Wind Farm

A wind farm or wind park is a group of wind turbines in the same location used to produce electricity. A large wind farm may consist of several hundred individual wind turbines and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm can also be located offshore.

Wind resource assessment is the process by which wind power developers estimate the future energy production of a wind farm. Accurate wind resource assessments are crucial to the successful development of wind farms.

To estimate the energy production of a wind farm, developers must first measure the wind on site. Meteorological towers equipped with anemometers, wind vanes, and sometimes temperature, pressure, and relative humidity sensors are installed. Data from these towers must be recorded for at least one year to calculate an annually representative wind speed frequency distribution.

Since onsite measurements are usually only available for a short period, data are also collected from nearby long-term reference stations (usually at airports). These data are used to adjust the onsite measured data so that the mean wind speeds are representative of a long-term period for which onsite measurements are not available. Versions of these maps can be seen and used with software applications such as wind Navigator.

The following calculations are needed to accurately estimate the energy production of a proposed wind farm project:

• Correlations between onsite meteorological towers:
  • Multiple meteorological towers are usually installed on large wind farm sites. For each tower, there will be periods of time where data is missing but has been recorded at another onsite tower. Least squares linear regressions can be used to fill in the missing data. These correlations are more accurate if the towers are located near each other (a few km distance), the sensors on the different towers are of the same type, and are mounted at the same height above the ground.
• Correlations between long term weather stations and onsite meteorological towers:
  • Because wind is highly variable year to year, short-term (< 5 years) onsite measurements can result in highly inaccurate energy estimates. Therefore, wind speed data from nearby longer term weather stations (usually located at airports) are used to adjust the onsite data. Least squares linear regressions are usually used, although several other methods exist as well.
• Vertical shear to extrapolate measured wind speeds to turbine hub height:
  • The hub heights of modern wind turbines are usually 80 m or greater, but cost effective meteorological towers are only available up to 60 m in height. The power law and log law vertical shear profiles are the most common methods of extrapolating measured wind speed to hub height.
• Wind flow modeling to extrapolate wind speeds across a site:
  • Wind speeds can vary considerably across a wind farm site if the terrain is complex (hilly) or there are changes in roughness (the height of vegetation or buildings). Wind flow modeling software, based on either the traditional WAsPlinear approach or the newer CFD approach, is used to calculate these variations in wind speed.
• Energy production using a wind turbine manufacturer’s power curve:
  • When the long term hub height wind speeds have been calculated, the manufacturer’s power curve is used to calculate the gross electrical energy production of each turbine in the wind farm.
• Application of energy loss factors:
  • To calculate the net energy production of a wind farm, the following loss factors are applied to the gross energy production:
    • wind turbine wake loss
    • wind turbine availability
    • electrical losses
    • blade degradation from ice/dirt/insects
    • high/low temperature shutdown
    • high wind speed shutdown
    • curtailments due to grid issues

Because of the variable magnitude and frequency outputs of the variable speed wind turbines, there must be power electronic grid interconnection to decouple the outputs of the wind turbines to the grid. Moreover, the rotor blades of the wind turbines rotate following to the wind speed in their vicinity areas. As the wind speed is different in each location of a wind farm, each wind turbine rotates 8 with different speed. Therefore, it is necessary that each wind turbine has individual connection to the power electronic grid interconnection.

There are two methods of grid connections in a wind farm-group and individual.

The group connection eliminates the variable speed concept since all wind turbines are connected to one converter. Hence they must operate with same speed. Contrarily, the individual connection supports the concept of the variable speed. Moreover, this type of the connection provides reliability to the power system since only one converter is 9 crucial, which is the grid-side converter. When one of the rotor-side converters is inoperable, other wind turbines can supply the power. For the group connection, both converters are critical. A fault in either converter results in failure of the wind farm.

Wind energy penetration refers to the fraction of energy produced by wind compared with the total available generation capacity. There is no generally accepted maximum level of wind penetration. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for energy storage, demand management and other factors. An interconnected electricity grid will already include reserve generating and transmission capacity to allow for equipment failures. This reserve capacity can also serve to compensate for the varying power generation produced by wind stations. Studies have indicated that 20% of the total annual electrical energy consumption may be incorporated with minimal difficulty. These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy or hydropower with storage capacity, demand management, and interconnected to a large grid area enabling the export of electricity when needed. Beyond the 20% level, there are few technical limits, but the economic implications become more significant. Electrical utilities continue to study the effects of large scale penetration of wind generation on system stability and economics.

A wind energy penetration figure can be specified for different durations of time, but is often quoted annually. To obtain 100% from wind annually requires substantial long term storage or substantial interconnection to other systems which may already have substantial storage. On a monthly, weekly, daily, or hourly basis—or less—wind might supply as much as or more than 100% of current use, with the rest stored or exported. Seasonal industry might then take advantage of high wind and low usage times such as at night when wind output can exceed normal demand. Such industry might include production of silicon, aluminum, steel, or of natural gas, and hydrogen, and using future long term storage to facilitate 100% energy from variable renewable energy. Homes can also be programmed to accept extra electricity on demand, for example by remotely turning up water heater thermostats.