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Research Papers

# Speed and Direction Shear in the Stable Nocturnal Boundary Layer

[+] Author and Article Information
Kevin Walter

Wind Science and Engineering Research Center, Texas Tech University, P.O. Box 41023, Lubbock, TX 79409-1023kwalter@tradewindenergy.com

Christopher C. Weiss

Department of Geosciences, Texas Tech University, P.O. Box 42101, Lubbock, TX 79409chris.weiss@ttu.edu

Andrew H. Swift

Wind Science and Engineering Research Center, Texas Tech University, P.O. Box 41023, Lubbock, TX 79409-1023andy.swift@ttu.edu

Jamie Chapman

Civil Engineering, Texas Tech University, P.O. Box 41023, Lubbock, TX 79409-1023jamie.chapman@ttu.edu

Neil D. Kelley

National Renewable Energy Laboratory, National Wind Technology Center, 18200 State Highway 128, Boulder, CO 80303neil_kelley@nrel.gov

www.wind.ttu.edu.

J. Sol. Energy Eng 131(1), 011013 (Jan 08, 2009) (7 pages) doi:10.1115/1.3035818 History: Received December 30, 2007; Revised July 27, 2008; Published January 08, 2009

## Abstract

Numerous previous works have shown that vertical shear in wind speed and wind direction exist in the atmospheric boundary layer. In this work, meteorological forcing mechanisms, such as the Ekman spiral, thermal wind, and inertial oscillation, are discussed as likely drivers of such shears in the statically stable environment. Since the inertial oscillation, the Ekman spiral, and statically stable conditions are independent of geography, potentially significant magnitudes of speed and direction shear are hypothesized to occur to some extent at any inland site in the world. The frequency of occurrence of non-trivial magnitudes of speed and direction shear are analyzed from observation platforms in Lubbock, Texas and Goodland, Indiana. On average, the correlation between speed and direction shear magnitudes and static atmospheric stability are found to be very high. Moreover, large magnitude speed and direction shears are observed in conditions with relatively high hub-height wind speeds. The effects of speed and direction shear on wind turbine power performance are tested by incorporating a simple steady direction shear profile into the fatigue analysis structures and turbulence simulation code from the National Renewable Energy Laboratory. In general, the effect on turbine power production varies with the magnitude of speed and direction shear across the turbine rotor, with the majority of simulated conditions exhibiting power loss relative to a zero shear baseline. When coupled with observational data, the observed power gain is calculated to be as great as 0.5% and depletion as great as 3% relative to a no shear baseline. The average annual power change at Lubbock is estimated to be $−0.5%$.

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## Figures

Figure 1

Probability distribution for power law speed shear exponents in the 10–116m layer for all data (gray bars), unstable observations only (dashed line), and stable observations only (solid line)

Figure 2

Average diurnal variation of 10–116m in the layer static atmospheric stability (solid line) and the power law shear exponent (dotted line)

Figure 3

Probability distribution for direction shear in the 10–116m layer for all data (gray bars), unstable observations only (dashed line), and stable observations only (solid line)

Figure 4

Probability distribution for direction shear between the 10m and 116m heights for Lubbock, TX (gray bars) and between the 49m and 90m heights for Goodland, IN (black line)

Figure 5

Observed joint-probability of power law shear exponent and direction shear with extrapolated 85m wind speed greater than 8m∕s at Lubbock, TX.

Figure 6

Percent power change simulated for ranges of simultaneous speed and direction shear at hub-height wind speeds of 8m∕s and 10m∕s. Power change contour interval is 0.5%; the thick black line indicates zero change contour.

Figure 7

Distribution of potential power change relative to the no shear case at Lubbock (gray bars) and Indiana (black line)

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