A versatile tool for remote measuring

Wed, 01/02/2017 - 12:32

Detlef Koenemann




Hardly any other technology in the offshore industry is evolving as quickly as the LiDAR measuring method. The handy devices have finally also become affordable, and they are increasingly being installed at sea where their nearly unlimited capabilities can be fully used.

Even though this measuring technology generates huge quantities of data, the principle that it is based upon is simple. A laser beam is fired into the sky where it hits aerosols and dust particles that scatter the light. Only a very small fraction of the laser light is reflected back to the measuring instrument. Nevertheless, it is sufficient to analyse the motion of the particles and determine the wind speed.

The movement of the particles causes a shift in the frequency of the light that is reflected back. This is known as the Doppler Effect in acoustics and is actually not a complicated principle. However, evaluating the large amounts of data requires significant computational power.

The travel time of the light signal is used to calculate the distance to the point being measured. This is not easy to do. Light only takes 1 microsecond to travel to a point 150 meters away and back again. This means that a lot depends upon being able to precisely detect the signal and evaluate the data.

Measuring is getting cheaper

The tried-and-tested cup anemometer technology provides approximately one reading per second. Light Detection and Ranging (LiDAR) provides 30,000 or more. The reason why LiDAR devices have reached market maturity is because there are now extremely fast, small, affordable computers. They have made LiDAR suitable for mobile, universal measurement of wind speed on land, at sea and in the air.

Only a few years ago LiDAR devices were still produced in small series, making them virtually unaffordable for the wind energy industry. The growing demand from air quality measurement experts such as weather services, climatologists and environmentalists has driven prices down sufficiently. More than 1,000 LiDAR devices are now available worldwide. The favourable market trend coincides with a growing demand from the offshore wind energy sector, which depends on LiDAR metrology. It is the only way to perform measurements at the wind farm sites at sea without incurring very high costs. All other types of anemometer (cup, ultrasonic and vein) require a mast that needs to be installed on a foundation in the seabed and rises at least 100 m above sea level. This makes them much too expensive.

Manufacturers accept the new technology

It took a while before the offshore industry dropped its reservations about this new technology, which is highly dependent on complex software. The tide only turned after numerous tests had shown that LiDAR devices mounted on floating buoys are effective. “Wind potential measurement using floating LiDAR is becoming increasingly popular”, said Axel Albers from Deutsche WindGuard. “LiDAR devices mounted on small buoys were not widely accepted just one year ago, but the measuring instruments have now been comprehensively verified.”

The results of the tests have been so positive that some clients are now using floating LiDAR as their primary measuring devices. One example is the Dutch government, which has commissioned the service provider Fugro to determine the wind potential at the site of the planned wind farm Borssele.

Measuring the power curve of individual wind turbines is also becoming more important because the yield of a wind farm sometimes falls short of the forecasts. The investors’ suspicion that the wind turbines are to blame can only be dispelled or confirmed by measuring the power curve. Numerous LiDAR devices have already been installed on nacelles to measure the wind speed horizontally upstream. Turbine manufacturers refused to accept the results of a LiDAR measurement for a long time. But that has changed: “Leading manufacturers now accept power curve measurements using LiDAR mounted on the nacelle”, Axel Albers said, “and the others are following suit.” They probably have no choice because almost all experts are now convinced of the accuracy of the LiDAR measurement method.

LiDAR devices installed on the tower base, i.e. on the platform of the transition piece (TP LiDAR), also deliver very good results. This applies to the wind speed at hub height as well as measuring the vertical wind profile, also called wind shear. This allows the wind speeds to be determined for a large portion of the rotor swept area. “Not all manufactures have accepted TP LiDAR yet, but it’s just a matter of time”, Axel Albers said.

Determining the wind direction more accurately

Like Deutsche WindGuard, GWU Umwelttechnik is a service provider that already has many years of experience using LiDAR metrology. “A LiDAR system is quite a versatile remote sensing tool”, Managing Director Ludwig Wagner said. It not only allows measurement of the wind speed at a distance of several kilometres but also of the wind direction. This is important for measuring the wind potential as well as for correctly aligning the wind turbine. If the rotor is not exactly perpendicular to the wind direction, this not only leads to a decrease in the yield but also puts unnecessary strain on the drivetrain.

For simplicity, the LiDAR system is installed on the nacelle. Avent LiDAR Technologies, a joint venture between Leosphere (France) and NRG (USA), has developed a device that is suitable for this type of installation called Wind Iris. The latest generation of devices uses four lasers for measuring instead of two. They are aligned 5° up and down, and at 15° right and left. This enables the device to measure a larger, more representative area in front of the rotor. A numerical model projects the measurement results onto the hub height. Wind speed and direction ca then be precisely calculated. Measuring upstream is of course more accurate than previous measuring methods using cup or vein anemometers that were mounted on the nacelle, which put them in the wake vortex of the rotor.

Guideline allows marketable measurements

Researchers are also very interested in the virtually unlimited capabilities of laser beams. The scientists at the Fraunhofer IWES were among the first to put LiDAR devices on floating buoys. In August 2013, they anchored a buoy near the measurement mast Fino 1 (see photo), which has been measuring wind speeds at heights between thirty-three and a hundred metres using twelve cup and three ultrasonic anemometers since 2003. The valuesdetermined using LiDAR were only minimally different than the values delivered by conventional measurement methods. These positive results were confirmed a year later by testing another LiDAR instrument on the same buoy.

One of the activities that was part of the IEA’s Wind Task 32 ‘Wind LiDAR Systems for Wind Energy Deployment’, which the Fraunhofer IWES spearheaded, formulated guidance for suppliers of LiDAR buoys and made recommendations for marketable measurements in February 2016. “This Recommended Practice Document is a guideline for offshore wind potential measurements, but it is still just a recommendation, not an IEC standard. It is comparable to the guideline for power curve measurements with LiDAR”, said Julia Gottschall, Senior Scientist in wind farm planning and operation at Fraunhofer IWES. Fraunhofer IWES has been carrying out a six-month measurement with its LiDAR buoy near the research platform Fino 1 in accordance with this guideline since February 2016.

Even the operators and planners of offshore wind farms are developing strategies to bring LiDAR technology to the market. The UK Carbon Trust has initiated a programme called Offshore Wind Accelerator (OWA). In late October, OWA published a paper called the ‘roadmap for the commercial acceptance of floating LiDAR technology’. Apparently, they aren’t leaving anything to chance. These trends show that LiDAR will be the dominant technology in the future.