Community Wind Handbook/Understand Your Wind Resource and Conduct a Preliminary Estimate
Understand Your Wind Resource, Conduct a Preliminary Estimate
Prior to selecting the size of the wind system for the project, it is important to understand the wind resource at the location. A preliminary estimate helps determine which model type will best suit the site prior to spending the time and energy researching the system pricing.
Some small wind project developers use state wind maps to conservatively estimate the wind resource at turbine hub height. While these maps can provide a general indication of good or poor wind resources, they do not provide a resolution high enough to identify local site features.
State wind maps cannot include information on complex terrain, ground cover, wind speed distribution, direction distribution, turbulence intensity, and other local effects. Purchased maps or services can often provide higher resolution and more flexibility with zooming, orientation, and additional features. Pay attention to a map’s height above ground as it relates to the potential project’s tower height. Adjusting the wind speed for the height difference between the map and the turbine height adds a potential source of error depending on the wind shear exponent that is selected, and the greater the height difference the greater the potential error. Therefore, for small wind generator applications, 30- to 40-m wind maps are far more useful than 10-, 60-, 80-, or 100-m wind maps. It is also important to understand the resolution of the wind map or model-generated data set. If the resolution is lower than the terrain features, adjustments will be needed to account for local terrain effects.
Local airport or weather stations can offer local wind data, but these data may be less reliable than actual site data. If airport data (typically recorded at 30 ft or 10 m above ground) or weather station data (typically recorded at 5 to 20 ft above ground) are used, inquire not only about the site’s current equipment and location but also if it is historically consistent with the data collection equipment and siting. Equipment at these sites is not primarily intended for wind resource assessment, so it may not be positioned at an appropriate height or in a location free of obstructions. Unfortunately, airport and weather stations are usually far from the site of interest, with considerably different orography, tree cover, and monitoring height, making these data of questionable usefulness. Given the expertise required to effectively establish and correlate wind resource data, the data provided by airport and weather stations may only provide a rough screening assessment.
Average wind speeds increase with height and may be 15% to 25% greater at a typical wind turbine hub height of 80 ft (24 m) than those measured at airport anemometer heights. The National Climatic Data Center collects data from airports in the United States and makes wind data summaries available for purchase.
An additional useful indirect measurement of the wind resource is the observation of an area’s vegetation. Trees, especially conifers or evergreens, can be permanently deformed by strong winds. This deformity, known as “flagging,” has been used to estimate the average wind speed for an area.