Cover story from DLRmagazine 173: DLR's WiValdi Wind Energy Research Farm enables full-scale scientific research

Winds of change

The OPUS 1 and OPUS 2 wind turbines surrounded by measuring masts
The DLR WiValdi Wind Energy Research Farm is located among fields and fruit trees on the Lower Elbe

According to the Federal Statistical Office, wind power was the second most important source of energy generation in Germany in 2022. But in order to accomplish the energy transition, wind power must be expanded even further. That means more turbines, but also improved designs. There is still much progress to be made regarding efficiency, cost effectiveness and noise generation.

Since May 2023, DLR has been operating two wind turbines in Krummendeich – amid the idyllic fields and fruit trees along the Lower Elbe, between Cuxhaven and Stade. OPUS 1 and OPUS 2 stretch 150 metres into the sky, not far from several older wind turbines rotating in the predominantly west-southwest wind. The strong breeze enables them to consistently generate electricity from a renewable source. At first glance, the two DLR turbines do not appear significantly different from their neighbours. But up close, the differences become clear. Clusters of black measurement dots are spread out across sections of the rotor blades. They are markings for optical sensors. Then, there are the bright red measurement masts. The third wind turbine, OPUS 3, and another measurement mast are in the planning stage, while the control room is already under construction. Together they form a very special ensemble – the DLR Wind Energy Research Farm, WiValdi.

Understanding and optimising wind power

DLR planned and implemented the research farm in conjunction with the Research Alliance Wind Energy (Forschungsverbund Windenergie; FVWE) and industrial partners. 'WiValdi' stands for 'Wind Validation' and reflects the goal of the researchers – to use scientific methods to determine as precisely as possible what happens to air as it is circulated through the facilities of the research farm. To do this, the researchers are investigating a wide range of flow processes over an extended period of time. These processes range from atmospheric variations to the slightest turbulence around the rotor blades.

Field research – live, on site and at full scale

"WiValdi enables us to conduct research at full scale under real environmental conditions for the first time and thus better understand wind power as a whole with all its influencing factors," says Jan Teßmer, Head of the DLR Wind Energy Experiments Facility, which is responsible for setting up and operating the research farm. In this way, DLR can develop technologies to further increase the efficiency, cost-effectiveness and acceptance of wind energy – together with companies and other research institutions to which the test farm in Krummendeich is also open. In setting up the research farm, DLR is working closely with Enercon, one of the leading manufacturers in the wind energy sector. The two large wind turbines, OPUS 1 and OPUS 2, each have a rated output of 4.26 megawatts.

Future wind turbines in commercial wind farms will not look fundamentally different from those that exist today. "Nevertheless, wind power is far from being entirely understood," says Michaela Herr of the DLR Institute of Aerodynamics and Flow Technology. "On the contrary, we have only just started exploring many important questions." Herr leads the newly founded Wind Energy Department at the Institute and has been working primarily in the field of aeroacoustics for many years. Aeroacoustics is the study of noise generated aerodynamically when air flows around or through components – such as the rotor blades of wind turbines.

"WiValdi enables us to conduct research at full scale under real environmental conditions for the first time and thus better understand wind power as a whole with all its influencing factors."

Jan Teßmer, Head of DLR Wind Energy Experiments Facility

Powerful, yet quiet

Measuring of the rotor blades
Before assembly, all six rotor blades were dynamically measured by the experts at the DLR Institute of Aeroelasticity. In addition, one blade was statically tested in collaboration with Fraunhofer IWES.

The noise generated by wind turbines is often a critical point during discussions about wind power, especially for local residents. According to a survey conducted by opinion research organisation 'forsa' in 2022, acceptance of more onshore wind power is high, at around 80 percent. It is important that it remains that way, particularly in view of the extensive expansion planned over the next few years. Germany's onshore wind turbines currently have a total output of just under 60 gigawatts. This is expected to increase to 115 gigawatts by the end of 2030.

The toothed trailing edges on the rotor blades are referred to as serrations. They reduce noise generation.

There are already many ways to further reduce noise emissions. The use of serrations – modified trailing edges on the rotor blades – is now standard and they can be found on OPUS 1 and OPUS 2 at the WiValdi research farm. These serrations reduce the noise where it originates – directly at the trailing edges of the rotor blades. Adjustments to the turbine control system could also allow for quieter, more efficient operation. It is precisely these aspects that Michaela Herr and her team are keen to investigate. How irritating people find noise pollution depends on the volume and also on the type and characteristics of the sound. For example, the temporally fluctuating sound caused by rotating turbine blades is more disturbing than a steady noise. "With WiValdi, we can investigate directly on site and over a longer period of time how the weather and other local conditions affect the generation and propagation of the sound. At the same time, we measure the performance of the turbines and other important parameters. This allows us to develop better models and create individual analyses for new wind energy sites. In the future, it will be possible to design new farms and position their turbines in such a way that they can be operated as efficiently and quietly as possible under the given conditions and with a maximised service life."

Rotor blades of the wind turbines
The rotor blades are each 57 metres long and weigh approximately 20 tonnes.

Welcome to the realm of sensor and measurement technologies

From their foundations to the tip of their blades, 150 metres up in the air, all of the WiValdi components are equipped with a large number of sensors. These measure temperature, humidity, wind speed, pressure and even the slightest deformation of the rotor blades. They generate a unique and growing amount of data with an unprecedented level of detail. Researchers from all over the world have already expressed to colleagues at DLR their interest in gaining an insight into this unique and ever-growing collection.

Yves Govers from the DLR Institute of Aeroelasticity checks the measurement markings on one of the rotor blades.

Approximately 1500 sensors are installed in the six high-tech rotor blades. They make it possible to comprehensively and scientifically investigate the vibration and load behaviour, of a wind turbine on a real scale during operation for the first time. "Large, lightweight bladesare good for efficiency, but they are also very elastic and flexible," says Yves Govers from the DLR Institute of Aeroelasticity. "They bend and vibrate under the loads caused by the wind, which leads to new technical challenges that we need to study in detail." Together with the DLR Institute of Lightweight Systems and Leibniz University Hanover, which is part of 'ForWind – the Center for Wind Energy Research of the Universities of Oldenburg, Hanover and Bremen', his team equipped the rotor blades with sensors during their manufacture. "Imagine the sensors to be like the human nervous system," says Lutz Beyland from the DLR Institute of Lightweight Systems. "They collect information, monitor the system and indicate where problems could arise." The sensors enable mechanical stresses and material fatigue to be detected at an early stage and the construction methods and system control to be optimised. Before attaching the sensors, the team practiced all of the necessary steps at the DLR Center for Lightweight-Production-Technology in Stade.

The lidar system on the ground measures the wind at an altitude of up to 2000 metres. The microwave radiometer measures temperature and humidity up to an altitude of 10 kilometres.

The first scientific measurement campaigns started in Krummendeich before construction work began. Using a lidar measurement system, the DLR Institute of Atmospheric Physics determined information on wind speed, wind direction and turbulence at the site. This dataset serves as a comparative value against which any changes to the local airflow conditions caused by the wind turbines can be detected. Another important reference is the acoustic site assessment carried out by the DLR Institute of Aerodynamics and Flow Technology.

Construction begins – WiValdi takes shape

Construction began in spring 2021 and marked the advent of an exciting, yet challenging time. All aspects of the large-scale project came together in the hands of a three-strong team from the DLR Wind Experiments Facility – from planning the research wind farm and coordinating processes and contractors to communicating with the citizens of the surrounding communities.

DLR researchers Jakob Klassen and Lukas Firmhofer, and their Department Head, Jan Teßmer, had to quickly familiarise themselves with some entirely new specialist areas and understand the local conditions. The soft marshy soil, for example, which is common near the coast, requires special construction measures to protect the surrounding fruit plantations from soil erosion. "This was something very special for us. It was very different from our usual routines as researchers," say overall Project Manager Jakob Klassen and Lukas Firmhofer from the construction team. In addition to enthusiasm for wind power, the project also called for strong nerves as construction was delayed as a result of the COVID-19 pandemic and a shortage of skilled workers. To keep local people informed about the project and ensure that progress was visible, the team maintained an online construction site diary and organised site visits and information events.

Measurement mast array
The measurement mast can measure the incoming wind from the ground to the tip of a turbine's rotor blade.

he first measurement mast was raised in April 2022. It is located at the western entrance to the research farm, stands 150 metres tall and is equipped with over 100 sensors. The sensors allow it to measure the incoming wind from the ground to the tip of a turbine's rotor blade. The measurement mast array followed in late 2022. This structure connects two outer 100-metre-high masts with a central 150-metre-high mast. They are equipped with sensors based on technology developed by the ForWind partners at the University of Oldenburg. These sensors determine exactly how the wind swirls after passing through the first turbine before hitting the second. "It is very turbulent," says Jan Teßmer. "It is as if the air were passing over cobblestones, so it gets shaken up significantly in the process." Work on the foundations of the wind turbines began at the turn of the year, with further components such as the tower segments, nacelle and generator arriving in spring 2023. All of the electrical components of the wind turbine are located within the container-shaped nacelle. A lidar system is installed on the roof of the nacelle. It measures the wind arriving at and leaving the rotor.

Installation of the high-tech rotor blades

In April and May 2023, the time came to install the six rotor blades. From the DLR team to the crane operators, ground marshals and workers who received the blades in the rotor hub and fastened them with more than 50 bolts each, this last step presented a particular challenge. It was also the most exciting phase, but before it could begin, there was a lot of waiting around. The 57-metre-long, 20-tonne blades could only be lifted and installed using a large crane in good weather and with calm wind conditions. If the wind speed was more than six kilometres per hour, as is often the case on the coast, the team had to wait. Once they made the decision to install a blade and fasten it to the structure, they had to move quickly. Once the blade was lifted by the crane, there was no turning back. The work was completed early in the afternoon of 13 May 2023 and OPUS 2 stood fully assembled in the early summer sunshine.

Lined up – a special arrangement

The WiValdi complex is not yet complete. OPUS 3, the third wind turbine, and the fifth measurement mast are expected to be completed during the course of 2024. The planning work, tenders and preparations are underway. However, the structure and composition of the research wind farm are already clearly unique. The components are positioned one behind the other, facing in the direction of the prevailing wind. Commercial wind farms would not be planned in this way today. Placing a second turbine immediately behind a first locates it within the wake. Here, the second turbine faces weaker wind and very urbulent air. But this is exactly what the researchers intended, as they can now investigate how close together such turbines can be positioned in the future in order to make better use of available space while still achieving the highest possible level of efficiency. Simulations created using the data collected at WiValdi will help answer these questions and serve as a basis for municipalities to designate new areas for wind power. This will prove important within the context of Germany's 'Onshore Wind Energy Act', which requires two percent of the land area in Germany to be allocated to wind energy generation by 2032. In addition to the construction of new wind farms, the further development of wind power is also focused on 'repowering' measures. This involves replacing old wind turbines with new, more powerful and more efficient systems. New research results and technologies, such as those developed at WiValdi can assist with this.

Take a deep breath and keep going

Robert Habeck, German Federal Minister for Economic Affairs and Climate Action, visits WiValdi
From left to right: Anke Kaysser-Pyzalla, Chair of the DLR Executive Board; Falko Mohrs, Lower Saxony State Minister for Science and Culture; Robert Habeck, German Federal Vice Chancellor and Minister for Economic Affairs and Climate Action; Karsten Lemmer, DLR Executive Board Member responsible for Innovation, Transfer and Research Infrastructure; Michaela Herr, Head of the Wind Energy Department at the Institute of Aerodynamics and Flow Technology; and Jan Teßmer, Head of the DLR Wind Energy Experiments Facility.

Just a few days after the assembly of OPUS 1 and OPUS 2, work continued at Krummendeich. Preparations for commissioning began in close cooperation with the turbine manufacturer, Enercon. The DLR Institute of Flight Systems makes sure that the WiValdi infrastructure is fit for research and is responsible for coordinating all the scientific contributions. Project Manager Henrik Oertel, for example, is working with his colleagues to ensure that all data generated at the farm end up in the synchronised data management system that will be available to the farm's users. Trial operations are now underway, during which the interactions between individual systems and installations are being tested. Meanwhile, Robert Habeck, German Federal Minister for Economic Affairs and Climate Action, paid a visit to the research wind farm, which was followed by the official inauguration of WiValdi on 15 August 2023 with partners and guests from government, industry and local authorities, as well as nearby residents, in attendance. Initial research projects and the evaluation of the measurement data collected so far have also begun. Further project applications have been written and submitted, and the first requests for joint tests with industrial companies have been reviewed.

The OPUS 1 system tentatively fed its first electricity into the grid in at the beginning of August. OPUS 2 followed a few weeks later, marking another milestone for the WiValdi team and a preview of the next 20 years, during which the research farm will make a significant contribution to improving sustainable energy supply and enable unique scientific insights.

Wind turbine
DLR developed the research farm with the Research Alliance Wind Energy (Forschungsverbund Windenergie; FVWE).

Interdisciplinary cooperation

The wind energy research area at DLR has an interdisciplinary structure, and benefits in particular from DLR's expertise and experience in aeronautics. Like an orchestra, individual instruments may sound beautiful, but only show their full effect in concert with others. The Wind Energy Experiments Facility at DLR's Braunschweig site operates the WiValdi Wind Energy Research Farm. It also coordinates wind energy research at DLR, to which the Institutes of Aerodynamics and Flow Technology, Aeroelasticity, Flight Systems, Atmospheric Physics and Lightweight Systems contribute.

Shared success – the WiValdi participiants

DLR developed and built the research farm together with its partners in the Research Alliance Wind Energy (Forschungsverbund Windenergie; FVWE). The FVWE combines the expertise of approximately 600 researchers to drive forward the energy supply of the future. It consists of three participating institutions: DLR, ForWind – the Center for Wind Energy Research of the Universities of Oldenburg, Hanover and Bremen – and the Fraunhofer Institute for Wind Energy Systems (IWES).

WiValdi is funded by the German Federal Ministry for Economic Affairs and Climate Action and the Lower Saxony Ministry of Science and Culture. Approximately 50 million euros are being invested in its development.

An article by Denise Nüssle from the DLRmagazine 173


Denise Nüssle

German Aerospace Center (DLR)
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Julia Heil

Editorial management DLRmagazine
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