Wind turbine ice protection

Abstract

A pitch controlled wind turbine has a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three blades. The wind turbine includes at least three blade connecting members, each blade connecting member extending between neighbouring blades. The wind turbine has at least three pre-tension members, each connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of the pre-tension member due to extension / retraction of the tensioning device, each pre-tension member thereby providing pre-tension in the blade connecting member to which it is connected. A de-icing system is coupled to one or more of the tensioning devices and configured to control the one or more tensioning devices to extend or retract to excite at least some of the wind turbine blades, blade connecting members and / or pre-tension members to displace ice therefrom.

Description

[0001] WIND TURBINE ICE PROTECTION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a pitch controlled wind turbine having a de-icing system.

[0004] BACKGROUND OF THE INVENTION

[0005] Wind turbines are often deployed in regions where cold temperatures and adverse weather conditions occur, leading to the formation of ice on various parts of the turbine structure.

[0006] The accumulation of ice on wind turbine components poses several significant challenges. First, the overall weight of the turbine can increase, potentially causing imbalances and mechanical stress on critical components, and aerodynamic impacts which can result in reduced efficiency and costly maintenance. Furthermore, ice throw can lead to structural damage, impacting the longevity of wind turbines. This is a particular issue in pitch controlled wind turbines having blade load sharing connecting members, which include various additional components intended to support larger blades. These additional components may be susceptible to ice build-up, which if not mitigated could result in a reduction in performance and structural integrity of the wind turbine.

[0007] As a result, de-icing systems aiming to remove accumulated ice have become important to ensure the reliable and continuous operation of wind turbines in cold climates.

[0008] SUMMARY OF THE INVENTION

[0009] A first aspect of the invention provides a pitch controlled wind turbine comprising a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end; the wind turbine further comprising at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade; at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pre-tension member thereby providing pre-tension in the blade connecting member to which it is connected; and a de-icing system coupled to one or more of the tensioning devices and configured to control the one or more tensioning devices to extend or retract so as to excite at least some of the wind turbine blades, the blade connecting members and / or the pretension members so as to displace ice therefrom.

[0010] It should be understood that the term “excite” in the present context refers to the initiation or generation of vibrations in a respective component.

[0011] The provision of a de-icing system can protect essential components of the pitch controlled wind turbine having blade load sharing connecting members from the accumulation of ice. It should be understood that the term “ice” as used herein refers to frozen water, e.g. including ice, snow, sleet, hail, slush, and the like. Reducing or removing ice build-up on such components helps avoid issues such as increased mechanical stress, reduced power generation efficiency, potential damage from ice throw, and increased downtime and maintenance as well.

[0012] Controlling the tensioning devices to displace ice from the wind turbine blades, the blade connecting members and / or the pre-tension members utilises pre-existing equipment to alleviate the negative impacts of ice build-up, providing for a simple means of minimising the impact of ice accumulation. In some arrangements, making use of pre-existing equipment makes it possible to retrofit existing arrangements with a simple update (e.g. via a software update to a control system).

[0013] The de-icing system may be configured to control one or more of the tensioning devices to move between an extended state and a retracted state in a substantially stepwise manner so as to alter a tension in a respective pre-tension member in a substantially stepwise manner.

[0014] A substantially stepwise change in tension in a pre-tension member can cause a sudden oscillation or jolt in a pre-tension member and in one or more blade connection members attached thereto, and in one or more wind turbine blades. The sudden jolt assists in displacing ice from a respective wind turbine blade and / or blade connecting member and / or pre-tension member.

[0015] The term “substantially stepwise” refers to the limitations of equipment to achieve a completely instantaneous change in state of the tensioning device. It should be understood that the change in state of the tensioning device is intended to be as instantaneous as is permitted by the equipment.

[0016] The de-icing system may be configured to control one or more of the tensioning devices to apply a tension to a respective pre-tension member. The tension may be at least 80% of a maximum tension generatable by the tensioning device, preferably the tension is between 80% and 90% of the maximum tension generatable by the tensioning device.

[0017] Increasing the tensioning in the pre-tension member before releasing the tension when moving to the retracted state has been found to increase the sudden oscillation or jolt in a respective pre-tension member, thereby further assisting in displacing ice from a respective wind turbine blade and / or blade connecting member and / or pre-tension member.

[0018] The de-icing system may be configured to control one or more of the tensioning devices to cyclically extend and retract so as to generate oscillations in at least some of the wind turbine blades, the blade connecting members and / or the pre-tension members.

[0019] Generating oscillations in the wind turbine blades, blade connecting members and pretension members provides for a consistent movement at a surface upon which ice may be present, increasing the displacement of ice therefrom.

[0020] Preferably the frequency of the oscillations is the same or similar to the edge-wise or fore-aft natural frequency (or eigen frequency) of the blade (dependent on pitch of the blade).

[0021] The de-icing system may be configured to control at least two of the tensioning devices to cyclically extend and retract synchronously or in a phased relationship. Cyclically extending and retracting two tensioning devices allows the oscillations to be applied to multiple pre-tension members, and thus multiple blade connecting members and multiple wind turbine blades.

[0022] Utilising a phased relationship facilitates the targeting of oscillations to a respective wind turbine blade (i.e. a blade between the two respective pre-tension members), increasing the movement of the blade and thus the ice displacement therefrom.

[0023] The de-icing system may be configured to control a first and a second of the tensioning devices to cyclically extend and retract synchronously, and to control a third of the tensioning devices to cyclically extend and retract out of phase with the first and second of the tensioning devices.

[0024] Such an arrangement can excite multiple wind turbine blades at the same time, further increasing ice displacement and improving the efficiency of the de-icing system.

[0025] The de-icing system may be configured to control one or more of the tensioning devices to cyclically extend and retract so as to generate flapwise or edgewise oscillations in at least some of the wind turbine blades.

[0026] The direction of the oscillations on the wind turbine blades can be adjusted based on the location of ice on the respective blade surface, further improving the efficiency of the de-icing system.

[0027] The de-icing system may be configured to control one or more of the tensioning devices to cyclically extend and retract at a frequency substantially equivalent to a natural frequency of the respective wind turbine blade, blade connecting member or pretension member.

[0028] Operating at such frequencies has been found to increase excitation of a respective wind turbine blade in a flapwise or edgewise manner. In this way, a greater level of ice removal is achieved, and less energy is required to initiate the vibration.

[0029] The de-icing system may be coupled to the pitch mechanism and may be configured to control the pitch mechanism to cyclically alter a pitch of a respective wind turbine blade so as to displace ice therefrom. In this way, the de-icing system utilises a pre-existing mechanism to further assist in the removal of ice. Utilising a change in pitch alongside excitation of the wind turbine blades has been found to further improve ice removal on wind turbine blades. Moreover, the pitch of a wind turbine blades can be adjusted to allow the excitation to have maximum impact. Such an arrangement is particularly advantageous when the rotor is static (e.g. because altering the pitch will influence the rotational speed of the blades if the turbine is operational).

[0030] The de-icing system may be coupled to the pitch mechanism and configured to control the pitch mechanism to alter a pitch of a respective wind turbine blade such that the wind turbine blade is in a substantially stalled condition.

[0031] Such an arrangement has been found to increase the level of excitation in the wind turbine blade. This is particularly advantageous when edgewise oscillations are generated in the wind turbine blades.

[0032] Moreover, in a preferred embodiment, the blade connecting members and / or the pretension members can be configured to provide damping which can reduce a level of edgewise oscillations acting on the rotor. As such, the risk of damaging the rotor is significantly reduced compared to a rotor of a standard wind turbine (i.e. without the blade connecting members and / or the pre-tension members). In this way, large edgewise oscillations can be generated in a wind turbine blade without significant risk of damage to the rotor.

[0033] The de-icing system may include one or more electrical heating elements and / or a fluid heating system.

[0034] The provision of such heating elements and / or systems has been found to further improve the efficiency of the de-icing system, as ice can be more easily removed from a respective wind turbine component in response to excitation if it is at least partially melted.

[0035] Electrical heating elements provide a precise and controlled temperature increase so as to melt ice. Electrical heating elements can generate heat quickly and efficiently, rapidly melting ice and restoring functionality to a component of the wind turbine. A fluid heating system is an effective means of providing heat to melt ice.

[0036] Each tensioning device may comprise an actuator having a first portion coupled to the hub and a second portion movable with respect to the first portion and coupled to the respective pre-tension member, wherein the tensioning device is configured to extend and retract by movement of the second portion with respect to the first portion.

[0037] The actuator provides a simple to control means of exciting at least some of the wind turbine blades, the blade connecting members and the pre-tension members.

[0038] The actuator may be a linear actuator, such as a hydraulic actuator, an electrical actuator or a mechanical actuator. The actuator may have a cylinder and a rod moveable into and out of the cylinder. The first portion of the actuator may include the cylinder, and the second portion of the actuator may include the rod.

[0039] The actuator may be a rotary actuator. The actuator may comprise a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension member, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor. The first portion of the actuator may include the motor, and the second portion of the actuator may include the tension element.

[0040] The de-icing system may include one or more sensors in or on one or more of the blade connecting members or the pre-tension members for detecting ice accumulation.

[0041] The provision of a sensor can provide improved control to the de-icing system, and can provide up-to-date information to an operator regarding the presence of ice. The energy requirements of the de-icing system may be reduced, in that the respective system only operates when required (e.g. when a predetermined amount of ice has accumulated, or when weather conditions suggest ice has accumulated), and is not continuously operating.

[0042] The blade connecting members may cause the wind turbine blades to mutually support each other, in the sense that loads on the wind turbine blades, in particular edgewise loads and to some degree flapwise loads, are ‘shared’ among the wind turbine blades. The connection points on the wind turbine blades may be arranged at a distance from the root end which is between 10% and 60% of the length of the wind turbine blades from the root end to the tip end, preferably radially inboard of 50% of the length of the respective wind turbine blade from the root end, and more preferably radially inboard of 45% of the length of the respective wind turbine blade from the root end.

[0043] The connection points on the wind turbine blades may be arranged at a position where a thickness-to-chord ratio of the wind turbine blade is between 20% and 50%.

[0044] The wind turbine blades may each comprise an inboard blade part comprising the root end and an outboard blade part comprising the tip end. The inboard blade part and the outboard blade part may be connected to each other at a split position. The inboard blade part may be joined to the outboard blade part by a connection joint. The connection joint may comprise a connector.

[0045] The connector may be a metallic component, preferably a cast component or a machined component. The connector may be a composite component. The connector may be a co-cured or co-bonded component. Providing such a connector may improve ease of manufacture of the connector and provide a lightweight, high strength connector.

[0046] The connector may be coupled to transfer load between a spar cap portion of the inboard blade part and a spar cap portion of the outboard blade part. The connector may be arranged to transfer load from the blade connecting members into the spar cap portion of the inboard blade part. This may improve the load transfer efficiency of the turbine blade as the spar cap portions may be designed to withstand higher loads relative to the blade shell. The connector may comprise a connection point for a blade connecting member.

[0047] Each wind turbine blade may comprise a leading edge, a leading edge extension, and a blade shell, wherein the leading edge extension extends forward of the leading edge, and the connection point of the respective wind turbine blade is located forward of the leading edge on the leading edge extension, and each wind turbine blade further comprises a respective fairing extending over at least the leading edge extension. The leading edge extension may be integrally formed with the connector. The fairing may cover the connection joint. The fairing may be secured to the connector. The connector may extend outside the profile of the blade shell at the connection joint. This may improve the load transfer across the connection joint but may require a larger fairing.

[0048] Each wind turbine blade may be coupled to two of the blade connecting members. Each of the two blade connecting members may extend from respective connection points of one of the blades. The connection points of the one of the wind turbine blades may be adjacent each other on the same leading edge extension. Each blade connecting member may be independently moveable at the respective first and second connection points to which it attaches. The connection point(s) may comprise a bearing structure.

[0049] The wind turbine may be an upwind wind turbine.

[0050] A second aspect of the invention provides a method of de-icing a pitch controlled wind turbine, the pitch controlled turbine comprising: a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end; at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade; and at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pretension member thereby providing pre-tension in the blade connecting member to which it is connected; the method comprising controlling the one or more tensioning devices to extend or retract so as to excite at least some of the wind turbine blades, the blade connecting members and / or the pre-tension members so as to displace ice therefrom. The method may comprise controlling one or more of the tensioning devices to move between an extended state and a retracted state in a substantially stepwise manner so as to alter a tension in a respective pre-tension member in a substantially stepwise manner.

[0051] The method may comprise controlling one or more of the tensioning devices to apply a tension to a respective pre-tension member. The tension may be at least 80% of a maximum tension generatable by the tensioning device, preferably the tension is between 80% and 90% of the maximum tension generatable by the tensioning device.

[0052] The method may comprise controlling one or more of the tensioning devices to cyclically extend and retract so as to generate oscillations in at least some of the wind turbine blades, the blade connecting members and / or the pre-tension members.

[0053] The method may comprise controlling at least two of the tensioning devices to cyclically extend and retract synchronously or in a phased relationship.

[0054] The method may comprise controlling a first and a second of the tensioning devices to cyclically extend and retract synchronously, and controlling a third of the tensioning devices to cyclically extend and retract out of phase with the first and second of the tensioning devices.

[0055] The method may comprise controlling one or more of the tensioning devices to cyclically extend and retract so as to generate flapwise or edgewise oscillations in at least some of the wind turbine blades.

[0056] The method may comprise controlling one or more of the tensioning device to cyclically extend and retract at a frequency substantially equivalent to a natural frequency of the respective wind turbine blade, pre-tension member, or blade connecting member.

[0057] The method may comprise controlling the pitch mechanism to cyclically alter a pitch of a respective wind turbine blade so as to displace ice therefrom.

[0058] The method may comprise controlling the pitch mechanism to alter a pitch of a respective wind turbine blade such that the wind turbine blade is in a substantially stalled condition. The pitch controlled wind turbine may comprise one or more electrical heating elements and / or a fluid heating system, and the method may further comprise controlling the one or more electrical heating elements and / or fluid heating system to de-ice the pitch controlled wind turbine.

[0059] The pitch controlled wind turbine may comprise a de-icing system including one or more electrical heating elements and / or a fluid heating system, and in that case, the method may further comprising the steps of detecting ice accumulated on at least a part of the wind turbine blades, the blade connecting members and / or the pre-tension members, and activating the one or more electrical heating elements and / or fluid heating system to weaken a connection between ice and the at least part of wind turbine blades, the blade connecting members and / or the pre-tension members. Then controlling the one or more tensioning devices to extend or retract so as to excite at least some of the wind turbine blades for example one or more of the ways described below to release from the wind turbine blades, the blade connecting members and / or the pre-tension members at least a part of the ice accumulated. This method was found to be particularly advantageous in that using the tensioning devices to shake off ice from the wind turbine already when the connection between the ice and the wind turbine is weakened by heating rather than wait for the ice to melt completely saved significant time and allowed for significant increased production time during icing conditions.

[0060] The pitch controlled wind turbine may include one or more sensors in or on one or more of the blade connecting members or the pre-tension members for detecting ice accumulation, and the method may comprise controlling the one or more tensioning device to extend or retract in response to a signal from one or more sensors.

[0061] A third aspect of the invention provides the use of a tensioning device for de-icing a pitch controlled wind turbine. The pitch controlled wind turbine comprises a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end. The wind turbine further comprises at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade. Furthermore, at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via the tensioning device, wherein the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pretension member thereby providing pre-tension in the blade connecting member to which it is connected, and a de-icing system coupled to one or more of the tensioning devices and configured to control the one or more tensioning devices to extend or retract so as to excite at least some of the wind turbine blades, the blade connecting members and / or the pre-tension members so as to displace ice therefrom. The concept of using the tensioning device of a pitch controlled wind turbine with blade connecting members to shake off ice may allow for omitting electrical heating elements and / or fluid heating system and thereby simplify the construction of this particular type of wind turbines.

[0062] Despite the advantage of simpler design of wind pitch controlled wind turbines with blade connecting members without electrical heating elements and / or fluid heating system, in some cases, pitch controlled wind turbines with blade connecting members may be provided with electrical heating elements and / or fluid heating system. In such cases, the use of a tensioning device for de-icing a pitch controlled wind turbine may be particularly efficient by allowing removal of ice from the wind turbine faster by forced shaking of part of the wind turbine using one or more of the tensioning devices for example in one or more of the ways described below to release from the wind turbine blades, the blade connecting members and / or the pre-tension members at least a part of the ice without having to fully melt the ice.

[0063] BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0065] Figure 1 shows a front view of a wind turbine according to a first example;

[0066] Figure 2 shows a side view of the wind turbine;

[0067] Figure 3 shows a partially side view of components of a de-icing system adjacent the hub of a wind turbine;

[0068] Figure 4 shows a schematic of a de-icing system according to an example; Figure 5A shows a schematic of a blade connecting member or pre-tension member according to a first example;

[0069] Figure 5B shows a schematic of a blade connecting member or pre-tension member according to a second example;

[0070] Figure 6 shows a wind turbine blade;

[0071] Figure 7 shows an exploded view of a connection joint;

[0072] Figure 8 shows a turbine blade having a fairing;

[0073] Figures 9A to 9D show examples of a wind turbine controlled by a de-icing system.

[0074] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0075] In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, and chord 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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. 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.

[0081] 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.

[0082] The term outboard refers to a radial direction from the hub of the blade towards the tip end of the blade. The term inboard refers to a radial direction from the tip end towards the hub.

[0083] 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.

[0084] 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.

[0085] Figures 1 and 2 show a pitch controlled wind turbine 1 according to a first example. Figure 1 is a front view of the wind turbine 1 , and Figure 2 is a side view of the wind turbine 1. The wind turbine 1 includes a tower 2 and a nacelle 3 mounted on the tower 2. A hub 4 is mounted rotatably on the nacelle 3, and carries three wind turbine blades 5 projecting outwardly from the nacelle 3. While the example wind turbine 1 shown in Figures 1 and 2 has three blades 5a, 5b, 5c, it will be appreciated that other numbers of blades 5 are possible. When wind blows against the wind turbine 1 , the wind turbine blades 5 generate a lift force which causes a generator (not shown) within the nacelle 3 to generate electrical energy.

[0086] It will be appreciated that the wind turbine 1 depicted may be any suitable type of wind turbine 1. The wind turbine 1 shown is an upwind wind turbine, although it will be appreciated the wind turbine 1 may be a downwind wind turbine. The wind turbine 1 may be an onshore wind turbine such that the foundation is embedded in the ground, or the wind turbine 1 may be an offshore installation in which case the foundation would be provided by a suitable marine platform. Blade connecting members 6 interconnect neighbouring wind turbine blades 5 between connection points 7a, 7b on the wind turbine blades 5. In the illustrated arrangement, three blade connecting members 6a, 6b, 6c are provided (i.e. one per blade 5). The connecting members 6 are cables, e.g. metallic (e.g. steel) or polymer (for example comprising ultra-high molecular weight polyethylene - LIHMWPE) cables. In some examples, each wind turbine blade 5 is coupled to two blade connecting members 6. Each two blade connecting members 6 extend from respective connection points 7a, 7b on one of the blades 5. Each blade connecting member 6 may be independently moveable at the respective first and second connection points 7a, 7b to which it attaches. The connection point(s) 7a, 7b may comprise a bearing structure.

[0087] A pre-tension member 8 extends from one of each of the blade connecting members 6 and towards a common point 25 (see Figure 3) arranged at or adjacent the hub 4. In the example shown in Figures 1 and 2, the pre-tension members 8 extend to the hub 4. The pre-tension members 8 are configured to provide pre-tension in the blade connecting members 6. The pre-tension members 8 are typically cables, e.g. metallic or polymer cables. In the illustrated arrangement, three pre-tension members 8a, 8b, 8c are provided (i.e. one per blade 5).

[0088] The pre-tension members 8 are coupled to the hub 4 by respective tensioning devices 9. Each tensioning device 9 provides radial movement of a radially inward end of the pre-tension member 8 with respect to an axis of rotation 10 (see Figure 3) of the hub due to extension or retraction of the tensioning device 9. Each pre-tension member 8 thereby provides pre-tension in the blade connecting member 6 to which it is connected. In the figures, three tensioning devices 9a, 9b, 9c are provided (i.e. one per pre-tension member 8).

[0089] The wind turbine blades 5 have a root end 11 proximal to the hub 4, adapted to be connected to the hub 4 via a pitch mechanism 50 (see Figure 3), and a tip end 12 distal from the hub 4. The blades 5 include a leading edge 13 and a trailing edge 14 that extend between the respective root end 11 and tip end 12. Each of the blades 5 include a suction side 15 and a pressure side 16 (see Figure 5). A thickness dimension of the blade 5 extends between the suction side 15 and the pressure side 16. The connection points 7a, 7b are provided between the root end 11 and the tip end 12 of a respective blade 5 (i.e. at a distance from the root end 11 and at a distance from the tip end 12). The connection points 7a, 7b may be between 10% and 60% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12 in the radial direction but are preferably radially inboard of 50% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12, and more preferably radially inboard of 45% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12, e.g. around 30-40%. It will be appreciated that the connection points 7a, 7b may be adjacent each other. Alternatively, the connection points 7a, 7b may be spaced from one another.

[0090] The wind turbine 1 includes a de-icing system 17. The de-icing system 17 is configured for protecting one or more of the blade connecting members 6, the pre-tension members 8 or the wind turbine blades 5 from the impact of ice accumulation. The deicing system 17 may be configured to remove or reduce ice accumulation on a given component of the wind turbine 1. It should be understood that the term “ice” as used herein refers to frozen water, e.g. including ice, snow, sleet, hail, slush, and the like.

[0091] The provision of a de-icing system can protect essential components of the pitch controlled wind turbine having blade load sharing connecting members from the accumulation of ice. Reducing or removing ice build-up on such components helps avoid issues such as increased mechanical stress, reduced power generation efficiency, and increased downtime and maintenance as well.

[0092] The de-icing system 17 is coupled to one or more of the tensioning devices 9. In Figure 3, only one tensioning device 9 is indicated, but it will be appreciated that the de-icing system 17 may be coupled to a plurality of the tensioning devices 9a, 9b, 9c in some arrangements. The de-icing system 17 is configured to control one or more of the tensioning devices 9 to extend or retract so as to excite at least some of the wind turbine blades 5, the blade connecting members 6 and / or the pre-tension members 8. The excitement of any of these components can cause any ice accumulated thereon to be displaced therefrom (i.e. ice is thrown or falls from a respective surface in response to an induced motion at the surface), thereby reducing a presence of ice on a surface of the component. The de-icing system 17 may be in the form of a control system that can convey a signal to a respective tensioning device 9, instructing the tensioning device 9 to extend and / or retract so as to excite a respective wind turbine 1 component. The extension and / or retraction of the tensioning device 9 may induce a movement in a respective pretension member 8, which may in turn induce a movement (e.g. cause excitation) in one or more respective blade connection members 6 and in one or more wind turbine blades 5.

[0093] Referring to Figure 3, the hub 4 of the wind turbine is generally shown. The hub 4 may include a hub member 57 extending from the hub substantially along a direction defined by a rotational axis of the hub 10. The pre-tension members 8 may be connected to the hub member 57. As can be seen, the tensioning devices 9 are connected at a radial inner end thereof to or towards the common region or point 25. The common point 25 may provide a pivotal connection to each of the tensioning devices 18. In Figure 3, the common point 25 is on a projection 37 extending forward of the hub 4 in an upwind direction of the wind turbine 1. The projection 37 may be a framework or structure rigidly coupled to the hub 4. A spinner 29 may be coupled to the hub to cover a radially inner portion of each tensioning device 9. The spinner 29 may shroud the common point 25. In this way, the spinner 29 can be seen as an important component in providing protection to the radially inner portion of each tensioning device 9.

[0094] The tensioning device 9 may include an actuator 51 having a first portion 53 coupled to the hub 4 and a second portion 54 movable with respect to the first portion 53 and coupled to the respective pre-tension member 8. The tensioning device 9 extends and retracts by movement of the second portion 54 with respect to the first portion 53. Extension and retraction of the tensioning device 9 changes the tension in pre-tension member 8. In such an arrangement, the de-icing system 17 may be coupled to the actuator 51 so as to control movement of the second portion 54 relative to the first portion 53 and thus induce a movement in a respective pre-tension member 8.

[0095] The actuator 51 may be a linear actuator having a cylinder and a rod moveable into and out of the cylinder. The actuator 51 may be hydraulic or electro-mechanical, for example. The cylinder may be the first portion 53 and the rod may be the second portion 54. The actuator 51 may be a rotary actuator. Although not shown, the actuator 51 may include a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension member 8, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor. The first portion 53 of the actuator 51 may include the motor, and the second portion 54 of the actuator 51 may include the tension element.

[0096] It will be appreciated that the tensioning device 9 may be in any suitable form to cause excitement in at least some of the wind turbine blades 5, blade connecting members 6 and the pre-tension members 6.

[0097] The de-icing system 17 is indicated schematically in Figure 3, and may be configured to control one or more of the tensioning devices 9 to cause excitation and ice displacement in targeted areas of the wind turbine 1 , or to increase the effects of excitation in displacing ice.

[0098] The de-icing system 17 may be configured to control one or more of the tensioning devices 9 to move between an extended state and a retracted state. In the retracted state, the tensioning device 9 may apply a tension to a respective pre-tension member 8. In the extended state, the tensioning device may release or reduce the tension in the pre-tension member 8.

[0099] The de-icing system 17 may be configured to control one or more tensioning devices 9 to move between the extended state and the retracted state in a substantially stepwise manner so as to alter a tension in a respective pre-tension member 8 in a substantially stepwise manner. Put another way, the de-icing system 17 may control one or more of the tensioning devices 9 to retract so as to generate a tension in a respective pre-tension member 8, before extending the tensioning device 9 so as to cause a sudden release or reduction of the tension. It may be preferable to control the tensioning devices 9 in such a way while the wind turbine is in an idle state (i.e. when the blades 5 are not rotating and the wind turbine 1 is not generating power). This reduces the area of ice throw from the blades 5.

[0100] The term “substantially stepwise” refers to the limitations of equipment to achieve a completely instantaneous change in state of the tensioning device. It should be understood that the change in state of the tensioning device is intended to be as instantaneous as is permitted by the equipment (e.g. by the actuator 51).

[0101] In some arrangements, the de-icing system 17 may be configured to control one or more tensioning devices 9 to extend or retract until the tensioning device 9 reaches its mechanical limit or end-point. Upon reaching such a limit, the tensioning device 9 will no longer be able to extend or retract, and so will suddenly decelerate to a stop. In this way, a sudden alteration in tension in a respective pre-tension member 8 is induced. Utilising the mechanical limit of the tensioning device 9 may generate a more stepwise change in tension in a respective pre-tension member 8 (e.g. compared with a motor or hydraulic system implementing a stepwise change in the tensioning device 9), thereby improving de-icing of the wind turbine 1.

[0102] The stepwise change between the extended and retracted state of a given tensioning device 9 causes a sudden oscillation or jolt in a pre-tension member 8 to which the tensioning device 9 is connected. This may further induce such a jolt in one or more of the blade connection members 6 connected to the pre-tension member 8, and in turn, may induce a jolt in a blade 5 connected to each blade connection member 6. For example, with reference to Figure 1 , a substantially stepwise change of the tensioning device 9a from the extended to the retracted state may cause a sudden oscillation in pre-tension member 8a, which in turn causes a sudden oscillation in blade connection member 6a. This may then cause a sudden oscillation in at least wind turbine blade 5a and 5b (i.e. at either end of the respective blade connection member 6a). The sudden movement in the blades 5a, 5b, the pre-tension member 8a and / or the blade connection member 6a can assist in displacing ice from each respective component surface.

[0103] In an example arrangement, the de-icing system 17 may be configured to control a plurality or all of the tensioning devices 9 (e.g. all three devices 9a, 9b and 9c may in the arrangement of Figure 1) to move between the extended and retracted state in a substantially stepwise manner at the same time. This may induce a sudden oscillation or jolt in multiple pre-tension members 8a, 8b, 8c, blade connection members 6a, 6b, 6c and wind turbine blades 5a, 5b, 5c at the same time, thereby increasing ice removal from multiple components of the wind turbine 1. The substantially stepwise change from the extended state to the retracted state may be repeated as required, e.g. to repeatedly induce a sudden oscillation or jolt in a respective wind turbine 1 component until sufficient ice removal has occurred.

[0104] The de-icing system 17 may be configured to control one or more of the tensioning devices 9 to cyclically extend and retract so as to generate oscillations in at least some of the wind turbine blades 5, the blade connecting members 6 and / or the pre-tension members 8. In this way, the repeated extension and retraction of the tensioning devices 9 causes repeated oscillations or vibrations to occur in the pre-tension members 8 connected to the tensioning devices 9 and, in turn, the respective blade connection members 6 and the wind turbine blades 5. The consistent movement at a surface of the pre-tension members 8 and / or blade connection members 6 and / or wind turbine blades 5 can cause an effective displacement of ice on said surface. It may be preferable to control the tensioning devices 9 in such a way while the wind turbine is in a parked or an idle state (i.e. when the blades 5 are not rotating or rotating very slowly).

[0105] The de-icing system 17 may be configured to control at least two of tensioning devices 9 to cyclically extend and retract, thereby generating oscillations in multiple pre-tension members 8. This can facilitate oscillations being generated in multiple blade connecting members 6 and multiple wind turbine blades 5, increasing ice removal from the wind turbine 1.

[0106] The de-icing system 17 may be configured to control at least two of the tensioning devices 9 to cyclically extend and retract synchronously or in a phased relationship. Controlling the tensioning devices 9 to extend and retract synchronously results in the two or more tensioning devices 9 being in the extended state at the same time and in the retracted state at the same time. In this way, the pre-tension members 8 connected to each tensioning device 9 may oscillate in sync with one another. Operating synchronously may include the tensioning devices 9 applying and releasing the same tension (i.e. the same force) to the pre-tension members 8 in the respective extended and retracted states. In some arrangements, a different tension may be applied to different pre-tension members 8.

[0107] Controlling the tensioning devices 9 to extend and retract in a phased relationship results in the two or more tensioning devices oscillating out of sync from one another. In some arrangements, the phased relationship may include one of the tensioning devices 9 being in the extended state while another of the tensioning devices 9 is in the retracted state, resulting in the oscillations in the respective pre-tension members 8 being out of sync with a phase shift of approximately 180°. It will be appreciated that the pre-tension members 8 may be in operated in any phase shift, e.g. less than 180°, in some arrangements.

[0108] Controlling the tensioning devices to cyclically extend and retract in a phased relationship may enhance the ability of the de-icing system to target specific wind turbine blades 5 for more effective ice-removal. For example, referring to Figure 1 , the de-icing system may control the tensioning devices 9a and 9c (i.e. on either side of the blade 5a) to cyclically extend and retract in a phased relationship. In some arrangements, the tensioning device 9a may be in the extended state when the tensioning device 9c is in the retracted state and visa versa, resulting in the pre-tension members 8a, 8b oscillating out of sync with one another, defining a phase shift of approximately 180°. This will generate similarly phased oscillations in the blade connecting members 6a, 6c, which will thus cause the blade 5 to be cyclically pulled and released by the blade connecting members 6a, 6c on either side thereof. In this way, oscillations may be predominantly generated in a targeted wind turbine blade 5a, and not in the other wind turbine blades 5. In this arrangement, tensioning device 9b may be inactive, meaning no oscillations are generated directly in pre-tension member 8b or blade connecting member 6b (although it will be understood that residual oscillations will occur since the blade connecting members 6a, 6b, 6c are interconnected).

[0109] Once sufficient ice has been removed from the blade 5a or once the oscillations have been occurring for a predetermined amount of time, the de-icing system 17 may be configured to repeat the process, targeting other blades. For example, the tensioning devices 9a and 9b may be controlled to cyclically extend and retract in a phased relationship to generate oscillations on blade connecting members 6a, 6b and on blade 5b. The tensioning devices 9b and 9c may be controlled to cyclically extend and retract in a phased relationship to generate oscillations on blade connecting members 6b and 6c and on blade 5c.

[0110] The de-icing system 17 may be configured to control all of the tensioning devices 9 to cyclically extend and retract. In some arrangements, the de-icing system 17 is configured to control two of the tensioning devices 9 to cyclically extend and retract synchronously, and to control the other of the tensioning devices 9 to cyclically extend and retract out of phase with the two synchronous tensioning devices 9. Such an arrangement assists in exciting multiple wind turbine blades 5 at the same time, increasing the efficiency of the de-icing system 17.

[0111] In an example arrangement, referring to Figure 1 , the de-icing system 17 may be configured to control tensioning devices 9a and 9b to cyclically extend and retract in sync with one another, and control tensioning device 9c to cyclically extend and retract in a phased relationship (i.e. out of sync) relative to tensioning devices 9a and 9b. Such an arrangement may cause the pre-tension members 8a and 8b to oscillate in synchrony with one another, while pre-tension member 8c oscillates in a phased relationship relative to pre-tension members 8a and 8b. Such a configuration has been found to induce oscillations in wind turbine blades 5a and 5c, resulting in a targeted de-icing of multiple blades 5. In some arrangements, the de-icing system 17 controls the tensioning devices 9 such that the pre-tension member 8c is out of sync with pretension members 8a, 8b with a phase shift of approximately 180°. It will be appreciated that the pre-tension members 8 may be in operated in any phase shift, e.g. less than 180°, in some arrangements.

[0112] In some arrangements, the tension generated in a respective pre-tension member 8 by the tensioning devices 9 (i.e. in the extended state) may be a maximum tension generatable by the tensioning device 9 and / or the maximum tension tolerable by the pre-tension member 8. Such an arrangement maximises the oscillation or jolt that induced in a respective pre-tension member 8 and / or an amplitude of the oscillations generated in the respective pre-tension member, thereby increasing ice displacement from wind turbine 1 components.

[0113] In some arrangements, the tension generated in a respective pre-tension member 8 by the tensioning devices 9 (i.e. in the retracted state) may be less than the maximum tension generatable by the tensioning device 9. For example, the tension may be at least 70%, preferably at least 75%, preferably at least 80%, preferably at least 85%, preferably at least 90%, preferably at least 95% of the maximum tension generatable by the tensioning device 9. The tension generated may be between 80% and 90% of the maximum tension generatable in some arrangements. Operating below the maximum tension generatable reduces the risk of damage to wind turbine components 1 (e.g. the tensioning devices 9, pre-tension members 8, blade connection members 6) if they are consistently operated at their maximum capability. In some arrangements, the maximum tension generatable in the pre-tension member

[0114] 8 by the tensioning device 9 is in the range of 100 kN to 600 kN, preferably in the range of 200 kN to 400 kN, preferably in the range of 250 kN to 350 kN, preferably in the range of 275 kN to 325 kN. The maximum tension generatable may be 300 kN in some arrangements.

[0115] The tension generated in a respective pre-tension member 8 by the tensioning devices

[0116] 9 may be lower in some arrangements. For example, the tensioning device 9 may move toward the extended state to reduce tension in a respective pre-tension member 8. Reducing a tension in a respective pre-tension member 8 can facilitate an increase in vibration amplitude of a respective wind turbine component (e.g. when excited). An increased vibration amplitude may be more beneficial to strain and crack ice present on a respective component. In this way, the removal of ice from a respective component may be improved.

[0117] It will be understood that the de-icing system 17 may be configured to control one or more of the tensioning devices 9 to apply varying tensions to the pre-tension members 8 based on various inputs (e.g. from a user or a sensor arrangement). The de-icing system 17 may be configured to control each tensioning device 9 to apply a different tension to each respective pre-tension member 8.

[0118] The de-icing system 17 may be configured to control one or more of the tensioning devices 9 to cyclically extend and retract so as to generate flapwise or edgewise oscillations in at least some of the wind turbine blades 5. The term “flapwise oscillations” refers to oscillations that occur in a flapwise direction of a respective blade 5, i.e. in a direction perpendicular to a plane defined by the rotor (i.e. by the rotating blades) of the turbine 1. The term “edgewise oscillations” refers to oscillations that occur in an edgewise direction of a respective blade 5, i.e. in a direction parallel to a plane defined by the rotor of the turbine 1. Such an arrangement can facilitate the adjustment of the direction of oscillation of a respective blade 5 based on, for example, the location of ice on a blade 5.

[0119] In some arrangements, the de-icing system 17 is configured to control one or more of the tensioning devices 9 to cyclically extend and retract at a frequency substantially equivalent to a natural frequency of the respective wind turbine blade 5. Such an arrangement results in the pre-tension members 8 and / or blade connection members 6 oscillating at a frequency substantially equivalent to that of a natural frequency of a respective wind turbine blade 5. Operating in this way has been found to increase excitation of a wind turbine blade 5, increasing ice removal and requiring less energy to initiate the oscillation. In some arrangements, the de-icing system 17 is configured to control one or more of the tensioning devices 9 to cyclically extend and retract at a frequency substantially equivalent to a natural frequency of a respective pre-tension member 8 or blade connecting member 6. In this way, the respective pre-tension members 8 and / or blade connection members 6 oscillate at a frequency substantially equivalent to the natural frequency of the pre-tension member 8 or blade connection member 6. The natural frequency may be in an edgewise or a flapwise direction. The natural frequency may be known by the de-icing system 17 (e.g. stored in a memory of the de-icing system 17). In some arrangements, the de-icing system 17 may be configured to control the tensioning devices 9 to cyclically extend and retract at a frequency substantially equivalent to a harmonic of the wind turbine blade (e.g. a first harmonic, a second harmonic, or more).

[0120] As shown in Figure 3, the de-icing system 17 may be coupled to the pitch mechanism 50 in some arrangements. The de-icing system 17 may be configured to control the pitch mechanism 50 to alter a pitch of a respective wind turbine blade 5 or blades 5. Such an arrangement can assist in displacing ice from the respective blade 5. Adjusting the pitch of a wind turbine blade 5 can improve the ice displacement obtained by generating an oscillation in the blade 5. The de-icing system 17 may be configured to control the pitch mechanism 50 to alter the pitch of all of the blades 5 simultaneously in some arrangements, so as to avoid an imbalanced wind turbine 1 . Utilising the pitch to impact ice displacement is most effective when the wind turbine 1 is not operational (i.e. when the blades 5 are not rotating), as altering the pitch will influence the rotational speed of the wind turbine 1 if operational. As used herein, it will be understood that the term “pitch” refers an angle of the wind turbine blade with respect to the oncoming wind in the wind turbine 1. In some arrangements, the de-icing system 17 is configured to control the pitch mechanism 50 to cyclically alter the pitch of a respective wind turbine blade 5. Cyclically altering the pitch of a blade 5 causes a repeated movement at a surface of the blade, which can cause a displacement of ice therefrom.

[0121] In an example arrangement, the de-icing system 17 may be configured to control the pitch mechanism 50 to cyclically alter the pitch of a wind turbine blade 5 between a pitch angle of about 70° and a pitch angle of about 90°. It should be understood that the de-icing system 17 may be configured to simultaneously alter the pitch angle and control the tensioning devices 9 to generates oscillations in the blades 5 (e.g. as discussed in the examples above). In some arrangements, the de-icing system 17 is configured to generate such oscillations at a frequency the same as a frequency of pitch alteration of the one or more blades 5 at the same frequency. In this way, oscillations of the blade 5 are generated at the same frequency as the change in the pitch of the blade 5, greatly improving the effectiveness of the de-icing system 17 in removing ice from the blade 5.

[0122] The de-icing system 17 may be configured to control the pitch mechanism 50 to alter a pitch of a wind turbine blades 5 such that the blade is in a substantially stalled condition. The term “substantially stalled condition” refers to a condition in which the wind turbine blade 5 is set at a pitch angle that results in a reduction or cessation of an aerodynamic lift force of the blade 5. For example, at low wind speeds (e.g. less than 6 m s'1), the blades 5 may be pitched to a more negative angle, e.g. toward 0° or toward -5° to be in the stalled condition. Some further examples of operating the de-icing system 17 will now be discussed.

[0123] In one example, the wind turbine 1 is configured in a power production mode, and so the wind turbine blades 5 are rotating. The de-icing system 17 is configured to control a first and a second of the tensioning device 9a, 9b to cyclically extend and retract synchronously, and to control a third of the tensioning devices 9c to cyclically extend and retract out of phase with the first and second tensioning devices 9a, 9b. The deicing system 17 may control the tensioning devices 9a, 9b, 9c to generate a flapwise oscillation in the pre-tension members 8a, 8b, 8c having a frequency substantially equivalent to the natural flapwise frequency (e.g. first harmonic or second harmonic) of the wind turbine blades 5. This arrangement increases excitation in the blades 5, leading to improved ice removal therefrom. The mean pitch angle may be between 0 and 25° (e.g. close to 0° at low wind speeds such as below 6 m s'1, and between about 20° and 25° at high wind speeds such as between 20-25 m s'1). This example is particularly effective at low wind speeds, e.g. below 6 m s'1, as high blade loads are avoided.

[0124] In another example, the de-icing system 17 is configured to generate the same oscillations as discussed in the previous example. Instead of the pitch angles discussed in the previous example, the de-icing system 17 is configured to control the pitch mechanism 50 to alter the pitch of a wind turbine blades 5 such that the blade is in a substantially stalled condition. This arrangement increases oscillations of the blade in an edgewise direction, thereby increasing the removal of ice. The presence of the blade connection members 6 and the pre-tension members 8 provide damping in this arrangement, which generally decrease the level of edgewise oscillations. In this way, the risk of having edgewise oscillations that damage the wind turbine 1 is reduced compared with a standard wind turbine 1 (i.e. without the blade connection members 6 and pre-tension members 8).

[0125] Referring to Figure 4, a schematic of the de-icing system 17 is represented. As has been discussed, the de-icing system 17 is coupled to the tensioning devices 9 and to the pitch mechanism 50. The de-icing system 17 may include a heating system 18 which may include one or more electrical heating elements and / or a fluid heating system. As will be discussed in more detail below, the heating system 18 may provide heat to various locations of the wind turbine 1 to assist in removing ice therefrom, as ice will be more easily displaced if it is partially melted.

[0126] The de-icing system 17 may include one or more sensors 19 for detecting ice accumulation. The de-icing system 17 may control one or more of the tensioning device 9, the pitch mechanism 50 and / or the heating system 18 based on data obtained by the one or more sensors 19. The sensors 19 may be provided within or on one or more of the blade connecting members 6 or the pre-tension members 8 (e.g. embedded within or positioned on the cable), see for example, Figures 5A and 5B. In some examples one or more sensors 19 may be arranged away from the blade connecting members 6 and pre-tension members 8, such as on a part of the blade or on the nacelle. The sensors 19 may be configured to detect ice accumulation. The sensors 19 may supply the de-icing system 17 with information regarding the status at the respective blade connecting member 6 or pre-tension member 8 or on other components of the wind turbine 1. The de-icing system 17 may only be operational when the sensors 19 detect ice accumulation. In this way, the energy requirements of the de-icing system 17 may be reduced as the system may only be operational when required. The de-icing system 17 may be configured such that the sensor furthermore or alternatively provides the system with information regarding safety aspects of the blade connecting member 6 and / or the pre-tension member 8, such as for example (change in) creep rate, amplitude and frequency of natural or induced oscillations / vibrations, or overheating and thereby prevents or reduces the risk of breakage of the blade connecting member 6 and / or the pre-tension member 8. The sensor 19 may be an accelerometer, temperature sensor, position sensor, load sensor, strain sensor or combinations thereof. Alternatively, the sensor may be an indirect sensor. For example, ice presence may be detected via one or more sensors that records power generation of the wind turbine and determine presence of ice on the blades by a reduction of power generation compared to the wind turbine generator power curve when ice is not present (in other words, a reduction in power output of the wind turbine may suggest ice presence).

[0127] Figures 5A and 5B schematically indicate an example blade connecting member 6 or a pre-tension member 8 (i.e. a cable that could serve either function). As can be seen, a heating system 18 is provided with the respective blade connecting member 6 or pretension member 8. The heating system 18 may be provided embedded, attached, or otherwise secured or coupled with / to a respective blade connecting member 6 or pretension member 8. A plurality of sensors 19 may be provided with the respective blade connecting member 6 or pre-tension member 8, e.g. as shown in Figure 5A, distributed along a length of a respective blade connecting member 6 or pre-tension member 8. A sensor 19 may alternatively be provided that extends along substantially the entire length of the respective blade connecting member 6 or pre-tension member 8, e.g. as shown in Figure 5B.

[0128] The heating system 18 may include one or more electrical heating elements. The electrical heating element may be of any suitable kind, for example the electrical heating element may be a wire heating element (e.g. a nickel-chromium wire), a tubular heater, an infrared heating element or the like. Electrical heating elements provide for a precise, efficient, and controlled temperature increase so as to melt ice and, evaporate water and / or assist in the displacement of ice from a respective surface.

[0129] In one example, the de-icing system 17 may be configured to protect blade load sharing components of the wind turbine 1 forming part of the wind turbine blade 5. An example of a wind turbine blade 5 is shown in Figure 6. The blades 5 include a suction side 15 and a pressure side 16. A thickness dimension of the blade 5 extends between the suction side 15 and the pressure side 16. Each blade 5 may have a cross section which has a substantially circular profile near the root end 11 . The blade 5 may transition from a circular profile to an aerofoil profile moving from the root end 11 of the blade 5 outboard. The blade 5 may comprise a “shoulder” 22 outboard of the root end 11 , which is the widest part of the blade where the blade 5 has its maximum chord. The blade 5 may have an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade. The progressively decreasing thickness may extend from the shoulder 22 to the tip end 12.

[0130] Each of the blades 5 may be a split blade formed of an inboard blade portion 23 and an outboard blade portion 24 coupled together. Each blade portion 23, 24 has a blade shell 52 that defines a respective leading edge 13a, 13b, trailing edge 14a, 14b, suction side 15a, 15b, and pressure side 16a, 16b.

[0131] The inboard portion 23 and outboard portion 24 of each blade 5 may be connected at a connection joint indicated by connection line 40. The connection line 40 between the inboard and outboard blade portions 23, 24 may be a spanwise split, with the connection line 40 being chordwise. The inboard blade portion 23 extends from the blade root 11 to the connection line 40. The outboard blade portion 24 extends from the blade connection line 40 to the blade tip 12.

[0132] It will be appreciated that the blade 5 may have any number of blade portions 23, 24, with respective connection joints between them. Alternatively, the blades 5 may not be split blades and may instead extend continuously from the root end 11 to the tip end 12 without any connection joint.

[0133] The split blade 5 may include a bolted connection in some examples. An example connection between the inboard and outboard blade portions 23, 24 is indicated in Figure 7. The blade portions 23, 24 are coupled by a connection joint that includes a connector 41. The connector 41 connects a first blade end surface 26 of the inboard blade portion 23 to a second blade end surface 27 of the outboard blade portion 24. The connector 41 is adapted to transfer load between the inboard blade portion 23 and the outboard blade portion 24.

[0134] The connector 41 may be a metallic component, preferably a cast component or a machined component. The connector 41 may be a composite component. The connector 41 may be a co-cured or co-bonded component. As explained in further detail below, the connection points 7a, 7b of the connecting members 6 may be on the connector 41 at the connection joint. A leading edge extension 42 may extend forward of the leading edge 13 of the blade 5. The leading edge extension 42 may be integrally formed with the connector 41 , although it will be appreciated that in alternative examples the leading edge extension 42 may be a separate component to the connector 41 . The leading edge extension 42 may include connection points 7a, 7b that attach to the connecting members 6. The first and second connection points 7a, 7b may be arranged forward of the leading edge 13 and adjacent the pressure side 16. This provides additional clearance for the connecting members 6 as the wind turbine blades 5 rotate with the hub 4 about the nacelle 3. In particular, sufficient clearance may be provided between the connecting members 6 and the blades 5 when the blades 5 are pitched between about -5 degrees and about +95 degrees. The connection points on the wind turbine blades 5 may be arranged at a position where a thickness-to-chord ratio of the wind turbine blade 5 is between 20% and 50%.

[0135] The wind turbine blade 5 may have a fairing 20 extending over at least the leading edge extension 42, such as shown in Figure 8. The fairing 20 may be secured to the connector 41. In some examples, the connector 41 may extend outside the profile of the blade shell 52 at the connection joint. This may improve the load transfer across the joint, but may require a larger fairing 20. With a split blade, the fairing 20 may span the gap between the two blade sections 23, 24 connected by the connector 41 (or any other form of connection). The fairing 20 may extend over the leading edge extension 42 and the connector 41. The fairing 20 may be sealed to the blade shell 52 by any suitable means, such as a sealant 46.

[0136] The fairing 20 is important in improving aerodynamic performance of a split wind turbine blade 5, i.e. compared to a blade 5 having a leading edge extension 42 coupled to the blade connecting member 6 with no fairing 20. The de-icing system 17 may be configured for protecting the fairing 20 in some examples. For example, oscillations induced in the blade 5 by the de-icing system 17 may serve to displace ice from the fairing 20 located on the blade surface.

[0137] The fairing 20 may include a heating system 18 in some examples, e.g. an electrical heating element as shown in Figure 8. The electrical heating element(s) provided to the fairing 20 may be substantially the same as that discussed in relation to the blade connecting member 6 or pre-tension member 8. The electrical heating element(s) may be provided embedded in the fairing 20 or otherwise attached to a surface of the fairing 20. The heating element in or on the fairing 20 assists in the displacement of ice from the fairing 20.

[0138] As shown in Figure 8, the heating system 18 may also include a heating element locating in or on a respective wind turbine blade 5. The heating element may further assist in removing ice from the blade 5.

[0139] Whilst electrical heating may be a preferred form of heating for the de-icing system 17, it is also contemplated that the components of the wind turbine blade load sharing that require heating may be heated by a fluid heating system. The fluid heating system may transfer heat energy from a working fluid to a component of the wind turbine 1 that requires heating as part of the de-icing system 17. The working fluid may circulate, e.g. by pumping, to transfer heat energy from a heat source to the component. An advantage of a fluid heating system over an electrical heating system is that the working fluid and a conduit for conveying the working fluid may be made of non- electrically conductive materials and which are therefore not susceptible to lightning strikes. This may avoid the cost and weight of lightning strike protection for at least some parts of the anti-icing and / or de-icing system 17. Examples of fluid for a fluid heating system are gas, such as dried air; ethylene glycol; ethylene glycol-water mixtures; and salt water.

[0140] Examples of operating the de-icing system 17 are indicated in Figures 9A to 9D. In the figures, arrows are provided on the pre-tension members 8 to indicate extension or retraction of a tensioning device 9 connected to the respective pre-tension member 8. Arrows are also provided proximal to a respective blade 5, to indicate a direction of movement of the blade 5 in response to extension or retraction of the tensioning devices 9.

[0141] In Figure 9A, tensioning device 9a is in the retracted state, while tensioning device 9c is in the extended state. In this way, a tension is increased in pre-tension member 8a, and a tension is released or reduced in pre-tension member 8c. As can be seen, this causes the blade 5a between the two tensioning devices 9a, 9c to move toward the more tensioned pre-tension member 8a. The inverse occurs in Figure 9B, in which tensioning device 9c is in the retracted state and tensioning device 9a is in the extended state. In this case, the blade 5a moves toward the more tensioned pre-tension member 8c. As will be understood, cyclically repeating the extension and retraction of the tensioning devices 9a, 9c such that one is extended while the other is retracted (i.e. in a phased relationship) will result in the blade 5a oscillating back and forth between the directions indicated in Figure 9A and 9B. Such an approach allows the de-icing to be targeted to a particular blade.

[0142] Referring to Figure 9C, tensioning devices 9a and 9b are in the retracted state, thereby applying a tension to pre-tension members 8a and 8b. Tensioning device 9c is in the extended state, thereby releasing or reducing a tension in pre-tension member 8c. In this way, tension in the pre-tension member 8c and respective blade connecting member 6c is less than a tension applied to the pre-tension members 8a, 8b and respective blade connecting members 6a, 6b. As is shown in the figure, such an arrangement results in wind turbine blades 5a and 5c moving toward the more tensioned pre-tension members 8a, 8b. Specifically, blade 5a moves toward pretension member 8a, while blade 5c moves toward pre-tension member 8b. The inverse occurs in the arrangement of Figure 9D, in which tensioning device 9c is in the retracted state while tensioning devices 9a, 9b are in the extended state. In this case, the blades 5a, 5c move toward the more tensioned pre-tension member 8c. As will be understood, cyclically repeating the extension and retraction of the tensioning devices 9a, 9b, 9c such two of the devices 9a, 9b are extended while the other 9c is retracted (i.e. two devices in synchrony and one in a phased relationship relative to the other two) will result in the blades 5a, 5c oscillating back and forth between the directions indicated in Figure 9C and 9D. Such an approach allows the de-icing to be achieved in multiple blades.

[0143] The above examples may be implemented while the wind turbine 1 is not in a power production mode, and so the wind turbine blades 5 are not rotating about the hub 4.

[0144] The de-icing system 17 has been described as providing protection to various components the wind turbine 1 utilising various mechanisms. It should be understood that any combination of the above examples may be implemented.

[0145] It will be understood that any references herein to the electrical heating element 18 may also be applicable to a fluid heating system. In this way, the electrical heating element 18 of the figures may be replaced or present in combination with a fluid heating system, e.g. the electrical heating element 18 may be replaced with or utilised in combination with a flexible pipe for carrying a working fluid.

[0146] 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 pitch controlled wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2), a hub (4) mounted rotatably on the nacelle (3), and at least three wind turbine blades (5), wherein each wind turbine blade (5) extends between a root end (11) connected to the hub (4) via a pitch mechanism (50), and a tip end (12), the wind turbine (1) further comprising: at least three blade connecting members (6), each blade connecting member (6) extending from a connection point (7a, 7b) on one wind turbine blade (5) towards a connection point (7a, 7b) on a neighbouring wind turbine blade (5), where the connection point (7a, 7b) on a given wind turbine blade (5) is arranged at a distance from the root end (11) and at a distance from the tip end (12) of the wind turbine blade(5); at least three pre-tension members (8), each pre-tension member (8) being connected to one of the blade connecting members (6) and to the hub (4) via a tensioning device (9), wherein the tensioning device (9) provides radial movement of a radially inward end of the pre-tension member (8) with respect to an axis of rotation (10) of the hub (4) due to extension or retraction of the tensioning device (9), each pre-tension member (8) thereby providing pre-tension in the blade connecting member (6) to which it is connected; and a de-icing system (17) coupled to one or more of the tensioning devices (9) and configured to control the one or more tensioning devices (9) to extend or retract so as to excite at least some of the wind turbine blades (5), the blade connecting members(6) and / or the pre- tension members (8) so as to displace ice therefrom.

2. The pitch controlled wind turbine (1) according to claim 1 , wherein the de-icing system (17) is configured to control one or more of the tensioning devices (9) to move between an extended state and a retracted state in a substantially stepwise manner so as to alter a tension in a respective pre-tension member (8) in a substantially stepwise manner.

3. The pitch controlled wind turbine (1) according to claim 2, wherein the de-icing system (17) is configured to control one or more of the tensioning devices (9) to apply a tension to a respective pre-tension member (8), wherein the tension is at least 80% of a maximum tension generatable by the tensioning device (9), preferably the tensionis between 80% and 90% of the maximum tension generatable by the tensioning device (9).

4. The pitch controlled wind turbine (1) according to any preceding claim, wherein the de-icing system (17) is configured to control one or more of the tensioning devices (9) to cyclically extend and retract so as to generate oscillations in at least some of the wind turbine blades (5), the blade connecting members (6) and / or the pre-tension members (8).

5. The pitch controlled wind turbine (1) according to claim 4, wherein the de-icing system (17) is configured to control at least two of the tensioning devices (9) to cyclically extend and retract synchronously or in a phased relationship.

6. The pitch controlled wind turbine (1) according to claim 4, wherein the de-icing system (17) is configured to control a first and a second of the tensioning devices (9a,9b) to cyclically extend and retract synchronously, and to control a third of the tensioning devices (9c) to cyclically extend and retract out of phase with the first and second of the tensioning devices (9a, 9b).

7. The pitch controlled wind turbine (1) according to any of claims 4 to 6, wherein the de-icing system (17) is configured to control one or more of the tensioning devices (9) to cyclically extend and retract so as to generate flapwise or edgewise oscillations in at least some of the wind turbine blades (5).

8. The pitch controlled wind turbine (1) according to claim 7, wherein the de-icing system (17) is configured to control one or more of the tensioning devices (9) to cyclically extend and retract at a frequency substantially equivalent to a natural frequency of a respective wind turbine blade (5), blade connecting member (6) or tension member (8).

9. The pitch controlled wind turbine (1) according to any preceding claim, wherein the de-icing system (17) is coupled to the pitch mechanism (50) and is configured to control the pitch mechanism (50) to cyclically alter a pitch of a respective wind turbine blade (5) so as to displace ice therefrom.

10. The pitch controlled wind turbine (1) according to any preceding claim, wherein the de-icing system (17) is coupled to the pitch mechanism (50) and configured to control the pitch mechanism (50) to alter a pitch of a respective wind turbine blade (5) such that the wind turbine blade (5) is in a substantially stalled condition.11 . The pitch controlled wind turbine (1) according to any preceding claim, wherein the de-icing system (17) includes one or more electrical heating elements (18) and / or a fluid heating system.

12. The pitch controlled wind turbine (1) according to any preceding claim, wherein the de-icing system (17) includes one or more sensors (19) in or on one or more of the blade connecting members (6) or the pre-tension members (8) for detecting ice accumulation.

13. A method of de-icing a pitch controlled wind turbine (1), the pitch controlled wind turbine (1) comprising: a tower (2), a nacelle (3) mounted on the tower (2), a hub (4) mounted rotatably on the nacelle (3), and at least three wind turbine blades (5), wherein each wind turbine blade (5) extends between a root end (11) connected to the hub (4) via a pitch mechanism (50), and a tip end (12); at least three blade connecting members (6), each blade connecting member (6) extending from a connection point (7a, 7b) on one wind turbine blade (5) towards a connection point (7a, 7b) on a neighbouring wind turbine blade (5), where the connection point (7a, 7b) on a given wind turbine blade (5) is arranged at a distance from the root end (11) and at a distance from the tip end (12) of the wind turbine blade (5); and at least three pre-tension members (8), each pre-tension member (8) being connected to one of the blade connecting members (6) and to the hub (4) via a tensioning device (9), wherein the tensioning device (9) provides radial movement of a radially inward end of the pre-tension member (8) with respect to an axis of rotation (10) of the hub (4) due to extension or retraction of the tensioning device (9), each pretension member (8) thereby providing pre-tension in the blade connecting member (6) to which it is connected; wherein the method comprises controlling the one or more tensioning devices (9) to extend or retract so as to excite at least some of the wind turbine blades (5), theblade connecting members (6) and / or the pre-tension members (8) so as to displace ice therefrom.

14. The method according to claim 13, further comprising controlling one or more of the tensioning devices (9) to move between an extended state and a retracted state in a substantially stepwise manner so as to alter a tension in a respective pre-tension member (8) in a substantially stepwise manner.

15. The method according to claim 13, further comprising controlling one or more of the tensioning devices (9) to cyclically extend and retract so as to generate oscillations in at least some of the wind turbine blades (5), the blade connecting members (6) and / or the pre-tension members (8).

16. The method according to any one of claim 13 to 15, wherein the pitch controlled wind turbine (1) comprises a de-icing system (17) including one or more electrical heating elements (18) and / or a fluid heating system, the method further comprising the steps of detecting ice accumulated on at least a part of the wind turbine blades (5), the blade connecting members (6) and / or the pre-tension members (8), activating the one or more electrical heating elements (18) and / or fluid heating system to weaken a connection between ice and the at least part of wind turbine blades (5), the blade connecting members (6) and / or the pre-tension members (8), and controlling the one or more tensioning devices (9) to extend or retract so as to excite at least some of the wind turbine blades (5) to release from the wind turbine blades (5), the blade connecting members (6) and / or the pre-tension members (8) at least a part of the ice accumulated.

17. Use of a tensioning device (9) for de-icing a pitch controlled wind turbine (1), the pitch controlled wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2), a hub (4) mounted rotatably on the nacelle (3), and at least three wind turbine blades (5), wherein each wind turbine blade (5) extends between a root end (11) connected to the hub (4) via a pitch mechanism (50), and a tip end (12), the wind turbine (1) further comprising: at least three blade connecting members (6), each blade connecting member (6) extending from a connection point (7a, 7b) on one wind turbine blade (5) towards a connection point (7a, 7b) on a neighbouring wind turbine blade (5), where theconnection point (7a, 7b) on a given wind turbine blade (5) is arranged at a distance from the root end (11) and at a distance from the tip end (12) of the wind turbine blade(5); at least three pre-tension members (8), each pre-tension member (8) being connected to one of the blade connecting members (6) and to the hub (4) via the tensioning device (9), wherein the tensioning device (9) provides radial movement of a radially inward end of the pre-tension member (8) with respect to an axis of rotation (10) of the hub (4) due to extension or retraction of the tensioning device (9), each pre-tension member (8) thereby providing pre-tension in the blade connecting member (6) to which it is connected; and a de-icing system (17) coupled to one or more of the tensioning devices (9) and configured to control the one or more tensioning devices (9) to extend or retract so as to excite at least some of the wind turbine blades (5), the blade connecting members(6) and / or the pre- tension members (8) so as to displace ice therefrom.

18. The use according to claim 17, wherein the de-icing system (17) includes one or more electrical heating elements (18) and / or a fluid heating system.

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