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# Wind profile power law

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### Wind profile power law

A wind turbine smoke test showing how the wind speed is higher at the top of the rotor than at the bottom.

The wind profile power law is a relationship between the wind speeds at one height, and those at another.

The power law is often used in wind power assessments[1][2] where wind speeds at the height of a turbine (>~ 50 metres) must be estimated from near surface wind observations (~10 metres), or where wind speed data at various heights must be adjusted to a standard height[3] prior to use. Wind profiles are generated and used in a number of atmospheric pollution dispersion models.[4]

The wind profile of the atmospheric boundary layer (surface to around 2000 metres) is generally logarithmic in nature and is best approximated using the log wind profile equation that accounts for surface roughness and atmospheric stability. The wind profile power law relationship is often used as a substitute for the log wind profile when surface roughness or stability information is not available.

The wind profile power law relationship is:

\frac{u}{u_r} = \bigg(\frac{z}{z_r} \bigg)^\alpha

where u is the wind speed (in metres per second) at height z (in metres), and u_r is the known wind speed at a reference height z_r. The exponent (\alpha) is an empirically derived coefficient that varies dependent upon the stability of the atmosphere. For neutral stability conditions, \alpha is approximately 1/7, or 0.143.

In order to estimate the wind speed at a certain height z, the relationship would be rearranged to:

u = u_r\bigg(\frac{z}{z_r} \bigg)^\alpha

The value of 1/7 for α is commonly assumed to be constant in wind resource assessments, because the differences between the two levels are not usually so great as to introduce substantial errors into the estimates (usually < 50 m). However, when a constant exponent is used, it does not account for the roughness of the surface, the displacement of calm winds from the surface due to the presence of obstacles (i.e., zero-plane displacement), or the stability of the atmosphere.[5][6] In places where trees or structures impede the near-surface wind, the use of a constant 1/7 exponent may yield quite erroneous estimates, and the log wind profile is preferred. Even under neutral stability conditions, an exponent of 0.11 is more appropriate over open water (e.g., for offshore wind farms), than 0.143,[7] which is more applicable over open land surfaces.

## Wind power density

Estimates of wind power density are presented as wind class, ranging from 1 to 7. The speeds are average wind speeds over the course of a year,[8] although the frequency distribution of wind speed can provide different power densities for the same average wind speed.[9]

Class 10 m (33 ft) 30 m (98 ft) 50 m (164 ft)
Wind power density (W/m2) Speed m/s (mph) Wind power density (W/m2) Speed m/s (mph) Wind power density (W/m2) Speed m/s (mph)
1

## References

1. ^ Elliott, D.L., C.G. Holladay, W.R. Barchet, H.P. Foote, and W.F. Sandusky, 1986, Pacific Northwest Laboratory, Richland, WA. Wind Energy Resource Atlas of the United States
2. ^ Peterson, E.W. and J.P. Hennessey, Jr., 1978, On the use of power laws for estimates of wind power potential, J. Appl. Meteorology, Vol. 17, pp. 390-394
3. ^ Robeson, S.M., and Shein, K.A., 1997, Spatial coherence and decay of wind speed and power in the north-central United States, Physical Geography, Vol. 18, pp. 479-495
4. ^ Beychok, Milton R. (2005).
5. ^ Touma, J.S., 1977, Dependence of the wind profile power law on stability for various locations, J. Air Pollution Control Association, Vol. 27, pp. 863-866
6. ^ Counihan, J., 1975, Adiabatic atmospheric boundary layers: A review and analysis of data from the period 1880-1972, Atmospheric Environment, Vol.79, pp. 871-905
7. ^ Hsu, S.A., E.A. Meindl, and D.B. Gilhousen, 1994, Determining the power-law wind-profile exponent under near-neutral stability conditions at sea, J. Appl. Meteor., Vol. 33, pp. 757-765
8. ^ Classes of wind power density at 10 m and 50 m
9. ^ Comparison of annual average wind power at three sites with identical wind speeds.
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