# Can LiDARs Replace Meteorological Masts in Wind Energy?
## 4 Wind Resource, Power and Load Comparison
A similar long-term comparison study between LiDAR and conventional tower-based measurements at a site with higher wind speeds will help to further evaluate the performance and accuracy of the LiDAR technology. Next we compare the power production, computed from the measured wind speed. To this end, the power curve of the NREL 5-MW reference wind turbine which has been used extensively in wind energy research [35], is employed. The NREL 5-MW turbine has a rotor diameter and hub height of 126 m and 90 m, respectively. The cut-in, rated and cut-out wind speeds are 3 m/s, 11.4 m/s and 25 m/s. Note however, that the 90 m hub height is higher than the highest measurement point, i.e., 57 m in the current measurement campaign. One way to overcome this issue can be to extrapolate the wind speed at 90 m height using the power law, i.e. \[\frac{\overline{V}(z)}{\overline{V}(z_{\text{ref}})}=\left(\frac{z}{z_{\text{ ref}}}\right)^{\alpha}, \tag{9}\]  
where \(\overline{V}(z)\) is the mean wind speed at height \(z\) which in this specific case is 90 m, \(\overline{V}_{\text{ref}}\) is the mean wind speed at height \(z_{\text{ref}}\) which is 57 m, and \(\alpha\) is the power law exponent. To apply the power law in the current study, we still need to decide a value for \(\alpha\). The study showed that \(\alpha\) is a highly variable  
Figure 16: Velocity distribution at 57 m height. (**a**) Histogram of frequency of occurrence of 10-min average wind speed; (**b**) Weibull distribution of the PDF obtained from the measured data. parameter, depending on the nature of the terrain, the atmospheric stability and other thermal and mechanical parameters [36]. It is possible to determine the value of a by fitting Equation (9) to the measured wind data, but an \(\alpha\) value obtained in such a way can only be verified against the LiDAR measurement, since the highest measurement point on the meteorological mast is at 57 m. To maintain the reliability of the comparison between the sonic anemometer and the LiDAR measurement, we therefore use the wind speed at 57 m height directly to estimate the power output for the wind turbine. Figure 17a compares histograms of the frequency of occurrence of power values estimated from the sonic anemometer and the LiDAR. Except for those times when the wind turbine is not in the power production mode, i.e., below the cut-in wind speed or above the cut-out wind speed, the LiDAR-measured wind speed produce slightly higher occurrence frequencies. Nevertheless, the power output obtained from both the sonic anemometer and the LiDAR show a similar tendency, and the difference in occurrence frequency between the two devices is less than 1% for all power production bins. The capacity factor estimated from the sonic anemometer-measured wind speed is 15%, and that obtained from the LiDAR-measured wind speed is 17%. The higher capacity factor estimated from the LiDAR measurement can be attributed to the fact that the LiDAR was not able to collect the same fraction of lower wind speed data (which corresponded to those below the cut-in wind speed) as that collected by the sonic anemometer. This resulted in a comparatively smaller duration of the 'no power production' mode and consequently in the higher capacity factor from the LiDAR measurement. Figure 17b compares the power duration curves which essentially show the number of hours in the measurement period (usually one year) for which the wind power and the power output of the given turbine are equal to or higher than each particular value. The available wind power values are also obtained for the swept area of the NREL 5-MW turbine. The Power duration curves obtained from the LiDAR-measured wind speed show a high level of agreement with those obtained from the sonic anemometer measurement. The corresponding curves for the two devices almost overlap with each other. Note that the sonic anemometer measurement data extends to 7841 h compared to only 6666 h for the LiDAR.