Paul Kühn is Group Manager for Onshore Site Assessment at the Fraunhofer IWES and is supervising the Inland Wind Measurement Mast research project. © Fraunhofer IWES

BINE interview with Paul Kühn from Fraunhofer IWES in Kassel 15.05.2014

The wind measurement mast is equipped with 39 measuring devices for recording the wind speed, turbulence and other meteorological parameters. © Fraunhofer IWES

During the course of the day, the wind conditions above 100 metres and below 70 metres differ significantly.© Fraunhofer IWES

“Wind energy has excellent prospects inland”One of the highest wind measurement masts in Europe has been standing on top of a wooded hill near Kassel since 2011. The mast is being used to investigate the wind layers between 100 and 200 metres in height and the influence of forest cover and terrain inclinations on the wind profile and generation of turbulence. The measured values, which are obtained using conventional sensors on the mast, are also used to validate and further develop laser-based LiDAR methods. In the interview, Paul Kühn, Group Manager for Onshore Site Assessment at the Fraunhofer Institute for Wind Energy and Energy System Technology IWES, talks about the project’s initial results and future prospects.

BINE Information Service: What special features do the inland meteorological conditions have in regards to wind turbines?Kühn: Compared with wind turbines on the coast, the average wind speed inland is slightly lower. The more complex surface structures make the wind conditions near the ground more turbulent. With increasing mast height, it is also possible here to reach air flows that enable economic operation. High towers and a large rotor surface to rated power ratio are recommended inland. Such a configuration increases the number of full load hours. Icing and lightning are also important issues. Since 2011, we have fortunately only suffered two lightning strikes that have affected the measurement technology on the mast.

How would you rate the wind quality between 100 and 200 metres in terms of using wind energy?Kühn: There is a considerable increase in the mean wind speed with height. Between 100 and 200 metres, the value increases by about 25 per cent. The lower rotor blade has a very different wind than the upper one. This causes considerable stress to the rotors.

What have you found out in terms of turbulence?

The LiDAR error map enables the optimal location for a LiDAR device to be identified before starting the measurement campaign. The red and blue areas indicate large deviations resulting from the measuring principles. © Fraunhofer IWES

Kühn: Above 80 metres, the turbulence roughly corresponds to the conditions on the coast. Among other things, the exact height depends on the forest cover; in other words on the extent to which trees drive turbulent air layers upwards. The time of day also has an influence. Depending on the solar irradiance, boundary layers form in the air. At heights below 70 metres, the average wind speed is highest at midday. It is then exactly at its lowest at heights above 100 metres (left image).

What influence do the forest areas have?Kühn: There was a lack of empirically reliable data available on the influence of forested surface areas for evaluating existing simulation models and the LiDAR measurement methods. Our investigations now enable us to supply this data. This therefore also enables us to quantify the influence of tree heights, the type of forest – in other words coniferous or deciduous – and the steepness of the terrain, and to integrate them into the models.

How did the comparison between the mast and the laser-based LiDAR method turn out?Kühn: We have compared conventional measurement technology using LiDAR not just on “our” large mast but also temporarily using several existing wind masts in other regions. The LiDAR measurement values are already of a very high quality. The errors in comparison to the mast measurements were somewhat smaller than we had expected, and they are dependent on the direction. To put it in quantifiable terms: between plus 2 and minus 4 per cent at the site of our 200 metre mast. The deviations in the LiDAR measurements are due to the measurement principle, and they can be easily corrected. Our results have now provided empirically reliable correction factors for this.

What does that mean for future planning projects?Kühn: With our investigations we want to help reduce measurement uncertainties. Around planned wind turbine sites there are points with higher and lower LiDAR error probabilities that are possibly closely adjacent to one another. We recommend measuring from the areas with the lowest possible error probability. For this purpose we have developed a LiDAR error map based on our own calculations and we are making ourselves available to the market as a partner for future planning projects (left image).

When planning a new wind farm inland, is it therefore sufficient to only measure the wind quality at the site using LiDAR?Kühn: Yes, that generally suffices. Many banks and financiers already accept LiDAR measurements. Technical Guideline 6 “Determination of wind potential and energy yields” from the Federation of German Wind Power and other Renewable Energies (FGW) is currently being correspondingly revised. The great advantage of LiDAR is that it is cheaper and easier to get approval than with masts. But even with LiDAR it’s best to measure over the course of a year in order to record the influence of the seasons. Besides that, however, there are also other recommended combinations of LiDAR with masts. For example, with shorter masts it’s possible to use an additional LiDAR over several months for measuring wind at large heights. Or it can be used for assessing sites when reference values for a neighbouring wind farm are available.

For what other regions in Germany are your results relevant?Kühn: Our project focuses on a fundamental investigation of the wind conditions in wooded low mountain ranges and forest areas. The findings, empirical data and improved and validated simulation models produced so far will be incorporated in all projects taking place in similar geographic regions. The major challenge is to draw conclusions about extensive areas based on small-scale measurements. Clearly, however, individual site assessments will still be indispensable in future.

What are you planning to do next in your research project during the coming months?Kühn: We will conclude our project on utilising inland wind energy, which also includes the mast, early next year and will then publish our final results. Until then, a wind farm already under construction will also have commenced operation. This will enable us to compare our calculations with the actual wind power production. Parallel to this, we will also work on the wind turbine technology work package in collaboration with a

We will conclude our project on utilising inland wind energy, which also includes the mast, early next year and will then publish our final results. Until then, a wind farm already under construction will also have commenced operation. This will enable us to compare our calculations with the actual wind power production. Parallel to this, we will also work on the wind turbine technology work package in collaboration with a manufacturer. We will also clarify the question as to whether recommendations for designing components for future inland wind turbines can be derived from our findings.

And what are your subsequent plans?Kühn: This summer we’re planning to meet at the tall mast on the Rödeser Berg hill for comparative measurements and to share experiences as part of a European-wide collaboration with LiDAR specialists from neighbouring countries. It’s also intended to use wind field scanners for the measurements – which are very expensive and scarce devices. With a special LiDAR system, we’re also planning plan to investigate wind layers up to 2,000 metres in height. Our results will be incorporated in the revision of the European Wind Atlas. Its regional relevance shall be improved and, for example, restrictive nature conservation requirements will be already indicated in the atlas.

What prospects do you see as a researcher for the future of inland wind energy?Kühn: Very good prospects: the sites are limited along the coast and offshore wind farms are still very cost-intensive. Inland wind farms generate electricity close to the consumer, thus reducing the problem of transmission lines over long distances. The energy turnaround is therefore taking place in the regions. This not only applies to Germany – many of our European neighbours also still have considerable potential inland.

(mi)