017) found the negative shear in the top half of a low-level jet to dampen motion, forces and moments on the turbine tower and nacelle. Figure 6a shows the height occurrence of local maximum at the onshore sites.…”

Section: Height Of Local Maximummentioning

confidence: 99%

Reply to Anonymous Referee 1

Møller¹

2019

Preprint

View full text Add to dashboard Cite

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 100m 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.

show abstract

Impacts of the low-level jet's negative wind shear on the wind turbine

Cited by 36 publications

References 22 publications

Low-level jets over the North Sea based on ERA5 and observations: together they do better

Low-level jets over the North Sea based on ERA5 and observations: together they do better

Comparing Abnormalities in Onshore and Offshore Vertical Wind Profiles

Reply to Anonymous Referee 1

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