Abstract:
A method of forming a structural connection between a spar cap  14  and an aerodynamic fairing  12 . A composite comprising an uncured matrix and a compressible solid is applied between the spar cap and fairing and is then compressed and cured to adhere the fairing to the spar cap. The cured matrix composite has a void volume of at least 20%. The high void volume means that as the fairing is compressed into place and compresses the composite, it has space in which to deform so as not to place undue stress on the fairing and to produce a lightweight connection.

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     This application is a continuation of Patent Cooperation Treaty International Patent Application PCT/GB2013/052508, filed Sep. 26, 2013, and entitled “A METHOD OF FORMING A STRUCTURAL CONNECTION BETWEEN A SPAR CAP AND A FAIRING FOR A WIND TURBINE BLADE,” which is incorporated by reference herein in its entirety, and which claims priority to United Kingdom Patent Application GB1217210.2, filed on Sep. 26, 2012. 
    
    
     BACKGROUND 
     Field 
     The present invention relates to a method of forming a structural connection between a spar and an aerodynamic fairing and in particular to a spar and an aerodynamic fairing for a wind turbine blade. 
     Description of the Related Art 
     Modern wind turbine blades are typically made by separately manufacturing a structural beam or spar which extends along the length of the blade and two half shells, or aerodynamic fairings, which are attached to the spar to define the aerodynamic profile of the blade. 
     A typical method of attaching the fairings to the spar is shown in  FIGS. 1A-1C . In this method, an adhesive  2  is applied to the upper and lower surfaces of the spar  4  and the fairings  6  are placed over the adhesive  2  to enclose the spar  4 , as shown in  FIG. 1B . Typically, a cavity with a thickness of between 5 mm and 50 mm will exist between the upper and lower surfaces of the spar  4  and fairings  6 , and the adhesive  2  fills this cavity. The fairings  6  are then clamped against the spar  4  to squeeze the adhesive  2  and bond the fairings  6  to the spar  4 . As the forces required to squeeze the adhesive  2  are high, the fairings  6  are typically kept in the tools in which they were formed for the step of bonding each fairing  6  to the spar  4 . 
     However, as the fairings  6  are pressed against the spar  4 , the adhesive applies pressure to the inner surface of the fairings  6 . This can lead to distortion of the fairings  6  and the tools (not shown) in which they are held, resulting in a distorted aerodynamic surface  8 , as shown in  FIG. 1C . Thus, the tools must be of sufficient structural stiffness to prevent such distortion. 
     Although acceptable for smaller blades, the above technique can be extremely expensive when used to bond larger blades, such as those which are 45 meters or more in length. This is due to the cost of building a tool large enough to accommodate the fairings, stiff enough not to distort, and which can be lifted and closed accurately. 
     US 2009/0226702 is directed to an adhesive joint for use in joining various wind turbine components. This document recognises a problem with excessive adhesive used in these joints. In particular, it is not possible to remove this excess adhesive in a closed structure. Such excess adhesive may break off in use and cause problems such as clogging drainage holes and causing damaging impact forces. In order to overcome this problem, this document proposes providing a porous layer which extends beyond the adhesive joint. Once the space in the region of the joint around the porous member is fully filled with adhesive, excess adhesive will be squeezed into the parts of the porous layer outside of the joint. It is then retained during use within this porous layer and the problems of the loose lumps of adhesive are solved. The document does not address the assured distortion of the fairings and tools which is addressed by the present invention. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided a method of forming a structural connection between a spar cap and an aerodynamic fairing for a wind turbine blade, comprising the steps of applying a composite between the spar cap and the fairing, the composite comprising an uncured matrix and a compressible solid, compressing the deformable solid so that the composite substantially occupies a space between the spar cap and the fairing, curing the matrix to maintain the compressible solid in its compressed state with the composite having a void volume of at least 30%, and adhering the fairing to the spar cap as the matrix is cured. 
     With this arrangement, the composite applies a pressure to the spar cap and the inside surface of the fairing which is sufficient to ensure that the bonding surfaces are well connected but which is low enough to prevent distortion of the fairing during assembly. The relatively low pressure exerted by the composite is largely a factor of the significant void volume. This means not only that the matrix must have a relatively low density, but also readily allows the matrix to be compressed as there is ample space into which the deformable material can be depressed without creating undue resistance against the fairing. The void volume also helps to reduce the weight of the joint. 
     A void volume of at least 20% allows the low pressure advantage set out above. However, in practice, the void volume can be significantly higher provided that the structural integrity of the joint is maintained. Lower void volumes provide additional weight benefits. Thus, the void volume may preferably be greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%. 
     Also, unevenness in the fairing, spar, or composite can be smoothed out by the compressible solid as it deforms in the space between the spar cap and fairing. For example, where the space between fairing and spar cap is locally reduced, the compressible solid will be compressed more, whereas an incompressible traditional adhesive is likely to distort the fairing. Thus, a sound structural connection can be formed without inducing large or uneven pressures on the fairing. 
     Further, by curing the matrix to hold the compressible solid in its deformed state, the composite can provide a structural connection between the fairing and the spar cap which resists buckling of the fairing. 
     The adhering of the fairing to the spar cap may be carried out by the matrix. The matrix may comprise an epoxy resin and/or a structural adhesive. 
     Alternatively, an additional adhesive may be provided, wherein the adhering of the fairing to the spar cap is carried out at least in part by the additional adhesive. This will increase the adhesion between the fairing, composite and spar cap. 
     In a preferred embodiment, the matrix is non-foaming. This prevents excessive expansion of the composite which may lead to fairing distortion. 
     The deformable solid is preferably approximately 20% thicker than the widest part of the cavity when in an uncompressed state. 
     Any suitable compressible solid may be used, although preferably the compressible solid comprises a first porous layer facing the fairing and a second porous layer facing the spar cap, the first and second porous layers being separated by a third layer which has a greater thickness and a lower density that the first and second layers. 
     Thus, this low density layer forms a significant proportion of the compressible solid and its lower density allows for the void volume. The relatively high density of the first and second layers which are porous means that resin or adhesive from the third layer can pass through the first and second layers into contact with the fairing and spar cap respectively, providing a large surface area which bonds with the fairing and spar cap respectively. 
     The first and second layers may be of any material which provides a relatively high surface area and allows the adhesive to pass through, such as a moulded plastic mesh. However, they are preferably a fibrous structure which may be a non-woven felt-like structure, but is preferably woven. 
     The third layer may be any compressible material which can support the upper and lower layers in their uncompressed state and which, in situ, has a high enough void space so that once the matrix is applied and the composite is compressed and cured, it is able to provide the required void volume. The third layer in its uncompressed state and prior to application of the matrix preferably has a void volume of at least 30%, more preferably 40% and most preferably 50%. The third layer may, for example, be an open cell foam, but is preferably formed of fibres which are woven or stitched between the first and second layers. For such a structure, the fibres of the third layer are generally perpendicular to the first and second layers such that, in use, they will bridge the gap between the fairing and spar cap thereby providing good support for the fairing once the matrix has cured. 
     The composite may be formed by adding the matrix once the three-dimensional fabric has been placed between the fairing and the spar cap. In a preferred embodiment, the three-dimensional fabric is impregnated with the matrix prior to the step of applying the composite between the spar cap and the fairing. This simplifies the assembly process. One way of applying the matrix is to pass the compressible solid through a bath containing the matrix. The impregnated composite is then passed through a pair of rollers, the space between which can be adjusted to squeeze the composite to a greater or lesser extent thereby removing as much of the matrix as necessary to achieve the required void volume in the finished product. 
     The method is suitable to produce a wind turbine blade of any length. In a preferred embodiment, the blade is at least 45 meters in length. 
     According to a second aspect of the present invention, there is provided a wind turbine blade comprising a spar with at least one spar cap, a fairing positioned over the spar cap, and a composite which substantially fills a space between the spar cap and the fairing, wherein the composite comprises a cured matrix, a compressed solid and a void volume of at least 20%. 
     The composite may be arranged to adhere the spar cap to the fairing. This reduces the assembly steps required. Alternatively, the blade further comprises an additional adhesive arranged to at least partially adhere the spar cap to the fairing. This supplements any adhesion provided by the composite, or, in the case where substantially no adhesion is provided by the composite, provides the adhesion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIGS. 1A to 1C  are schematic views of a fairing and a spar of a wind turbine blade connected using a conventional method; 
         FIG. 2  is a perspective view of a wind turbine blade according to the present invention; 
         FIG. 3  is a schematic partial section view through line  3 - 3  of  FIG. 2 , showing the structural connection between fairing and spar; 
         FIG. 4  is a schematic perspective view of a deformable solid for use with the blade of  FIG. 2 ; and 
         FIG. 5  is a schematic partial section view of a first alternative structural connection between fairing and spar, showing the deformable solid in a deformed state. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , the wind turbine blade  10  has an aerodynamic fairing  12  which encloses and is attached to a structural spar  14  extending along the length of the blade  10 . The aerodynamic fairing  12  is a semi-rigid structure, such as a GFRP comprising fibreglass fibres, for instance PPG Hybon® 2002, and a supporting matrix of epoxy resin, for instance Dow Airstone® 780. 
     As shown in  FIG. 3 , the fairing  12  is positioned over the spar  14  such that a cavity  16  is defined between the fairing  12  and a spar cap  18  at the upper surface of the spar  14 . A composite  20  is disposed between the fairing  12  and the spar cap  18  and substantially fills the cavity  16 . The composite  20  comprises a three-dimensional woven fibreglass fabric, such as Parabeam® PG18, and a cured supporting matrix of epoxy resin, such as Dow Airstone® 780, which is bonded to both the fairing  12  and the spar cap  18  to form a structural connection between the fairing  12  and the spar  14 . By “structural connection” it is meant that the composite  20  forms a connection with sufficient shear strength, compressive strength, adhesion and compressive stiffness that the fairing  12  remains attached to the spar  14  during use and is not caused to buckle by the forces induced on the blade  10  during use. 
     With reference to  FIG. 4 , the three-dimensional fabric  22  comprises two substantially parallel fibrous face sheets  24  between which extend resilient connecting fibres  26  woven into the face sheets  24 . The fibres  26  are arranged to provide a very high void space between the first and second sheets (in this case, 94-97%). This is a measure of the total volume occupied by the air between the sheets  24  as a percentage of the total volume between the sheets  24 . A space entirely filled with fibres, would represent a void volume of 0%, while the absence of any fibres represents a void volume of 100%. This is a measure of the void space in the uncompressed fabric prior to the application of the resin. 
     To form the structural connection, a layer of the three dimensional fabric  22 , which is thicker than the cavity  16 , is pre-impregnated with an uncured epoxy resin  23 . This is done by passing the fabric through a bath of uncured resin and then passing the coated fabric through the pair of rollers. This allows the gap between the rollers to be adjusted to achieve the required amount of resin desired to give the required void space in the finished article. The resin will cling to the fibres (as shown in  FIG. 4  which shows just three such coated fibres—the remainder will be coated in practice), but does not fill the space between the sheets  24 . The density of the sheets  24  is such that most, if not all, of the space between the fibres in the sheets  24  is filled by the resin in order to ensure a good contact surface to both the spar  14  and fairing  12 . 
     The composite is placed on the spar cap  18  of the spar  14 . In this example, the layer of fabric  22  is approximately 20% thicker than the widest point of the cavity  16 . The fairing  12  is then placed over the composite  20  to form the cavity  16  and held in place by assembly jigs (not shown) until the resin has cured to form the structural connection. 
     As the connecting fibres  26  of the fabric  22  are deformable, the uncured composite  20  behaves like a mattress and can be likened to an elastic foundation. Thus, the composite  20  is compressed between the fairing  12  and the spar cap  18  such that it takes the shape of the cavity  16 . When compressed, the fabric  22  of the composite  20  applies an even pressure to the inside of the fairing  12  which is sufficient to ensure that bonding surfaces are well connected but not so great that the aerodynamic surface of the fairing  12  is distorted. In this manner, the structural connection is formed without the need to apply large external forces to squeeze the adhesive and risk damaging the tooling and fairing  12 . 
     Once the resin has cured, the hardened composite  20  should form a sufficient structural connection between the spar  14  and the fairing  12 . In other words, the structural properties of the fabric  22  and the resin should be selected such that the cured composite  20  has sufficient shear strength, compressive strength, and compressive stiffness characteristics for a given installation. In this manner, the fairing  12  will remain attached to the spar cap  18  and can resist buckling which may otherwise result during use of the blade  10 . 
     In order to provide the required deformability and compressive strength in the finished article, the composite should have a void volume of at least 30% in its compressed and cured state. In this example, the void space is 80%. The cured, compressed material preferably has a density of 160-300 kg/m 3 . This allows plenty of space for the fabric to deform when compressed as set out above. It should be noted that the void volume is the void volume of the composite material. Any region of the material which contains only fibres and no matrix material is a single phase material and not composite. Thus, any such regions are excluded when determining the void volume. Thus, for example, US 2009/0226702 has certain regions which have a 0% void volume where the adhesive is present and other regions where only the porous layer is present which do not represent part of the composite. 
     To allow the fairing  12  to be connected to the structural spar  14  without any significant deformation, the relationship between the stiffness of the fairing  12  and that of the composite  20  in its uncured state should be in the region of: 
     
       
         
           
             
               
                 K 
                 s 
               
               
                 K 
                 f 
               
             
             ≤ 
             
               1.25 
               
                 - 
                 10 
               
             
           
         
       
         
         
           
             Where: 
             K s  is the foundation stiffness of the composite  20  in its uncured state, in this case corresponding to the stiffness of the fabric  22 , measured as elastic modulus per unit thickness (Pa/M) and defined as the quotient of the elastic modulus of the foundation material and the total thickness of the foundation material; and 
             K f  is the bending stiffness of the fairing  12 , measured in units of pressure (Pa) and defined as the product of the elastic modulus and area moment of inertia of the constitutive elements of the fairing  12 . Bending stiffness can also be considered as a structure&#39;s resistance to curvature under an applied loading. 
           
         
       
    
     Referring to  FIG. 5 , a traditional structural adhesive  28 , such as a regular filled epoxy adhesive, could be applied to the three dimensional fabric  22  to assist in the bonding of the fairing  12  to the spar  14 . 
     Although the spar has been described as comprising a spar cap, it may be a simple beam, for example a box-section beam. 
     Rather than being formed from upper and lower shells, the aerodynamic fairing could be formed from any number of shells. 
     Although the deformable solid has been described in the first embodiment as a three dimensional fabric, any suitable resilient and compressible material may be used. 
     In addition, although the composite has been described as three dimensional fabric with a resin matrix, the matrix could be a structural adhesive. 
     The uncompressed thickness of the three dimensional fabric could be more or less than 20% thicker than the thickness of the cavity between fairing and spar, depending on the compressibility characteristics of the fabric. 
     Although the three dimensional fabric is described as being pre-impregnated with resin, the resin could be added to the fabric in situ, for example by injecting into the cavity between fairing and spar cap while they are held in place by the assembly jig. 
     The composite could be placed on the fairing prior to placing the fairing on the spar cap, rather than being placed on the spar cap.