Abstract
The high-resolution bistatic lidar developed at the Physikalisch-Technische Bundesanstalt (PTB) aims to overcome the limitations of conventional monostatic lidar technology, which is widely used for wind velocity measurements in wind energy and meteorology applications. Due to the large measurement volume of a combined optical transmitter and receiver tilting in multiple directions, monostatic lidar generally has poor spatial and temporal resolution. It also exhibits large measurement uncertainty when operated in inhomogeneous flow; for instance, over complex terrain. In contrast, PTB’s bistatic lidar uses three dedicated receivers arranged around a central transmitter, resulting in an exceptionally small measurement volume. The coherent detection and modulation schemes used allow the detection of backscattered, Doppler shifted light down to the scale of single aerosols, realising the simultaneous measurement of all three wind velocity components. This paper outlines the design details and theory of operation of PTB’s bistatic lidar and provides an overview of selected comparative measurements. The results of these measurements show that the measurement uncertainty of PTB’s bistatic lidar is well within the measurement uncertainty of traditional cup anemometers while being fully independent of its site and traceable to the SI units. This allows its use as a transfer standard for the calibration of other remote sensing devices. Overall, PTB’s bistatic lidar shows great potential to improve the capability and accuracy of wind velocity measurements, such as for the investigation of highly dynamic flow processes upstream and in the wake of wind turbines.
Highlights
Accurate wind velocity measurements are an essential prerequisite for many applications in the field of wind energy and meteorology, such as wind potential analysis [1], the power curve evaluations of wind turbines [2] and atmospheric turbulence analysis [3].For example, low measurement uncertainties are especially desired for reliable resource assessments of projected wind farms because the wind turbine power output scales with the third power to the wind velocity [4]
Physikalisch-Technische Bundesanstalt (PTB)’s high-resolution bistatic lidar facilitates non-invasive wind velocity measurements at a sampling rate of up to 10 Hz and is fully traceable to the SI units based upon the set-up’s geometry, laser wavelength, and the digital time base used for frequency measurements
The comparative measurements between the PTB bistatic lidar and the cup anemometers mounted on the met masts show that the measurement uncertainty of PTB’s bistatic lidar is well within the measurement uncertainty of traditional cup anemometers while being fully independent of its site
Summary
Accurate wind velocity measurements are an essential prerequisite for many applications in the field of wind energy and meteorology, such as wind potential analysis [1], the power curve evaluations of wind turbines [2] and atmospheric turbulence analysis [3]. PTB lidar (cf Figure 2b) uses three dedicated receivers arranged at a radius of 1 m around a central transmitter, with all units focused into a spatially highly resolved, ellipsoid-shaped measurement volume (at a height of 5 m: diameter 300 μm, length 2 mm; at a height of 200 m: diameter 12 mm, length 4 m) This facilitates the simultaneous measurement of the three-dimensional wind velocity with exceptionally high spatial resolution, down to the scale of single aerosols. PTB’s high-resolution bistatic lidar facilitates non-invasive wind velocity measurements at a sampling rate of up to 10 Hz (depending on the weather conditions at the measurement height) and is fully traceable to the SI units based upon the set-up’s geometry, laser wavelength, and the digital time base used for frequency measurements. Preliminary results indicate that the measurement uncertainty amounts to less than 1% at sufficiently high rates of detected particles
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