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Naturally, wind energy developers are very interested in the energy that can be extracted from the wind, and how this varies by location. Wind is ubiquitous, and in order to make the choice of potential project sites an affordable and manageable process, some indication of the relative size of the “wind resource” across an area is very useful. The wind resource is usually expressed as a wind speed or energy density, and typically, there will be a cut-off value below which the energy that can be extracted is insufficient to merit a wind farm development.
The best, most accurate indication of the wind resource at a site is through on-site measurement, using an anemometer and wind vane (described in detail later in this chapter). This is, however a fairly costly, and time-consuming process.
On a broader scale, wind speeds can be modelled using computer programmes, which describe the effects on the wind of parameters such as elevation, topography and ground surface cover. These models must be primed with some values at a known location, and usually this role is fulfilled by local meteorological station measurements or other weather-related recorded data or extracted from numerical weather prediction models, such as those used by national weather services.
Typically, these wind-mapping programmes will derive a graphical representation of mean wind speed (for a specified height) across an area. This may take the form of a 'wind atlas,' which represents the wind speed over flat homogenous terrain, and requires adjustments to provide a site-specific wind speed prediction to be made with due consideration of the local topography. In some areas, 'wind maps' may be available which include the effects of the terrain and ground cover. Wind atlases and wind maps have been produced in a very wide range of scales, from the world level down to the local government region, and represent the best estimate of the wind resource across a large area. They do not substitute for anemometry measurements – rather they serve to focus investigations and indicate where on-site measurements would be merited.
As a further stage in investigations, theoretical wind turbines can be placed in a chosen spacing, within a geographical model containing wind speed values as a gridded dataset. This is usually computed in a Geographical Information System (GIS). Employing assumptions on the technology conversion efficiency to units of energy, it is possible to derive an energy estimate that corresponds to a defined area. This is typically expressed as Region X having a wind energy potential of Y units of energy per year.
Most wind energy resource studies start with a top-level theoretical resource, which is progressively reduced through consideration of so-called constraints. These are considerations which tend to reduce the area that in reality will be available to the wind energy developer. For instance, they can be geographically-delineated conservation areas, areas where the wind speed is not economically viable, or areas of unsuitable terrain. Areas potentially available for development are sequentially removed from the area over which the energy resource is summed.
Different estimates of the potential energy resource can be calculated according to assumptions on the area that will be available for development. The resource without constraints is often called the “theoretical” resource; consideration of technical constraints results in an estimation of a ‘technical’ resource; and consideration of planning, environmental and social issues results in the estimation of a so-called “practical” resource. Such studies were common in the 1980s and 1990s, when wind energy penetration was relatively low, but have been overtaken somewhat by events, as penetrations of wind energy are now substantial in many European countries.
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