Lidar Technology

 

WIND DOPPLER LIDAR

Why Windcube technology is based on pulsed Doppler heterodyne laser principle?

Doppler principle is the simplest and the more accurate way to measure the speed of a target. Light frequency Doppler shift is directly proportional to radial wind speed. Heterodyne detection downshifts light frequency to radiofrequency where it can be easily measured by a detector. Moreover, it is the more sensitive of all detections, allowing low power and reliable lasers. Pulsed operation provides long range thanks to the high peak power and good spatial resolution by the short pulse duration.

What are the differences with other Wind Lidar technologies like :

  • Continuous Wave Lidar

To measure wind speed, CW lidars need to focus the laser beam at successive distances. Pulsed Lidars have a fixed focus and measure wind at all distances with the same pulse. Range resolution increases with the square of the distance in CW Lidars. So max range is limited to 300m. Pulsed Lidars have a constant range resolution, proportional to the pulse duration, whatever the distance, even 10km. However, CW Lidars can measure at very short range, while pulsed Lidars have a blind zone, about twice the range resolution.

  • Direct detection Lidar

Direct detection Lidars use correlation of aerosol concentration on different lines of sight to derive transverse wind speed. They need very low noise detection and are more sensitive to direct  sun light than coherent Lidars. Specific atmospheric conditions are required, with contrasted backscatter variations. Measurement time is longer than with Doppler Lidars, and up to now, turbulence parameters are not derived.

  • Doppler Rayleigh Lidar

Rayleigh scattering occurs on small particles and molecules. Doppler shift is measured with complex interferometers, requiring demanding temperature management, sensitive cameras and complex image processing. They are also sensitive to sun light. Their main advantage is that they don’t need aerosol load and can be used for high altitude wind measurement in the stratosphere.

What is the purpose and the benefits of the 5th vertical beam of the Windcube technology?

The purpose of the 5th beam of the Windcube vertical profiler is to directly measure the vertical component of the wind speed. It is used to improve both direct vertical wind speed measurement and flow inclination accuracy, as well as direct vertical and horiztontal turbulence intensity. Unlike the other four beams, the fifth beam is vertically oriented and therefore provides direct measurements of the w component of the wind vector. Its purpose is to provide extremely accurate vertical wind speed measurements at all Windcube measuring heights. Vertical wind speed measurements are critical for detecting off-axis flow, which can have an effect on wind turbine siting and selection as well as energy production. When taken in conjunction with horizontal wind speed measurements at various heights, vertical wind speed measurements can also help determine atmospheric stability, which can also have significant effects on energy production.  

Why use FCR and how does the FCR Flow complexity Recognition work?

FCR allows for more accurate Windcube measurements in complex terrain sites, where the normal measurement mode may introduce a measurement bias due to lack of flow homogeneity across the measured volume. The causes of the bias have been well documented by existing studies.

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The FCR is different from the normal mode because it does not make the assumption of homogeneity of the wind that is used by all remote sensors and also by the Windcube in normal mode. FCR mode measures the complexity of the flow and is used to provide wind data non-biased directly, which requires no post-correction. The FCR uses all the radial velocities / raw measured at all altitudes, including the vertical velocities of the fifth beam, to force the entire profile of wind speed and direction.This is possible because the radial velocities / raw Windcube are measured very precisely.Other remote sensors and also the normal mode Windcube measures speed and wind direction of each altitude independently.

Do the Windcube Lidars need a calibration?

Doppler Lidar system needs no speed calibration, since the Lidar uses the speed of light (constant and universal) to process the wind speed. However, it needs a zero-distance calibration to define the output of the laser beam (window). It is done during the manufacturing process once and for all.

In regards of quality tests. All new Lidars strictly follow below quality tests :

  • A stage of qualification of the instruments, in which the manufactured system follows environmental tests (vibration, waterproofness) and functional tests (quality & performance of all sub electronic, opto electronic and mechanical sub modules).
  • A stage of metrological validation. After 48h and 96h, data are compared to an instrument reference; during this stage, metrological performances of the manufactured system are compared to a reference Lidar system. The metrological performances of this reference Lidar have been certified by a reference institute every year.
  • An endurance phase of 10 days, in which the system is continuously operating, in order to identify any premature aging of the instrument or one of its components.