WO2010083921A2

WO2010083921A2 - A pre-form and a spar comprising a reinforcing structure

Info

Publication number: WO2010083921A2
Application number: PCT/EP2009/067191
Authority: WO
Inventors: Jakob Hjorth Jensen
Original assignee: Vestas Wind Systems A/S
Priority date: 2009-01-23
Filing date: 2009-12-15
Publication date: 2010-07-29
Also published as: CN201376388Y; WO2010083921A3

Abstract

A pre-form (6) comprising a plurality of layers of fibres, e.g. in the form of fibre tows, each layer of fibres being arranged in a layer of resin is disclosed. The layers of fibres and resin define a thickness which is at least 12 mm. The pre-form can be used as a reinforcing structure, e.g. in the form of a slab (6), in a spar (1) or a blade shell for a wind turbine blade. Due to the large thickness, the pre-form can replace several prior art reinforcing structures, which previously had to be positioned individually in the spar or blade shell in order to provide the required strength. Thereby the number of manufacturing steps required for manufacturing the spar is reduced. Furthermore, the capacity of the manufacturing equipment used for producing the pre-form is utilised to a greater extent. Furthermore, a method for manufacturing such a pre-form is disclosed.

Description

A PRE-FORM AND A SPAR COMPRISING A REINFORCING STRUCTURE

FIELD OF THE INVENTION

The present invention relates to semi -finished components with fibres in layers of resin, also known as pre-forms. The present invention further relates to a spar for a wind turbine blade, the spar comprising a reinforcement structure which is formed from such a pre-form, and to a blade shell for a wind turbine blade, the blade shell comprising a reinforcement structure formed from such a pre-form. Finally, the present invention relates to a method for manufacturing such a pre-form.

BACKGROUND OF THE INVENTION

When making large wind turbine blades, a spar is sometimes included in the blade as a load carrying element. Such a spar may be an elongate member arranged along a longitudinal axis of the blade, and defining a substantially rectangular, hollow cross section. The spar may be provided with a number of reinforcing structures, also known as slabs, e.g. in the form of pre-forms. Most often a plurality, such as three adjacently arranged pre-forms are used in order to provide sufficient strength to the spar. When the pre-forms are positioned in a fibre reinforced spar they are positioned individually and fibre glass is wound between the preforms. The manufacture of the spar thereby involves a relatively high number of processing steps. Furthermore, the capacity of the production system used for manufacturing the preforms is not utilised in an optimum manner.

As an alternative to arranging a spar in a wind turbine blade, a reinforcing structure may be arranged directly in a blade shell of the wind turbine blade.

It has previously been the prevalent opinion among persons skilled in the art of wind turbine blade manufacturing that it is not possible to manufacture a slab having a thickness which is larger than approximately 7 mm, because thicker structures will not have a sufficiently high quality. This is primarily because it becomes increasingly difficult to evacuate air from the structure as the thickness, and thereby the number of layers, increases. Accordingly, it has been believed that sufficient strength of the spar can only be obtained by individually positioning a number of slabs in the spar or the blade shell.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a single pre-form which is capable of providing sufficient strength to a spar for a blade for a wind turbine. It is a further object of embodiments of the invention to provide a spar for a wind turbine blade, where the spar can be manufactured with a reduced number of manufacturing steps as compared to prior art spars, without compromising the strength of the spar.

It is an even further object of embodiments of the invention to provide a method for manufacturing a pre-form, the pre-form being capable of providing sufficient strength to a spar for a blade for a wind turbine.

According to a first aspect the invention provides a pre-form comprising a plurality of layers of fibres, each layer of fibres being arranged in a layer of resin, wherein the layers of fibres and resin define a thickness which is at least 12 mm.

In the present context the term 'pre-form' should be interpreted to mean a structure of fibres arranged in layers of resin, where the structure has been pre-consolidated to form a single unit which can be moved and stored. It is still possible to shape the pre-form into a desired shape to fulfil a specific purpose, and the pre-form needs to be cured when the final shape has been obtained.

The fibres may, e.g., be in the form of individual fibres, fibre tows, tow-pregs or prepregs. The fibres may be oriented within each layer, or they may be arranged in the layers in random directions.

The layers of fibres and resin define a thickness which is at least 12 mm. The thickness is preferably measured substantially perpendicularly to the layers of fibres, e.g. being the smallest distance between the two outermost layers of fibres.

As mentioned above, it has previously been the prevalent opinion among persons skilled in the art of wind turbine blade manufacturing that it is not possible to manufacture pre-forms for reinforcing structures, such as slabs, with a thickness which is larger than approximately 7 mm. This is primarily due to the fact that it is difficult to control evacuation of the pre-form, partly because the risk that resin is sucked out of the pre-form along with the evacuated gas increases as the number of layers of the pre-form increases. This results in a pre-form of a quality which is too poor to provide sufficient strength to a spar or a blade shell for a wind turbine blade.

Contrary to this, the inventor of the present invention has surprisingly found that it is possible to manufacture such structures with a significantly larger thickness, such as at least 12 mm, such as at least 15 mm, such as at least 18 mm, such as at least 20 mm, such as at least 25 mm, or even thicker, by selecting the manufacturing process and various processing parameters in a careful manner. This is described in further detail below. Thus, it is possible to manufacture a pre-form which is sufficiently thick, and of a sufficient quality, to provide the required strength to a spar for a wind turbine blade. Accordingly, it is only necessary to position a single reinforcing structure in the spar in order to obtain the required strength. This is a great advantage because it is no longer necessary to individually position several reinforcing structures in the spar with intermediate windings of fibre glass there between. Thereby the number of processing steps of the manufacturing process for the spar is considerably reduced, and the process is much easier to perform.

Furthermore, the capacity of the manufacturing equipment used for manufacturing the pre- forms can be utilised to a greater extent, because it is possible to produce a larger amount of reinforcing structure before the structure needs to be pre-consolidated and removed from the manufacturing equipment. For instance, if one pre-form is made sufficiently thick to replace three prior art pre-forms, two pre-consolidating steps and two removing steps can be saved.

At least some of the fibres may be carbon fibres. Alternatively or additionally, other types of fibres may be used, e.g. glass fibres, synthetic fibres, bio fibres, mineral fibres and metal fibres, depending on the final use of the pre-form.

The resin of at least one of the layers of resin may be a thermosetting resin, i.e. a resin which cures in response to an increase in temperature above a given resin specific point. The resin may be an organic polymeric liquid which, when converted into its final state for use, consolidates and becomes at least partly or completely solid. As an example, the resin may be an epoxy-based resin or a polyester-based resin, though other resin types may also be applied. Furthermore, one or more different resin types may also be applied for preparation of a pre-form. If using different types of resin, it may however be an advantage to use compatible resins.

The pre-form may comprise at least 20 layers of fibres, such as at least 25 layers of fibres, such as at least 28 layers of fibres, such as at least 30 layers of fibres, such as at least 32 layers of fibres, such as at least 35 layers of fibres, such as at least 38 layers of fibres, such as at least 40 layers of fibres, or even 50, 60, 70, 80, 90 or 100 layers of fibres. The number of layers should be sufficiently high to provide a desired thickness of the pre-form. In some pre-forms, a thickness of 1 mm is obtained by 2 layers of fibres and two layers of resin. Thus, in this case a pre-form having a thickness of 12 mm has 24 layers of fibres and 24 layers of resin.

The fibres may be provided in the form of fibre tows. In the present context the term 'fibre tow' should be interpreted to mean a bundle of a large number of individual fibres. The fibres may be arranged substantially in parallel within each layer of fibres. According to this embodiment, the fibres define a longitudinal direction of the pre-form along the parallel fibres. The pre-form will be very strong and substantially incompressible along this direction. Furthermore, such a unidirectional fibre layer provides the pre-form with maximum strength and stiffness per fibre and per weight unit of the pre-form.

According to a second aspect the invention provides a spar for a blade for a wind turbine, the spar comprising two reinforcing structures, each of said reinforcing structures being formed from a pre-form according to the first aspect of the invention.

As described above, the pre-form according to the first aspect of the invention is sufficiently thick to replace mulitple prior art reinforcing structures or slabs, e.g. two, three, four or even more slabs, while providing the same reinforcement to the spar. Thereby it is no longer required to individually position a number of reinforcing structures in the spar with intermediate windings of fibre glass there between, and the spar can therefore be manufactured in fewer processing steps than similar prior art spars.

The spar, except for the reinforcing structures, may be made from a fibre glass material. The fibre glass material may be wound around a shape defining element in order to form the spar with a desired shape.

The spar may define a substantially rectangular, hollow cross section, and the reinforcing structures may be arranged in two opposing wall parts defining the rectangular, hollow cross section. The two opposing wall parts may be the wall parts which face the pressure side and the suction side, respectively, of a blade having the spar arranged therein.

According to one embodiment, each of the reinforcing structures may be arranged between an inner layer of spar material, said inner layer defining an interface towards a cavity of the hollow cross section, and an outer layer of spar material defining an outer surface of the spar.

According to this embodiment the spar may advantageously be produced in the following manner. Initially a layer of spar material is wound around a shape defining element, defining the desired substantially rectangular, hollow cross sectional shape. The two reinforcing structures are then arranged along two opposing sides of the rectangular shape, and another layer of spar material is wound around the shape defining element, the first layer of spar material and the two reinforcing structures. Thereby the reinforcing structures are fixated and positioned inside the spar. The spar material may be a fibre glass material.

According to a third aspect the invention provides a blade shell for a blade for a wind turbine, the blade shell comprising at least one reinforcing structure, said reinforcing structure(s) being formed from a pre-form according to the first aspect of the invention. According to the third aspect of the invention, a reinforcing structure is arranged directly in a blade shell. This may be an alternative to providing the wind turbine blade with a spar.

According to a fourth aspect the invention provides a wind turbine blade comprising a spar according to the second aspect of the invention and/or a blade shell according to the third aspect of the invention.

According to a fifth aspect the invention provides a method of manufacturing a pre-form, the method comprising the steps of:

alternatingly providing layers of fibres and layers of resin, until a stack of layers of fibres and resin defines a thickness which is at least 15 mm,

- evacuating the stack of layers of fibres and resin, and

- pre-consolidating the evacuated stack of fibres and resin.

Performing the method steps of the fifth aspect of the invention preferably results in a preform according to the first aspect of the invention being manufactured, in which case the remarks set forth above are equally applicable here.

According to the fifth aspect of the invention a pre-form is manufactured in the following manner. Initially alternating layers of fibres and resin are provided. This step may, e.g., be performed by providing a first layer of fibres, applying a first layer of resin onto the first layer of fibres, arranging a second layer of fibres on top of the first layer of resin, applying a second layer of resin onto the second layer of fibres, arranging a third layer of fibres on top of the second layer of resin, etc., thereby forming a stack of alternating layers of fibres and resin. Layers of fibres and resin are repeatedly added to the stack until the stack defines a thickness which is at least 15 mm. As described above, the thickness is preferably measured substantially perpendicularly to the layers of the fibres, e.g. being the smallest distance between the two outermost layers of fibres. Performing the evacuating and pre-consolidating steps results in a decrease of the thickness of the stack. It should be noted that the thickness of the stack prior to performing the evacuating and pre-consolidating steps should be sufficient to ensure that the thickness of the stack after having performed the evacuating and pre-consolidating steps is at least 12 mm. Thereby the advantages described above with reference to the first aspect of the invention are obtained.

When a stack having a desired thickness has been provided, the stack is evacuated. During this step, gas inside the stack is removed. The gas may, e.g., be entrapped atmospheric air and/or gaseous products, by-products and starting materials related to the preparation process.

When the evacuation step has been completed, the evacuated stack of fibres and resin is pre-consolidated. This may, e.g., include heating the stack to a temperature which is sufficient to decrease the viscosity of the resin, but not sufficient to cause the resin to cure. During the pre-consolidation step the porosity of the stack is reduced, and the resin, and possibly the fibres, may be redistributed. The pre-consolidation may involve a limited curing of the resin. After the pre-consolidation step has been performed, the pre-form constitutes a single unit which can be moved and stored, but which can still be shaped into a desired shape to fulfil a specific purpose.

As described above with reference to the first aspect of the invention, the inventor of the present invention has surprisingly found that it is possible to manufacture pre-forms with a thickness of at least 12 mm, and with a quality which is sufficient to allow the pre-form to be used as a reinforcing structure, e.g. in a spar for a wind turbine blade. This is obtained by carefully selecting the manufacturing process and various process parameters, notably during the evacuation and pre-consolidation steps. This will be described in further details below.

The step of alternatingly providing layers of fibres and layers of resin may comprise arranging the fibres of the layers of fibres substantially unidirectionally. According to this embodiment, the resulting pre-form will be very strong and substantially incompressible along a longitudinal direction defined by the unidirectional fibres. Furthermore, a maximum strength and stiffness per fibre and per unit weight is obtained.

As described above, the fibres may be provided in the form of individual fibres, fibre tows, tow-pregs, prepregs, etc.

The step of alternatingly providing layers of fibres and layers of resin may comprise applying resin in a substantially liquid or semisolid form onto the layers of fibres. In the present context the term 'semisolid' should be interpreted to mean a material which is capable of supporting its own weight (similar to solid materials), and which has the ability to flow under pressure (similar to liquid materials). According to this embodiment, it is easy to apply the resin in a desired pattern and at desired positions. For instance, the resin may be sprayed onto the fibre layers, or it may be applied by means of an extrusion technique. In order to obtain a substantially liquid or semisolid resin, the resin may be heated before it is applied to the fibre layers. This may, e.g., be necessary in the case that the resin is of a kind which is solid or has a relatively high viscosity at room temperature.

The step of evacuating the stack of layers of fibres and resin may comprise positioning the stack of layers of fibres and resin under vacuum. According to this embodiment gas is sucked out of the stack due to the vacuum. The stack may advantageously be arranged in an enclosure, and the pressure in the enclosure may subsequently be lowered, thereby creating the vacuum.

The step of evacuating the stack of layers of fibres and resin should be performed for a time period which is sufficient to ensure that sufficient air or gas is allowed to be evacuated from the stack. In previous methods the evacuating step was performed for approximately 10 minutes. However, the inventor of the present invention surprisingly found that performing the evacuation step during a significantly longer time period has a beneficial effect on the quality of the resulting pre-form. Thereby a pre-form is provided having a strength which is sufficient to allow a reinforcing element to be manufactured from the pre-form, and the reinforcing element is capable of, in itself, providing sufficient reinforcement to a slab for a blade for a wind turbine.

According to one embodiment, a predetermined time may be allowed to lapse between performing the step of alternatingly providing layers of fibres and layers of resin and performing the step of evacuating the stack of layers of fibres and resin. As described above, the resin is sometimes heated prior to applying the layers of resin, in order to obtain a sufficiently low viscosity of the resin to allow it to be easily applied. If the step of evacuating the stack of layers of fibres and resin is, in this case, performed immediately after the stack has been formed, then the viscosity of the resin will probably still be so low that there is a substantial risk that resin is drawn out of the stack along with the gas being evacuated. However, if a predetermined time is allowed to lapse between forming the stack of layers of fibres and resin and the evacuation step, then the resin is allowed to cool, and the viscosity of the resin is thereby increased. Accordingly, the risk of resin being drawn out of the stack during evacuation is reduced. The predetermined time is preferably sufficiently long to allow the temperature of the resin to reach a level where the viscosity of the resin is sufficiently high to substantially prevent resin from being drawn out of the stack during evacuation. As the number of layers of fibres and resin, and thereby the thickness of the stack, increases, the time required for the resin to reach a desired temperature level must be expected to increase. This is primarily due to the fact that the heat capacity of the stack increases. It is therefore increasingly important to allow a predetermined time to lapse between forming the stack and evacuating the stack when a thick pre-form is manufactured, as it is the case in the method according to the present invention.

The step of evacuating the stack of layers of fibres and resin may be performed while the layers of resin have a temperature which ensures that the resin has a viscosity which substantially prevents resin from being drawn out of the stack of layers of fibres and resin during the evacuation step. The advantages described above are thereby obtained. According to this embodiment, the temperature of the resin may be monitored, and the evacuation step may be performed when the temperature of the resin reaches a desired level, instead of simply allowing a fixed time interval to lapse between the two steps.

Thus, the method may advantageously comprise the following steps. The resin is initially heated to a temperature at which the viscosity of the resin is sufficiently low to allow the resin to wet the fibres, and the stack of layers of fibres and resin is formed while applying the heated resin. The stack is then allowed to cool to a temperature at which the viscosity of the resin is sufficiently high to substantially prevent resin from being drawn out of the stack during evacuation. Finally, the stack is evacuated until a sufficient amount of air or gas has been evacuated from the stack to provide a preform of a desired quality. The temperatures and the evacuating time depend on a number of factors, primarily on the kind of resin used for the stack. However, once the resin has been chosen, the skilled person will be able to select appropriate temperatures and evacuation times.

The method may further comprise the step of arranging the pre-form in a spar for a blade for a wind turbine. This may, e.g., be done as described previously.

As an alternative, the method may comprise the step of arranging the pre-form in a blade shell for a blade for a wind turbine.

It should be noted that a skilled person would readily recognise that any feature described in combination with the first, second, third, fourth or fifth aspect of the invention could also be combined with any of the other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawing, in which

Fig. 1 is a cross sectional view of a prior art spar for a wind turbine blade, Fig. 2 is a cross sectional view of a spar for a wind turbine blade according to an embodiment of the invention, and

Fig. 3 is a perspective view of the spar of Fig. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross sectional view of a prior art spar 1 for a wind turbine blade. The spar 1 defines a substantially rectangular cross section with a hollow part 2. The spar 1 comprises an inner layer 3 of fibre glass material and an outer layer 4 of fibre glass material. Along a first wall part 5 of the rectangular cross section, three individual slabs 6 are arranged between the inner layer 3 of fibre glass material and the outer layer 4 of fibre glass material. Between the individual slabs 6, intermediate layers 7 of fibre glass material are arranged.

Similarly, along a second, oppositely arranged, wall part 8, three individual slabs 6 and intermediate layers 7 of fibre glass are arranged.

The spar 1 shown in Fig. 1 may be manufactured in the following manner. Initially the inner layer 3 of fibre glass material is wound around a shape defining element. A slab 6 is arranged along the first wall part 5, and a slab 6 is arranged along the second wall part 8, and an intermediate layer 7 of fibre glass material is wound around the slabs 6 and the inner layer 3 of fibre glass material. Subsequently two further slabs 6 are arranged along the first wall part 5 and the second wall part 8, respectively, on top of the intermediate layer 7 of fibre glass material. Another intermediate layer 7 of fibre glass material is then wound around the further slabs 6 and the first intermediate layer 7 of fibre glass material. Finally, the last two slabs 6 are arranged along the first wall part 5 and the second wall part 8, respectively, on top of the second intermediate layer 7 of fibre glass material, and the outer layer 4 of fibre glass material is wound around the slabs 6 and the second intermediate layer 7 of fibre glass material.

It is clear from the description above, that manufacturing of this prior art spar 1 having the prior art slabs 6 arranged therein requires a significant number of processing steps.

Fig. 2 is a cross sectional view of a spar 1 according to an embodiment of the invention, and having two slabs 6 according to an embodiment of the invention positioned therein.

The spar 1 defines a substantially rectangular cross section with a hollow part 2. The spar 1 comprises an inner layer 3 of fibre glass material and an outer layer 4 of fibre glass material 4. Along a first wall part 5 of the rectangular cross section, a single slab 6 is arranged between the inner layer 3 of fibre glass material and the outer layer 4 of fibre glass material. Similarly, a single slab 6 is arranged along a second, oppositely arranged, wall part 8 of the rectangular cross section, between the inner layer 3 of fibre glass material and the outer layer 4 of fibre glass material.

When comparing the spar 1 of Fig. 1 and the spar 1 of Fig. 2, it is clear that the slabs 6 according to the invention, and which are shown in Fig. 2, are considerably thicker than the prior art slabs 6 shown in Fig. 1.

The spar 1 of Fig. 2 may be manufactured in the following manner. Initially the inner layer 3 of fibre glass material is wound around a shape defining element. A slab 6 is arranged along the first wall part 5, and a slab 6 is arranged along the second wall part 8. Subsequently, the outer layer 4 of fibre glass material is wound around the slabs 6 and the inner layer 3 of fibre glass material. It is clear that manufacturing the spar 1 of Fig. 2 requires significantly fewer steps than manufacturing of the prior art spar 1 of Fig. 1. However, the slabs 6 positioned in the spar 1 of Fig. 2 provide approximately the same reinforcement to the spar 1 as the slabs 6 positioned in the spar 1 of Fig. 1. Hence the manufacturing process of the spar 1 is significantly facilitated without compromising the strength of the resulting spar 1.

Fig. 3 is a perspective view of the spar 1 of Fig. 2. It is clear from Fig. 3 that the spar 1 extends in a direction being substantially perpendicular to the cross section of Fig. 2, the direction being indicated by arrow 9.

Claims

1. A pre-form comprising a plurality of layers of fibres, each layer of fibres being arranged in a layer of resin, wherein the layers of fibres and resin define a thickness which is at least 12 mm.

2. A pre-form according to claim 1, wherein at least some of the fibres are carbon fibres.

3. A pre-form according to claim 1 or 2, wherein the resin of at least one of the layers of resin is a thermosetting resin.

4. A pre-form according to any of the preceding claims, the pre-form comprising at least 20 layers of fibres.

5. A pre-form according to any of the preceding claims, wherein the fibres are provided in the form of fibre tows.

6. A pre-form according to any of the preceding claims, wherein the fibres are arranged substantially in parallel within each layer of fibres.

7. A spar for a blade for a wind turbine, the spar comprising two reinforcing structures, each of said reinforcing structures being formed from a pre-form according to any of the preceding claims.

8. A spar according to claim 7, the spar defining a substantially rectangular, hollow cross section, wherein the reinforcing structures are arranged in two opposing wall parts defining the rectangular, hollow cross section.

9. A spar according to claim 8, wherein each of the reinforcing structures is arranged between an inner layer of spar material, said inner layer defining an interface towards a cavity of the hollow cross section, and an outer layer of spar material defining an outer surface of the spar.

10. A spar according to claim 9, wherein the spar material is a fibre glass material.

11. A blade shell for a blade for a wind turbine, the blade shell comprising at least one reinforcing structure, said reinforcing structure(s) being formed from a pre-form according to any of claims 1-6.

12. A wind turbine blade comprising a spar according to any of claims 7-10 and/or a blade shell according to claim 11.

13. A method of manufacturing a pre-form, the method comprising the steps of:

- alternatingly providing layers of fibres and layers of resin, until a stack of layers of fibres and resin defines a thickness which is at least 15 mm,

evacuating the stack of layers of fibres and resin, and

- pre-consolidating the evacuated stack of fibres and resin.

14. A method according to claim 13, wherein the step of alternatingly providing layers of fibres and layers of resin comprises arranging the fibres of the layers of fibres substantially unidirectionally.

15. A method according to claim 13 or 14, wherein the step of alternatingly providing layers of fibres and layers of resin comprises applying resin in a substantially liquid or semisolid form onto the layers of fibres.

16. A method according to any of claims 13-15, wherein the step of evacuating the stack of layers of fibres and resin comprises positioning the stack of layers of fibres and resin under vacuum.

17. A method according to any of claims 13-16, wherein a predetermined time is allowed to lapse between performing the step of alternatingly providing layers of fibres and layers of resin and performing the step of evacuating the stack of layers of fibres and resin.

18. A method according to any of claims 13-17, further comprising the step of arranging the pre-form in a spar for a blade for a wind turbine.