DK178313B1

DK178313B1 - Wind turbine blade with air leakage protection

Info

Publication number: DK178313B1
Application number: DK201470769A
Authority: DK
Inventors: Per Langfeldt
Original assignee: Envision Energy Denmark Aps
Priority date: 2014-12-09
Filing date: 2014-12-09
Publication date: 2015-11-30
Also published as: CN105697232B; CN105697232A; JP2016109138A

Abstract

The present invention relates to a wind turbine and a method of operating a wind turbine. The wind turbine comprises at least one wind turbine blade having an inner blade section and an outer blade section separated by a pitch junction. A first sealing element is arranged over an air gap in the pitch junction at the pressure sides of the blade sections. At least one second sealing element is arranged at the suction side of at least one blade section. The first and second sealing elements acts as air barrier to prevent air leakage from the pressure side to the suction side when pitching the outer blade section equal to or less than a predetermined pitch angle.

Description

Wind turbine blade with air leakage protection Field of the Invention

The present invention relates to a wind turbine comprising: a wind turbine tower having a top end, a nacelle arranged at the top end of the wind turbine tower, a rotor hub rotatably connected to the nacelle, at least one wind turbine blade connected to the rotor hub, the at least one wind turbine blade has a first side surface defining a pressure side and a second side surface defining a suction side, wherein the at least one wind turbine blade comprises an inner blade section connected to an outer blade section via a pitch junction, the outer blade section is configured to pitch relative to the inner blade section using a pitch bearing system, and wherein the pitch junction comprises a first end located on the inner blade section facing a second end located on the outer blade section.

The present invention also relates to a method for operating a wind turbine as described above comprising the steps of pitching the outer blade section relative to the inner blade section at wind speeds above a first nominal wind speed under normal operation.

Background of the Invention

It is known that the pitch junction of a partial-pitch wind turbine comprises an air gap situated between the blade extender/inner blade section and the outer blade section. The air gap is formed by a pitch bearing system having a first and a second bearing element/ring mounted to each of the blade sections allowing the outer blade section to pitch relative to the inner blade section. This air gap causes an air leakage from the pressure side to the suction side, particularly during operation, thus sucking air around the pitch junction away from the profile of the pressure side and out onto the suction side. This, in turn, reduces the efficiency of the wind turbine blade and thus the power production of the wind turbine.

US 8403642 B1 discloses a conventional full-span wind turbine blade to which one or two add-on aerodynamic edge extensions are mounted at the blade root. A continuous row of brushes is arranged along the peripheral edge of the combined profile of the blade root and the edge extensions. The bristles of the brushes allow air to pass from the pressure side to the suction side through the air gap between the wind turbine blade and the rotor hub. Furthermore, if bristles of equal lengths are used then the brushes are not able to substantially close off the air gap along the chord of the wind turbine blade due to the outer profile of the rotor hub. The bristles do not provide an effective solution for reducing air leakage between two blade sections further out on the wind turbine blade, since the air pressure as well as air flow on the pressure side during operation increases from the blade root towards the tip end which means that the bristles would flex more and thus allow more air to pass through the air gap.

Another solution is disclosed in DK 177305 B1 wherein one or two bridging elements are arranged inside the air gap for enclosing the air gap between the inner and outer blade sections. The bridging element forms a solid element or a shell element extending along both the pressure and suction sides. If a single bridging element is attached to both blade sections, then the bridging element has to stretch/deform considerably at the trailing edge during pitching or comprise sufficient extra material to accommodate for the pitching. This extra material or the highly elastic material would tend to flex uncontrollably when influenced by the incoming wind. The use of two bridging elements increases the potential contact surface area between the two bridging elements, which in turns increasing the potential wear on the bridging elements and the risk that one of the elements is forced out of engagement with the respective blade section.

Yet another solution is disclosed in WO 2010/046760 A2 wherein a projecting collar is arranged at the pitch junction between the inner and outer blade sections. The collar comprises an aerodynamic fairing extending over most of the air gap and over an anchor plate to which the interconnecting wires located between the individual collars are attached. However, this collar is not able to pitch with the blade sections due to this anchoring, thus an air gap is needed between the collar and either one of the blade sections to allow the blade sections to pitch relative to the fairing.

It is also known to arrange one or more stall fences at or near pitch junction. However, such stall fences are specifically designed to function outside the normal pitching range and at much higher wind speed.

DK 201370434 Al discloses a wind turbine blade having a main blade element and a plurality of airfoil shaped elements mounted to a trailing edge area of the main blade element. Said airfoil shaped elements are spaced apart to form a gap, wherein a rubber lip is arranged along both the pressure and suction sides to seal off the gap thus preventing dirt and moisture from entering the gap.

Object of the Invention

An object of this invention is to provide a wind turbine blade with an alternative solution that reduces air leakage between the inner blade section and the outer blade section.

An object of this invention is to provide a wind turbine blade that reduces air leakage within a normal pitching range.

An object of this invention is to provide a quick and simple method for maintaining the aerodynamic effect of a wind turbine blade within a normal pitching range.

Description of the Invention

An object of the invention is achieved by a wind turbine characterised in that at least one first sealing element is arranged on only one of the side surfaces, e.g. the pressure side, of at least one of the inner and outer blade sections, the at least one first sealing element extends over the pitch junction and contacts the other blade section, e.g. the same side surface thereof, wherein the at least one first sealing element is substantially air impermeable.

The term “arranged at” is defined as the respective element being located directly on a side surface or on an end surface, and no more than 150 millimetres from the adjoining edge between that end and the side surface. The term “air impermeable” means that the respective element is able to substantially prevent air/wind from passing through the element.

This configuration reduces the amount of air leaking from the pressure side onto the suction side when the outer blade section is pitched relative to the inner blade section, thereby increasing the efficiency of the wind turbine blade as well as the power production. The set of first sealing elements is arranged along only the pressure side and optionally further along the trailing or leading edge of the wind turbine blade, unlike conventional skins or covers which extend along the entire peripheral edge of the blade, i.e. along both the pressure and suction sides. This reduces the amount of material needed and simplifies the assembly process. This further enables the first sealing elements to regain their original form even if the outer blade section is pitched beyond the normal pitching range; this is not possible with a conventional skin or cover.

The first sealing elements may be mounted to or integrated into one of the blade sections, e.g. the inner blade section, and extend over the entire pitch junction/air gap. The first sealing elements further contact the pressure side and/or the end of the other blade section, e.g. the outer blade section, thereby effectively closing off the air gap and reducing air leakage. This allows the total contact area between the sealing elements and the blade sections, and thus the wear, to be reduced compared to the use of bridging elements arranged within the air gap.

When the blade sections are aligned with each other, i.e. the pitch angle of the outer blade section is zero, the contact area between the first sealing elements and the other blade section is substantially equal along the pressure side. As the outer blade section starts to pitch within a normal pitching range, this contact area is gradually reduced in the chordwise direction following the angular rotation/displacement of the outer blade section around a central pitching point of the pitch bearing system. If the pitching continues past the normal pitching range in either direction, the first sealing elements are gradually moved out of contact with the other blade section beginning at the trailing edge and ending at the leading edge. When the outer blade section is pitched back towards zero degrees, the first sealing elements are gradually moved into contact with the other blade section again. The outer blade section may be pitched in a clockwise or anti-clockwise direction, and the normal pitching range may be defined by two end values/pitch angles.

According to one embodiment, at least one second sealing element is further arranged at the other side surface, e.g. the suction side, of at least one of the inner and outer blade sections, the at least one second sealing element extends at least towards the other blade section.

The air leakage can be further reduced by placing at least a set of second sealing elements at the other side surface of the respective blade section. The set of second sealing elements is arranged along only the suction side and optionally further along the trailing or leading edge of the wind turbine blade. The second sealing element may be mounted to or integrated into one of the blade sections, e.g. the inner blade section, thereby allowing its free end to move freely relative to the other blade section, e.g. the outer blade section. The second sealing elements are configured to reduce the amount of air entering the air gap and passing out onto the suction side when the first sealing elements are gradually moved out of contact with the other blade section due to the pitching. Air is thereby prevented from leaking from the pressure side to the suction side by, in part, the first sealing elements and, in part, the second sealing elements.

The first and second sealing elements may be configured to reduce the air leakage when the outer blade section is pitched within the normal pitching range of a steady-state, e.g. between zero degrees and a positive or negative pitch angle or between a positive pitch angle and a negative pitch angle. The pitching range may be between 5 and 15 degrees, e.g. 8 or 10 degrees.

Two or more rows of second sealing elements may be arranged at the suction side and/or at an end surface of one blade section. At least one of the rows may be arranged on the end surface so that it substantially follows the pressure side of the other blade section when the outer blade section is placed in a predetermined pitch angle. This allows for an increased reduction of the air leakage.

According to a special embodiment, at least one of the inner and outer blade sections comprises a truncated trailing edge profile, where the at least one first or second sealing element is further arranged at the truncated trailing edge profile.

One or both blade sections may have a truncated trailing edge profile on which the first or second sealing elements are further arranged. Alternatively, one or both blade sections may have a sharp and well-defined trailing edge profile. This prevents air from entering the air gap through the gap at the trailing edge.

Another sealing arrangement, e.g. one or more deformable lips or stall fences, may be provided around the pitch bearing system. The first and/or second sealing elements may more or less abut this sealing arrangement at the pressure and/or suction side so that they together form a more or less continued sealing arrangement around the peripheral edge of the pitch junction. Alternatively, one of the first and second sealing elements may extend further around the leading edge so that dirt and/or moisture are prevented from entering the pitch junction and further into the interior of the blade sections.

According to a preferred embodiment, the at least one first or second sealing element is shaped as a deformable solid element, e.g. a lip.

At least one of the first and second sealing elements, e.g. the first sealing element, is configured as an air/wind impermeable element in the form of a deformable or flexible solid element. The solid element may be a lip, a sleeve, or a plate. The first sealing element may be made of a single or multiple layers of materials, such as polyethene, polyolefin, polyurethane or any other suitable material. The first sealing element may alternatively be provided with any type of air impermeable structure that substantially prevents air/wind from passing through the element. This allows the first sealing element to act as an air barrier at the pitch junction.

According to a preferred embodiment, the at least one second sealing element is shaped as a brush comprising a plurality of deformable bristles, where the bristles face the other blade section.

At least one of the first and second sealing elements, e.g. the second sealing element, is configured as a brush having a support part to which one or more rows of bristles are attached. The bristles may alternatively be integrated into the support part. The second sealing element may be provided with another air/wind permeable structure that substantially allows air/wind to pass through the element. This allows the second sealing element to better adapt to the contours of the other blade section, e.g. the end profile thereof, during pitching.

Alternatively, the second sealing element also comprises an air/wind impermeable structure that substantially prevents air/wind from passing through the element similar to that of the first sealing element. In this configuration, the second sealing element may be a deformable or flexible solid element, such as a lip, a sleeve, or a plate. This allows both the first and second sealing elements to act as air barriers at the pitch junction.

The first and/or second sealing elements may be shaped as a single element or a plurality of adjoining segments. The segments may be overlapping segments. The wind turbine blade, e.g. the inner and outer blade sections, may at the pitch junction have a chord length of at least 2 metres, preferably between 2 and 5 metres or between 3 and 4 metres. The first sealing element may have a width, i.e. length in a spanwise direction, of 0.1 to 1 metre, preferably between 0.2 and 0.5 metre.

The first and/or second sealing element may alternatively be mounted to or integrated into the other blade section, e.g. the outer blade section. In a further alternative embodiment, the first and/or second sealing element may be provided on both blade sections and optionally aligned with each other. In a preferred embodiment, at least another set of second sealing elements may be provided on the other blade section, e.g. at the pressure side or on an end surface thereof.

A gap may be formed between the free end of the second sealing elements and an opposite facing end surface of the other blade section. The gap may be between 5 to 15 millimetres, preferably no more than 10 millimetres. Alternatively, the second sealing element may extend over the edge, e.g. at the suction side, of the other blade section and contact an end surface thereof, e.g. a third end surface facing the pressure side. The second sealing element may have a width, i.e. length in a spanwise direction, at least corresponding to more or less the width of the air gap, e.g. between 1 and 10 centimetres. If one or both blade sections comprise a retracted end surface, i.e. a second end surface, as described later, then the second sealing element may have a width of no more than the width between the retracted end surface and the opposite facing end surface, e.g. a retracted end surface of the other blade section, preferably between 10 and 100 centimetres or between 15 and 50 centimetres.

According to one embodiment, at least one of the sealing elements is made of a hydrophobic material, preferably a superhydrophobic material.

The first and/or second sealing elements may be made of a hydrophobic or superhydrophobic material, i.e. a material that repels water such as glass, silica, titania, or any other suitable material. Alternatively, the outer surface(s) of the first and/or second sealing elements may be processed so that they achieve a hydrophobic property, e.g. by nano-machining, coating, or by applying nanoparticles. This prevents moisture and ice from accumulating on the sealing elements.

According to one embodiment, at least one of the first and second ends comprises a first end surface and a second end surface, wherein the second end surface is placed in a retracted position relative to the first end surface.

The end of the inner and/or outer blade section may comprise a first end part connected to the adjoining side surfaces and optionally the truncated trailing edge. The first end part may further be connected to a second/middle end part. The first end part may have a first end surface facing the other blade section which defines the edge of that blade section. The second end part may have a second end surface facing the other blade section which is placed in a retracted position relative to the first end surface. A third end surface may be located between the first and second end surfaces. The third end surface may be substantially parallel to the pressure side and/or the suction side.

The second sealing elements may be located on the third end surface where their free ends extend towards an opposite facing end surface of the other blade section, e.g. a retracted second end surface. Alternatively, the second sealing elements or another set of second sealing elements may be located on the second end surface.

The partial-pitch wind turbine blade may have a length from the blade root to the tip end of at least 35 metres corresponding to a relative length of 1. The pitch junction may be positioned at a relative length between 0.20 and 0.50 relative to the root end, preferably between 0.30 and 0.40.

An object of the invention is also achieved by a method for operating a wind turbine as described above characterised by providing at least one first sealing element on at least one of the side surfaces of at least one of the inner and outer blade sections, where the at least one first sealing element contacts the other blade section and maintains the aerodynamic effect of the at least one wind turbine blade when the outer blade section is pitched equal to or less than a first pitch angle.

This provides a quick and simple method for maintaining the aerodynamic effect of a wind turbine blade within a normal pitching range. The first sealing element is made of flexible or deformable material allowing it to bend/flex when influenced by the incoming wind. The first sealing element is mounted to one blade section, e.g. the inner blade section, while the free end of the first sealing element will be pushed towards a side surface, e.g. the pressure side, of the opposite blade section, e.g. the outer blade section, when the incoming wind hits the pressure side of the blade sections. This effectively closes off the air gap, thereby preventing air leakage from the pressure side to the suction side.

Pitching the outer blade section during the normal operation, e.g. no further than the first pitch angle, enables the first sealing element to adapt to the angular displacement between the two blade sections due to its flexible characteristics while remaining in contact with the side surface, e.g. the pressure side, of the opposite blade section. The contact area between the first sealing element and the opposite blade section, when seen from the leading edge, will vary along the chord length when pitching the outer blade section.

The pitching system is activated at wind speeds equal to or greater than a nominal or rated wind speed and operated in the normal operation mode. At wind speeds below the nominal wind speed and optionally equal to or greater than a cut-in wind speed, the outer blade section may be in a neutral position, e.g. having a pitch angle of zero. In this operation mode, the first sealing element lies against the side surface, thus maintaining the aerodynamic effect of the wind turbine blade.

According to one embodiment, the at least one first sealing element is moved out of contact with the other blade section, e.g. gradually, when the outer blade section is pitched greater than the first pitch angle, where this pitching reduces the aerodynamic effect of the at least one wind turbine blade.

The first sealing element starts to move out of contact with the other blade section at the trailing edge when the outer blade section is pitched further than the first pitch angle. Pitching further in the same direction means that the first sealing element is moved further out of contact with the opposite blade section, when seen from the leading edge. This allows air to enter the air gap at the pitch junction, thereby causing an air leakage from the pressure side to the suction side. This results in a reduction of the aerodynamic effect of the wind turbine blade and thus a reduction of the power output.

According to a special embodiment, the pitching of the outer blade section is limited to be equal to or less than the first pitch angle during normal operation for preventing air leakage from the pressure side to the suction side.

Pitching in normal operation may be limited to the first pitch angle so that the first sealing element is able to act as an air barrier in more or less the entire normal operation range. The pitching of the outer blade section in normal operation is done with wind speeds equal to or less than a cut-out wind speed. By limiting the normal pitching range to the first pitch angle, the first sealing element remains in contact with the opposite blade section and prevents an air leakage from the pressure side to the suction side. This allows the wind turbine blade to maintain its aerodynamic effect at wind speeds up to the cut-off wind speed, whereas conventional partial-pitch wind turbine blades will experience a reduction in the aerodynamic effect and thus in the power output.

According to one embodiment, the method further comprises providing at least one second sealing element on the other side surface of at least one of the inner and outer blade sections, where the at least one second sealing element extends towards at least the other blade section.

This allows both sides of the pitch junction to be closed off while allowing the outer blade section to be pitched relative to the inner blade section. The first and second sealing elements function together to prevent air leakage during pitching. If placed on the same blade section, then the first and second sealing elements are both moved relative to the opposite facing end of the other blade section, or vice versa. If the first and second sealing elements are placed on either blade section, the first sealing element is moved relative to the second sealing element, or vice versa.

According to a special embodiment, the at least one second sealing element reduces the loss of the aerodynamic effect of the at least one wind turbine blade when the outer blade section is pitched greater than the first pitch angle and equal to or less than a second pitch angle, where the suction side of one of the inner and outer blade sections intersects with the pressure side of the other blade section when the outer blade section is pitched greater than the second pitch angle.

Pitching in normal operation may be limited to the second pitch angle so that the first sealing element acts as an air barrier in a part of the normal operation range. The first sealing element acts together with the second sealing element as air barriers in the other part of the normal operation range to at least reduce the loss of aerodynamic effect. Pitching towards the second pitch angle means that the second sealing element gradually takes over from the first sealing element as the first sealing element is moved out of contact with the opposite blade section. As the second sealing element sees a reduced amount of air compared to the first sealing element, the sealing element may have a different structure and/or configuration that the first sealing element.

Air leaking from the pressure side to the suction side may be further reduced by providing at least two sets of second sealing elements on the same blade section or on either blade sections. Alternatively, the second sealing element may be provided with an air permeable structure, e.g. similar to that of the first sealing element. This allows the suction effect between the pressure and suction sides to be further reduced, thus allowing the first and second sealing elements to substantially maintain the aerodynamic effect of the wind turbine blade.

According to one embodiment, the first pitch angle is between 5 and 15 degrees and/or the second pitch angle is between 6 and 35 degrees.

Pitching in normal operation, e.g. the normal pitching range, may be between zero degrees or a negative pitch angle and the first or second pitch angle. The negative pitch angle may be between -15 and -5 degrees.

The rated or nominal wind speed may be between 10 and 15 metres per second (m/s). The cut-out wind speed may be between 20 and 25 m/s. These two end values define the normal operation range.

Description of the Drawing

The invention is described by example only and with reference to the drawings, wherein:

Fig. 1 shows an exemplary embodiment of a wind turbine according to the invention;

Fig. 2 shows the wind turbine blade of fig. 1 seen from the pressure side with first sealing elements arranged relative to the pitch junction;

Fig. 3 shows a first exemplary embodiment of the pitch junction and the first and second sealing elements;

Fig. 4 shows a second exemplary embodiment of the pitch junction and the first and second sealing elements;

Fig. 5 shows a third exemplary embodiment of the pitch junction and the first and second sealing elements;

Fig. 6 shows a fourth exemplary embodiment of the first and second sealing elements in an un-pitched position;

Fig. 7 shows the first and second sealing elements of fig. 6 in a first pitched position; and

Fig. 8 shows the first and second sealing elements of fig. 6 in a second pitched position.

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.

Reference list 1 Wind turbine 2 Wind turbine tower 3 Nacelle 4 Rotor hub 5 Wind turbine blades 6 Inner blade section 7 Outer blade section 8 Pitch junction 9 First end, blade end 10 Second end, blade end 11 First side surface, pressure side 12 Second side surface, suction side 13 First sealing elements 14 Trailing edge 15 Leading edge 16 First end surface 17 Second end surface 18 Second sealing elements 19 Third end surface 20 End surface

Detailed Description of the Invention

Fig. 1 shows an exemplary embodiment of a wind turbine 1 with a wind turbine tower 2 and a nacelle 3 arranged on top of the wind turbine tower 2. A yaw bearing (not shown) is arranged between the nacelle 3 and the wind turbine tower 2 allowing the nacelle 3 to yaw relative to the wind turbine tower 2 using a yaw system. A rotor hub 4 is arranged relative to the nacelle 3, e.g. rotatably connected to a generator (not shown) inside the nacelle 3 via a rotation or main shaft. At least one wind turbine blade 5, preferably two or three, is mounted to the rotor hub 4. Each wind turbine blade 5 extends outwards from a centre of the rotor hub 4 and comprises a blade root and a tip end.

The wind turbine blades 5 are partial-pitch blades comprising an inner blade section 6 and an outer blade section 7. The inner blade section 6 is coupled to the outer blade section 7 via a pitch bearing (not shown) arranged between opposite facing ends of the two blade sections 6, 7. The pitch bearing allows the outer blade section to pitch relative to the inner blade section using a pitch system. The pitch bearing or pitch junction is located at the relative distance of 0.2 to 0.4 measured from the blade root. The wind turbine blades 5 have a length of at least 35 metres.

Fig. 2 shows the pitch junction 8 located between the inner blade section 6 and the outer blade section 7. The pitch junction 8 comprises a first end 9 located on the inner blade section 6 which faces a second end 10 located on the outer blade section 7. The first and second ends 9, 10 are spaced apart to form an air gap extending from a first side surface 11, e.g. the pressure side, of the blade sections 6, 7 to a second side surface 12, e.g. the suction side, of the blade sections 6, 7.

At least one or more first sealing elements 13 are arranged at the pressure side 11 and extend from a trailing edge 14, e.g. a truncated trailing edge profile, of the wind turbine blade 5, e.g. the blade sections 6, 7, towards a leading edge 15 of the wind turbine blade 5. The first sealing element 13 has a width that is greater than the width of the air gap so that it covers the entire pitch junction 8. The width of the first sealing element 13 is between 0.1 and 1 metre, preferably between 0.2 and 0.5 metre. This prevents air from leaking the pressure side 11 onto the suction side 12 when the outer blade section 7 is pitched relative to the inner blade section 6, thus maintaining the aerodynamic effect of the wind turbine blade 5.

Figs. 3-8 show the pitch junction 8 seen from the trailing edge 14 where the pitch bearing system and other items are omitted for illustrative purposes. Fig. 3 shows a first embodiment of the pitch junction 8 where at least one of the blade ends 9, 10 comprises a first end part, e.g. first end surface 16, and a second end part, e.g. second end surface 17. Both blade ends 9, 10 may comprise the first and second end parts as shown in fig. 3. The first end surface 16 is connected to the adjoining pressure and suction sides 11, 12 while the second end surface 17 is placed in a retracted position relative to the first end surface 16.

The first sealing element 13 is shaped as an air impermeable element, e.g. a solid deformable or flexible lip. The first sealing element 13 is made of polyethene, polyolefin, polyurethane or any other suitable material. In this embodiment, the first sealing element 13 is mounted to the pressure side 11 of the inner blade section 6, e.g. using fastening means like screws or bolts, and contacts the pressure side 11 of the outer blade section 7.

One or more second sealing elements 18 are arranged at the suction side 12 and extend from the trailing edge 14 of the wind turbine blade 5 towards the leading edge 15 of the wind turbine blade 5. The second sealing element 18 is in this embodiment mounted to a third end surface 19 of the blade end 9, e.g. using fastening means like screws or bolts. The third end surface 19 is arranged between the first and second end surfaces 16, 17 and faces the pressure and/or suction side 11, 12.

The second sealing element 18 is shaped as an air impermeable element, e.g. a solid deformable or flexible lip, made of polyethene, polyolefin, polyurethane or any other suitable material. The free end of the second sealing element 18 extends at least over the air gap of the pitch junction 8 and contacts the third end surface 19 of the blade end 10. This further prevents air from leaking from the pressure side 11 to the suction side 12 when the outer blade section 7 is pitched relative to the inner blade section 6, thus further reducing the loss of aerodynamic effect of the wind turbine blade 5.

Fig. 4 shows a second embodiment of the pitch junction 8’ where at least one of the blade ends 9, 10 forms a single end part having a continuous end surface 20. Both blade ends 9, 10 may have a single end part as shown in fig. 4.

In this embodiment, the first sealing element 13’ is mounted to the pressure side 11 of the outer blade section 7 and contacts the pressure side 11 of the inner blade section 6. The second sealing element 18’ is mounted to the end surface 20 of the inner blade section 6. The second sealing element 18’ is in this embodiment shaped as an air permeable structure in the form of a brush with a plurality of bristles. The free end of the bristles extends at least towards the end surface 20 of the outer blade section 7. This reduces the amount of air passing through the air gap and out onto the suction side 12 while allowing the sealing element 18’ to better conform to the contours of the blade end 9. Thereby further reducing the loss of aerodynamic effect of the wind turbine blade 5.

Fig. 5 shows a third embodiment of the first and second sealing elements where the first sealing element 13 is mounted to the pressure side 11 of the inner blade section 6. At least two sets of second sealing elements 18” are arranged at the pitch junction 8.

One set of second sealing elements 18” is arranged at the suction side 12 and mounted to the third end surface 19 of the blade end 10. The free end of this set faces the end surface 17 of the opposite blade end 9. Another set of second sealing elements 18” is arranged at the pressure side 11 and mounted to the third end surface 19 of the blade end 9. The free end of this set faces the end surface 17 of the opposite blade end 10.

The second sealing elements 18” are in this embodiment shaped as air permeable structures in the form of brushes with a plurality of bristles. This further reduces the amount of air passing through the air gap and out onto the suction side 12 while allowing the sealing elements 18” to better conform to the contours of the blade ends 9, 10, thereby even further reducing the loss of aerodynamic effect of the wind turbine blade 5.

Fig. 6 shows a fourth embodiment of the first and second sealing elements where the first sealing element 13 is mounted to the pressure side 11 of the inner blade section 6. The second sealing element 18”’ is arranged at the suction side 12 and mounted to the third end surface 19 of the inner blade section 6.

The second sealing element 18”’ is in this embodiment shaped as an air permeable structure in the form of a brush with a plurality of bristles. The free end of the second sealing element 18’” extends at least over the air gap of the pitch junction 8 and contacts the third end surface 19 of the outer blade section 7. This reduces the amount of air passing through the air gap and out onto the suction side 12 while allowing the sealing element 18’” to better conform to the contours of the blade end 9 of the outer blade section 7, thereby further reducing the loss of aerodynamic effect of the wind turbine blade 5.

The second sealing elements 18’, 18”, 18”’ shown in the embodiments of figs. 4-6 are made of a hydrophobic or superhydrophobic material or any other suitable material.

The operation of the invention will now be described with reference to figs. 6-8. The outer blade section 7 is in fig. 6 pitched to a neutral position, e.g. pitch angle of zero degrees. In this neutral or un-pitched position, the first sealing element 13 contacts the pressure side 11 of the outer blade section 7 substantially evenly in the chordwise direction (indicated in fig. 2). The first sealing element 13 acts as an air barrier over the pitch junction 8 and thus prevents air from leaking from the pressure side 11 to the suction side 12.

Pitching the outer blade section 7 within a predetermined pitching range, e.g. equal to or less than a first pitch angle, causes the contact area between the first sealing element 13 and the pressure side 11 of the outer blade section 7 to decrease as function of the angular displacement of the outer blade section 7 relative to the inner blade section 6, when seen from the trailing edge 14. This prevents any air leakage and thus enables the wind turbine blade 5 to maintain its aerodynamic effect within the pitching range. Pitching the outer blade section 7 beyond the predetermined pitching range, e.g. equal to or less than a second pitch angle, causes the first sealing element 13 to gradually move out of contact with the outer blade section 7, as shown in fig. 7. Air then gradually enters the air gap of the pitch junction 8 via the opening between the first sealing element 13 and the pressure side 11. The second sealing element 18’” then at least reduces the amount of air that passes through the air gap and out onto the suction side 12. This allows the first and second sealing elements 13, 18’” to act together to at least reduce the loss of aerodynamic effect of the wind turbine blade 5.

Pitching the outer blade section 7 even further beyond the predetermined pitching range, e.g. greater than the second pitch angle, causes the second sealing element 18”’ to gradually intersect the first end part of the inner blade section 6, e.g. the pressure side 11 of the outer blade section 7 intersects the suction side 12 of the inner blade section 6, as shown in fig. 8. This causes an air gap being formed at the pitch junction 8, thereby allowing air to leak from the pressure side 11 to the suction side 12. This in turn reduces the aerodynamic effect of the wind turbine 5.

Pitching the outer blade section 7 back towards the neutral position causes the first sealing elements 13, 18”’ to gradually move into contact with the inner blade section 6 again. The air gap at the pitch junction 8 is thereby closed off, thus preventing any air leakage. This allows the wind turbine blade 5 to regain its aerodynamic effect.

The present invention is not limited to the illustrated embodiment or the described embodiments herein, and may be modified or adapted without departing from the scope of the present invention as described in the patent claims below.

Claims

1. A wind turbine (1) comprising: - a wind turbine tower (2) having a top end, - a nacelle (3) arranged at the upper end of the wind turbine tower (2), - a rotor hub (4) rotatably connected to the nacelle (3), - at least one wind turbine blade (5) connected to the rotor hub (4), wherein the at least one wind turbine blade (5) has a first side surface (11) defining a pressure side and a second side surface (12) defining a suction side, wherein the at least one wind turbine blade (5) comprises an inner wing section (6) connected to an outer wing section (7) via a pitch joint (8), wherein the outer blade section (7) is configured to rotate in the pitch relative to the inner arm section (6 ) using a pitch bearing system, in which the pitch assembly (8) comprises a first end (9) located on the inner wing section (6) facing towards a second end (10), which is located on the outer wing section (7), characterized in that that - at least a first sealing element (13) is arranged on only one side of the side surfaces (11, 12), for example. the pressure side, of the at least one of the inner and outer wing sections (6, 7), wherein the at least one first sealing element (13) extends over the pitch assembly (8) and is in contact with the second wing section, for example. a single side surface thereof, wherein the at least one first sealing element (13) is substantially air-impermeable.

2. Wind turbine according to claim 1, characterized in that the at least one second sealing element (18) is further arranged on the other side surface, for example. the suction side of at least one of the inner and outer wing sections (6, 7), wherein the at least one second sealing element (18) extends at least towards the second blade section.

3. Wind turbine according to claim 2, characterized in that at least one of the inner and outer wing sections (6, 7) comprises a truncated trailing edge profile (14), wherein the at least one first or second sealing member (13, 18) is further arranged at the truncated trailing edge profile (14).

4. Wind turbine according to claim 2 or 3, characterized in that the at least one first or second sealing member (13, 18) is designed as a deformable, fixed element, for example. a lip.

5. Wind turbine according to claim 2 or 3, characterized in that the at least one second sealing element (18) is designed as a brush, comprising a plurality of deformable bristles in which the bristles face towards the second blade section.

6. Wind turbine according to any one of claims 1 to 5, characterized in that at least one of the first and second sealing elements (13, 18) is made of a hydrophobic material, preferably a superhydrofobisk material.

7. A wind turbine according to any one of claims 1 to 6, characterized in that at least one of the first and second ends (9, 10) comprises a first end face (16) and a second end face (17), wherein the second end face (17) is arranged in a retracted position relative to the first end face (16).

8. A method of operating a wind turbine according to any one of claims 1 to 7, comprising the steps of: - turning of the outer wing section (7) in the pitch relative to the inner arm section (6) at wind speeds above a first nominal wind speed during normal operation, characterized in that: - providing the at least one first sealing element (13) of only one of the side surfaces (11, 12) of the at least one of the inner and outer wing sections (6, 7), wherein the at least one first sealing element (13) are in contact with the second arm section over the entire length of the at least one first sealing element (13) and retains the aerodynamic effect of the at least one wind turbine blade (5), when the outer blade section (7) is rotated equal to or less than a first pitch angle.

9. A method according to claim 8, characterized in that the at least one first sealing element (13) is moved out of contact with the second wing section, for example. gradually when the outer blade section (7) is rotated in pitch than the first pitch angle, wherein the rotation in the pitch reduces the aerodynamic effects of the at least one wind turbine blade (5).

10. The method of claim 8 or 9, characterized in that the rotation in the pitch of the outer wing section (7) is limited to be equal to or less than the first pitch angle during normal operation to prevent air leakage from the pressure side to the suction side.

11. A method according to any one of claims 8 to 10, characterized in that the method further comprises providing at least one second sealing element (18) on the other side surface of at least one of the inner and outer wing sections (6, 7), wherein the at least one the second sealing element (18) extends at least towards the second blade section.

12. A method according to claim 11, characterized in that the at least one second sealing element (18) reduces the loss of the aerodynamic effect of the at least one wind turbine blade (5), when the outer blade section (7) is rotated more in pitch than the first pitch angle and equal to or lower than a second pitch angle, wherein the suction side of one of the inner and outer wing sections (6, 7) intersecting the pressure side of the second arm section when the outer blade section (7) is rotated more in pitch than said second pitch angle.

13. A method according to any one of claims 8 to 12, characterized in that the first pitch angle is between 5 and 15 degrees and / or the second pitch angle is between 6 and 35 degrees.