Publishers version - DTU Orbit

13 Turbulence measurements by wind
lidars
Ameya Sathe
DTU Wind Energy, Risø Campus, Roskilde, Denmark
13.1 Introduction
It is now well established that wind lidars (henceforth referred to as lidars) measure the 10min
mean wind speed with acceptable accuracy. Several measurement campaigns have been
carried out in this regard, where cup anemometers are used as reference instruments (Smith
et al., 2006; Kindler et al., 2007; Peña et al., 2009). Turbulence measurements using lidars
is still a subject of research, and an acceptable method is yet to be established. At first it is
important to understand which turbulence measurements we refer to in this chapter. They
are the second-order moments of wind speeds. It is then interesting to know why turbulence
measurements are useful for wind energy. Amongst a whole range of applications, we can list
a few, such as,
• Load calculations of wind turbines - The driving loads causing fatigue of wind turbines is
atmospheric turbulence. Currently, the three dimensional spectral tensor model of Mann
(1994) is used to quantify turbulence. Expressions of turbulence spectra from Kaimal
et al. (1972) are also used in load calculations. These theoretical and empirical models
are obtained for neutral and homogeneous conditions. Wind turbines operate under all
terrain types and atmospheric conditions. Hence, the best input of turbulence for load
calculations is by directly measuring it at the site where wind turbines will operate.
• Powercurve measurements - The powercurve of a wind turbine is sensitive to turbulence
intensity, especially in the region around the rated wind speed. Turbulence measurements
will enable accurate power curve measurements that are vital for a wind farm developer.
• Validation of wind profile models - In recent years with the increase in the size of the
wind turbines, wind profile models that extend in the entire boundary layer are developed
(Gryning et al., 2007; Peña et al., 2010a). These models are based on the assumption
that momentum flux changes linearly with height. If we are able to measure turbulence
at several heights, we can verify these assumptions. Alternatively, empirical relations of
the variation of momentum flux with height can also be derived.
The next interesting question is what is the current standard for the measurement of turbulence
in wind energy. The answer is the sonic anemometer. They are compact instruments
that can measure all three components of wind velocity in relatively small sample volume
that for all practical purposes can be considered a point. They need to be mounted on a
meteorological mast (met-mast), such that the flow distortion due to the mast itself is kept
to a minimum. Despite this there are disadvantages of using sonic anemometers in turbulence
measurements, the most important being that tall met-masts are very expensive, and
offshore, the costs increase significantly. We thus have to look for alternatives. Remote sensing
methods such as sodars and lidars are viable alternatives, but they are still a subject of
research for turbulence measurements. In this chapter we restrict the discussion to turbulence
measurements using lidars only.
Although lidars have been introduced in wind energy recently, for meteorology they have
been investigated previously to measure turbulence using different scanning techniques. A
common technique is by conical scanning and using the velocity azimuth display (VAD)
technique of processing the data. One of the first remote sensing (Doppler radar) turbulence
studies using a full 360 ◦ scan in a horizontal plane was carried out by Browning and
242 DTU Wind Energy-E-Report-0029(EN)