A multirotor wind turbine

Abstract

A multirotor wind turbine (1) comprising a tower (2) and at least two load carrying structures (3) is disclosed, each load carrying structure (3) carrying an energy generating unit (4). Each load carrying structure (3) comprises a primary structure (5) and at least two secondary structures (6), the secondary structures (6) extending on opposing sides of the primary structure (5) between a first attachment position (8) at or near an end of the primary structure (5) and a second attachment position (13) at the tower (2). Each secondary structure (6) comprises two or more tension members (7), and the tension members (7) are connected to the primary structure (5) at the first attachment position (8) via a load sharing yoke structure (9).

Description

[0001] A MULTIROTOR. WIND TURBINE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a multirotor wind turbine with at least two load carrying structures, each load carrying structure carrying an energy generating unit.

[0004] BACKGROUND OF THE INVENTION

[0005] Wind turbines normally comprise one or more energy generating units, each energy generating unit comprising a hub carrying one or more wind turbine blades. The wind acts on the wind turbine blades, thereby causing the hub to rotate. The rotational movements of the hub are transferred to a generator, either via a gear arrangement or directly, in the case that the wind turbine is of a so-called direct drive type. In the generator, electrical energy is generated, which may be supplied to a power grid.

[0006] Some wind turbines are provided with two or more energy generating units in order to increase the total power produced by the wind turbine, without having to provide the wind turbine with one very large, and therefore heavy, energy generating unit. Such wind turbines are sometimes referred to as 'multirotor wind turbines'.

[0007] In multirotor wind turbines the energy generating units may be carried by one or more load carrying structures which are, in turn, connected to a tower structure. Thereby at least some of the energy generating units are not mounted directly on the tower structure, and they may have a centre of gravity which is displaced with respect to a longitudinal axis defined by the tower structure. The load carrying structure may comprise a primary structure, e.g. in the form of a compression member, such as a rigid arm or beam, and one or more secondary structures, e.g. in the form of tension members, such as wire or cables. In this case the secondary structures need to be designed and dimensioned to handle expected loads. Furthermore, measures need to be taken for handling situations where a secondary structure breaks or fails, or needs to be replaced.

[0008] DESCRIPTION OF THE INVENTION

[0009] It is an object of embodiments of the invention to provide a multirotor wind turbine, wherein the load carrying structures carrying the energy generating units are capable of handling expected loads without excessive costs.

[0010] It is a further object of embodiments of the invention to provide a method for replacing a tension member of a multirotor wind turbine, where the replacement can be performed in an easy and cost effective manner.

[0011] According to a first aspect, the invention provides a multirotor wind turbine comprising a tower and at least two load carrying structures, each load carrying structure carrying an energy generating unit, where each load carrying structure comprises a primary structure and at least two secondary structures, the secondary structures extending on opposing sides of the primary structure between a first attachment position at or near an end of the primary structure and a second attachment position at the tower, wherein each secondary structure comprises two or more tension members, and wherein the tension members are connected to the primary structure at the first attachment position via a load sharing yoke structure.

[0012] Thus, according to the first aspect, the invention provides a multirotor wind turbine, i.e. a wind turbine comprising at least two energy generating units. In the present context the term 'energy generating unit' should be interpreted to mean a part of the wind turbine which actually transforms the energy of the wind into electrical energy. Each energy generating unit thereby typically comprises a rotor, carrying a set of wind turbine blades, and a generator. The energy generating unit may further comprise a gear arrangement interconnecting the rotor and the generator. The generator, and possibly the gear arrangement, may be arranged inside a nacelle.

[0013] The multirotor wind turbine comprises a tower and at least two load carrying structures. Each load carrying structure carries an energy generating unit, and is connected to the tower. Thus, the load carrying structures form connections between the respective energy generating units and the tower, and they are capable of handling the loads involved with carrying the energy generating units.

[0014] Each load carrying structure comprises a primary structure and at least two secondary structures. The primary structure extends away from the tower along a direction being transverse with respect to a length direction defined by the tower, i.e. along a direction being non-parallel to the length direction defined by the tower. The at least two secondary structures extend on opposing sides of the primary structure, also away from the tower, between a first attachment position at or near an end of the primary structure and a second attachment position at the tower. The first attachment position may, e.g., be at or near a free end of the primary structure, opposite to an end of the primary structure being connected to the tower.

[0015] Thus, one of the secondary structures may extend between the end of the primary structure and the tower 'in front of' the primary structure, while another one of the secondary structures extends between the end of the primary structure and the tower 'behind' the primary structure. This provides a construction which is capable of handling thrust loads on the energy generating units, and on the load carrying structures.

[0016] Each secondary structure comprises two or more tension members. In the present context the term 'tension member' should be interpreted to mean a member being suitable for receiving and handling pull. The tension members could, e.g., be in the form of wires, ropes, cables or similar flexible members. The two or more tension members provide redundancy in the sense that should one of the tension members break or fail, at least one remaining tension member will be in place, so that the load carrying structure continues to be operational until the broken or failing tension member can be replaced. Furthermore, providing two or more tension members rather than a single tension member allows for sufficient strength and durability to be obtained by means of standard tension members that would be unable to, on their own, handle the expected loads on the secondary structures. This reduces the costs of the secondary structures.

[0017] When the secondary structures each comprises two or more tension members, there is a risk that, during operation of the wind turbine, the tension members of a given secondary structure are unevenly loaded. In order to avoid, or at least minimise, such uneven loading, the tension members are connected to the primary structure at the first attachment position via a load sharing yoke structure. This load sharing yoke structure ensures that loads on the secondary structure are substantially evenly distributed among the tension members forming the secondary structure. Thereby the risk of exposing one of the tension members to excessive loads is minimised, thus minimising the risk of one or more of the tension members breaking or failing.

[0018] Each tension member may be connected to the corresponding load sharing yoke structure via a hinge, thus allowing the tension members to pivot individually relative to the load sharing yoke structure. According to this embodiment, the tension members forming part of a given secondary structure are able to pivot relative to the load sharing structure independently of any pivoting movements of the other tension member(s). This allows the tension members to adapt to any differences in forces applied thereto, e.g. due an angle of the incoming wind, so as to ensure that the loads on the secondary structure are substantially evenly distributed among the tension members. Furthermore, differences in lengths of the tension members, e.g. due to tolerances, can be compensated for by the individual hinges.

[0019] Alternatively or additionally, each load sharing yoke structure may be connected to the corresponding primary structure via a hinge, thus allowing the load sharing yoke structure to pivot relative to the primary structure. According to this embodiment, the entire load sharing yoke structure, and thus the element to which the tension members are connected, is able to pivot relative to the primary structure. This allows the load sharing yoke structure to adapt to any impact on the load carrying structure which could potentially cause uneven loading of the tension members. Furthermore, differences in lengths of the tension members, e.g. due to tolerances, can be compensated for. Thereby a substantially even load distribution among the tension members is obtained.

[0020] Each load sharing yoke structure may be provided with at least one mounting interface configured for attachment of a service tension member to the load sharing yoke structure. According to this embodiment, a service tension member can easily be mounted on the load carrying structure, and essentially in the same manner as the tension members forming part of the secondary structures are mounted on the load carrying structure. This allows for easy and safe replacement of one of the tension members, e.g. in the following manner.

[0021] When replacement of a tension member is required, a service tension member is mounted on the load sharing yoke structure having the tension member to be replaced mounted thereon, via the mounting interface. Furthermore, the service tension member is attached to the tower at the second attachment position, so that the service tension member is mounted essentially in parallel to the tension members forming part of the relevant secondary structure.

[0022] Once the service tension member is in position, it will, thus, function essentially as one of the tension members forming part of the secondary structure.

[0023] Accordingly, the tension member to be replaced can safely be removed, because the service tension member is essentially taking over for the tension member being replaced, and therefore the tension member being replaced can be removed without compromising the total strength of the secondary structure. A replacement tension member can then be mounted on the load carrying structure at the attachment points where the removed tension member was attached, and once the replacement tension member is in place, the service tension member can be removed. The mounting interface may be arranged on the load sharing yoke structure between attachment points of two tension members. According to this embodiment, it is ensured that the service tension member is mounted at a substantially central position of the load sharing yoke structure, or at least not in an extreme or outer position of the load sharing yoke structure. This ensures that the service tension member is able to appropriately take over for a tension member being replaced, regardless of which of the tension members is being replaced.

[0024] Each tension member may be connected to the tower at the second attachment position via a bracket structure. This allows the tension members to be easily and fast mounted on the load carrying structure and tightened.

[0025] Each bracket structure may be connected to the tower, or to an intermediate structure being connected to the tower, via a hinge, thus allowing the bracket structure to pivot relative to the tower and / or to the intermediate structure. This allows for even easier installation of the tension members. Furthermore, according to this embodiment, the bracket structure contributes to ensuring load sharing among the tension members, e.g. by compensating for differences in length of the tension members, possibly due to tolerances, essentially as described above with reference to the load sharing yoke structure.

[0026] Each bracket structure may be provided with at least one mounting interface configured for attachment of a service tension member to the bracket structure. According to this embodiment, the service tension member can be easily attached to the bracket, in the case that one of the tension members forming part of the secondary structure needs to be replaced. The remarks set forth above with reference to the mounting interface formed on the load sharing yoke structure are equally applicable here.

[0027] According to a second aspect, the invention provides a method for replacing a tension member of a multirotor wind turbine, the multirotor wind turbine comprising a tower and at least two load carrying structures, each load carrying structure carrying an energy generating unit, where each load carrying structure comprises a primary structure and at least two secondary structures, the secondary structures extending on opposing sides of the primary structure between a first attachment position at or near an end of the primary structure and a second attachment position at the tower, wherein each secondary structure comprises two or more tension members, each tension member being connected to the primary structure at the first attachment position via a load sharing yoke structure, the method comprising the steps of:

[0028] - attaching a service tension member to the load sharing yoke structure having the tension member to be replaced attached thereto,

[0029] - attaching the service tension member to the tower at an attachment position near the second attachment position of the tension member to be replaced,

[0030] - removing the tension member to be replaced from the multirotor wind turbine,

[0031] - mounting a replacement tension member on the multirotor wind turbine by attaching the replacement tension member at the first and second attachment positions where the removed tension member was previously attached,

[0032] - tensioning the replacement tension member, and

[0033] - removing the service tension member from the multirotor wind turbine.

[0034] The method according to the second aspect may advantageously be performed on a multirotor wind turbine according to the first aspect of the invention. The remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here.

[0035] Thus, according to the second aspect, the invention provides a method for replacing a tension member of a multirotor wind turbine, the multirotor wind turbine comprising a tower and at least two load carrying structures, each load carrying structure carrying an energy generating unit, and each load carrying structure comprising a primary structure and at least two secondary structures, each secondary structure comprising two or more tension members. This has already been described in detail above with reference to the first aspect of the invention.

[0036] When one of the tension members needs to be replaced, a service tension member is initially attached to the load sharing yoke structure having the tension member to be replaced attached thereto. This could, e.g., include attaching the service tension member at a mounting interface formed on the load sharing yoke structure, e.g. between attachment points of two of the at least two tension members. This has already been described in detail above.

[0037] The service tension member is further attached to the tower at an attachment position near the second attachment position of the tension member to be replaced. This could, e.g., include attaching the service tension member to a mounting interfaced formed on a bracket structure to which the tension member to be replaced is attached. This has already been described in detail above.

[0038] Once the service tension member is attached to the load sharing yoke structure and to the tower, as described above, it is essentially arranged in parallel to the tension members forming part of one of the secondary structures, notably the secondary structure which the tension member to be replaced forms part of. Therefore, the service tension member is now able to operate as if it formed part of the secondary structure, in the sense that it is able to contribute to the total strength of the secondary structure, essentially in the same manner as the tension members forming part of the secondary structure. Accordingly, the tension member to be replaced can now be safely removed without risking that the total strength of the secondary structure is decreased to a level where there is a risk that it is no longer capable of handling expected loads.

[0039] Thus, the tension member to be replaced is now removed from the multirotor wind turbine, and a replacement tension member is mounted on the multirotor wind turbine, essentially in the manner which the removed tension member was mounted on the multirotor wind turbine. Accordingly, the replacement tension member is attached at the first and second attachment positions, at the load sharing yoke structure and the tower, respectively, where the removed tension member was attached. The replacement tension member is further tensioned, so as to enable the replacement tension member to handle loads in the same manner as the removed tension member was, prior to its removal. Finally, the service tension member is removed from the multirotor wind turbine, since it has now fulfilled its purpose.

[0040] Thus, the method according to the second aspect of the invention allows one the tension members forming part of one of the secondary structures to be replaced in an easy, safe and cost effective manner.

[0041] The method may further comprise the step of tensioning the service tension member after attaching the service tension member to the load sharing yoke structure and to the tower, and before removing the tension member to be replaced from the multirotor wind turbine. Such tensioning ensures that the service tension member is indeed capable of temporarily replacing the tension member to be replaced, in terms of contributing to the total strength of the secondary structure.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The invention will now be described in further detail with reference to the accompanying drawings in which

[0044] Fig. 1 is a schematic front view of a multirotor wind turbine according to an embodiment of the invention,

[0045] Figs. 2 and 3 are perspective views of a multirotor wind turbine according to an embodiment of the invention from two different angles, Fig. 4 shows a first attachment position of the multirotor wind turbine of Figs. 2 and 3,

[0046] Fig. 5 shows a second attachment position of the multirotor wind turbine of Figs. 2 and 3, and

[0047] Figs. 6 and 7 show details of the second attachment position of Fig. 5.

[0048] DETAILED DESCRIPTION OF THE DRAWINGS

[0049] Fig. 1 is a schematic front view of a multirotor wind turbine 1 according to an embodiment of the invention. The multirotor wind turbine 1 comprises a tower 2 and two load carrying structures 3, each load carrying structure 3 carrying an energy generating unit 4. Each load carrying structure 3 comprises a primary structure 5 in the form of an arm extending away from the tower 2, and with the energy generating unit 4 mounted at a free end thereof.

[0050] Each load carrying structure 3 further comprises at least two secondary structures 6 extending on opposing sides of the primary structure 5. In Fig. 1 only one secondary structure 6 is visible for each load carrying structure 3. Each of the secondary structures 6 comprises two or more tension members (not visible) extending between a first attachment point near the free end of the primary structure 5, and thus near the position where the energy generating unit 4 is mounted, and a second attachment position at or near the tower 2. This will be described in further detail below.

[0051] Figs. 2 and 3 are perspective views of a multirotor wind turbine 1 according to an embodiment of the invention. Fig. 2 is a perspective front view of the multirotor wind turbine 1, and Fig. 3 is a perspective view from behind of the multirotor wind turbine 1. It can clearly be seen that each load carrying structure 3 comprises one primary structure 5 and two secondary structures 6, and that the two secondary structures 6 extend on opposing sides of the respective primary structures 5. Accordingly, for a given load carrying structure 3, one of the secondary structures 6 extends in front of the primary structure 5, and the other secondary structure 6 extends behind the primary structure 5.

[0052] Fig. 4 is a detail of the multirotor wind turbine of Figs. 2 and 3, showing a free end of the primary structure 5 of one of the load carrying structures 3, and thus the part of the load carrying structure 3 where the energy generating unit 4 is mounted on the load carrying structure 3. It can further be seen that each of the secondary structures 6 comprises two tension members 7 extending in parallel. The tension members 7 may, e.g., be in the form of wires or cables. This introduces redundancy in the sense that if one of the tension members 7 fails or breaks, the entire secondary structure 6 will not fail. Furthermore, this allows sufficient strength and durability of the secondary structures 6 to be obtained by means of standard tension members, such as standard wires or cables, and thus at low cost.

[0053] The tension members 7 of each secondary structure 6 are connected to the primary structure 5 at respective first attachment positions 8, via load sharing yoke structures 9. Each load sharing yoke structure 9 is connected to the primary structure 5 via a hinge 10, and each of the tension members 7 is connected to the respective load sharing yoke structure 9 via a hinge 11. Thus, the load sharing yoke structures 9 are able to pivot relative to the primary structure 5, and the tension members 7 are able to individually pivot relative to the respective load sharing yoke structures 9. This introduces some degrees of freedom which ensure that loads are appropriately distributed among the tension members 7, thus avoiding that one of the tension members 7 is excessively loaded. For instance, the load sharing yoke structure 9 and the hinges 10, 11 ensure that the tension members 7 are able to adapt to any differences in forces applied thereto, e.g. due an angle of the incoming wind or to differences in lengths of the tension members 7, e.g. due to tolerances.

[0054] A mounting interface 12 is provided on the load sharing yoke structure 9 between the hinges 11 connecting the respective tension members 7 to the load sharing yoke structure 9. This allows a service tension member to be attached to the load sharing yoke structure 9, e.g. in the case that one of the tension members 7 needs to be replaced.

[0055] Fig. 5 is another detail of the multirotor wind turbine of Figs. 2 and 3, showing the part where the secondary structures 6 are connected to the tower 2. The tension members 7 of the respective load carrying structures 3 'meet' at second attachment positions 13 arranged in front of and behind the tower 2, respectively. The tension members 7 are mounted in bracket structures 14, each being connected to the respective attachment positions 12 via hinges 15. This allows for easy mounting of the tension members 7, and contributes to the load distribution among the tension members 7.

[0056] Figs. 6 and 7 are details of Fig. 5. Fig. 6 shows the attachment position 13 arranged in front of the tower 2, and Fig. 7 shows the attachment position 13 arranged behind the tower 2.

Claims

CLAIMS1. A multirotor wind turbine (1) comprising a tower (2) and at least two load carrying structures (3), each load carrying structure (3) carrying an energy generating unit (4), where each load carrying structure (3) comprises a primary structure (5) and at least two secondary structures (6), the secondary structures(6) extending on opposing sides of the primary structure (5) between a first attachment position (8) at or near an end of the primary structure (5) and a second attachment position (13) at the tower (2), wherein each secondary structure (6) comprises two or more tension members(7), and wherein the tension members (7) are connected to the primary structure (5) at the first attachment position (8) via a load sharing yoke structure (9).

2. A multirotor wind turbine (1) according to claim 1, wherein each tension member (7) is connected to the corresponding load sharing yoke structure (9) via a hinge (11), thus allowing the tension members (7) to pivot individually relative to the load sharing yoke structure (9).

3. A multirotor wind turbine (1) according to claim 1 or 2, wherein each load sharing yoke structure (9) is connected to the corresponding primary structure (5) via a hinge (10), thus allowing the load sharing yoke structure (9) to pivot relative to the primary structure (5).

4. A multirotor wind turbine (1) according to any of the preceding claims, wherein each load sharing yoke structure (9) is provided with at least one mounting interface (12) configured for attachment of a service tension member to the load sharing yoke structure (9).

5. A multirotor wind turbine (1) according to claim 4, wherein the mounting interface (12) is arranged on the load sharing yoke structure (9) between attachment points of two tension members (7).

6. A multirotor wind turbine (1) according to any of the preceding claims, wherein each tension member (7) is connected to the tower (2) at the second attachment position (13) via a bracket structure (14).

7. A multirotor wind turbine (1) according to claim 6, wherein each bracket structure (14) is connected to the tower (2), or to an intermediate structure being connected to the tower (2), via a hinge (15), thus allowing the bracket structure (14) to pivot relative to the tower (2) and / or to the intermediate structure.

8. A multirotor wind turbine (1) according to claim 6 or 7, wherein each bracket structure (14) is provided with at least one mounting interface configured for attachment of a service tension member to the bracket structure (14).

9. A method for replacing a tension member (7) of a multirotor wind turbine (1), the multirotor wind turbine (1) comprising a tower (2) and at least two load carrying structures (3), each load carrying structure (3) carrying an energy generating unit (4), where each load carrying structure (3) comprises a primary structure (5) and at least two secondary structures (6), the secondary structures(6) extending on opposing sides of the primary structure (5) between a first attachment position (8) at or near an end of the primary structure (5) and a second attachment position (13) at the tower (2), wherein each secondary structure (6) comprises two or more tension members (7), each tension member(7) being connected to the primary structure (5) at the first attachment position(8) via a load sharing yoke structure (9), the method comprising the steps of:- attaching a service tension member to the load sharing yoke structure (9) having the tension member (7) to be replaced attached thereto,- attaching the service tension member to the tower (2) at an attachment position near the second attachment position (13) of the tension member (7) to be replaced,- removing the tension member (7) to be replaced from the multirotor wind turbine (1),- mounting a replacement tension member (7) on the multirotor wind turbine (1) by attaching the replacement tension member (7) at the first (8) and second (13) attachment positions where the removed tension member (7) was previously attached,- tensioning the replacement tension member (7), and- removing the service tension member from the multirotor wind turbine (1).

10. A method according to claim 9, further comprising the step of tensioning the service tension member after attaching the service tension member to the load sharing yoke structure (9) and to the tower (2), and before removing the tension member (7) to be replaced from the multirotor wind turbine (1).

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