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W i nd Measurement    101


              sector-wise wind energy density (see Fig. 6-15). Wind energy rose in
              Fig. 6-15c is a sector-wise plot of average wind energy density (propor-
              tional to the cube of wind speed) multiplied by the relative frequency
              of wind in each sector.

        Computed Quantities
              After the data has been validated, the following quantities are com-
              puted. These derived quantities are used to determine, among other
              items, the turbine class and uncertainty in resource.

              Turbulence
              Turbulence has a very specific meaning when describing wind condi-
              tions. It is the standard deviation of horizontal wind speed, vertical
              wind speed, and wind direction around the 10-min average. Turbu-
              lence intensity (TI) is defined as the ratio of standard deviation to
              the average. In wind projects, the horizontal wind speed turbulence
              intensity is most commonly used. For horizontal wind speed, the TI is:
                                            σ
                                      TI =                         (6-5)
                                           v avg
              where σ is the standard deviation of wind speed and v avg is the av-
              erage wind speed. In most wind applications, the standard deviation
              and average are based on 10-min observations. For example, with
              3-s measurement interval and 10-min recording interval, the standard
              deviation and average used in the above formula are for a sample
              size of 200 values.
                 Turbulence intensity is a measure of the atmospheric stability;
              specifically, it measures rapid changes in wind speed over short inter-
              vals. A value of TI that is 0.1 or less is considered low turbulence; TI
              in the range of 0.1 to 0.25 is considered moderate turbulence; 0.25 or
              higher is considered high turbulence. TI is used to determine the tur-
              bine category appropriate for the site. High turbulence causes energy
              output to diminish and affects the loading, durability, and operation
              of a turbine.
                 According to IEC 61400-1, there are three categories of turbines A,
              B, and C with values of I ref = 0.16, 0.14, and 0.12, respectively. I ref is the
              expected turbulence intensity at 15 m/s wind speed. In the normal
              turbulence model (NTM), the standard deviation of wind speed at
              hub is described as:

                             σ = I ref (0.75v hub + 5.6)           (6-6)
                            TI = σ/v hub = I ref (0.75 + 5.6/v hub )  (6-7)

              The three smooth curves in Fig. 6-16 are graphs of the NTM, repre-
              sented by Eq. (6-7), for different values of I ref . Statistics of the measured
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