Wind EnErgy rotor bladEs

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Flow isseparated

Flow isseparated

Flow isattached

Flow isattached

VortexGenerators

The aerodynamic performance around the blade root is relatively poor, because the wing pro file there gradually becomes cylindrical in order for

a connection to be made to a circular flange. In the early days of wind power not much attention was paid to this, as the main aim was to increase the rotor diameter.

As an interim solution, cylindrical extensions were thus even used between the rotor blade and the hub, without taking any account of the fact that these “extenders” themselves had no aerodynamic effects at all. They were simply there to increase the rotor swept area.

Back then it was believed that you could neglect the aerodynamics at the blade root because the speed of blade rotation at that point was low, with the effects of wind speed thus being lowest there. It was only when Enercon introduced rotor blades which were aerodynamically shaped right to the blade root (and were thus widest there), that people realised how terribly this blade root area had been neglected.

Enercon proved that the yield gain from the new blade shape was so high that it outweighed the higher costs of the relatively complicated blade

construction. It has so far not been copied because Enercon protected this innovation in the usual way by taking out comprehensive patents.

Aerodynamic attachments

But the aerodynamics of the inner rotor section, including the blade root, can also be considerably improved with relatively little effort. It mainly comes down to holding off the stall effect that inevitably comes with higher wind speeds for as long as possible .

This is achieved by using vortex generators, which are glued to the rotor blade from the blade root out-wards like a “fence” in a long line (see diagrams). These measures are not new, as they have proven themselves for years on commercial aircraft. They have the same effect on the rotor blades: a stabilisa-tion of the airflow and an improvement in lift, which contributes most to the torque of the rotor.

The vortex generators which are now coming onto the market were jointly developed by Smart Blade GmbH and the TU Berlin, and are available from 3M. The chemicals company developed an especially weather-resistant plastic as a material for this, as well

From passive to active

Passive elements can improve the

aerodynamics at the blade root. This

increases the yield and reduces noise

generation. Even better effects can be

achieved using active blade elements.

This is still some way off, however.

Diagrammatic representation of the effects of vortex generators on a rotor blade Graphic: 3M

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as an extremely long-lasting glue. Tests in the wind tunnel at the TU Berlin showed that through an exact positioning of the vortex generators on the rotor blades, a stalling is delayed and lift can be increased. Furthermore, because the turbulences caused by rotor movement are “smoothed”, less noise is gener-ated and the wind turbine should run more quietly. This still has to be proven in field tests, however.

From passive to active

Siemens reworks the blade root

Last April a 2.3 MW turbine in Iowa (USA) lost one of its 53 m long rotor blades. After a rotor blade of the same type came off just a few weeks later in California, the manufacturer Siemens started a comprehensive investigation and reduced the power of all turbines fitted with this rotor blade. That both accidents occurred in the USA is not surprising, as the Siemens SWT-2.3-108 has mainly been erected there – approximately 700 times so far.

Siemens is one of the wind turbine manufacturers with the most experience in rotor blade construction. It has been manufacturing rotor blades in Denmark for its own use for approximately 15 years and developed a special process based on manufacturing the whole blade in one piece. It is clear that there can be no overshadowing of this process, and is why considerable effort went into finding out the causes.

All the SWT-2.3-108 turbines were inspected, that is a total of approximately 2,100 blades, and it was discovered that 20 blades had been damaged through delamination at the blade root. The origin of this damage was the cylinder, which is part of the blade root and, like the rotor blade itself, is made using glass fibre reinforced epoxy (GFRE) composite. This is pushed between the inner and outer skins of the blade before lamination and glued to these. It thus forms part of the core of the blade root. Numerous radial drill holes around the blade root serve to take up the T-bolts, which fix the blade to the hub.

Because the strongest forces are in effect here, the glueing of the cylinder to the blade skin must be completely reliable. The cylinder is thus cleaned inside and out before glueing and sand-blasted. The roughness of the surface was obviously not sufficient in some cases, however, leading to the disastrous delamination .

As a consequence of this, Siemens is now preparing and checking all cylinders more intensively before glueing. The surface roughness is being tested using an improved method. The damaged rotor blades have been replaced, and as of September, all of the SWT-2.3-108 turbines are back in normal operation.

Numerous bolts connect the rotor blade to the hub. Siemens has discovered that unexpected faults can arise here, in this case on the blade of a 2.3 MW turbine. Photo: Siemens

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Wind EnErgy rotor bladEs

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Boundary layer barrier and air suction

BayWa r.e. Rotor Service GmbH has also proven that you can get more energy out of the blade root area of existing blades. The company found two partners for this: Spitzner Engineers GmbH and the team of Prof. Henry Seifert, who works at the wind power institute fk-wind at Bremerhaven University of Applied Sciences. There is two years of development work in the retrofittable components, which have meanwhile proven their effectiveness in field tests.

The rotors of a stall-regulated Nordex N60 wind turbine were fitted with the retrofit components of the e-ro system in May 2012. Over the one year test period, BayWa r.e. claims to have achieved a perfor-mance increase of 15 %. The retrofit set, certified by DEWI-OCC, mainly consists of a blade root profile, a boundary layer barrier and a winglet, that is a right- angled blade tip.

The blade root is extended and aerodynamically optimised by fitting a profile with a blunt trailing edge (dark blue in the diagram). A boundary layer barrier, together with a passive boundary layer air suction de-sign, improves the flow behaviour such that a laminar flow can be maintained for longer.

The air sucked in at the blade root is emitted through the winglet attached to the blade tip (light blue in the diagram), which reduces the turbulence there. BayWa r.e. stresses that this is a purely passive system with absolutely no electrical or mechanical support, which affects the airflow purely through the centrifugal forces created by the rotor blade. It is thus a low-maintenance design.

BayWa r.e. now wishes to prove that the desired effect is not only limited to stall-regulated wind tur-bines. Before the end of the year, a field test with a pitch-regulated GE 1.5 MW turbine is to begin.

Flexible trailing edge

The aerodynamics experts not only have to look at the blade root, however. Although the rotor blades are very elastic and can bend around the long axis, they are still really a bit too stiff compared to the turbulent air flow. This becomes increasingly important with a

growing rotor diameter. The rotors of current offshore prototypes have a diameter of 150 to 170 m. Espe-cially in turbulent winds they experience very differ-ent loads from one moment to the next.

The pitch regulation common today can only adjust the blade as a whole, however, and thus only reduce some of the load. Vortex generators and other passive elements can play a part in reducing loads by smoothing turbulence, but such measures have limited effects.

Active elements could lower the stresses on the blade structure much more effectively. However, the move from a rigid to a moveable rotor blade requires a lot of risk-taking, because moveable elements are fundamentally less reliable than rigid ones. It is so far completely uncertain what an active system should look like if it is to work reliably for at least 20 years under all weather conditions.

Additionally, active elements require power, while passive ones do not. Sensors and actuators are required to make controlled movements at the trail-ing edge of the blade, for example. This development task is so appealing that several teams are already working on bringing a “smart blade” to market maturity.

Looking to nature

The companies Smart Blade and Tembra are develop-ing a flexible trailing edge which is integrated into the structure of the rotor blade and uses the principles of biomechanics. Variable-shape structures, of which nu-merous examples can be seen in nature, are being tak-en as inspiration. Fish fins are a good example of this.

The trailing edge of the Smart Blade is controlled by a “pneumatic muscle” developed by the company Festo, using bionics for their inspiration. The compa-ny is known around the world for its pneumatic ac tuators and recently created a furore when it pre-sented an artificial seagull that could be controlled like a model plane. The principle is simple: feeding in pressurised air leads to the pneumatic actuators pulling together and exerting a force on a flexible structure. The shape of the trailing edge of the smart blade can thus be controllably altered.

Through continuous changes to the shape of the trailing edge while the blade rotates in the airflow, it would be possible to continuously alter the aerody-namic behaviour. “Turbulence and other operational irregularities can be quickly and controllably counter-balanced,” say the Smart Blade developers: “This reduces the permanent and extreme loads on the rotor blade and reduces the loads on the wind turbine as a whole.”

But this is not all. The flexible trailing edge should not only reduce the loads, but should at some point also be able to take on the complete performance management. “This would make the traditional pitch-ing systems in current wind turbines obsolete,” prom-ise the engineers. It is still a long way until we reach that point, though.

Detlef Koenemann

Extending the trailing edge at the blade root (dark blue) and the winglet fitted to the blade tip (light blue), consider­ably improve the yields. Graphic: BayWa r.e.