Method of repairing a wind turbine blade and respectively re- paired wind turbine blade

Field of invention

The present invention relates to the field of wind turbine blades, in particular to a method of repairing a wind turbine blade and a respectively repaired wind turbine blade.

Art Background

Wind power is considered one of the cleanest, most environ- mentally friendly energy sources presently available, and wind turbines have gained increased attention for utilizing this energy source. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades . The rotor blades capture kinetic energy of wind and transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox or directly to the generator. The generator then converts the mechanical energy to electrical energy that may be fed into a power grid.

During operation, wind turbine blades typically suffer from a continuous exposure to influences from the surrounding, such as impact of airborne particles and water rain at high speed, which may result in damages of the wind turbine blades. Also repeated temperature and humidity changes may cause damages, such as cracks, of the wind turbine blades. Small repairs of blades in service require certain outside conditions to be properly conducted. The standard repair materials for epoxy based blades are epoxy-amine systems as well. The minimum use temperature of repair materials is usually down to 15 C, but not below. This limits the available time for repair work on site and may lead to extended downtime which is linked with massive cost for the turbine operator. Therefore, only a short time window for repair is often available. If the temperature is only slightly below the re- quired minimum temperature, a heating tent may be used to raise the laminate and surrounding temperature to some ex- tent, which causes however additional costs and efforts. If the temperature or humidity is clearly below (or above) the process window, then the repair cannot be conducted.

Thus, there may be a need for a repair process of wind tur- bine blades with reduced equipment required and which can be applied independently from the outside conditions, such as temperature and humidity.

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to an aspect of the invention, there is provided a method of (on-site) repairing a wind turbine blade comprising a resin containing a cleavable functional group (in particu- lar an acid-cleavable functional group), the method compris- ing applying an acidic aqueous liquid (having a pH of less than 7) comprising an acid to a portion (to be repaired, e.g. a damaged area) of a surface of the wind turbine blade, heat- ing the portion of the surface of the wind turbine blade (where the acidic aqueous liquid has been applied), applying a basic (alkaline) aqueous liquid (having a pH of more than 7) comprising a base (alkaline compound) to the portion of the surface of the wind turbine blade (where the acidic aque- ous liquid has been applied), optionally (in particular in case of large cracks) applying a (reactive) hot melt adhesive comprising an isocyanate or a blocked isocyanate to the por- tion of the surface of the wind turbine blade (where the acidic aqueous liquid and the basic aqueous liquid have been applied), optionally drying that portion of the surface of the wind turbine blade.

According to a further aspect of the invention, there is pro- vided a (repaired) wind turbine blade obtainable (or ob- tained) by a method as described herein.

These aspects of the invention are based on the idea that cracks in a surface of a wind turbine blade comprising a res- in containing a cleavable functional group (in particular an acid-cleavable functional group) may be repaired by applying an acidic aqueous liquid to the surface portion to be re- paired, followed by heating the surface portion where the acidic aqueous liquid has been applied. Without wishing to be bound to any theory, the present inventors assume that by heating an acid-treated surface the cleavable functional groups of the resin in an outermost layer of the wind turbine blade can be cleaved, thereby generating for instance free hydroxy groups at the resin. Moreover, hereby at least part of the resin in an outermost layer of the wind turbine blade may become dissolved in the acidic aqueous liquid and/or a thermoset structure in an outermost layer of the wind turbine blade may be at least partially converted into a thermo- plastic material. As a result of these changes, smalls cracks may be closed or sealed upon heating, in particular due to flow or entry of dissolved resin into the cracks and/or the developed thermoplastic properties or behaviour of the resin. Subsequently, a basic aqueous liquid is applied so as to neu- tralize the applied acidic aqueous liquid. Moreover, any dis- solved resin may be precipitated again (while remaining ther- moplastic), which further promotes sealing of cracks. In case of larger cracks, a (reactive) hot melt adhesive comprising (optionally blocked) isocyanate functional groups may be ap- plied such that covalent bonds between the (unblocked) isocy- anate functional group of the hot melt adhesive and hydroxy groups of the resin (which may be generated as a result of the cleavage described above) may be formed, i.e. the thermo- plastic may be re-crosslinked to further enhance the mechani- cal properties. As a result, the wind turbine blade may be efficiently repaired on-site with reduced equipment required and independently from the outside conditions, such as tem- perature and humidity.

Detailed Description

Hereinafter, details of the present invention and other fea- tures and advantages thereof will be described. However, the present invention is not limited to the following specific descriptions, but they are rather for illustrative purposes only.

It should be noted that features described in connection with one exemplary embodiment or exemplary aspect may be combined with any other exemplary embodiment or exemplary aspect, in particular features described with any exemplary embodiment of a method of repairing may be combined with any other exem- plary embodiment of a method of repairing and with any exem- plary embodiment of a wind turbine blade and vice versa, un- less specifically stated otherwise.

Where an indefinite or definite article is used when refer- ring to a singular term, such as "a", "an" or "the", a plural of that term is also included and vice versa, unless specifi- cally stated otherwise.

The expression "comprising", as used herein, includes not on- ly the meaning of "comprising", "including" or "containing", but may also encompass "consisting essentially of" and "con- sisting of".

Unless specifically stated otherwise, the expression "at least partially", "at least a partial" or "at least (a) part of", as used herein, may mean at least 5 % thereof, in par- ticular at least 10 % thereof, in particular at least 15 % thereof, in particular at least 20 % thereof, in particular at least 25 % thereof, in particular at least 30 % thereof, in particular at least 35 % thereof, in particular at least

40 % thereof, in particular at least 45 % thereof, in partic- ular at least 50 % thereof, in particular at least 55 % thereof, in particular at least 60 % thereof, in particular at least 65 % thereof, in particular at least 70 % thereof, in particular at least 75 % thereof, in particular at least

80 % thereof, in particular at least 85 % thereof, in partic- ular at least 90 % thereof, in particular at least 95 % thereof, in particular at least 98 % thereof, and may also mean 100 % thereof.

In a first aspect, a method of (on-site) repairing a wind turbine blade comprising a resin containing a cleavable func- tional group (in particular an acid-cleavable functional group) comprises applying an acidic aqueous liquid (having a pH of less than 7) comprising an acid to a portion (to be re- paired) of a surface of the wind turbine blade, heating the portion of the surface of the wind turbine blade (where the acidic aqueous liquid has been applied), applying a basic (alkaline) aqueous liquid (having a pH of more than 7), com- prising a base (alkaline compound) to the portion of the sur- face of the wind turbine blade (where the acidic aqueous liq- uid has been applied).

The method of repairing may be carried out on-site, i.e. while the wind turbine blade is installed at a wind turbine, which has the great advantage that downtime of the wind tur- bine can be kept to a minimum. The method of repairing may however also be suitable for repairing cracks or other de- fects that occurred during the manufacturing, storage and/or transport of the wind turbine blade. Thus, the method of re- pairing may also be performed before the wind turbine blade is installed at a wind turbine. Moreover, the method of re- pairing may also be suitable for repairing a wind turbine blade after its uninstallation from a wind turbine, for in- stance if the wind turbine blade is intended to be reused (recycled) in another (or the same) wind turbine. The wind turbine blade to be repaired by the method described herein comprises a resin containing a cleavable functional group. In particular, the wind turbine blade may comprise a (bulk) material or substrate comprising a (glass) fiber rein- forced polymer resin composite comprising a resin containing a cleavable functional group.

In an embodiment, the resin comprises at least one of an epoxy-based resin and an epoxy-amine-based resin. Commercial- ly available examples thereof are based on Recyclamine® (from Aditya Birla Chemicals) or Cleavamine® (from Adesso Advanced Materials) technology for instance.

In the context of the present application, the term "cleava- ble functional group" may particularly denote a functional group having a covalent bond that may be cleaved under cer- tain conditions (e.g. temperature, pH value, reagent). For instance, the cleavable functional group may in particular be an acid-cleavable functional group, i.e. a functional group having a covalent bond that may be cleaved by an acid or un- der acidic conditions (pH of less than 7).

In an embodiment, the cleavable functional group comprises at least one of an acetal and a ketal functional group. An ace- tal or a ketal is generally represented by the following chemical formulas: wherein R ¹ , R ² , R ³ and R ⁴ independently from each other repre- sent an organic (carbon atom-containing) moiety. As it is known to a person skilled in the art, an acetal or a ketal can be cleaved by hydrolysis under acidic conditions to yield two alcohols (R ¹ -OH and R ² -OH in the above general formulas) and an aldehyde (R ³ -CHO in the above general formula of an acetal) or a ketone (R ³ -C(=O)-R ⁴ in the above general formula of a ketal).

In an embodiment, the acidic aqueous liquid has a pH value of less than 7, in particular of less than 6, in particular of less than 5, in particular of less than 4. Moreover, the acidic aqueous liquid may have a pH value of more than 0, in particular of more than 1.

In an embodiment, the acid is preferably an organic acid or a carboxylic acid, rather than a mineral acid. An organic acid or a carboxylic acid may not only be particularly suitable for adjusting an appropriate pH, but may additionally also act as a solvent for a reaction product of the resin after cleavage of the cleavable functional group. Moreover, it may be advantageous if the organic acid or carboxylic acid has a good miscibility with water or a water/solvent-mixture and may also dissolve non-polar substances.

In an embodiment, the acid comprises a monocarboxylic acid, a dicarboxylic acid, a tricarboxylic acid and/or a polycarbox- ylic acid, in particular selected from the group consisting of aliphatic monocarboxylic acids, aliphatic dicarboxylic ac- ids, aliphatic tricarboxylic acids and aliphatic polycarbox- ylic acids. For instance, the acid may be selected from the group consisting of acetic acid, lactic acid, maleic acid, fumaric acid, tartaric acid, oxalic acid, malonic acid, suc- cinic acid, adipic acid and citric acid. Combinations of two or more of these acids are suitable as well.

In an embodiment, the acid comprises at least one selected from the group consisting of acetic acid, lactic acid, citric acid, oxalic acid, tartaric acid or combinations of two or more of foregoing. In particular, the acid may comprise ace- tic acid (for instance in a concentration of 30% by mass of the acidic aqueous liquid), which has proven particularly suitable for the purposes of the present invention. In an embodiment, the acidic aqueous liquid further comprises a solvent, in particular a polar solvent, preferably a polar protic solvent. The term "polar solvent", as used herein, re- fers to a solvent that has a good miscibility with water. The term "protic solvent", as used herein, refers to a solvent that is capable of donating protons, for instance via hydro- gen bonding. It may be advantageous if the acidic aqueous liquid comprises a solvent that is capable to migrate into a thermoset structure of the resin and swell it, preferably in combination with the ability to form strong hydrogen bonds, thereby cleaving (or at least weakening) the hydrogen bonds of the thermoset structure. Suitable examples of the solvent include ethanol, propanol (1-propanol or 2-propoanol), buta- nol, dimethyl sulfoxide (DMSO) and combinations of two or more thereof.

In an embodiment, the acidic aqueous liquid is immobilized on and/or in a carrier and/or in form of a hydrogel when ap- plied. For instance, the acidic aqueous liquid may be soaked in a carrier, such as a fleece (non-woven fabric) or sponge. Additionally or alternatively, the acidic aqueous liquid may be immobilized in form of a hydrogel, such as a chitosan- based hydrogel. By taking this measure, it may be avoided that the acidic aqueous liquid runs off or drains off from the portion of the surface of the wind turbine blade to be repaired. Moreover, the acidic aqueous liquid may be selec- tively applied to the portion to be repaired by adjusting the carrier or the hydrogel to the size of the damaged area, thereby avoiding that undamaged portions of the wind turbine blade surface are affected.

After the acidic aqueous liquid has been applied to the por- tion to be repaired of the surface of the wind turbine blade, a heating step is carried out. To this end, the portion where the acidic aqueous liquid has been applied may be covered with a heating device, such as a heating blanket. In an embodiment, the heating of the portion of the surface of the wind turbine blade is carried such that at least part of the cleavable functional groups of the resin in an outer- most layer of the wind turbine blade are cleaved (thereby forming (free) hydroxy groups at the resin). An example of cleaving a ketal functional group of an epoxy-amine-based resin is illustrated by the following chemical cleavage reac- tion :

As it is evident from the above reaction scheme, an epoxy- amine-based resin having a cleavable ketal functional group is hydrolyzed in the presence of an acid (catalyst), thereby forming cleaved resin parts having a (free) hydroxy group and a ketone (more specifically, acetone in the depicted exam- ple) . The generated hydroxy groups may be used for coupling with isocyanate functional groups of a hot melt adhesive, as will be described in further detail below.

In an embodiment, the heating of the portion of the surface of the wind turbine blade is carried such that at least part of the resin in an outermost layer of the wind turbine blade is (gets, becomes) dissolved in the acidic aqueous liquid, for instance because of a protonation of an amine group in an epoxy-amine-based resin in the above depicted example. Resin in dissolved form may enter or flow into cracks in the dam- aged surface of the wind turbine blade and may subsequently be precipitated therein (by applying a basic solution as ex- plained in further detail below), thereby contributing to a sealing of the cracks.

In an embodiment, the heating of the portion of the surface of the wind turbine blade is carried such that a thermoset structure in an outermost layer of the wind turbine blade is (at least partially) converted into a thermoplastic material. Due to the developed thermoplastic properties, smalls cracks may be closed or sealed upon heating.

In the context of the present application, the term "outer- most layer of the wind turbine blade" may particularly denote a surface layer (of the wind turbine blade) having a thick- ness of up to 1 mm, in particular up to 0.5 mm.

In an embodiment, the portion of the surface of the wind tur- bine blade is heated to a temperature in a range of from 70°C to 100°C, such as from 80°C to 90°C, and/or for a heating time in a range of from 1 min to 1 h, such as from 5 min to 45 min. By taking this measure, the cleavage of the cleavable functional groups of the resin may be controlled such that a sufficient amount of cleavable functional groups resin in an outermost layer of the wind turbine blade is cleaved, while avoiding cleavage in the bulk material of the wind turbine blade (i.e. underneath the outermost layer), Moreover, by taking this measure, the cleavage of the cleavable functional groups of the resin may be controlled such that a sufficient amount of the resin in an outermost layer of the wind turbine blade is dissolved and/or such that a thermoset structure in an outermost layer of the wind turbine blade is sufficiently converted into a thermoplastic material.

If the acidic aqueous liquid is applied in immobilized form, such as by means of a carrier and/or as a hydrogel, the immo- bilization means (e.g. carrier and/or hydrogel matrix) is preferably removed after the heating step.

After the portion to be repaired of the surface of the wind turbine blade has been acid-treated and heated, and optional- ly an applied acid immobilization means has been removed, a basic (alkaline) aqueous liquid comprising a base (alkaline compound) is applied to that portion of the surface of the wind turbine blade. In an embodiment, the basic aqueous liquid has a pH value of more than 7, in particular of more than 8, in particular of more than 9, in particular of more than 10. Moreover, the basic aqueous liquid may have a pH value of less than 14, in particular of less than 13.

In an embodiment, the basic aqueous liquid is applied such that the applied acidic aqueous liquid is neutralized. In particular the pH at the portion to be repaired portion of the surface of the wind turbine blade may be raised, for in- stance to more than 5, in particular to more than 6, such as to pH 7 or more.

In an embodiment, the basic aqueous liquid is applied such that dissolved resin is precipitated (while remaining thermo- plastic). As explained above, at least part of the resin in an outermost layer of the wind turbine blade may be dissolved in the acidic aqueous liquid, for instance because of a pro- tonation of an amine group in an epoxy-amine-based resin, and such resin in dissolved form may enter or flow into cracks in the damaged surface of the wind turbine blade. The basic aqueous liquid may be applied such that dissolved resin is precipitated again, for instance because of a deprotonation of a previously protonated amine group in an epoxy-amine- based resin. As a result, such precipitated resin present in cracks of the damaged surface of the wind turbine blade may contribute to and improve a sealing of the cracks, all the more as the precipitated resin may still be thermoplastic and thereby further seal the cracks.

In an embodiment, the base (alkaline compound) is selected from the group consisting of a hydroxide, a hydrogen car- bonate (bicarbonate) and a carbonate. For instance, sodium salts thereof, such as sodium hydrogen carbonate (NaHCO ₃ ), may be particularly suitable and cost-efficient.

In an embodiment, the method further comprises, after the step of applying a basic aqueous liquid, a step of applying (one or more layers of) a (reactive) hot melt adhesive com- prising an isocyanate functional group or a blocked isocya- nate functional group to the portion of the surface of the wind turbine blade (where the acidic aqueous liquid and the basic aqueous liquid have been applied). By taking this meas- ure, also larger cracks may be efficiently sealed. The hot melt adhesive may be applied in sheet form or in powder form. The hot melt adhesive may be solid and may in particular com- prise a polyurethane (PU) hot melt adhesive, which may have a similar melting point as the thermoplastic material obtaina- ble from the resin of the wind turbine blade. The hot melt adhesive comprises an isocyanate functional group or a blocked isocyanate functional group. Suitable examples for a blocked isocyanate functional group include an isocyanate functional group blocked with an alcohol (i.e. as a urethane or carbamate functional group) and/or an amine (i.e. as a urea functional group). Prior to a subsequent reaction as will be described below, a blocked isocyanate functional group has to be unblocked by cleavage of the blocking moie- ties (such as alcohol or amine), for instance by heating the blocked isocyanate functional group to an appropriate temper- ature for the release of the respective blocking moieties, as known to a person skilled in the art.

In an embodiment, the (reactive) hot melt adhesive is applied such that covalent bonds between the (unblocked) isocyanate functional group of the hot melt adhesive and hydroxy groups of the resin are formed; in other words, such that (un- blocked) isocyanate functional groups of the hot melt adhe- sive react with hydroxy groups of the resin that have been formed from the cleavable functional groups of the resin. By taking this measure, a strong covalent bond between adhesive and the newly formed hydroxy groups can be established, and the thermoplastic can be re-crosslinked to enhance the me- chanical properties of the thermoplastic further.

In an embodiment, the method further comprises, after the step of applying a basic aqueous liquid and optionally the step of applying a hot melt adhesive, a step of drying the portion of the surface of the wind turbine blade. Preferably, the drying may involve an active drying, such as by heating. The drying may however also involve a passive drying, i.e. allowing to dry.

In a further aspect, a (repaired) wind turbine blade obtaina- ble or obtained by a method as described herein. By the meth- od of repairing as described herein, not only cracks of a wind turbine blade to be repaired may be closed or sealed, but a thus-repaired wind turbine blade also differs from an original (brand-new) wind turbine blade in that initially present cleavable functional groups have been consumed by the repairing process, in particular by cleavage of bonds and re- lease of for instance aldehydes and/or ketones, as evident from the cleavage reaction scheme depicted above.

The invention or the embodiments of the invention described in this document can be descriptively summarized as follows:

The surface of the damaged area on a wind turbine blade may be wetted with an aqueous acid, e.g. 30 % acetic acid which can act both as solvent and acid and which was proven to work well for the purpose as described herein. Other acids can be e.g. lactic acid, citric acid, oxalic, tartaric acid, which can be used with or without an additional solvent e.g. a po- lar solvent like ethanol, butanol or DMSO. Generally, the ac- id shall have good solubility/miscibility in water or wa- ter/solvent mix, and shall preferably be able to act as a solvent as well. Ideally the acid is able to interact with and dissolve also non-polar substances. The optional solvent shall be able to migrate into the thermoset structure and swell it, ideally combined with the ability to form strong hydrogen bonds (polar protic solvents) and thus cleaving the hydrogen bonds found in the thermoset structure.

According to an embodiment of the invention, the acidic solu- tion is preferably soaked in a carrier (fleece, sponge or similar) or immobilized in form of a hydrogel (e.g. chitosan based) to avoid the solution to run off. The carrier or gel containing the acid catalyst may be adjusted to the size of the damaged area to avoid undamaged areas being affected. The damaged area may be covered with a heating device e.g. a heating blanket. The damaged area is preferably heated to 80- 90 C to activate the surface by cleavage of acetal/ketal groups in the network. The heating time shall be enough to cleave the acetal bonds in the outermost layer of the blade to convert the thermoset structure into a thermoplastic mate- rial. But a heating time should be chosen that avoids cleav- age in the bulk material. Based on experiments, the heating time shall preferably not be more than one hour.

After heating, the acidic solution may be neutralized. In case a carrier or immobilized acid has been used, the materi- al is removed, and the surface is treated with a slightly basic solution, e.g. a hydroxy or carbonate based base such as NaHCO ₃ and the surface may be completely dried afterwards.

Depending on the size of the damage, the formed thermoplastic material may be used to stabilize a small crack by welding. For larger damages one or several layers of a reactive hot melt polyurethane (PU) solid adhesive (in sheet or powder form) are added, which have a similar melting point as the thermoplastic received from the laminate matrix. The reactive PU hot met adhesive contain an isocyanate or blocked isocya- nate component that can react with the free alcohol groups of the cleaved epoxy resin (which are depicted in the cleavage reaction scheme above) that is obtained in the acid and se- quential base treatment. With this strategy a strong covalent bond between adhesive and the newly formed reactive groups can be established, and the thermoplastic can be re- crosslinked to enhance the already good mechanical properties of the thermoplastic further.

While the present invention has been described in detail by way of specific embodiments and examples, it should be appre- ciated the invention is not limited thereto and various al- terations and modifications are possible, without departing from the scope of the invention.