Patent Description:
Wind turbines are required to be very efficient in order to be competitive with other forms of energy and to reduce the cost of energy (COE). The main approach to increasing the efficiency of a wind turbine is to increase the size of its generator rotor and to also increase the size of its aerodynamic rotor by using longer rotor blades that can extract more energy from the wind.

However, manufacturing limitations as well as other issues such as transportation, installation and repair effectively place constraints on the length of a rotor blade, and can increase the overall COE of a wind turbine.

A wind turbine is generally made by moulding a composite material using a vacuum resin infusion technique. There are two principle ways of moulding a rotor blade. In one approach, two blade halves are moulded separately and then glued together along their outer edges. In another approach, the entire blade is moulded as one piece. Moulding a rotor blade in one piece can be preferred because manufacturing tolerances are less tight, the moulding procedure is less prone to error, and the finished blade does not have a large adhesive joint.

However, to manufacture a very long one-piece rotor blade, the resin flow process can be very complex to design and optimize on account of the large dimensions, the longer spans and the thicker structural components. These all increase the risk of extended manufacturing errors such as transverse wrinkles, poor resin impregnation, air pockets, dry fibre regions, etc. Even if it is possible to repair such a manufacturing error, the repair may be prohibitively expensive. However, the cost of scrapping an entire blade structure may be even higher.

Regardless of the manner in which a rotor blade is manufactured, the storage, transport and installation of a very long rotor blade are costly. In particular, conventional road transportation may not be possible for very long rotor blades.

It is known to manufacture rotor blades in two or more segments, and to assemble these segments on site. This approach can at least reduce the costs associated with storage, transport and installation. The structural stability of such a rotor blade depends very much on the nature of the joint between the segments. In one approach, each blade segment has an essentially cylindrical shape, even though the joint is at a distance from the root end, and a suitably shaped aerodynamic part is attached to one side of the blade to create an airfoil shape. This approach imposes severe limitations on the airfoil shape, and the extensive joint regions between blade segment and aerodynamic attachment make it difficult to achieve a smooth outer surface. In another approach, blade segments are made with an airfoil cross-section and are joined using complementary inserts that are secured inside the cavities of the moulded blade segments. <CIT> for example proposes bolts extending perpendicularly through the ends of each blade segment, and fixing the positions of the bolts by rods. Other ways of connecting rotor blade segments are known from the prior art. For example, it is also known from <CIT>, <CIT> and <CIT> to connect rotor blade segments using inserts and bolts. Additional prior art solutions are also available in <CIT> and <CIT>.

However, the ability of such connections to transfer flapwise loads is limited, so that the blade lifetime may be shortened. The prior art segmented blades generally also require complex assembly measures in order to install a down conductor for a lightning protection system.

It is therefore an object of the invention to provide an alternative way of joining rotor blade segments to overcome the problems outlined above.

This object is achieved by the wind turbine rotor blade of claim <NUM>; by the method of claim <NUM> of manufacturing a wind turbine rotor blade; by the method of claim <NUM> of assembling a wind turbine rotor blade; and by the wind turbine of claim <NUM>.

According to the invention, the wind turbine rotor blade comprises at least a first rotor blade segment and a second rotor blade segment connected by means of a connecting interface. The connecting interface comprises a plurality of bores formed in a connecting face of each rotor blade segment, and each bore extends in a longitudinal direction of the rotor blade. The bores are formed so that the bores of the second rotor blade segment align with bores of the first rotor blade segment. Each bore terminates in a cavity dimensioned to accommodate a threaded nut. The connecting interface further comprises a plurality of double-end stud bolts, wherein a double-end stud bolt comprises a shaped portion formed between a first threaded end and a second threaded end, and shaped to engage with a tightening means. The first threaded end of each double-end stud bolt extends through a bore of the first rotor blade segment and is screwed into a threaded nut held in a cavity of the first rotor blade segment; the second threaded end of each double-end stud bolt extends through a bore of the second rotor blade segment and is screwed into a threaded nut held in a cavity of the second rotor blade segment.

In the context of the invention, the arrangement of bores, cavities, double-end stud bolts and threaded nuts are collectively referred to as the "connecting interface" between two rotor blade segments.

An advantage of the wind turbine rotor blade according to the invention is that the connecting interface can be realised in a more economical manner compared to prior art solutions. Furthermore, the inventive rotor blade can very effectively withstand flapwise bending loads that arise mainly due to wind pressure. Such loads result in tensile and compressive stresses in the blade material, and it is important to design a wind turbine rotor blade to be able to withstand these loads in order to achieve a favourably long lifetime. Any joint between rotor blade segments must be able to transfer these loads correctly. The inventive rotor blade fulfils this requirement, since the arrangement of stud bolts can very effectively transfer flapwise loads from one blade segment to the other.

According to the invention, the method of manufacturing such a wind turbine rotor blade comprises the steps of moulding at least a first rotor blade segment and a second rotor blade segment, wherein the rotor blade segments are moulded to comprise complementary connecting faces. The method further comprises the steps of forming a plurality of bores in a connecting face of each rotor blade segment, each bore extending in a longitudinal direction of the rotor blade and dimensioned to accommodate one end of a double-end stud bolt of a connecting interface; forming a cavity at the end of each bore, wherein a cavity is dimensioned to accommodate a threaded nut corresponding to the threaded end of the double-end stud bolt; and arranging a threaded nut in each of the plurality of cavities. The order given here for the method steps is arbitrary, and the steps can be performed in any suitable order when carrying out the inventive method.

An advantage of the inventive manufacturing method is that rotor blade segments can be prepared with less effort that comparable rotor blade segments manufactured using a prior art technique.

According to the invention, such a wind turbine rotor blade is assembled by aligning the bores of a first rotor blade segment with the bores of a second rotor blade segment; arranging a double-end stud bolt in each pair of aligned bores; engaging a tightening means or tightening tool with the shaped portion of a double-end stud bolt; and actuating the tightening means to screw the threaded ends of each double-end stud bolt into the corresponding threaded nuts.

The wind turbine according to the invention comprises a number of rotor blades as described above in the context of the invention. Usually, a wind turbine comprises three rotor blades mounted to a hub.

In the following, it may be assumed that a wind turbine rotor blade is for a large wind turbine, for which an assembled rotor blade can have a length of <NUM> or more. The rotor blade segments can be moulded using any appropriate moulding technique. In one possible approach, a rotor blade segment can be manufactured by moulding two halves and gluing the cured segment halves along their outer edges using a suitable adhesive. However, in a particularly preferred embodiment of the invention, a rotor blade segment is moulded using a closed-mould technique in which the entire blade segment is moulded in one moulding step, thus eliminating the problems associated with adhesive bonds.

A double-ended stud bolt is in the form of a rod with a threaded portion at each outer end. Since the purpose of the double-ended stud bolt is to pull the connecting faces of the rotor blade segments towards each other, one of the threaded ends has a left-hand thread, and the other threaded end has a right-hand thread. When the ends of a stud bolt are inserted into the threaded nuts contained in the cavities, and the stud bolt is turned in the appropriate direction, the resulting inward force is transferred via the threaded nuts to the connecting faces of the rotor blade segments.

In a particularly preferred embodiment of the invention, bores and cavities of a connecting interface are formed in a precast component, and the precast component is embedded in the rotor blade segment during the moulding procedure. Preferably, the precast component is a precast spar cap. This can be prepared in advance of the rotor blade segment moulding procedure. The precast spar cap is preferably realised to comprise fibreglass composite material at least in a region near the connecting face of a rotor blade segment. A precast spar cap can be made entirely of a fibreglass composite material. Alternatively, the fibreglass composite material can transition to a carbon fibre material in a region further away from the connecting face of a rotor blade segment.

A precast spar cap can be made using a vacuum assisted resin transfer technique to obtain a robust and lightweight spar cap that can be embedded between composite layers of the rotor blade segment. The bores and cavities of the connecting interface are preferably machined in the body of the spar cap using appropriate tools such as a drill, a milling tool, etc. The bores of a connecting interface are machined into an outer face of a spar cap to extend in a longitudinal direction of the rotor blade. The cavities of the connecting interface can be milled from the outside surface of the spar cap, so that a threaded nut can be easily placed into the cavity. Once the threaded nuts have all been placed in the cavities, the cavities can be sealed to prevent the threaded nuts from falling out.

Preferably, a threaded nut is realised as a transverse nut. The body of the transverse nut is essentially cylindrical and arranged so that the long axis of the cylindrical nut is at right angles to the stud bolt and essentially perpendicular to the outer surface of the rotor blade. In the case that the cavities are formed in a precast component such as a spar cap, the long axis of the cylindrical nut is essentially perpendicular to the outer surface of the precast component.

The number of double-end stud bolts required to reliably connect two blade segments may depend on various factors such as the dimensions of the rotor blade segments, the diameter and length of the double-end stud bolts, etc. In a preferred embodiment of the invention, the connecting interface is realised to comprise <NUM> - <NUM> double-end stud bolts on the pressure side and on the section side to connect two blade segments.

In a preferred embodiment of the invention, the connecting interface comprises a means of spring-loading a threaded nut in its cavity. The spring-loading means can comprise any suitable kind of spring element that serves to press the threaded nut against the bore aperture so that, when the double-end stud bolt is inserted into the bore, the bolt end will easily find the threaded bushing of the transverse nut. In this way, the spring-loading means can facilitate and speed up the assembly process.

As mentioned above, wind turbine rotor blades are subject to different types of bending loads. In addition to flapwise loading, a wind turbine rotor blade is also subject to edgewise loading. The edgewise bending load is caused mainly by the force of gravity acting on the rotor blade, and is highest at the blade root. It is important for the wind turbine rotor blade also to be able to withstand edgewise bending loads. Therefore, in a particularly preferred embodiment of the invention, the connecting interface further comprises an edgewise load transfer assembly realised to transfer edgewise loads between the rotor blade segments. For example, in a preferred embodiment, the edgewise load transfer assembly comprises a first bracket embedded in the first rotor blade segment and a second bracket embedded in the second rotor blade segment, and a connecting means for connecting the first bracket to the second bracket. The brackets can be embedded in the laminate of the rotor blade segments and preferably align along the leading edge of the rotor blade. Each bracket can extend from the rotor blade segment by a short distance. The outer ends of the brackets can be joined together using suitable fasteners. To effectively transfer the edgewise loads, outer ends of the brackets are preferably shaped to lie against each other. For example, each bracket can undergo a right angle or <NUM>° bend at its outer end, and the bent portions of two brackets are shaped to abut each other. Fasteners can extend through the abutting ends of the brackets to connect these in a secure manner. The brackets can be embedded in the laminate of the rotor blade segments and preferably align along a longitudinal axis of the blade. One bracket pair can be embedded in the laminate to extend along the leading edge of the rotor blade. A further bracket pair can be embedded in the laminate to extend parallel to the trailing edge of the rotor blade.

The rotor blade of a wind turbine is generally manufactured to incorporate at least one down conductor of a lightning protection system (LPS). A down conductor generally serves to provide a path for lightning current from a receptor at some point on the blade's outer surface to ground. An uninterrupted down conductor can be installed in a one-piece rotor blade. However, when a rotor blade is made of two or more segments, the down conductor may also comprise segments that need to be connected. In a particularly preferred embodiment of the invention, one or more double-ended stud bolts are used to connect two parts of a down conductor. The two parts of the down conductor will be arranged in two connected rotor blade segments. This can be achieved by forming an electrical connection between one end of a down conductor part and the threaded nut at one end of a stud bolt. In the same way, the threaded nut at the other end of the stud bolt is electrically connected to the down conductor part arranged in the other rotor blade segment. In the event of a lightning strike to the assembled rotor blade, the stud bolt forms part of the path for the lightning current.

As described above, a rotor blade is assembled by aligning the bores of the blade segments and arranging double-end stud bolts in the aligned bore pairs. A tightening tool is then engaged with the shaped portion of a double-end stud bolt and actuated to screw the threaded ends of each double-end stud bolt into the corresponding threaded nuts.

Preferably, the shaped portion is formed integrally with the body of the stud bolt, i.e. the bolt and the shaped portion are formed in one piece. Preferably, the shaped portion is formed at some point along the bolt to divide the bolt into two parts or bolt ends, one of which extends into a bore in the first rotor blade segment, while the other extends into a corresponding bore in the second rotor blade segment.

In one preferred embodiment of the invention, the shaped portion of a double-end stud bolt is realised in the form of a hexagonal nut that can engage with a correspondingly sized wrench. To connect two rotor blade segments, the wrench is used to successively tighten the bolts until all bolts have been tightened.

In an alternative preferred embodiment of the invention, the shaped portion is realised in the form of a sprocket to engage with links of a chain or belt comprising part of the tightening means. A suitable chain or belt can be arranged to engage with the outer teeth of all sprockets and then actuated so that all sprockets (and therefore all stud bolts) are simultaneously turned. Instead of a sprocket, the shaped portion of a stud bolt can be in the form of a cogwheel. In this case, the tightening means can comprise one or more suitably shaped cogwheels to engage with the stud bolt cogwheels, and may be realised to effect simultaneous tightening of all stud bolts.

Regardless of how the shaped portion of a stud bolt is realised, the bolt tightening means will generally require some room to manoeuvre in the space between rotor blade segments. Therefore, in a particularly preferred embodiment of the invention, the method of manufacturing a rotor blade comprises a step of forming a number of recesses at the connecting face of a rotor blade segment. The shape of the recess is determined by the type of tightening means that will be used, and also by the form of the shaped portion on the stud bolt. During assembly of the blade segments, a technician can deploy the tightening means until all bolts have been tightened and the blade segments are firmly joined at their connecting faces. The installation method preferably also includes a step of closing any gaps remaining after the tightening procedure is complete. The gaps can be closed by arranging appropriately shaped elements over the gaps and using adhesive to attach these elements to the rotor blade, for example. A sheet of adhesive film may be applied about the blade circumference to cover any resulting seam and to provide a smooth surface.

<FIG> shows an exemplary embodiment of the inventive rotor blade <NUM>, connected using double-ended stud bolts <NUM> shown in <FIG>. The bolts <NUM> extend in the direction of the longitudinal axis L of the rotor blade <NUM>. <FIG> shows two rotor blade segments <NUM>, <NUM> and a connecting interface <NUM>. One rotor blade segment <NUM> comprises the root end <NUM> of the blade, while the other blade segment <NUM> forms at least a section of the airfoil part of the blade <NUM>. The connecting interface <NUM> comprises several double-ended stud bolts <NUM> inserted into aligned bore pairs <NUM> of the rotor blade segments <NUM>, <NUM>. The threaded end <NUM>, <NUM> of each stud bolt <NUM> is screwed into a transverse nut <NUM> contained in a cavity <NUM>.

The bolt <NUM> shown in <FIG> has two co-linear ends <NUM>, <NUM> and an integral shaped portion <NUM>. Each of the two bolt ends <NUM>, <NUM> terminates in a threaded portion. The threaded portions <NUM>, <NUM> have opposite directions as indicated by the curved arrows. In this embodiment, the shaped portion <NUM> is in the form of a hexagonal nut <NUM>, and this stud bolt <NUM> can be tightened by using a suitable hex wrench. During tightening, the opposite threaded ends will "pull" the threaded nuts <NUM> towards the centre of the bolt <NUM> as indicated by the inwardly pointing straight arrows. In this exemplary embodiment, the transverse nuts <NUM> each have an elliptical cross-section (when viewed from above) so that, when placed in a correspondingly shaped machined cavity, the nut <NUM> stays aligned in the cavity <NUM> until the assembly procedure can commence. Each threaded end <NUM>, <NUM> of a stud bolt <NUM> enters a corresponding threaded bushing <NUM> in a transverse nut <NUM>. The diagram also indicates an additional threaded bushing <NUM> through the upper part of a transverse nut <NUM>. This will be used to connect a down conductor segment as will be explained below.

Returning to <FIG>, the diagram also indicates segments 21W, 22W of a web or spar in the interior of the rotor blade. In a one-piece rotor blade, the web would generally be formed as a single uninterrupted piece. Here, the rotor blade <NUM> is made by assembling two or more segments <NUM>, <NUM>, and the web therefore also be realised by structurally separate segments 21W, 22W. The diagram indicates a favourable shape for the ends of the web segments 21W, 22W so that loads are effectively transferred at the joint region.

The diagram also shows an edgewise load transfer means <NUM>, in the form of brackets <NUM>, <NUM> embedded in the rotor blade segments <NUM>, <NUM> and shaped with abutting outer ends. The outer ends of the brackets <NUM>, <NUM> can be secured by suitable fasteners. For example, the brackets <NUM>, <NUM> may be machined to have aligned bores or threaded bushings, and may be joined by a suitable combination of bolts, metal screws, nuts, etc..

<FIG> is a more detailed view of part of a connecting interface <NUM> and shows the orientation of the machined cavities <NUM> at the interior ends of the bores <NUM>. For the sake of clarity, only two bores are shown, one of which is used in a path of the LPS. At least three stud bolts, more generally ten or more, would be used to connect two blade segments. The diagram shows transverse nuts <NUM> contained in the cavities <NUM>. The elliptical body of a transverse nut <NUM> is held in place by the elliptically machined cavity <NUM> so that the transverse nut <NUM>, once it is placed in its cavity <NUM>, will not be able to turn. A nut <NUM> is placed in its cavity <NUM> so that the threaded bushing is aligned with the bore <NUM>.

In this embodiment, the stud bolts <NUM> have one long end <NUM> and one shorter end <NUM>, i.e. the integral nut <NUM> is not midway along the bolt <NUM>. The diagram also shows a spring-loading means <NUM> in the cavities <NUM> of the first rotor blade segment <NUM>. In this exemplary embodiment, the spring-loading means <NUM> is realised as a coiled spring <NUM> with a diameter that is slightly larger than the diameter of the threaded end <NUM> of the stud bolt <NUM>, and is arranged between the rear of the cavity <NUM> and the transverse nut <NUM> so that the annular shape of the coiled spring <NUM> is centred about the threaded bushing <NUM>. The spring <NUM> acts to press the nut <NUM> in the direction of the bore <NUM>, and simplifies the process of joining the two rotor blade segments <NUM>, <NUM>. As a stud bolt <NUM> is tightened against the transverse nuts <NUM>, its long end <NUM> will pass through the back end of a transverse nut <NUM> and into the empty space formed by the coiled spring <NUM>.

This diagram also shows a segmented down conductor 21C, 22C comprising a first segment 21C and a second segment 22C. One end of the first segment 21C is mechanically and electrically connected to the transverse nut <NUM> at one end of a stud bolt <NUM>, and the transverse nut <NUM> at the other end of that stud bolt <NUM> is mechanically and electrically connected to one end of the second segment 22C of the down conductor. The end of a down conductor segment 21C, 22C is preferably secured to the transverse nut <NUM> by preparing a threaded bushing at one of the outer ends of a transverse nut as shown in <FIG>. Metal screws can then be used to fasten the down conductor segments 21C, 22C to the transverse nuts <NUM>. When the nuts <NUM> and bolt <NUM> are made of steel, for example, they provide a safe route for lightning current in the event of a lightning strike to the rotor blade <NUM>.

<FIG> shows a view onto a connecting face 21F of a first rotor blade segment <NUM>. The diagram indicates the positions of precast composite components 21P, in this case precast spar caps 21P, embedded in the laminate of the rotor blade segment <NUM>, which may be assumed to be manufactured in a closed-mould technique so that there are no glue joints along the leading edge LE and trailing edge TE of the blade segment <NUM>. The bores <NUM> of a connecting interface are formed in the precast spar caps 21P before these are embedded in the laminate. A web segment 21W extends perpendicularly between the spar caps, and may be formed as shown in <FIG> for effective load transfer at the joint between blade segments.

The diagram also shows precast composite components 21B embedded as bracket holders 21B along the leading edge LE and parallel to the trailing edge TE. A bracket <NUM> of an edgewise load transfer means <NUM> is embedded in each bracket holder 21B.

<FIG> illustrates a stage in the in-situ assembly of the inventive rotor blade <NUM> for a wind turbine <NUM>. Initially, three inner blade segments <NUM> are mounted to the hub <NUM>, and this "star" assembly can be raised in one step and mounted at the top of the tower <NUM>. In this diagram, one rotor blade <NUM> has already been assembled, as shown in the upper left-hand side. The hub <NUM> has been turned to bring the next first blade segment <NUM> into a vertical orientation. A second blade segment <NUM> is being hoisted into place using a crane (not shown). The diagram indicates bolts <NUM> of a connecting interface extending from the connecting face of the second blade segment <NUM>. These bolts <NUM> will align with bores of the first blade segment <NUM>.

<FIG> shows two blade segments <NUM>, <NUM> shortly before the bolts <NUM> have been fully tightened. After tightening, the connecting faces 21F, 22F will lie against each other. The diagram indicates recesses <NUM> formed in the precast composite parts 21P, 22P to facilitate tightening of the bolts <NUM> using a hex wrench. To obtain a smooth aerodynamic surface without gaps or holes, the outer ends of the rotor blade segments <NUM>, <NUM> are designed to form a wide gap or recess with the same thickness and width as a cover ply. This shape can be obtained during the casting stage, or can be formed after casting in a milling step. Finally, a precast ply cover <NUM> is arranged to close this wide gap <NUM> around the perimeter of the rotor blade. This cover <NUM> can be glued into place using a suitable structural adhesive. The precast ply cover <NUM> can be realised as one or more thin flexible composite precast panels that can be bonded or even riveted into place on the outside surface of the assembled blade <NUM>.

Claim 1:
A wind turbine rotor blade (<NUM>) comprising at least a first rotor blade segment (<NUM>) and a second rotor blade segment (<NUM>) connected by means of a connecting interface (<NUM>), which connecting interface (<NUM>) comprises
- a plurality of bores (<NUM>) formed in a connecting face (21F, 22F) of each rotor blade segment (<NUM>, <NUM>), each bore (<NUM>) extending in a longitudinal direction (L) of the rotor blade (<NUM>) and terminating in a cavity (<NUM>) dimensioned to accommodate a threaded nut (<NUM>);
- a threaded nut (<NUM>) arranged in each of the plurality of cavities (<NUM>); and
- a plurality of double-end stud bolts (<NUM>), wherein a double-end stud bolt (<NUM>) comprises a shaped portion (<NUM>) formed between a first threaded end (<NUM>) and a second threaded end (<NUM>), which shaped portion (<NUM>) is shaped to engage with a bolt tightening means; and wherein the first threaded end (<NUM>) of each double-end stud bolt (<NUM>) is screwed into a threaded nut (<NUM>) held in a cavity (<NUM>) of the first rotor blade segment (<NUM>), and the second threaded end (<NUM>) of each double-end stud bolt (<NUM>) is screwed into a threaded nut (<NUM>) held in a cavity (<NUM>) of the second rotor blade segment (<NUM>).