Wind measurements that reduce electricity prices
By Claude Olsen
Extensive testing in three countries has proven that laser measurements of wind provide precise, reliable wind data. The research findings will make it more profitable to develop and operate wind farms.
Profit margins from wind farms are small and sometimes even negative. Detailed knowledge about the wind conditions around each turbine can be critical for a wind farm’s profitability. However, there has been a lack of reliable data on how meteorological conditions affect power production, particularly in offshore installations. Up to now, measurement data have been limited to spot measurements taken with mast-mounted instruments. These only provide measurements for a specific location and do not measure conditions at the same elevation as the large turbines, which can be 200 metres tall.
NORCOWE researchers have recently tested methods for accurately measuring wind using lasers. The methods can give wind farm owners and energy suppliers an entirely new understanding of how wind affects energy production.
Revolutionary new knowledge
The main objective behind the four years of extensive testing in three countries has been to characterise wind flow patterns around a single wind turbine and an entire wind farm. The wind interaction is different for turbines in the front row of a wind farm than for those placed behind. Turbines behind the first row will be exposed to less wind and more turbulence than if they had stood in isolation. This phenomenon, known as the wake effect, can lead to far lower energy production in practice than theory would dictate, and can cause considerable wear on the turbines.
At the core of the new method is a lidar, an instrument that measures wind speed using lasers.
“We wanted to understand the wake effect in detail and needed something better than a mast which remains fixed when wake and wind are in constant flux. We needed instruments that were more flexible and lidars are perfect for this purpose,” says Professor Joachim Reuder of the Geophysical Institute at the University in Bergen.
He has headed a large group of researchers affiliated with the Norwegian Centre for Offshore Wind Energy (NORCOWE) in Bergen.
Initial testing at Stavanger Airport
Lidar instruments have been on the market for a number of years, but to find out whether laser measurements were useful for wind farms the NORCOWE researchers travelled to Stavanger Airport in 2013. On site they tested a scanning lidar capable of examining a large slice of airspace and two static Doppler lidars which measure air column wind speed directly above the instrument up to nearly 300 metres.
Wind measurements at Stavanger Airport showed that a scanning lidar’s laser technology could be used to measure wind speeds. Photo: University of Bergen
The measurements taken with the scanning Doppler lidar were compared with those taken by equipment that meteorologists use when deploying weather balloons to measure wind speeds at different altitudes. It was a perfect match.
“We wanted to find out how well the lidar could provide wind profiles up to two to three kilometres and how it measured land-sea boundary layer transitions. The tests showed a high correlation,” Professor Reuder says.
On the ground in the Netherlands
In Wieringermeer, north of Amsterdam, the Energy Research Centre of the Netherlands (ECN) has installed a number of test turbines. In autumn 2013, NORCOWE researchers set up three static lidars and one scanning lidar around a standard 2.5 MW wind turbine. In addition, two lidars were placed at the top of the turbine to measure horizontal wind in front (inflow) and behind (wake) the turbine. The measurements were compared with wind measurements taken by the traditional method at a nearby mast. The measurements, recorded from November 2013 to May 2014, generated a vast amount of data that researchers continue to analyse and can further delve into.
The WINTWEX-W campaign: Measurements taken by a lidar are compared with measurements taken by a number of other methods at the wind farm in Wieringermeer in the Netherlands. Photo: Benny Svardal
“This yielded very important information about what the wake (the wind field behind the turbine) looks like, its movements and how it changes over time, as well as the role played by the temperature distribution over height (i.e. atmospheric stability). We learned a great deal about how to use the lidar correctly,” Professor Reuder adds.
One interesting discovery is that the wake effect is greatest under stable wind conditions, which is known as stable atmospheric stratification. This occurs primarily during the winter and at night. The least amount of wake occurs when turbulence is high, for example during the daytime in the summer.
At sea in Germany
After documenting that lidars are well-suited for measuring wind speeds at different elevations in front of and behind a wind turbine, the NORCOWE researchers were ready to take the next leap and focus on an offshore wind farm under the OBLEX-F1 measurement campaign in that kicked off in spring 2015.
In cooperation with several German research institutions, NORCOWE partners and equipment suppliers, researchers from the University of Bergen and CMR have carried out extensive air and sea measurements by the Alpha Ventus wind farm off the Northwest Coast of Germany. The wind farm has 12 turbines and a platform with a wind-measuring mast (FINO1).
During the experiment, one scanning lidar measured winds flowing into the wind farm, while another measured the wake behind the turbines. A static lidar measured wind speeds at various heights along the mast on FINO1. In addition, two static lidars were placed on buoys floating outside the wind farm. The researchers also collected vast amounts of other meteorological and oceanographic data. The experiments were carried out from May 2015 to October 2016.
The equipment is being rigged to the measurement platform FINO1 near the Alpha Ventus wind farm, part of the presently largest measurement campaign using lidars. Photo: Benny Svardal
“These experiments were by far the greatest accomplishment of the NORCOWE centre. The measurement campaign was unique and the most extensive campaign of its kind undertaken so far. We were able to test several new methods specifically targeting the characterisation of turbulence. The results are promising, but we must take a closer look before we can say anything more than that,” says Professor Reuder.
The experiments near the Alpha Ventus wind farm have made the NORCOWE centre and its partners world leaders in the use of scanning lidars at sea. A number of scientific and other articles about the experiments will be published. The industry is eagerly awaiting the results and hoping for fast answers. One of the interested companies is Statoil.
“Both the research on lidar technology and the trial campaigns are very interesting. As part of the efforts to model wind fields correctly and, thus, simulate the right wind loads for the wind turbines, we are looking forward to the results of analyses based on data from the measurement campaign at FINO1,” states Marte Godvik, Principal Researcher at Statoil. She also held a presentation on wake and large floating wind turbines during the NORCOWE conference, Science Meets Industry, in Stavanger in spring 2016.
Important results for the industry
The industry uses the expression "levelised cost of energy" (LCOE), which is a calculation of the cost of each kilowatt-hour produced over a wind farm’s lifetime. Lidar technology helps both to increase revenue by improving wind farm planning and management and to reduce the costs of producing wind turbines.
“The knowledge gained from the OBLEX-F1 experiments will provide better information for wind farm designers and guidance on how to operate wind farms under different atmospheric conditions. In future, we hope we will also be able to provide key input for a new offshore wind standard for the design of turbines and turbine blades,” Professor Reuder says.
A standard that is excessively restrictive requires producers to use an unnecessarily large amount of steel and carbon fibre. A more accurate standard based on new knowledge will enable producers to use less material and to manufacture turbines at a lower cost.
Major material stresses
The blades on modern wind turbines can reach close to 100 metres in length. This means that wind conditions are not the same along the entire blade. In the worst-case scenario, wind on one end of the turbine blade can flow in one direction while at the other end the wind is from the opposite direction. This is a source of major stress that must be incorporated as a safety factor when turbine producers design wind farm turbines. Suppliers are therefore interested in knowing the wind speed at a given point along the blade compared with the wind speed at another point.
At present, data from wind measurements over land are used when producers are to perform load calculations. The problem is that these measurements are taken on masts which are not as high as large turbines.
“This means that the standards in use today are not suitable for the bigger, modern turbines, which makes it an important research topic,” Professor Reuder says.
Temperature and turbulence
Temperature variation within the 200 meters separating the ocean’s surface and the top of the turbines can have a major impact on a wind farm’s energy production. If the temperature rises as elevation increases – known as inversion – the air will generally be stable and there is little chance of wind layers mixing. This means that the wake is more powerful and that the rows of wind turbines behind the first one will produce much less energy. Under unstable conditions the air currents move more quickly and production increases. The measurement data currently available are not sufficient to qualify the data models used in wind farm planning. As such, it is uncertain whether wind turbines are optimally placed to produce the greatest amount of energy possible.
Laser measurements will also be useful in wind farm operations. Operators can retrieve information about the wake effect and steer their wind turbines so that more air passes through the first row of turbines to reduce the wake effect on the turbines behind. This can improve overall production from a wind farm by a few percent, which may have significant bearing for a wind farm owner’s bottom line.
An important component of the experiments was testing lidars mounted on buoys. They were equipped with motion-compensation systems to enable them to measure the exact airspace targeted by the researchers. These tests were also successful and have contributed to the commercialisation of new technology developed by NORCOWE partners.
“Setting up a mast offshore is expensive and generates a limited amount of information. Putting a lidar on a buoy with motion compensation and towing it out is both a faster and cheaper way to map wind resources for new offshore installations. It also generates more knowledge about wind layers than using a mast. There is now proof of concept for Lidar technology and we believe lidars will become the new gold standard for offshore wind measurement,” says Benny Svardal, researcher at CMR and project manager of the OBLEX-F1 campaign.
He has received considerable attention from Norwegian and international industry stakeholders and research partners interested in the findings from the experiments.
Spin-off: Measuring wind-related loads on bridges
The lidar experiments have not been limited to wind farms.
“We have mostly focused on offshore wind, but we see many other applications for lidar technology in Norwegian industry. For example, identifying turbulence at airports and wind conditions in areas where bridge construction is planned,” Benny Svardal explains.
Project manager of the OBLEX-F1 campaign, Benny Svardal. Photo: CMR
In order to check whether laser measurements can be used in bridge construction, the NORCOWE researchers travelled to Lysefjord accompanied by a 3D-scanning lidar. In cooperation with the University of Stavanger (UiS) and the Norwegian Public Roads Administration, they monitored wind conditions in the area surrounding the Lysefjord bridge site.
Detailed data on wind and turbulence along the entire bridge span and how this changes with the wind direction in the fjord provide important information both for quality-assuring data models that are used in designing new bridges and for adapting bridge constructions to local conditions.
UiS, the Technical University of Denmark (DTU) and CMR have also recently carried out advanced lidar measurements in Bjørnafjord, and the results may be an important contribution to the challenges facing the long fjord-crossings under the “Ferry-free coastal route E39” project.
More information about the measurement campaigns can be found in the presentations from the NORCOWE 2016 Conference.