Wind Turbines

Technical Field of the Invention

[001] This application relates to a multilayer adhesive system comprising a damping layer and an outer skin layer. The multilayer adhesive system is particularly suited for wind turbine leading edge erosion protection. State of the art

[002] US 2014/0050581 describes an erosion protection system and method applicable to foils or aerodynamic surfaces. Erosion protective elements having joggles are provided. Each single element is alignable with the next to provide a contiguous erosion protection.

[003] US 3,275,295 describes a turbine blade with a tapered one-piece erosion shield.

[004] EP 3 037 655 describes a rotor blade extension realized for mounting over a tip of a wind turbine rotor blade, comprising an airfoil extension portion realized to extend the length of the rotor blade. Said rotor blade extension is able to protect leading edge from erosion.

[005] ES 2,333,929 describes a dismountable leading edge protection made of polymeric materials.

[006] F 2,994,708 discloses a sacrificial patch that can be used for leading edge erosion protection.

[007] US 2008/0107540 describes a damping element for a wind turbine rotor blade. The damping element comprises a laminate material made of at least one viscoelastic layer and at least one stiff layer adhered to said viscoelastic layer, wherein the damping element is adapted to be attached to a body of the rotor blade so that the at least one viscoelastic layer is in contact with the body of the rotor blade.

[008] US 2015/0132140 discloses an erosion shield for a wind turbine blade having a plurality of layers. The layers have an adhesive bond strength between adjacent layers less than the cohesive tensile strength of the layers, such that the outer layers of erosion resistant material are arranged to peel away or delaminate from the erosion shield under the action of the wind once the particular layer is ruptured or eroded. This dynamic removal of the outer layers of the erosion shield provides for increased shield lifetime, and a reduction in the maintenance operations required for a wind turbine blade having such an erosion shield.

[009] WO 2008/157013 discloses a method for protecting an airfoil leading edge surface from sand and water erosion comprising the steps of: first applying to said leading edge surface a preformed molded covering having a complementary shape to said leading edge surface; secondly bonding said preformed molded covering to said leading edge surface; and last applying a sand erosion resistant topcoat. Problem solved by the invention

[010] Having regard to the state of the art summarized above, there remains a need for a composition and method for more effective and cost-effective prevention of leading edge erosion in wind turbines. In particular, there is a need for lighter and more durable systems for protecting wind turbine blades from leading edge erosion. Summary of the Invention [Oil] The above objectives are accomplished by the invention disclosed herein, which provide a layered structure capable to resist the impact energy of incoming particles and/or rain drops and to damp the kinetic energy of such impacting object so that the covered structure does not suffer any negative effect upon the impact. The layered structure of the present invention is beneficial since, amongst other benefits, it reduces operating cost and wind turbine downtime.

[012] The layered structure of the present invention is characterized by the features defined in appended claim 1. Preferred embodiments thereof are characterized in appended claims 2 to 10.

[013] The present invention also provides a method for producing the layered structure of the present invention. The essential elements of this method are characterized in appended claim 11.

[014] The present invention also pertains to the use of the layered structure of the present invention for protecting the leading edge of wind turbine blades against erosion. This aspect is characterized by appended claim 12. Finally, it pertains to a method for repairing eroded wind turbine blades, as specified in appended claim 13. rief Description of the Drawings

[015] Figure 1 depicts one embodiment of the invention. In this figure, an erosion protection system is depicted, which comprises a damping layer (3) fixed to its substrate via an adhesive layer (4). The damping layer is further protected by an outer skin (1) bonded to damping layer (3) via an adhesive layer (2).

[016] Figure 2 depicts another embodiment of the invention. In this figure, an erosion protection system is depicted, which comprises a damping layer (3) fixed to its substrate via an adhesive layer (4). The damping layer is further protected by a self-adhesive outer skin (1).

[017] Figure 3 depicts a further embodiment of the invention. In this figure, an erosion protection system is depicted, which comprises two independent damping layers (3) and (5) bonded together via an adhesive layer (4). The upper damping layer (3) is further protected by an outer skin (1) bonded via an adhesive layer (2). The assembly is immobilized on its substrate via adhesive layer (6).

[018] Figure 4 depicts yet another embodiment of the invention. In this figure, an erosion protection system is depicted, which comprises a damping layer (3) fixed to its substrate via an adhesive layer (4). The damping layer is further protected by an outer skin (1) bond via adhesive layer (2). In this embodiment, the thickness of the layer is not uniform and a maximum thickness is found in the centre of the device where impact probability is the highest.

[019] Figure 5 depicts another embodiment of the invention. In this figure, an erosion protection system is depicted, which comprises a damping layer (3) fixed to its substrate via an adhesive layer (4). The damping layer is further protected by a self adhesive outer skin (1). In this embodiment, the thickness of the layer is not uniform and a maximum thickness is found in the centre of the device where impact probability is the highest.

[020] Figure 6 depicts the simplest embodiment of the invention. In this figure, an erosion protection system is depicted, which comprises a damping layer (3) fixed to its substrate via an adhesive layer (4). [021] Figure 7 depicts as cross-sectional view of an airfoil covered with the layered structure as depicted in Figure 2 (adhesive layer not shown for reasons of clarity). For simplification reasons, the embodiments depicted in Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 are shown on flat substrates; the invention disclosed within this application should however be understood as foldable in such a manner that it can adopt complex geometries such as the geometry of a leading edge of a wind turbine (as depicted in Figure 7). etailed Description of the Invention

[022] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For exam ple, an adhesive system that comprises a list of components is not necessarily l imited to only those components but may include other components that are not expressly listed or inherent to such a com position. That said, the terms "comprises", "comprising", "includes", "including", "has", "having" or any other variation thereof also cover the disclosed embodiment having no further additional components (i.e. consisting of those components). By way of example, a system comprising an adhesive layer and a damping layer discloses the system with just these components as well as a system comprising these components along with other unmentioned components (e.g. an outer skin, an optional second damping layer, an embedded sensor and/or a release liner).

[023] Also, the indefinite articles "a" and "an" preceding an element or component of the invention are intended to be non-restrictive regarding the num ber of instances (i.e. occurrences) of the element or component. Therefore "a" or "an" should be read to include one or at least one, and the singular word form of the element or component also incl udes the plural unless the number is obviously meant to be singular. By way of example, reference to a dam ping layer comprising a polymer should be understood to mean that the damping layer comprises one or at least one type of that polymer unless specified otherwise.

[024] Further, when an aspect of the invention is described as being "preferred", it should be understood that this preferred aspect of the invention can be combined with other preferred aspects of the invention. Combinations of two or more preferred features are thus to be understood as particularly preferred embodiments of the present invention.

[025] As used herein, the term "low weight material" characterizes a material that has a density of less than 1.5 g/cm ³ , preferably less than 1.0 g/cm ³ .

[026] As used herein, the term "hydrophobic" relates to a material showing a water contact angle of greater than 90°. This indication is meant to characterize the hydrophobicity of the material in general, and not the hydrophobicity of the specific surface of interest. Therefore, any impact of surface roughness on the contact angle is to be eliminated/disregarded.

[027] As used herein, indications of "density" refer to density indications that may be determined in accordance with J IS Z 8807 (Version of 2012).

[028] As used herein, indications of "Young's modulus" refer to Young's modul us indications that may be determined in accordance with J I S K 7161 (Version of 1994). amping layer

[029] The damping layer has the function of absorbing and dissipating a major fraction of the kinetic energy of any object hitting the surface of the wind turbine blade. At the same time, it is supposed not to lead to a significant increase of the overall weight of the wind turbine blade.

[030] The materials employed for the damping layer are foamed materials. The type of foam is not particularly limited providing that the constituents of the damping layer are able to shield effectively the substrate by dissipating the energy, and more particularly the kinetic energy, of the incoming object entering in contact with the system assembly.

[031] Foams suitable for use in the present invention as damping layer materials are characterized by one or more of the following characteristics:

- density of from 0.1 to 2 g/cm ³ , preferably from 0.2 to 1.5 g/cm ³ and more preferably from 0.2 to 1.0 g/cm ³ ;

- Young's modulus of from 0.1 to 4 GPa preferably from 0.15 to 1.5 GPa and more preferably from 0.2 to 1.0 GPa;

- tensile elongation at break measured according to ASTM D638 - 14 comprise preferably between 10 and 1500 %, more preferably between 50 and 1000 % and even more preferably between 100 and 800 %.

- breaking strength (i.e. stress value at rupture) measured according to ASTM D638 - 14 comprise preferably between 0.01 and 10 N/mm ² , more preferably between 0.05 and 6 N/mm ² and even more preferably between 0.1 and 3 N/mm ² .

[032] It is preferred that the material exhibits at least density and Young's modulus characteristics in accordance with the above indications.

[033] Examples of suitable materials include PTFE and expanded PTFE, PU and PU foam, PE foam, foamed polyisocyanurate, expanded PS, polyester foam and/or any combination thereof. Examples of polymer materials that can be used for obtaining suitable foam materials include but are not limited to epoxy, fluoropolymer, latex, polyimide, polyolefin, polystyrene, polyurethane, polyvinyl chloride), silicone, polyester, urea-formaldehyde and/or any mixture thereof.

[034] Examples of suitable foam structures encompass closed cell, semi-closed cell and open cell foams. Closed cell materials are preferred since they prevent fluid migration throughout the structured assembly.

[035] In a preferred embodiment, the material employed for the damping layer is a syntactic foam.

[036] In a preferred embodiment, in combination with any of the above or below embodiment, the material employed for the damping layer is a hydrophobic foam.

[037] In a preferred embodiment, the damping material is composed of expanded PTFE or polyethylene-based foam.

[038] In another preferred embodiment, the damping layer is composed of a mixture of an expanded fluoropolymer and a second foam material.

[039] In a preferred embodiment, in combination with any of the above or below embodiment, low weight material are employed for the damping so that the system overall weight is minimized in order to reduced the load apply to the airfoil. [040] In a preferred embodiment, in combination with any of the above or below embodiments, the damping layer material is selected in such a way that its behaviour contributes in a synergetic effect to the overall device impact resistance.

[041] The damping layer may consist of an individual material as disclosed above, but it may also contain such a material in an amount of 50 wt.% or more, preferably 80 wt.% or more, more preferably 90 wt.% or more (with respect to the total weight of the damping layer) in combination with one or more further materials. Such further materials may also be in accordance with the above description of the damping layer material, but they may also be different materials. Such different materials are not particularly limited as long as it does not preclude accomplishment of the essential effects of the damping layer of the present invention, i.e. accomplishment of damping effect at low additional weight.

[042] The thickness of the damping layer is preferably comprised between 10mm and 0.05 mm, more preferably between 5mm and 0.1mm; even more preferably between 3mm and 0.2mm. It is also possible in accordance with the present invention, to provide a layered structure, wherein the damping layer has a non-uniform thickness, wherein thickness at the centre (i.e. the part to be positioned at the leading edge of the blade) is greatest and decreases towards the rim. It is also possible that the thickness of the damping layer decreases to 0 mm before the rim of the layered structure, such that there is an outer area at and near to the rim, where only outer skin and adhesive layer are present. Outer Skin

[043] The function of the outer skin is to protect the damping layer from external elements including but not limited to salt, chemical attack, blood and residues from insects, faeces from birds, ice, water attack, hail and snow, rain erosion, salinity, wear from sand and other small particles UV irradiation, and/or any combination thereof.

[044] The materials employed for the outer skin are not particularly limited providing that the constituents of the outer skin are able to provide the above protection effect.

[045] The main function of the outer skin is the above-mentioned protection effect and not to damp energy of incoming objects entering in contact with the system assembly, yet to the skill person in the art it is obvious that part of the energy of the incoming object will be absorbed by this layer.

[046] Materials suitable for use in the present invention as outer skin materials are characterized by one or more of the following characteristics:

- density of from 0.5 to 10 g/cm ³ , preferably from 0.6 to 9 g/cm ³ and more preferably from 0.8 to 8.0 g/cm ³ ;

- Young's modulus of from 0.5 to 100 GPa preferably from 1.0 to 50 GPa and more preferably from 1.5 to 30 GPa;

- water vapour transmission rate at 23°C and 85% relative humidity of less than 1000 g/m ² d (wherein d stands for "day"), more preferably less than 100 g/m ² d, and even more preferably 10 g/m ² d.

- a root mean square surface roughness of less than 1 mm, preferably less than 0.5 mm and even more preferably less than 0.25 mm

[047] It is preferred that the material exhibits at least a Young modulus and root mean square surface roughness characteristics in accordance with the above indications. [048] The materials that can be employed for the outer skin are not particularly limited. Also preferred are materials that are able to resist impact as defined in standard ASTM G76 - 13 "Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets", ASTM G73-10 "Standard Practice for Liquid Impingement Erosion Testing", and/or ASTM G76-13 "Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets" , wherein impact resistance in these tests is to be determined for a material having the same layer thickness as it is envisaged for the outer skin of the layered structure of the invention. Preferably, a material is selected that complies with the requirements of more than one of these tests.

[049] In a preferred embodiment, the outer skin is a ceramic material, which is advantageously applied by aerosol deposition process. Examples of suitable ceramic material include but are not limited to Al ₂ 0 ₃ , Ti0 ₂ , BaTi0 ₃ and/or mixture thereof. Aerosol deposition process is well known in the art and is, for instance, described in Hanft et al. J. Ceramic. Sci. Tech. (2015), 06, 147-182.

[050] In a preferred embodiment, the outer skin is a graphene based material, which is advantageously applied by Chemical vapour deposition process or spraying for instance.

[051] A resin component selected from epoxy resin, polyester resin, polyurethane resin, which is optionally reinforced by glass fiber and/or carbon fiber.

[052] In a preferred embodiment, in combination with any of the above or below embodiments, a low weight material is employed for the outer skin so that the system overall weight is minimized in order to reduce the load apply to the airfoil.

[053] In a preferred embodiment, in combination with any of the above or below embodiments, the material employed for the outer skin is selected in such a way that its behaviour contributes in a synergetic effect to the overall device impact resistance.

[054] In a preferred embodiment, in combination with any of the above or below embodiment, the outer skin is covered with a coating providing anti-icing properties. Examples of suitable anti-icing coatings are disclosed in US 2015/0376483.

[055] In a preferred embodiment, in combination with any of the above or below embodiments, the material employed for the outer skin is selected or treated in such a way that it exhibits hydrophobic properties.

[056] In a preferred embodiment, in combination with any of the above or below embodiments, the system is covered with a coating providing self cleaning properties.

[057] The outer skin typically has a thickness of from 0.01 to 1 mm, preferably from 0.01 to 0.5 mm, more preferably from 0.01 to 0.25 mm .

[058] In a preferred embodiment, in combination with any of the above or below embodiments, especially in cases where the outer skin is of dark (e.g. black) color, the outer skin is covered with a paint in such a way that if the outer skin is based on a colored material leading to system with a non suitable color for its final usage, this property can be adjusted. Adhesive layer

[059] The adhesive layers to be used for the invention described herein are not specifically restricted provided that they can withstand the conditions encountered on a wind turbine blade. These conditions include, but are not limited to extreme temperatures and temperature variations, high UV exposure, high humidity variations and exposure to the elements like rain, hail, snow and the like. The following information applies independently to each of the adhesive layers of the present invention.

[060] The adhesive layer material is not particularly limited and examples of suitable adhesives include those based on acrylic polymers; rubber-based adhesives such as styrene/diene/styrene block copolymers (e.g., styrene/isoprene/styrene block copolymers and styrene/butadiene/styrene block copolymers), polyisoprene, polyisobutylene, and polybutadiene; silicone type adhesives such as silicone rubbers, dimethylsiloxane-based polymers, and diphenylsiloxane-based polymers; vinyl ether type adhesives such as polyvinyl methyl ether), polyvinyl ethyl ether), and polyvinyl isobutyl ether); vinyl ester type adhesives such as vinyl acetate/ethylene copolymers; and polyester type adhesives produced from a carboxylic acid ingredient such as dimethyl terephthalate, dimethyl isophthalate, or dimethyl phthalate and a polyhydric alcohol ingredient such as ethylene glycol. The adhesive layer may be either a cross-linked adhesive layer or an uncross-linked adhesive layer. Preferably, the adhesive layer is a pressure-sensitive adhesive layer. From the standpoint of adhesion to the airfoil hydrophobic adhesives are preferred.

[061] If a fluoropolymer is selected as the material for the damping layer, it is preferred to use an adhesive layer material suitable for low surface energy substrate. The technical difficulties associated with adhesion on low surface energy substrate are known to the person skilled in the art. An example of a suitable adhesive system includes but is not limited to silicone based adhesives. A surface treatment, such as etching, may also be advantageous in order to promote the adhesion toward the fluoropolymer based damping layer.

[062] As noted above, there is no particular limitation regarding the adhesive systems that can be used for making the structure of the present invention. Adhesive systems may in particular be pressure-sensitive adhesive systems. Typical representatives of such systems, which are all useful for the present invention, are acrylics, rubbers, silicones, polyurethanes, polyesters, polyethers and EVA systems. For instance, any of the pressure-sensitive adhesive systems described in "Adhesion Science and Engineering" edited by M. Chaudhury and A.V. Pocius, Elsevier B.V., 2002; "Pressure-sensitive Adhesives and Applications" by I. Benedek, 2nd Ed., Marcel Dekker Inc., 2004; as well as "Technology of Pressure-Sensitive Adhesives and Products" edited by I. Benedek and M.M. Feldstein, C C Press, 2009; can be used in the present invention.

[063] In a preferred embodiment, in combination with any of the above or below embodiments, a switchable adhesive system is employed to secure the device on its substrate. Using switchable adhesive system is beneficial for the present invention since it allow easy dismounting of the device in case it needs to be replaced. Switchable adhesive system are known in the art and examples of suitable technology are disclosed in WO 2001/005584 or US 2013/0196143.

[064] In a preferred embodiment, in combination with any of the above or below embodiments, the adhesive is selected is such a way that its viscoelastic behaviour contributes in a synergetic effect to the overall device impact resistance.

[065] In a preferred embodiment, in combination with any of the above or below embodiments, a low weight material is employed for the adhesive layer so that the system overall weight is minimized in order to reduce the load applied to the airfoil.

[066] In a preferred embodiment, the thickness of the adhesive layer is 10 - 200 μιη, and more preferably 15 - 50 μιη. [067] A release liner may also be disposed on the pressure-sensitive adhesive layer before the adhesive layer is used, to improve ease of handling such as, for example, protect the adhesive layer and provide additional support (e.g. stiffness). Examples of the material that can be used as a release liner include materials which are known in this field. Suitable release liner materials may, for instance, include bases having a release layer of a plastic film or paper which surface is treated with a release agent such as a silicone, long-chain alkyl, fluorine, or molybdenum sulfide release agent; low-adhesive bases comprising fluorine- containing polymers; and low adhesive bases comprising non-polar polymers including olefinic resins such as polyethylenes and polypropylenes to improve release property from an adhesive layer. Specific examples thereof include films of plastics such as polyesters including poly(ethylene terephthalate), polyvinyl chloride), poly(vinylidene chloride), various acrylic and methacrylic polymers, polystyrene, polycarbonates, polyimides, cellulose acetate (acetate), regenerated cellulose (cellophane), and celluloid and laminated films composed of wood-free paper, glassine paper, or the like and a polyolefin. It is preferred to use a polyester film. A preferred option is a fluorinated siliconized material.

[068] The thickness of the release liner is generally 10 - 200 μιη, preferably 25 - 100 μιη.

[069] It is preferred to use a release liner having maximum peel force of less than 0.5 N/25mm when measured at a 180° peeling angle.

6.4 Optional feature

[070] In a preferred embodiment, in combination with any of the above or below embodiments, the system is coupled of a sensing system. Said sensor may or may not be included within the layered structure of the present invention.

[071] In a preferred embodiment, in combination with any of the above or below embodiments, the system is coupled with a sensing system for condition monitoring.

[072] Said sensor can be used to follow properties of both the eorsion protection system of the overall airfoil. Properties measured by said sensor include but are not limited to fatigue, stiffness, crack and/or any combination thereof.

[073] Examples of suitable sensors to be used in combination of the present invention include but are not limited to Acoustic emission sensor, Strain sensor, optical fibers based system and/or any combination thereof. Suitable sensory systems are described, for instance, in EP 2 676 113 or US 2013/0089463.

[074] In a preferred embodiment, in combination with any of the above or below embodiments, the system is coupled with an anti-icing system. Said anti-icing system may or may not be included within the layered structure of the present invention. System feature

[075] When providing a leading edge of a wind turbine with the layered structure of the present invention, this is typically done such that the layered structure covers the following area of the blade: the leading edge, the suction side and the pressure side. More particularly the film or sheet of the present invention are meant to cover the leading edge of the tip section and the mid section of blade turbine. Usage of the present invention on the max cord section and/or the root section is of limited benefit since these areas are known to the person skilled in the art to be less prone to erosion.

[076] The length and width dimensions of the film or sheet of the present invention are not particularly limited. They reflect the size and the geometry of the wind turbine rotor blade to be protected. Having regard to typical sizes of such blades, the length of the film or sheet may range from 0.1 m to 100 m, typically from 0.5 m to 30 m and more preferably from 1 m to 10 m.

[077] Similarly, the width depends on the thickness of the blade airfoil type to be protected. Typical widths range from 5 cm to 1 m, preferably from 10 cm to 80 cm.

[078] It is of course possible to provide the blade with a protective film or sheet of the invention in two, three, four or more parts. This can be easier to apply. By consequence, the size and preferably the length of the film or sheet of the invention is reduced accordingly, e.g. lengths of the individual parts can be 1/2, 1/3 or 1/4 of the lengths indicated above.

[079] In a preferred embodiment, the invention disclosed herein can be folded in such manner that it can adopt the complex geometry such as that of a leading edge of a wind turbine blade.

[080] In a preferred embodiment, the thickness of the layer used to construct the device and hence, the thickness of the overall system is not uniform. In this embodiment, the maximum thickness is found where impact probability is the highest. Differences in thickness may be present for each of the layers and especially the damping layer and/or the outer skin.

[081] In a preferred embodiment, the thickness of the device at its rim is minimum such as is does not impair with the aerodynamic properties of the airfoil on which the system is installed. The thickness of the device at its rim is preferably less than 200μιη, more preferably less than 120μιη and even more preferably less than 80μιη.

[082] In view of aerodynamic properties, it is furthermore preferred that the thickness increases gradually and steadily from the rim towards the centre, i.e. the foremost position at the edge of the wind turbine blade. At the centre, the thickness of the overall system is preferably from 0.05 to 10 mm, more preferably from 0.1 to 4 mm. At its edges, the thickness of the overall system is preferably from 0.01 to 1 mm, more preferably less than 0.5 mm, even more preferably less than 0.25 mm and most preferably less than 0.1 mm. Optionally, it is possible to smoothen the transitition at the rim of the layered structure by means of paste and/or putty, for instance relying on a silicone-based material.

[083] According to another preferred embodiment in combination with any of the above or below embodiments, the adhesive layer between damping layer and outer skin has the effect of smoothing any surface irregularities and roughness of the damping layer. For this purpose, it is possible to provide a discontinuous adhesive layer that supports the outer skin in regular or irregular intervals. For instance, said adhesive layer may be patterned. ethod of manufacture

[084] The layered structure of the present invention may be produced (in the absence of the wind turbine blade) by a method comprising the steps of

(i) Providing a damping layer;

(ii) Applying an adhesive layer on one side of the damping layer;

(iii) Optionally applying a second adhesive layer on the other side of the damping layer; (iv) Applying the outer skin to the other side of the damping layer or to the second adhesive layer, if present.

[085] In the above method, the relative order of steps (ii), (iii) and (iv) can be modified, such that step (ii) is carried out simultaneously or after step (iii) or even after step (iv), such that the possible sequences of steps are as follows: (i)-(ii)-(iii)-(iv); (i)-(ii)-(iv); (i)-(iii)=(ii)-(iv); (i)-(Mi)-(ii)-(iv); (i)-(Mi)-(iv)-(ii); and (i)-(iv)-(ii).

[086] Application of the adhesive layer or adhesive layers can be carried out using conventional methods and equipment. For instance, the adhesive layer can be provided on a release liner on one of its surfaces and adhered to the damping layer via the other one of its surfaces. The release liner may then be removed immediately or later, for instance shortly before the layered structure is applied onto the wind turbine blade.

[087] Alternatively, the adhesive layer can be provided in form of a solution, which is subsequently left to dry. Such solutions can be coated onto the damping layer by spraying or using a doctor blade or the like.

[088] Application of the outer skin can be accomplished by conventional coating or laminating techniques. A preferred technique is application by aerosol deposition. This technique is suitable for ceramic materials and also for blend of inorganic and organic materials (such as polymer/ceramic beads). Other preferred technique for application of the outer skin include, but are not limited to spraying, dip coating, lamination and roll-to-roll processes, vapor deposition, plasma deposition as well as chemical and electrochemical techniques.

[089] It is also possible to form the layered structure of the present invention in situ on the wind turbine blade. In this case, the relative order of formation of layers begins with the innermost adhesive layer, followed by damping layer, optional adhesive layer and outer skin.

[090] Yet another possibility is to pre-form a layered structure comprising a damping layer, optionally an adhesive layer and an outer skin (in this order), to apply an adhesive onto the wind turbine blade and finally to apply the pre-formed layered structure onto the wind turbine blade provided with adhesive.

[091] The last two approaches are particularly suitable for application of the layered structure in the course of repair of a pre-existing wind turbine blade. This is because the direct and separate application of the adhesive onto the wind turbine blade is a procedure well-suited for filling any holes in the surface (surface defects) of the wind turbine blade. ses and applications

[092] The layered structure of the present invention is suitable for protecting the leading edge of wind turbine blades against erosion.

[093] It can be applied to new wind turbine blades at the manufacturing stage, but also outdoor to wind turbine blades before or after that they already have suffered erosion during use.

[094] Any application method available to the skilled person may be used for applying the layered structure to the wind turbine blades. A preferred application method involves provision of a layered structure, wherein the outer adhesive layer is protected by a release layer, followed by removal of the release layer and adhering the adhesive layer onto the front edge of the wind turbine blade. Adhesion may be improved by subsequent application of pressure onto the layered structure.

] When repairing a wind turbine blade having suffered erosion, it is possible to first coat the surface of the blade with an adhesive and/or filler to smoothen the surface and subsequently apply the layered structure of the present application (or merely the remaining layers of the structure of the present application if an adhesive has already been applied) onto the adhesive-covered surface. The remaining layers may be applied either individually (in sequence) or jointly in the form of a pre-formed laminate.