Patent Publication Number: US-2022213867-A1

Title: Method of forming a wind turbine blade

Description:
TECHNICAL FIELD 
     The present invention relates generally to a method of forming a wind turbine blade and, in particular, to a method of forming a wind turbine blade comprising a main blade module and a separate edge module such as a pre-manufactured trailing edge. 
     BACKGROUND 
     In modern wind turbines, the provision of serrations along the trailing edge of the rotor blades is a commonly-used technique for reducing noise from the blades. In particular, such serrations act to reduce the noise generated at the turbulent trailing edge boundary layer while minimising any reduction in aerodynamic efficiency of the blades. 
     It is known to provide a separate part including serrations which is retrofitted to a wind turbine blade. Often, the separate part is attached at the trailing edge of the blade, typically by bonding it to the pressure side of the blade, so that the serrations extend outward from the trailing edge. Despite efforts to minimise the thickness of such separate serrated trailing edge parts, attaching such separate parts in this way can lead to a step at the point where the separate part attaches to the surface of the rotor blade, in particular where the edge of the separate part attaches to the surface of the rotor blade. Such a step can create turbulence and noise. 
     In order to address these issues, it has been proposed to manufacture a wind turbine blade that includes serrations at the trailing edge, thus obviating the need to retrofit a separate part. In particular, it has been proposed to provide a rotor blade formed of two parts: a main blade part or module that forms the majority of the aerodynamic profile of the blade; and, a separate edge part or module including serrations at the trailing edge. 
     Joining the main blade and separate edge modules together during the manufacturing process can pose difficulties. For example, moving the main blade and separate edge modules to bring them together can be challenging because of, for example, the scale of the parts, the complex or asymmetric geometry of the parts, and the fact that the parts are formed from delicate materials. Furthermore, the parts need to be joined together in such a way that there is a smooth transition, i.e. no step, at the surface of the rotor blade to avoid the generation of noise or turbulence at this transition point. 
     It is against this background to which the present invention is set. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention there is provided a method of forming a wind turbine blade. The wind turbine blade comprises a main blade module defining a main body of the blade and including a first mating feature. The wind turbine blade comprises a separate edge module defining at least part of a trailing edge of the blade and including a second mating feature. The method comprises applying an adhesive to at least one of the first mating feature and the second mating feature. The method also comprises arranging the separate edge module relative to the main blade module such that the first and second mating features are mutually adjacent. The method also comprises applying a pressure force to squeeze the adhesive to bond the first and second mating features together. The pressure force is caused by removing air from, or injecting air into, an air sealed region. 
     The method may comprise locating a pressure distributor against the separate edge module to distribute the pressure force that is applied to the first and second mating features. 
     The method may comprise providing a consolidator at an interface between the main blade module and the separate edge module at an outer surface of the blade. The consolidator may be arranged to ensure that the separate edge module is flush with the main blade module at the interface when the pressure force is applied. 
     The first mating feature may be a tongue and the second mating feature may be a recess complementary to the tongue. Arranging the separate edge module relative to the main blade module may include receiving the tongue into the recess. 
     The separate edge module may comprise at least one vent hole extending between the recess and an outer surface of the separate edge module, and wherein squeezing the adhesive causes the adhesive to flow through the at least one vent hole. 
     Arranging the separate edge module relative to the main blade module may comprise: positioning the main blade module so that the tongue extends in a generally upwards direction; and, placing the separate edge module on the main blade module, wherein the tongue and the recess are shaped so that the separate edge module self-locates relative to the main blade module when placed thereon. The main blade module may be positioned in a cradle, guide or gauge, which may be shaped to verify the profile of the completed blade. 
     The air sealed region may encapsulate at least the first and second mating features. The method may comprise forming the air sealed region after the separate edge module has been arranged relative to the main body module, and the method may comprise removing air from the sealed region to create a vacuum and cause the pressure force to be applied. In this way the mating features are compressed, and therefore bonded, together. 
     The air sealed region may be defined by a deformable vacuum bag. 
     The method may comprise providing an end of the separate edge module that defines the at least part of the trailing edge with a protective cover prior to forming the air sealed region. Advantageously, this guards against damage being caused to the trailing edge, or a trailing edge feature such as serrations, when the vacuum is created and the vacuum bag is compressed against the separate edge module. 
     The air sealed region may be defined by one or more inflatable airbags, also referred to as bonding airbags. The method may comprise injecting air into the one or more inflatable airbags to cause the force to be applied by the inflatable airbags applying pressure to an outer surface of the blade in the vicinity of the first and second mating features. 
     The pressure force applied to the outer surface may be applied progressively in a chordwise direction of the blade towards the trailing edge of the blade. For example, the pressure force may be applied progressively from an open end of the recess to a closed end of the recess. This may be achieved by the provision of a row or series of bonding airbags adjacent to the blade outer surface that are pressurised sequentially or progressively in a direction towards the closed end of the recess. 
     The method may comprise injecting the one or more inflatable airbags with air at a temperature greater than ambient temperature. 
     Arranging the separate edge module relative to the main blade module may comprise positioning the main blade module so that the first mating feature extends in a generally upwards direction. The arranging step may comprise clamping the separate edge module in a clamp. The clamp may comprise inflatable airbag clamps, wherein clamping the separate edge module comprises injecting air into the air sealed inflatable airbag clamps. The arranging step may comprise positioning the clamped separate edge module generally above the main blade module. The arranging step may comprise lowering the separate edge module onto the main blade module and using an alignment device to maintain alignment between the main blade module and the separate edge module when arranging the first and second mating features to be mutually adjacent, the alignment device being coupled to the clamp, for example by a frame. 
     The alignment device may comprise a plurality of alignment rollers that roll against an outer surface of the main blade module as the separate edge module is lowered to maintain alignment of the separate edge module with the main blade module. 
     According to another aspect of the invention there is provided a wind turbine blade. The blade comprises a main blade module defining a main body of the blade and including a first mating feature. The blade comprises at least one separate edge module defining at least part of a trailing edge of the blade and each including a second mating feature. The wind turbine blade is formed according to the method described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a wind turbine having a plurality of wind turbine blades formed according to the invention; 
         FIG. 2( a )  is a schematic view of a prior art wind turbine blade arrangement having a serrated trailing edge piece attached at a trailing edge of a wind turbine blade; FIG.  2 ( b ) is a schematic view of a part of the wind turbine blade of  FIG. 1  and having a separate edge module mounted to a main blade module; 
         FIG. 3  shows a schematic view of the separate edge module of  FIG. 2( b ) ; 
         FIG. 4  shows schematic views of a frame having inflatable airbags and rollers for arranging and bonding together the main blade module and separate edge module of  FIG. 2( b ) : in particular,  FIGS. 4( a ) and 4( b )  show perspective and sectional views of the frame, respectively; 
         FIG. 5( a )  is a schematic view of the frame of  FIG. 4  in which the separate edge module is clamped by the frame and positioned above the main blade module, and  FIG. 5( b )  shows the frame and separate edge module lowered onto the main blade module; 
         FIGS. 6( a )-( c )  show partial sectional views of the main blade module, separate edge module, and inflatable airbags of  FIG. 4 : in particular,  FIG. 6( a )  shows the main blade module and the separate edge module arranged to be mutually adjacent, and the airbag in a deflated state,  FIG. 6( b )  shows the airbag in a partially inflated state so that a pressure force is applied to the separate edge module at an end opposite to the trailing edge, and  FIG. 6( c )  shows the airbag in a fully inflated state in which the pressure force is applied to a greater amount of the separate edge module; 
         FIGS. 7( a ) and 7( b )  show schematic views of a cradle supporting a plurality of the main blade modules and a plurality of the separate edge modules of  FIG. 2 , where the main blade modules and separate edge modules are arranged to be mutually adjacent; 
         FIG. 8  shows a schematic sectional view of the main blade module and separate edge module encapsulated by a deformable vacuum bag; and, 
         FIG. 9  shows the steps of a manufacturing method to form the wind turbine blade(s) of  FIG. 1  from the main blade module and separate edge module of  FIG. 2( b ) . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a wind turbine wind turbine ( 10 ). The wind turbine  10  includes a tower  12 , a nacelle  14  rotatably coupled to the top of the tower  12 , a rotor including a rotor hub  16  mounted to the nacelle  14 , and a plurality of wind turbine rotor blades  18  and which are coupled to the rotor hub  16 . The rotor  16  comprises three blades  18 , but in other examples the rotor  16  may have any suitable number of blades  18 . The nacelle  14  and rotor blades  18  are turned and directed into the wind direction by a yaw system. The nacelle  14  houses generating components (not shown) of the wind turbine  10 , including the generator, gearbox, drivetrain and brake assembly, as well as convertor equipment for converting the kinetic energy of the wind into electrical energy for provision to the grid. The wind turbine  10  is shown in its fully-installed form suitable for operation; in particular, the rotor  16  is mounted on the nacelle  14  and each of the blades  18  are mounted on the rotor and rotor hub  16 . 
       FIG. 2( a )  shows part of a prior art wind turbine rotor blade  100  having attached thereto a separate part  102  including a plurality of serrations  104 . In particular, the separate part  102  is bonded to the blade  100  at a pressure side  106  of the trailing edge  108  of the blade  100 , so that the serrations  104  extend outwards, i.e. downstream, from the trailing edge  108 . It is noted that a step  110  is formed at the transition between the pressure-side surface  106  of the blade  100  and the separate part  102 . This step change in thickness causes noise and/or turbulence. 
       FIG. 2( b )  shows a part of one of the rotor blades  18  of the wind turbine  10 . The blade  18  is formed by a plurality of parts or modules that are manufactured separately and then joined together. In particular, the blade  18  is formed by one or more main blade modules  20  and a plurality of separate edge modules  22 . The one or more main blade modules  20  form the majority of the aerofoil shape or profile of the blade  18  and define a leading edge (not shown) of the blade  18 . The edge modules  22  are premanufactured modules, i.e. they are made separately from the main blade module  20 , and attached to the main blade module  20  to form the wind turbine blade  18 . Each of the separate edge modules  22  define part of a trailing edge  24  of the blade  18 , and the separate edge modules  22  are connected or arranged end-to-end along the main blade module  20  to define the trailing edge  24 . In particular, the separate edge modules  22  may be connected or clicked together prior to joining the main blade module  20 . Hence each edge module  22  may be referred to as a premanufactured trailing edge (PMTE) module. For example, each of the separate edge modules  22  may be approximately 250 mm in length (in a spanwise direction of the blade). The separate edge modules  22  may be non-identical to conform to the profile of the blade  18 . In the described example, the separate edge modules  22  each include a plurality of serrations  26  at the trailing edge end. 
     The main blade module  20  is preferably formed from composite materials, for example fibre-reinforced plastic such as glass-fibre reinforced plastic (GFRP). The main blade module  20  may be formed using any suitable technique known for forming wind turbine blade shells, e.g. vacuum-assisted resin transfer moulding (VARTM). The edge module  22  may also be formed from composite materials, e.g. GFRP or just plastic, and may be a moulded part. This may also be formed by VARTM, injection moulding or other suitable technique. Alternatively, the edge module  22  could be made from an elastomeric material, such as rubber. 
     With continued reference to  FIG. 2( b ) , and additional reference to  FIG. 3 —which shows the edge module  22  without the serrations—the main blade and separate edge modules  20 ,  22  include respective complementary mating features  28 ,  30  for joining the modules  20 ,  22  together. In particular, the mating feature of the main blade module  20  is in the form of a tongue  28 , and the mating feature of the separate edge module  22  is in the form of a recess  30  (as shown in  FIG. 3 ) arranged to receive the tongue  28 . The recess  30  of the edge module  22  is at, and faces, an end of the edge module  22  opposite to an end that defines the trailing edge  24  of the blade  18 . 
     Methods of bringing together and bonding the main blade and separate edge modules  20 ,  22  will now be described. 
       FIGS. 4( a ) and 4( b )  show schematic perspective and sectional views of an apparatus or arrangement  32  for forming the wind turbine blade  10  from the main blade and separate edge modules  20 ,  22 . The arrangement  32  includes a jig or frame  34  with frame bars defining a cuboid frame shape that is open at a lower side  36  so as to receive the modules  20 ,  22  therein. 
     The arrangement  32  includes a clamping mechanism or arrangement, or simply a clamp, for gripping and moving the separate edge modules  22 . In particular, the clamp includes at least one pair of clamping airbags or bladders  38  attached to the frame  34  and facing each other, and between which the edge module  22  is received. The clamping airbags  38  define an air sealed region  39  and may be inflated by injecting air into the sealed region  39 . This causes the clamping airbags  38  to apply a pressure force to both sides of the edge module  22  to grip or clamp the edge module in place relative to the frame  34 . This allows the edge module  22  to be moved via movement of the frame  34 . In particular, lifting and moving the separate edge module  22  using the inflatable and flexible clamping airbags  38  reduces the risk of the separate edge modules  22  being crushed or otherwise damaged during this process. The use of airbags also allows the jig  32  to lift and move edge modules  22  that have different geometries. 
     The arrangement  32  includes an alignment device or arrangement for maintaining the relative positions of the main blade and separate edge modules  20 ,  22  when they are being brought together and/or bonded together. In particular, the alignment device in the described example is in the form of pairs of rollers or wheels  40   a ,  40   b  attached to the frame  34 , and between which the main blade and separate edge modules  22  are received. Specifically, two pairs of the rollers  40   a  are located and spaced apart to receive the edge module  22  therethrough so that the rollers  40   a  contact the edge module  22  when it is received therethrough. Similarly, two more pairs of the rollers  40   b  are located lower in the frame  34  and spaced apart to receive the main blade module  20  therethrough so that the rollers  40   b  contact the main blade module  20  when it is received therethrough. The rollers  40   a ,  40   b  provide a flexible means for ensuring alignment of the modules  20 ,  22  that also guards against damage being caused to the modules  20 ,  22 . 
     The apparatus  32  also has an angled receptor  42  attached to a horizontal bar  44  internal to the frame  34  which is arranged to receive, and maintain the position of, the end of the edge module  22  that defines the trailing edge  24  of the blade  18 . 
     The apparatus  32  also includes pairs of clips  46  along an upper end of the frame  34 . These allow the apparatus  32  to be connected to a hoist, for example, so as to move the apparatus  32  with clamped edge module  22  into position relative to the main blade module  20 , where the edge module  22  may be lowered onto the main blade module  20 , as described below. 
     The apparatus  32  also includes at least one pair of bonding airbags or bladders  48  attached to the frame  34  and facing each other, and located adjacent to the tongue  28  and recess  30  when the main blade and separate edge modules  20 ,  22  are brought together. The bonding airbags  48  define an air sealed region  49  and may be inflated by injecting air into the sealed region  49 . This causes the bonding airbags  48  to apply a pressure force to both sides of the modules  20 ,  22  to bond them together, as described in greater detail below. 
     With reference to  FIGS. 5( a ) and 5( b ) , a method of forming the wind turbine blade  10  is now described.  FIG. 5( a )  shows in schematic form the main blade module  20  arranged or positioned such that the end of the blade having the tongue  28  extends in a generally upwards direction. The main blade module  20  may be held or supported in a structure for maintaining this orientation. 
     A layer of adhesive  52 , for example an adhesive resin, is applied to a surface of the tongue  28 , which will act to bond the tongue  28  and recess  30  together when the main blade and separate edge modules  20 ,  22  are brought together. In addition, or alternatively, the adhesive may be applied to a surface of the recess  30 . 
     The apparatus  32  is used to securely clamp or grab the separate edge module  22 . In particular, the edge module  22  is received into the apparatus so that the trailing edge  24  is received into the angled receptor  42  so as to guard against bending of the trailing edge  24 . The rollers  40   a  roll against the surface of the edge module  22  as it is received into the apparatus so as to maintain the position and orientation of the edge module  22 . When the edge module  22  is received into the apparatus  32  the clamp airbags  38  are in a deflated state. Once the edge module  22  is in position it is clamped in place by inflating the clamp airbags  38  so that a pressure force is applied by the airbags  38  on either side of the edge module  22 . 
     The apparatus  32  is then moved, for example by a hoist connected to the clips or hooks  46 , to position the edge module  22  generally above the main blade module  20 , with the recess  30  of the edge module  22  extending generally downwards towards the tongue  28  of the main blade module  20 . Of course, the adhesive layer may instead be applied at this point instead of prior to the edge module  22  being clamped and moved into position. 
     The apparatus  32  is then lowered from the position generally shown in  FIG. 5( a )  to the position generally shown in  FIG. 5( b ) . In particular, the edge module  22  is lowered onto the main blade module  20  such that the tongue  28  and recess  30  fit together and are mutually adjacent. As the apparatus  32  is lowered, the rollers  40   b  contact, and roll relative to, the surface of the main blade module  20  to maintain alignment between the main blade module  20  and the separate edge module  22 . 
     The main blade and separate edge modules  20 ,  22  are then bonded together by injecting air into the bonding airbags  48  which forces the tongue  28  and recess  30  together and bonds them by means of the adhesive. This is described in greater detail with reference to  FIGS. 6( a ), 6( b ) and 6( c ) . 
       FIG. 6( a )  shows a schematic sectional view of the tongue  28  and recess  30  when they have been brought together to be mutually adjacent by means of the apparatus  32  so that the bonding airbags  48  are adjacent thereto. The bonding airbags  48  are in a deflated state such that they do not apply a pressure force to the main blade or separate edge modules  20 ,  22 .  FIG. 6( a )  shows that there is a gap  50  defined between the tongue  28  and recess  30  and that the tongue  28  has a layer of adhesive  52  applied to it. 
       FIG. 6( b )  shows the bonding airbags  48  in a partially inflated state. In particular, the bonding airbags  48  are inflated in a manner such that the airbags  48  contact, and apply a pressure force at an open end  54  of the recess  30  of the edge module  22  opposite to a closed end  56  of the recess  30 , as shown in  FIG. 6( b ) . This forces the open end  54  of the recess  30  of the edge module  22  towards the tongue  28  and to squeeze the adhesive  52  between them. Specifically, by applying the pressure force at the recess open end  54  in the first instance the adhesive  52  is squeezed further into the recess  30  towards the closed end  56 . At this stage, the closed end  56  of the recess (also referred to as a glue or adhesive chamber  56 ) starts to fill up with the adhesive  52 . The pressure force is then applied progressively from the open end  54  to the closed end of the recess  30  by sequential inflation of a series of the airbags  48  from the open end  54  towards the closed end  56  Expressed differently, the pressure force is spread out in a chordwise direction of the blade  18  in a direction towards the trailing edge  24  of the blade. Advantageously, such progressive application of the pressure force causes the flow of adhesive  52  to be in a direction into the recess  30  towards its closed end  56  and therefore both prevents the adhesive  52  flowing out of the open end  54  of the recess  30  (which can cause a step at the blade surface) and facilitates complete filling of the gap  50  between the tongue  28  and recess  30 , thereby improving the quality of the bond. In addition, application of the force in this sequential or progressive manner may prevent the formation of undesirable air cavities in the bond. 
       FIG. 6( c )  shows the bonding airbags  48  in a fully inflated state, in which the airbags  48  apply a pressure force all the way along the surface of the edge module  22  in the vicinity of the recess (not labelled in  FIG. 6( c ) ), i.e. all the way from the recess open end  54  to the recess closed end  56  of the recess  30 . This forces the gap (not labelled in  FIG. 6( c ) ) between the tongue  28  and recess  30  to be closed, and the main blade and separate edge modules  20 ,  22  to be bonded all the way along the tongue  28  and recess  30  from the open end  54  to the closed end  56 . The adhesive  52  is squeezed further into the recess  30  until the adhesive chamber  56  is completely full. The edge module  22  may include one or more vent holes  58  that extend from the closed end  56  of the recess  30  to the outer surface of the edge module  22 . Once the closed end  56  is filled with the adhesive  52 , continued application of the pressure force causes the adhesive  52  to flow through the vent holes  58 . Once the adhesive  52  reaches the surface of the edge module  22 , and the adhesive  52  has cured to a sufficient degree to maintain a bond between the main blade and separate edge modules  20 ,  22 , the bonding process is complete and inflation of the airbags  48  may be stopped, or deflation of the airbags  48  may be permitted. The provision of the vent hole(s)  58  is a relatively simple way to determine when the adhesive has completely filled a gap between the mating features  28 ,  30 . Any adhesive  52  that exits the vent holes  58  at the surface of the blade  18  may easily be removed to ensure the rotor blade surface remains smooth. This may be achieved by, for example, abrading the cured adhesive  52  away from the surface of the blade  18 . 
     The bonding airbags  48  may include a number of embedded heating elements (not shown) at or near a surface of the airbags  48  that contacts the separate edge module  22 . Such heating elements would increase the temperature at which the bonding process between the main blade and separate edge modules  20 ,  22  takes place, which can reduce the time taken for the bond to cure. Alternatively, instead of using embedded heating elements, the bonding airbags  48  may be injected with heated air which would also increase the temperature at which the bonding process takes place, thereby reducing the cure time between the main blade and separate edge modules  20 ,  22 . 
     The bonding airbags  48  have a flexible surface and so application of the force by the airbags  48  to the surface of the blade  18  is less likely to cause damage to the blade  18  than application of a force by a solid part. Application of a force using such a method requires relatively little effort and manpower. 
       FIGS. 7( a ) and 7( b )  show perspective views of the wind turbine blade  18  being supported in a cradle or guide  60 . In this example, the cradle  60  supports the main blade module  20  when the main blade and separate edge modules  20 ,  22  are being bonded, as will be described below. In particular, instead of applying a pressure force by inflating one or more airbags as in the above-described example, in the example described below a deformable bag encapsulates the tongue  28  and recess  30  (not shown in  FIGS. 7( a ) and 7( b ) ) to form an air sealed region that includes the tongue and recess. Air is then removed from the sealed region to create a vacuum therein, which causes a pressure force that squeezes the tongue and recess together. Such an approach is a particularly simple and inexpensive way to provide an air pressure force to cause bonding of the mating features, and needs minimum tooling. 
     In the described example, the cradle  60  is formed by separate cradle components spaced apart and arranged and shaped to receive the main blade module  20  therein. In particular, the main blade module  20  is positioned in the cradle  60  so that its tongue  28  extends in a generally upwards direction. Specifically, a side of each cradle  60  includes a hinge  61  that allows the cradle  60  to be opened up to receive the main blade module  20  therein. 
     The separate edge modules  22  may then be placed on the main blade module  20  to complete the aerodynamic profile (as shown in  FIGS. 7( a ) and 7( b ) ). The tongue  28  and recess  30  (not shown in  FIGS. 7( a ) and 7( b ) ) of the main blade and separate edge modules  20 ,  22 , respectively, are shaped such that the separate edge module  22  self-locates on the main blade module  20  when placed thereon. The cradle  60  is also shaped to support the separate edge module  22  in its self-locating position when placed on the main blade module  20 . Once the main blade module  20  and separate edge module  22  are in place, the cradle  60  is closed back up to verify the shape of the blade  18 . 
     The cradle  60  includes a datum reference, e.g. in the form of a cable, bar or laser projection between each of the separate cradle components, which indicates how far the separate edge module  22  is to be lowered when being placed on the main blade module  20 . 
     The separate components of the cradle  60  may equally be connected together by further components therebetween. In addition, the cradle  60  may include a platform or walkway to allow an operator to access the separate edge modules  22  when they are placed on the main blade module  20 . 
     Prior to placing the separate edge module  22  on the main blade module  20 , and similarly to the above-described example, a layer of adhesive is applied to the surface of the tongue  28  and/or the recess  30  (not shown in  FIGS. 7( a ) and 7( b ) ), to be used to bond the modules  20 ,  22  together. 
     With additional reference to  FIG. 8 , a method of bonding the main blade module  20  and separate edge module  22  together is now described when the modules  20 ,  22  are positioned in the cradle  60  such that the tongue  28  and recess  30  are mutually adjacent.  FIG. 8  shows a schematic sectional view of the main blade module  20  positioned in the cradle (not shown) so that the tongue  28  extends in a generally upwards direction. The separate edge module  22  is placed on the main blade module  22  so that the tongue  28  is received in the recess  30 .  FIG. 8  also shows the presence of the gap  50  when the tongue  28  and recess  30  are mutually adjacent, but prior to them being bonded together by means of the adhesive layer. 
     In the described example, a consolidator  62  may optionally be placed at the interface between the main blade module  20  and the separate edge module  22  at the outer surface of the blade  18 . That is, the consolidator  62  is placed in the region of the opening of the gap  50  at the blade surface. The consolidator  62  is in the form of strips that overlap the interface between the modules  20 ,  22  at the blade surface. In the described example, the consolidator strips  62  may be formed from plywood covered with polytetrafluoroethylene (PTFE), e.g. Teflon. The consolidator strips  62  aim to ensure that the separate edge module  22  is flush with the main blade module  20  at the interface when a force is applied to bond the modules  20 ,  22  together. The provision of the consolidator  62  helps to guard against the formation of a step or ridge at the interface between the modules  20 ,  22 , the presence of which at the surface of the blade  18  would be a source or noise and/or turbulence when the blade  18  is in operation. The consolidator  62  can also assist in guarding against the adhesive  52  flowing out of the interface during the bonding process. 
     In the described example, a pressure distributor  64  may optionally be placed against the outer surface of the separate edge module  22 . In particular, the pressure distributor  64  is located adjacent the recess  30  of the separate edge module  22 . In the described example, the pressure distributor  64  is in the form of a layer of perforated hardboard, e.g. pegboard, covered by a layer of mesh formed by connected strands of metal, fibres, or any other suitable flexible material. The pressure distributor acts to distribute the force that is applied to join the main blade and separate edge modules  20 ,  22  together, in particular to distribute the force applied to the separate edge module  22  adjacent to the recess  30 . Advantageously, the provision of such a distributor  64  may distribute the applied force across the outer surface of the blade  18 , or a particular region thereof, to ensure the force is applied in an even manner and so provide an even bond between the modules  20 ,  22 . 
     With the consolidator  62  and pressure distributor  64  in place, a deformable vacuum bag  66  is then placed around the arrangement. In particular, the vacuum bag  66  encapsulates the separate edge module  22  and part of the main blade module  20  including the tongue  28 . The vacuum bag  66  is sealed by attaching it to the surface of the main blade module  20  along a sealing line or at sealing points  68 . Note that the consolidator  62  and pressure distributor  64  are therefore also encapsulated by the vacuum bag  66 . Although in the described example the vacuum bag  66  encapsulates the entire separate edge module  22  including the trailing edge  24 , in different examples the vacuum bag  66  may encapsulate the recess  30 , but not the trailing edge  24 , of the edge module  22 , and have sealing points attached to the surface of the separate edge module  22 . 
     A tube (not shown) or other means for removing air is provided to draw air out from the air sealed region  70  defined by the vacuum bag  66 . In particular, the tube is used to remove air from the sealed region  70  to create a vacuum in the sealed region  70 . This results in the pressure in the sealed region being lower than outside of the sealed region, and this difference in pressure causes a pressure force to be applied. The arrows  72  in  FIG. 8  indicate schematically the direction of the force caused by the pressure difference. Specifically, the pressure force causes the tongue  28  and recess  30  to be forced together thereby closing the gap  50 . In turn, the adhesive layer on the tongue  28  and/or recess  30  is therefore squeezed, causing the main blade and separate edge modules  20 ,  22  to be bonded together. 
     As mentioned above, when the pressure force is applied the consolidator  62  acts to ensure that the main blade and separate edge modules  20 ,  22  are bonded in such a manner that the interface between these modules  20 ,  22  at the surface of the blade  18  is flush. The consolidator  62  also acts to ensure that the adhesive does not spew or flow out at the surface of the blade  18  in a manner that would result in a ridge or step at the blade surface. 
     In order to protect the trailing edge  24 , in particular the serrations  26  (not shown in  FIG. 8 ), from being damaged when the vacuum pressure force is applied, a protective cover or shield (not shown) may be provided to cover the end of the separate edge module  22  that defines the trailing edge  24  prior to forming the air sealed region  70  with the vacuum bag  66 . In particular, the protective cover may be in the form of a material which guards against the vacuum bag  66  pressing against, damaging and/or crushing the serrations  26  when air is removed from the sealed region  70  to cause the vacuum. 
       FIG. 9  summarises steps of the method  80  performed to form the wind turbine blade  18  in the above-described example. At step  82  an adhesive is applied to either or both of the mating features, i.e. the tongue  28  and recess  30 , of the main blade and separate edge modules  28 ,  30 . At step  84  the main blade and separate edge modules  20 ,  22  are arranged relative to each other so that the mating features  28 ,  30  are mutually adjacent. In the example shown in  FIGS. 4-6 , this step involves clamping the separate edge module  22  using the clamping airbags  38  and lowering the frame  34  and clamped edge module  22  onto the upwardly-facing main blade module  20 . In the example shown in  FIGS. 7-8 , this step involves positioning the main blade module  20  in the cradle  60  and placing the self-locating edge module  22  on the main blade module  20 . At step  86  a pressure force is applied to squeeze the adhesive  52  to bond the first and second mating features  28 ,  30  together. In the example shown in  FIGS. 4-6 , the pressure force is caused by injecting air into the bonding airbags  48  whose surface then presses on the surface of the separate edge module  22  to press the mating features  28 ,  30  together. In the example shown in  FIGS. 7-8 , the pressure force is caused by removing air from the sealed region  70  to create a vacuum which results in the mating features  28 ,  30  being forced together. 
     In each of the described examples, a pressure force is applied to squeeze the adhesive to bond mating features together, in particular by removing air from, or injecting air into, an air sealed region. Application of a force that is caused by a change of air pressure is advantageous for the bonding process because it facilitates force being applied to parts having complex geometry, e.g. a curved surface such as that of the aerofoil profile of a rotor blade, and being formed of delicate, lightweight materials. In particular, application of an air pressure force to a localised region may be advantageous compared with application of force caused by, for example, a solid part pressing against the main blade and/or separate edge module in that such a solid part is unlikely to conform to the shape of the surface of the complex-shaped modules, potentially causing uneven application of the force and therefore a sub-optimal bond between the modules. Also, such a solid part is unlikely to provide a flexible application of force, thus increasing the risk of causing damage to the modules during the bonding process. 
     Many modifications may be made to the above-described examples without departing from the scope of the present invention as defined in the accompanying claims. 
     In the above-described example, the separate edge modules include serrations at the trailing edge. In different examples, however, the separate edge modules may additionally or alternatively include different features for controlling or disrupting the flow at the trailing edge, e.g. vortex generators. Indeed, in certain examples the separate edge modules may simply define the trailing edge of the rotor blade without any features such as serrations. 
     In the above-described examples, the mating features of the main blade and separate edge modules are a tongue and recess, respectively. In different examples, however, the main blade module may instead have a recess and the separate edge module may instead have a tongue. Indeed, in further different examples the mating features need not be in the form of a tongue and recess, and any suitable mating features for joining the modules together may be used. 
     The pressure distributor and consolidator are described above in connection with the example in which the vacuum is used to bond the main blade and separate edge modules together. Note, however, that one or both of the pressure distributor and consolidator may also be used in connection with the other example that uses the bonding airbags to bond the modules together. 
     The vent holes are described above in connection with the example in which the bonding airbags are used to bond the modules together; however these may also be present in the above-described vacuum bag example. In both of these example, any adhesive that spews out from the vent holes during the bonding process may be removed, e.g. using a knife, to ensure the outer surface of the rotor blade is smooth.