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Wind Resource Assessment
Resource Assessment and Siting
Just as in real estate, in wind development, location is everything. The first consideration in choosing a site is the wind resource. Without a good wind resource, no wind project will ever be viable. There are many tools available to examine the wind resource, and this examination goes on throughout project development. The wind resource average wind speed, however, is not the only determining factor when choosing a site. Proximity to transmission lines and roads, terrain, and land use are just a few of the other factors that require consideration when siting a wind project.
Resource Assessment - Wind Vocabulary
Describing the wind resource at a site requires the use of “wind vocabulary.” The following terms are necessary to explain exactly how the wind behaves, which in turn leads to an estimate of the energy produced by a project.
Wind Power Density (Wind Class): The power density of the wind determines how much energy can be extracted by a wind turbine and is influenced by two factors: wind speed and air density. The power density is proportional to the cube of the wind speed, which means that every time the wind speed doubles, the power density increases by a factor of eight. As air density decreases at higher altitudes and temperatures, the power density decreases proportionally. Wind farms built in the mountains and in hot climates must take this into account when estimating energy production.
To simplify the comparison of the wind resource at different sites, wind power density has been standardized in Wind Power Classes. These classes are based on wind speeds taken at specific heights, generally using sea level air densities. Since the wind speed at any site will vary with height due to the effects of the terrain on the wind flow, the wind class is often defined at more than one height. A site with a measured average wind speed of 5.8 m/s at a height of 10 m and 7.2 m/s at a height of 50 m has a Class 4 wind resource. This table displays the wind speeds associated with Wind Power Classes at two heights, assuming sea level air density.
Shear: Friction created by a site’s vegetation and terrain slows the wind close to the ground. The shear of a site determines how quickly the wind speed increases with increasing height above the ground. This number can be used to estimate the wind speed at the height of a proposed turbine even if the wind speed data were taken at a lower height.
Turbulence: Wind obstacles such as trees, buildings, and large land features affect the smoothness of the wind. When the wind is turbulent, the forces on the turbine blades can vary, affecting the production of the turbine.
Frequency (Weibull/Rayleigh) Distribution: The basic tool for estimating energy production at a site is the frequency distribution. A frequency distribution shows the percentage of time that the wind is blowing at certain speeds over the course of a study period. The wind speeds are binned, meaning that wind speeds between 0 and 1 m/s are binned as 1 m/s, wind speeds between 1 and 2 m/s are binned as 2 m/s, and so on. Once the wind speeds are binned, a count of all the wind speeds falling within each bin can be made and the frequency of the wind occurring in a particular bin calculated. The shape of the curve created by these bins is often referred to as a Weibull distribution. Average wind speed at a site can be influenced by prevailing winds or by storm-driven winds. Wind turbine performance is best in an environment with smooth, prevailing winds. A frequency distribution can clarify which type of wind is contributing to the average wind speed. Manufacturers often use a simplified version of the Weibull distribution, known as the Rayleigh distribution, to show estimates of annual energy production when only an annual average wind speed is known. The Rayleigh distribution assumes a generic distribution and should only be used for preliminary energy production estimates. This figure shows examples of frequency distributions at a site with strong prevailing winds and at a site with storm-driven winds.
Wind Rose: A wind rose is a useful tool for delineating the directions from which the wind blows. It displays not only wind direction, but also the percentage of the power in the wind that comes from that direction. Alternatively, it can display the average wind speed by direction. This is a valuable tool for project layout and micro-siting. This figure shows an example of a wind rose.
Initial Assessment
A quick and easy initial assessment of the wind resource at a potential site is essential to prevent wasting time and money preparing an unsuitable site. There are a couple of ways an initial assessment can be done, but the best way is to use both of the tools listed below.
- Wind maps: Contemporary computer climate models can estimate average wind speeds at a location: http://www.windmaps.org/default.asp
- Site visit: A site visit cannot be used to definitively determine the wind resource, but can be used to look for signs that there may or may not be a good wind resource.
If one of the above resources indicates a Class 3 or greater wind resource, it may be worthwhile to investigate the site further either through a preliminary study with a low-cost system, or a more in depth system with a measurement system at the height of the proposed wind turbine. A preliminary study using a relatively short meteorological tower, i.e. between ten and twenty meters tall, can serve to confirm or refute the indicated wind resource. If none of these resources indicate an adequate annual wind speed, an alternate site should be considered.
These tools as well as other siting factors such as terrain, transmission access, land use and utility service territory are discussed in greater detail here.
