Adjusting Wind Speeds
When the wind blows close to the ground, obstructions near the ground create turbulence and friction, lowering the average wind speed. The higher the obstructions, the greater the turbulence and the lower the windspeed. As a general rule, windspeed increases with height.
If we know the windspeed at a particular height, we can calculate what the wind speed should be at a lower elevation. In order to determine the probable wind speed at a proposed turbine elevation for a particular site, it is necessary to adjust the published wind speed data for the lower elevation. The approximate change of speed with height for different types of topography can be calculated from the following equation:
V2 = V1 * (H2/H1)n
where V2 is the unknown wind speed at our tower height H2 above ground, V1 is the known wind speed (from the wind chart) at a second height H1, and the exponent n is the change in wind velocity with height. Values for n are listed in the following table for different types of ground cover and topography. If the wind comes across a fallow crop field or body of water, you do not have to reach as high for greater wind speeds as you would in a forest or suburb where obstructions tend to cause turbulence in wind flow.
Table 1. Height Scaling Exponents (shear exponent)
|Low grass or fallow ground||0.15|
|High grass or low row crops||0.18|
|Tall row crops of low woods||0.20|
|High woods with many trees, suburbs, small towns||0.30|
|General rule of thumb (independent of ground cover)||0.14|
As can be seen from the table, higher values of n indicate higher wind shear and greater levels of wind turbulence. It should be pointed out that the shear exponents shown are approximate. Local topography can have substantial effects on turbulence, introducing uncertainty in determining the correct exponent. The only certain way to know the expected wind speed for a particular site is to erect meteorological tower at the height of the turbine and measure wind speeds and direction. Wind speed studies are usually made for a year or more in order to account for seasonal variations.
Let us consider the calculation for a hyphothetical site. We have the following information:
1. The wind speed map shows an average wind speed of 14 MPH at 230 feet above ground level.
2. We plan to install a turbine on a 100 foot tower.
3. The area surrounding the turbine consists of high grass.
From the table, we see that high grass corresponds to a wind shear exponent of 0.18. Our formula would be as follows:
V2 = 14 * ( 100 / 230 )0.18
V2 = 12.05 MPH
So, using our formula we see that we lose about 2 MPH of average wind speed by going from a height of 230 feet down to 100 feet.
Aerostar® Wind Turbines www.aerostarwind.com