WO2011161442A2

WO2011161442A2 - A wind turbine blade de-icing system based on shell distortion

Info

Publication number: WO2011161442A2
Application number: PCT/GB2011/051153
Authority: WO
Inventors: Mark Hancock; Paul Hibbard
Original assignee: Vestas Wind Systems A/S
Priority date: 2010-06-22
Filing date: 2011-06-21
Publication date: 2011-12-29
Also published as: WO2011161442A3

Abstract

The invention relates to a wind turbine blade de-icing system in which one or more actuators are used to apply a force to the blade interior to flex the blade surface into a curvature that can no longer support ice. While ice adheres quite easily to blade surfaces, it is brittle in comparison to the materials from which blades are typically made, and can be made to shear off from the blade surface if the surface flexes or distorts sufficiently. As blade surfaces are designed to flex a great deal before damage to the blade occurs, the ice can typically be dislodged without undue stress to the blade. Actuation can be achieved through a pressure source, or by mechanical actuators. Webs can be provided in the blade interior to increase the curvature of the surface as it flexes, and to divide the blade into regions that can then be independently controlled according to the propensity of ice to build up in those regions. Hinge lines in the blade surface or an ice-phobic surface can be used to increase the efficacy of the system.

Description

A WIND TURBINE BLADE DE ICING SYSTEM BASED ON SHELL DISTORTION

The invention relates to a wind turbine blade de-icing system, and in particular to a system that operates based on shell distortion.

Wind turbines are located in windy, exposed locations, and as such locations are typically cold, wind turbines can often become covered in ice. The formation of ice is a particular problem for wind turbine blades, as the ice increases the weight and detrimentally affects the aerodynamic performance of the blade. Furthermore, inhomogeneous ice formation across a blade surface can lead to blade imbalance and damage. All of these effects lead to a significant reduction in the efficiency of the wind turbine, and as such it is desirable to reduce the amount of ice on wind turbine blades.

Many techniques are known for reducing the formation of ice on wind turbine blades, such as passive systems, which do not require energy to be applied to the wind turbine blades, and active systems, which require energy, typically electrical energy, to be applied.

A known passive system is to coat the surface of a wind turbine component, such as a blade, all over with ice resistant paint, such as an epoxy or polyurethane paint, with which ice forms a lower strength bond. This is not a complete solution, however, as ice can still adhere, albeit less tenuously.

Active systems can perform more reliably than passive systems, but often require the installation of additional thermal or electrical components in the wind turbine blades, adding to their weight. The propensity of lightning to strike wind turbines blades can complicate the installation and operation of thermal and electrical components in blades.

We have therefore appreciated that it would be desirable to provide a system for actively removing ice from the surface of a wind turbine blade.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described by way of example and with reference to the drawings in which:

Figure 1 is a schematic illustration of a wind turbine;

Figure 2 is a schematic illustration in cross-section through a wind turbine blade;

Figure 3 is a schematic illustration in cross-section through a further wind turbine blade;

Figure 4 is a top elevation view of a wind turbine blade according to an example of the invention;

Figure 5 is a schematic illustration in cross-section of a wind turbine blade according to an example of the invention; Figure 6 is a schematic illustration in cross-section of a further wind turbine blade according to an example of the invention;

Figure 7 is a schematic illustration in cross-section of a wind turbine blade showing a build-up of ice on the blade surface;

Figure 8 is a schematic illustration in cross-section of the wind turbine blade of

Figure 7 during a de-icing process;

Figure 9 is a schematic illustration in cross-section of a wind turbine blade according to a further example of the invention;

Figure 10 is a schematic illustration of a wind turbine blade according to an example of the invention, divided into zones;

Figure 1 1 is a schematic illustration of a wind turbine blade surface have hinge lines for ease of folding.

Figure 12 is a schematic illustration in cross-section of a further wind turbine blade according to an example of the invention;

Figure 13 is a schematic illustration in cross-section of a further wind turbine blade according to an example of the invention;

Figure 14 is a schematic illustration of a wind turbine blade, using a mechanical actuator to apply a force directly to the inner surface of the blade;

Figure 15 is a schematic illustration of a wind turbine blade, using a mechanical actuator to apply a force directly to an inner web in the interior of the blade.

SUMMARY OF THE INVENTION

The invention is defined in the independent claims to which reference should be made. Advantageous features are set forth in the appendent claims.

According to a preferred aspect of the invention, there is provided a wind turbine blade de-icing system, comprising a pressure source connected to the interior of a wind turbine blade, wherein the pressure source is arranged to modulate the internal pressure of the blade, such that the surface of the blade is deflected by an extent sufficient to weaken the adhesion of ice on the blade surface.

As the adhesion between the ice and the blade surface is weakened or broken by induced movement of the blade surface itself, the de-icing effect can be provided over the entire surface of the blade and without the need for the system itself to be complicated.

A controller can be provided for controlling the modulation provided by the pressure source, as this allows the de-icing operation to be activated when the need arises.

In one embodiment, the wind turbine blade de-icing system comprises a wind turbine blade having a blade surface provided with surface flex lines, wherein movement of the surface is either restricted or promoted along the flex lines. This allows the curvature of the blade surface to be increased, providing greater efficiency in removing ice.

In one embodiment, the surface flex lines comprise hinge lines along which the blade surface is disposed to fold more easily than the surrounding surface regions. The hinge lines further result in sharper folding of the surface.

The hinge lines may comprise surface regions of lesser thickness, as these are straight forward to implement in the surface of the blade.

In a further embodiment, the wind turbine blade comprises one or more spaced internal webs, wherein where the webs attach to the internal surface of the wind turbine blade they restrict movement of the surface to provide the surface flex lines. The webs have the effect of restricting movement of the surface to the regions between the webs, causing the surface deformation to be greater for a given pressure differential or actuation.

The spaced internal webs may divide the interior of the blade into one or more regions, and each region may be pressurised independently of other regions. In this way the blade surface may be de-iced more selectively, or the pressure differential applied to the region may be tailored to more effectively address ice formation in that region. The regions include at least a tip region, a region forward of the spar in the wind turbine blade, and a region aft of the spar in the wind turbine blade.

A thickness of the blade surface at leading edge is thinner than the thickness of the blade surface in a neighbouring region to the leading edge. By "neighbouring region" is meant a region of the blade surface than is located adjacent to the leading edge.

The leading edge may be formed as an aerodynamic skin bonded to a structural leading edge such that a cavity is formed between the aerodynamic skin and the structural leading edge. The aerodynamic skin may be formed from reinforced plastic material.

An inflatable tube may be provided in the cavity between the aerodynamic skin and the structural leading edge

In one embodiment, the system comprises a wind turbine blade with a surface having regions that are ice-phobic. This discourages ice adhesion, and where ice does form, allows the flexing of the blade surface to be more effective.

The pressure source may be a source of positive pressure, or of negative pressure.

In an alternative example, the de-icing system of any preceding claim, comprises one or more actuators for applying a force to the inner blade surface or a spaced inner web. These may apply additional force to the surface to cause it to flex.

In a further embodiment, the system comprises a sealed wind turbine blade, for ensuring that the pressure differential between the interior of the blade and the exterior can be easily maintained. In a further embodiment, the wind turbine blade de-icing comprises a lead-hole and one-way valve arrangement for drawing air into the blade interior due to pressure effects as the blade rotates. This passive system provides a further way of pressurising the blade interior. The pressure effects can be increased using a pressure amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of the invention relate to a wind turbine blade de-icing system in which one or more actuators are used to apply a force to the blade interior to flex the blade surface into a curvature that can no longer support ice. While ice adheres quite easily to blade surfaces, it is brittle in comparison to the materials from which blades are typically made, and can be made to shear off from the blade surface if the surface flexes or distorts sufficiently. As blade surfaces are designed to flex a great deal before damage to the blade occurs, the ice can typically be dislodged without undue stress to the blade. In the examples that follow, actuation can be achieved through a pressure source, or by mechanical actuators.

In further examples, webs can be provided in the blade interior to increase the distortion curvature of the surface as it flexes, and to divide the blade into regions that can then be independently controlled according to the propensity of ice to build up in those regions. Hinge lines in the blade surface or an ice-phobic surface can be used to increase the efficacy of the system.

Figure 1 illustrates a wind turbine 1 , comprising a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade 5 is mounted on a hub 6. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in Figure 1 may be a small model intended for domestic or light utility usage, or may be a large model used, such as those that are suitable for use in large scale electricity generation on a wind farm for example. In the latter case, the diameter of the rotor could be as large as 150 metres or more.

A first example of a known blade construction is illustrated in Figure 2 in cross section. The blade 10 comprises upper 12 and lower 14 outer shell halves and a central, inner beam (also know as a spar) 16, which extends longitudinally through the interior of the blade 10. The beam 16 is of typically of a generally quadrangular cross section and is connected to the outer shell halves 12, 14 along upper 18 and lower 20 surfaces, known as "spar caps". These can be shaped such that the contact area between the beam and the outer shell portions is maximised. To account for the decreasing size of the cross-section of the blade towards the tip end, there is a corresponding decrease in the cross-section of the inner beam towards the tip end.

The inner beam 16 is the main load-bearing component and it is important to optimise its stiffness, particularly at the tip end of the blade, as the outer shell portions may or may not contribute significantly to the overall stiffness of the blades. The beam 16 itself is typically formed from an epoxy resin composite material including carbon reinforcement fibres aligned in the longitudinal direction of the beam. The majority of the carbon fibres are incorporated in the spar caps 18, 20 of the beam, and have a Young's modulus of around 250 GPa. In embodiments where additional stiffness is required, the outer shell portions may include one or more different types of carbon fibres.

Other cross-sections of beam 16 may be used, such as a circular, l-shaped or C- shaped cross-section. Inner beams of l-shaped or C-shaped cross-section are sometimes referred to as webs.

As will be appreciated by those skilled in the art, carbon fibres are fibres in which the main constituent is carbon, and as such include fibres containing graphite, amorphous carbon or carbon nano-tubes. The carbon fibres may be produced from polyacrylonitrile (PAN), pitch or rayon precursors. Advantageously, carbon fibres have a significantly higher stiffness to density ratio than glass fibres and therefore can provide the same or a higher value of elastic modulus to a composite material as glass fibres with a much lower weight of fibres. Although the cost per unit mass of carbon is greater than that of glass, since a lower weight of carbon than glass is required to provide the required elastic modulus, the total cost of the blades according to the invention need not be much higher than that of standard blades of corresponding length.

A further example of a known blade 30 is illustrated in Figure 3 in cross section. In this example, the blade 30 comprises an upper layer 32 and a lower layer 34, each formed from an epoxy resin composite material incorporating strips of pultruded carbon fibres 40 which extend in a longitudinal direction along the blade. Each layer 32, 34 is sandwiched between a thin inner layer 36 and outer layer 38 of glass and epoxy resin skin. A pair of C- beams 42 formed of a glass reinforced fibrous web extends between the upper 32 and lower 34 composite layers. In both Figures 2 and 3, the spar caps may be completely covered by the outer shell portions, or may be at least partially exposed, so that their surfaces form a part of the exterior surface of the blade.

Preferred examples of the invention relate to mechanisms for de-icing a wind turbine blade, such as either of the blades shown in Figures 2 and 3. A first example of the invention will now be described with reference to Figures 4 to 8.

Figure 4 is a schematic illustration of a wind turbine blade 40 viewed in elevation from the top. The blade has a blade surface 42, a blade section tip 44 and a blade root section 46. Although not shown in the diagram, it will be appreciated that in operation, the blade 40 is connected to a blade rotor hub at the blade root section 46. The blade's inner beam 48 or spar is indicated in the diagram by means of the dotted line running sparwise along the length of the blade. The internal construction of the blade 40 can be as indicated in Figures 2 or 3, or can be other alternatives known in the art.

As shown in the diagram, a pressure source, which in this case is an air or fluidic pump 50, is connected to the interior of the blade and to a controller 52, for pressurising or depressurising the blade interior. The pump and the controller can be located in the rotor hub, nacelle, or in the blade itself as desired.

The interior of the blade is sealed so that a pressure differential with the atmosphere can be maintained for at least short periods of time. This may be achieved by closing off sections of the blade with bulkheads or internal membranes for example. Under the action of a pressure differential between the interior and the exterior of the blade, it will be appreciated that the blade surface will flex or distort in the manner discussed above. The curvature of the flexed or distorted blade has been found sufficient to cause any ice adhering to the surface to lose traction.

Figure 5 schematically illustrates the blade of Figure 4 in cross-section. The inner beam 48 has been simplified for the purposes of the illustration to a box shaped structure, and the blade surface 42 (whether formed from opposing shells or from another structure) is shown in an equilibrium, un-flexed position by way of a bold line. In normal use, the blade interior 45 has an equilibrium pressure that is determined by the fluid that it contains. This is usually air, though in alternative examples the blade can be filled with other gases, such as inert, noble gases. In this example, the effects of any changes in the internal pressure that are a result of deformation of the rotor blade as it turns or the influence of the incident wind on the blade shall be ignored.

In the example of Figure 5, the controller 52 operates the pump 50 to pressurise the internal cavity 45 of the blade 40. The change in pressure is shown in the diagram by the +P notation. The inner beam 48 will not be substantially affected by changes in the internal pressure of the blade, but the thinner, flexible surface 42, made up of the upper and lower shells, will flex under the change in pressure and adopt an expanded position, shown by means of the dotted line.

Alternatively, in Figure 6, the controller operates the pump 50 to depressurise the internal cavity of the blade 40. Here, the change in pressure is shown in the diagram by the notation -P. Again, the thinner, flexible surface 42, made up of the upper and lower shells, will flex under the change in pressure and adopt a contracted position, shown by means of the dotted line. A change in pressure inside the blade 40 of as little as l OOmbar has been found by analysis to be sufficient to shear the ice bond, depending on the ice span, the thickness, and adhesion.

In Figures 5 and 6 it will be appreciated that the inner beam 46 is located in the blade cavity 45 and is in fluid communication with it. In this way, the controller 52 and pump 50 can cause the entire blade surface 40 to flex outwards or inwards, by the application of a pressure to the blade interior.

In Figure 7, the blade 40 is shown in cross-section carrying a coating of accumulated ice 70. Accumulation of ice is problematic for wind turbines, reducing efficiency as it changes the aerofoil shape and increases the blades weight, as well as reducing the blade lifespan, by hindering the operation of control surfaces and adding to strain on the blade components. Techniques and sensors for detecting a condition representative of ice accumulation on the surface of a wind turbine blade are therefore known for use with wind turbines and so shall not be discussed in detail here.

The controller 52 is therefore arranged to receive an input indicating the blade's surface condition. On receiving an indication that there is a build-up of ice on the blade 40, the controller instructs the pump 50 to pressurise or depressurise the blade interior 45 and flex the blade surface 42.

As shown in Figure 8, flexing the blade surface 42 has at least two effects on the ice. Movement of the surface 42, and any ice adhering to it, firstly causes the ice to both shear at the ice-blade interface, and, secondly, causes the ice layer to bend and fail in tension. This is because the blade surface is designed to flex naturally in operation and so can flex without damage compared with the ice, which is stiff but essentially brittle. Flexing of the blade surface 42, particularly outwards, also supplies a propulsive force to the ice 70, which acts to propel ice off the surface of the blade. In combination, these effects mean that ice can be removed from the blade surface merely through pressure alone.

In other examples of the invention, the change in pressure applied to the interior 45 of the wind turbine blade 40 can be cycled so that the blade surface is expanded and contracted in succession thereby weakening any ice adhering to the blade surface though outward and inward motion as it is folded backwards and forwards. In this regard, the pressure may be cycled approximately say once every few seconds.

A further example of the invention will now be described with respect to Figure 9, which shows the blade 40 of Figures 3, 4 or 5 in cross-section. In this example, the blade comprises inner ribs or webs 90 which connect to the outer shells of the surface 42 and extend longitudinally through the interior of the blade. The webs 90 provide additional load bearing support for the blade surface 42 along the length of the blade, but also restrain the surface 42 of the blade so that only the surface regions between the connecting inner webs 90 are free to move. In this way, the inner webs form flex lines on the blade surface, where the motion of the surface is restricted. The use of webs enables thinner panels to be used in the skin, which are then able to flex more and be more effective at shedding the ice. One or more of the webs may divide the blade interior into non-fluid communicable compartments, while other webs allow fluid communication.

For the same pressure signal applied to the interior of the blade, the presence of the inner webs 90 results in a greater bending radius or distortion curvature, that is a greater deflection of the blade surface between webs 90 than when the webs are omitted. As a result, the deflection of the blade surface creates sharper or more pronounced waves and troughs that provide a greater shearing and shedding effect on the ice.

In the examples discussed above the pressure throughout the blade interior 45 has been assumed to be substantially uniform, that is neither the ribs 90 or the inner beam 48 significantly obstruct fluid communication in the interior 45 of the blade, allowing the pressure to equalise.

In alternative examples, the provision of the inner webs 90 is used to divide the interior of the blade to be divided into zones or compartments. In this case, at least some of the inner ribs or webs 90 (and also possibly the spar) are not fluid communicable, but seal off the area which they enclose. The system then comprises separate pumps for pressurising each zone or compartment independently to the others, or pipes and valves for controlling the pressure in each zone. In one example, also illustrated in Figure 9, the interior of the blade is divided into a region 92 forward of the inner beam 48, that is between the beam spar and the leading edge of the blade, and a region 94 to the rear of the spar, that is between the spar and the trailing edge of the blade. Each of these regions can be pressurised to different degrees to take into account the different flexibility of the blade shells adjacent those regions, due to the different stresses on the blade resulting from its shape, as well as the likely thickness of the ice in that region. For locations where ice is more likely to accumulate, such as the leading edge, the pump can supply a higher pressure for more effective removal.

The inner webs 90 can also be used to divide the blade in the span wise direction into compartments. Figure 10 shows a simplified example in which a lateral inner web 95 is used to divide the blade into a tip section 96, and a main body section. As before the main body portion can contain a region 92 forward of the spar, and a region 94 aft of the spar. For clarity, regions 92 and 94 are shaded using diagonal lines to distinguish them from the tip section 96. As will be appreciated, the stress in the blade surface at the tip section can be very different from the inner areas of the blade. Providing a separate blade region at the tip, for separate pressurisation to the main body of the blade, allows differences in the equilibrium surface stress of the blade surface 42 to be taken into account. The blade can be divided up into zones that are configured differently to that shown in Figure 10. It can for example be useful to divide the span wise direction of the blade into several lateral sections, particularly along the leading or trailing edges. Each zone can then be pressurised separately with an identical or differing pressure, depending on the presence and thickness of any ice that is detected.

In the discussion of the de-icing mechanism given so far, the construction of the blade surface 42 has been assumed to be largely uniform across the surface of the blade. However, in the example of Figure 1 1 , the blade surface 42 is provided with hinge lines 100 and 102. These are discontinuities in the blade surface 42 that have a lower stiffness than the surrounding surface area, and along which the blade surface can bend or move more easily. The hinge lines 100 and 102 form alternative and opposite flex lines for the blade surface 42, to those provided by the presence of any inner webs 90.

In the example of Figure 1 1 , the hinge lines 100 and 102 are shown formed by a section of reduced shell laminate width along lines 100 and 102. In practice such discontinuities are relatively straight forward to manufacture during the blade production process. In alternative embodiments, the hinge lines 100 and 102 can be formed of regions of material with a different density to the rest of the blade surface, or of different materials altogether, providing the resulting hinge line section 100 and 102 has the desired flexibility.

As a result of the hinge lines 100 and 102, the blade surface bends more easily along the hinge line than at neighbouring sections. This has the effect of increasing the angle through which the blade section is deflected under pressure, creating sharper waves and troughs, and increasing the shear effect on the accumulated ice.

In Figure 1 1 , hinge lines are shown parallel 100 and perpendicular 102 to the ribs 90. In practice, hinge lines could be provided in any two directions on the surface, and need not be straight. The parallel and perpendicular directions are however useful as the hinge lines then cooperate with the direction in which the ribs 90 are arranged. In alternative examples, the hinge lines may be used with or without the inner ribs 90, or if used with ribs in orientations that are different to those shown in the above diagram.

In further examples of the invention, the surface 42 of the blade is provided with one or more regions bearing an ice-phobic or ice-resistant coating 104. The coating can be implemented in a number of ways, such as by ice resistant paints, like an epoxy or polyurethane paint, or by covering the surface with a plastic, in particular, synthetic plastics, such as at least one of polysilicone, polysiloxane or polydimethylsiloxane (PDMS), fluorinated polymers such as Teflon® or polytetrafluoroethylene (PTFE), epoxy or polyurethane.

The ice-phobic regions 104 of the blade are shown in Figure 1 1 in a chess board pattern with regions of the blade 106 that have not been so coated. In alternative examples of the invention, regions 106 could be regions of the blade with an entirely untreated surface, or could have an ice prone surface applied to them to attract ice. An ice-prone surface finish could also be made by roughening the surface, such as by rubbing with an abrasive material or by chemical etching or other chemical treatment. The combination of ice-phobic and ice-prone regions has been found effective in de-icing blades, as ice that naturally forms in the ice prone regions expands onto the ice-phobic regions and subsequently loses adhesion to the surface, eventually detaching altogether from the surface under the usual cycles of strains and stress. In other examples, the ice-phobic regions of the blade surface could extend over all of the blade surface, or at least an extended region of the blade surface, and could further be arranged with a fixed relationship to the surface flex lines, such as at locations away from the lines, or at locations over the lines.

In alternative examples, ice-phobic regions 104 could also be provided by smoothing the outermost, exposed surface of the blade or by providing heating elements under the blade surface 42.

In a wind turbine blade the blade surface at the leading edge may be of such a stiffness that it is not flexible enough to weaken the adhesion of ice at the blade surface, i.e. the stiffness of the leading edge blade surface is too high. Figure 12 shows a partial cross section of the wind turbine blade 40 at the leading edge. The surface of the leading edge is provided as an aerodynamic leading edge surface 42a which is bonded through adhesive to the blade surface 42. The blade surface 42b forms an internal structural leading edge surface whereas the aerodynamic leading edge surface 42a forms an aerodynamic leading edge which is flexible enough to distort under pressure to break the ice off, but stiff enough to hold shape in order to resist the normal aerodynamic loads experienced by the rotor blade 40 in use. The aerodynamic leading edge surface 42a is bonded to the internal structural leading edge surface 42b by adhesive at 54.

The aerodynamic leading edge surface 42a is formed from composite material such as glass or carbon reinforced plastic which is adhered to the blade surface 42 and faired in. The composite material of the aerodynamic leading edge surface 42a has a fibre orientation to allow maximum deflection, for example with biaxial fibres running at +/-45 degrees to leading edge. The surface 42a is also thin, for example between 1.5mm and 4mm to enable sufficient flexing.

As can be seen in Figure 12, a gusset 55 (or web) is provided in the blade cavity at the leading edge. The gusset is bonded through adhesive 56 to the internal structural leading edge surface 42b and through adhesive 57 to the aerodynamic leading edge surface 42a. When the leading edge region is inflated, the gusset 55 prevents the aerodynamic leading edge surface 42a from peeling at the adhesive 54 from the internal structural leading edge surface 42b. In effect, the gusset 55 reduces the amount of peel in the adhesive joints 54 is reduced when the blade cavity is inflated. Furthermore, when the interior is inflated through internal pressure, the adhesive joints 56 and 57 will be in compression rather than peel which increases the structural strength of these joints.

An inflatable tube 60 may be used in the blade interior to transmit the pressure. In Figure 12, the tube 60 is shown in a deflated state.

Figure 13 shows a partial cross section of the wind turbine blade 40 at the leading edge. The beam 16 is formed from spar caps 18 and 20 and two webs 90 (although only one web is shown in Figure 13). The blade surface 42c at the leading edge is thinner than the blade surface 42 in the rest of the blade. By providing a thinner blade surface allows it to flex more readily such that the pressure required to weaken the adhesion of ice is reduced. For example, the thickness of the blade surface 42c at the leading edge may be 10% of the thickness of the blade surface 42 in neighbouring parts of the blade. The skilled person will appreciate that other regions of the blade may have a reduced thickness blade surface.

The above examples relate to active pressurisation of the interior of the blade using the active system of a pump 50 and controller 52. It will be appreciated however that pressurisation could also be achieved using a passive system comprising a lead hole in the blade surface and a one-way valve. The lead hole draws air into the interior of the blade 40 from the external environment as the blade turns around the hub, while the valve ensures that air does not leak back through the lead hole on intake to reduce the pressure differential.

By configuring the lead hole and blade surface appropriately, a sufficient pressure differential can be created. A return path for the air is also needed provided so that the air can escape from the blade interior at a different point in time to the intake point. In further embodiments, it can be advantageous to include an apparatus to increase the pressure created by air entering the blade though the lead hole. This can be achieved by providing a piston chamber in the blade interior connected both to the exterior of the blade by the lead hole, and to the blade interior. Inside the chamber is a piston having a large and a small piston head.

The large piston head moves in the chamber under the action of the air entering the blade as the blade rotates. As the large piston head is acted upon by the air pressure, it moves the smaller piston head inside a piston chamber of smaller diameter connected to the blade interior. The arrangement therefore acts like a pressure amplifier.

In all of the examples above, the deflection of the blade surface has been effected solely by the action of pressure. In the examples described below with reference to Figures 14 and 15, however, the deflection can be effected by mechanical actuators 120 and 124, which may include without limitation, electric motor actuators, rotary actuators, piezo electric actuators, hydraulic actuators and pneumatic actuators. In Figure 14, mechanical actuators 120 have an actuator member connected directly to the interior of the blade surface 42, for pushing or pulling the surface away from its equilibrium position. In Figure 15, the same effect is achieved by locating the actuator 124 between two of the webs 90 so that the action of the actuator 124 deforms the webs and subsequently the surface 42.

Examples of the invention have been described by way of illustration only. The examples are not therefore to be taken as limiting in any way the scope of protection defined in the claims. In particular, it will be appreciated that features and techniques described separately in respect of individual examples could be used together or in isolation with other examples. Furthermore, although the invention has been described with respect to a horizontal axis wind turbine, it could be used equally with a vertical axis wind turbine.

Claims

1 . A wind turbine blade de-icing system, comprising a pressure source connected to the interior of a wind turbine blade, wherein the pressure source is arranged to modulate the internal pressure of the blade, such that the surface of the blade is deflected by an extent sufficient to weaken the adhesion of ice on the blade surface.

2. The wind turbine blade de-icing system of claim 1 , comprising a controller for controlling the modulation provided by the pressure source.

3. The wind turbine blade de-icing system of claim 1 or 2 comprising a wind turbine blade having a blade surface provided with surface flex lines, wherein movement of the surface is either restricted or promoted along the flex lines.

4. The wind turbine blade de-icing system of claim 3, wherein the surface flex lines comprise hinge lines along which the blade surface is disposed to fold more easily than the surrounding surface regions.

5. The wind turbine blade de-icing system of claim 4, wherein the hinge lines comprise surface regions of lesser thickness.

6. The wind turbine blade de-icing system of any preceding claim, wherein the wind turbine blade comprises one or more spaced internal webs, and wherein where the webs attach to the internal surface of the wind turbine blade they restrict movement of the surface to provide surface flex lines.

7. The wind turbine blade de-icing system of claim 6, wherein one or more spaced internal webs divide the interior of the blade into two or more regions, and wherein each region can be pressurised independently of other regions.

8. The wind turbine blade de-icing system of claim 7, wherein the regions include at least a tip region, a region forward of the spar in the wind turbine blade, and a region aft of the spar in the wind turbine blade.

9. The wind turbine blade de-icing system of any preceding claim, comprising a wind turbine blade having a blade surface, and the thickness of the blade surface at leading edge is thinner than the thickness of the blade surface in a neighbouring region to the leading edge.

10. The wind turbine blade de-icing system of claim 9, wherein the leading edge is formed as an aerodynamic skin bonded to a structural leading edge such that a cavity is formed between the aerodynamic skin and the structural leading edge.

1 1 . The wind turbine blade de-icing system of claim 9, wherein the aerodynamic skin is formed from reinforced plastic material.

12. The wind turbine blade de-icing system of claim 10 or 1 1 , wherein an inflatable tube is provided in the cavity between the aerodynamic skin and the structural leading edge

13. The wind turbine de-icing system of any preceding claim, comprising a wind turbine blade with a surface having regions that are ice-phobic.

14. The wind turbine blade de-icing system of any preceding claim, wherein the pressure source is a source of positive pressure.

15. The wind turbine blade de-icing system of any preceding claim, wherein the pressure source is a source of negative pressure.

16. The wind turbine blade de-icing system of any preceding claim, comprising one or more actuators for applying a force to the inner blade surface or a spaced inner web.

17. The wind turbine blade de-icing system of claim 1 or 2, comprising a sealed wind turbine blade.

18. The wind turbine blade de-icing system of any preceding claim, comprising a lead- hole and one-way valve arrangement for drawing air into the blade interior due to pressure effects as the blade rotates.

19. The wind turbine blade de-icing system of claim 18, comprises a pressure amplifier for enhancing the pressure effects as the blade rotates.

20. A wind turbine comprising the wind turbine de-icing system of any preceding claim.