Wind: Technology Overview

Wind power systems convert the movement of the air to power by means of a rotating turbine and a generator.  Wind power has been the fastest growing energy source of the last decade in percentage terms and has realized around a 30 percent annual growth in worldwide capacity for the last five years.  Cumulative worldwide wind capacity is now estimated to be over 39,000 MW.  Europe now leads in wind energy, with over 28,000 MW installed; Germany, Denmark, and Spain are the leading European markets. Installations of wind turbines have outpaced all other energy technologies in Europe for the past two years.  

Applications

Typical utility-scale wind energy systems consist of multiple wind turbines that range in size from 0.60 MW to 2 MW.  Typically sized wind energy system installations may total 5 to 300 MW, although single and small groupings of turbines are common in some places, such as Denmark and Germany.  Newly emerging off-shore wind energy development is encouraging the development of both larger turbines (up to 5 MW) and larger wind farm sizes. 

Wind is an intermittent resource with annual capacity factors usually ranging from 25 to 40 percent.  The capacity factor of an installation depends on the wind regime in the area and energy capture characteristics of the wind turbine.  Capacity factor directly impacts economic performance; thus, reasonably strong wind sites are a must for cost effective installations.

Because wind is intermittent, it cannot be relied upon as firm capacity for peak power demands.  To provide a dependable resource, wind energy systems may be coupled with some type of energy storage to provide power when required, but this adds considerable expense and is not common.  For larger wind farms numerous studies have shown that relatively low levels of wind grid penetration will not necessitate additional backup generation.  Efforts are currently underway by research agencies to forecast wind speeds more accurately, thereby increasing confidence in wind power as a generation resource and dependability in utility dispatching.

Resource Availability

Wind speed increases significantly with height and wind turbine power output rises with the cube of wind speed, making small differences in wind speed very significant. Wind strength is rated on a scale from Class 1 to Class 7 (see the table below).  Wind speeds and power densities (watts per meter squared, W/m²) at a Class 1 site and at a 50 m hub height can go as high as 5.5 m/s and 200 W/m².  In comparison, wind speeds and power densities at a Class 7 site and at the same hub height may reach 11.9 m/s and 2000 W/m².  Class 4 sites and higher are usually considered economically viable for wind project development.  At these sites wind speeds may reach 7.5 m/s with a power density of 500 W/m² at a 50 m hub height.  Regardless of the existence of high resolution resource maps for some regions, a minimum of one-year of site data collection is typically required to determine if utility-scale wind energy is viable at a specific location.

Environmental Impacts

Wind is a clean generation technology from the perspective of emissions.  However, there are still environmental considerations associated with wind turbines.  First, opponents of wind energy frequently cite visual impacts as a drawback.  Turbines are approaching and exceeding 300 feet tall and, for maximum capture of resource, tend to be located on ridgelines and other elevated topography.  Combining turbines of different type, manufacturer, color and rotation can improve the visual impact of turbine developments.  Second, turbines can cause avian fatalities if they are located in areas populated by native birds or on migratory flyways.  To some degree, these issues can be partially mitigated through proper siting, environmental review, and the involvement of the public during the planning process.