Wind energy: Measurement flights in the USA successfully completed Large-scale experiment on wind wakes in the Great Plains

Technische Universität Braunschweig has participated in the international wind energy measurement campaign AWAKEN in the USA with aerial measurements. The Klaus Tschira Foundation made it possible for TU’s research aircraft, a Cessna F406, to fly to the USA at the end of August to measure the wakes of wind farms in a large-scale experiment in the Great Plains. The experiment investigated how quickly the wind recovers behind onshore wind farms.

The aim of the measurement campaign was to investigate the wake behind large onshore wind farms. The project used a variety of instruments on and around the wind turbines, including lasers and radars to measure wind flow. The aircraft measurements complement these continuous measurements in areas where no data are available. They can be flexibly adapted to conditions such as wind direction.

Stronger influence of terrain

Compared to the measurements over the North Sea, the influence of the terrain is much greater. Although the Great Plains are flat, the terrain slopes to the east, which causes special wind phenomena. In addition, the ground heats up during the day and cools down at night, causing strong turbulence during the day and a rapid recovery of wind speed.

The D-ILAB research aircraft took off from Braunschweig-Wolfsburg airport on 22 August 2023. Its destination was the Great Plains in Oklahoma, USA, to take part in the measurement campaign as part of the international AWAKEN project. During the five-week measurement campaign, the Braunschweig scientists recorded the characteristics of wind farm wakes at different distances in systematic flights at hub height. In previous years, TU Braunschweig and the Institute of Flight Guidance had already collected important data from flights over offshore wind turbines in the German Bight.

Logistical and aeronautical challenges of the northern route

In just over three days, pilots Maik Angermann, Thomas Feuerle and Matthias Cremer ferried the aircraft west across the Atlantic via the so-called northern route. The ferry flight consisted of a total of seven individual flights. The stopovers were used for refuelling, pilot changes and overnight accommodation for the crew. At the first stop, in Wick, Scotland, the necessary survival equipment, consisting of a life raft and dry suits, was collected. After this stop, the journey continued to Reykjavik in Iceland. On the second day, the crew continued their journey to Narsarsuaq in Greenland and Goosebay in Canada. In Canada, the survival equipment was returned. On the third day, the flight continued to the United States with stopovers in Burlington, Vermont and Fort Wayne, Indiana, before reaching Enid-Woodring, Oklahoma. In Oklahoma, the Braunschweig crew met with scientists and flight measurement engineers Konrad Bärfuss, Jonas Füllgraf and Mark Bitter.

The aircraft was then prepared for the measurement task. The nose mast, which is used to record atmospheric parameters, was transported in the aircraft during the transfer and had to be reassembled in the USA. After a further inspection by the local aviation authority, the first reconnaissance flights took place.

Important meteorological data in 50 flight hours

Flight profiles were planned in consultation with international scientists and adapted to the prevailing weather conditions. In addition, so-called profiles, i.e. ascent and descent flights from the lowest possible altitude to around 1000 metres, were flown regularly to record the distribution of temperature and wind with altitude. In particular, the temperature profile with altitude provides information on how strongly the atmosphere is mixed, so that wind farm wakes are quickly reduced.

Wind farm wakes are most pronounced at night and in the early hours of the morning. The flights therefore took place shortly after sunrise.

“By the end of September, a total of 50 flight hours had been flown and a large amount of important meteorological data had been recorded,” says pilot Maik Angermann. This data was backed up daily to a server at home. While it was night in the USA and the crew slept soundly, the time difference meant that the scientific staff on the other side of the Atlantic were able to carry out initial analyses of the measurement data during the day. The results were immediately incorporated into the flight plan for the following day. On 3 October 2023, the crew returned safely to Braunschweig with D-ILAB.

The data will be analysed at the Institute of Flight Guidance in the coming months and years to determine the recovery of wind speed behind onshore wind farms. The data will also be made available to international research teams. The data is particularly important for checking model calculations and improving models.