Field of the Invention

The present invention relates to a wind turbine rotor blade suitable for a wind turbine, said wind turbine rotor blade having a length extending from a first end, e.g. a root end, to a second end, e.g. a tip end, said wind turbine rotor blade further comprises a leading edge and a trailing edge, where a pressure side and a suction side extend between said leading edge and trailing edge and thus defines an airfoil shaped cross section, where said leading edge comprises a leading edge protection area along at least a part of said length of the wind turbine rotor blade, where a leading edge protection means is provided in said leading edge protection area, said protection means comprises a protective layer having a first surface facing against the leading edge and a second surface facing away from the leading edge, said first surface is attached to the leading edge protection area.

Background of the Invention

During the last 15 years or so, the tip speed of wind turbines has increased. The old and smaller wind turbines typically had tip speeds about 60 m/s. The new and larger turbines run typically at tip speeds about 80-90 m/s.

As the tip speed has increased, the problems with leading edge erosion have also become more severe. This is primarily due to the increase in impact energy caused by rain drops hitting the leading edge of the wind turbine. These impacts are often referred to as the water hammer effect.

Since the speed of the blades on the modern turbines has become so high, this water hammer effect has a damaging effect on blade leading edges due to fatigue.

The damages of the blade leading edges cause the aerodynamic efficiency of the tur- bines to drop - often 5% or more. Furthermore, if the leading edges are not repaired, the erosion will continue into the glass fiber structure of the blade, causing severe damages. In worst case the blade structure will open up and the structural strength of the blade will be reduced.

Wind turbine blades often suffer from leading edge erosion after a few years opera- tion. The erosion is mainly caused by fatigue in the leading edge protection coating or tape. Traditionally the leading edge has a relatively stiff structure underneath the protection coating or tape.

Rain erosion at the leading edge of wind turbine blades is one of the most costly quali- ty problems at today's large wind turbine blades.

The fatigue damage occurs primarily because of rain drops hitting the leading edge blade surface at very high speeds. The impact causes very large stresses in the coating, and after a period of 1-4 years, the coating is fatigued, and fall off.

Therefore wind turbine service companies' need to repair the leading edges as soon as possible, after the damages are identified. This is very costly because typically the blades are situated 50 - 100 meters above ground level. Expensive cranes or lifts are needed for workers to get access to the blades. For offshore turbines a rappelling (robe access) technique is often used to save the extremely expensive offshore crane costs. Rappelling can be used in cases where the blades only need a new coating or tape protection. In more severe cases the blades need to be transported to a factory for repair.

The cost of wind turbines can be reduced by increasing the blade tip speed. That is the main reason why blade tip speeds have increased. In not-noise sensitive areas like mountain or offshore sites, the tip speed could be increased more, and thereby the cost of the wind turbines could be reduced more, if a safe solution is found to solve the current erosion problems.

Many paint and tape companies are working hard to solve the problem. New coating systems with better fatigue properties are developed, but little improvements have been achieved so far. Many paint companies have tried to develop coating systems with higher fatigue resistance properties, but even with these new coatings, the leading edges get damaged severely within the first few years of operation.

3M developed an anti-erosion polyurethane tape for helicopters 20 or 30 years ago. This tape has been used at wind turbine blades for 15 years or more, and it is still commonly used on many new blades every day. This is still considered the best or one of the best solutions at the market, although it can only withstand the water hammer effect for a few years in areas with heavy rain like a monsoon area.

WO 2103/092211 Aldescribes a wind turbine rotor blade suitable for a wind turbine with the construction described in the opening clause. This document describes that, in the leading edge protection area, an adhesive layer is provided between a first surface of the protective layer and the original blade leading edge. The document does not describe hat a space can be formed between the original blade leading edge and the first surface of the protective layer in the leading edge protection area. The protective layer is intended to rupture and to be detached from the blade. The protective layer is not intended to last for the lifetime of the turbine.

Therefore a solution that can last for the lifetime of the turbine (20-25 years design lifetime) is urgently needed.

Object of the Invention

It is the object of the present invention to obtain a wind turbine rotor blade with leading edge protection means and in which rotor blade fatigue loads on the leading edge are reduced, which fatigue loads are due to rain drop impact (the water hammer effect).

Description of the Invention

This object is obtained with a wind turbine rotor blade mentioned by way of introduction and which is peculiar in that said first surface of the protective layer comprises a first and a second edge area for attachment, e.g. by adhering, to a first attachment area on the pressure side and a second attachment area on the suction side of said wind turbine rotor blade and that a space is formed between the original blade leading edge in the leading edge protection area and the first surface of the protective layer.

By using a the leading edge protection means as an anti-erosion shield consisting of a very flexible thin shell which can spread out the elastic wave, the fatigue loads in the coating or tape are reduced, resulting in longer lifetime of the leading edge protection.

To achieve this effect a small space which is an air space or a light structure (ex. a PVC foam) should be present between the original blade leading edge surface and the inner part of the leading edge protection means. It has been found, that the under laying blade properties also have a very large influence on the lifetime time of the erosion protection coating. The leading edge stiffness, structural damping and weight are important factors. All of them should be low.

Since the existing blade leading edges usually are quite thick, have a relatively large damping and a relatively high weight, the invention is a leading edge protection means to be mounted on the existing blade leading edges that have lower stiffness, damping and weight.

An air gap or a light foam (ex. a 60 kg/m3 PVC foam) should separate the leading edge from the leading edge protection means. Hereby the fatigue loads at the leading edge coating or tape will be decreased, and the lifetime of the leading edge will be increased.

In a preferred concept of the invention, the leading edge protection means consists of a glass/epoxy laminate at 1.5 mm thickness and a +/- 45 degree fiber orientation. The leading edge protection means is painted with a good quality, relatively soft, anti- erosion coating, ex. Mankiewicz LEP9.

The leading edge protection means may be bonded to the blade about 50-100 mm chord wise direction from the leading edge, leaving an airspace of 1-2 mm between the leading edge of the blade and the inner surface of the anti-erosion shield. When a water drop hits the leading edge protection means with high speed (typically up to 80 - 110 m/s when adding the blade speed and the falling speed of the rain drop), the energy will dissipate more easily into the glass fiber structure of the leading edge protection means and thereby reduce the peak stresses in the anti-erosion coating. When the peak stresses in the coating are reduced, the lifetime of the coating is increased.

The leading edge protection means can be made from any material that can dissipate the energy from the impacts from rain drops, hail and other objects in the incoming air.

Glass fiber epoxy is good, because it has good fatigue properties and low damping properties. It should be relatively thin to achieve a light and flexible leading edge pro- tection means.

The coating can be any coating that has a good resistance to water erosion. It does not need to be a coating. It could also be a tape, similar to the 3M anti-erosion tape, commonly used for wind turbine blades, and originally developed to protect helicopter blades from sand erosion.

According to one embodiment the wind turbine rotor blade is peculiar in that the protective layer is attached to the leading edge protection area, which is provided as an original leading edge surface of the wind turbine rotor blade and which is not provided with recesses or the like to adapt to the protective layer within the wind turbine rotor blade.

With such a construction it is possible to apply the protection layer to a wind turbine rotor blade which does not need any sort of machining or preparation. Hereby an efficient method of attaching the protective layer to an existing wind turbine blade is possible, and furthermore efficient production of a new wind turbine blade is established.

According to a further embodiment the wind turbine rotor blade is peculiar in that the protective layer is a flexible membrane. With such a constat ction a membrane or shell is used for spreading out the elastic wave. Accordingly the fatigue loads are reduced whereby a longer life is possible for the leading edge.

According to a further embodiment the wind turbine rotor blade is peculiar in that the flexible membrane is made of a glass/epoxy laminate, e.g. with a +1-45 degree fibre orientation and has a thickness in the range 1 to 2 mm, preferably around 1,5 mm.

With such a construction of the flexible membrane it is possible to have a long lifetime, and simultaneously the small thickness will not influence remarkably on the effectivity of the wind turbine blade.

According to a further embodiment the wind turbine rotor blade is peculiar in that the flexible membrane is painted with a soft anti-erosion coating.

When painting the flexible membrane with a soft anti-erosion coating, a long lifetime is established, and simultaneously it is ensured that the elastic wave is spread out in an efficient way.

According to a further embodiment the wind turbine rotor blade is peculiar in that the protective layer comprises a third and a fourth edge area for attachment, e.g. by adhering, to a third and a fourth attachment area on the wind turbine rotor blade, and that the third and fourth edge areas connect the first and a second edge areas so that the space is a closed space.

When the protective layer forms a closed space, it is possible to have a space which is solely filled with air or partly filled with air and a flexible foam material. With such a construction the leading edge will have a structure underneath the protective layer which is yielding. Such construction, which is not stiff, contributes to an efficient spreading of the elastic wave. Hereby the fatigue loads are reduced. According to a further embodiment the wind turbine rotor blade is peculiar in that the space is an air-filled space e.g. as one single room or divided into a number of rooms or alternatively as a space. The space may contain light structures e.g. a PVC foam.

As mentioned above it is possible that the space is filled by air as one single room or divided into a number of rooms. The space could contain any other light foam structure which provides a number of air-filled rooms. The important thing is to have a construction which reduces the stiffness and establishes a more yielding leading edge surface.

According to a further embodiment the wind turbine rotor blade is peculiar in that the light structure have a density between 40 and 80 kg/m3, preferably around 60 kg/m3.

Tests have shown that such density would remarkably influence on the lifetime of the wind turbine blade.

The protective layer may be attached to the original leading edge surface of the wind turbine rotor blade by adhering.

It is preferred that the protective layer is adhered to the wind turbine rotor blade. Any suitable glue could be used. It is also possible to establish a mechanical attachment between the protective layer and the wind turbine rotor blade. However, gluing is preferred.

According to a further embodiment the wind turbine rotor blade is peculiar in that the first attachment area on the pressure side and the second attachment area on the suction side of the wind turbine rotor blade are arranged between 5 and 100 mm chord wise direction form the leading edge.

From experience it is known that erosion occurs from 5-10 mm in the chord wise direction. It extends from this area up to 100 mm or more. Accordingly when the first and the second attachment areas of the pressure side and the suction side of the wind turbine rotor blades extend in the area between 5 or 100 mm, a good protection against erosion is obtained. Preferably the leading edge protection means is bonded to the blade about 50-100 mm chord wise direction from the leading edge.

According to a further embodiment the wind turbine rotor blade is peculiar in that the space has a thickness between 1 and 2 mm between the leading edge and the first surface.

With such thickness of the space a reduction of the density is obtained, and the yielding effect is obtained, which protects the yielding edge. Moreover, such thickness of the space together with the above-mentioned thickness of the membrane would not have an influence on the efficiency of the wind turbine rotor blade which could be neglected.

According to a further embodiment the wind turbine rotor blade is peculiar in that the density of a glue joint at the leading edge of the wind turbine rotor blade is less than 2000 kg/m ³ (2 g/cm ³ ), preferably less than 1000 kg/m ³ (l g/cm ³ ), said glue joint comprises the glue layer and the part of the two shells which are arranged opposite each other at each side of the glue layer.

With such density of the glue joint it is possible to reduce the fatigue loads in the leading edge.

According to a further embodiment the wind turbine rotor blade is peculiar in that the glue joint comprises the glue layer and the overlaying composite layers of the wind turbine rotor blade at such glue layer.

When measuring the density it is not the glue itself which is important, it is the combined structure of the glue layer and the overlaying composite layers at the leading edge which are important in order to determine the density at the leading edge of the wind turbine rotor blade. According to a further embodiment the wind turbine rotor blade is peculiar in that the glue joint does not comprise the protective layer.

It is important that the protective layer is not a part of the calculation of the density of the glue joint at the leading edge. Accordingly it is the original leading edge of the wind turbine rotor blade which is used for the calculation of the density.

When using anti-erosion coating today usually the outermost third of the blade will be covered. This is based on a tip speed of about 80-90 m/s. In case the tip speed is in- creased, it is necessary to protect a larger part of the blade. The same consideration will also be valid when applying the protective layer.

It is mentioned that the protective layer could have four attachment areas such that the space between the protective layer and the leading edge is a closed space. However, the space may also be open between the first and the second edge areas of the protective layer.

Different possibilities exist for attaching the protective layer to the leading edge. The protective layer could have an end part in the direction of the root of the blade which is cut off perpendicular to the longitudinal direction and thus is sharp-edged, and the space in the end area could be filled with glue. Alternatively this end part could be gradually reduced in size and be glued to the leading edge of the wind turbine blade. In this way the outermost part of the protective layer would be in close contact with the leading edge of the blade.

The gradual reduction is especially important at the tip of the blade seeing that this is the area where the erosion mainly occurs. Accordingly the structure at the end of the protective layer facing the tip is important.

At the end of the protective layer facing the tip it is possible to use a sandwich foam which has been painted with a flexible anti-erosion coating. This way the end of the space is closed. However, the anti-erosion effect of the protective layer is maintained. Alternatively, the end part of the space facing the tip could be open. This way it is possible for water to leave the space. In some situations this may give rise to problems with dirt which could be accumulated in the space.

The end part of the space facing the tip could also be glued and gradually reduced. Moreover, a new small section of a protective layer could be applied over the open end of the protective layer, and this section could have an open end. Accordingly only a very small part of the space would have problems with accumulation of dirt.

When forming the protective layer, which forms a space, it is also desired to provide the space with drain holes. It is preferred that the drain holes are provided at the leading edge of the blade, whereby the water may be drained into the blade. This way it is possible to have rather large drain holes, which would not have the risk of being stopped by accumulated dirt. Often a standard ring has drain holes in the area close to the tip. It is believed that the drain holes could be circular and have a diameter between 4 and 8 preferably approximately 6 mm.

When drain holes are established in the protective layer, water enters into the space. Such accumulation of water in the space between the protective layer and the leading edge would be obviated. Therefore it is believed that the holes should be established in the leading edge of the blade. There will be no risk of water entering the space between the protective layer and the leading edge seeing that water is almost non- existing inside the blade.

Description of the Drawing

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Fig. 1 shows a wind turbine,

Fig. 2 shows a cross section of a wind turbine rotor blade,

Fig. 3 shows a schematic view of the waves in a product, as a result of a falling rain drop,

Fig. 4 shows a wind turbine rotor blade provide with a leading edge protection means, Fig. 5 shows a curve illustrating the relation between the density of the blade parts at the leading edge and the lifetime og the blade,

Fig. 6 shows a section through the wind turbine rotor blade provided with a leading edge protection means,

Fig. 7 shows a partial view of the wind turbine rotor blade provided with attachment areas for the leading edge protection means,

Fig. 8 shows a protective layer as seen from the first surface thereof,

Fig. 9 shows a partial view of an end part of the leading edge protection means according to a first embodiment,

Fig. 10 shows a partial view of an end part of the leading edge protection means according to a second embodiment,

Fig. 11 shows a partial view of an end part of the leading edge protection means according to a third embodiment,

Fig. 12 shows a partial view of an end part of the leading edge protection means according to a fourth embodiment,

Fig. 13 shows a partial view of an end part of the leading edge protection means according to a fifth embodiment, and

Fig. 14 shows a section through the glue joint at the leading edge of a wind turbine rotor blade.

Detailed Description of the Invention

In the following text the figures will be described one by one, and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

Position number list

1 wind turbine

2 tower

3 foundation

4 nacelle

5 hub

6 wind turbine rotor blade

7 first end of blade (root end) 8 second end of blade (tip end)

9 cross section of a wind turbine blade

10 leading edge

11 trailing edge

12 pressure side

13 suction side

14 leading edge protection area

15 leading edge protection means

16 protective layer

17 first surface of protective layer

18 second surface of protective layer

19 first edge area of first surface

20 second edge area of first surface

21 first attachment area

22 second attachment area

23 space

24 third edge area of first surface

25 forth edge area of first surface

26 rain drop

27 surface

28 schematic part of wind turbine blade

29 shear wave

30 head wave

31 compressional wave

32 reflected wave

33 curve showing relation between lifetime and density 34 drain hole

35 towards root end

36 sharp edged end part

37 glue or foam

38 gradually reduced end part

39 glue

40 towards tip end

41 foam 42 open end of space

43 small section of protective layer

44 glue

45 open end of space in small section of protective layer

46 glue joint

47 glue layer

48 outer overlaying composite layer

49 inner overlaying composite layer

50 third attachment area

51 fourth attachment area

In fig. 1 a typical wind turbine 1 is seen comprising a tower 2 installed at a foundation 3. At the top of the tower 2 a nacelle 4 comprising e.g. a gearbox, a generator and other components is seen. At the nacelle 4 a shaft for carrying a rotor comprising a hub 5 and three wind turbine rotor blades 6 is also installed. The rotor blades 6 are arranged at the hub 5 at a first end 7 called the root end of the rotor blade 6. The second end 8 of the rotor blades 6 constitutes a tip end.

Fig. 2 shows a cross section of a wind turbine rotor blade 6 with a leading edge 10 and a trailing edge 11 connected by a pressure side 12 and a suction side 13 of the specific air foil profile. Both the leading edge 10 and the trailing edge 11 are depicted as an area behind a line as the terms "leading edge 10" and "trailing edge 11" more or less refer to an area and not a well-defined line. One could argue that at least the trailing edge 11 is very well-defined as it is the outermost extreme edge of the blade 6, where the air flow from the pressure side 12 meets with the air flow from the suction side 13, but the trailing edge 11 is very hard to manufacture and to handle with a sharp edge, which is the theoretical optimum, and sometimes the airfoil is designed as a truncated profile, where the trailing edge 11 is blunt. In this case the terms leading edge 10 and trailing edge 11 are to be understood as the area adjacent the extreme outer edges. Fig. 3 shows a situation where a rain drop 27 hits the surface of a wind turbine blade 28. The rain drop 27 will be compressed, and due to the stiffness in a structure the effect of the rain drop will be a shear wave 29. The shear wave results in a head wave and a compressional wave 31. At the opposite side of the wind turbine blade 28, where the rain drop hits, a reflected wave 32 is created.

Accordingly one could say that there is a "back-kick" effect from the structure, when a rain drop hits the surface. This "back-kick" effect causes erosion on the surface of the wind turbine blade hit by the rain drop 27.

Fig. 4 shows a wind turbine rotor blade 6 having a root end 7 and a tip end 8. The blade comprises a leading edge 10 and a trailing edge 11. At the outer end closest to the tip end 8, a leading edge protection area 14 is provided. This area is provided with a leading edge protection means 16 in the form of a protective layer 15 according to the present invention.

Fig. 5 shows a curve 33 which combines the lifetime in relation to the density of the structure of the leading edge of a wind turbine blade. One can see that the curve 33 has a very steep increase at the left hand side when the density increases. Moreover, one can see that the curve 33 at the right hand side has a longer lifetime, when the density decreases. The density of 2 g/ cm ³ involves a lifetime of 3 years. If the density is 1 g/cm ³ or less, one can see that the lifetime of the wind turbine blade would be seven years or more. Fig. 6 shows a section through a wind rotor blade 6 provided with a protective layer 16. The protective layer 16 is attached to the pressure side 12 through the cooperation of a first attachment area 21 and a first edge area 19 of the protection layer 16. The protective layer is attached to the suction side 13 at a second attachment area 22 which cooperates with a second edge area 20 of the protective layer.

The leading protection area 14 is covered by the application of the protective layer 16. When the protective layer is arranged on the leading edge, a space 33 is provided between the leading edge protection area 14 of the wind turbine blade and the first surface of the protective layer, which surfaces against the wind turbine blade. In the space drain holes are provided in order to drain possible water from the space 14 into the interior of the wind turbine blade.

Fig. 7 shows a partial view of the wind turbine rotor blade with the leading edge pro- tection area 14 arranged at the leading edge 10 between the suction side 13 and the pressure side 12. A first attachment area 21 is provided at the pressure side, and a second attachment area 22 is provided at the suction side. A third and fourth attachment area 50, 51 are arranged between the first and second attachment areas 21, 22 thereby forming an enclosed area inside the attachment areas.

Fig. 8 illustrates a protective layer 16 having a first surface 17 provided with a first and second edge area 19, 20 for attachment to the first and second attachment areas 21 and 22. Furthermore, the protective layer 16 comprises a third and a fourth edge area 24, 25 for attachment to the third and fourth attachment areas 50, 51. Hereby a closed space 23 is provided.

As mentioned earlier there need not be the third and the fourth attachment areas and edge areas in which situation the space 23 will be open at the outer end of the protective layer seen in the longitudinal direction of the wind turbine blade.

Fig. 9 shows a partial view of a wind turbine blade 6 provided with a protective layer 16. Only the end part of the protective layer 16 facing the root end as indicated by arrow 35 is illustrated. The end part of the protective layer 16 has a sharp-edged end part 36 which is cut off in a direction substantially perpendicular to the longitudinal direction through the wind turbine rotor blade 6. In this embodiment the open space at the end of the protective the layer is closed with a glue or a foam 37. It is preferably a foam painted with a flexible coating which is used for closing the hole. Hereby the anti-erosion effect would be obtained of the major part of the protective layer which forms the space 23 in the area between the protective layer and the leading edge 10 of the wind turbine rotor blade 6.

Fig. 10 illustrates an alternative form where the end of the protective layer is a gradually reduced end part 38. This end part 38 is through a glue 39 attached to the leading edge 10 of the blade 6. Fig. 11 illustrates the end part of the protective layers facing the tip end as illustrated with the arrow 40. The end part is closed with a foam 41. This foam is preferably painted with a flexible coating in order to close the hole. Also in this situation the anti- erosion effect of the protective layer is obtained seeing that the space 23 exists over the major part of the leading edge protective area covered by the protective layer 16.

Fig. 12 shows an end part of the protective layer 16 facing in the direction towards the tip end as indicated by arrow 40. In this embodiment the end part is left with an open- ing 42. Hereby it is possible for water to drain off.

Fig. 13 shows an alternative embodiment in which the protective layer 16 has a gradually reduced end part 38 which through the glue 39 is attached to the leading edge 10 of the blade 6.

The gradually reduced end part 38 is covered with a small section of a protective layer 43.

This small section of the protective layer 43 is with a glue 44 attached to the gradually reduced end part 38 and provides an open end 45 of the space 23 inside the small section of protective layer 43.

Fig. 14 shows a partial section through a glue joint 46 at the leading edge 10 of a wind turbine rotor blade 6. The glue joint 46 comprises a glue layer 47 and an outer over- laying composite layer 48 and an inner overlaying composite layer 49.

It is noted that the leading edge could also have a glue joint, which has a glue layer arranged between two half shells which are attached to each other through the glue layer. In such construction the glue joint comprises the glue layer and the part of the two shells which are arranged opposite each other at each side of the glue layer.

It is noted that the embodiments illustrated above are examples, and that modifications are possible. It is also possible to combine the features from different embodiments. E.g. it will be possible to provide drain holes 34 in any of the embodiments illustrated.