A wind turbine blade with ballast unit and method for its manufacture

Abstract

The present disclosure relates to a wind turbine blade (7) comprising: a root end and a tip end; a leading edge (17) and a trailing edge (18); and a shell structure, the shell structure comprising a leeward shell (14) and a windward shell (15), the shell structure comprising an inner skin (29a, 29b) and an outer skin (30a, 30b), wherein the outer skin defines an outer surface of the wind turbine blade and the inner skin faces an interior volume of the blade. The wind turbine blade comprises a ballast unit (32) defining an internal ballast volume (33) for receiving a ballast material, wherein the ballast unit is integrated in the shell structure and sandwiched between the inner skin and the outer skin. The disclosure further relates to a method of manufacturing a wind turbine blade, and a rotor comprising three wind turbine blades.

Description

[0001] A WIND TURBINE BLADE WITH BALLAST UNIT AND METHOD FOR ITS MANUFACTURE

[0002] Technical field

[0003] The present invention relates to manufacturing of a wind turbine blade, particularly a wind turbine blade for a horizontal-axis wind turbine. The invention further relates to a wind turbine blade and a rotor for a wind turbine blade.

[0004] Background of the invention

[0005] Modern manufacturing processes of wind turbine blades do typically not yield perfectly identical blades with regard to mass and mass distribution. In case a wind turbine rotor is assembled of blades having different masses and / or different mass distributions, the natural centre of rotation of the rotor will be displaced from the actual rotational axis during operation of the wind turbine. As a result, the wind turbine rotor may suffer an unbalance.

[0006] To account for this, wind turbine blades may be equipped with ballast tanks. Subsequent to manufacturing the blades, the masses or mass distributions of the blades are determined, and the ballast tanks of the individual blades can be filled to different levels to thereby provide a rotor having a natural centre of rotation as close as possible to the actual rotational axis.

[0007] However, adding a ballast tank to a blade may entail various problems. Attaching a ballast tank within the interior volume of a blade requires a separate manufacturing step during production of the blade. The ballast tank may detach during operation of the blade. The ballast tank can be difficult to access, and thereby there is a risk of the ballast mass being incorrectly positioned. Further, a hole drilled through the entire thickness of the shell of the blade for access to the ballast tank may introduce a structural weakness in the blade.

[0008] Accordingly, there is a need for improved ballast tanks which address the above-outlined problems associated with conventional ballast tanks.

[0009] Summary of the invention

[0010] On the above background, it is an object of preferred embodiments of the invention to manufacture a wind turbine blade having a ballast unit with a decreased risk of detachment. It is further an object of preferred embodiments of the invention to manufacture a wind turbine blade in which integration of the ballast unit is simplified.

[0011] A first aspect of the present disclosure relates to a method of manufacturing a wind turbine blade, the method comprising the steps of: providing a shell mould; arranging a first skin layup onto the shell mould; positioning a ballast unit onto the first skin layup, the ballast unit defining an internal ballast volume for receiving a ballast material; arranging a second skin layup onto the ballast unit, wherein the first skin layup, the ballast unit and the second skin layup provides a shell layup; providing resin to the shell layup to impregnate the first skin layup and the second skin layup, wherein the ballast volume is sealed from the resin during the step of providing the resin to preserve the internal ballast volume for receiving the ballast material; curing the resin to provide a shell structure for the wind turbine blade, the shell structure comprising a leeward shell and a windward shell, the shell structure comprising an inner skin, an outer skin, and the ballast unit, wherein the ballast unit is sandwiched between the inner skin and the outer skin, wherein the outer skin is formed by the first skin layup and defines an outer surface of the blade, wherein the inner skin is formed by the second skin layup and faces an interior volume of the blade; and forming the wind turbine blade from the shell structure such that the wind turbine blade extends in a spanwise direction between a root end and a tip end of the blade, in a chordwise direction between a leading edge and a trailing edge of the blade, and in a thickness direction between the leeward shell and the windward shell.

[0012] The shells may be formed in the shell moulds using a vacuum assisted resin infusion (VARTM) method as is well known in the art. Conventional wind turbine blades comprise a core material in the shell sandwiched between an inner skin and an outer skin. The present disclosure introduces the concept of substituting some of this core material with one or more ballast units. In the resulting blade, such ballast units may be sandwiched between the inner skin and the outer skin, in a manner similar to the way in which the remaining core material is sandwiched between the skins.

[0013] The provision of ballast units sandwiched between the inner skin and outer skin can ensure a decreased risk of detachment of the ballast tank as compared to a ballast tank attached to a structure within the interior of the blade, such as a ballast tank attached to the inner skin and facing the blade interior.

[0014] Further, by positioning a ballast unit onto the first skin layup during manufacturing, it is possible to simplify integration of ballast units into the wind turbine blade. Attachment of a ballast tank as a separate manufacturing step is not required. Instead, the ballast unit may be added in parallel with providing and positioning core material onto the first skin layup. This will also improve cost-efficiency of manufacturing the blade since a separate ballast tank is not required and the amount of core material required may be reduced slightly.

[0015] Moreover, the mass of the blade can be lower than in a conventional blade with a separately attached ballast tank. For example, in a rotor comprising three blades, at least one blade will typically not have any ballast material in the ballast volume. At least this blade will have a lower mass than a corresponding conventional blade with a separately attached ballast tank.

[0016] Since the resulting ballast unit will be integrated in the shell instead of being positioned within the interior volume of the blade, access to the ballast unit is improved. The ballast unit may be filled with ballast material by penetrating the outer skin only (and, typically, a fluid- impermeable outer cover layer of the ballast unit as well) and then introducing the ballast material into the ballast unit. In contrast, filling a conventional ballast tank with ballast material requires drilling through the outer skin, the core material, the inner skin, and a wall of the ballast tank. Access to a ballast unit according to the present disclosure thereby yields a reduced structural weakness in comparison with the structural weakness introduced by penetrating the shell to access a conventional ballast tank.

[0017] Furthermore, for conventional ballast tanks, it can be difficult to verify whether a hole drilled through the shell provides proper access to the ballast tank. In contrast, for typical examples of the present disclosure, access to the ballast unit can immediately be verified simply by evaluating whether core material is present or not. A typical blade according to the present disclose comprises just a few ballast units, for example one ballast unit, two ballast units, or three ballast units. Accordingly, the core material will typically still occupy a majority of the shell between the inner skin and the outer skin.

[0018] In examples of the present disclosure, the first skin layup and / or the second skin layup may be formed from glass fibre.

[0019] In examples of the present disclosure, the method further comprises positioning core material onto the first skin layup. In such examples, the step of arranging a second skin layup onto the ballast unit may further comprise arranging the second skin layup onto the core material. Moreover, in such examples, the shell structure provided by curing the resin may comprise the core material, wherein the ballast unit and the core material are sandwiched between the inner skin and the outer skin.

[0020] In general, a ballast unit is not necessarily in direct contact with the first skin / first skin layup and / or the second skin / second skin layup. For example, core material may be located between the first skin / first skin layup and the ballast unit, and / or core material may be located between the second skin / second skin layup and the ballast unit. Hence, the ballast unit may also be arranged on top of and / or below core material, relative to the thickness direction.

[0021] Moreover, examples according to the present disclosure are not limited to two skins / two skin layups. The ballast unit and at least some of the core material may, for example, be separated by an auxiliary skin layup layer / auxiliary skin layer.

[0022] The ballast material may, for example, comprise an adhesive, for example a PUR adhesive.

[0023] Typically, the leeward shell is the portion of the shell of the wind turbine blade located at a leeward side of the wind turbine blade. Similarly, the windward shell is typically the portion of the shell of the wind turbine blade located at the windward side of the wind turbine blade. The leeward shell and the windward shell may be interconnected at the trailing edge and / or at the leading edge.

[0024] The leeward shell portion and the windward shell portion typically define an airfoil profile of the wind turbine blade configured to generate an aerodynamic lift caused by a pressure difference between the windward side and the leeward side of the blade, as defined by the respective shell portions. Wind turbine blades according to the present disclosure may further comprise other elements, for example reinforcement structures, such as spar caps, engaging the leeward shell and the windward shell. Such reinforcement structures may be interconnected by one or more webs.

[0025] In examples of the present disclosure, the method comprises a step of adding the ballast material to the ballast unit to at least partially fill the ballast volume by the ballast material, wherein the step of adding the ballast material is performed after the step of forming the wind turbine blade.

[0026] Thus, the ballast material may be provided separately from, e.g., providing resin to the shell layup impregnating the first and second skin layups.

[0027] The ballast material may be added based on measurements of one or more blades, for example measurements indicative of mass or mass distribution of one or more blades.

[0028] By partially filling the ballast volume, the mass and mass distribution of the wind turbine blade can be adjusted, for example relative to other wind turbine blades to be included in the same rotor.

[0029] In examples of the present disclosure, the method comprises a step of piercing the outer skin to provide a filling hole which fluidly connects the ballast volume with exterior surroundings allowing ballast material to be added to the ballast unit to at least partly fill the ballast volume, wherein the step of piercing the outer skin is performed prior to the step of adding the ballast material to the ballast unit.

[0030] In examples of the present disclosure, the inner skin remains sealed during the step of adding the ballast material.

[0031] In contrast to how ballast material is typically added to conventional ballast tanks, ballast material may be added by only piercing the outer skin. This simplifies the entire process of actually adding the ballast material.

[0032] In examples according to the present disclosure, the ballast unit is provided with a filling hole as the blade is moulded and / or prior to moulding the blade. Regardless of how a filling hole is provided, it may be closed after ballast material has been added to the ballast unit.

[0033] A second aspect of the present disclosure relates to a wind turbine blade comprising: a root end and a tip end, the blade extending in a spanwise direction between the root end and the tip end; a leading edge and a trailing edge, the blade extending in a chordwise direction between the leading edge and the trailing edge; and a shell structure, the shell structure comprising a leeward shell and a windward shell, the blade extending in a thickness direction between the leeward shell and the windward shell, the shell structure comprising an inner skin and an outer skin, wherein the outer skin defines an outer surface of the wind turbine blade and the inner skin faces an interior volume of the blade, wherein the wind turbine blade comprises a ballast unit defining an internal ballast volume for receiving a ballast material, wherein the ballast unit is integrated in the shell structure and sandwiched between the inner skin and the outer skin.

[0034] A wind turbine blade according to the second aspect may generally provide any of the same or similar technical effects and advantages as provided by the method of manufacturing a wind turbine blade according to the first aspect of the present disclosure.

[0035] In examples of the present disclosure, the ballast volume is at least partially filled by the ballast material.

[0036] In examples of the present disclosure, ballast volume is at least partially filled by a gas, such as air.

[0037] A ballast volume partially filled by a ballast material and / or a gas may adjust the mass and mass distribution of the wind turbine blade, for example relative to other wind turbine blades included in the same rotor.

[0038] In examples of the present disclosure, the wind turbine blade has an outer section closer to the tip end than to the root end, the outer section defined as the outermost 30 % of the blade relative to the spanwise direction, wherein the ballast unit is arranged within the outer section.

[0039] A ballast unit may typically provide ballast more efficiently if it is positioned in an outer portion of the wind turbine blade. In examples of the present disclosure, the ballast unit comprises a rigid internal framework.

[0040] In examples of the present disclosure, the rigid internal framework comprises a plurality of straight force-carrying members connected by nodes.

[0041] In examples of the present disclosure, the rigid internal framework comprises a cellular structure or a rib structure, such as a perforated cellular structure or a perforated rib structure, for example a perforated honeycomb structure.

[0042] A rigid internal support may improve the capability of the ballast unit to resist a collapse during the moulding process of the wind turbine blade. In particular, the shell layup is subjected to a vacuum in a VARTM process, and the rigid internal support prevents the collapse of the ballast unit under the vacuum pressure.

[0043] Examples of a rigid internal framework is a plurality of straight force-carrying members connected by nodes, and a cellular structure or a rib structure, such as a perforated cellular structure or a perforated rib structure, for example a perforated honeycomb structure.

[0044] An assembly of straight force-carrying members connected by nodes may also be referred to as a truss. Such a structure may advantageously serve to resist collapse while the shell layup is under vacuum.

[0045] Providing a perforated cellular structure or a perforated rib structure may at the same time ensure that ballast material can be properly distributed within the ballast volume.

[0046] In examples of the present disclosure, the ballast unit comprises a fluid-impermeable outer cover layer for sealing the ballast volume during moulding of the wind turbine blade.

[0047] A fluid-impermeable outer cover layer may ensure that the ballast unit is sealed sufficiently while providing resin to manufacture the wind turbine blade. Accordingly, the ballast volume will be fully useable for ballast material if required.

[0048] Further a fluid-impermeable outer cover layer may improve the capability of the ballast unit to resist a collapse during vacuum step of manufacturing the wind turbine blade. In particular, gas, such as air, enclosed in the ballast volume may ensure that the pressure of this gas within the ballast unit aids in resisting the collapse. A fluid impermeable outer cover layer may, for example, be implemented as a container with rigid outer walls, or a flexible membrane, for example a flexible membrane wrapped around a rigid internal framework.

[0049] The ballast unit may be produced with an additive manufacturing process. For example, the ballast unit may be 3D printed. Additive manufacturing allows for complex internal geometries of the ballast unit.

[0050] In examples of the present disclosure, the ballast unit is a first ballast unit, wherein the wind turbine blade comprises a second ballast unit defining a ballast volume for receiving a ballast material, wherein the second ballast unit is integrated in the shell structure and arranged between the inner skin and the outer skin.

[0051] A further advantage of ballast units according to the present disclosure is that it is easily possible to integrate several of these ballast units throughout the blade, which may, in particular, be relevant for larger blades where more ballast may be required, although it is can also be relevant for smaller blades.

[0052] Some conventional ballast tanks are adapted to be integrated in a particular position in a particular type of blade. In contrast, ballast units according to the present disclosure may be integrated anywhere in the blade, easily permitting several ballast units in the same blade. Typically, it is however most straightforward to install a ballast unit in a relatively flat section of a shell, although examples of the disclosure are not limited to a particular position.

[0053] In examples of the present disclosure, the first ballast unit and the second ballast unit are positioned at different nodes of one or more vibrational eigenmodes of the wind turbine blade.

[0054] Integrating several ballast units at different nodes of one or more vibrational eigenmodes may ensure that these ballast units minimally affect the vibrational properties of the blade. Since the amount of ballast material to be added typically varies amongst different blades, it is preferable to add the ballast material in a manner in which it does not affect the vibrational properties. When having several ballast units, this can be achieved by positioning the ballast units at different nodes of one or more vibrational eigenmodes. In examples of the present disclosure, the wind turbine blade comprises an outside marking visible on the outer skin, the outside marking indicative of the placement of the ballast unit, for example relative to the spanwise direction and / or the chordwise direction.

[0055] The provision of such an outside marking may ensure that it is straightforward to pierce the skin and add ballast material in the correct location, i.e. , at the ballast unit.

[0056] An outside marking may, e.g., be a sticker, paint, an indentation, a protrusion, a transparent shell portion, or an imprint provided on the outer skin.

[0057] Since the ballast unit is integrated in the blade shell, it may, in some examples, be visible from the exterior of the blade during manufacture of the blade prior to painting the blade. This permits easily providing the outside marking, for example in relation to painting the blade.

[0058] In some examples, the ballast unit is provided with an outer shape which promotes an indentation or a protrusion on the outer surface of the blade.

[0059] Accordingly, a ballast unit according to the present disclosure is particularly well suited to facilitate the provision of an outside marking.

[0060] In examples of the present disclosure, the wind turbine blade comprises a drain hole fluidly connecting the ballast volume with exterior surroundings, the drain hole piercing the inner skin or the outer skin.

[0061] A drain hole may ensure drainage of fluid such as water, ventilation of the ballast volume, pressure equalization between ballast volume and exterior surroundings, or any combination thereof.

[0062] A drain hole may be built into the ballast unit prior to integration in the wind turbine blade, or it may be added subsequent to manufacturing the blade.

[0063] In examples of the present disclosure, a core material is sandwiched between the inner skin and the outer skin.

[0064] In examples of the present disclosure, the ballast unit is different from the core material.

[0065] In examples of the present disclosure, the core material is based on foam or balsa. The core material may, for example, be polyethylene terephthalate foam, polyvinyl chloride foam, or balsa wood.

[0066] In examples of the present disclosure, the ballast material is an adhesive.

[0067] The ballast material may be a resin. This resin may be the same as or different from the resin provided during manufacture of the blade to impregnate the first skin layup and the second skin layup.

[0068] In examples of the present disclosure, the ballast unit has a length in the spanwise direction between 1 meter and 8 meters, for example between 2 meters and 6 meters, such as between 3 meters and 5 meters.

[0069] The ballast unit should be able to receive at least a certain amount of ballast material and should thereby at least have a certain size. Since it is integrated in the shell, it may have some restrictions regarding its size in the thickness direction of the blade. Further, the ballast unit may have some restrictions regarding its size in the chordwise direction of the blade, due to the curvature of the shell. Accordingly, to provide a certain volume, a significant length of the ballast unit in the spanwise direction may thereby be required.

[0070] However, at the same time, if the length of the ballast unit in the spanwise direction is too great, the distribution of ballast material in a partially filled ballast volume is highly uncertain. Whether the ballast material is concentrated in one end of the ballast unit or the other can impose a significant difference on the mass moment of the blade.

[0071] Accordingly, the length of the ballast unit in the spanwise direction preferably has to strike a balance. The above exemplified lengths seemingly do so.

[0072] In examples of the present disclosure, the ballast volume of the ballast unit provides a fluidly connected volume of at least 50 litres, for example of at least 100 litres, such as of at least 200 litres.

[0073] Accordingly, the ballast unit has a ballast volume sufficient for ballasting the wind turbine blade.

[0074] In examples of the present disclosure, a thickness of the ballast unit in the thickness direction is greater than a thickness of the core material adjacent to the ballast unit. Even though the ballast unit is integrated in the shell in manner which is similar to how the core material is integrated in the shell, it is not necessarily restricted to the same geometry.

[0075] A greater thickness of the ballast unit ensures a greater ballast volume with minimal increase in surface area of the ballast unit. In turn, this provides a more efficient utilization of the limited surface area of the shell. This may also improve flexibility as to where the ballast unit can be placed in the shell, given the restrictions provided by the curvature of the shell. For example, considering first and second ballast units each having a ballast volume of 100 litres, with the first ballast unit having a small thickness and the second ballast unit having a greater thickness, the first ballast unit must typically have a greater surface area, and may thereby be more restricted as to where it can be easily integrated in the shell.

[0076] In examples of the present disclosure, the ballast unit is integrated in the leeward shell, for example between a main leeward spar cap and a rear leeward spar cap.

[0077] The leeward shell may often provide a shell portion which is substantially flat, thereby permitting easy integration of the ballast unit. For example, the portion of the shell between the main leeward spar cap and the rear leeward spar cap (in case the blade comprises such two spar caps) may be particularly suitable for a ballast unit. A spar cap may alternatively be referred to as a reinforcement structure and provides reinforcement of the blade.

[0078] A third aspect of the present disclosure relates to a rotor for a wind turbine, the rotor comprising three separate wind turbine blades, each of the wind turbine blades being a wind turbine blade comprising a ballast unit according to the second aspect, wherein the ballast volume of the ballast unit of one blade of the three wind turbine blades is filled with more ballast material than the ballast volume of the ballast unit of another blade of the three wind turbine blades.

[0079] The three separate wind turbine blades may comprise: a first wind turbine blade comprising a first ballast unit defining a first ballast volume; a second wind turbine blade comprising a second ballast unit defining a second ballast volume; and a third wind turbine blade comprising a third ballast unit defining a third ballast volume. The each of the first ballast volume, the second ballast volume, and the third ballast volume may be filled with a different amount of ballast material. An amount of ballast material may also be no ballast material. For example, the heaviest blade of the three may not be provided with ballast material.

[0080] The ballast volume of the ballast unit of each of the blades may be filled with ballast material to provide each of the separate wind turbine blade with the same mass moment relative to a rotational axis of the rotor.

[0081] The rotor may further comprise a central hub, wherein the root end of each of the three separate wind turbine blades are connected to the hub such that three separate wind turbine blades extend radially outward from the hub.

[0082] The wind turbine may further comprise a tower; a nacelle, the nacelle mounted at an upper position of the tower; and a generator configured to be operatively coupled to the rotor within the nacelle via a drive train for converting mechanical kinetic energy of the rotor into electrical energy.

[0083] A fourth aspect of the present disclosure relates to use of a wind turbine blade comprising a ballast unit according to the second aspect, the ballast volume of the ballast unit being at least partially filled with a ballast material to provide ballast to the wind turbine blade.

[0084] Brief description of the drawings

[0085] Embodiments of the invention will now be further described by reference to the accompanying drawings, in which:

[0086] Fig. 1 illustrates a cross-sectional view of a wind turbine blade comprising a ballast unit according to the present disclosure,

[0087] Fig. 2 illustrates a windward shell of the wind turbine blade illustrated in Fig. 1 ,

[0088] Fig. 3 illustrates a flow chart of a method of manufacturing a wind turbine blade according to the present disclosure,

[0089] Fig. 4a-d illustrate method steps of manufacturing a wind turbine blade according to the present disclosure, Fig. 5 illustrates an angled cutaway view of a ballast unit according to the present disclosure, and

[0090] Fig. 6 illustrates main structural components of an exemplary horizontal-axis wind turbine comprising three separate wind turbine blades.

[0091] Detailed description of the drawings

[0092] It should be understood that the detailed description and specific examples, while indicating embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description.

[0093] Fig. 1 illustrates a cross-sectional view of a wind turbine blade 7 comprising a ballast unit 32 according to the present disclosure.

[0094] The blade comprises a shell structure, the shell structure comprising a leeward shell 14 and a windward shell 15 joined at a leading edge 17 and a trailing edge 18. In examples of the present disclosure, the two shells 14, 15 may be formed as a single piece.

[0095] The blade 7 extends in a chordwise direction between the leading edge 17 and the trailing edge 18. This chordwise direction is indicated by an arrow labelled with the letter “C”.

[0096] Further, the illustrated blade 7 extend in a thickness direction between the windward shell 15 and the leeward shell 14. The thickness direction is indicated in the figure by an arrow labelled with the letter “T”. The thickness direction is substantially perpendicular to the chordwise direction.

[0097] The illustrated blade further comprises a main reinforcement arrangement 20 and a rear reinforcement arrangement 25 reinforcing the blade, i.e. , resisting deformation of the blade. The rear reinforcement arrangement 25 is located between the main reinforcement arrangement 20 and the trailing edge 18.

[0098] The main reinforcement arrangement 20 comprises a main windward spar cap 21 integrated in the windward shell 15 and a main leeward spar cap 22 integrated in the leeward shell 14. These main spar caps are interconnected by a main web 23. In a similar manner, the rear reinforcement arrangement 25 comprises a rear windward spar cap 26 integrated in the windward shell 15 and a rear leeward spar cap 27 integrated in the leeward shell 14. These rear spar caps may be interconnected by a rear web 28.

[0099] Each of the spar caps 21 , 22, 26, 27 may comprise layers of pultruded fibres, e.g. carbon fibres in a resin matrix, aligned in a spanwise direction of the blade. The spar caps engage the shell structure to counteract loads on the blade during operation.

[0100] The exemplary wind turbine blade also comprises a ballast unit 32. The ballast unit defines an internal ballast volume 33. Here, the internal ballast volume 33 is simply the interior of the ballast unit 32.

[0101] The ballast volume 33 is configured to be able to receive a ballast material, such as adhesive. For example, the interior volume is at least partially vacant or empty, such that ballast material may be provided to at least partially fill this interior volume. Further, the ballast volume 33 is preferably a fluidly connected volume, such that fluid ballast material added within one part of the ballast volume 33 can redistribute to another part of the ballast volume 33.

[0102] In an alternative exemplary blade according to the present disclosure, the ballast volume 33 is at least partially filled by a ballast material 33. That is, ballast material has already been added.

[0103] The ballast unit 32 is integrated in the shell structure 14, 15. In the illustration, the unit 32 is integrated in the windward shell 15.

[0104] The shell structure comprises an inner skin 29a, 29b and an outer skin 30a, 30b. More specifically, the leeward shell 14 comprises an inner leeward skin portion 29a and an outer leeward skin portion 30a, and the windward shell 15 comprises an inner windward skin portion 29b and an outer windward skin portion 30b, wherein the inner skin comprises the inner leeward skin portion 29a and the inner windward skin portion 29b, and the outer skin comprises the outer leeward skin portion 30a and the outer windward skin portion 30b.

[0105] The outer skin 30a, 30b defines an outer surface of the blade 34, and the inner skin 29a, 29b faces an interior volume 35 of the blade. The outer skin may further be coated by paint.

[0106] The ballast unit 33 is sandwiched between the inner skin and the outer skin, namely between the inner windward skin portion 29b and the outer windward skin portion 30b. The shell structure further comprises core material 31 sandwiched between the inner skin 29a, 29b and the outer skin 30a, 30b.

[0107] As illustrated in this example, the ballast unit 32 is arranged between the windward spar caps 21 , 26 and next to core material 31. However, in other examples, a ballast unit may also be arranged elsewhere in a wind turbine blade, for example between the leeward spar caps 22, 27, between a main windward / leeward spar cap and the leading edge, between a rear windward / leeward spar cap and the trailing edge, in a cross-sectional segment of the blade without spar caps, or in a cross-sectional segment of the blade with just one reinforcement arrangement.

[0108] Further, a ballast unit is not necessarily arranged next to core material, although this is typically the case. The ballast unit may also be arranged on top of and / or below core material, relative to the thickness direction.

[0109] Fig. 2 illustrates a windward shell 15 of the wind turbine blade illustrated in Fig. 1. In the figure, the horizontal dashed line with reference to Fig. 1 indicates the plane of the cross-sectional view of the blade provided in Fig. 1. In the figure, components inside the shell 15 are shown for the sake of providing an illustrative example of how such components may be arranged relative to each other in the shell. In the actual manufactured shell, the components themselves are typically not visible viewing from the outside.

[0110] As in Fig. 1 , the chordwise direction of the windward shell is indicated in the figure by an arrow labelled with the letter “C”.

[0111] Further, the windward shell 15 and the wind turbine blade extend in a spanwise direction between a root end 10 and a tip end 12 of the wind turbine blade. The spanwise direction is indicated in the figure by an arrow labelled with the letter “S”. The spanwise direction is substantially perpendicular to the chordwise direction.

[0112] As also described in relation to Fig. 1 , the windward shell 15 comprises a main windward spar cap 21 and a rear windward spar cap 26. As illustrated in Fig. 2, they extend in the spanwise direction of the blade.

[0113] Fig. 2 also illustrates the placement and extent of the ballast unit 32 in the windward shell 15. In this particular example, the ballast unit 32 has a greater extent in the spanwise direction relative to the extent in the chordwise direction and in the thickness direction. This may ensure simple integration in the shell while still providing sufficient ballast volume.

[0114] Fig. 3 illustrates a flow chart of a method of manufacturing a wind turbine blade according to the present disclosure.

[0115] In a first step S1 , a shell mould is provided. The shell mould may be arranged to mould the leeward shell and the windward shell as separate shells which are then joined subsequently, or it may by arranged to mould the windward shell and the leeward shell as a single part. The shell mould may comprise a windward shell mould part and a leeward shell mould part.

[0116] In another step S2, a first skin layup is arranged onto the shell mould. The first skin layup may comprise a first windward skin layup portion and a second windward skin layup portion. The first windward skin layup portion is then arranged onto the windward shell mould part and the first leeward skin layup portion is then arranged onto the leeward shell mould part.

[0117] In another step S3, a ballast unit and core material are positioned onto the first skin layup. Further, one or more spar caps may also be positioned onto the first skin layup. The ballast unit defines an internal ballast volume for receiving a ballast material. In case a single ballast unit is used, it may for example be positioned onto the first windward skin layup portion or the first leeward skin layup portion. Core material is typically added to both the first windward and leeward skin layup portions. Generally, many different layouts of core material and spar caps are well known. A ballast unit may then, for example, replace part of the core material in a well-known layout. One possible distribution of ballast unit, core material and spar caps are provided in Figs. 1 and 2, although many different layouts of components are possible.

[0118] In another step S4, a second skin layup is arranged onto the ballast unit and onto the core material. The second skin layup may also be arranged onto any spar caps. The second skin layup may comprise a second windward skin layup portion and a second leeward skin layup portion. The second windward skin layup portion is then arranged onto the first windward skin layup portion and the windward shell mould part, sandwiching any core material previously positioned onto the first windward skin layup portion, and the second leeward skin layup portion is arranged onto the first leeward skin layup portion and the leeward shell mould part, sandwiching any core material previously positioned onto the first leeward skin layup portion. The first skin layup, the ballast unit, the core material, and the second skin layup collectively provide / form a shell layup. In another step S5, resin is provided to the shell layup to impregnate the first skin layup and the second skin layup. Prior to providing resin, the shell layup may be covered with a vacuum bag. Further, supplementary mould parts and / or flanges may be provided to prepare the shell layup and the shell mould for the resin, for example at the leading edge and / or the trailing edge. During this step of providing the resin, the ballast volume is sealed from the resin to preserve the internal ballast volume for receiving the ballast material. In other words, subsequent to providing the resin to the shell layup, the ballast volume remains suitable for receiving ballast material, for example receiving ballast material provided via a hole pierced in an outer skin of the wind turbine blade after forming the wind turbine blade.

[0119] In another step S6, the resin is cured to provide a shell structure for the wind turbine blade. The shell structure comprises a leeward shell and a windward shell. The shell structure also comprises an inner skin, an outer skin, core material and the ballast unit. The ballast unit and the core material are sandwiched between the inner skin and the outer skin. The outer skin is formed by the first skin layup and defines an outer surface of the blade. The inner skin is formed by the second skin layup and faces an interior volume of the blade.

[0120] In case a vacuum bag is used while providing the resin, this vacuum bag is removed after curing. Generally, a process relying on providing the resin using a vacuum bag may be referred to as vacuum assisted resin transfer moulding (VARTM), but the shell of the blade may also be manufactured using other approaches such as prepreg technology.

[0121] In another step S7, the wind turbine blade is formed from the shell structure, such that the wind turbine blade extends in a spanwise direction between a root end and a tip end of the blade, in a chordwise direction between a leading edge and a trailing edge of the blade, and in a thickness direction between the leeward shell and the windward shell.

[0122] Thereby, a wind turbine blade comprising a ballast unit is manufactured. Optionally, the method may comprise additional steps, such as a step of piercing the outer skin to provide a filling hole which fluidly connects the ballast volume with exterior surrounds allowing ballast material to be added to the ballast unit, and / or a step of adding ballast material to the ballast unit to at least partially fill the ballast volume by ballast material, for example via the filling hole. Subsequently, the filling hole may be sealed.

[0123] Fig. 4a-d illustrate method steps of manufacturing a wind turbine blade according to the present disclosure. The method described and illustrated here may for example correspond to the method described in relation to Fig. 3. The illustrations in Fig. 4a-d are provided as cross-sectional views in a manner similar to the cross-sectional view provided in Fig. 1.

[0124] In Fig. 4a, a first skin layup 42 is arranged onto a windward shell mould part 40 of a shell mould.

[0125] In Fig. 4b, various components are positioned onto the first skin layup 42. These various components comprise a main spar cap 21 , a rear spar cap 26, a ballast unit 33, and core material 31.

[0126] Fig. 4c illustrates how these various components are arranged on the first skin layup 42 after being positioned. Further, this figure illustrates that a second skin layup 43 is arranged onto the ballast unit 33 and the core material 31 .

[0127] In subsequent manufacturing steps, resin is provided and cured. Accordingly, a windward shell comprising core material 31 , a ballast unit 33, and spar caps 21 , 26 is manufactured. Similar steps are carried out to manufacture a leeward shell comprising core material 31 and spar caps, albeit relying on a leeward shell mould part instead of a windward shell mould part.

[0128] In Fig. 4d, the windward shell mould part 40 and the leeward shell mould part 41 are positioned on top of each other to facilitate joining of the windward shell 15 and the leeward shell 14 at the leading edge and at the trailing edge. For example, adhesive can be applied to the leading edge and trailing edge to join the two shells 14, 15.

[0129] However, note that according to some other blade shell moulding processes, the windward shell and the leeward shell are formed as a single part.

[0130] Next, the mould parts 40, 41 are opened to demould the blade. Finally, finishing may be applied to the surface of the blade, such as applying paint.

[0131] Generally, blade manufacturing processes may include additional steps, such as attaching one or more webs 23, 28 to the windward spar caps 21 , 26 and the leeward spar caps 22, 27, and / or installation of root reinforcements for securely fastening the manufactured blade onto a hub of a wind turbine.

[0132] Fig. 5 illustrates an angled cutaway view of a ballast unit 32 according to the present disclosure. The figure further indicates the spanwise, chordwise, and thickness direction of the blade upon installation of the ballast unit 32 in a wind turbine blade by the letters “S”, “C”, and “T”, respectively. The ballast unit 32 has a greater extent in the spanwise direction than in the chordwise direction, and the ballast unit 32 has a greater extent in the chordwise direction than in the thickness direction. However, other examples of ballast units within the scope of the present disclosure have other dimensions.

[0133] The ballast unit 32 may comprise a fluid-impermeable outer cover layer 36 which seals the interior ballast volume during moulding of the wind turbine blade. In an example, the outer cover layer 36 may be a plastic sheet.

[0134] The figure provides a cutaway view in which part of the cover layer 36 has been omitted to reveal a rigid internal framework 39. In this particular example, the rigid internal framework 39 is provided as a honeycomb structure, as visibly indicated by the hexagonal pattern. Further, the honeycomb structure is perforated to fluidly connect the hexagonal cells of the honeycomb structure such that internal ballast volume is a fluidly connected volume. Exemplary perforation holes 44 are illustrated for cell walls of one hexagonal cell in Fig. 5. Typically, hexagonal cells of the honeycomb structure are perforated to provide perforation holes 44 to each neighbouring hexagonal cell.

[0135] The ballast unit 32 may be further provided with a drain hole 37. In this particular example, the drain hole ensures drainage of fluid such as water. However, generally, one or more drain holes may provide drainage of fluid such as water, ventilation of the ballast volume, pressure equalization between ballast volume and exterior surroundings, or any combination thereof.

[0136] In addition, the ballast unit 32 may be provided with a filling hole 38. This filling hole 38 is typically provided just prior to filling the unit 32 with ballast material such as adhesive, which may even be performed on site. However, in principle, the filling hole 38 may be provided at any stage, albeit preferably after moulding the shell. The filing hole may be provided by piercing a skin, such as the outer skin, for example using a drill.

[0137] Fig. 6 illustrates main structural components of an exemplary horizontal-axis wind turbine 1 comprising three wind turbine blades 7 constituting the rotor 4 of the wind turbine. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted at top of the tower 2. The rotor is operatively coupled to a generator 5 within the nacelle 3 via a drive train (not shown) for converting mechanical kinetic energy harvested from the wind into electrical energy. In addition to the generator 5, the nacelle 3 may house additional components required to operate and optimize the performance of the wind turbine 1. The tower 2 supports the load presented by the nacelle 3, the rotor 4, and other wind turbine components within the nacelle 3.

[0138] The rotor 4 includes a central hub 6 and three elongated wind turbine blades 7 extending radially outward from the central hub 6, i.e., longitudinally in a lengthwise direction, from a root section of the blades 7 at the hub 6 to a tip section of the blades. In operation, the blades 7 are configured to interact with the passing air flow to produce lift that causes the central hub 6 to rotate about the longitudinal axis of the rotor 4. Wind speed in excess of a minimum level will activate the rotor 4 and allow it to rotate within a plane substantially perpendicular to the direction of the wind. The rotation is converted to electric power by the generator 5 and is usually supplied to the utility grid.

[0139] According to an example within the scope of the present disclosure, each of the three separate blades 7 of the wind turbine comprises a ballast unit, such as the ballast unit described in relation to Fig. 5. In this example, the ballast volume of the ballast unit of one blade of the three wind turbine blades 7 is filled with more ballast material than the ballast volume of the ballast unit of another blade of the three wind turbine blades 7.

[0140] Various versions and elements of the invention have been exemplified for the purpose of clarification rather than limitation. Well-known details of methods and systems have been omitted to not obscure the content of the disclosure with redundancy. Various elements and features of the invention and this disclosure may be combined in any way possible within the scope of the claims.

Claims

CLAIMS1. A method of manufacturing a wind turbine blade, the method comprising the steps of: providing a shell mould; arranging a first skin layup onto the shell mould; positioning a ballast unit onto the first skin layup, the ballast unit defining an internal ballast volume for receiving a ballast material; arranging a second skin layup onto the ballast unit, wherein the first skin layup, the ballast unit and the second skin layup provides a shell layup; providing resin to the shell layup to impregnate the first skin layup and the second skin layup, wherein the ballast volume is sealed from the resin during the step of providing the resin to preserve the internal ballast volume for receiving the ballast material; curing the resin to provide a shell structure for the wind turbine blade, the shell structure comprising a leeward shell and a windward shell, the shell structure comprising an inner skin, an outer skin, and the ballast unit, wherein the ballast unit is sandwiched between the inner skin and the outer skin, wherein the outer skin is formed by the first skin layup and defines an outer surface of the blade, wherein the inner skin is formed by the second skin layup and faces an interior volume of the blade; and forming the wind turbine blade from the shell structure such that the wind turbine blade extends in a spanwise direction between a root end and a tip end of the blade, in a chordwise direction between a leading edge and a trailing edge of the blade, and in a thickness direction between the leeward shell and the windward shell.

2. A method according to claim 1 , wherein the method comprises a step of adding the ballast material to the ballast unit to at least partially fill the ballast volume by the ballast material, wherein the step of adding the ballast material is performed after the step of forming the wind turbine blade.

3. A method according to claim 2, wherein the method comprises a step of piercing the outer skin to provide a filling hole which fluidly connects the ballast volume with exterior surroundings allowing ballast material to be added to the ballast unit to at least partly fill the ballast volume, wherein the step of piercing the outer skin is performed prior to the step of adding the ballast material to the ballast unit.

4. A method according to claim 3, wherein the inner skin remains sealed during the step of adding the ballast material.

5. A wind turbine blade comprising: a root end and a tip end, the blade extending in a spanwise direction between the root end and the tip end; a leading edge and a trailing edge, the blade extending in a chordwise direction between the leading edge and the trailing edge; and a shell structure, the shell structure comprising a leeward shell and a windward shell, the blade extending in a thickness direction between the leeward shell and the windward shell, the shell structure comprising an inner skin and an outer skin, wherein the outer skin defines an outer surface of the wind turbine blade and the inner skin faces an interior volume of the blade, wherein the wind turbine blade comprises a ballast unit defining an internal ballast volume for receiving a ballast material, wherein the ballast unit is integrated in the shell structure and sandwiched between the inner skin and the outer skin.

6. A wind turbine blade according to claim 5, wherein the ballast volume is at least partially filled by the ballast material.

7. A wind turbine blade according to any of claims 5-6, wherein the ballast volume is at least partially filled by a gas, such as air.

8. A wind turbine blade according to any of claims 5-7, wherein the wind turbine blade has an outer section closer to the tip end than to the root end, the outer section defined as the outermost 30 % of the blade relative to the spanwise direction, wherein the ballast unit is arranged within the outer section.

9. A wind turbine blade according to any of claims 5-8, wherein the ballast unit comprises a rigid internal framework.

10. A wind turbine blade according to claim 9, wherein the rigid internal framework comprises a plurality of straight force-carrying members connected by nodes.11 . A wind turbine blade according to any of claims 9-10, wherein the rigid internal framework comprises a cellular structure or a rib structure, such as a perforated cellular structure or a perforated rib structure, for example a perforated honeycomb structure.

12. A wind turbine blade according to any of claims 5-11 , wherein the ballast unit comprises a fluid-impermeable outer cover layer for sealing the ballast volume during moulding of the wind turbine blade.

13. A wind turbine blade according to any of claims 5-12, wherein the ballast unit is a first ballast unit, wherein the wind turbine blade comprises a second ballast unit defining a ballast volume for receiving a ballast material, wherein the second ballast unit is integrated in the shell structure and arranged between the inner skin and the outer skin.

14. A wind turbine blade according to any of claims 5-13, wherein the wind turbine blade comprises an outside marking visible on the outer skin, the outside marking indicative of the placement of the ballast unit, for example relative to the spanwise direction and / or the chordwise direction.

15. A rotor for a wind turbine, the rotor comprising three separate wind turbine blades, each of the wind turbine blades being a wind turbine blade comprising a ballast unit according to any of claims 5-14, wherein the ballast volume of the ballast unit of one blade of the three wind turbine blades is filled with more ballast material than the ballast volume of the ballast unit of another blade of the three wind turbine blades.

Images