Blade runner – wind’s role in post-nuclear Germany

Germany has set itself the goal of eliminating nuclear power by the beginning of the 2020s. In order to compensate for the amount of power currently coming from nuclear power plants while remaining clean, independent and capable of meeting the population’s energy needs, different technologies must be explored and further developed. To achieve this goal, the country has decided to award huge investment to the research and development (R&D) of renewable energy, especially wind power technologies.

The primary motivation for investing in wind power R&D is that different and larger rotors will be necessary to satisfy future energy demands. These rotors will need to be designed with a different approach compared with current methods, since contemporary technologies would simply be incapable of satisfying design requirements.

These new types of rotors will need to adapt themselves to variable wind conditions, ranging from relatively soft midland wind to severe offshore conditions. Besides being capable of withstanding gust loads in diverse environments, wind rotors will also need to become larger in order to generate more power while remaining light enough to be feasible, transportable and mountable. On top of this, they have to guarantee a long service life, low maintenance and repair needs and life cycle costs equal to or lower than those of current rotors. Contemporary prevailing wind rotor technologies are not yet capable of fulfilling all of these conflicting goals, thus motivating the development of new technologies and the design of the next generation of lightweight blades.

Smart solutions

To meet the challenges that come with a nuclear phase-out, a consortium of research and industry partners – including the German Aerospace Center (DLR), the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) and the Joint Center for Wind Energy Research (ForWind) at the University of Hannover and University of Oldenburg – has brought together experts in aerospace and wind energy technologies to develop the wind rotor blades of the future.

New rotor blades use so-called smart devices – many of which are mechanical components used mainly in the aerospace sector – that are capable of modifying themselves seamlessly, or morphing. This allows components to achieve the optimal shape for the task at hand or for the given aerodynamic or load conditions, thereby increasing the overall system’s performance. Adopting these technologies will not only enable the use of blades that can adapt themselves to different conditions, but also enlarge the envelope at which the blades can work, and bring additional benefits such as low maintenance and repair needs.

The Smart Blades project that ran 2012–16 had the goal of studying the potential of using smart technologies for the development of new wind-rotor-blade concepts. To this end, three different technologies were explored. The first part of the project focused on the development of passive smart blades that would be capable of coupling bending and torsion within the structure for modifying global shape while maintaining integrity. The second studied the potential of rotor blades with an active trailing edge to alleviate loads. The third and final part of the project considered the use of rotor blades with an adaptive leading edge to alleviate loads while changing instream airflow conditions. In addition, the consortium explored a series of interdisciplinary topics that are helpful for studying and evaluating the technologies as a whole system, and for analysing other challenges faced by the new blades, such as control and aeroelasticity.

At the ready

Technology that enables the coupling of bending and torsion in the structure makes it possible to construct rotor blades that are capable of adjusting themselves to variable wind conditions in a passive manner. This means they can rotate themselves along their axis in order to achieve an optimal angle of operation, providing, for example,

a smaller exposed surface when there are strong winds so as to prevent damage while still offering good performance. Different approaches for solving this problem were explored in the Smart Blades project, including specially tailored fibre-reinforced composite structures designed to bend under specific loads so that the desired angle for the blade’s profile can be achieved automatically. Having a system that can adapt to this kind of geometry variation without the need for a special actuation system implies not only great weight and cost savings, but also an improvement in performance.

Smart Blades’ successor project, Smart Blades 2, has ambitions to demonstrate and test smart-blade technologies in wind tunnel experiments, as well as on a real wind turbine, so that the performance of the blades can be studied and validated under various conditions, especially those that are difficult to replicate in a test setting.

Another important part of the new project focuses on the validation of the developed tools and methods for the design of the blades to assess the suitability of their introduction and use in industrial applications, showing their feasibility and evaluating their economic potential. In addition, promising concepts resulting from the Smart Blades project will be further studied and developed, and their usability evaluated. This step is necessary not only for knowing the behaviour of the system in greater detail, but also for gaining industry confidence in the technologies, achieving a higher technology readiness level and making preparations for mass production of smart blades.

Within the given time frame, one of the biggest challenges for the Smart Blades 2 project relates to manufacturing processes, as many of the technologies will need to be produced or implemented in ways that are not yet standard in the industry. To meet this challenge, DLR is calling upon the expertise of its Institute of Composite Structures and Adaptive Systems and its Center for Lightweight Production Technology (ZLT), located in the German city of Stade, where the first smart blades will be manufactured.

Working closely with the wind industry, these institutions are leaders in their respective research areas and possess a high level of expertise in the existing process chain for the development and manufacturing of composite material components. ZLT focuses on industrial-scale production technologies and is equipped to handle the entire process chain for components made of fibre-reinforced composites, allowing the scientific and industrial sectors to enhance cooperation in the research of new methods for developing ideal production technologies. The Smart Blades 2 project is an optimal platform for the collaboration of the working

research and industry partners, and for the production of the new blades. Using unique, multifunctional, large-scale facilities, the research centre develops strategies for manufacturing large, high-quality carbon fibre-reinforced plastics (CFRP) components in a cost-effective manner. Every step in the process is studied, integrated and, using the appropriate technical infrastructure, reproduced on an industrial scale, qualities that will further the aims of Smart Blades 2.

State-of-the-art production technologies that will be used for the manufacture of smart blades include highly productive fibre-placement methods and sensor-guided, component-based control of thermally inert curing processes in autoclaves or open moulds. These can be used to manufacture extremely large and complex parts, such as the bending torsion-coupled rotor blades. Moreover, researchers are working on the development of fully automated manufacturing of high-volume components, using dry textile semi-finished products in resin-transfer moulding (RTM) processes. These processes will be further explored in Smart Blades 2 with the intention of implementing them in the production of rotor blades for wind energy systems.

Once the blades are manufactured, the testing phase will begin with the transportation of the newly made blades to the laboratory testing facility of Fraunhofer IWES in Bremerhaven for static and dynamic certified tests of single blades. After completion of these tests, the smart blades will be mounted on an experimental wind turbine where their behaviour can be studied and measured in real environmental conditions.

With the help of the results of these experimental tests, the developed computer models for the aerodynamic and structural behaviour of the blades will be tested, evaluated and validated. These results will allow enhancement of the models and technologies, enabling the development of methods for designing smart blades of different scales, expanding the design potential, and pushing manufacturers towards bigger, more powerful and more efficient wind rotor blades – the final objective of the project.

Active duty

The second and third technologies studied in the Smart Blades project focused on the use of active elements capable of modifying the leading and trailing edges of the blades in a controlled manner, a technology inspired by aircraft high-lift devices such as trailing edge flaps, leading edge slats and droop noses.

It was successfully shown that the implementation of these devices on wind rotor blades can be used not only to reduce aerodynamic loads that would otherwise damage the blades and the wind rotor’s structure, but also to increase the overall performance of the wind rotor. Flow separation along the blades, which often leads to enormous load fluctuations, can be minimised along the blade surface with these technologies.

The main advantage of the new blade system is its acquired ability to change the airfoil’s shape in a rapid manner, allowing the blades to adapt themselves to highly variable wind conditions, such as gusts, and improving their performance under all required circumstances.

The use of an active trailing edge complements the bending torsion technology by allowing the blade to respond in a flexible and fast manner, making it capable of constantly adapting itself to the flow even after sudden changes coming from airflow or structural load variations. In addition, the flaps behave independently and can therefore be individually controlled and automatically react to the heterogeneities of the inflow conditions, a common issue for wind engines with larger rotor diameters.

The technology used for the leading edge, an integral slat system, can be used in two ways. As a passive device, it increases the maximum lift, which reduces the occurrence of flow separation. As an active device, it regulates the gap between the blade’s profile and the slat, allowing control of air inflow to obtain an optimal load distribution over the blade, thereby complementing the use of the first and second technologies for the blade’s profile.

All these technologies will be manufactured and tested as part of the Smart Blades 2 project, which aims to use the results of its precursor in a fruitful manner, further developing technologies and paving the way for implementation in the industry. Despite great technological challenges that still need to be overcome, particularly in production, the nexus between researchers and industry partners in the project provides an optimal foundation for developing and manufacturing the next generation of wind rotor blades to meet the needs of the future.