Wind Resource Assessment - Calculations

Calculations

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 WAsP linear 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