A wind turbine with blade connecting tension members with varying diameter

Abstract

A wind turbine (1) comprising a tower (2), a nacelle (3), and at least three wind turbine blades (5) is disclosed The wind turbine (1) further comprises blade connecting tension members (8) extending between a connection point (9) at one wind turbine blade (5) and a connection point (9) at a neighbouring wind turbine blade (5). Each blade connecting tension member (8) comprises a plurality of first sections (12) having a first cross sectional diameter and a plurality of second sections (13) having a second cross sectional diameter, where the first cross sectional diameter is smaller than the second cross sectional diameter. The first sections (12) and the second sections (13) are arranged alternatingly along a length direction of the blade connecting tension member (8). This reduces the noise generated during operation of the wind turbine (1).

Description

[0001] A WIND TURBINE WITH BLADE CONNECTING TENSION MEMBERS WITH VARYING DIAMETER

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a wind turbine comprising at least three wind turbine blades, where blade connecting tension members extend between connection points at neighbouring wind turbine blades.

[0004] BACKGROUND OF THE INVENTION

[0005] During operation of a wind turbine, as well as during standstill, the components of the wind turbine are subjected to various loads. For instance, the wind turbine blades of the wind turbine are subjected to loads originating from gravity acting on the wind turbine blades, loads originating from wind pressure on the wind turbine blades, loads originating from changes in wind direction, turbulence, etc.

[0006] As the size of wind turbine blades increases, the loads on the wind turbine also increase. In order to handle such increased loads, the amount of material used for manufacturing the wind turbine may be increased. However, this increases the weight as well as the manufacturing costs of the wind turbine.

[0007] As an alternative to increasing the amount of material used for the wind turbine, in particular for the wind turbine blades, the wind turbine may be provided with blade connecting tension members, e.g. in the form of wires, i.e. blade connecting tension members which interconnect the wind turbine blades. Such blade connecting tension members cause the wind turbine blades to mutually support each other, in the sense that a part of the loads on the wind turbine blades are 'shared' among the wind turbine blades, via the blade connecting tension members.

[0008] Introducing such blade connecting tension members may give rise to noise induced due to air passing the blade connecting tension members during operation of the wind turbine. Since the blade connecting tension members move along when the rotor rotates, the angle of attack between the air and the respective blade connecting tension members changes during operation of the wind turbine. This is particularly an issue when the ratio of rotor angular rotation speed changes with respect to the wind speed, such as when the turbine transitions from part load to full-load operation modes, or due to the presence of wind sheer, turbulence, or yaw error. Furthermore, the airflow over a given blade connecting tension member will not be perpendicular to a length direction of the blade connecting tension member. Accordingly, the airflow over the blade connecting tension member has a component along the length direction of the blade connecting tension member. This makes it difficult to predict the expected level of noise as a result of fitting blade connecting tension members of a given kind on a given wind turbine. For instance, modifications to blade connecting tension member which appear to have a noise reducing effect on a non-moving blade connecting tension member may not necessarily result in a reduction of noise once the blade connecting tension member is mounted on an operating wind turbine.

[0009] DESCRIPTION OF THE INVENTION

[0010] It is an object of embodiments of the invention to provide a wind turbine with blade connecting tension members extending between the wind turbine blades, in which the noise generated by the wind turbine is reduced as compared to similar prior art wind turbines.

[0011] The invention provides a wind turbine comprising a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub, and a tip end arranged opposite to the root end, the wind turbine further comprising blade connecting tension members, each blade connecting tension member extending between a connection point at one wind turbine blade and a connection point at a neighbouring wind turbine blade, where the connection point at a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade, wherein each blade connecting tension member comprises:

[0012] - a plurality of first sections having a first cross sectional diameter and a plurality of second sections having a second cross sectional diameter, where the first cross sectional diameter is smaller than the second cross sectional diameter, wherein the first sections and the second sections are arranged alternatingly along a length direction of the blade connecting tension member.

[0013] Thus, the invention provides a wind turbine comprising a tower with a nacelle mounted thereon, preferably via a yaw system. A hub carrying at least three wind turbine blades is mounted rotatably on the nacelle. The hub and the wind turbine blades may be referred to as a rotor. During operation, the rotor is positioned in accordance with the wind direction by appropriately operating the yaw system. The wind turbine blades catch the wind and cause the hub to rotate. This rotation is then transferred to a generator, possibly via a gear system, where the mechanical energy is transformed to electrical energy, which may then be supplied to a power grid.

[0014] Each wind turbine blade extends between a root end connected to the hub, possibly via a pitch system, and a tip end arranged opposite to the root end. Thus, each wind turbine blade extends radially outwards from the hub optionally at a smaller coning angle.

[0015] The wind turbine further comprises blade connecting tension members. Each blade connecting tension member extends between a connection point at one wind turbine blade and a connection point at a neighbouring wind turbine blade. Accordingly, each blade connecting tension member interconnects two neighbouring wind turbine blades, i.e. wind turbine blades which are connected to the hub adjacent to each other. The blade connecting tension members may, e.g., be in the form a wire, a cable, or any other suitable kind of blade connecting tension member. The connection point at a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end. Accordingly, the connection point is neither positioned at the root end, nor at the tip end, i.e. it is not positioned at an extremity of the wind turbine blade. Rather, it is positioned at an appropriate intermediate position between the root end and the tip end. This will be described in further detail below.

[0016] Thus, the blade connecting tension members cause the wind turbine blades to mutually support each other, in the sense that loads on the wind turbine blades, in particular edgewise loads and optionally to some degree flapwise loads, are 'shared' among the wind turbine blades, via the blade connecting tension members. Thereby the loads on the wind turbine blades, during operation as well as during standstill, can be handled by a lower material thickness, and thereby decreased weight and lower manufacturing costs.

[0017] Each blade connecting tension member comprises a plurality of first sections having a first cross sectional diameter and a plurality of second sections having a second cross sectional diameter. For example, each blade connecting tension member may have 20 to 400 first sections and 20 to 400 second sections, such as 40 to 200 first sections and 40 to 200 second sections. The first cross sectional diameter is smaller than the second cross sectional diameter.

[0018] Accordingly, the cross sectional diameter of the blade connecting tension member is not constant throughout the entire length of the blade connecting tension member, but is instead smaller in the parts of the blade connecting tension member corresponding to the first sections than in parts of the blade connecting tension member corresponding to the second sections.

[0019] The first sections and the second sections are arranged alternatingly along a length direction of the blade connecting tension member. In the present context the term 'arranged alternatingly' should be interpreted to mean that each of the first sections is arranged between two of the second sections or between one of the second sections and an end of the blade connecting tension member, and that each of the second sections is arranged between two of the first sections or between one of the first sections and an end of the blade connecting tension member. It is, however, not ruled out that one or more further sections having a cross sectional diameter which differs from the first cross sectional diameter as well as from the second cross sectional diameter are arranged between one of the first sections and one of the second sections.

[0020] Thus, along the length direction, the cross sectional diameter of the blade connecting tension member varies between the first cross sectional diameter and the second cross sectional diameter in an alternating manner, possibly with further variations of the cross sectional diameter as described above.

[0021] It has been found by the inventors of the present invention that such alternating variations in the cross sectional diameter of the blade connecting tension members result in a reduction of the generated noise during operation of the wind turbine. More particularly, the inventors of the present invention have found that the alternating variations in cross sectional diameter interferes with a part of the airflow flowing near the blade connecting tension members in a manner which results in reduced noise caused by air passing the blade connecting tension members while the rotor rotates. Realising this is by no means trivial, since, as described above, the interaction between the airflow and the rotating blade connecting tension members, leading to the noise generation, is unpredictable.

[0022] The first sections may have a first length and the second sections may have a second length, and the first length may be substantially equal to the second length. According to this embodiment, all of the first sections have substantially identical lengths, i.e. the first length, and all of the second sections have substantially identical lengths, i.e. the second length. Moreover, the lengths of the first sections are substantially equal to the lengths of the second sections.

[0023] The first and second lengths may, e.g., be within the interval 1-6 times the first cross sectional diameter, such as within the interval 2-5 times the first cross sectional diameter, such as within the interval 2-4 times the first cross sectional diameter, such as approximately 4 times the first cross sectional diameter.

[0024] For instance, the first and second lengths may be within the interval 5 cm to 30 cm, such as within the interval 10 cm to 25 cm, such as approximately 20 cm. In an alternative embodiment, the first length may differ from the second length. According to this embodiment, similarly to the embodiment above, all of the first sections have substantially identical lengths and all of the second sections have substantially identical lengths. However, in this case the length of the first sections differs from the length of the second sections. For instance, the second length may be smaller than the first length. In this case the sections with larger cross sectional diameter are shorter than the sections with smaller cross sectional diameter. For instance, the first length may be within the interval 2 cm to 10 cm, such as approximately 5 cm, and the second length may be within the interval 5 cm to 30 cm, such as approximately 20 cm. As an alternative, the second length may be larger than the first length. For instance, the first length may be within the interval 5 cm to 30 cm, such as approximately 20 cm, and the second length may be within the interval 2 cm to 10 cm, such as approximately 5 cm.

[0025] In another alternative embodiment, the lengths of the first sections and / or the lengths of the second sections may vary from one first / second section to another. For instance, the lengths of the first and / or second sections may vary randomly, or they may increase or decrease systematically along the length direction of the blade connecting tension member.

[0026] The first cross sectional diameter may be between 70% and 90% of the second cross sectional diameter, such as between 75% and 85% of the second cross sectional diameter, such as approximately 80% of the second cross sectional diameter.

[0027] According to this embodiment, the difference in cross sectional diameter between the first sections and the second sections is sufficiently large to create the desired aerodynamical effect causing the reduction in noise generation, but not so large that unwanted side effects in terms of strength of the blade connecting tension members or performance of the rotor occur.

[0028] For instance, the difference between the first cross sectional diameter and the second cross sectional diameter may be within the interval 5 mm to 50 mm, such as within the interval 10 mm to 30 mm, such as approximately 20 mm. Alternatively or additionally, the first cross sectional diameter may be within the interval 70 mm to 110 mm, such as within the interval 80 mm to 100 mm, such as approximately 90 mm, and / or the second cross sectional diameter may be within the interval 90 mm to 130 mm, such as within the interval 100 mm to 120 mm, such as approximately 110 mm.

[0029] Each blade connecting tension member may comprise a plurality of transition sections, each transition section interconnecting one of the first sections and one of the second sections, and the transition section may have a length, L3, within a range defined as L3 / (LI + L2) <0.05, such as L3 / (LI+ L2) <0.03, where Li and L2are the lengths of the first section and the second section, respectively, being interconnected by the transition section.

[0030] According to this embodiment, a first section and an adjacent second section are interconnected by a transition section. Thus, this transition section provides a transition between the smaller cross sectional diameter of the first section and the larger cross sectional diameter of the second section. Since L3 / (LI+L2)<0.05, the length, L3, of the transition section is significantly smaller than the length, Li, of the first section and / or the length, L2, of the second section. Accordingly, the transition between the two cross sectional diameters occurs in a relatively abrupt or sharp manner. Such an abrupt or sharp transition has been found to be beneficial in terms of obtaining the aerodynamic effects which provide the desired reduction in noise generation.

[0031] For instance, the transition section may be defined as a section between the first section and the second section, where the cross sectional diameter, D, is within a range defined by (DI+0.1 -(D2-DI)) < D< (D2-0.1 - (D2-DI)), where Di is the first cross sectional diameter and D2is the second cross sectional diameter. In this case the length, L3, of the transition section may be regarded as the distance between a position on the blade connecting tension member where the cross sectional diameter is larger than the first cross sectional diameter by 10% of the difference between the first and second cross sectional diameters, and a position on the blade connecting tension member where the cross sectional diameter is smaller than the second cross sectional diameter by 10% of the difference between the first and second cross sectional diameters. For instance, the length, L3, of the transition section may be smaller than 30 mm, such as smaller than 20 mm, such as smaller than 10 mm.

[0032] Each blade connecting tension member may comprise:

[0033] - a tension member core, and

[0034] - a surface shaping layer arranged circumferentially with respect to the tension member core, the surface shaping layer defining the first sections and / or the second sections.

[0035] Since the surface shaping layer is arranged circumferentially with respect to the tension member core, it defines the shape and texture of the outer surface of the blade connecting tension member. The first and / or second sections may, e.g., be provided by applying the surface shaping layer in some sections of the blade connecting tension member and not in other sections, or by varying the thickness of the surface shaping layer along the length direction of the blade connecting tension member.

[0036] The tension member core may be made from a polymer material. The polymer material may, e.g., be an Ultra High Molecular Weight Polyethylene, e.g. of the kind being manufactured under the tradename 'Dyneema'. Ultra High Molecular Weight Polyethylene fibres have a high strength / weight ratio, a high Young's modulus, and good fatigue properties.

[0037] As an alternative, the polymer material may be based on polyester, polyamid, nylon, polypropylene, aramid, etc. As another alternative, the polymer material may be a composite material, e.g. a liquid crystal polymer, such as polybenzoxazole (PBO).

[0038] As an alternative to a polymer material, the tension member core may be made from steel, e.g. in the form of a steel wire, or carbon fibres. In the latter case the tension member core may be provided as a composite member by means of carbon pultrusion. The surface shaping layer may be or comprise a woven or braided layer. According to this embodiment, the surface shaping layer may be woven or braided directly onto the tension member core, e.g. by arranging yarn in an appropriate pattern. Alternatively, the woven or braided layer may be manufactured separately, and subsequently arranged circumferentially with respect to the tension member core. In one embodiment, the surface shaping layer may be or comprise a 3D woven material, e.g. made from synthetic polymer fibres.

[0039] The surface shaping layer may be or comprise a coating. According to this embodiment, the surface shaping layer may be applied directly onto the tension member core, e.g. by brush, spray or roll in an appropriate pattern. It is preferred that the coating is a highly viscous UV resistant coating material applied in one or more layers with a total layer thickness of 1-6 mm. The coating may for example be a polyurethane based coating.

[0040] The surface shaping layer may be or comprise a self-amalgamating tape. According to this embodiment, the surface shaping layer may comprise at least two layers arranged on or around the core such that at least one layer of the self-amalgamating tape is attached on top of an inner layer so selfamalgamating takes place.

[0041] The surface shaping layer may be or comprise a shrink wrap tape or shrink tube. According to this embodiment, the surface shaping layer may comprise shrink wrap tape wound in one or more layers circumferentially around the tension member core followed by heating of the shrink wrap tape so the tape shrinks and form a tight layer around the tension member core, or the surface shaping layer may comprise a shrink tube slid circumferentially over the tension member core from one end of the tension member core followed by heating of the shrink tube so the tube shrinks and form a tight layer around the tension member core.

[0042] The surface shaping layer may be or comprise an open or closed cell foam. According to this embodiment, the surface shaping layer may be prepared by arranging spray foam or other expanding fluid, such as expanding polyurethane, onto the tension member core, foaming and thereafter curing the open or closed cell foam. Alternatively, the open or closed cell foam may be manufactured separately, and subsequently arranged circumferentially with respect to the tension member core.

[0043] The surface shaping layer may be or comprise a porous or dense polyurethane material. According to this embodiment, the surface shaping layer may preferably be manufactured separately from the tension member core, and subsequently arranged circumferentially with respect to the tension member core.

[0044] The surface shaping layer may comprise one or more surface shaping elements arranged between the tension member core and the woven or braided layer, the coating, the self-amalgamating tape, the shrink wrap tape or shrink tube, the open or closed cell foam, and / or the porous or dense polyurethane material, respectively. According to this embodiment, the woven or braided layer, the coating, the self-amalgamating tape, the shrink wrap tape or shrink tube, the open or closed cell foam, and / or the porous or dense polyurethane material may have a substantially uniform thickness, and the first and second cross sectional diameters of the first and second sections, respectively, may be defined by the surface shaping elements. For instance, surface shaping elements may be provided in the second sections, but not in the first sections, or the first sections may be provided with surface shaping elements which are smaller than surface shaping elements provided in the second section.

[0045] The surface shaping layer may comprise an inner layer arranged circumferentially with respect to the tension member core and an outer layer arranged circumferentially with respect to the inner layer, and the inner layer may define the first sections and / or the second sections.

[0046] According to this embodiment, the surface shaping layer comprises two layers, i.e. the inner layer and the outer layer, arranged concentrically with respect to each other and with respect to the tension member core, and with the inner layer arranged between the tension member core and the outer layer. Furthermore, an outer surface of the outer layer forms the outer surface of the surface shaping layer, and thereby the outer surface of the blade connecting tension member. On the other hand, the inner layer defines the first sections and / or the second sections. Thus, the inner layer may be selected in a manner which allows the first sections and / or the second sections to be easily and suitably defined, while the outer layer may be selected so as to obtain appropriate and desirable surface properties of the blade connecting tension member.

[0047] According to this embodiment, the blade connecting tension member may, e.g., be made by applying the inner layer to the tension member core, and subsequently applying the outer layer to the inner layer.

[0048] The outer layer may be made from a silicon-based material, a polyurethane based material, a polyurea-based material, or ethylene propylene rubber. These materials potentially have suitable resistance to ultraviolet light exposure, are potentially soft enough to absorb impacts from rain drops to enable suitable rain erosion performance, and have demonstrated the ability to create a smoothed surface shaping layer with improved noise and drag properties for stepped cylinders at relevant flow conditions for a cable stayed rotor.

[0049] Alternatively or additionally, the outer layer may be in the form of a selfamalgamating tape, such as a tape based on ethylene propylene rubber or silicon. According to this embodiment, the outer layer can easily be applied to the inner layer by simply wrapping the self-amalgamating tape onto the inner layer.

[0050] For instance, the outer layer may be in the form of a moulded shell, such as a polyurethane-based injection-moulded profile, e.g. a C profile, or it may be in the form of an over-braiding that builds up to a larger overall thickness.

[0051] The surface shaping layer may be made at least partly from a porous material. In the present context the term 'porous material' should be interpreted to mean a material which, at least to some extent, allows airflow to pass through the material. Due to the porous properties of the surface shaping layer of the blade connecting tension member, an airflow passing the blade connecting tension member during operation of the wind turbine will thus partly pass through the surface shaping layer of the blade connecting tension member rather than passing along an outer surface thereof. Accordingly, the porous outer layer interferes with the airflow.

[0052] It has been found by the inventors of the present invention that the interference with the passing airflow provided by a porous surface shaping layer of the blade connecting tension member results in a reduction of the generated noise during operation of the wind turbine. Thus, the noise generated by the wind turbine during operation is even further reduced.

[0053] The porous material of the surface shaping layer may define a porosity of at least 0.5, such as within the interval 0.8 to 1.0, such as within the interval 0.9 to 1.0.

[0054] The porosity is a non-dimensional parameter, o, defined as o = 1 - — , where ptis Ps the density of the actual porous material and psis the density of the material without pores. Thus, a porosity of 0 indicates that there are no pores in the material, i.e. pt= ps, whereas a porosity approaching 1 indicates that the density of the actual porous material is significantly lower than the density of the material without pores, i.e. pt« ps. A high porosity, e.g. a porosity approaching 1, provides good acoustic properties, and may therefore be expected to result in reduction of the generated noise to a great extent.

[0055] The porous material of the surface shaping layer may define an airflow resistivity within the interval 1000 Pa-s / m3to 10000 Pa-s / m3. The airflow resistivity is a measure for the resistance an air particle experiences when passing through a material, and therefore provides a measure for how difficult or easy it is for air particles to pass the material. It is also a parameter which governs the acoustic behaviour of porous materials for sound absorption.

[0056] The porous material may be or comprises an open-celled foam, such as polyurethane foam or polyethylene foam. The predominant advantage of using open-celled foam is that they are porous by nature, so no sophisticated processes manufacturing process, such as additive manufacturing or secondary machining operations are necessary to create the porous flow channels. Furthermore, open-celled foams are generally lightweight.

[0057] In one embodiment, the porous material is an open-celled foam and porous material is arranged onto the tension member core by spraying a spray foam or providing an expanding fluid onto the tension member core. After foaming, the spray foam or expanding fluid is cured. Optionally, a mould may be arranged onto the tension member core prior to arranging the porous material onto the tension member core, and the spray foam or other expanding fluid may be provided into the mould to ensure a desired outer shape and / or positioning of the porous material relative to the tension member core. After curing of the porous material, the mould is removed from the tension member optionally followed by surface finish e.g. by grinding, of the porous material. Providing of an open-celled foam by spraying a spray foam or providing an expanding fluid onto the tension member core was found to also provide an advantages method of preparing a partially degraded porous material arranged on the tension member core. Specifically, the repair method includes the step of spraying spray foam or providing an expanding fluid onto the tension member core and optional remaining old porous material arranged on the tension member core. Due to the nature of spray foam, partially connected residues of old porous material may be secured to the tension member core again by the applied spray foam, so preparation of the surface of the tension member core with old porous material prior to application of the spray foam may be limited or even not be required. However, optionally, the repair method may also include removing partially released porous material prior to the spray foam or providing the expanding fluid onto the tension core member and optional old porous material. Furthermore, the method may optionally include a step of arranging a mould onto the tension member core prior to arranging the porous material onto the tension member core with optional old porous material still attached to the tension member core.

[0058] Airflow resistance is defined as the ratio between differential pressure, or pressure difference, across a specimen of the material and volumetric airflow rating passing through the test specimen. Specific airflow resistance applies to a specific thickness of the material. Airflow resistivity is defined as specific airflow resistance per unit thickness of the material in the direction of airflow. Accordingly, airflow resistivity is a material specific parameter, and a low airflow resistivity indicates that an airflow can easily pass through the material, whereas a high airflow resistivity indicates that an airflow is not allowed to easily pass the material.

[0059] An airflow resistivity within the interval 1000 Pa-s / m3to 10000 Pa-s / m3, such as within the interval 1000 Pa-s / m3to 8000 Pa-s / m3, such as within the interval 1000 Pa-s / m3to 5000 Pa-s / m3, represents a relatively low airflow resistivity. This is desirable, since this allows the airflow to pass easily through the surface shaping layer of the blade connecting tension member, and the noise reducing effect described above is therefore obtained to a great extent.

[0060] Each blade connecting tension member may have an outer surface with a surface roughness within an interval defined as 0.100 mm<Ra<0.400 mm, such as 0.200<Ra<0.300, and / or within an interval defined as 0.200 mm<Rz<0.500 mm, such as 0.300<Rz<0.400. According to this embodiment, a surface texture and / or surface roughness of the blade connecting tension member is obtained, which further contributes to reducing the noise generated by the interaction between the blade connecting tension members and the passing air.

[0061] The values mentioned above are particularly relevant for a blade connecting tension member with a diameter of approximately 70 mm. In the case that the diameter is smaller, a lower surface roughness may be appropriate, and in the case that the diameter is larger, a higher surface roughness may be appropriate. An appropriate normalized measure for the roughness could be 0.0014<Ra / D<0.0057, such as 0.0029<Ra / D<0.0043, and / or 0.0029<Rz / D<0.0071, such as 0.0043<Rz / D<0.0057, where D is a representative diameter of the blade connecting tension member. It should be observed that the above preferred ranges for Ra and Rz are for 'macroscopic' roughness (see below) for the outer surface of the surface texture providing layer. The intervals above may be defined on an evaluation scale or filter scale, which is comparable to the lengths of the blade connecting tension members, between the relevant connection points. Accordingly, these parameters and intervals may be regarded as a 'macroscopic' roughness of the surface shaping layer, or as a 'low frequency' roughness. A sample length, or cut-off length,c, for the 'macroscopic' roughness may bec=60 mm and not include a transition section. Correspondingly, a sample length, or cut-off length, for a 'microscopic' roughness may bec=0.8 mm and not include a transition section.

[0062] According to ISO standards 16610-1 and 16610-21, the parameter Ra is defined as a mean value of deviations in surface position from an average surface position. Accordingly, Ra is a measure for the size of hills and valleys formed on the surface, as an average consideration. Outliers in the form of very large or very small deviations will have only little impact on the parameter Ra.

[0063] Furthermore, according to ISO standards 16610-1 and 16610-21, the parameter Rzis defined as the difference between a representative sample of the largest deviations from the average surface position and a representative sample of the smallest deviations from the average surface position. Accordingly, Rzreflects extremities in the deviations from the average surface position and is therefore a measure for the size of fluctuations in surface positions. Outliers in the form of very large or very small deviations have a significant impact on the parameter Rz.

[0064] It has been found by the inventors of the present invention that some surface roughness will improve the aerodynamic properties of the blade connecting tension member, but that too high a surface roughness may have a detrimental effect. A roughness as defined by the intervals above is therefore suitable.

[0065] For instance, Ra may be within the interval 0.100 mm-0.400 mm, such as within the interval 0.200 mm-0.300 mm, such as within the interval 0.220 mm-0.280 mm, such as within the interval 0.230 mm-0.270 mm. Similarly, Rzmay be within the interval 0.200 mm-0.500 mm, such as within the interval 0.300 mm-0.400 mm, such as within the interval 0.320 mm-0.380 mm, such as within the interval 0.330 mm-0.370 mm.

[0066] The surface roughness of the surface shaping layer on a microscopic scale, or a high frequency scale, may be significantly lower than the surface roughness on the macroscopic scale, such as 1-3 orders of magnitude lower. For instance, on the microscopic scale, Ra may be within the interval 0.700 .m-1.400 .m, such as within the interval 0.800 .m-1.200 .m, such as approximately 1.000 .m.

[0067] The connection points at the wind turbine blades may be arranged at a distance from the root end which is between 15% and 50% of the length of the wind turbine blade from the root end to the tip end. According to this embodiment, the connection points on the wind turbine blades are arranged at a position which is well clear of the root end as well as the tip end of the wind turbine blades.

[0068] The position of the connection points along the wind turbine blades may be selected in a manner which suitably balances various issues which need to be taken into consideration. For instance, positioning the connection point close to the tip end of the wind turbine blade results in very efficient support to the wind turbine blades by the blade connecting tension members. However, this comes at a price of a high drag caused by the blade connecting tension members during rotation of the rotor, and thereby decreased energy production. On the other hand, positioning the connection point close to the root end of the wind turbine blade results in a low drag caused by the blade connecting tension members, thereby minimising the adverse impact on the energy production of the wind turbine. However, the support to the wind turbine blades by the blade connecting tension members will not be very efficient. By positioning the connection points at a distance from the root end which is between 15% and 50% of the length of the wind turbine blade, these considerations are balanced in such a manner that efficient support is obtained without introducing an unacceptable drag. Furthermore, by positioning the connecting points within this region it is ensured that the blade connecting tension members are attached to the wind turbine blades where a structural stiffness of the wind turbine blade is sufficiently high. For instance, the structural stiffness of the wind turbine blade decreases towards the tip end, and connecting the blade connecting tension members too near the tip end may therefore create a significant pre-bending of the wind turbine blade.

[0069] The wind turbine may be a pitch controlled wind turbine. According to this embodiment, the wind turbine blades are connected to the hub via respective pitch systems and the connection points at the wind turbine blade for the blade connecting tension member cable are arranged to allow for pitching of the blades so the wind turbine blades are able to perform pitching movements relative to the hub, about a longitudinal axis arranged along the length of the wind turbine blades, in order to adjust an angle of attack between the wind turbine blades and the incoming wind.

[0070] The wind turbine may further comprise a pre-tension system arranged with the hub, and each blade connecting tension member may comprise a tensioner leg and the tensioner leg may be connected to the pre-tension system.

[0071] According to this embodiment, a pre-tension is introduced in each of the blade connecting tension members by appropriately operating the pre-tension system to act on the blade connecting tension members, via the respective tensioner legs. By appropriately applying pre-tension to the blade connecting tension members, it is possible to obtain a desired level of mutual support of the wind turbine blades. The pre-tension applied to the blade connecting tension members may be adjustable.

[0072] Each tensioner leg may comprise:

[0073] - a plurality of first sections having a first cross sectional diameter and a plurality of second sections having a second cross sectional diameter, where the first cross sectional diameter is smaller than the second cross sectional diameter, wherein the first sections and the second sections are arranged alternatingly along a length direction of the tensioner leg. According to this embodiment, the tensioner legs exhibit acoustical properties which are similar to the acoustical properties of the blade connecting tension members, due to the varying cross sectional diameter. Accordingly, the noise level generated due to air passing the tensioner legs is also reduced essentially in the manner described above with reference to the blade connecting tension members.

[0074] BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The invention will now be described in further detail with reference to the accompanying drawings in which

[0076] Figs. 1 and 2 illustrate a wind turbine according to an embodiment of the invention,

[0077] Figs. 3 and 4 illustrate a blade connecting tension member for a wind turbine according to a first embodiment of the invention,

[0078] Figs. 5 and 6 illustrate a blade connecting tension member for a wind turbine according to a second embodiment of the invention,

[0079] Figs. 7 and 8 illustrate a part of a blade connecting tension member for a wind turbine according to a third embodiment of the invention,

[0080] Figs. 9 and 10 illustrate a part of a blade connecting tension member for a wind turbine according to a fourth embodiment of the invention,

[0081] Fig. 11 illustrates transition sections between first sections and second sections of a blade connecting tension member for a wind turbine according to an embodiment of the invention, and

[0082] Fig. 12 illustrates a wind turbine according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

[0083] Figs. 1 and 2 illustrate a wind turbine 1 according to an embodiment of the invention. Fig. 1 is a front view of the wind turbine 1 and Fig. 2 is a side view of the wind turbine 1.

[0084] The wind turbine 1 comprises a tower 2, a nacelle 3 mounted on the tower 2 and a hub 4 mounted on the nacelle 2. Three wind turbine blades 5 are connected to the hub 4. Each wind turbine blade 5 extends between a root end 6 connected to the hub 4 and an oppositely arranged tip end 7.

[0085] The wind turbine 1 further comprises three blade connecting tension members 8. Each blade connecting tension member 8 interconnects two neighbouring wind turbine blades 5 by being connected to connection points 9 at the respective wind turbine blades 5. The wind turbine blades 5 are able to mutually support each other via the blade connecting tension members 8, in the sense that loads on the wind turbine blades 5, in particular edgewise loads and flapwise loads, are shared among the wind turbine blades 5, via the blade connecting tension members 8.

[0086] The blade connecting tension members 8 are of a kind which comprises a plurality of first sections having a first cross sectional diameter and a plurality of second sections having a second cross sectional diameter, where the first cross sectional diameter is smaller than the second cross sectional diameter. The first sections and the second sections are arranged alternatingly along a length direction of the blade connecting tension members 8.

[0087] Accordingly, the cross sectional diameter of the blade connecting tension members 8 is not constant throughout the entire length of the respective blade connecting tension members 8, but rather varies between the first cross sectional diameter and the second cross sectional diameter. Such alternating variations in cross sectional diameter interrupt the airflow along the length direction of the blade connecting tension members in a manner which results in reduced noise caused by air passing the blade connecting tension members while the rotor rotates and the angle of attack between the air and the blade connecting tension members therefore changes. Accordingly, the noise generated due to air passing the blade connecting tension members 8 during operation of the wind turbine 1 is reduced.

[0088] Figs. 3 and 4 illustrate a blade connecting tension member 8 for a wind turbine according to a first embodiment of the invention. Fig. 3 is a perspective view of the blade connecting tension member 8 and Fig. 4 is a side view of the blade connecting tension member 8.

[0089] The blade connecting tension member 8 of Figs. 3 and 4 comprises a plurality of first sections 12 and a plurality of second sections 13. In the first sections 12 the blade connecting tension member 8 has a first cross sectional diameter, and in the second sections 13 the blade connecting tension member 8 has a second cross sectional diameter. The first cross sectional diameter is smaller than the second cross sectional diameter. The first sections 12 and the second sections 13 are arranged alternatingly along a length direction of the blade connecting tension member 8.

[0090] Thus, in the length direction of the blade connecting tension member 8, the cross sectional diameter alternatingly varies between the first, smaller, cross sectional diameter and the second, larger, cross sectional diameter. It can also be seen that the transition between the first sections 12 and the second sections 13 are relatively sharp or abrupt. Such a variation in the cross sectional diameter interrupts an airflow parallel to the length direction of the blade connecting tension member 8. This results in a reduction in the noise generated due to an airflow impacting the blade connecting tension member 8 during operation of the wind turbine.

[0091] The first sections 12 and the second sections 13 may, e.g., be provided by varying a thickness of an outer or surface shaping layer of the blade connecting tension member 8.

[0092] In the blade connecting tension member 8 of Figs. 3 and 4, the length of the first sections 12 and the length of the second sections 13 are substantially identical. Figs. 5 and 6 illustrate a blade connecting tension member 8 for a wind turbine according to a second embodiment of the invention. Fig. 5 is a perspective view of the blade connecting tension member 8 and Fig. 6 is a side view of the blade connecting tension member 8.

[0093] The blade connecting tension member 8 of Figs. 5 and 6 is very similar to the blade connecting tension member 8 of Figs. 3 and 4, in the sense that it comprises a plurality of first sections 12 with a first cross sectional diameter and a plurality of second sections 13 with a second cross sectional diameter. The remarks set forth above with reference to Figs. 3 and 4 are therefore equally applicable here.

[0094] However, in the blade connecting tension member 8 of Figs. 5 and 6, the length of the first sections 12 is significantly smaller than the length of the second sections 13.

[0095] In another embodiment (not shown), the length of the first sections may vary so some or all of the first sections are of different lengths, for example varying randomly or gradually. Similarly, in another embodiment (not shown), the length of the second sections may vary so some or all of the second sections are of different lengths, for example varying randomly or gradually.

[0096] Figs. 7 and 8 illustrate part of a blade connecting tension member 8 for a wind turbine according to a third embodiment of the invention. Fig. 7 is a perspective view of the blade connecting tension member 8 and Fig. 8 is a side view of the blade connecting tension member 8.

[0097] The blade connecting tension member 8 of Figs. 7 and 8 comprises a tension member core 10 and a surface shaping layer 11 arranged circumferentially with respect to the tension member core 10. In Fig. 7 part of the surface shaping layer 11 has been removed in order to reveal the tension member core 10 and provide a cross sectional view of the surface shaping layer 11.

[0098] The surface shaping layer 11 is in the form of a braided layer, which may be applied at various thickness along the length direction of the blade connecting tension member 8, so as to form first sections having a first cross sectional diameter and second sections having a second cross sectional diameter, e.g. in the manner described above with reference to Figs. 3-6. As described above, this reduces the noise generated due to air impacting the blade connecting tension member 8 during operation of the wind turbine.

[0099] Figs. 9 and 10 illustrate a part of a blade connecting tension member 8 for a wind turbine according to a fourth embodiment of the invention. Fig. 9 is a perspective view of the blade connecting tension member 8 and Fig. 10 is a cross sectional view of the blade connecting tension member 8.

[0100] The blade connecting tension member 8 of Figs. 9 and 10, similarly to the blade connecting tension member 8 of Figs. 7 and 8, comprises a tension member core 10 and a surface shaping layer 11 arranged circumferentially with respect to the tension member core 10. The surface shaping layer 11 is made from a porous material in the form of an open-cell polyester foam, defining relatively large and visible pores allowing an incoming airflow to at least partly pass through the surface shaping layer 11. This reduces the noise generated due to air impacting the blade connecting tension member 8 during operation of the wind turbine.

[0101] Furthermore, the thickness of the surface shaping layer 11 may be varied along the length direction of the blade connecting tension member 8, so as to form first sections having a first cross sectional diameter and second sections having a second cross sectional diameter, e.g. in the manner described above with reference to Figs. 3-6.

[0102] Fig. 11 illustrates transition sections 14 between first sections 12 and second sections 13 of a blade connecting tension member 8 for a wind turbine according to an embodiment of the invention.

[0103] Similarly to the blade connecting tension members 8 described above with reference to Figs. 3-6, the blade connecting tension member 8 of Fig. 11 comprises a plurality of first sections 12 having a first cross sectional diameter, Di, and a plurality of second sections 13 having a second cross sectional diameter, D2. The first sections 12 have a first length, Li, and the second sections 13 have a second length, L2, and the first sections 12 and the second sections 13 are arranged alternatingly along the length direction of the blade connecting tension member 8.

[0104] The alternatingly arranged first sections 12 and second sections 13 are connected to each other via transition sections 14, each having a third length, L3. Thus, each transition section 14 defines a transition between the smaller cross sectional diameter, Di, of a first section 12 and the larger cross sectional diameter, D2, of an adjacent second section 13.

[0105] It can be seen that the third length, L3, of the transition sections 14 is significantly shorter than the first length, Li, of the first sections 12, as well as significantly shorter than the second length, L2, of the second sections 13. Thus, the transition between the first cross sectional diameter, Di, and the second cross sectional diameter, D2, takes place in a relatively abrupt or sharp manner. This ensures that the desired aerodynamic effects providing the desired reduction in generated noise are obtained. The third length, L3, may, e.g., be within a range defined as L3 / (LI + L2)<0.05, such as L3 / (LI + L2) <0.03.

[0106] For instance, the transition section 14 may be defined as a section between the first section 12 and the second section 13, where the cross sectional diameter, D, is within a range defined by (DI+0.1 - (D2-DI)) < D <(D2-0.1 - (D2-DI)).

[0107] Fig. 12 illustrates a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 of Fig. 12 is similar to the wind turbine 1 of Figs. 1 and 2, in the sense that it is provided with blade connecting tension members 8. The remarks set forth above with reference to Figs. 1 and 2 are therefore equally applicable here.

[0108] The wind turbine 1 of Fig. 12 is further provided with a pre-tension system 15 at the hub 4. Tensioner legs 16 interconnect each of the blade connecting tension members 8 with the pre-tension system 15, so as to introduce pre-tension in each of the blade connecting tension members 8. By appropriately selecting the introduced pre-tension, a desired level of mutual support of the wind turbine blades 5 can be obtained. The tensioner legs 16 may, similarly to the blade connecting tension members 8, comprise a plurality of first sections having a first cross sectional diameter and a plurality of second sections having a second cross sectional diameter, where the first cross sectional diameter is smaller than the second cross sectional diameter, and where the first sections and the second sections are arranged alternatingly along a length direction of the tensioner leg 16. In this case the tensioner legs 16 exhibit acoustical properties similar to the acoustical properties of the blade connecting tension members 8, and therefore a reduction of the noise generated due to air passing the tensioner legs 16 is also obtained.

Claims

CLAIMS1. A wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2), a hub (4) mounted rotatably on the nacelle (3), and at least three wind turbine blades (5), wherein each wind turbine blade (5) extends between a root end (6) connected to the hub (4), and a tip end (7) arranged opposite to the root end (6), the wind turbine (1) further comprising blade connecting tension members (8), each blade connecting tension member (8) extending between a connection point (9) at one wind turbine blade (5) and a connection point (9) at a neighbouring wind turbine blade (5), where the connection point (9) at a given wind turbine blade (5) is arranged at a distance from the root end (6) and at a distance from the tip end (7) of the wind turbine blade (5), wherein each blade connecting tension member (8) comprises:- a plurality of first sections (12) having a first cross sectional diameter and a plurality of second sections (13) having a second cross sectional diameter, where the first cross sectional diameter is smaller than the second cross sectional diameter, wherein the first sections (12) and the second sections (13) are arranged alternatingly along a length direction of the blade connecting tension member (8).

2. The wind turbine (1) according to claim 1, wherein the first sections (12) have a first length and the second sections (13) have a second length, and wherein the first length is substantially equal to the second length.

3. The wind turbine (1) according to claim 1 or 2, wherein the first cross sectional diameter is between 70% and 90% of the second cross sectional diameter.

4. The wind turbine (1) according to any of the preceding claims, wherein the first cross sectional diameter is within the interval 70 mm to 110 mm and / or the second cross sectional diameter is within the interval 90 mm to 130 mm.

5. The wind turbine (1) according to any of the preceding claims, wherein each blade connecting tension member (8) comprises a plurality of transition sections (14), each transition section (14) interconnecting one of the first sections (12) and one of the second sections (13), wherein the transition section (14) has a length, L3, within a range defined as L3 / (LI + L2)<0.05, where Li and L2are the lengths of the first section (12) and the second section (13), respectively, being interconnected by the transition section (14).

6. The wind turbine (1) according to any of the preceding claims, wherein each blade connecting tension member (8) comprises:- a tension member core (10), and- a surface shaping layer (11) arranged circumferentially with respect to the tension member core (10), the surface shaping layer (11) defining the first sections (12) and / or the second sections (13).

7. The wind turbine (1) according to claim 6, wherein the surface shaping layer (11) is or comprises a woven or braided layer, a coating, a self-amalgamating tape, a shrink wrap tape or shrink tube, an open or closed cell foam, and / or a porous or dense polyurethane material.

8. The wind turbine (1) according to claim 7, wherein the surface shaping layer (11) comprises one or more surface shaping elements arranged between the tension member core (10) and the woven or braided layer, the coating, the selfamalgamating tape, the shrink wrap tape or shrink tube, the open or closed cell foam, and / or the porous or dense polyurethane material.

9. The wind turbine (1) according to any of claims 6-8, wherein the surface shaping layer (11) comprises an inner layer arranged circumferentially with respect to the tension member core (10) and an outer layer arranged circumferentially with respect to the inner layer, and wherein the inner layer defines the first sections (12) and / or the second sections (13).

10. The wind turbine (1) according to claim 9, wherein the outer layer is made from a silicon-based material, a polyurethane based material, a polyurea-based material or ethylene propylene rubber.

11. The wind turbine (1) according to claim 9 or 10, wherein the outer layer is in the form of a self-amalgamating tape.

12. The wind turbine (1) according to any of claims 6-11, wherein the surface shaping layer (11) is made at least partly from a porous material.

13. The wind turbine (1) according to any of the preceding claims, wherein each blade connecting tension member (8) has an outer surface with a surface roughness within an interval defined as 0.100 mm<Ra<0.400 mm and / or within an interval defined as 0.200 mm<Rz<0.500 mm.

14. The wind turbine (1) according to any of the preceding claims, wherein the connection points (9) at the wind turbine blades (5) are arranged at a distance from the root end (6) which is between 15% and 50% of the length of the wind turbine blade (5) from the root end (6) to the tip end (7).

15. The wind turbine (1) according to any of the preceding claims, wherein the wind turbine (1) is a pitch controlled wind turbine.

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