Abstract
A comparative exercise has been organised within the International Energy Agency (IEA) Wind Annex 32 in order to test the Rotor Equivalent Wind Speed (REWS) method under various conditions of wind shear and measurement techniques. Eight organisations from five countries participated in the exercise. Each member of the group has derived both the power curve based on the wind speed at hub height and the power curve based on the REWS. This yielded results for different wind turbines, located in diverse types of terrain and where the wind speed profile was measured with different instruments (mast or various lidars). The participants carried out two preliminary steps in order to reach consensus on how to implement the REWS method. First, they all derived the REWS for one 10 minute wind speed profile. Secondly, they all derived the power curves for one dataset. The main point requiring consensus was the definition of the segment area used as weighting for the wind speeds measured at the various heights in the calculation of the REWS. This comparative exercise showed that the REWS method results in a significant difference compared to the standard method using the wind speed at hub height in conditions with large shear and low turbulence intensity.
Highlights
The performance of large wind turbines is known to be influenced by the vertical wind shear across the rotor [1][2][3][4][5]
Eight organisations from five different countries have participated in a comparative exercise to explore the application of the Rotor Equivalent Wind Speed (REWS) method for power curve measurement proposed in the Committee Draft of the IEC 61400-121
The exercise revealed that the main challenge in using the REWS method was to determine the number of measurement heights to be used, the distribution of these measurement heights in space and the definition of the segment areas which represent the weighting of each measurement height in the REWS
Summary
The performance of large wind turbines is known to be influenced by the vertical wind shear across the rotor [1][2][3][4][5]. The Rotor Equivalent Wind Speed (REWS) method consists of averaging the weighted wind speed over the swept rotor area [6] It provides a more accurate estimate of the kinetic energy flux passing through the rotor than considering only the wind speed measured at hub height. The REWS method is one of the main modifications proposed to the IEC 61400-12-1 standard for wind turbine power performance measurement currently under revision [7] It requires measurements of the wind speed at several heights distributed between the lower and higher tip heights. These measurements can be performed with a tall mast or a ground based remote sensing wind profiler such as a lidar.
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