Wind Resource Assessment Overview
The first step in developing a wind project is to locate and quantify the wind resource. The magnitude of the wind and the characteristics of the resource are the largest factors in determining a potential site’s economic and technical viability. There are three basic steps to identifying and characterizing the wind resource: prospecting, validating, and micrositing.
The process of locating sites for wind energy development is similar to exploration for other resources, such as minerals and petroleum. Thus, the term prospecting is often used to describe the identification and preliminary evaluation of a wind resource area. Prospecting includes identifying potentially windy sites within a fairly large region — such as a state, county, or utility service area — and investigating the development and general suitability of these sites for wind energy projects. Wind maps provide a good starting point for obtaining a general sense of a region’s wind characteristics. Wind maps are available on a national scale and in greater detail for many states. Such maps can easily miss good wind sites, however, and are inappropriate to use alone for site selection purposes. For example, many sites with higher wind speeds are found within areas specified as a lower wind speed class. An experienced meteorologist can help determine whether a more detailed look is warranted.
Once a general sense of the wind resource characteristics is obtained, additional information can be garnered from a variety of sources to help identify less obvious resources or obtain more information on favorable sites. Topographic maps are used to identify passes, ridges, and other features that may enhance the wind speed. The following features may provide insight into the quality of the wind resources:
High Wind Speeds
- Gaps, passes, and gorges
- Long valleys extending down from mountains
- High-elevation plains and plateaus
- Exposed ridges and mountain summits
- Coastlines and immediate inland strips with a minimum of relief and vegetation
- Upwind and crosswind corners of islands.
Low Wind Speeds
- Valleys perpendicular to the prevailing winds
- Sheltered basins
- Short and/or narrow valleys and canyons
- Areas of high surface roughness (e.g., dense vegetation).
Existing wind speed data can also help provide a picture of the wind characteristics in a region. Existing data are available from weather stations, airports, and other organizations. Most existing and publicly available wind data have been collected for purposes other than wind energy, and the source and accuracy of existing data should be considered before using it. Nonetheless, such data are often valuable for generally characterizing the resource and providing a long-term record for correlations with short-term data. The data may also provide important information about the seasonal and diurnal pattern of the wind in a particular vicinity.
For areas that appear promising based on the maps, existing data, or topography, a site visit generally provides additional insight to the development potential. In areas where no data exist, terrain or vegetation indicators may be used to identify a suspected resource. Valuable information may also be gained by talking to farmers or ranchers who live in the area. A site visit also provides an opportunity to survey the location to site a meteorological tower and begin the validation process.
Validating is the process of obtaining high-quality data to verify the magnitude and characteristics of the wind resource at a site. It begins with the development of a monitoring plan and includes the installation of wind measurement equipment as well as the analysis of site-specific wind and other data. During this phase, instrumented meteorological (met) towers are deployed to collect time-series data. Measurement sensors typically include anemometers at multiple heights (for wind speed), wind vanes (for direction), temperature gauges, and barometers (for pressure). For wind energy purposes, it is important to collect data at heights as high as possible to be able to identify the wind characteristics likely to be experienced by a wind turbine in that location. It is also necessary to collect wind speed data at multiple heights so that the wind shear (variation in wind speed with height above the ground) can be determined.
Wind has seasonal and diurnal patterns. As a result, monitoring programs must last for a minimum of 1 year. Longer monitoring programs will help to reduce the uncertainty in the data because of year-to-year variations. Data are commonly sampled at intervals of 1 second or more and averaged in increments of 10 minutes or, at most, 1 hour.
Micrositing is a more detailed level of investigation and analysis, usually conducted using a combination of complex modeling tools and high-density met tower data. Micrositing is intended to identify the best location for individual wind turbines within a project site. Shorter data collection periods are common if long-term data are available for the vicinity.
WINDExchange provides residential-scale 30-meter wind maps, 80-meter land-based wind maps and wind resource potential estimates, 90-meter offshore wind resource maps and wind resource potential estimates, 110- and 140-meter potential wind capacity maps, and information about state anemometer loan programs and data to help states and regions build capacity to support and accelerate wind energy deployment.