Technical field

The present invention relates to wind turbine blades and more specifically to a reinforcing structure such as a spar cap used within a wind turbine blade to provide the blade with additional strength and rigidity.

Background

The design of composite wind turbine blades is a compromise that strikes a balance between aerodynamic and structural properties. The drive is now towards larger blades in order to maximise energy capture of the turbine installation. However, larger blades experience increased forces during use which increases the likelihood of the blades suffering damage.

A modern utility-scale wind turbine blade comprises an outer shell that is typically formed from two half shells joined together along their respective leading and trailing edges. The blade is reinforced to prevent it from bending excessively and, usually, each half shell incorporates one or more relatively stiff spar caps that run along the length of the blade. To provide the blade with the necessary strength to withstand the shear forces acting on it during operation, the opposing spar caps are interconnected by a shear web.

The spar caps may comprise a plurality of strips of fibre-reinforced polymer, such as carbon fibre reinforced plastic (CFRP) pultrusions, arranged in a stacked configuration. The spar caps may be formed in-situ during manufacture of the half shells, usually by stacking the strips in a blade mould together with the other blade materials in a shell layup. Resin may then be supplied to the lay-up in a resin-infusion process, such as a vacuum-assisted resin-infusion process. The resin must infiltrate between all of the layers of blade materials including, crucially, between the various fibre-reinforced polymer strips forming the spar cap. After infusing the lay-up, the resin is cured, typically by applying heat to the mould.

The strips forming the spar cap often have very flat and smooth surfaces. When the strips are stacked together there may be relatively little space between opposed surfaces of adjacent strips in the stack. Consequently, it can be difficult for resin to penetrate between the stacked strips during an infusion process. The structural integrity of the spar cap may be compromised if the strips are not sufficiently bonded together as a result of insufficient resin infiltrating between the stacked strips.

Bonding between stacked strips can be improved by using infusion-promoting layers, which may be interposed between the stacked strips of the spar cap. For example, WO2013/015736 describes an infusion-promoting layer. Such layers promote resin flow between the stacked strips during the infusion process.

Whilst the use of infusion-promoting layers such as those described in WO2013/015736 can increase the bond strength between the stacked strips of a spar cap, there still remains room for further improvement. In particular, there is a continual desire to further improve the adhesion between the stacked strips and to maximise the fracture toughness in the interfaces between the strips.

When designing wind turbine blades, it is also important to consider the electrical conductivity of the blade materials. Carbon-fibre is electrically conductive. However, the interfaces between the stacked strips of carbon fibre reinforced plastic can be problematic because the resin or adhesive in these interfaces has relatively low electrical conductivity. Therefore, the electrical conductivity between adjacent strips in the spar cap may be insufficient. This may lead to a high voltage potential occurring between adjacent strips in the event of a lightning strike which is undesirable.

It is against this background that the present invention has been developed.

Summary of the invention

According to a first aspect of the present invention there is provided a wind turbine blade comprising an elongate reinforcing structure. The reinforcing structure comprises a plurality of strips of fibre-reinforced polymer arranged in a stack, and an interface layer arranged between two adjacent strips in the stack. The interface layer comprises a fabric with primary fibres and secondary fibres. The primary fibres are mutually-aligned within the plane of the interface layer, and the secondary fibres are interspersed with the primary fibres. The secondary fibres are shorter than the primary fibres.

The primary fibres may also be referred to as ‘in-plane fibres’. The term ‘mutually aligned’ means that the fibres are aligned in one or more directions, for example they may be unidirectional or multi-directional (e.g. biaxial or triaxial), but they are not randomly- orientated fibres, such as in a chopped-strand mat. The secondary fibres may have any orientation including being oriented in the same plane as the primary fibres. Preferably, however, the secondary fibres extend transverse to the plane of the interface layer, in this case, the secondary fibres may therefore be referred to herein as ‘out-of-plane’ fibres. Preferably, the secondary fibres extend substantially perpendicular to the plane of the interface layer.

The primary fibres may be biaxial fibres. Preferably, if the primary fibres are biaxial fibres, the secondary fibres may extend transverse to, and preferably substantially perpendicularly to, the primary fibre directions of the biaxial primary fibres.

The secondary fibres may be mutually aligned. The secondary fibres may be mutually aligned in a direction that is transverse to the plane of the interface layer.

The primary fibres may be relatively long, whereas the secondary fibres may be relatively short. The primary fibres may typically be over 10cm in length. The primary fibres may have lengths in the range of 10cm to 10m. The secondary fibres may typically have lengths in the range of 10 micrometres to 2mm. Preferably, the secondary fibres have a length of less than 1mm.

The stacked strips of fibre-reinforced polymer preferably comprise carbon-fibres. The stacked strips of fibre-reinforced polymer may additionally or alternatively comprise other fibres, such as glass or aramid fibres. Preferably the strips are pultruded strips. Preferably, the strips of fibre-reinforced polymer are carbon-fibre pultrusions.

The primary fibres may be selected from carbon fibres, glass fibres or aramid fibres. Preferably the primary fibres are glass fibres. The primary fibres may be virgin fibres, preferably virgin glass fibres. “Virgin fibres” refers to fibres that have not been previously used.

The secondary fibres may be carbon fibres. The secondary fibres may be recycled fibres, preferably recycled carbon fibres. “Recycled fibres” means fibres that have been previously used.

The interface layer may be in the form of a mat. Preferably the interface layer is in the form of a fibre mat.

The interface layer may be made up predominantly from the primary fibres.

The length to diameter ratio of the secondary fibres may be less than 60:1 . The reinforcing structure may comprise a plurality of interface layers arranged between two adjacent strips in the stack.

The elongate reinforcing structure may be a spar cap. The elongate reinforcing structure may be embedded within an outer shell of the blade.

The strips of fibre-reinforced polymer may be pultrusions. Preferably, the strips of fibre- reinforced polymer are carbon-fibre pultrusions.

Brief description of the figures

Embodiments of the present invention will now be described by way of non-limiting example only, with reference to the accompanying figures, in which:

Figure 1 is a schematic cross-sectional view of a wind turbine blade according to an example of the present invention;

Figure 2 is a schematic perspective view of a wind turbine blade spar cap comprising a stack of strips with interface layers interposed between strips, according to an example of the present invention;

Figures 2a and 2b show alternative ways of forming the spar cap of Figure 2;

Figure 3 is a schematic plan view of an interface layer in accordance with an example of the present invention;

Figure 4a is a schematic side view of the interface layer of Figure 3; and

Figures 4b and 4c are schematic side views of variants of the interface layer.

Detailed description

Figure 1 shows a cross sectional view of a wind turbine blade 10 comprising a hollow outer shell 12 that has an airfoil profile. The outer shell 12 may be formed of a first half shell 14a and a second half shell 14b that are joined together to form the complete outer shell 12. Each half shell 14 may be a composite laminate structure comprising inner and outer laminate layers 16, 18 or ‘skins’ of material. In some examples, the laminate layers 16, 18 are formed of fibre reinforced plastic, such as glass reinforced plastic (GFRP). Lightweight core material 20, such as structural foam or balsa wood, may be provided between the inner and outer skins 16, 18 to form a sandwich-panel construction. In other examples, the core material 20 may be omitted, or included only in certain regions of the shell 20 where a sandwich structure is desirable to increase rigidity.

The wind turbine blade 10 comprises an elongate reinforcing structure 22. In some examples, the first and second half shells 14a, 14b each comprise an elongate reinforcing structure 22 in the form of a spar cap, as shown in Figure 1 . The spar caps 22 may also be known by other terminology in the art, such as ‘beams’ or ‘girders’ for example. In some examples, as shown in Figure 1 , the spar caps 22 are embedded between the inner and outer skins 16, 18, thereby forming an integral part of the outer shell 12. As such, the blade 10 may comprise a so-called “structural shell”. In other examples, the spar caps 22 may be connected to an inner surface 24 of the shell laminate. Preferably, the spar caps 22 of the respective half shells 14a, 14b are mutually opposed.

The spar caps 22 extend longitudinally in a spanwise direction (S), perpendicular to the plane of Figure 1 . Each spar cap 22 is configured to absorb bending loads experienced by the blade 10 in use, and may extend along part of, or substantially the entire length of the blade 10. A shear web 26 is connected between the spar caps 22 to take up shear loads experienced by the blade 10 in use. The shear web 26 may be connected between the spar caps 22 by adhesive 28 in some examples.

Preferably, the spar caps 22 are relatively lightweight and have a high tensile strength, i.e. the spar caps 22 are extremely stiff. As such, the spar caps 22 may be formed of carbon fibre reinforced plastic (CFRP) in some examples. Carbon fibres are generally preferred due to their very high strength-to-weight ratio.

Each of the spar caps 22 is formed of a plurality of strips of fibre-reinforced polymer 30 arranged in a stack 32. The strips 30 are preferably formed of carbon fibre reinforced plastic (CFRP), and are substantially flat, having a generally rectangular cross-sectional profile. The strips 30 have a high tensile strength, and therefore have a high load bearing capacity. The number of strips 30 in the stack 32 depends upon the thickness of the strips 30 and the required thickness of the spar cap 22. The number of strips 30 and/or thickness of the spar cap 22 may also be dependent on the required thickness of the shell 12 in examples where the spar caps 22 are embedded in the shell laminate. Typically, the strips 30 each have a thickness of a few millimetres and the spar cap 22 may comprise any number of strips 30 in the stack 32. The fibre-reinforced polymer strips 30 are preferably pre-fabricated before being integrated with, or connected to, the shell 12. The strips 30 may advantageously be formed in a pultrusion process in which resin-soaked reinforcing fibres are drawn through a pultrusion die so as to form a strip 30 having a substantially uniform cross section along its length. The strip 30 is passed through curing apparatus, such as an oven, to cure the resin. Strips 30 that are formed in a pultrusion process are referred to as ‘pultruded strips’ or ‘pultrusions’. The pultrusion process ensures that the reinforcing fibres in the strips 30 are aligned relative to one another and fixed in place in a cured resin matrix before the strips 30 are stacked to form the spar cap 22.

The half shells 14 of the wind turbine blade 10 may be formed in a composite moulding process in which components of the shell 14 are arranged in a blade mould to form a layup. One or more layers of fibrous reinforcing material may be arranged in the mould initially to form the outer skin 18 of the shell 14. Panels of core material 20, such as structural foam, may then be arranged on top of the fibrous material of the outer skin 18. In examples where the spar cap 22 is integrated with the shell 14, the panels of core material 20 may be spaced apart relative to one another to define a channel configured to receive the strips of fibre-reinforced polymer 30. In such examples, a plurality of strips of fibre-reinforced polymer 30, such as the pultruded strips of CFRP 30 described above, may then be arranged in the channel in a stack 32 to form the spar cap 22. In some examples, the strips 30 may be arranged together in a stack 32 first, i.e. pre-assembled, before the stack 32 as a whole is arranged in the channel.

One or more additional layers of fibrous reinforcing material may be arranged in the mould, on top of the shell components already in the mould, to form the inner skin 16 of the half shell 14. The fibrous reinforcing material forming the inner and outer skins 16, 18 of the half shell 14 may be dry fibrous material, such as dry glass fibre in chopped strand mats. Alternatively, the fibrous material may be pre-preg material which is pre-impregnated with resin. In some examples, the fibrous material may comprise both dry fibrous material and pre-preg fibrous material.

The shell materials in the mould are integrated with resin. In examples where the fibrous reinforcing material of the inner and outer skins 16, 18 comprises dry fibrous material, resin may be supplied to the lay-up in a resin-infusion process. Examples where the fibrous reinforcing material of the inner and outer skins 16, 18 comprises pre-preg fibrous material may also involve a resin infusion process to supply additional resin to the lay-up. Typically, the lay-up in the mould is integrated with resin in a vacuum-assisted resininfusion process. As such, a vacuum bagging film may be placed over the mould to cover the lay-up. Sealing tape may be used to seal the vacuum bagging film to a flange of the mould before air is evacuated from between the mould and the bagging film by a vacuum pump. Once a suitable partial vacuum has been established, resin may be introduced to the sealed volume through one or more inlets. The resin infuses between the various laminate layers and fills any gaps in the lay-up to integrate the various shell components. In some examples, once the lay-up has been infused with resin, the mould may be heated whilst the vacuum is maintained to cure the resin and bond the various layers together to form the half shell 14. In other examples, the resin may be allowed to cure naturally, i.e. without applying heat to the mould.

It will be appreciated that both the first and second half shells 14a, 14b may be formed in accordance with the above-described manufacturing process. The complete outer shell 12 may be formed in a join-up process wherein the half shells 14a, 14b are arranged one on top of the other with adhesive applied along leading and trailing edges 34, 36 to join the half shells 14a, 14b together. In some examples, the shear web 26 may be arranged between the half shells 14a, 14b and connected between the opposed spar caps 22 during the join-up process.

The resin-infusion process relies on there being suitable pathways for resin to infiltrate through and between the various components of the shell lay-up. As described by way of background, opposed surfaces 38, 40 of adjacent stacked strips 30 may be relatively smooth as a result of the process used to pre-manufacture the strips 30. As such, there may be very little space for resin to infiltrate between the opposed surfaces 38, 40 when resin is introduced to the lay-up. If resin cannot sufficiently permeate the interfaces between the stacked strips 30 of the spar cap 22, this may result in insufficient adhesion between the strips 30. In order to improve adhesion between adjacent strips 30, wind turbine blades 10 in accordance with the present invention comprise an interface layer 42 (shown in Figure 2) arranged between adjacent stacked strips 30 in the spar cap 22 as will now be described in further detail with reference to Figure 2.

Figure 2 shows an elongate reinforcing structure 22 for a wind turbine blade 10. In the examples described below, the elongate reinforcing structure 22 is a spar cap like that described above with reference to Figure 1 , and will be referred to as such from now on. In Figure 2 the spar cap 22 is shown on a larger scale so that its individual components can be seen more clearly. The spar cap 22 comprises a plurality of strips of fibre-reinforced polymer 30 (preferably CFRP) arranged in alignment one on top of another in a stack 32. The longitudinally-extending strips 30 comprise upper and lower surfaces 38, 40 which are opposed to corresponding lower or upper surfaces 40, 38 of adjacent strips 30 in the stack 32.

The reinforcing structure 22 comprises an interface layer 42 interleaved between adjacent strips 30 in the stack 32. In some examples, the stack 32 may comprise interface layers 42 interleaved between each of the adjacent strips 30. Further, in some examples (not shown), the reinforcing structure 22 may comprise a plurality of interface layers 42 between each of the adjacent stacked strips 30. The interface layers 42 may have substantially the same dimensions as the strips 30 when seen in a plan view. As such, the interface layers 42 may cover substantially the entirety of the upper or lower surface 38, 40 of a strip 30 in some examples.

Figures 2a and 2b show two possible methods by which the stack 32 of reinforcing strips 30 may be assembled. For example, as shown in Figure 2a, the strips 30 may be stacked on top of one another with separate interface layers 42 arranged between the adjacent strips 30 of the stack 32. The strips 30 and the layers 42 may be aligned to form the stack structure 32. In other examples, the strips of fibre-reinforced polymer 30 may comprise an interface layer 42 connected to a surface 38 or 40 of the strip 30 as shown in Figure 2b. For example, the strips 30 may include an interface layer 42 integrated with the lower surface 40 of the strip 30. Alternatively, the strips 30 may comprise an interface layer 42 connected to or integrated with the upper surface 38 of the strip 30. As such, the interface layers 42 and strips 30 may not all be separate components when arranging the stack 32. This may improve alignment between the strips 30 and interface layers 42, and may also reduce the number of process steps required to arrange the stack 32, making manufacture of the spar cap 22 and shell 14 more time-efficient.

Typically, the spar cap 22 is formed in-situ during manufacture of the shell 14 as described above with reference to Figure 1 , i.e. where the resin used to integrate the shell lay-up also infuses between the strips of fibre-reinforced polymer 30 to form an integrated spar cap 22 and shell 14 in a single process. However, in other examples, the spar cap 22 may be formed separately, for example in a separate composite moulding process before being connected to, or integrated with, the shell 14. For example, the spar cap 22 may be formed separately by arranging the strips of fibre-reinforced polymer 30 in a stack 32 in a spar cap mould, with interface layers 42 between adjacent strips 30 in the stack 32 as previously described.

Regardless of whether the spar cap 22 is formed separately or in-situ with the shell 14, the stack 32 of strips 30 is consolidated to form the spar cap 22 by providing and curing resin to integrate the strips 30. When providing resin to the stack 32, the resin infiltrates around the stack 32 and through the interface layers 42 between the strips 30. The presence of the interface layers 42 establishes an interstice or “infusion region” between a pair of adjacent strips of fibre-reinforced polymer 30. Because the interface layer 42 separates the opposing surfaces 38, 40 of the adjacent strips 30, the flow and infusion of resin between the strips 30 is improved, and any dry spots between the strips 30 are avoided. The interface layer 42 will now be described in more detail with reference to the remaining figures.

Figure 3 shows a portion of an interface layer 42 in one example in a schematic plan view. The interface layer 42 is a fabric or ‘mat’ comprising primary fibres 44 and secondary fibres 46. The primary fibres 44 are mutually-aligned (i.e. non-random) within the plane of the interface layer 42. As such, the primary fibres 44 may be referred to as “in-plane fibres”. The primary fibres 44 may be unidirectional fibres, but in preferred examples the primary fibres 44 are multi-axial fibres. For example, as shown in Figure 3, the primary fibres 44 may be biaxial fibres, which are oriented at +/- 45 degrees. The primary fibres 44 may be any suitable reinforcing fibres, such as carbon fibres, glass fibres or aramid fibres for example. In preferred examples, the primary fibres 44 are glass fibres, and most preferably the primary fibres 44 are virgin glass fibres.

The secondary fibres 46 are interspersed with the primary fibres 44 in the interface layer 42. In examples where the primary fibres 44 are multi-axial fibres, such as biaxial fibres, these primary fibres 44 effectively form a lattice which is particularly beneficial for containing the secondary fibres 46. The secondary fibres 46 are shorter than the primary fibres 44, as shown most clearly in Figure 4a, for example, which shows a schematic side view of the interface layer 42. The short secondary fibres 46 may be any suitable reinforcing fibres, such as carbon fibres, glass fibres or aramid fibres. In preferred examples, the secondary fibres 46 are carbon fibres, and more preferably the secondary fibres 46 may be recycled carbon fibres. Recycled fibres are readily available from decommissioned wind turbine blades and can be advantageously re-used as the secondary fibres 46 in this invention. When composites structures are decommissioned, the structure may be cut up and the fibres may be separated from the polymer matrix so that they can be recycled. As the fibres have been cut, they cannot normally be used in a new structure as they do not have the required length. However, in this case, as the secondary fibres are short they can advantageously comprise cut recycled fibres.

The short secondary fibres 46 may have any orientation, including random orientations. Preferably, however, the secondary fibres 46 are mutually aligned. The secondary fibres 46 may be arranged in the plane of the interface layer 42 in some examples, however, preferably at least some of secondary fibres 46 extend transverse to the plane of the interface layer 42. As such, the secondary fibres 46 may be referred to as “out-of-plane fibres”. In particularly preferred examples, a majority or substantially all of the secondary fibres 46 may extend transverse to the plane of the interface layer 42.

In the example shown in Figures 3 and 4a, the secondary fibres 46 are mutually aligned substantially perpendicular to the plane of the interface layer 42. Figure 4b shows an example of an interface layer 42 in which the secondary fibres 46 are mutually-aligned in a direction transverse to, but not perpendicular to, the plane of the interface layer 42. Figure 4c shows a further example in which the secondary fibres 46 extend transverse to the plane of the interface layer 42 but are randomly orientated. In other examples, the secondary fibres 46 may include both in-plane and out-of-plane fibres.

As mentioned above, the secondary fibres 46 are relatively short in comparison to the primary fibres 44. The primary fibres 44 may have lengths in the range of 10cm to 10m. The secondary fibres 46 may have lengths in the range of 10 micrometres to 2mm, preferably less than 1mm. Preferably, the length to diameter ratio of the secondary fibres 46 is less than 60:1.

The interface layer 42 is preferably made up predominantly from the primary fibres 44. The secondary fibres 46 therefore preferably constitute a minority of the volume percentage of the interface layer 42.

It has been found that the inclusion of short fibres 46 in the interface layer 42 increases the fracture toughness of the interface between the strips of fibre-reinforced polymer 30. The short fibres 46 help to distribute and transfer stresses between adjacent strips 30, and thereby ensure that problematic stress concentrations do not develop in the interfaces between strips 30. The reduction or elimination of stress concentrations in the interfaces makes the interfaces less susceptible to developing cracks. In the event that any cracks should still develop, the distributed short fibres 46 also serve to limit the propagation of such cracks. The advantages are particularly pronounced when the short fibres 46 are oriented out of the plane of the interface layer 42, i.e. transverse to the direction(s) of the primary fibres 44. In such examples, the interface layer 42 includes both in-plane and out- of-plane reinforcing fibres 44, 46. This further increases the strength and fracture toughness of the spar cap 22 because the interface between the strips 30 is able to resist loads acting in a plurality of different directions.

As discussed by way of background, although carbon pultrusions 30 or other polymer strips 30 reinforced with carbon fibres, are electrical conductors, electrical conductivity may be poor in the resin-rich interfaces between adjacent strips 30. For example, the surfaces 38, 40 of fibre-reinforced polymer strips 30, such as pultrusions, tend to be formed of cured polymer resin. In combination with the resin in-between the strips 30, the stack 32 may therefore comprise a relatively thick insulating region between the carbon fibres of adjacent pultrusions 30.

In some examples, the electrical conductivity of the interface may be increased by the inclusion of an interface layer 42 between the fibre-reinforced polymer strips 30. In particular, the electrical conductivity of the interface is increased in examples where the secondary fibres 46, i.e. the short fibres, in the interface layer 42 are electrically conductive fibres, such as carbon fibres for example. The electrical conductivity is particularly improved when the short fibres 46 are oriented transverse to the plane of the interface layer 42. Orienting the short, secondary fibres 46 substantially perpendicular to the plane of the interface layer 42 is most preferable because the fibres 46 provide the most direct electrical pathway between the adjacent strips/pultrusions 30 in such an orientation.

The interface layers 42 described herein may be provided in the form of a dry fabric, which is subsequently infused by resin during a resin infusion process as described above. In other examples, one or more of the interface layers 42 may be provided in the form of prepreg material, which is fibrous material that is pre-impregnated with resin. Providing the interface layer 42 in the form of pre-preg material ensures that resin is present between the strips of fibre-reinforced polymer 30 and may therefore provide a particularly reliable bond between adjacent strips 30 in the stack 32 and further minimise dry spots between the strips 30. In some examples, the plan dimensions of the or each interface layer 42 may not correspond exactly to the plan dimensions of the strips of fibre reinforced polymer 30 in the stack 32. For example, the interface layers 42 may be wider and/or longer than the strips 30 in the stack 32 such that the interface layer 42 forms a flange around the stack 32. This may help to draw in resin from around the stack 32 in some examples. Alternatively, the interface layer 42 may be smaller than the strips 30 in length and/or width to ensure that the layer 42 does not overlap any other shell components in the lay-up. This also reduces the risk of the interface layer 42 becoming wrinkled.

Examples of how the stack 32 of fibre-reinforced polymer strips 30 may be formed have been described previously with reference to Figures 2a and 2b. In some other examples, the reinforcing structure 22 may be formed of a stack 32 of strips 30 wherein one or more strips 30 are connected to an interface layer 42, and one or more strips 30 are not connected to an interface layer 42 before forming the stack 32. For example, the stack 32 may comprise a strip 30 that is not connected to an interface layer 42, and which is sandwiched between adjacent strips 30 that do have an interface layer 42 connected to their upper surface 38 and lower surface 40 respectively such that, when the stack 32 is arranged, the interface layers 42 are between each of the pairs of adjacent strips 30. In other examples, a strip 30 having interface layers 42 connected to both its upper and lower surfaces 38, 40 may be interposed between strips 30 that do not have an interface layer connected to their respective lower and upper surface 40, 38. Interface layers 42 may therefore only be connected to every second strip 30 in some examples whilst still providing that interface layers 42 are between each of the adjacent strips 30 when the stack 32 is arranged.

It will be appreciated that in examples wherein the reinforcing structure 22 is connected to an inner surface 24 of the shell 14, the method for making the shell 14 is substantially the same as described above except for the inclusion of the stack 32 of strips 30 in the lay-up. In such an example, the shell 14 is therefore formed by arranging layers of fibrous material and core material 20 in the mould, with the core material 20 sandwiched between the layers of material forming the inner and outer skins 16, 18 of the shell 14. The components in the mould may then be integrated with resin which is cured, or allowed to cure, to form the shell 14 as described above. A spar cap 22 as described above may be formed separately before being connected to the inner surface 24 of the shell 14, for example by adhesive. Many modifications may be made to the examples described above without departing from the scope of the present invention as defined in the accompanying claims. It will be appreciated that features described in relation to each of the examples above may be readily combined with features described with reference to any other examples described without departing from the scope of the invention as defined by the following claims.