Exclusive: Fraunhofer IWES unveils 115-metre-plus blade test rig for large wind turbine rotor blades

The German Fraunhofer Institute for Wind Energy Systems (IWES) has inaugurated a new test rig for 115-metre-plus rotor blades and for current and next-generation offshore turbines. Eize de Vries was at the event and behind the scenes in Bremerhaven.

The huge hydraulic cylinders (yellow) move the blade during testing (pic credit: Hauke Müller)

Fraunhofer IWES officially inaugurated its new blade test rig in Bremerhaven, north-west Germany, on 30 June, with German economic affairs and climate change minister Robert Habeck among the guest speakers (see below).

The rig is now awaiting its first client. Vestas will soon ship in its latest 115.5-metre offshore blade, like those fitted on the V236-15.0 MW prototype operating at the Østerild National Test Centre in western Jutland, Denmark.

Fraunhofer IWES’s 115m test rig was designed and built by CFM Schiller, a German specialist supplier, also specialising in vibration-isolated mounting and engineering projects. The German economic affairs and climate action ministry funded the construction of the test rig, to the tune of around €18 million, while the education and research ministry, the European Regional Development Fund and the state of Bremen, together contributed about another €5 million to build the hall for the test rig.

The new system could be described as a semi-open-air rotor blade test facility. ‘Only’ the weather-sensitive test-rig blade support and blade-clamping area with operating gear such as the rear-mounted hydraulic cylinders are fully covered and protected by the hall against adverse environmental conditions. The largest part of the blades being tested operate in the open air. Another key test facility element are T-slot rails incorporated in the “strong” floor area allowing linkages of cables and additional testing equipment with specific predetermined areas along a blade axis during specific tests.

Only the weather-sensitive test-rig blade support and blade-clamping area with operating gear such as the rear-mounted hydraulic cylinders are fully covered, while the bulk of the blade is outside the hall. Note that for the inauguarion, a failrly small blade was mounted to the rig (pic credit: Hauke Müller)

The test rig is huge from a visual perspective and, more importantly, capable of generating and withstanding maximum bending moments amounting to 160MNm during testing trials. This, according to Steffen Czichon, head of Fraunhofer IWES’s rotor blade department, would correspond to a 2km long fictive “mass-less” rod linking the Fraunhofer IWES test rig to Bremerhaven centre, with 120 people, each weighing around 68kg, hanging at the other outer end.

Size leap

This new test rig is currently one of the world’s largest blade testing devices outside China. It represents a next leap in size Fraunhofer IWES, set against the previous largest test rig for rotor blades up to 90m, inaugurated 12 years ago. This was the world’s largest test rig at the time (2011), when the world’s longest commercial rotor blade measured “only” 61.5 metres. These were the blades fitted on the offshore-dedicated 5MW Repower 5M turbine and its successor, the 6.15MW 6.2M126 model. Remarkably, the company, renamed Senvion, retained the world’s largest size record during 2004-2011, and the size gap with Fraunhofer’s 90-metre test rig capabilities – designed with envisaged future state-of-the-art 8-10MW turbines in mind – closed only years later.

“Before commencing with actual rig development and construction we had to seek public funding, which is a time-consuming process, and we had to find the right sweet spot too,” recalls Czichon. “The ‘115m-plus’ number is a direct reference to the test rig’s strong reinforced-concrete floor space in front of the blade clamping device, but it will allow much larger future blades as well. This can either be accomplished by allowing blade tip sections to stick out over the strong floor, and/or alternatively cutting off the tip sections that, from a blades-testing perspective, are often non-critical.”

Unconventional projects

Besides the 90-metre test rig, Fraunhofer IWES commissioned a smaller testing device in 2009 for 5-6MW turbine blades up to 70m lengths, and both still in active use. The 90-metre device is used most frequently, while the smaller rig is particularly useful for unconventional projects, for validation trials and to explore new test methods. The realisation of each of these three test rigs, and the stepwise scaling leaps in between, had to match available financial public funding conditions and boundaries, Czichon explains.

The individual steps also aligned remarkably well with turbine scaling pace over the years, but the available scaling margin left with the latest and largest 115m-plus test rig has definitely shrunk. This is due to the fast wind-industry scaling pace evident in the latest 14-15MW turbine developments in Europe, and even more considering what is happening in China. Here prototypes of 14MW, 16MW and 18MW with up to 126m-long blades have already or will be installed by Goldwind, MingYang and CSSC respectively. A 17MW Dongfang prototype is planned for late this year or early 2024 with the so far largest firmly announced rotor – at 262 metres, with 128-metre blades. MingYang says it is developing an 18MW-plus turbine with a 280m-plus rotor, and with the scaling race heating up, others are already speaking of 22-25MW and bigger, with corresponding 300- 320MW-plus rotors.

Czichon: “The 115m-plus test rig’s combined blade support and clamping structure is a modular welded steel structural design. This design philosophy offers several benefits, such as allowing future enhancement through structural reinforcement in a rather easy straightforward manner to accommodate larger blade sizes with again higher corresponding loads. Conducting such a mid-life upgrade for equivalent test rigs with the support clamping structure in steel-reinforced concrete would be far more difficult, if not impossible.”

Carried over

Another clever feature carried over from the smaller Fraunhofer IWES test rigs is the blade clamping area (below) that can accommodate multiple blade root sizes and can also be tilted by 12.5 degrees. A novel addition to the latest test rig is a pitch-able front plate. This allows much easier blade mounting and disassembly in a horizontal position, Czichon says, and the blade can be rotated towards the most favourable position for conducting these tasks.

(pic credit: Fraunhofer IWES)

Conventional rotor blade validation up to type certification typically involves sequential testing of a full blade in both flap-wise and edge-wise (within the rotor plane) directions. Conducting increased fatigue loading until failure occurs could thereby be very interesting to gain fresh insight in fatigue limits as an integral part of R&D trials, Czichon explains.

In February 2020, the technology institute conducted an “elliptical biaxial resonant fatigue test” with a full blade and using the 70-metre test rig. This method combines and synchronises the two uniaxial tests in flap-wise and edge-wise directional modes, and is expected to offer reduced testing time – allowing faster time-to-market plus perhaps some cost savings in parallel. The actual testing trials were preceded by a major modelling effort, and the main conclusions were that the combined loading methodology would offer more realistic test outcomes. Another test is planned for this year using the 90m rig.

Czichon: “The physical design involved adding decoupled masses and spring elements, synchronise flap-wise and edge-wise frequencies, and excite the actuators simultaneously to simulate realistic elliptical blade movement. The idea of dual-axis testing itself is old and has been picked up by others as well more recently, but it comes with an inherent complexity: Since natural frequencies of wind turbine blades are very different in flap- and edge-wise direction, the resulting tip deflection pattern is chaotic when blades are tested in resonance. Our approach has overcome this weakness, achieving a 1:1 elliptical excitation, which is more representative of field loads.” 

Low eigenfrequencies

He adds that with the latest blades generation becoming increasingly longer, they show very low eigenfrequencies (natural frequencies), and this translates into correspondingly longer testing times. Test duration has increased from about one year for blades at Fraunhofer’s 90-metre test rig to roughly two years for blades on the new 115m-plus device. This has led internally to fresh thinking on counteracting the issue from new and different directions. One promising approach is a switch to conducting sequential blade fatigue testing in segments, whereby each shorter segment comes with higher eigenfrequencies and the potential to achieve a predefined fatigue damage level within a shorter time.

An essential precondition for this new testing approach is that before the testing starts, the blade designer and certification body predefine a specific blade section along the longitude axis for conducting these tests. Most importantly, this sequential segmented blade testing method has proven to reduce over-testing in the physical test, compared with the ‘minimum’ prescribed target loads by 65%, plus a substantial reduction in testing time. This is also considerably more than achievable with dual-axis elliptical testing, while offering far greater flexibility and cost savings in parallel.

“What third-party clients expect from any blade test is to learn as much as possible within a short period because this saves time and money, and enables getting a blade certification stamp fastest. And because the ongoing trend is for blade development cycles to become increasingly shorter, this also helps Fraunhofer IWES to optimise and utilise its combined test rig facilities to the full”, Czichon concludes.

High-profile inauguration

The inauguration was attended by Robert Habeck, Germany’s economic affairs and climate action minister, who highlighted his government’s ambition to increase the country’s installed offshore wind base from 8GW today to 30GW in 2030. He pointed to the major role science and technology advancement must play to make this ambitious challenge possible.

German climate action and economic affairs minister Robert Habeck at the inauguration (pic credit: Hauke Müller)

 Additional speakers were president of the senate and mayor of Bremen Andreas Bovenschulte, and the city’s senator or science and ports, Claudia Schilling.

Fraunhofer IWES managing director Andreas Reuter, director of research infrastructures and sustainability of the Fraunhofer Society Hans-Otto Feldhütter, and Steffen Czichon represented the technology institute.

Several speakers hinted at much larger envisaged future turbines being developed in the years to come, considering the huge environmental and climate challenges ahead. Fraunhofer IWES’s new test rig would then play a key role in the testing and validation of the corresponding, ever bigger rotor blades. The final speaker, Vestas chief specialist for test and validation Christian Fenselau, said future turbines should not only be bigger but also better, and with a technology maturity level that actually enables meeting 25-30 years design life.