Debris catcher

Abstract

A wind turbine blade having a root end and a tip end and comprising a blade shell defining an internal cavity, the internal cavity having inner surfaces, the wind turbine blade further comprising a debris capture system with a debris capture device on at least one of the inner surfaces of the internal cavity, the debris capture device defining a capture volume and having an open end providing an opening into the capture volume and facing the root end of the wind turbine blade, and a closed end facing the tip end of the wind turbine blade, wherein the closed end has a surface inclined away from the inner surface in a direction towards the root end of the wind turbine blade.

Description

[0001] DEBRIS CATCHER

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a system for capturing debris within an internal cavity of a wind turbine blade.

[0004] BACKGROUND OF THE INVENTION

[0005] During manufacture and / or maintenance of hollow wind turbine blades, debris may enter the internal cavity within the blade. Examples of this debris are adhesive used to attach internal components within the blade, and foam or other wrappings which become loose during operation of the wind turbine.

[0006] As the wind turbine blade rotates, gravity and / or centrifugal force may cause the loose debris to move within the internal cavity. Falling debris may interfere with other internal components, for example impacting upon components or blocking vents in the blade. This may damage the blade and so increase maintenance required on the wind turbine.

[0007] SUMMARY OF THE INVENTION

[0008] A first aspect of the invention provides a wind turbine blade having a root end and a tip end and comprising a blade shell defining an internal cavity, the internal cavity having inner surfaces, the wind turbine blade further comprising a debris capture system with a debris capture device on at least one of the inner surfaces, the debris capture device defining a capture volume and having an open end providing an opening into the capture volume and facing the root end of the wind turbine blade, and a closed end facing the tip end of the wind turbine blade, wherein the closed end has a surface inclined away from the inner surface in a direction towards the root end of the wind turbine blade.

[0009] Defining a capture volume having an opening facing the root end of the blade may allow debris travelling along the inner surface in the tip-wise direction to be captured in the capture volume. Debris may travel in the tip-wise direction due to centrifugal force as the blade rotates or due to gravity once the blade is pointing generally downwards. Capturing debris in the capture volume may prevent the free movement of debris in the blade internal cavity from passing to the tip end of the blade where it may get stuck or may fall the spanwise length of the blade with high inertia and cause interference with other internal components, such as the root end or hub. The closed end having the surface inclined away from the inner surface in the direction towards the root end of the blade may allow debris falling in a root-wise direction along the blade shell inner surface (e.g. due to gravity once the blade is pointing generally upwards) to more easily pass over the debris capture device rather than becoming trapped on the tip-ward side of the device.

[0010] The wind turbine blade may have a leading edge and a trailing edge and a chordwise direction extending between the leading edge and the trailing edge. The wind turbine blade may also have a windward surface and a leeward surface each extending between the leading edge and a trailing edge, and a thickness direction extending between the windward surface and the leeward surface normal to the chordwise direction. The opening may extend either substantially in the chordwise direction, or substantially in the thickness direction.

[0011] Providing an opening generally perpendicular to the spanwise direction of the blade may improve the likelihood of debris falling in the spanwise direction being captured in the debris capture device.

[0012] The debris capture device may be a panel having the inclined surface. The panel may be attached to the inner surface so that the inner surface and the panel together define in part the capture volume.

[0013] The panel may have an attachment portion for attaching the debris capture device to the inner surface.

[0014] The debris capture device may be fixed to the inner surface using adhesive.

[0015] The debris capture device may be fixed to the inner surface during manufacture of the wind turbine blade or during assembly of the wind turbine. Alternatively, the debris capture device may be retrofitted to the inner surface once the turbine has been assembled.

[0016] The device may be fixed to the inner surface using a common adhesive such as that used to attach a shear web extending between windward and leeward sides of the blade shell, which may simplify the assembly process and further allow for the device to be easily retrofitted to blades already installed in turbines. The inclined surface may be substantially planar.

[0017] The inclined surface may be formed in a curved portion of the debris capture device.

[0018] The opening may have a height normal to the inner surface of less than approximately 200 millimetres.

[0019] The height of the opening may be selected to limit the size of debris that may be captured.

[0020] The capture volume may have a height normal to the inner surface. The maximum height of the capture volume may be the same or greater than a height of the opening.

[0021] Providing a capture volume with a greater maximum height than a height of the opening may provide an overhang between the capture volume maximum height position and the opening. This may reduce the likelihood of multiple pieces of debris escaping the capture volume once they have entered the debris capture device.

[0022] The wind turbine blade may further comprise a web extending between windward and leeward sides of the blade shell. The inner surface may be a blade shell inner surface. The debris capture device may extend up to the web such that the web at least partially defines the capture volume.

[0023] The web may prevent debris from escaping the capture volume in a chordwise direction of the blade.

[0024] The wind turbine blade may further comprise a second shear web extending between the windward and leeward sides of the blade shell. The debris capture device may extend between the web and the second web such that the web and the second web at least partially define the capture volume.

[0025] Providing two webs connected to the device creates a capture volume which prevents escape of debris in a chordwise direction from the debris capture device, only allowing movement of debris in the spanwise direction. The debris capture device may further comprise a cap arranged at one side of the opening to at least partially define the capture volume.

[0026] A cap may be used to prevent movement of debris in the chordwise direction once it has entered the capture volume. It may be particularly beneficial to use a cap on a debris capture device adjacent the leading edge and / or the trailing edge of the blade where no web is present.

[0027] The wind turbine blade may have an internal personnel working zone defined as a distance from the root end towards the tip end in a spanwise direction. The zone may extend from the root end of the blade up to a region of the blade where the maximum height of the internal cavity between the windward and leeward sides of the blade shell is at least 1 metre. The debris capture device may be located either at the boundary of the working zone or just outside the working zone towards the tip end and accessible by personnel from within the working zone. The internal personnel working zone may be defined as height between the two shells which allows a person to access in a knelling position as defined in European Standard EN547-1.

[0028] Positioning the device outside of the internal personnel working zone may reduce the risk of the debris capture system becoming a trip hazard for work personnel.

[0029] The device may comprise plastics material. The material may be selected to withstand impacts upon the debris capture device from debris.

[0030] The debris capture device may be provided on the inner surface of the windward or leeward side of the blade shell.

[0031] The debris capture system may comprise a plurality of the debris capture devices arranged chordwise and / or arranged spaced spanwise along the blade.

[0032] Providing a plurality of debris capture devices may increase the likelihood of debris in the internal cavity of the blade being captured.

[0033] The debris capture system may comprise: a first debris capture device on at least one of the inner surfaces of the internal cavity, the first debris capture device defining a first capture volume and having a first open end providing a first opening into the first capture volume and facing the root end of the wind turbine blade, and a first closed end facing the tip end of the wind turbine blade, wherein the first closed end has a first surface inclined away from the inner surface in a direction towards the root end of the wind turbine blade; and a second debris capture device on at least one of the inner surfaces of the internal cavity, the second debris capture device defining a second capture volume and having a second open end providing a second opening into the second capture volume and facing the tip end of the wind turbine blade, and a second closed end facing the root end of the wind turbine blade, wherein the first opening faces the second opening, and wherein the second closed end has a second surface inclined away from the inner surface in a direction towards the tip end of the wind turbine blade.

[0034] The second debris capture device may be arranged to capture debris previously captured in the first capture volume as the blade tip points upwards again. In this case, debris may fall out from the first capture volume due to gravity, but instead of falling to the root end of the blade it may be captured by the second capture volume.

[0035] A spanwise distance between the first opening and the second opening may be between 2 metres and 20 metres.

[0036] The distance between the first opening and second opening may be specified such that debris falling from one end of the blade may pass over one of the first and second debris capture devices and follow a path to enter the other of the debris capture devices.

[0037] The first debris capture device may be the debris capture device of the first aspect of the invention.

[0038] A further aspect of the invention provides a method of maintaining a wind turbine blade, wherein the wind turbine blade is the wind turbine blade of the first or second aspect of the invention, and wherein the wind turbine blade has an internal personnel working zone with a debris capture device arranged at the boundary or just outside the boundary of the internal personnel working zone. The method comprises: work personnel accessing the internal cavity of the wind turbine blade, optionally through the root end of the blade, further optionally through a hatch in the root end of the blade; and work personnel removing debris from the debris capture device(s), wherein the work personnel operate from inside the internal personnel working zone.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0041] Figure 1 shows a wind turbine;

[0042] Figure 2 shows a wind turbine blade;

[0043] Figure 3 shows a perspective view of a chordwise section of a wind turbine blade comprising a debris capture system;

[0044] Figure 4 shows a perspective view of a chordwise section of a wind turbine blade comprising another debris capture system;

[0045] Figure 5 shows a debris capture device;

[0046] Figure 6 shows a further debris capture device;

[0047] Figure 7 shows a perspective view of a chordwise section of a wind turbine blade comprising a debris capture system;

[0048] Figure 8 shows a perspective view of a chordwise section of a wind turbine blade comprising another debris capture system;

[0049] Figure 9 shows a perspective view of a chordwise section of a wind turbine blade comprising another debris capture system;

[0050] Figure 10 shows an end view of the chordwise section of Figure 9;

[0051] Figure 11 shows a perspective view of a chordwise section of a wind turbine blade comprising another debris capture system;

[0052] Figure 12 shows a planform view of a section of a wind turbine blade;

[0053] Figure 13 shows a planform view of a section of a further wind turbine blade;

[0054] Figure 14A-14E shows a path of a piece of debris through a debris capture system;

[0055] Figure 15 shows a further debris capture system;

[0056] Figure 16 shows a further debris capture system; and

[0057] Figure 17 shows a yet further debris capture system.

[0058] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0059] In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, chord, planform and personnel working zone are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt. The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

[0060] The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

[0061] The chord of a blade is the straight-line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction. The term chordwise is used to refer to a direction from the leading edge to the trailing edge, or vice versa.

[0062] A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when the blade is in use, has a higher pressure than a suction surface of the blade.

[0063] A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when the blade is in use.

[0064] The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

[0065] The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same.

[0066] A view which is perpendicular to both of the spanwise and chordwise directions is known as a planform view. This view looks along the thickness dimension of the blade.

[0067] The term web or shear web is used to refer to a longitudinal, generally spanwise extending, reinforcing member of the blade that can transfer load from one of the windward and leeward sides of the blade to the other of the windward and leeward sides of the blade. The term internal personnel working zone is used to refer to a region of a wind turbine blade interior intended for personnel to enter and work in, e.g. for maintenance purposes, and beyond which personnel should not access. An internal personnel working zone may be defined as a distance from the root end of a turbine blade in a spanwise direction. The spanwise length of the internal personnel working zone may be determined based on the maximum height and / or width of the internal cavity of the wind turbine blade at a threshold value of that maximum cavity height and / or cavity width. The blade cavity height threshold value may be 1 metre. The blade cavity width threshold value may be 0.8 meters. These values may be derived from European Standard EN547-1. It may be beneficial to remove potential trip hazards from the personnel working zone to reduce the likelihood of work personnel falling whilst working on the blade.

[0068] Figure 1 shows a wind turbine 10 including a tower 12 mounted on a foundation and a nacelle 14 disposed at the apex of the tower 12. The wind turbine 10 depicted here is an onshore wind turbine such that the foundation is embedded in the ground, but the wind turbine 10 may be an offshore installation in which case the foundation would be provided by a suitable marine platform.

[0069] A rotor 16 is operatively coupled to a generator (potentially via a gearbox) (not shown) housed inside the nacelle 14. The rotor 16 includes a central hub 18 and a plurality of rotor blades 20, which project outwardly from the central hub 18. It will be noted that the wind turbine 10 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 16 is mounted at the nacelle 12 to rotate about a substantially horizontal axis defined at the centre at the hub 18. While the example shown in Figure 1 has three blades, it will be realised by the skilled person that other numbers of blades are possible.

[0070] When wind blows against the wind turbine 10, the blades 20 generate a lift force which causes the rotor 16 to rotate, which in turn causes the generator within the nacelle 14 to generate electrical energy.

[0071] Figure 2 shows an example of one of the wind turbine blades 20 for use in such a wind turbine. The blade 20 has a root end 21 proximal to the hub 18 and a tip end 22 distal from the hub 18. The blade 20 includes a leading edge 23 and a trailing edge 24 that extend between the root end 21 and tip end 22. The blade 20 includes a suction surface 25 and a pressure surface 26. A thickness dimension of the blade extends between the suction surface 25 and the pressure surface 26.

[0072] The blade 20 has a cross section that may be substantially circular near the root end 21. The blade portion near the root must have sufficient structural strength to support the blade portion outboard of that section and to transfer loads into the hub 18. The blade 20 may transition from a circular profile to an aerofoil profile moving from the root end 21 of the blade towards a "shoulder" 59 of the blade, which is the widest part of the blade 20 where the blade 20 has its maximum chord. The blade 20 has an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade, which extends from the shoulder to the tip end 22.

[0073] The wind turbine blade 20 includes an outer blade shell 29 defining a hollow interior space referred to as an internal cavity. A least one shear web may extend internally between upper and lower parts of the blade shell 29. The wind turbine blade 20 may further comprise a plate 27 fastened to the blade shell 29 at the root end 21 of the blade. The plate 27 caps the internal cavity, and in some cases is used to help mount the blade 20 to the hub 18. The plate 27 may comprise a hatch 27a to selectively permit access to the internal cavity. At least one inner surface 28 is defined within the internal cavity. This may for example be the inner surface of the blade shell 29 or the wall of a shear web, for example.

[0074] Debris such as excess adhesive fragments, loose internal components (e.g. screws, brackets, foam pieces etc.) may become trapped in the internal cavity. This may occur during operation of the wind turbine 10, for example as adhesive fragments break off under load. Once trapped inside the blade 20, the debris is free to move due to gravity and / or centrifugal force as the blade 20 rotates about the hub axis. When moving under gravity and / or centrifugal force, debris may be described as falling. Falling debris may interfere with or damage the blade shell 29 and internal components of the blade 20 such as webs, cables, sensors and other electronic components, the plate 27 (where present) or components within the hub (particularly when the plate 27 is not present).

[0075] The wind turbine blade 20 therefore comprises a debris capture system as shown for example in Figures 3 and 5, with a debris capture device 30 on the inner surface 28. The debris capture device 30 is arranged to limit movement of falling debris by defining a capture volume 32 in which debris may become trapped. The debris capture device 30 has an open end providing an opening 34 into the capture volume 32 and facing the root end 21 of the blade 20, and a closed end 36 facing the tip end 22 of the blade. As such, debris falling in a tip-wise direction may pass through the opening 34 and into the capture volume 32 where further movement in the tip-wise direction is prevented by the closed end of the device 30.

[0076] As the blade 20 rotates, debris within the blade will move relative to the blade due to gravity in a direction determined by the orientation of the blade. When the tip end 22 of the blade 20 is pointing generally upwards, debris will fall towards the root end 21 of the blade 20. Conversely, when the tip end 22 of the blade 20 is pointing generally downwards, debris will fall towards the tip end 22 of the blade 20. In this way debris will alternate between falling towards the tip end 22 and towards the root end 21 as the blade 20 rotates about the hub axis.

[0077] Since the direction in which the debris falls will alternate as the blade 20 rotates, it is beneficial to encourage the debris to pass over the debris capture device 30 when it is falling in a root-wise direction such that it can enter the capture volume 32 once its direction of falling changes. Therefore, the closed end of the debris capture device 30 has a surface 38 inclined away from the inner surface 28 in a direction towards the root end 21 of the blade 20. Debris falling generally along the inner surface 28 in a rootwise direction will encounter the surface 38. Inclining the surface 38 towards the root end 21 of the blade 20 may provide a path for the debris to move away from the inner surface 28 and over the debris capture device 30, before falling back towards the inner surface 28. This may reduce the likelihood of debris becoming trapped outside of the debris capture device 30 where further movement is possible.

[0078] The blade 20 has a leading edge 23 and a trailing edge 24 which define a chord of the blade. The opening 34 of the debris capture device 30 may extend substantially in the chordwise direction. Arranging the opening 34 in the chordwise direction may increase the likelihood of debris falling in the spanwise direction entering the opening 34. Figure 3 shows an example of a debris capture device 30 attached to an inner surface 28 of the blade shell 29 with the opening 34 extending substantially in the chordwise direction. The debris capture device 30 may be a panel having the inclined surface 38. The panel may be manufactured as a single component. The panel may be attached to the blade shell 29 so that the inner surface 28 of the blade shell 29 and the panel together define the capture volume 32. The panel may have an attachment portion 37 as shown in Figure 3 for attaching the debris capture system 30 to the inner surface 28. The debris capture device 30 may be attached to the inner surface 28 using adhesive 31 , or by fastening for example.

[0079] The debris capture device 30 may be attached to the inner surface 28 during manufacture of the blade 20 or may be retrofitted once the wind turbine 10 has been assembled.

[0080] The debris capture system may comprise a plurality of debris capture devices 30. Each debris capture device 30 may be provided on the inner surface 28 of the windward 26 or leeward side 25 of the blade shell 29, such as illustrated in Figure 4. Each debris capture device 30 may be substantially identical and each may be arranged with its opening 32 facing the root end 21 of the blade 30. Providing debris capture devices on both the windward 26 and leeward 25 sides of the blade shell 29 may be beneficial for capturing debris moving in the spanwise direction and across the thickness of the blade 20.

[0081] The inclined surface 38 may be substantially planar as illustrated in Figure 5. Where the inclined surface 38 is substantially planar, the surface may extend at a constant angle from the inner surface 28. Alternatively, the inclined surface 38 may be formed in a curved portion of the debris capture device 30 as shown in Figure 6.

[0082] The opening 34 has a height H measured normal to the inner surface 28. The height H of the opening 34 may be less than approximately 500mm, preferably less than approximately 300mm, further preferably less than approximately 200mm. Limiting the height H of the opening 34 limits the size of debris that may enter the capture volume 32. The capture volume also has a height measured normal to the inner surface 28, and a maximum height H2 defined as the greatest height of the capture volume 32. Figure 5 illustrates an example where the maximum height H2 of the capture volume 32 is equal to the height H of the opening 34. Alternatively, Figure 6 illustrates an example where the maximum height H2 of the capture volume 32 is greater than the height H of the opening 34. The maximum height H2 of the capture volume 32 may be equal to or greater than the height H of the opening 34. Providing a capture volume 32 with a greater maximum height H2 than the height H of the opening 34 may create an overhang between the opening 34 and the capture volume 32. This overhang may be beneficial in reducing the likelihood of debris escaping the debris capture device 30 once it has entered the capture volume 32.

[0083] The wind turbine blade 20 may further comprise a web 40 extending between the windward and leeward sides of the blade. The internal surface 28 may be a surface of the web 40 as illustrated in Figure 7, such that the debris capture device 30 is attached to the web 40. In this arrangement, the opening 24 may extend in a thickness direction perpendicular to the chord of the blade 20. The debris capture device may extend up to the windward surface 26 and / or the leeward surface 25 of the blade shell 29. The blade shell 29 may entirely block one or two sides of the capture volume 32.

[0084] Where the debris capture device 30 is attached to the inner surface of the blade shell 29, the debris capture device 30 may extend up to the web 40 such that the web 40 at least partially defines the capture volume 32 as illustrated in Figure 8. In some cases, the web 40 may entirely block one side of the capture volume 32. Where the web 40 extends in the spanwise direction, the portion of the web adjacent the debris capture device may be substantially perpendicular to the opening 34.

[0085] In some cases, the wind turbine blade 20 comprises a web 40 and a second web 42 as illustrated in Figures 9 and 10. The debris capture device 30 may extend between the web 40 and the second web 42 such that the web 40 and the second web 42 at least partially define the capture volume 32. The web 40 and the second web 42 may entirely block two sides of the capture volume 32. This may prevent debris from escaping the capture volume 32 in a chordwise direction.

[0086] It will be appreciated that the blade 20 may comprise any number of webs such as a web 40, a second web 42 and a third web 44 as shown in Figure 11. Each web may be arranged parallel to the other webs. The blade 20 may comprise a plurality of debris capture devices 30 each extending between two webs 40, 42, 44.

[0087] The debris capture device 30 may comprise a cap 39 arranged at one side of the opening 34 to at least partially define the capture volume 32. This arrangement may be particularly beneficial where the debris capture device 30 is arranged adjacent the leading edge 23 or the trailing edge 24 of the blade 20. The cap 39 may entirely block one side of the capture volume 32. The cap 39 may be manufactured as an integral part of the debris capture device 30 or may be a separate component attached to the debris capture device 30 during installation of the device. Although only shown on debris capture devices 30 attached to the blade shell 29, it will be appreciated that a debris capture device 30 attached to a web 40 may also comprise a cap 39.

[0088] The debris capture device 30 may be positioned in the blade 20 to improve ease of maintenance, for example removing debris from the capture volume 32. The blade 20 may have an internal personnel working zone 50 defined as a distance from the root end 21 towards the tip 22 end in a spanwise direction. This zone is intended for personnel 52 to enter and operate in, for example when conducting maintenance on the blade 20. Figures 12 and 13 illustrate a person 52 operating in the internal personnel working zone 50, including a dashed line to indicate the boundary of the working zone 50. As shown in Figure 12, the debris capture device 30 may be located at the boundary of the working zone. Alternatively, the debris capture device 30 may be positioned just outside the working zone 50, just outside here defined as being accessible by personnel 52 from within the working zone as illustrated in Figure 13.

[0089] The debris collection system may comprise a plurality of the debris capture devices 30 arranged chordwise and / or arranged spaced spanwise along the blade 20. Each debris capture device 30 may be substantially identical, or each may be a different size and / or have a different profile. Where the debris capture system comprises a plurality of debris capture devices 30 arranged spaced in the spanwise direction, the distance between adjacent debris capture devices may be between around 2 metres and 10 metres. The distance between adjacent debris capture devices 30 may be measured between the opening 34 of the device located closer to the tip end 22 of the blade 20 (the tip-ward device) and the inclined surface 38 of the device located closer to the root end 21 of the blade (the root-ward device). It may be preferable to define the distance between debris capture devices 30 in this way as this is the distance over which debris falling over the tip-ward device may fall back towards the inner surface 28 before it encounters the inclined surface 38 of the root-ward device.

[0090] Figures 14A to 14E illustrate how a piece of debris 60 may cascade in a generally rootward direction through the blade 20 by being captured in adjacent debris capture devices 30. It is beneficial to direct debris in a generally root-ward direction as debris closer to the root end 21 of the blade 20 is easier to remove than debris at the tip end 22. Each figure shows a blade 20 with a debris capture system comprising a plurality of debris capture devices 30. Between each figure, the blade 20 completes one half rotation about the hub 18 such that the blade 20 alternates between the tip end 22 pointing upwards and downwards. This is illustrated by the root and tip labels and the direction of gravity in each figure. The illustrated example has a tip-ward device 30a located towards the tip end 22 of the blade 20 and a root-ward device 30b located towards the root end 21 of the blade. It will however be appreciated that the steps may generally be applied to any debris capture system comprising more than one debris capture device 30.

[0091] A piece of debris 60 is initially located towards the tip end 22 of the blade as shown in Figure 14A. As the blade 20 completes one half turn, the tip end 22 points upwards and gravity causes the piece of debris 60 to fall towards the root end 21 of the blade 20. As the piece of debris 60 encounters the tip-wards device 30a, the inclined surface 38 provides a path to move the piece of debris 60 away from the inner surface 28. Once passed beyond the root-ward device 30a, the piece of debris 60 may fall back to the inner surface 28. Each debris capture device 30a, 30b may be positioned on an inner surface 28 such that the direction of rotation of the blade 20 encourages the piece of debris 60 to fall back towards the inner surface 28. The dashed outlines and arrows in each of Figures 14B to 14E illustrate the path of the piece of debris 60.

[0092] Depending on the speed of rotation of the blade 20 and the distance between the tipward device 30a and the root-ward device 30b, the piece of debris 60 may fall back to the inner surface 28 between the two devices 30a, 30b as shown in Figure 14B. In this case, as the blade 20 continues to rotate such that the tip end 22 is pointing downward gravity will cause the piece of debris 60 to fall into the capture volume 32 defined by the tip-ward device 30a as shown in Figure 14C. Further tip-ward movement of the piece of debris 60 is prevented by the closed end 36 of the tip-ward device 30a.

[0093] The blade 20 completes another half rotation between Figures 14C and 14D, such that the tip end 22 of the blade is pointing upwards, and gravity causes the piece of debris 60 to fall towards the root end 21 of the blade. The piece of debris 60 encounters the root-ward device 30b and the inclined surface 38 of the device provides a path to move the piece of debris away from the inner surface 28. Once past the root-ward device 30b, the piece of debris 60 may fall back towards the inner surface 28. In the final step illustrated in Figure 14E, the blade 20 completes a final half rotation such that the tip end 22 is pointing downward and gravity acts to pull the piece of debris 60 towards the tip end of the blade. The piece of debris 60 then enters the capture volume 32 of the root-ward device 30b and is captured. Further tip-wise movement of the piece of debris 60 is prevented by the closed end 36 of the root-ward device 30b. The piece of debris 60 may then be more easily removed from the root-wards device 30b during maintenance of the blade 20.

[0094] In a further embodiment of the invention, the debris capture system comprises a first debris capture device 70 at the inner surface 28 and a second debris capture device 80 at the inner surface 28. The first debris capture device 70 defines a first capture volume 72 and has a first open end providing a first opening 74 into the first capture volume and facing the root end 21 of the turbine blade 20. The first debris capture device 70 also comprises a closed end 76 facing the tip end 22 of the wind turbine blade 20. The first debris capture device 70 may be the debris capture device 30 previously described.

[0095] The second debris capture device 80 defines a second capture volume 82 and has a second open end providing an opening 84 into the second capture volume 82 and facing the tip end 22 of the blade 20. The second debris capture device 80 also comprises a second closed end 86 facing the root end 21 of the blade 20. The first debris capture device 70 and the second debris capture device 80 are arranged such that the first opening 74 faces the second opening 84. Examples of this debris capture system are illustrated in Figures 15 and 16.

[0096] The spanwise distance between the first opening 74 and the second opening 84 may be approximately 2 metres. The two debris capture devices 70, 80 may limit spanwise movement of debris to the region between the two devices. Debris captured in the first debris capture device 70 may escape the first capture volume 72 when the blade tip end 22 points upward and gravity pulls the debris towards the root end 21 of the blade 20. The debris may then enter the second capture volume 82 through the second opening 84, where further movement in the root-wards direction is prevented by the closed end 86 of the second debris capture device 80. The same process may occur in reverse when the blade 20 rotates to point the tip end 22 of the blade downwards and debris is free to fall from the second debris capture device 80 into the first debris capture device 70. Limiting movement of debris to the region between the two devices may reduce inertia of the debris whilst falling and limit the area and / or components that the debris may interfere with.

[0097] Where the blade 20 comprises an internal personnel working zone as described with reference to Figures 12 and 13, the spanwise distance between the first opening 74 and the second opening 84 may be equal to the length of the working zone. For example, the first debris capture device 70 may be located at the boundary of the working zone and the second debris capture device 80 may be located at the root end 21 of the blade 20.

[0098] It may be beneficial to provide a first capture volume 72 having a greater maximum height measured normal to the inner surface 28 than the height of the first opening 74, as illustrated in Figure 16 and described previously with reference to Figure 6. This may result in the first debris capture device 70 having an overhang between the first opening 74 and the first capture volume 72. The overhang may be beneficial in reducing the likelihood of debris escaping the first debris capture device 70 once it has entered the first capture volume 72. In some cases (not shown) the second capture volume 82 may also have a greater maximum height measured normal to the inner surface 28 than the height of the second opening 74.

[0099] The debris capture system may comprise at least one further debris capture device 90 at the inner surface 28 and arranged towards the tip end 22 of the blade 20 relative to the first debris capture device 70. The further debris capture device 90 defines a further capture volume 92 and has a further open end providing a further opening 94 into the further capture volume and facing the root end 21 of the turbine blade 20. The further debris capture device 90 also comprises a closed end 96 facing the tip end 22 of the wind turbine blade 20. Figure 17 illustrates a blade 20 comprising two further debris capture devices 90.

[0100] The further debris capture devices 90 may be used to capture debris at different spanwise locations along the blade 20. Once captured in a further debris capture device 90 towards the tip end 22 of the blade 20, the debris may cascade towards the root end 21 of the blade by the process described with reference to Figures 14A-14E. As such, it may be preferable to provide further debris capture devices 90 comprising an inclined surface 98 inclined away from the inner surface 28 in a direction towards the root end 21 of the blade 20. This may provide a smooth path for the debris falling towards the root end 21 of the blade 20 to move away from the inner surface 28 and over the further debris capture device 90, before falling back towards the inner surface 28.

[0101] It may be desirable to allow debris captured in the further debris capture devices 90 to escape once captured, such that the debris may cascade towards the root end 21 of the blade 20 and be more easily removed by personnel 52. As such, the further debris capture devices 90 may comprise a further capture volume 92 having a maximum height measured normal to the inner surface 28 that is equal to the height of the further opening 94, as illustrated in Figure 5 and Figure 17. This may allow debris captured in the further capture volume 92 to escape the further debris capture devices 90 more easily when the tip end 22 of the blade points upwards. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims

Claims

CLAIMS1 . A wind turbine blade having a root end and a tip end and comprising a blade shell defining an internal cavity, the internal cavity having inner surfaces, the wind turbine blade further comprising: a debris capture system with a debris capture device on at least one of the inner surfaces of the internal cavity, the debris capture device defining a capture volume and having an open end providing an opening into the capture volume and facing the root end of the wind turbine blade, and a closed end facing the tip end of the wind turbine blade, wherein the closed end has a surface inclined away from the inner surface in a direction towards the root end of the wind turbine blade.

2. The wind turbine blade of claim 1 , wherein the wind turbine blade has a leading edge and a trailing edge and a chordwise direction extending between the leading edge and the trailing edge, a windward surface and a leeward surface each extending between the leading edge and the trailing edge, and a thickness direction extending between the windward surface and the leeward surface normal to the chordwise direction, wherein the opening extends either substantially in the chordwise direction, or substantially in the thickness direction.

3. The wind turbine blade of claim 1 or claim 2, wherein the debris capture device is a panel having the inclined surface and is attached to the inner surface so that the inner surface and the panel together define in part the capture volume.

4. The wind turbine blade of claim 3, wherein the panel has an attachment portion for attaching the debris capture device to the inner surface.

5. The wind turbine blade of any preceding claim, wherein the inclined surface is substantially planar.

6. The wind turbine blade of any preceding claim, wherein the inclined surface is formed in a curved portion of the debris capture device.

7. The wind turbine blade of any preceding claim, wherein the opening has a height normal to the inner surface of less than approximately 200 millimetres.

8. The wind turbine blade of any preceding claim, wherein the capture volume has a height normal to the inner surface, wherein the maximum height of the capture volume is the same or greater than a height of the opening.

9. The wind turbine blade of any preceding claim, wherein the wind turbine blade further comprises a web extending between windward and leeward sides of the blade shell, the inner surface is a blade shell inner surface, and wherein the debris capture device extends up to the web such that the web at least partially defines the capture volume.

10. The wind turbine blade of claim 9, wherein the wind turbine blade further comprises a second web extending between the windward and leeward sides of the blade shell, and wherein the debris capture device extends between the web and the second web such that the web and the second web at least partially define the capture volume.11 . The wind turbine blade of any preceding claim, wherein the debris capture device further comprises a cap arranged at one side of the opening to at least partially define the capture volume.

12. The wind turbine blade of any preceding claim, wherein the wind turbine blade has an internal personnel working zone defined as a distance from the root end towards the tip end in a spanwise direction, and wherein the debris capture device is located either at the boundary of the working zone or just outside working zone towards the tip end and accessible by personnel from within the working zone.

13. The wind turbine blade of any preceding claim, wherein the debris capture device is provided on the inner surface of windward or leeward sides of the blade shell.

14. The wind turbine blade of any preceding claim, wherein the debris capture system comprises a plurality of the debris capture devices arranged chordwise and / or arranged spaced spanwise along the blade.

15. A wind turbine blade of any preceding claim, wherein the debris capture device is a first debris capture device, the capture volume is a first capture volume, the open end is a first open end, the opening is a first opening into the first capture volume and facing the root end of the wind turbine blade, the closed end is a first closed end facing the tip end of the wind turbine blade, the surface is a first surface inclined away from theinner surface in a direction towards the root end of the wind turbine blade; and the debris capture system further comprises: a second debris capture device on at least one of the inner surfaces of the internal cavity, the second debris capture device defining a second capture volume and having a second open end providing a second opening into the second capture volume and facing the tip end of the wind turbine blade, and a second closed end facing the root end of the wind turbine blade, wherein the first opening faces the second opening, and wherein the second closed end has a second surface inclined away from the inner surface in a direction towards the tip end of the wind turbine blade.

16. The wind turbine blade of claim 15, wherein a spanwise distance between the first opening and the second opening is between 2 metres and 20 metres.

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