Ready for universal use

Tuesday, 30 June, 2015 - 09:15
In 2013, the use of a laser beam emitting from a buoy to take measurements was tested for the first time. The picture shows a floating LiDAR system in the Firth of Forth, Scotland. (Photo: FLiDAR)
In 2013, the use of a laser beam emitting from a buoy to take measurements was tested for the first time. The picture shows a floating LiDAR system in the Firth of Forth, Scotland. (Photo: FLiDAR)

Measuring wind speeds with the help of laser beams has made great strides over the last few years and now meets most needs. Because it does not require a met mast, LiDAR is ideal for measurements out at sea.

To justify the extremely large investments in offshore wind energy, project developers and operators want to know as much as possible about wind conditions. After all, just as the power from the wind increases in line with the cube of the wind speed, so do the measuring errors. For this reason, even minor mistakes can have dramatic effects on yield and eat away at the profits.

Wind regimes are accurately known only at some points of the North Sea and the Baltic Sea. The site with the best known conditions is probably the alpha ­ventus wind farm. There, you can find the FINO1 platform, which has been in operation with a 100-metre met mast since 2003, has its own helipad and stands on a four-legged jacket structure. All of this makes FINO1 the most expensive measuring station in the history of offshore wind energy – and a symbol for the fact that costly masts should be the exception rather than the rule out at sea.

Offshore experts want to know as much as possible – not just wind speeds and direction at a certain point in a wind farm. This thirst for knowledge is growing along with the options to quench it, and LiDAR provides those in abundance. It is an extraordinarily versatile measuring tool that ten years ago was still unaffordable.

Wind potential and power curve

A LiDAR system can simply be placed on the platform that is already located at the base of an offshore wind turbine. To prevent the wind turbine from disturbing the measurements, the laser beam is pointed upwards at an angle so that the measuring point is between two and three rotor diameters distant at hub height. In this way, wind potential can thus be measured quite precisely, and the wind turbine’s power curve can also be measured.

The LiDAR procedure has not yet been approved by the IEA because the current standard only allows the ­power curve to be measured with cup anemometers (cups). ­However, some customers do not want to wait for the standard to be revised. If a wind farm’s yield falls short of expectations, they want to know if this is due to the power curve – in which case measuring with LiDAR is the best method to get to the bottom of the mistake. ­Apparently, wind turbine manufacturers also accept this.

The power curve only relates to wind speed at hub height – that is, at one point. However, rotors move in an air layer that is 120 to 150 meters thick, so measuring the height profile of the wind speed is becoming more and more important. This measurement is not possible from a wind turbine platform. In August 2013, ­Fraunhofer IWES therefore anchored a buoy equipped with a LiDAR system near the FINO1 platform in order to compare the measurement results with the measurement of the masts. Because the laser beam rocks back and forth with the waves’ motions along with the buoy, the buoy was fitted with acceleration sensors. Motions are accurately recorded so that the laser beam’s oscillations can be subtracted out from the signal.

The positive results of the first measuring campaign were confirmed one year later by a second campaign with another LiDAR system, making it possible to measure wind speeds between 40 and 200 m above sea level. The LiDAR buoy is expected to be available soon for commercial measurements, at which point Fraunhofer IWES will widen the circle of suppliers of this special service. The trailblazer here was FLiDAR, a Dutch company which positioned a LiDAR buoy at the site of the planned Neart Na Gaoithe wind farm off the Scottish coast in ­November 2013. AXYS Technologies, Automasjon og Data, Babcock, Eolos and Fugro are also players. The broad range of ­offerings suggests a strong demand.

Very versatile

It is also essential to know more about how wind turbines in a wind farm affect each other. It has been established that the wake behind a rotor can be several kilometres long. Researchers at the University of Oldenburg therefore deploy a “multi LiDAR” system to investigate wind flows, turbulence and shadowing effects within wind farms...

You can read the complete article written by Detlef Koenemann

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