As we have discussed in the past, a wind turbine’s output is dependent on its height, since taller turbines are exposed to greater wind speeds. These elevated rotors become far more reliable in generating electricity, since they enhance the efficiency of the system. However, determining the wind speed interacting with the system can be challenging when assessing larger wind turbines, due to the presence of wind shear, or the “change of wind speed with height across the wind turbine rotor”.
Wind gradient (also known as shear) is a relatively simple concept: the closer to the Earth’s surface, the slower the wind. This is due to the friction between the wind and the Earth’s surface. Wind shear is also sometimes used to refer to the change in wind direction deriving from altitude. Just as wind speeds may be affected by a variety of factors influencing turbine performance, such as geography and climate, wind shear is an integral consideration when calculating the output of wind power systems.
Such a factor is represented in the following video. It was created by a user of IEC 61400-1 Ed. 3.0 b:2005 – Wind turbines – Part 1: Design requirements, with the assistance of that standard’s guidelines. The models demonstrate the relative wind experienced by a point near the tip of a large wind turbine blade.
As identified in the video, wind speed can vary among the different parts of a rotor. It is evidenced here that the system is experiencing wind shear.
As stated in the Introduction of IEC 61400-12-1 Ed. 2.0 b:2017 – Wind energy generation systems – Part 12-1: Power performance measurements of electricity producing wind turbines, “a key element of power performance testing is the measurement of wind speed.” Therefore, as wind turbines are designed substantially larger, measuring wind speed at a single height may not be suitable for accurately measuring wind speed throughout the entire rotor, and thus would not be useful for determining a wind turbine’s performance.
As a remedy to this issue, the IEC 61400-12-1 Ed. 2.0 b:2017 standard makes use of what is known as rotor equivalent wind speed (REWS). REWS is an arithmetic combination of simultaneous measurements of wind speed at various heights spanning the complete rotor between lower tip and upper tip, and, in certain instances, it gains greater insight on performance than a single measurement of wind speed at hub height.
According to IEC 61400-12-1 Ed. 2.0 b:2017, rotor equivalent wind speed can be calculated if the wind speed is measured at three or more heights across the wind turbine rotor (more than three is preferred). For the standard user, there are several options for measuring wind speeds throughout the rotor, with the utilization of anemometers at different locations.
This calculation, along with the wind shear correction factor, are to be used to generate an accurate representation of anticipated wind speed.
It is also important that users of the IEC 61400-12-1 Ed. 2.0 b:2017 standard remain aware that a variety of meteorological factors influence the performance of an electricity generating wind turbine, and it is important to refer to the standard’s guidance for measuring and assessing these additional weather patterns.