Lightweight Design worldwide

, Volume 11, Issue 4, pp 42–47 | Cite as

Bend-twist Coupling on Rotor Blades for Wind Turbines

  • Zhuzhell Montano
  • Michael Kühn
  • Elia Daniele
  • Jan Stüve
Design Rotor Blade

Scientists at DLR and Fraunhofer IWES have developed a rotor blade with a bend twist coupling rotor blade that turns out of the wind independently when strong loads occurs. This passive measure eases loads on the rotor blades and the entire turbine.

Load-optimized Rotor Blades

Wind energy industry trends are moving towards wind energy plants with longer rotor blades to increase energy yield, especially onshore. However, larger rotor blades mean larger loads for the wind turbine to withstand, this while needing to preserve weight as low as possible. The strongest structural loading conditions usually happen under gust loads. One way of reducing these induced loads is to use the Bend-Twist Coupling (BTC) effect, where rotor blades independently position themselves optimally towards the wind to minimize loads.

This approach was realized in a rotor blade design and was one of the results of the SmartBlades1 project , which ran between 2012 and 2016. The project also examined two additional technologies used to design and construct large rotor blades. Within the frame of the project a passive load-reduction concept leveraging the geometric BTC principle was implemented. This means that the blades bend more strongly when gusts occurs, but also twist increasingly along their span owing to their geometry. Like leaves on a tree, this reduces the exposed area of the rotor blade and its angle of attack; thus avoiding damage while maintaining a high energy yield. One of the project’s outcomes was a design conceived by the Fraunhofer IWES for 20 m long rotor blades, which paved the way for the follow-up project, SmartBlades2.

Over the course of the SmartBlades2 project, four of these demonstrator blades have been produced over the last few months. The first is a demonstrator blade used for performing mechanical tests on a test bench. The last three form a blade set which will be tested on a wind energy plant, Figure 1.
Figure 1

First demonstrator blade (© DLR)

All blades were built at DLR’s Center for Lightweight Production Technology (Zentrum für Leichtbauproduktionstechnologie, ZLP) in Stade (Germany). After the first blade was manufactured in 2017, it was subject to static and dynamic tests at the testing center of the Fraunhofer IWES in Bremerhaven (Germany) between December 2017 and June 2018. These tests made it possible to investigate and verify the behavior and integrity of the designed blades.

In addition to the tests at the Fraunhofer IWES, a complete three blade rotor will be installed in an open-air research system to investigate and measure how the system behaves under real environmental and meteorological conditions. A comprehensive range of tests will be performed on the rotor, which is fitted with many different instruments and its behavior will be mapped out in detail.

A prefab concept was chosen to construct the blades.

The measurement results from the two tests will then be used to validate the design and simulation methods developed in the course of SmartBlades1 and SmartBlades2. The tests will also show how the new technology concepts and production methods hold up. Furthermore, the investigations will demonstrate the economic potential of the BTC concept at a significantly higher technology readiness level (TRL). Ultimately, the research results will be used to improve both the models and the technologies. This will reduce the risk of deploying new methods for designing larger, smarter blades, and extend the design spectrum for such systems. The goal is to provide support for industry partners on their way to manufacturing intelligent blades in mass production processes for future, larger, load-optimized and more efficient rotor blades.

Production of the Blades

The ZLP in Stade and Augsburg (Germany), are the DLR research centers where new types of production technologies for fiber- reinforced composite materials are developed at an industrial scale in collaboration with the industry. These centers have the infrastructure to make components of a length of 20 m and more.

At the end of the SmartBlades1 project, a rotor blade mold was realized to construct the demonstrator blades with geometric BTC. This mold is made of glass fiber-reinforced plastic and includes an electric modeling tool heating unit divided in different heating areas. Before the construction of the rotor blades could start, the design underwent verification by Aero Dynamik Consult (ADC). A prefab concept was chosen to construct the rotor blades, given the lack of any other molding tools available for constructing the four prototypes than the form for the rotor blade itself.

Compared with rotor blades commonly used in the wind energy sector, which are now more than 80 m long, the demonstrator rotor blade, at 20 m long and weighing approximately 1.75 t in total, appears small. However, the difference is slight from a production engineering perspective. Like most modern rotor blades, the demonstrator blades comprise glass fiber-reinforced plastic and were manufactured using a semi-shell sandwich construction method. Here, the two half shells — the pressure and suction sides of the rotor blade — are produced individually, then joined together in a bonding process, Figure 2. Unlike in industrial manufacturing, where the main components of the half shells like the spar cap, the flange inlay and base are prefabricated in specially made molding tools, no other molding tools were used in this project apart from an electrically heated shell tool.
Figure 2

Bonding of the pressure and suction sides of the blade (© DLR)

The production of the base was reduced to the production of a slab. The sandwich panels made in a vacuum infusion process were cut to shape, using a cut wooden template and a template router. The 120 layers of glass fiber of the flange inlay were pre-cut using a CNC cutter and manually laid in and infused in the appropriate area of the shell tool. A try square made from simple chipboard and integrated into the shell tool was used to assist layering. The 18 m long spar cap was also prefabricated in the shell tool. The problem here was laying the 34 individual layers, which, owing to the precision requirements of the intelligent rotor blades, had to be positioned to the exact millimeter. Since such positioning precision is well known from DLR projects in the aerospace industry, various quality assurance measures from aeronautical research were adapted to the production of the rotor blades. Individual layers were positioned using a laser positioning system. A thermographic system that detected not only the temperature distribution of the entire blade half but also the flow front of the resin during infusion was also used. This system helped identifying leakage in the vacuum leakage in the vacuum superstructures. These measures allowed production quality requirements to be fulfilled without any additional tools.

Quality assurance measures from aeronautical research were adapted to produce the rotor blades.

The production of the half shells begun after the prefab components had been manufactured. The flange inlays were positioned first, before the outer laminate of the blade, comprising two layers, was laid up. The spar caps and the sandwich material, consisting of balsa wood and foam, were then integrated. Once the inner laminate of the blade, which also comprised two layers, had been laid up, the blade halves were vacuum-infused. The final prefabricated part, the base, was then positioned in the shell of the suction side using a six-part positioning device. In a final step, the pressure and suction sides were bonded to form the finished rotor blade. This is the most critical production step in rotor blade production. A dry fit was performed beforehand to minimize the risk of a faulty component. This involved trial adhesion using a substitute glue for the adhesive resin, in this case modeling clay. After using this method to determine the exact bonding gap, it was possible to perform adjustments, e.g. through overlamination, and modify the amount of adhesive accordingly.

Test Bench Experiment

After the first blade had been constructed, Figure 3, at the ZLP, it was sent to the certified test center for rotor blades [1] of the Fraunhofer IWES in Bremerhaven, where it was subject to static and cyclical tests between December 2017 and June 2018.
Figure 3

Removal of the demonstrator blade from the mold (© DLR)

The inputs for designing a typical test bench test comprise extreme and fatigue loads based on the blade design [2]. The design test loads were determined by the Fraunhofer IWES using multi-body simulations [2] in accordance with currently valid standards [3, 4]. The loads applied correspond to extreme load cases and the estimated fatigue damage over 20 years of operation, including the appropriate safety factors, and represent the essential operating conditions of a wind turbine generator system [4]. They were also coordinated with industry partners.

Preparation for the tests also involved measuring the external test blade geometry precisely and calculating the position of the center of gravity. They serve to identify manufacturing tolerances. The test had to be redesigned to allow for these tolerances. The fact that the load in the direction of turn reacts very sensitively to the position of the center of gravity meant that the correct application of the pulling forces in terms of amplitude and direction had to be guaranteed. An important part of the test program on the test bench is validation of the structure under extreme loads, Figure 4. The extreme load test for the demonstrator blade comprised six different test phases:
  • ▸ positive and negative flap direction (“out-of-plane” or “flap-wise”), with compression being applied to the suction and pressure sides

  • ▸ positive and negative turn direction (“in-plane” or “lead-lag”), with compression being applied to the leading and trailing edges

  • ▸ positive and negative torsion.

However, the last two tests are not part of a standard configuration. These tests had to be performed, given that validating the behavior of a BTC blade is contingent on calculating the torsional stiffness distribution with sufficient precision. The results will make it possible to calibrate the corresponding strain gauges for the tests on the open-air system, thus guaranteeing the reliable collection of measurement data.
Figure 4

While testing extreme loads, the loads are applied via three hydraulic cylinders (© Fraunhofer IWES, Pascal Hancz)

Up to four different load frame shears were used in all the aforementioned measurement configurations — at 6.7, 9.7, 14.0 and 17.7 m along the length of the blade — to apply the loads. The optimum positioning of the load frames used results from a load distribution that minimizes the differences with respect to the numeric design values, which, in this case, is always below 10 %.

A total of 163 strain gauges were installed on the blade and on four blade bolts. The strains on the blade were measured using cable sensors, an optical measuring system from Aicon and also the SSB Bladevision system, Figure 5. The angles were measured using the same sensors on the basis of gravity and also indirectly through calculating path measurements using the cable sensors.
Figure 5

The optical measurement plates on the hall wall detect the deformation in the three main axes of the blade (© Fraunhofer IWES, Pascal Hancz)

The successful completion of the extreme load test at the end of January 2018 was followed by fatigue tests (positive and negative flap and turn direction) in April 2018. These tests reproduced the entire life cycle of the demonstrator blade by generating accelerated load oscillations over six days and 1,000,000 and 1,500,000 cycles for flap and turn directions respectively. Furthermore, a detailed modal analysis of the test blade was performed in February 2018 by the DLR Institute of Aeroelasticity in Göttingen (Germany), which underpins the planned update of the finite element models by the DLR Institute of Composite Structures and Adaptive Systems in Braunschweig. Comparing the model with the measurements will reveal possible manufacturing deviations and/or imperfections, thereby enabling error tracing and optimization of the developed models.

The applied loads correspond to extreme load cases and fatigue damage from 20 years of operation.

System Tests

One of the main project objectives is to facilitate the introduction of the developed rotor blade technologies to the industry. This can be achieved by developing the technologies to a higher TRL. TRLs allow assessing the degree of maturity of a technology and show how mature and ready for deployment a technology is. This is particularly important when intending to use the researched technology in industrial applications or under real operating conditions.

After the successful construction and test stand testing of the demonstrator blades, the next step en route to a higher TRL is to test out and validate the BTC technology in a relevant environment. To this end, the set of blades produced by DLR will be installed and tested on the Cart 3 (Controls Advanced 600 kW Research Turbine with 3 Blades) test wind energy plant of the National Renewable Energy Laboratory (NREL) in Boulder, Colorado (USA), in the coming months. This special test turbine and the location were chosen because they offer unique test conditions: The Cart 3 is fitted with wide ranging instruments, making it ideally suited for the test program. The wind conditions there range from very low to very high wind speeds as well as gusts. This will allow performing detailed measurements, making it possible to study the behavior of the blades in detail. The main objectives of the test program are:
  • ▸ to test the blades under real conditions and measure their behavior

  • ▸ to test the controls specifically developed for the plant

  • ▸ to test instruments specially developed for the project and integrated into the structure of the rotor blades

  • ▸ to assess and validate the developed blades, instruments, tools and models based on the test data measured during the test program.

The SmartBlades2 consortium is collaborating with the NREL test engineers team in order to achieve these objectives. Rotor blades and measurement instruments are currently being prepared, calibrated and installed for the tests. The test campaign will start at the beginning of the wind season in winter 2018. The validation tasks will already start with data evaluation during the test program and continue until the end of the project in 2019. The results will be made available to the industry for supporting further development of BTC rotor blades and pave the way to introduce this technology in the industry for larger rotor blades.

SmartBlades2 Consortium

The project partners of the Research Alliance Wind Energy are DLR, Fraunhofer IWES and ForWind. Industry partners are GE Global Research, Henkel, Nordex Energy, Senvion, SSB Wind Systems, Suzlon Energy and WRD Wobben Research and Development.


  1. [1]
    IEC 61400-1, Ed. 3: Wind Turbine — Part 1: Design Requirements, EN 61400-1:2005(E)Google Scholar
  2. [2]
    Bätge, M.: Design of a 20 m Blade for the Demonstration Turbine, Deliverable Nos. and of the SmartBlade Project, Fraunhofer IWES, FKZ 0325601 B, 2016Google Scholar
  3. [3]
    IEC 61400-23, Ed. 1: Wind Turbine Generator System — Part 23: Full-Scale Structural Testing of Rotor Blades for WTGSs, April 2014Google Scholar
  4. [4]
    Rules and Guidelines, IV Industrial Services, 1. Guideline for the Certification of Wind Turbines, Edition 2010, Germanischer Lloyd Industrial Services GmbHGoogle Scholar

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Zhuzhell Montano
    • 1
  • Michael Kühn
    • 2
  • Elia Daniele
    • 3
  • Jan Stüve
    • 2
  1. 1.Department of Adaptive SystemsInstitute of Composite Structures and Adaptive Systems of the German Aerospace CenterBraunschweigGermany
  2. 2.Department of Composite Process TechnologyInstitute of Composite Structures and Adaptive Systems of the German Aerospace CenterStadeGermany
  3. 3.Division of Wind Park Planning and OperationFraunhofer Institute for Wind Energy Systems IWESOldenburgGermany

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