Patent Description:
Wind turbine blades must be able to efficiently convert wind into spinning movement of the wind turbine blades, so that energy of the wind can be converted into rotary mechanical movement of a rotor to which the wind turbine blades are attached. It is preferable, to use materials having a high specific modulus (elastic modulus per mass density of a material), also known as stiffness to weight ratio, in wind turbine blades. This is particularly important in the spar caps of wind turbine blades because the spar caps are subjected to high bending loads (also referred to as flapwise loads) occurring in the operation of the wind turbine and transfer these to the wind turbine hub and ultimately to the foundations of the wind turbine.

When parts of a spar cap made from carbon fiber-reinforced plastic are damaged due to manufacturing defects, transportation damages or even due to fatigue, for example, it is very challenging to repair the spar caps mainly due to the following two requirements. On the one hand, what must be taken into account is that carbon fiber-reinforced plastic is an electrically conductive material and must be integrated within a lightning protection system of the wind turbine for a case where the wind turbine blade intercepts a lightning strike. The lightning protection system may comprise electrical terminals embedded in the spar cap and a down-conductor, for example. In the case of a lightning strike interception by the spar cap, the carbon fiber-reinforced plastic leads the electrical current from the lightning strike to the ground. In a pristine state of the spar cap, the carbon fiber-reinforced plastic is a good electrical conductor in its fiber direction, due to the continuity of the carbon fibers. Repair methodologies require interrupting the aforementioned continuity and electrical conductivity and determine a discontinuity for the electrical path along the spar cap. In case of a lightning strike, the current lead by the spar cap sparks across such discontinuity. Therefore, if a repaired part of the spar cap is not properly integrated in the lightning protection system, the carbon fiber-reinforced plastic can fail due to direct lightning strike and/or flashovers from the main down-conductor while leading the current to the ground. On the other hand, the high stiffness and structural integrity of the spar caps must be maintained after a repair is performed.

Generally, methods for repairing structures made from carbon fiber-reinforced plastic in the state of the art are known. For example, <CIT> relates to such a method. However, this method relates to wing assemblies of aircrafts and is thus not applicable to repairing a wind turbine blade for a wind turbine.

<CIT> describes a patched wind turbine rotor blade comprising a prepared surface, an adhesive region disposed on the prepared surface and a repair disposed on the adhesive region, wherein the adhesive region sufficiently bonds the repair to the prepared surface to substantially match the mechanical properties of the bulk of the turbine rotor blade.

Therefore, there is a need for a simple and cost-efficient method of repairing a spar cap of a wind turbine blade of a wind turbine, the spar cap comprising carbon fiber-reinforced plastic by which the electrical conductivity, high stiffness and structural integrity of the spar cap are maintained.

This problem is solved by the subject-matter of the claims. Therefore, this object is solved by a method of repairing a spar cap comprising carbon fiber-reinforced plastic according to independent claim <NUM>. Further details of the invention unfold from the dependent claims as well as the description and the drawings.

According to the invention, the problem is solved by a method of repairing a damaged spar cap of a wind turbine blade of a wind turbine, the damaged spar cap comprising carbon fiber-reinforced plastic and the method having the steps of: (a) removing damaged carbon fiber-reinforced plastic from the damaged spar cap to obtain a corresponding recess in the damaged spar cap, (b) applying an adhesive to the recess, wherein the adhesive is an electrically conductive adhesive film, (c) fitting at least one patch comprising carbon fiber-reinforced plastic into the recess, and (d) joining the at least one patch with the spar cap to obtain a repaired spar cap.

The method of the invention is particularly applied to a spar cap of a wind turbine blade of a wind turbine. In particular, the spar cap, and more particularly the carbon fiber-reinforced plastic of the spar cap, is connected to a down-conductor of a wind turbine. The carbon fiber-reinforced plastic of the spar cap may be connected to electrical terminals and the electrical terminals may be connected to the down-conductor. Carbon conductive mats may be arranged to connect the carbon fiber-reinforced plastic of the spar cap to the electrical terminals.

In particular, the carbon fiber-reinforced plastic is a unidirectional carbon fiber-reinforced plastic. This means that all or at least <NUM>% of the fibers are directed in only one direction. Moreover, in particular, the carbon fiber-reinforced plastic is a continuous carbon-fiber reinforced plastic. Moreover, the carbon fiber-reinforced plastic, especially the carbon fiber-reinforced plastic of the patch, may comprise pre-impregnated fibers and in particular be a prepreg. For example, a thermoset polymer matrix material, such as epoxy, or a thermoplastic resin may already be present in the pre-impregnated carbon fiber-reinforced plastic.

Prior to removing the damaged carbon-fiber reinforced plastic part, the damaged part or an area of the damaged part of the spar cap may be identified by means of a non-destructive technology such as ultrasonic scanning, for example. The damaged part or area may further be cleaned and/or paint attached to the spar cap may be removed in the damaged area prior to removing the damaged carbon-fiber reinforced plastic. Also, the repaired spar cap may be non-destructively tested to determine a quality of the repair.

According to the invention, the adhesive is an electrically conductive adhesive film. Thereby, a lightning current from a lightning strike can be transferred across the patch. In particular, the conductive adhesive film may comprise an adhesive layer and a conductive material layer. The conductive material layer may comprise carbon fibers or metal scrims, for example.

In a preferred embodiment of the invention, the recess is tapered in at least one direction with a tapering angle Θ<NUM> and/or at least one of the at least one patches is chamfered in at least one direction with a chamfering angle Θ<NUM>. Preferably, the tapering angle Θ<NUM> is in the range of <NUM>° to <NUM>°, preferably in the range of <NUM>° to <NUM>° and more preferably in the range of <NUM>° to <NUM>°. Preferably, the chamfering angle Θ<NUM> is in the range of <NUM>° to <NUM>°, preferably in the range of <NUM>° to <NUM>° and more preferably in the range of <NUM>° to <NUM>°. The tapering angle Θ<NUM> and/or chamfering angle Θ<NUM> ensures a smooth transition of the stiffness from the spar cap to the patch. Generally, a small angle is preferred since the load is transferred via shear rather than peel and provided adhesives are much tougher and stronger in shear mode. Moreover, the effective adhesive surface is increased and thereby the stability of the joint between the at least one patch and the spar cap is increased.

Moreover, it is preferred that at least one of the at least one direction of the tapering and/or at least one of the at least one direction of the chamfering is a fiber direction F of the carbon fiber-reinforced plastic. Preferably, the fiber direction F is a unidirectional direction of unidirectional carbon fiber-reinforced plastic. Thereby, the transition of the stiffness from the spar cap to the patch is further improved.

Furthermore, it is preferred that the tapering angle Θ<NUM> corresponds to the chamfering angle Θ<NUM>. This means, that the tapering angle Θ<NUM> is equal or substantially equal to the chamfering angle Θ<NUM>, meaning that the tapering angle Θ<NUM> may be a value of <NUM>% to <NUM>% of the chamfering angle Θ<NUM>. Thereby, the fitting of the at least one patch into the recess is facilitated and the thickness of a connection means, such as an adhesive, is controlled. In particular, the patch is fitted form-fitting into the corresponding recess.

It is an option that the at least one patch comprises, in particular along a single chamfer of the at least one patch, at least two different chamfering angles Θ<NUM>, Θ<NUM>. In particular, the chamfering angles Θ<NUM>, Θ<NUM> may deviate from one another by at least <NUM>°, preferably by at least <NUM>° and more preferably by at least <NUM>° and up to a maximum of <NUM>°. One of the chamfering angles Θ<NUM>, in particular the chamfering angle Θ<NUM> provided closer to a long end of the patch, is smaller than the other one of the chamfering angles Θ<NUM>, in particular the chamfering angle Θ<NUM> provided closer to a middle portion of the patch. The chamfering angle Θ<NUM> provided closer to the long end of the patch may be in the range of <NUM>° to <NUM>°, for example. The chamfering angle Θ<NUM> provided closer to the middle portion of the patch may be in the range of <NUM>° to <NUM>° for example. Thereby, two adhesive surfaces of different incline are provided, which further increases the stability of the joint between the at least one patch and the spar cap.

In a preferred embodiment of the invention, the adhesive is applied continuously on at least <NUM>% of a surface of the recess and in particular at least <NUM>% of the surface of the recess. More particularly, the adhesive may be applied to the entire surface of the recess. In other words, the adhesive extends continuously along the surface of the recess. Thereby, the adhesive extends continuously along a full repair section, in which the carbon fibers of the spar cap have been interrupted after removal of the damaged part and connected to the adhesive.

In a further preferred embodiment of the invention, the at least one patch is provided with a peel-ply on a top surface and/or a bottom surface of the patch, whereby the peel-ply is removed prior to fitting the at least one patch into the recess. The peel-ply may be a sacrificial nylon, polyester or non-porous Teflon ply. Thereby, the surface of the patch is protected from contamination and activation of the surface upon removal of the peel-ply for enhancing adhesive bonding is achieved.

In another preferred embodiment of the invention, the carbon fiber-reinforced plastic of the spar cap and/or the carbon fiber-reinforced plastic of the patch are provided as pultruded carbon elements. This are in particular pre-cured carbon fibers, where the matrix is already hardened via, for instance, a pultrusion process. By means of using pultruded carbon elements, the risk of introducing wrinkles during repair, which might lead to further structural damage of the spar cap during repair, is eliminated. The pultruded carbon elements may have a width in the range of <NUM> to <NUM>, preferably in the range of <NUM> to <NUM>. The pultruded carbon elements may have a thickness in the range of <NUM>,<NUM> to <NUM>, preferably in the range of <NUM> to <NUM>. Preferably, the pultruded carbon elements of the spar cap and of the patches have an equal or substantially equal thickness.

It is an option that the spar cap and/or the at least one patch are provided with an array of at least two pultruded carbon elements arranged parallel to each other in a fiber direction F of the carbon fiber-reinforced plastic. Since the relatively stiff carbon fiber-reinforced plastic is arranged farther away from a neutral axis of the wind turbine blade, whereby a sectional inertia of the wind turbine blade is increased, the stiffness distribution along the spar cap is improved.

Moreover, it is an option that the spar cap and/or the at least one patch are provided with a stack of at least two pultruded carbon elements arranged stacked on top of each other. Thereby, the stiffness of the spar cap can be improved.

In yet another optional embodiment of the invention, at least two patches are being fitted into the recess, whereby adhesive is applied on top of at least one of the at least two patches. In particular, the adhesive may be one that can be applied to the recess as previously described. The at least two patches may have equal or substantially equal chamfering angles Θ<NUM>. Thereby, the stability of the joint between the patches and the spar cap is further increased.

In a preferred embodiment of the invention, damaged carbon fiber-reinforced plastic is removed from the spar cap by means of abrasion and/or sanding. A tool for abrasion and/or sanding may be led by hand or preferably by a CNC machine. Thereby, the damaged carbon fiber-reinforced plastic is removed in a precise and cost-efficient manner.

In a further preferred embodiment of the invention, the at least one patch is joined with the spar cap by means of vacuum bagging using a vacuum bagging assembly. In this way, the joint between the patches and the spar cap is established uniformly across the damaged area and thereby becomes very stable. The vacuum bagging assembly may comprise: a vacuum source, a vacuum bag, a vacuum port, a breather material, a non-perforated release foil, a bleeder material and a perforated release foil. In particular, the vacuum bag is an airtight flexible sheet placed over a lay-up comprising the breather material, the non-perforated release foil, the bleeder material and the perforated release foil. The lay-up may be sealed by means of sealants along its edges. The vacuum bag may be fitted with the vacuum ports, which may be connected to the vacuum source. During the cure, the vacuum bag is evacuated and the lay-up is compacted under atmospheric or autoclave pressure. Vacuum may be applied to the vacuum bag during the entire cure cycle. Preferably, the lay-up comprises the breather material, the non-perforated release foil, the bleeder material and the perforated release foil are arranged in the listed order with the perforated release foil being arranged in contact with the top surface of the patch. In particular, the breather material is a loosely woven or nonwoven material that acts as a continuous vacuum path over a part but does not come in contact with adhesive from the patch, for example. Particularly, the bleeder material is a nonstructural layer of material used to allow an escape of excess gas and resin during cure. The bleeder material can be removed after the curing process is completed and any excess resin taken with it. The perforated release film may be a solid release film that has been perforated with a hole pattern, which may be uniform. The effect of the perforated release film is to restrict the amount of resin bleed that is able to pass through the perforated release film. The non-perforated release foil may be used in the vacuum bagging process that can be in direct contact with a part without bonding. The vacuum bagging assembly can easily be removed after joining the patch with the spar cap.

Furthermore, it is preferred that the at least one patch is joined with the spar cap by applying heat to the at least one patch, in particular by means of a heating blanket. The application of heat may be provided additionally to the vacuum bagging. In particular, the heating blanket and/or a thermocouple may be enclosed by the vacuum bag of the vacuum bagging assembly. Thereby, the process of vacuum bagging may be controlled and certified.

In yet another preferred embodiment, at least one doubler plate is being arranged on top of the at least one patch and the spar cap and the at least one doubler plate is being joined with the at least one patch and the spar cap. The doubler plate may comprise or be made from carbon fiber-reinforced plastic. In particular, the doubler plate may be made from pultruded carbon elements. The doubler plate may be designed like the patch as previously described. Adhesive as previously described may be applied between the doubler plate and the patch and the spar cap. The doubler plate may be arranged overlapping a boundary line between the patch and the spar cap. The doubler pate may be provided with a tapering having a tapering angle Θ<NUM> in the range of <NUM>° to <NUM>°, preferably in the range of <NUM>° to <NUM>° and more preferably in the range of <NUM>° to <NUM>°. The length of the doubler plate may be in the range of <NUM> to <NUM> and preferably in the range of <NUM> to <NUM>. The doubler plate provides a redundant second load path across the repair and thereby provides a particularly stable joint between the patch and the spar cap at the cost of minimal weight addition to the spar cap.

Further advantages, features and details of the invention unfold from the following description, in which by reference to drawings <FIG> embodiments of the present invention are described in detail. Thereby, the features from the claims as well as the features mentioned in the description can be essential for the invention as taken alone or in an arbitrary combination. In the drawings, there is schematically shown:.

Same objects in <FIG> are denominated with the same reference number. If there is more than one object of the same kind in one of the figures, the objects are numbered in ascending order with the ascending number of the object being separated from its reference number by a dot.

<FIG> is a sectional view along a transversal plane of a first embodiment of a wind turbine blade <NUM> according to the invention. The wind turbine blade <NUM> has a trailing edge <NUM> and a leading edge <NUM>. The wind turbine blade <NUM> comprises a shell <NUM> and a spar <NUM>. The spar <NUM> comprises two spar caps <NUM>, <NUM>, which face each other and are connected to one another by means of a spar web <NUM>.

<FIG> is a perspective view of a patch <NUM> according to a first embodiment, which may be used in the method according to the invention. The patch <NUM> is a pultruded carbon element <NUM> with unidirectional fibers arranged in the direction of the arrow F. The patch <NUM> comprises a peel-ply <NUM> arranged on a top surface of the patch <NUM> and a peel-ply <NUM> arranged on a bottom surface of the patch <NUM>. The patch <NUM> has a thickness T<NUM> in the range of <NUM> to <NUM> and a width W<NUM> in the range of <NUM> to <NUM>.

<FIG> is a sectional long side view of the patch <NUM> from <FIG> along cutting line III-III. <FIG> is a sectional short side view of the patch <NUM> from <FIG> along cutting line IV-IV.

<FIG> is a perspective view of a patch <NUM> according to a second embodiment, which may be used in the method according to the invention. The patch <NUM> is a pultruded carbon element <NUM> with unidirectional fibers in the direction of the arrow F. The patch <NUM> comprises a peel-ply <NUM> arranged on a top surface of the patch <NUM> and a peel-ply <NUM> arranged on a bottom surface of the patch <NUM>. The patch <NUM> has a thickness T<NUM> in the range of <NUM> to <NUM> and a width W<NUM> in the range of <NUM> to <NUM>. The patch <NUM> is chamfered on the bottom surface of the patch <NUM> in the fiber direction F. The chamfering is provided at the long sides of the patch <NUM> with a middle portion of the patch <NUM> not being chamfered, i.e. arranged parallel to the top surface.

<FIG> is a sectional long side view of the patch <NUM> from <FIG> along cutting line VI-VI. Here, the chamfering angles Θ<NUM>, Θ<NUM> of the chamfers are marked. Both chamfering angles Θ<NUM>. , Θ<NUM> are in the range of <NUM>° to <NUM>° and are equal to each other in this embodiment.

<FIG> is a sectional short side view of the patch <NUM> from <FIG> along cutting line VII-VII. Because the middle portion of the patch <NUM> along cutting line VII-VII is not chamfered but arranged parallel to the top surface of the patch <NUM>, <FIG> is equal to <FIG>.

<FIG> is a perspective view on a step of the method according to the invention. In this step, two patches <NUM>, <NUM> are fitted into a recess <NUM> of the spar cap <NUM>, which is one of the spar caps <NUM>, <NUM> of the wind turbine blade <NUM> from <FIG>. The spar cap <NUM> comprises an array of a first stack of pultruded carbon elements <NUM>, <NUM>, <NUM>, <NUM> and a second stack of pultruded carbon elements <NUM>, <NUM>, <NUM>, <NUM>, the first and second stack being arranged parallel to each other in the fiber direction F of the pultruded carbon elements <NUM>. Further, the patches <NUM>, <NUM> comprise an array of pultruded carbon elements <NUM>, <NUM>, with the pultruded carbon elements <NUM> of the patch <NUM> not being visible in this perspective because they are covered by the patch <NUM>. Wedge elements <NUM> and <NUM> are arranged in a transverse direction adjacent to the pultruded carbon elements <NUM>. The recess <NUM> within both stacks of the pultruded carbon elements <NUM>. <NUM> corresponds to fit the patches <NUM>, <NUM>. In particular, the recess <NUM> is tapered in the fiber direction F with tapering angles Θ<NUM>, Θ<NUM>, which are equal to each other in this embodiment. The tapering angles Θ<NUM>, Θ<NUM> correspond to chamfering angles Θ<NUM>. Θ<NUM> of the patches <NUM>, <NUM>. The patches <NUM>, <NUM> are fitted with their lengths L<NUM>, L<NUM> measured in the fiber direction F into the recess <NUM>.

<FIG> is a side sectional view on the step of the method according to the invention from <FIG> along cutting line IX-IX of the spar cap <NUM> and the patch <NUM>. In this embodiment, portions of the first two pultruded carbon elements <NUM>, <NUM> of the first array of the spar cap <NUM> have been removed to form the recess <NUM>. Adhesive <NUM> has been applied between the patches <NUM>, <NUM> and adhesive <NUM> has been applied between the patch <NUM> and the spar cap <NUM>. The thickness T<NUM> of the patch <NUM> corresponds to the thickness T<NUM> of the pultruded carbon element <NUM>. Further, the thickness of the patch <NUM> corresponds to the thickness of the pultruded carbon element <NUM>.

<FIG> is a side view on a further step of the method according to the invention following the step from <FIG>. Here, the patches <NUM>, <NUM> are joined with the spar cap <NUM> by means of vacuum bagging using a vacuum bagging assembly <NUM> arranged on top of the patches <NUM>, <NUM> and the spar cap <NUM>. The vacuum bagging assembly comprises a vacuum source (not shown), a vacuum bag <NUM>, two vacuum ports <NUM>, <NUM> fitted to the vacuum bag <NUM> and a lay-up consisting of a breather material <NUM>, <NUM>, <NUM>, a non-perforated release foil <NUM>, a bleeder material <NUM> and a perforated release foil <NUM>. The lay-up is enclosed by the vacuum bag <NUM>, which is sealed by means of sealants <NUM>, <NUM>, <NUM>, <NUM> to the spar cap <NUM>. A thermocouple <NUM> and a heating blanket <NUM> are arranged between the breather material <NUM>, <NUM>, <NUM> and the non-perforated release foil <NUM>. Power and control lines of the thermocouple <NUM> and heating blanket <NUM> are led out of the vacuum bag <NUM>. The thermocouple <NUM> and heating blanket <NUM> may be connected to a control unit (not shown) for temperature-adjusted control of the heating blanket <NUM>.

In operation, the vacuum bag <NUM> is evacuated to compact the lay-up under atmospheric pressure. The breather material <NUM>, <NUM>, <NUM> acts as a continuous vacuum path but does not come in contact with adhesive from the precured patches <NUM>, <NUM>. The bleeder material <NUM> allows the escape of excess gas and resin during cure. The effect of the perforated release film <NUM> is that it restricts the amount of resin bleed that is able to pass through the perforated release film <NUM>. The non-perforated release foil <NUM> is in direct contact with the heating blanket <NUM>, so that it is not bonded. After operation, the vacuum bag assembly <NUM> can be removed and the repair has been performed, so that a repaired spar cap <NUM> is obtained.

<FIG> is a side sectional view on a further step of the method according to the invention following the step from <FIG>. Here, the patches <NUM>, <NUM> have been joined with the spar cap <NUM> and a repaired spar cap <NUM> was obtained. Further, doubler plates <NUM>, <NUM> made from pultruded carbon elements have additionally been arranged on top of the patch <NUM> and the spar cap <NUM>, in particular on top of the pultruded carbon element <NUM> of the spar cap <NUM>, to strengthen the joint of the patches <NUM>, <NUM> with the spar cap <NUM>. By means of adhesive <NUM>, <NUM> the doubler plates <NUM>, <NUM> are joined with the spar cap <NUM> and the patch <NUM>. The doubler pates <NUM>, <NUM> are provided with a tapering having a tapering angle Θ<NUM>.

Claim 1:
Method of repairing a damaged spar cap (<NUM>) of a wind turbine blade (<NUM>) of a wind turbine, the method having the steps of:
(a) removing a damaged carbon fiber-reinforced plastic part from the damaged spar cap (<NUM>) to obtain a corresponding recess (<NUM>) in the damaged spar cap (<NUM>),
(b) applying an adhesive (<NUM>) to the recess (<NUM>),
(c) fitting at least one patch (<NUM>) comprising carbon fiber-reinforced plastic into the recess (<NUM>), and
(d) joining the at least one patch (<NUM>) with the spar cap (<NUM>) to obtain a repaired spar cap (<NUM>),
characterized in
that the damaged spar cap (<NUM>) comprises carbon fiber-reinforced plastic and the adhesive (<NUM>) is an electrically conductive adhesive film.