Prof. Dr.-Ing. Andreas Reuter & Dr. Stephan Barth & Dr.-Ing. Jan Teßmer: Intelligent rotor blades reduce the pressure on wind energy giants

What is really noticeable about the new generation of wind turbine prototypes is their particularly long rotor blades. These enormous structures are intended to increase the energy yield. Each time the turbine rotates it covers an area as large as several soccer fields and with very different wind conditions in different places. The Wind Energy Research Alliance is developing “intelligent rotor blades”, which regulate the differing loads across this area and thus improve the profitability of wind turbines.

 

The prototypes for the new generation of offshore turbines – an eight-megawatt turbine with a rotor diameter of 164 me­­tres in Denmark and a seven-megawatt turbine with a rotor diam­eter of an incredible 171 metres in Scotland – were erected at the start of the year. The developers have made a considerable leap forward when it comes to the size – the rotor blades are around 20 metres longer than those of current standard turbines. Rotor diameters in the onshore segment are now commonly around 120 metres. The only way to comprehend dimensions like these is to compare them with other technical products: long-haul aircrafts have a wing span of 80 metres.

Rotorblätter sind Hightech-Produkte mit Abmessungen, die die Spannweite eines Langstrecken-Passagierflugzeuges bei weitem übertreffen.

Rotor blades are high-tech products that exceed the wing span of long-distance airliners by far.

Growth in wind turbines is currently mainly driven by the growth in the rotor area. This trend was motivated by the desire to achieve the maximum possible energy yield within the available areas and by the increasing use of locations with weaker wind conditions. The expan­sion of installed wind power capacity around the world is creating a need to develop locations as close to load centres as possible in order to keep energy transport costs down. Many of these locations have only moderate wind conditions, which is why the rotor area relevant for the “wind harvest” has been increased.

In offshore wind energy, the devel­­opment has also been driven by the need to achieve the maximum possible yield, given the infrastruc­ture that has to be set up (founda­tions, grid connection). The higher wind speeds at sea mean that the most cost-effective way to do this is with a com­­­­bination of a large rotor and a high nominal capacity.

The author studied Aerospace Technology at the TU Berlin and gained his doctorate in 1995. He then held managerial positions at various wind turbine manufacturers and engineering offices. Reuter is the Director of the Fraunhofer Institute for Wind Energy and Energy System Technology IWES Northwest.

 

Measuring up to 85 metres in length, the new rotor blades now cover an area the size of several soccer fields with every rotation. However, the gusty nature of the wind within this large area creates very different wind conditions, so that simply adjusting the entire rotor blade us­ing pitch systems is not enough to achieve opti­­mum regula­tion. Innovative technologies such as adjust­­able slats, trailing edges and other systems could in­­fluence the local air flow on rotor blades more accurately and quickly.

Wind turbine manufactures are currently reluctant to de­­velop and use smart blades technology, however, as it presents massive challenges. Completely new models need to be developed for describing the air flow conditions alone, as the previous approaches in wind power are too general and are unable to map many local phenomena. The formation of the actuators cannot be adopted directly from aviation, as the loads and deform­ations on rotor blades are very different from those on wings. A life span of 20 years with minimal mainten­ance must also be achieved. Because wind power components made from composite materials have to be around 10 to 100 times cheaper than aviation components, there is of course a very limited budget for active mechanisms in rotors.

Barth_bearbeitetThe author has been Managing Director of ForWind, the Center for Wind Energy Research of the universities of Oldenburg, Hannover and Bremen, since 2008. Before this, the doctor of physics conducted research in the field of rotor and wind park aerodynamics at the ECN in the Netherlands. Alongside various other roles, Barth is a member of the board of the European Wind Energy Association (EWEA)

In the Smart Blades research project funded by the BMUB, the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES), the Center for Wind Energy Research of the universities of Oldenburg, Han­nover and Bremen (ForWind) and the German Aero­space Center (DLR) aim to prove that the use of active flap systems or passive structural control on rotor blades can achieve more efficient turbine behaviour and ultimately a longer life span. These changes to the design have the potential to allow the entire turbine to be designed in a lighter way with optimum aerodynamic properties, thus saving on material and logistics costs. The key challenge here is to prevent the active mechanisms from ultimately making the rotor blades more fault-prone, heavier and higher-maintenance, which would increase the cost of generating electricity. The goal of the joint research project, which has received 12 million euros in funding, is therefore to prove the feasibility, efficiency and reliability of smart blades.

Passive smart blades are designed in such a way that, when subjected to different aerodynamic load, they not only bend but twist on their axes. This changes the angle of inflow and automatically protects the blade from an excessive load. This technology is the closest to commercial use and the plan is to demonstrate it on a re­search wind turbine in another project directly following this one.

Active smart blades achieve the same effect with the help of deformable components or adjustable flaps, which can be located either at the leading or trailing edge.

Compared to passive systems, the implementation of active systems for adjusting the trailing edge requires significantly more research, especially with regard to the actuator systems, as well as into coupling the active systems to the blade. It is hoped that a blade set can be built and tested for this technology in the fu­­ture, too.

The author studied Construction Engineering at the University of Hannover and gained his doctorate in numerical mechanics in the year 2000. He held managerial positions in research and industry in the field of structural mechanics. He also is the coordinator of wind energy research of the DLR since 2012.

Both technologies will be continued up to the development of finished construction documents during the three-year project.

Fraunhofer IWES has developed a reference turbine with an 80-metre rotor blade in order to allow the comparison of the individual concepts which are in line with state-of-the-art research and industrial use. The methods for the design and load calculation of active and passive intelligent blades have also been modified and developed further. The different behaviours of the blades mean that individual components such as the controller have to be investigated and adjusted with great precision. The behaviour of the innovative reference blade results in a very distinct load situation that needs to be taken into account. New test methods for this have been designed and tested in application.

The work of the researchers at the DLR includes the conceptual implementation of the active elements. This involves investigating the suitability of various concepts for a flexible leading or trailing edge and using generated aerodynamic or structural conditions to evaluate them. The effectiveness of a leading or trailing edge with a variable shape is also investigated numerically in order to define the necessary dimensions and the control be­­haviour of the elements. In order to make production more efficient, new material systems are being tested and work on the automation of production processes is also being conducted at the DLR.

Experiments in a wind tunnel are being used to test a third technology: the implementation of adaptive slats or active leading edges, conducted in the ForWind wind tunnel at the University of Oldenburg.

The researchers at ForWind are also working on interdisciplinary topics such as the validation of flow simulations and the constructive design of the blades, con­trol concepts and reliability analyses of wind turbines with smart blades as a complete system.

The project period will end with a detailed evaluation of the various approaches to identifying potential and weaknesses, to which both the experimental and the numerical results will contribute.

The aim is to develop and assess the expertise, tools and resources that rotor blade manufacturers and the wind power industry need to bring more effective, cost-efficient and reliable solutions onto the market and thus strengthen their competitiveness.