Windshear and Windveer Characterization for Resource Assessment Improvement

Introduction

As rotor size increases, wind speed and wind direction measurements at one point are no longer representative of the wind conditions in the area swept by the rotor.
Effects like windshear o windveer are to be considered in the resource assessment stage, in order to allow for proper wind farm design.
Moreover, those effects significantly affect the wind turbine performance and should also be considered when estimating energy production.
Nowadays, windshear is calculated from wind speed measures at two heights (usually hub height and lower blade-tip) leading to errors in the wind speed characterization above hub height. On the other hand, windveer is rarely considered.
This work proposes new methods for characterizing windshear and windveer and analyses the benefits of using these methods in the resource assessment stage.

Approach

To measure windshear and windveer, a ground-based LiDAR has been used. This equipment has been installed in an experimental windfarm for monitoring an AW125/3000 TH120 wind turbine (a 3MW wind turbine with a 125m rotor on a 120m concrete tower).
LiDAR allows obtaining measurements of different wind parameters at different heights, being possible to measure wind properties as high as the upper blade-tip.
New parameters have been defined to characterize windshear and windveer. These parameters are analyzed and compared with the standard parameters used to define these effects.

Main body of abstract

Based on LiDAR measurements, new parameters have been defined to better characterize windshear and windveer.
For windshear, the ratio Rotor Equivalent Wind Speed (REWS) / Wind Speed at hub height (Vhh) may be a good indicator of the effect of windshear on the available wind energy, but it is hub height and rotor size dependent.
The study shows that windshear coefficient calculated from wind speed measurements at two heights (usually hub height and lower blade tip) leads to wind speed miss-estimations at points above hub height.
Assuming a power law shear profile (as stated in IEC 61400-1), a new approach is proposed for the calculation of the shear coefficient (alpha). This approach consists on calculating alpha by least squares approximation considering, not only wind speed at two heights, but the wind speed distribution up to the upper blade tip height. The so calculated coefficient demonstrates to be a much better indicator of the real wind speed distribution in the analyzed height interval, and it is neither hub-height nor rotor size dependent.
For windveer, three parameters are defined, each of them containing important information about wind direction variation with height:
- Slope of the linear regression of the points Wind Direction / Height. Provides information about the orientation and magnitude of windveer.
- Total Sum of Squares (TSS) or total variance in the wind direction changes with the height. Provides a quantitative idea of wind direction variation.
- Ratio REWSveer / REWS, where REWSveer is the Rotor Equivalent Wind Speed accounting for wind direction variation along the rotor height. Indicates the energy loss due to windveer, is rotor size dependent.
A comparison of energy production estimation using former and proposed parameters is performed for different scenarios.

Conclusion

Accurate characterization of windshear and windveer is of growing importance for wind resource assessment, and affects wind farm design and energy production estimation.
Different methods for characterizing windshear and windveer have been proposed. Some of them are helpful for wind farm design while some others are useful for energy production estimation.


Learning objectives
Understand constraints of actual methods of characterizing windshear and windveer and its effect in wind resource assessment.
Learn new alternative methods for the characterization of windshear and windveer, its pros and cons.
Be able to select the appropriate method or combination of methods according to the application.
Improve resource assessment and wind farm design accounting for windshear and windveer.