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Accurate wind velocity measurements

Especially interesting for
  • the wind energy sector

Wind potential analyses, used to assess the cost effectiveness of wind farms, are currently based on the measurement of wind velocity by means of cup anemometers mounted on meteorological masts. As an alternative to these expensive met masts which are difficult to erect, a doppler lidar system for traceable, groundbased laser-optic wind velocity measurements has been developed at PTB.

Left: Conventional wind lidar system determining the wind vector by measuring individual velocity components

The planning of wind farms is essentially based on wind potential analyses to assess the wind energy regime at the site planned for the wind farm. Together with the characterized power curve of the wind turbines which are to be installed in such wind farms, the annual mean of the expected production of electric energy can thus be computed. This is indispensable when it comes to assessing the profitability of a wind farm.

Both for wind potential analyses and for power curve characterization, it is necessary to measure the wind velocity profile at the level of the rotor blades. At present, this is still done using traceable anemometers – i.e. cup anemometers calibrated by accredited calibration laboratories – mounted onto meteorological masts. Since wind turbines tend to become ever higher, erecting such met masts is becoming correspondingly difficult and expensive. According to investigations results published in the wind power sector, remote measurement procedures based on the Doppler lidar technique could be more economical, but for lidar systems, no suited procedures of measurement traceability are available yet.

Conventional wind lidar systems used in the wind power sector usually contain common emitting/receiving optics (see Fig. left), so that the emitting and the receiving beam overlap (monostationary and monoaxial). As only the flow velocity component in the direction of the beam is measured, with a height resolution of approx. 20 m, the beam is conically tilted into different directions in order to compute a velocity vector. For the planned application, a higher resolution would, however, be desirable. Furthermore, flow inhomogeneities via the scanning range of the beam can lead to deviations in the order of 10 %, especially on uneven terrain, preventing lidar wind velocity measurements from being traceable with the required measurement uncertainty.

The bistatic Doppler lidar system with spatially separate emitting/receiving devices (see Fig. right), which was developed at PTB, allows spatial resolutions in the cubic centimetre range and the detection of the complete velocity vector in one location. This system can be used both to calibrate a conventional system in the field and to elaborate traceable wind potential analyses and power curves.

Contact

Michael Eggert
Department 1.4 Fluid Flow Measuring Techniques/Gas Flow
Phone: +49 (0)531 592-1317
michael.eggert(at)ptb.de

Scientific publication

M. Eggert, H. Müller, H. Többen: Doppler- Lidar-Transfernormal zur Windgeschwindigkeitsmessung: Aktueller Entwicklungsstand. 20. Fachtagung „Lasermethoden in der Strömungsmesstechnik“, S.10/1–10/6 (2012)