Wind resource assessment is the process of measuring and analyzing wind before building a wind farm. It matters because small differences in wind speed can cause large differences in energy production. Developers use instruments on meteorological masts and lidar systems to find sites with strong, steady, and safe winds.
Good measurements reduce financial risk and help place turbines where they will produce the most electricity.
Key Facts
- Wind power available in moving air is P = 0.5 rho A v^3.
- A turbine cannot capture all wind power, so P_electric = 0.5 rho A v^3 C_p eta.
- Wind speed usually increases with height, often estimated by v2 = v1(h2/h1)^alpha.
- Capacity factor = actual energy produced over a time period / maximum possible energy over that time period.
- Turbulence intensity = standard deviation of wind speed / mean wind speed.
- Longer measurement campaigns, often 12 months or more, give better estimates of yearly wind conditions.
Vocabulary
- Meteorological mast
- A tall tower that holds wind sensors at several heights to measure wind speed, wind direction, temperature, pressure, and turbulence.
- Lidar
- A remote sensing device that uses laser light scattered by air particles to measure wind speed and direction above the ground.
- Wind shear
- Wind shear is the change in wind speed or direction with height above the ground.
- Capacity factor
- Capacity factor is the fraction of maximum possible energy that a wind turbine or wind farm actually produces over time.
- Turbulence intensity
- Turbulence intensity is a measure of how gusty or variable the wind speed is compared with the average wind speed.
Common Mistakes to Avoid
- Using only average wind speed, then ignoring the v^3 relationship. This is wrong because energy production depends very strongly on wind speed and on how often high wind speeds occur.
- Measuring wind too close to the ground and assuming it represents hub height. This is wrong because wind speed often increases with height, especially over rough terrain or forests.
- Ignoring wind direction and terrain effects. This is wrong because hills, valleys, buildings, and tree lines can speed up, slow down, or swirl the wind before it reaches a turbine.
- Treating short-term data as a complete climate record. This is wrong because one windy month or one calm season may not represent the long-term wind resource at the site.
Practice Questions
- 1 A lidar measures an average wind speed of 7.0 m/s at 80 m. If air density is 1.2 kg/m^3 and a turbine rotor has an area of 5000 m^2, what wind power is available in the air using P = 0.5 rho A v^3?
- 2 A met mast measures 6.0 m/s at 40 m. Estimate the wind speed at 100 m using v2 = v1(h2/h1)^alpha with alpha = 0.14.
- 3 A developer has two possible sites. Site A has stronger average wind but high turbulence from nearby hills, while Site B has slightly lower average wind but smoother flow and easier turbine access. Explain which factors should be compared before choosing the site.