Power curve performance of coastal turbines subject to low turbulence intensity offshore winds

The power curve performance of wind turbines located on the northeast coast of Brazil was investigated. The coastal site has wind characteristics typical of the ofshore environment, where both turbulence intensity (TI) and wind shear are consistently low. The wind farm has 20 turbines with 0.9 MW of rated power, hub height and rotor diameter of 55 and 44 m, respectively. The experiment was carried out from August 2013 to July 2014 and included a 100-m met-mast, with fve instrumented levels including two 3D sonic anemometers, and a lidar wind profler. Both were installed upstream of the wind turbines in the prevailing wind direction. Three turbines near the met-mast were used to determine the average power curve for this site by separating the SCADA dataset according to the quartiles of TI and wind shear. The TI was calculated at 60 m and the wind shear between 40 and 98 m. In addition, at 100 m, the 3D sonic anemometer measured wind speed and sonic temperature with an acquisition rate of 20 Hz, to compute atmospheric stability using the eddy covariance method. The results show that TI is the parameter that has the most signifcant efect on the power curve performance. At high TI (> 7.5%), the measured power coefcient (CP) curve was similar to that provided by the manufacturer, whereas for low TI (< 5.4%) the CP curve showed a poorer performance relative to that of the manufacturer’s. Since the winds blow consistently from the ocean, very stable and very unstable conditions are less frequent, and thus the typically unstable, nearly unstable and neutral conditions produce winds with low TI and wind shear. This stationary condition of low TI causes the measured power curve to show systematically poorer performance than the manufacturer’s power curve, which leads to an annual energy production difer ence of - 2.0%. In the worst scenario for a consistently low TI (< 5.4%), the diference may reach up to - 5.6%. Thus, the typical low TI of the ofshore environment could negatively impact the energy production of wind turbines.