Your Wind Resource - Finding Your Wind Speeds
The most important question to be answered before seriously considering the purchase and installation of a wind turbine is whether or not you have sufficient wind to produce enough power for your needs. The higher your average wind speed, the more power your turbine will produce and the shorter the payback period on your investment. The following discussion will provide you with information to help you make an informed decision about your wind resource.
A primary souce of wind speed data can be found at the U.S. Department of Energy "Windpowering America" web site http://www.windpoweringamerica.gov/wind_maps.asp. Clicking on the map on the right will allow you to view wind maps for your state. Other maps are avilable at truewind.com. The maps show wind speeds according to wind "Class". Generally, if you are in at least a class 3 area, you probably have enough wind to consider a wind turbine.
It is important to understand that the maps show wind speeds at certain elevations above ground, such as 50 meters or 70 meters. Most small wind turbines are installed on towers that are 80 to 120 feet (24-36 meters) so it is necessary to adjust the wind speeds shown on these maps for the lower elevation. We will show you how to do this in our article on Adjusting Windspeed Data.
Tree flagging can provide us with information about the wind resource. If local wind speeds are high enough, vegetation can become deformed. By looking at the amount of deformity, we can make a rough estimate of the average wind speed.
In order to quantify the amount of wind a method called the Griggs-Putnam Index of Deformity can be used. The image below shows the index. The first tree shows no deformity - it is symetrical. As the average wind speed increases the tree is subject to greater deformity, from slight flagging through complete flagging. As the average wind speed increases even further, trees are unable to grow vertically and can only survive close to the ground.
Brushing occurs when small tree limbs are bent downward so the tree has a brushy appearance. Flagging is when large limbs bend downward and limbs which are upwind are shorter than limbs which are downwind. Throwing happens when the actual trunk of the tree curves downwind. Carpeting occurs when the wind is so strong that the tree trunk cannot rise up but instead grows along the ground.