Anemometer Calibration Requirements for Wind Energy Applications

Wind measurements are critical in all areas of wind energy such as wind turbine power performance evaluations, site assessments, and wind plant operations. From a wind turbine power performance evaluation, wind speed readings are matched with the resulting power output of a wind turbine in order to produce a power curve, which is used as the primary selling specification for a wind turbine. In a site assessment for wind energy potential, the distribution of the measured wind speed is used to determine the predicted annual energy production, which is a critical value used in power purchase agreements. Since wind power is proportional to the cube of the wind speed, a slight error in the wind measurement translates to a much greater error in the predicted wind power, which emphasizes the importance of having accurate wind speed readings. In wind plant operations, wind speed measurements are used to validate the power output of the turbine and to trigger the start-up and shut-down of a turbine. For some large turbines, it is necessary to provide a “kick-start” once the necessary level of atmospheric winds is reached for wind power generation. When the winds are too strong, a rotating turbine becomes a safety hazard, thus requiring a shut-down.

To lower the uncertainty in wind data, it is recommended that individually calibrated anemometers be employed. In the wind power industry, the most commonly referred standard is IEC 61400-12-1, “Power Performance Measurements of Electricity Producing Wind Turbines”, originally introduced by MEASNET (the international Measuring Network of Wind Energy Institutes). This particular document primarily provides the procedures in conducting the performance evaluation of a wind turbine. It also discusses the requirements for performing anemometer calibrations along with the various tests that would evaluate the instrument's sensitivity to certain terrain and atmospheric conditions. Anemometer calibration procedures in IEC 61400-12-1 originally only pertained to cup anemometers. Although the reference primarily pertains to wind turbine performance testing, it is also commonly referenced for wind site assessments, wind plant monitoring, and other wind energy areas related to meteorology. Currently, the IEC standard is being updated to also include test standards for sonic anemometers and use of remote sensing. Other standards that are also commonly referenced for anemometer performance testing include:

ASTM D5096-02, “Determining the Performance of a Cup Anemometer or Propeller Anemometer”

ISO 17713-1, “Meteorology – Wind Measurements Part 1: Wind Tunnel Test Methods for Rotating Anemometer Performance”

ASTM D6011-96, “Determining the Performance of a Sonic Anemometer/Thermometer”

ISO 16622, “Meteorology – Sonic Anemometers/Thermometers – Acceptance Test Methods for Mean Wind Measurements”

In general, the purpose of an anemometer calibration is to relate the raw output of the anemometer under test to a corresponding measure of a controlled reference wind speed. Thus, all of the standards listed above require that calibrations are to be performed in a controlled uniform-flow wind tunnel. Of the five standards noted above, IEC 61400-12-1 requires that the reference wind speed is measured using a Pitot-static tube system and that the uncertainty in the measurements is presented in calibration reports. This paper discusses the anemometer calibration requirements for wind energy applications as defined in IEC 61400-12-1 and a comparison to other related standards.