WIND TURBINE BLADE

Abstract

There is provided a wind turbine blade at least 50% of the surface of which is covered with a self-adhesive thermoplastic film. By applying a self-adhesive thermoplastic film to the blade, the need for a gelcoat or the paint is eliminated. It is estimated that the thermoplastic film will take a similar time to apply as the gelcoat and/or paint. However, it does not require any further treatment once it has been applied thereby reducing significantly the work involved in finishing the blade. Also, the thickness of the film is precisely controlled in advance of its application to the blade ensuring that a surface with a uniform thickness is produced.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Great Britain patent application serial number 0805713.5, filed Mar. 28, 2008, which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wind turbine blade, and more specifically, to an improved surface coating for a wind turbine blade.

The coating on the surface of a wind turbine blade is exposed to a harsh environment of abrasion, UV, humidity, corrosion, cyclical stresses, and extreme temperature fluctuation and requires a high performance material. Coatings need to retain performance for up to 20 years, putting them in a higher performance and different specification bracket than most civil/automotive applications and additionally be of a cost significantly lower than products typically found in aerospace applications.

2. Description of the Related Art

Wind turbine blades are typically coated using either a gelcoat, or are painted. Gelcoat is applied directly into the mould during manufacture of the blade and is formulated from a chemical backbone compatible with the substrate laminate, which is usually a polyester, vinylester or an epoxy resin. Painted finishes are normally achieved using variations of cross-linked polyurethane paint, usually supplied as two components (with a polyol and, or polyester resin base and an aliphatic isocyanate curing agent). These are mixed prior to application and the chemical reaction produces the cross linked polyurethane polymer. Some blades use a combination of both gelcoat applied into the mould and paint applied to the blade after demoulding. This gives additional service life to the surface.

Some wind turbine blades have their leading edges taped with impact resistant tape, which is often applied to older blades to repair them.

The shortcomings of current technology are as follows:

With so-called ‘in mould’ technology such as gelcoat it is very difficult, if not impossible, to achieve a perfect surface straight out of the mould. Practically, any small variances in mixing quality, viscosity, humidity, substrate condition and operator skill can lead to a large number of cosmetic defects out of the mould but also a number of adhesion problems when in service. In reality a large amount of time and labour (around a third of the labour required to produce the blade) is spent repairing defects in the surface of these blades prior to use and filling manufacturing joins from the mould itself resulting in a cost increase in the blade. These problems, and the probability of a defect in the surface only increases with increasing blade size.

Painting as a process, in particular spraying, is a very wasteful process and requires a lot of operator skill in order to ensure a consistent coating. Polyurethane coating systems used for wind turbine blades are solvent based and when these are sprayed a large amount of hazardous organic solvent (typically 50% by weight), is released into the atmosphere Spraying of polyurethanes is also a potentially hazardous operation both for workers and the environment due to the isocyanate component in the curing agent which is a sensitising agent and great care has to be taken to prevent fumes of isocyante being inhaled by the operators involved in the spraying operation.

In addition it is found that, although aliphatic polyurethanes are the highest performing paints available for coating wind turbines, it is often necessary to repair such a coating after little more than five years in service (depending on operating conditions). This is extremely costly and adds additional cost to the service life of the blade as the necessary working life of a wind turbine blade is 20 years.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a wind turbine blade at least 50% of the surface of which is covered with a self-adhesive thermoplastic film.

By applying a self-adhesive thermoplastic film to the blade, the need for a gelcoat or the paint is eliminated. It is estimated that the thermoplastic film will take a similar time to apply as the gelcoat and/or paint. However, it does not require any further treatment once it has been applied thereby reducing significantly the work involved in finishing the blade. Also, the thickness of the film is precisely controlled in advance of its application to the blade ensuring that a surface with a uniform thickness is produced. Film manufacturing techniques allow the composition of the film to be precisely controlled and even to vary across the thickness of the film. The possibility of having variable or poor weathering performance over the lifetime of the blade due to variability in coating production/application processes is therefore almost completely eliminated.

Some advantages would be achieved by the film being applied to a significant proportion of the wind turbine blade with conventional techniques being used to coat the remainder of the blade. However, preferably, substantially all of the blade is covered with the film. The thermoplastic film may be alphatic polyurethane, vinyl, acrylic or fluorinated thermoplastics such as, PVDF; PVDF+HFP copolymer; THV (PVDF, HFP, TFE); PVF; FEP (TFE, HFP); PFA (TFE, PFVE); CTFE; CTFE+VF2/HFP or a combination of these.

The thermoplastic may be a single layer, but it is preferably formed of a two layer structure having an outer layer with enhanced UV, erosion, dirt shedding and weather resistant (latterly referred to throughout as ‘weather resistant’)properties compared to the inner layer. This allows weather resistant material which may be relatively expensive, to be concentrated towards the outer surface of the film where it is most effective. With the two layer structure, the inner layer preferably has enhanced adhesion properties compared to the outer layer. This facilitates the adhesion of the film to the blade.

One way of achieving the enhanced weather resistant properties of the outer layer and enhanced adhesion properties of the inner layer is for the inner and outer layers to be made of polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA) with the outer layer having more PVDF than PMMA and the inner layer having more PMMA than PVDF.

The two layers may be manufactured separately and fused or adhered together. However, preferably, the two layers are co-extruded. This is particularly suitable for a PVDF and PMMA composition as they are very suitable for co-extrusion.

The thermoplastic film preferably contains pigmentation, and/or fillers to give the film the desired colour. As most wind turbine blades are required to be white this pigmentation is normally achieved by the addition of suitable surface coated grade of rutile titanium dioxide. The film preferably also contains amounts of a UV absorber present in levels from 0.1% to 5% based on the film weight. The purpose of the UV absorber is to prevent the passage of damaging UV radiation through the film and into the adhesive layer. The UV absorber may be used singly or may be a combination of two different types of UV absorber to obtain optimal results. Examples of such a combination would be a benzophenone and a hindered amine light stabiliser that can act together in a synergistic manner. Another suitable UV absorber for the thermoplastic film is nano titanium dioxide containing surface modified inorganic oxide particles. This can be extremely effective in such a film and has the additional benefit that it is complete stable in the polymer and cannot suffer from “migration” effects. Such migration effects can be volatilisation during film manufacture causing plate-out effects on the extrusion die, or migration effects in service that can lead to reduced weathering performance or even dissbondment of the film. Such a nano titanium dioxide would be present in the film at between 0.1% and 8% of the film weight (excluding the adhesive).

All of the percentages recited for the composition of the film material are by weight percentages for the film excluding the adhesive layer.

For a wind turbine blade, it is desirable that the blades do not have a high gloss and/or high reflectance as this causes an unacceptable nuisance in the finished product when the blades are in service. Therefore, preferably, this effect is minimised by surface treating the film, for example by applying a cold roller to the film as it is extruded and/or by a matting agent incorporated into the film. A suitable matting agent would be a light stable acrylic resin of controlled particle size.

The thickness of the thermoplastic film (excluding the adhesive) is preferably less than 300 μm, and preferably between 50 and 150 microns thick.

When applying the film to the blade, care must be taken to avoid air bubbles becoming trapped between the film and the blade. The film may therefore be porous such that it is air permeable and water impermeable as this helps prevent the formation of air bubbles during the manufacturing process.

The adhesive is preferably a pressure sensitive adhesive such as a rubber, acrylic, modified acrylic (tackifier modified) or silicone adhesive.

The invention also extends to a method of manufacturing a wind turbine blade comprising moulding the blade body and adhering a thermoplastic film to at least 50% of the surface of the body.

The film is preferably applied to the blade body in a number of strips running between the leading and trailing edges of the blade. The film can also be preferably applied in a manner with the strips oriented such that the complexity of the curvature in which the film is applied can be markedly reduced.

The edge of one strip may overlap with the edge of an adjacent strip. Alternatively, the edges of adjacent strips do not overlap and the join is covered with a further strip of thermoplastic film, painted with acrylic or epoxy adhesive, painted with a PVDF paint or hot welded together

Similar considerations may apply at the leading and trailing edges where the adjacent strips may either overlap or the join may be covered with a further strip of thermoplastic film extending along the edge.

The method preferably also includes the step of heating the thermoplastic film shortly before when/or during its application to the blade body. This is preferably done by blowing hot air onto the film. This increases the flexibility of the film allowing it to be applied more easily to the surface of the blade.

The film can be applied ‘dry’ to the blade surface or ‘wet’ utilising water or other suitable fluid to enable the film to be more easily positioned without creasing or trapping air.

The film may be supplied in a number of sections each being specially shaped to fit on an appropriate section of the blade. Preferably, however, the film is applied from a roll. In this case, the film may be trimmed before its application to the blade body. This is particularly useful, for example, around the root of the blade which has a complex shape.

The thermoplastic film may be applied to a full length moulding of the blade. However, it is also possible that the blade is made up of a plurality of modules as disclosed in our earlier application GB 0717690.2. In this case, the thermoplastic film may either be applied to the individual modules before they are assembled into the finished blade, or the modules may be assembled before the film is applied.

According to a third aspect of the invention, there is provided a two layer thermoplastic film comprising an upper layer and a lower layer, wherein: the upper layer comprising

(a) 50% to 85% of polyvinylidene fluoride (PVDF), wherein up to 30% of the PVDF may be replaced by hexafluoropropylene (HFP);
(b) 10% to 45% polymethyl methacrylate (PMMA);
(c) optionally up to 8% UV stabilisers and/or absorbers;
(d) optionally up to 10% matting agent; and
(e) optionally up to 40% of an inorganic pigment;
the lower layer comprising
(f) a polymer of 10% to 45% of (PVDF), wherein up to 30% of the PVDF may be replaced by hexafluoropropylene (HFP);

(g) 50% to 85% PMMA;

(h) optionally up to 8% UV stabilisers/absorbers;
(i) optionally up to 10% matting agent; and
(j) optionally up to 40% of an inorganic pigment;

wherein the film has an initial gloss of less than 30% when measured with a reflectometer at an angle of 60° with respect to the film surface.

Preferably the upper layer has a thickness between 40 and 240 microns and the lower layer has a thickness between 10 and 60 microns.

Preferably the UV stabilisers are based on ultrafine ‘nano’ titanium dioxide materials containing surface modified inorganic oxide particles.

Preferably the PVDF contains up to 30% HFP.

Preferably the film further comprises adhesive on the lower layer.

It should be understood that any of the preferred features of the method referred to above may be applied in combination with any of the preferred features of the blade referred to above.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is a schematic plan view of an entire blade;

FIG. 2 is a cross-section of a first example of a join between adjacent strips;

FIG. 3 is a cross-section to a second example of a join between adjacent strips;

FIGS. 4A-E show a number of different configurations of strips that could be used;

FIGS. 5A-E show similar configurations to those of FIGS. 4A-E but include an edge protection strip;

FIG. 6 is a cross-section that refers to an example of a film and the underlying blade; and

FIG. 7 is a cross-section through a second example of a film and the underlying blade.

DETAILED DESCRIPTION

A wind turbine blade is shown in FIG. 1. The basic body of the blade may be formed in accordance with conventional techniques in which full length mouldings of each half are made and the two halves are joined together in a clam shell-like construction. Alternatively, the blades may have a modular construction as described in our earlier GB application number 0717690.2.

The present invention is concerned only with the surface coating. As can be seen from FIG. 1, the blade is covered with a number of strips 1 of self-adhesive of thermoplastic material 6 and adhesive 4. Each strip extends from the leading edge 2 to the trailing edge 3. The opposite side of the blade corresponds to this. At these leading and trailing edges 2, 3 the strip on one side may overlap slightly with the strip on the opposite side or a further thin strip may be provided along the edge to cover the join between strips in a similar manner to that described in the reference to FIGS. 2 and 3 below.

As can be seen from FIG. 1, each strip overlaps with an adjacent strip. The join between the two is shown in more detail in FIG. 3. Adhesive 4 is provided on the lower surface of each strip and will adhere to the underlying blade surface 5. At an overlap portion strip 1 adheres to the surface of the adjacent thermoplastic film 6 as shown. An alternative is shown in FIG. 2 in which adjacent strips 1, 1 abut one another and a further strip 7 with adhesive 8 of the same or a similar material runs along the join. The thickness of the material is such that the overlap portion or the further thin strips do not have a significant effect on the performance of the blade. As an alternative, the join shown in FIG. 3 may be painted, for example with a PVDF paint and, indeed, this is the current preference.

The strips 1 are supplied on a roll. The strips may have a backing material covering the adhesive that is peeled off before the strip is applied to the blade surface. However, no backing material is necessary if the top surface of the film 1 is of a material that does not adhere to the adhesive. An appropriate amount is unrolled and, if necessary, trimmed to the correct shape. Hot air is then blown onto the strip to make it flexible and the strip is then applied to the blade surface. The strip is initially adhered at a location close to one of the edges 23 and are progressively adhered across the blade with the operator being careful to ensure that no air is trapped as the film is progressively adhered.

FIGS. 4A-E show various configurations of the strips which may be applied to a blade. The blades may have the same configuration of strips on both sides, or they may be different. The strips may run across the blade (FIG. 4A), along the blade (FIG. 4B) or diagonally (FIG. 4C). Alternatively, the root end of the blade, which has the greatest curvature may be provided with a different configuration of strips from the remainder of the blade. In FIG. 4D the root end of the blade is covered with a number of triangular strips which converge adjacent to the root end. These strips offer the greatest degree of conformity with the blade curvature and this example will be most useful for a relatively un-pliable material. However, in the example of FIG. 4D the strips either need to be supplied pre-cut, or if they are taken from a roll, require a considerable amount of trimming and this example will be more difficult to produce in practice. As a compromise, the example of FIG. 4E provides reasonably good conformity in the curved regions, but the strips can be used from a roll with relatively little trimming.

The examples of FIGS. 5A to 5E are similar to those shown in the corresponding FIG. 4 representations. The only different is that the leading edge is provided with a protective strip 1A. This extends to both sides of the blade and therefore provides good weather proofing at the leading edge where it is most required.

The current preference is for the configuration shown in FIG. 5A as this has a weather proofing strip 1A on the leading edge, and also the transverse arrangement of strips 1 ensures that the seams between adjacent strips lie substantially in the direction of travel of the blade thereby minimising any turbulence.

The nature of the thermoplastic film and adhesive will now be described in greater detail with reference to FIGS. 6 and 7. FIG. 6 is a cross-section through the blade surface and a first film consisting of adhesive 4 layer and a thermoplastic film 6 which has a single layer. FIG. 7 is similar except that the thermoplastic film 6 has separated into upper 9 and lower 10 layers.

In all cases, the thermoplastic film 6 is preferably between 50 and 300 microns thick.

For the single layer of FIG. 4, the thermoplastic film preferably consists of 45.9% of polyvinylidene fluoride, 25.5% PMMA, 1.5% UV stabilisers and absorbers, 1.5% matting agent and 25.6% inorganic pigment.

For the double layer of FIG. 5, the upper layer preferably consists of 52.8% of polyvinylidene fluoride (15% of which is HFP), 22% PMMA, 1.5% UV stabilisers and absorbers, 1.5% matting agent and 22.2% inorganic pigment to give sufficient colour and opacity. The lower layer preferably consists of 22% of polyvinylidene fluoride (15% of which is HFP), 52.8% PMMA, 1.5% UV stabilisers and absorbers, 1.5% matting agent and 22.2% inorganic pigment to give sufficient colour and opacity.

For the double layer of FIG. 5 the upper layer is of a thickness between 5 and 295 microns and the lower layer is of a thickness between 5 and 295 microns. With the upper layer preferably being between 40 and 240 microns and the lower layer preferably being between 10 and 60 microns.

The film may be extruded (in the case of the FIG. 4 example) or co-extruded (in the case of the FIG. 5 example) using an extruder which is well known, for example the type of co-extruder used to manufacture UPVC windows. The extruded material may then be subjected to a second surface treatment such as a cold roller to produce the desired lack of reflectiveness of the upper surface. The film can then also preferably pass through a second process to coatadhesive onto the lower surface before the film is wound on to a roll ready for transportation.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A wind turbine blade at least 50% of the surface of which is covered with a self-adhesive, thermoplastic film.

2. The blade according to claim 1, wherein substantially all of the blade is covered with the film.

3. The blade according to claim 1, wherein the thermoplastic film comprises a two layer structure having an outer layer with enhanced weather resistant properties compared to the inner layer.

4. The blade according to claim 3, wherein the inner layer has enhanced adhesion properties compared to the outer layer.

5. The blade according to claim 3, wherein the inner and outer layers include polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA) with the outer layer having more PVDF than PMMA and the inner layer having more PMMA than PVDF.

6. The blade according to claim 3, wherein the layers are co-extruded.

7. The blade according to claim 1, wherein the thermoplastic film has an initial gloss of less than 30% when measured with a reflectometer at an angle of 60° with respect to the film surface.

8. The blade according to claim 1, wherein the thermoplastic layer is less than 300 μm thick.

9. The blade according to claim 1, wherein the film is porous such that it is air permeable and water impermeable.

10. The blade according to claim 1, wherein the adhesive is an acrylic adhesive.

11. A method of manufacturing a wind turbine blade comprising moulding the blade body and adhering a thermoplastic film to at least 50% of the surface of the body.

12. The method of manufacturing according to claim 11, wherein the film is applied in a number of strips running between the leading and trailing edges of the blade.

13. The method of manufacturing according to claim 11, wherein the film is applied in a number of strips arranged to reduce the complexity of curvature of each strip.

14. The method of manufacturing according to claim 12, wherein the edge of one strip overlaps with the edge of an adjacent strip.

15. The method of manufacturing according to claim 12, wherein edges of adjacent strip do not overlap and the join is covered with a further strip of thermoplastic film.

16. The method of manufacturing according to claim 12, wherein edges of adjacent strip do not overlap and the join is painted with a PVDF paint.

17. The method of manufacturing according to claim 11, wherein the thermoplastic film is heated shortly before and/or during its application to the blade body.

18. The method of manufacturing according to claim 11, wherein the blade body is wetted with a suitable fluid shortly before the application of the thermoplastic film to the blade body.

19. The method of manufacturing according to claim 11, wherein the film is trimmed before its application to the blade body.

20. The method of manufacturing according to claim 11 of making a blade according to claim 1.

21. A two layer thermoplastic film comprising an upper layer and a lower layer, wherein:

the upper layer comprising

(a) 50% to 85% of polyvinylidene fluoride (PVDF), wherein up to 30% of the polyvinylidene fluoride may be replaced by hexafluoropropylene (HFP);

(b) 10% to 45% polymethyl methacrylate (PMMA);

(c) optionally up to 8% UV stabilisers and/or absorbers;

(d) optionally up to 10% matting agent; and

(e) optionally up to 40% of an inorganic pigment.

the lower layer comprising

(f) a polymer of 10% to 45% of (PVDF), wherein up to 30% of the polyvinylidene fluoride may be replaced by hexafluoropropylene (HFP);

(g) 50% to 85% PMMA;

(h) optionally up to 8% UV stabilisers/absorbers;

(i) optionally up to 10% matting agent; and

(j) optionally up to 40% of an inorganic pigment;

wherein the film has an initial gloss of less than 30% when measured with a reflectometer at an angle of 60 degrees.

22. The two layer film according to claim 21 wherein the upper layer has a thickness between 40 and 240 μm and the lower layer has a thickness between 10 and 60 μm.

23. The two layer film according to claim 21, wherein the UV stabilisers are based on ultrafine ‘nano’ titanium dioxide materials containing surface modified inorganic oxide particles.

24. The two layer film according to claim 21, wherein the PVDF contains up to 30% HFP.

25. The two layer film according to claim 21, further comprising adhesive on the lower layer.

26. The two layer film according to claim 21, wherein there is at least 0.5% matting agent in at least one layer.