Speed and Direction Shear in the Stable Nocturnal Boundary Layer

Walter, Kevin; Weiss, Christopher C.; Swift, Allegra; Chapman, Jamie; Kelley, Neil

doi:10.1115/1.3035818

Cited by 47 publications

(72 citation statements)

References 9 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…4f) when calculated either across the turbine rotor disk (between 40 and 120 m) or across the extent sampled by the lidar (between 40 and 220 m). Note that the lowest measurement available from the lidar is at 40 m above the surface, and so these α values are lower than would be expected for comparison to other studies that calculate shear between a surface-layer measurement (10 m, for example) and altitudes at the turbine rotor disk (Walter et al 2009). The marked diurnal variations in atmospheric stability are an important trait of atmospheric behaviour in the US Midwest region.…”

Section: Atmospheric Boundary-layer Properties As Observed With Lidarmentioning

confidence: 80%

The Effect of Wind-Turbine Wakes on Summertime US Midwest Atmospheric Wind Profiles as Observed with Ground-Based Doppler Lidar

Rhodes

Lundquist

2013

Boundary-Layer Meteorol

105

104

View full text Add to dashboard Cite

We examine the influence of a modern multi-megawatt wind turbine on wind and turbulence profiles three rotor diameters (D) downwind of the turbine. Light detection and ranging (lidar) wind-profile observations were collected during summer 2011 in an operating wind farm in central Iowa at 20-m vertical intervals from 40 to 220 m above the surface. After a calibration period during which two lidars were operated next to each other, one lidar was located approximately 2D directly south of a wind turbine; the other lidar was moved approximately 3D north of the same wind turbine. Data from the two lidars during southerly flow conditions enabled the simultaneous capture of inflow and wake conditions. The inflow wind and turbulence profiles exhibit strong variability with atmospheric stability: daytime profiles are well-mixed with little shear and strong turbulence, while nighttime profiles exhibit minimal turbulence and considerable shear across the rotor disk region and above. Consistent with the observations available from other studies and with wind-tunnel and large-eddy simulation studies, measurable reductions in wake wind-speeds occur at heights spanning the wind turbine rotor (43-117 m), and turbulent quantities increase in the wake. In generalizing these results as a function of inflow wind speed, we find the wind-speed deficit in the wake is largest at hub height or just above, and the maximum deficit occurs when wind speeds are below the rated speed for the turbine. Similarly, the maximum enhancement of turbulence kinetic energy and turbulence intensity occurs at hub height, although observations at the top of the rotor disk do not allow assessment of turbulence in that region. The wind shear below turbine hub height (quantified here with the power-law coefficient) is found to be a useful parameter to identify whether a downwind lidar observes turbine wake or free-flow conditions. These field observations provide data for validating turbine-wake models and wind-tunnel observations, and for guiding assessments of the impacts of wakes on surface turbulent fluxes or surface temperatures downwind of turbines.

show abstract

Section: Atmospheric Boundary-layer Properties As Observed With Lidarmentioning

confidence: 80%

The Effect of Wind-Turbine Wakes on Summertime US Midwest Atmospheric Wind Profiles as Observed with Ground-Based Doppler Lidar

Rhodes

Lundquist

2013

Boundary-Layer Meteorol

105

104

View full text Add to dashboard Cite

show abstract

“…Furthermore the wind veer, i.e. variation of the direction with altitude, also influences the turbine power performance (Walter et al, 2009). Similarly to the wind shear, the wind veer modifies the angle of attack of the wind on the turbine blade, which then varies with the altitude.…”

Section: Summary and Discussionmentioning

confidence: 99%

Accounting for the speed shear in wind turbine power performance measurement

Wagner¹,

Courtney²,

Gottschall³

et al. 2011

Wind Energy

147

109

View full text Add to dashboard Cite

Abstract:The power curve of a wind turbine is the primary characteristic of the machine as it is the basis of the warranty for it power production. The current IEC standard for power performance measurement only requires the measurement of the wind speed at hub height and the air density to characterise the wind field in front of the turbine. However, with the growing size of the turbine rotors during the last years, the effect of the variations of the wind speed within the swept rotor area, and therefore of the power output, cannot be ignored any longer. Primary effects on the power performance are from the vertical wind shear and the turbulence intensity. The work presented in this thesis consists of the description and the investigation of a simple method to account for the wind speed shear in the power performance measurement. Ignoring this effect was shown to result in a power curve dependant on the shear condition, therefore on the season and the site. It was then proposed to use an equivalent wind speed accounting for the whole speed profile in front of the turbine. The method was first tested with aerodynamic simulations of a multi-megawatt wind turbine which demonstrated the decrease of the scatter in the power curve. A power curve defined in terms of this equivalent wind speed would be less dependant on the shear than the standard power curve. The equivalent wind speed method was then experimentally validated with lidar measurements. Two equivalent wind speed definitions were considered both resulting in the reduction of the scatter in the power curve. As a lidar wind profiler can measure the wind speed at several heights within the rotor span, the wind speed profile is described with more accuracy than with the power law model. The equivalent wind speed derived from measurements, including at least one measurement above hub height, resulted in a smaller scatter in the power curve than the equivalent wind speed derived from profiles extrapolated from measurements at hub height and below only. It is well established that the turbulence intensity also influences the power performance of a wind turbine. Two ways of accounting for the turbulence were tested with the experimental data: an adaptation of the equivalent wind speed so that it also accounts for the turbulence intensity and the combination of the equivalent wind speed accounting for the wind shear only with the turbulence normalising method for turbulence intensity suggested by Albers. The second method was found to be more suitable for normalising the power curve for the turbulence intensity. Using the equivalent wind speed accounting for the wind shear in the power performance measurement was shown to result in a more repeatable power curve than the standard power curve and hence, in a better annual energy production estimation. Furthermore, the decrease of the scatter in the power curve corresponds to a decrease of the category A uncertainty in power, resulting in a smaller uncertainty in estimated AEP.The thesis is submitted to the Danish Tec...

show abstract

“…Therefore, if we preferentially increase wind speeds near the surface, low-level shear should decrease. Percentage changes in shear are large for convective (−23 %) and near-neutral (−13 %) conditions, though this result is mainly a consequence of the minimal shear present to begin with during those periods (shear is larger during stable conditions, and largest in very stable conditions Walter et al 2009). Large reductions in shear were also produced for very stable conditions (−15 %), mainly because wind speeds increase at 35 m, but the roughness effect is typically capped below 125 m. Therefore, shear throughout the rotor layer is reduced.…”

Section: Impacts Across the Wind Farm In Various Stability Regimesmentioning

confidence: 93%

“…As the power law is simpler than the logarithmic-wind profile, it is often used in the wind industry to represent the rotor-layer shear (Peterson and Hennessey 1977). Values of α are known to vary considerably with boundary-layer stability (Walter et al 2009). …”

Section: Impacts Across the Wind Farm In Various Stability Regimesmentioning

confidence: 99%

Could Crop Height Affect the Wind Resource at Agriculturally Productive Wind Farm Sites?

Vanderwende

Lundquist

2015

Boundary-Layer Meteorol

View full text Add to dashboard Cite

The collocation of cropland and wind turbines in the US Midwest region introduces complex meteorological interactions that could influence both agriculture and wind-power production. Crop management practices may affect the wind resource through alterations of land-surface properties. We use the weather research and forecasting (WRF) model to estimate the impact of crop height variations on the wind resource in the presence of a large turbine array. A hypothetical wind farm consisting of 121 1.8-MW turbines is represented using the WRF model wind-farm parametrization. We represent the impact of selecting soybeans rather than maize by altering the aerodynamic roughness length in a region approximately 65 times larger than that occupied by the turbine array. Roughness lengths of 0.1 and 0.25 m represent the mature soy crop and a mature maize crop, respectively. In all but the most stable atmospheric conditions, statistically significant hub-height wind-speed increases and rotor-layer wind-shear reductions result from switching from maize to soybeans. Based on simulations for the entire month of August 2013, wind-farm energy output increases by 14 %, which would yield a significant monetary gain. Further investigation is required to determine the optimal size, shape, and crop height of the roughness modification to maximize the economic benefit and minimize the cost of such crop-management practices. These considerations must be balanced by other influences on crop choice such as soil requirements and commodity prices.

show abstract

Speed and Direction Shear in the Stable Nocturnal Boundary Layer

Cited by 47 publications

References 9 publications

The Effect of Wind-Turbine Wakes on Summertime US Midwest Atmospheric Wind Profiles as Observed with Ground-Based Doppler Lidar

The Effect of Wind-Turbine Wakes on Summertime US Midwest Atmospheric Wind Profiles as Observed with Ground-Based Doppler Lidar

Accounting for the speed shear in wind turbine power performance measurement

Could Crop Height Affect the Wind Resource at Agriculturally Productive Wind Farm Sites?

Contact Info

Product

Resources

About