GB2489477A - Spar for a water driven turbine blade and manufacture thereof - Google Patents

Abstract

A spar, especially for use in a water-driven turbine blade, and a method of its manufacture, the spar 32 comprises substantially planar stack of first sheets 34 secured together, each first sheet comprising a fibre reinforced composited material, wherein the opposed edges of the first sheets define opposed edges of the stack, the edges of the stack defining a mounting portion for mounting the spar to a turbine member and at least one elongate blade supporting portion. The method of manufacture includes the steps of laying up and curing a plurality of plies of fibre reinforced resin to form a laminate sheet, cutting the sheet to form the opposed edges and assembling the sheets in a stack to form the spar. Also claimed is a water driven turbine blade which may be adapted for use in an axial or cross flow tidal turbine, formed of the spar located between first and second blade surfaces 40, 42.

Description

SPAR FOR A TURBINE BLADE AND MANUFACTURE THEREOF

The present invention relates to a spar for a turbine blade and to a method of manufacturing a spar for a turbine blade, and in particular to a turbine blade for use as a turbine blade in a tidal generator. The present invention also relates to a turbine blade incorporating such a spar, and to a method of manufacturing such a turbine blade.

Many current axial-flow tidal turbine designs resemble horizontal axis wind turbines (HAWTs) in their general configuration. Typically, two, three or four blades radiate outwardly from a central hub. Other designs use helical blades in a cross-flow configuration.

In almost all cases there is a tendency for blades to be designed with relatively slender shapes for optimum hydrodynamic properties, typically to achieve high lift/drag ratios.

Compared to wind turbine blades, tidal turbine blades generate the same power with a smaller area, due to the higher density of the fluid (water for tidal turbines, versus air for wind turbines). For a similar thickness/chord ratio of the blade section, this means that bending moments and shear forces must be carried by a smaller cross-section, To avoid excessive stresses in the material, more of the blade cross-section must be filled with material. If the blade is made of metal, e.g. steel, it can become prohibitively heavy. If the blade is made of composite material, such a fibre reinforced resin composite material, this is not a problem from a weight perspective, because the blade is supported to a large extent by its buoyancy in the water. However, there is a challenge in fifing sufficient material into a small cross-section using a typical hollow spar designs as used on wind turbines. As well as high bending moments, tidal turbine blades see relatively high shear forces because those bending moments arc developed over a shorter length than in a wind turbine blade.

If the loads are high enough, it may be necessary to make the spar solid for at least a part of its length. This raises challenges in manufacture, as both surfaces of the blade (suction side and pressure side) must be of fair shape; typically this is achieved by moulding the blade shell andlor spar in two halves. Joining the two halves of a solid spar results in high shear stresses at the joint, which may exceed the capacity of typical high-strength adhesives or biaxial secondary bonding laminate, particularly towards the tip where the blade is thinnest.

Furthermore, because the spar is tapered in thickness along its length and of non-rectangular cross-section, the process of layering composite materials into moulds is time-consuming and difficult to automate for efficient production.

Finally at the inboard (root) end of the blade, it must be joined to the rest of the rotor.

Achieving a strong and reliable connection which can withstand many years' fatigue loads and extreme loads under water is a challenge. If conventional known wind-turbine blade connections are used, such as T-bolts or bonded inserts to a pitch bearing or similar hub connection, the root end of the blade needs to be disproportionately large compared to the aerofoil part of the blade: for instance a 1MW wind turbine may have blades of approximately 30m length whereas a tidal turbine of similar power output will need blades of approximately only Him length. All the other dimensions will scale roughly in the same proportion except the root diameter. Because the bending moments at the root of the tidal turbine blade are similar to those of the wind turbine blade, with similar joint design it will be necessary for both types of blade to have a root diameter of the order of 1,Sm. The aerofoil section of the 1MW wind turbine blade might be almost im in thickness whereas the 1MW tidal turbine blade may be only about O.3m thick at the start of the aerofoil section of the blade. Thus the tidal turbine requires a disproportionately steep transition from the (smaller) aerofoil section to the root section, to avoid a large part of the blade length being taken up by this transition region, which is thicker than the optimum aerofoil hence would cause excessive drag if it extended too far outboard. The problem with such a steep transition using hollow spar designs such as used on wind turbine blades is that the curvature of the faces of the spar generates Brazier loads that tend to deform the spar under load.

In a conventional wind turbine blade, as shown in Figure 1 which is a cross-section through a blade 2, there is typically a central spar 4 formed as a hollow box section, which is made by laminating composite material onto a male mandrel, which after removal provides a central longitudinal cavity 6. The blade surfaces 8, 10 are affixed to opposed sides of the spar 4. The spar 4 includes two opposed outer spar cap portions 12, 14 integrally connected by two opposed transversely oriented inner shear web portions 16, 18 Alternatively, as shown in Figure 2, the spar 20 is made from two opposed mutually spaced bodies known in the art as spar caps 22. The spar caps 22 are joined together to form a unitary spar 20 by opposed shear webs 24, 26. The spar caps 22 are composed primarily of unidirectional fibres running longitudinally along the blade axis; their purpose is to carry bending moments. The shear webs 24, 26 are composed primarily of fibres laid at +1-45 degrees to the axial direction; their purpose is to carry shear forces.

In both of these known structures there is a limit to how thick the spar portions or spar caps can become because, for a given blade thickness, as the spar caps are progressively thickened the shear webs become progressively less deep in the transverse direction and hence become unable to carry the shear forces. In the box spar method of Figure 1 it becomes impractical to make the spar if the spar caps are too thick because the mandrel becomes very thin. If the box spar is instead made in upper and lower parts in a female mould the joint between them becomes too weak to carry shear forces adequately. Furthermore in Figure 2 the adhesive joint between the spar caps and the shear webs becomes too weak to carry very high shear forces to which tidal turbine blades are subjected during use.

Attempts have been made by various companies to manufacture blades for 1MW tidal turbines. Currently, to the knowledge of the Applicant, none of these have been successftil.

So there is a need for an economically viable method of making blades for tidal turbines that are strong enough to work at the megawatt scale.

The present invention aims at least partially to meet that need.

Accordingly, the present invention provides spar for a water-driven turbine blade, the spar comprising a substantially planar stack of first sheets secured together, each first sheet comprising fibre reinforced resin composite material, wherein opposed edges of the first sheets define opposed edges of the stack, the edges of the stack defining a mounting portion for mounting the spar to a turbine member and at least one elongate blade supporting portion.

The present invention fi.irther provides a water-driven turbine blade including a spar according to the present invention, the turbine blade including opposite first and second surfaces each defining a respective load bearing blade surface, and the spar being located between the first and second surfaces, the first sheets of the stack lying in a plane extending between the first and second surfaces.

The present invention further provides a method of manufacturing a spar for a water-driven turbine blade, the method comprising the steps of: a) laying up a plurality of plies of fibre reinforced resin composite material to form a substantially planar laminate thereof; b) curing the resin to form a cured sheet from the laminate; c) cutting the sheet to form opposed edges which define a part of a mounting portion for mounting the spar to a turbine member and a part of at least one elongate blade supporting portion; and d) assembling together a substantially planar stack of the cut sheets to form a spar, the opposed edges of the sheets being arranged to form opposed edges of the stack, the edges of the stack defining the mounting portion for mounting the spar to a turbine member and the at least one elongate blade supporting portion.

The present invention further provides a method of manufacturing a water-driven turbine blade, the method comprising manufacturing a spar according to the method of the invention, and assembling a blade shell around the spar.

Preferred features are defined in the dependent claims.

Preferred embodiments of the present invention provide a tidal turbine blade whose spar is made up of cured plies arranged in a plane approximately perpendicular to the aerofoil chord line or blade surface, i.e. in a plane parallel to the primary lift force on the blade.

In this specification, the turbine blades incorporating the spars of the present invention may be used in any type of turbine which generates electrical power from flow of water relative to the turbine blade, which causes motion of the turbine blade which in turn drives an electrical generator, either directly or via a hydraulic pump. Depending upon the turbine design, the turbine is most typically disposed in the sea, but may be disposed in a river, or even a lake or other body of moving water. Most typically, the turbine is a tidal turbine which is adapted to cause electrical power generation from tidal motion, either on a rising sea tide or a reverse falling sea tide, with the turbine moving by water flow relative thereto in either of two opposite directions.

The turbine is typically an axial flow turbine, in which plural radial turbine blades are fixed to a rotatable hub which is caused to rotate about an axis by water movement over the turbine blade surfaces. Alternatively, the turbine is a cross flow turbine, in which the turbine blades are fixed to a support and are caused to rotate, for example in an oscillating motion, about a longitudinal axis of the turbine blade by water movement over the turbine blade surfaces, the turbine blade therefore acting similar to an aerofoil. However, the turbine into which the spars of the spars of the present invention may be incorporated may have any configuration adapted for use with moving water providing the driving force for motion of the turbine in order to generate electrical power.

In particular, the preferred embodiments of the present invention relate to a tidal turbine blade spar, and its method of manufacture, in which the cured plies of composite material in the spar are arranged substantially perpendicular to the aerofoil chord line, instead of being substantially parallel to it, as used in the prior art. That is to say the plies are arranged in the plane of the main lift force on the blade, so that off-axis fibres in those plies can carry the primary shear forces better. The orientation of the plies circumvents the problems of achieving sufficient shear strength and of laborious laminating of a spar, and also carries efficiently the Brazier loads in the root transition part of the blade. The present invention has particular application to blades in which the spar must be solid or substantially solid to achieve sufficient strength. However, the present invention also has application to blades incorporating more lightly-loaded spars if the benefits in manufacturing efficiency outweigh any increase in material usage.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic transverse cross-section through a first known wind turbine blade; Figure 2 is a schematic transverse cross-section through a second known wind turbine blade; Figure 3 is a schematic transverse cross-section through a tidal turbine blade in accordance with a first embodiment of the present invention; Figure 4 is a schematic perspective view of a spar for a tidal turbine blade in accordance with a second embodiment of the present invention; Figure 5 is a schematic plan view of a spar for a tidal turbine blade in accordance with a third embodiment of the present invention; Figure 6 is a schematic perspective view of a first attachment mechanism at a root end of a spar for a tidal turbine blade in accordance with a fourth embodiment of the present invention; Figure 7 is a schematic perspective view of a tidal turbine blade in accordance with a fifth embodiment of the present invention, the spar incorporating a second attachment mechanism at a root end of the spar; Figure 8 is a schematic cross-section through a tidal turbine blade in accordance with a sixth embodiment of the present invention, the spar incorporating a third attachment mechanism at a root end of the spar; Figure 9 is a schematic cross-section through a tidal turbine blade in accordance with a seventh embodiment of the present invention, the spar incorporating a fourth attachment mechanism at a root end of the spar; Figure 10 is a schematic cross-section through a tidal turbine blade in accordance with an eighth embodiment of the present invention, the spar incorporating a fifth attachment mechanism at a root end of the spar; Figure 11 is a schematic perspective view of a sixth attachment mechanism at a root end of a spar for a tidal turbine blade in accordance with a ninth embodiment of the present invention; Figure 12 is a schematic perspective view of a seventh attachment mechanism at a root end of a spar for a tidal turbine blade in accordance with a tenth embodiment of the present invention; Figure 13 is a schematic cross-section through a tidal turbine blade mounted to a hub in accordance with an eleventh embodiment of the present invention, the spar incorporating an eighth attachment mechanism at a root end of the spar; Figure 14 is a schematic perspective view of the root end of the spar of Figure 13; Figure 15 is a schematic cross-section of a ninth attachment mechanism at a root end of a spar for a tidal turbine blade in accordance with a twelfth embodiment of the present invention; Figure 16 is a schematic perspective view of a spar for a tidal turbine blade in accordance with a thirteenth embodiment of the present invention; Figures 17(a) and (b) are each a schematic cross-section through a spar for a tidal turbine blade in accordance with, respectively, fourteenth and fifteenth embodiments of the present invention; Figure 18 is a schematic cross-section through a spar for a tidal turbine blade in accordance with a sixteenth embodiment of the present invention; Figure 19 is a schematic plan view showing how a spar for a tidal turbine blade may be formed from a layer of plies of composite material in accordance with a seventeenth embodiment of the present invention; Figure 20 is a schematic plan view showing how a spar for a tidal turbine blade may be formed from a layer of plies of composite material in accordance with an eighteenth embodiment of the present invention; Figure 21 is a schematic plan view showing how a spar for a tidal turbine blade may be formed from a layer of plies of composite material in accordance with a nineteenth embodiment of the present invention; Figure 22 is a schematic perspective view of a spar for a tidal turbine blade in accordance with a twentieth embodiment of the present invention; Figures 23 and 24 are respectively a schematic side cross section and a schematic plan cross section of a mounting structure for the spar of Figure 22; Figure 25 is a schematic plan view of a tidal turbine blade in accordance with a twenty first embodiment of the present invention; and Figure 26 is a schematic perspective view of the tidal turbine blade of Figure 25 mounted on an axle.

Referring to Figure 3, there is shown a schematic transverse cross-section through a tidal turbine blade in accordance with a first embodiment of the present invention. The tidal turbine blade 30 includes a central spar 32 formed as a solid body of parallel laminated sheets 34 of fibre-reinforced resin composite material. Each sheet comprises a plurality of stacked plies (not shown) of the fibre-reinforced resin composite material produced, as described hereinafter, from prepreg or infused plies. The opposed blade surfaces 36, 38 are affixed to opposed sides 40, 42 of the spar 32 and are joined together at a leading edge 44 and at a trailing edge 46 of the blade 30.

The sheets 34 extend in the longitudinal direction of the blade 30, along the blade axis, substantially from the root to the tip. The sheets 34 therefore extend substantially transverse to the chord direction C-C. Accordingly, the sheets 34 each substantially lie in a plane of maximum load for the tidal turbine blade 32.

A proportion, typically around 60-90%, of the plies in each sheet 34 comprise longitudinally oriented unidirectional fibres, extending substantially along the blade axis, which resist any bending moments acting transversely to the blade length. A proportion, typically the remainder, of the plies in each sheet 34 comprise a biaxial fabric having fibres aligned at +1-degrees to the blade axis. These biaxial sheets carry shear forces. The biaxial plies carrying the shear forces are interleaved with the unidirectional plies carrying bending moment. The sheets 34 are individually fabricated from plural plies and then cured. Then the cured sheets 34 are bonded together to form the cross-section of the spar 32. The resins present in the plural plies laid-up within each sheet 34 are all co-cured together to form a single integral fibre-reinforced resin matrix within the respective sheet 34. Accordingly, there is sufficient inter-ply bond strength within each sheet 34 to transfer the shear loads from the unidirectional fibres to the biaxial fibres.

In a modified embodiment, rather than comprising both unidirectional plies and biaxial plies which are interleaved together, the plies in each sheet 34 comprise a triaxial fabric having fibres aligned at 0, +45 and -45 degrees to the blade axis.

In another modified embodiment, the plies in each sheet 34 comprise unidirectional plies having fibres aligned at 0, +45 and -45 degrees to the blade axis, Further embodiments may use multiaxial fabrics or unidirectional plies with fibres in other combinations of directions that give the spar sufficient shear and axial strength, such as fibres oriented at +10 and -10 degrees to the blade axis.

In the illustrated embodiment of the water-driven turbine blade, the opposed edges of the stack are attached to opposed inner faces of the first and second surfaces. However, in alternative embodiments, the opposed edges of the stack each form a respective part of the first and second surfaces, and may be shaped accordingly.

Figure 4 is a schematic perspective view of a spar for a tidal turbine blade in accordance with a second embodiment of the present invention and shows how the spar may be assembled. A sheet 48 of plural plies of fibre-reinforced resin composite material is laid up with the selected plies having the desired fibre angles, typically at 0, +45 and -45 degrees to the blade axis as described above. The resin in the sheet 48 is subsequently cured.

The sheet 48 is then cut to form a desired external profile shape 52 colTesponding to that of the final spar. In Figure 4, longitudinally extending edges 54, 56 define portions of forward and rear edges of the resultant spar, and the edges coincide at a tip end 58 corresponding to a tip of the spar. The opposite root end 60 corresponds to a root end of the spar.

Plural sheets 48 are then stacked together and adhesively bonded on the adjacent major faces, so as to form a stack 50 having a spar cross-section shown in Figures 3 and 4. The stack 50 may be held together by bolts 51.

Figure 5 is a schematic plan view of a spar for a tidal turbine blade in accordance with a third embodiment of the present invention This shows that the cut pattern for the external profile shape 62 may also incorporate alignment through-holes 64 for aligning the profiles of the stacked sheets 48 and bolt through-holes 66 for clamping the sheets 48 together during the adhesive bonding process using bolts, as shown for example by bolts 51 in Figure 3. The provision of such alignment through-holes 64 also saves weight in the resultant spar.

Various spar attachment mechanisms may be employed to mount the tidal turbine blade of the various embodiments of the present invention to a rotatable hub.

Referring to Figure 6, the attachment of the blade 70 may use the T-bolt method, which is well known in the wind turbine blade industry. A plurality of threaded bolts extend from the hub, each bolt being received in an axially oriented bolt hole 72 at an annular root end 74 of the spar 73. The threaded free end of the bolt is threaded into a radially extending cylindrical nut member 76 received in a corresponding radial hole in the root end 74.

Referring to Figure 7, the attachment of the blade 70 may use the bonded studs method, which is also well known in the wind turbine blade industry. A plurality of threaded studs extend from the hub, each stud being received in an axially oriented internally threaded insert 78 which is adhesively bonded into an axial hole 80 in the annular root end 74 of the blade 70. The threaded free end of the stud is threaded into the insert 78. The insert 78 is typically of metal.

Either of these methods illustrated in Figures 6 and 7 may be implemented using a known cylindrical root moulding 81, which is adhesively bonded to the root end 74 of the spar 73, as illustrated in Figure 8. Alternatively, the attachment may be directly connected to the spar 73. In Figure 9 the nut member 76 for the T-bolt method is directly received in a corresponding transverse hole in the root end 74. A central cavity 82 is located at the root end 74, to reduce material and weight. In Figure 10, such a central cavity is not provided and the root end 74 of the spar 73 includes radially inner and radially outer T-bolt mounting arrays 84, 86. Each of Figures 8 to 10 illustrates a blade shell 85 surrounding the spar 73. In the case of T-bolts, plural T-bolt mounting arrays 88 may share a common barrel nut 90, as shown in Figure 11. In the embodiment of Figure 12, the array of the threaded insert 78 may be curved to form an are.

In any of these T-bolt embodiments, the transverse hole for receiving the barrel nut(s) can be cut during the sheet profile cutting operation, assisting with alignment and avoiding a further process step.

Figures 13 and 14 illustrate a tidal turbine blade 92 rotatably mounted to a hub 94 in accordance with a further embodiment of the present invention, A spar 98 is surrounded by a blade shell 99. The root end 96 of the spar 98 includes a relatively large diameter bearing outer mount surface 100 and a relatively small diameter bearing inner mount surface 102.

The surfaces 100, 102 are mutually spaced in a longitudinal direction along the spar 98. A correspondingly shaped portion of the hub 94 includes a relatively large diameter bearing mount surface 104 and a relatively small diameter bearing mount surface 106. Annular bearings 108, 110 rotatably mount the tidal turbine blade 92 to the hub 94. One large diameter bearing 108 is fitted between mount surfaces 100, 104 and one smaller diameter bearing 110 is fitted between mount surfaces 102, 106. In a modified embodiment, the spaced bearings may have the same diameter.

Additional chordal-shaped packing pieces 112, 114 may be incorporated on opposite sides of the spar 98 to define fully annular bearing mount surfaces 100, 102, and to spread the bearing load evenly around the bearing mount surfaces 100, 102, as shown in Figure 14. The pieces 112, 114 may be manufactured independently by a stack-forming process similar to that for the spar as described above, or by using off-cuts of the stack produced during the spar manufacture. Such packing pieces 112, 114 may be located in correct position on the opposite sides of the spar 98 using lugs 116 inserted into complementary slots 118 (both shown in phantom) cut in the main spar 98 to improve the strength of the connection to the spar 98.

Such a bearing mount structure can avoid the problems associated with large diameter slewing ring bearings used, for example, on wind turbine blades, and the sealing of such bearings under water. Such an arrangement generates very high shear forces in the spar 98 between the two bearings 108,110, and high crush loads on the spar 98 as it passes through the bearings 108,110. However, the spar structure of the invention allows the spar 98 to be constructed that is much better at carrying high shear forces and crushing loads than a conventional hollow spar, so is well suited to such an arrangement, In particular, the spar construction provides a spar that is capable of carrying the high shear and crushing loads inherent in this longitudinally spaced bearing design without having to put the bearings impracticably far apart to reduce these loads sufficiently to be carried by a conventional hollow spar. The reduced bearing spacing then offers benefits in reduced hub size, weight and/or cost, and also improved hydrodynamics.

In a turbine construction in which two turbine blades are mounted diametrically opposite to each other, a through-spar design may be used to connect two opposite blades. Such a through-spar design may be used for a blade which has controllably variable pitch, such as an oscillating foil-type blade, in which the blades pitch in the same direction, As shown in Figures 22 to 24, in one embodiment a single integral construction of composite material comprises two opposed spar elements 120, 122 each having formed, on a radially inner end 124, 126, an annular bearing mount surface 128, 130. Each bearing mount surface 128, 130 has a structure as described above with respect to Figure 14. This provides two longitudinally spaced bearing mount surfaces 128, 130, which can be mounted to a hub by respective annular bearings, forming an oscillating foil-type blade which rotates about the longitudinal axis of the blade. As shown in Figures 23 and 24, the annular bearings 132, 134 are directly mounted to the bearing mount surfaces 128, 130, and engage the spacer elements 138, 140 on opposite sides of the spar elements 120, 122.

The spar structure includes composite material sheets orthogonal to the plane of the blade, and accordingly the spar mount at the root end is able to carry bending and shear loads effectively in the plane of the laminate. Therefore the spar mount at the root end 142 may alternatively have the structure shown in Figure 15, in which the root end 142 includes opposed integral mounting flanges 144 extending outwardly in the plane of the laminate. The laminate structure permits such mounting flanges 144, which have a corner 146 between the mounting flanges 144 and the major portion 148 of the spar 150. The flanges 144 can be mounted to the hub 147 by bolts 149. This provides a spar mounting connection that is simpler, lighter and more compact than the T-bolt structure described above.

Figure 16 is a schematic perspective view of a spar for a tidal turbine blade in accordance with a fttrther embodiment of the present invention. For some tidal turbine blades, such as those used on cross-flow turbines, the loads may not always act in the same direction. In the blade structure of Figure 16, the spar 152 includes a central part 154 extending between the major blade surfaces with the laminate sheets 156 being transverse and extending substantially orthogonally to the major blade surfaces. The spar 152 additionally includes opposed edge portions 158, 160 on opposite sides of the central part 154. The edge portions 158, 160 extend towards respective leading and trailing edges of the blade, to carry edgewise loads. The laminated composite material sheets 162 in the edge portions 158, 160 are orthogonal to the laminate sheets 156 of the central part 154, and extend substantially in the plane of the blade. The edge portions 158, 160 may be located and attached to the central part 154 of the spar 152 by complementary lugs 164 and slots 166 as shown in phantom. The central part 154 and the edge portions 158, 160 may incorporate root fixings 161 at the inboard root end, for example having the structure of the various embodiments described above.

Figures 17(a) and (b) are each a schematic cross-section through a spar for a tidal turbine blade in accordance with respective further embodiments of the present invention. In Figure 17(a) the longitudinal edges 168 of the laminate sheets 170 form a stepped configuration, with the sheets 170 having varying width in the transverse direction extending between the major blade surfaces. Alternatively, in Figure 17(b) the longitudinal edges 172 of the laminate sheets 174 form a smoothly arcuate configuration. The edges 172 are cut at an angle to the plane of the sheet 174 to form a cut edge which is not perpendicular to the sheet 174, as in Figure 17(a). Such an arcuate configuration allows a smoother cross-section of the spar 176 to be formed, which may improve the strength of the spar 176 by maximising the amount of material in the spar 176, particularly those fibres furthest from the neutral axis of bending. Furthermore, such an arcuate configuration may maximise the bond area between the profiled sheets 174. An arcuate configuration may alternatively be achieved by shaping the cross-section after assembling the profiled sheets 174 to achieve smoothly arcuate longitudinally extending opposed surfaces of the spar 176.

Figure 18 is a schematic cross-section through a spar 180 for a tidal turbine blade in accordance with a further embodiment of the present invention. For achieving increased strength, particularly with high edgewise or torsional loading, the central portion 183 of the spar 180 is wrapped circumferentially with additional composite laminate material 182 prior to fitting the shells forming the opposed blade surfaces. The central portion 183 includes orthogonally oriented sheets 185. The composite laminate material 182 typically comprises fibres at +1-45 degrees to the axial direction of the spar 180. The composite laminate material 182 may additionally comprise, integral with the wrapped material or as one or more inserted separate layers, additional longitudinally extending fibres, oriented axially along the spar 180, which may be bonded or laminated onto the spar 180. Such a reinforced configuration may be beneficial for instance in the arrangement shown in Figure 8, in which a cylindrical root moulding is adhesively bonded to the root end of the spar, and the load should be transferred gradually to the root laminate.

Figure 19 is a schematic plan view showing how a spar for a tidal turbine blade may be formed from a layer of sheets of composite material in accordance with a further embodiment of the present invention. In the present invention the sheets are profiled by a cuffing step to form a desired planar shape from a larger area sheet. This can lead to potential material wastage, as compared to known methods where sheets are cut to fill a mould. Figure 19 shows how the profile 184 is cut from a central part of the larger area sheet 186. It may be seen that the material wastage is significant even if care is taken in the nesting of plural profiles, two being shown in Figure 19, to be cut from a common sheet 186. This wastage may comprise as much as three times the composite material actually used in the spar of the blade.

Figure 20 illustrates a manufacturing method in which each sheet profile is formed from plural overlapping plies. A first stack of plies 190 is employed to provide composite material for the root portion 192 of the spar and a second stack of plies 194 is employed to provide composite material for the longitudinally extending portion 196 of the spar. The first and second stacks of plies 190, 194 overlap to form a series of staggered buff joints. A frill-width stack of plies 190 is only used at the broad root end part 198 of the spar, and a narrower stack of plies 194 is used at the narrower blade surface-supporting part 200 of the spar. As illustrated, the narrower stack of plies 194 may be used to produce two adjacent blade surface-supporting parts 200, each for a respective spar, and each end 202, 204 of the narrower stack of plies 194 overlaps a respective full-width stack of plies 190. A single narrower stack of plies 194 is centrally located between a pair of opposed full-width stack of plies 190, the three stacks of plies 190, 194 forming two sheet profiles 206, 208 after the cuffing of the profiles 206, 208 from the stacks of plies 190, 194.

Although such a manufacturing step requires a series of staggered butt joints in the plies, where the first and second stacks of plies 190, 194 overlap, such a manufacturing configuration does offer the advantage of an ability to change the fibre type and direction at the overlap regions 210. The fibre in the first stack of plies 190 and the second stack of plies 194 may be different, for example glass in the first stack of plies 190 and carbon in the second stack of plies 194. The fibre orientation in the first stack of plies 190 and the second stack of plies 194 may be different, for example multiaxial, at 0, +45 and -45 degrees to the longitudinal axis, in the first stack of plies 190 and priniarily unidirectional, at 0 degrees to the longitudinal axis, in the second stack of plies 194.

Thus in the highly-loaded and thin outboard section of the spar of the blade, formed from the blade surface-supporting parts 200, the laminate may consist primarily of fibres, such as carbon fibres, laid in the axial direction, whereas in the inboard section of the spar of the blade, formed from the root end parts 198, where loads are more varied in their direction, a greater proportion of off-axis fibres, such as +1-45 degrees to the axial direction, may be used. In order to reduce the material cost of the spar, additionally the wider inboard part formed from the root end parts 198 may be of glass fibres instead of carbon fibres. Such a structure provides an added benefit of avoiding galvanic corrosion between carbon fibres and any metallic components used in the root fixing, because carbon fibres are present only in the blade part remote from the root, when the blade is mounted in water, particularly sea water.

Figure 21 shows an alternative ply arrangement to provide a cutting pattern for the plies in order to minimise material wastage. Similar to the arrangement shown in Figure 20, each ply profile is formed from plural overlapping plies. A first full-width stack of plies 212 is employed to provide composite material for the root portion 214 of the spar and a second narrower stack of plies 216 is employed to provide composite material for the longitudinally extending portion 218 of the spar. The first and second stacks of plies 212, 216 overlap to form staggered butt joints. The narrower stack of plies 216 is used to produce two adjacent blade surface-supporting parts 222, each for a respective spar, and each end 224, 226 of the narrower stack of plies 216 overlaps a respective full-width stack of plies 212.

A single narrower stack of plies 216 is centrally located between a pair of opposed fttll-width stacks of plies 212, the three stacks of plies 212, 216 forming two sheet profiles 228, 230 after the cutting of the sheets 232, 234 from the stacks of plies 212, 216. In order to reduce material wastage from the narrower stack of plies 216, the narrower stack of plies 216 is oriented at an acute angle a to the axis of the longitudinally extending portion 218. This provides that reduced material is wasted from the longitudinal edges of the narrower stack of plies 216. Such a pattern can be readily nested if fabric widths arc chosen appropriately.

Also, the full-width stack of plies 212 may have a plan shape pre-cut to match that of the root portion 214, but oversized.

The narrow diagonal stack of plies 216 for the outboard part of the profiles 228, 230 may be formed from collimated unidirectional prepreg material, in which unidirectional fibres are within a resin matrix. In one arrangement a single diagonal stack of plies 216 is used for both of longitudinally extending portions 218. In an alternative arrangement, a pair of transversely overlapping ply portions are employed to form the diagonal stack of plies 216, the ply portions being joined by a transverse butt joint 232 indicated by the dashed line A of Figure 21. Such a collimated unidirectional prepreg material is not weakened by such transverse butt joints.

The full-width stacks of plies 212 for the inboard part of the blade spar may be cut from a roll 236 of fabric in which the fibres are oriented at 90/451-45 degrees to the fabric edge, i.e. 0/45/-45 degrees to the blade axis, the roll edges being shown by the dashed lines B, C of Figure 21. The same roll 236 may be used for the embodiment of Figure 20, which includes the dashed lines B and C. Material wastage is therefore reduced, and comparable to conventional spar manufacturing methods.

Figures 25 and 26 illustrate a non-pitching tidal turbine blade 240 in accordance with a further embodiment of the present invention. The blade 240 of this embodiment incorporates, like the embodiment of Figures 22 to 24, a through-spar 242. The through-spar 242 has a central portion 244 mounted to a lower portion 246 of a hub 248 fitted to a rotatable axle 250.

An upper portion 252 of the hub 248 is fitted over the central portion 244 and clamped by an annular array of bolts 254 to the lower portion 246. As for the previous embodiments, each of the opposed end portions 256, 258 of the through-spar 242 is covered by a respective blade shell 260, 262 bonded thereto.

In any of these embodiments, the spar can be constructed by the following method, which is merely illustrative. A sheet of constant thickness laminate is produced, by stacking plural plies, which may be nominally rectangular in plan. The laminate consists of a plurality of fibre reinforced composite plies with the fibres predominantly oriented along the long (axial) direction of the sheet, After the laminate has the required number of plies to provide the desired final thickness, the laminate is then cured so as fully to cure the resin in the fibre reinforced resin composite material. The curing step is typically carried out under vacuum, in a press, or in an autoclave. Typically there may be around 50%-90% of fibres oriented axially, i.e. at 0 degrees to the axial direction, and the remainder at +1-45 degrees to the axial direction.

The cured laminate is then cut, typically by laser cutting or water jet cutting, to form a series of one or more sheet profiles resembling the front view of the spar. The sheet profiles may have internal holes cut to make them lighter, to assist joining them together, or to make the connection to the rest of the turbine at the blade root, as described earlier.

By making these plural sheet profiles of different shape and stacking them together, a spar with an approximately aerofoil cross-section can be assembled.

The sheet profiles are adhesively bonded together, optionally under pressure to ensure a strong bond. They may additionally be bolted together, as shown in Figure 3, to provide a clamping force for the adhesive joint and to ensure correct alignment of the sheet profiles.

The use of a flat mould shape during the lay-up and bonding process in such a method facilitates the manufacturing method as compared to known methods for manufacturing spars using a mould having a curved moulding surface. This forms a spar of desired shape and configuration.

If required chordal-shaped packing pieces as illustrated in Figures 13 and 14 or edge pieces as illustrated in Figure 16 may be bonded to the spar.

After formation of the spar, root connections are fitted to the spar, for example T-bolts or inserts as described earlier.

The spar is then incorporated into the tidal turbine blade, A shell, typically comprising opposed shall halves which are bonded together at the leading and trailing edges of the combined blade, is fitted over the spar and bonded to it, to give the blade the required smooth aerofoil shape.

An alternative method to manufacture the sheets is automated tape laying, as known in the state of the art, which may provide reduced material wastage and reduced labour during manufacture, and permit further optimised fibre orientations. Again, the use of a flat mould shape in such a method would facilitate the manufacturing method as compared to known methods of laying tape into a mould having a curved moulding surface.

The present invention has application in the manufacture of oscillating foil tidal turbine blades, cross-flow tidal turbine blades and axial flow tidal turbine blades, because all of these blade types have very high loading and relatively thin blade sections and their root connections can generate high shear loads. The invention also has application for tidal turbine blade designs having a rudder-style attachment such as shown in Figure 14.

The manufacturing process may readily be automated because all the plies may have the same regular shape, and are laid onto a flat surface instead of a curved mould.

Ply drop-offs, necessary for the tapering of a conventional spar, can be avoided. Thus thicker plies can be used without incurring the strength penalties associated with terminating thick plies. By using thicker plies the laminating time can be reduced. For instance the Germanischer Lloyd Guidelines for Certification of Wind Turbines 2010 limit the weight of plies to l300g/m2 of fibre at ply drop-offs unless separate proof of strength is provided. By avoiding ply drop-offs in the manufacture of the preferred tidal turbine blades of the present invention, fabrics having a higher weight per unit area can be employed.

In the manufacture of a conventional spar, exothermic reactions during curing of the resin may impose a practical limit on the thickness of spar that can be produced and the speed at which it can be cured. In the present invention however, the initial sheet of laminate is cured, rather than the entire spar. Accordingly, such thickness limits only apply to the initial sheet of laminate, which can therefore have significant thickness. Additionally, the thickness of the initial sheet of laminate may be selected according to the maximum thickness which can effectively be cut by the technique selectively used to cut the profiles.

Since the shape of the sheets is simple, flat or nearly flat, and typically rectangular, off-axis plies can be easily made from collimated prepreg rather than stitched biaxial layers.

Collimated prepreg is stronger and cheaper than stitched fabric because it avoids the stress concentrations and process costs of the stitching operation.

Furthermore, since the plies are aligned in the plane of the maximum load direction, the spar carries shear forces through fibres rather than just resin or adhesive. Accordingly, the spar according to the preferred embodiments of the invention can withstand much higher shear forces than even a solid laminated spar consisting of plies laminated in the span-chord plane.

The spar according to the preferred embodiments of the invention is suited to carrying Brazier loads caused by steep changes of shape typically seen at the transition between the root and the aerofoil section of a tidal turbine blade. This means that the transition can be steeper than would be the case with a conventional wind-turbine-style design, allowing the aerofoil section to be extended further inboard towards the root section. The result is a tidal turbine blade which is capable of generating more power from the same blade length.

Integrated root connection solutions are easily incorporated during the culling process, eliminating the extra step of drilling holes for T-bolts or bonded inserts after fabricating the blade.

Alternative methods for a pitch bearing connection, such as the twin radial bearings, axially spaced apart, as used in the embodiments of Figures 13 to 14 and Figures 22 to 24, can be employed in the spars according to the various embodiments of the invention. In contrast, typical known hollow spar constructions may not achieve sufficient shear strength between the spaced bearings without spacing the bearings much further apart.

Spars that taper at both ends, called a "through spar", can easily be constructed in one integral piece according to preferred aspects of the invention. Such a construction would be difficult using a known male mandrel method, as described above with respect to Figure 1 and as used on many known wind turbine blades, because of the problem of extracting the mandrel from the centre of a through spar which would be closed at both opposite ends. A through-spar produced according to the embodiments of the invention is useful in oscillating foil tidal turbines and in tidal turbines which use twin radial bearings for attachment to the hub, as illustrated in Figures 22 to 24.

Through-spars produced according to the embodiments of the invention are also useful when incorporated into non-pitching blades, where a pitch bearing is not required and a rotor with an even number of blades can have opposite blades directly connected together, thereby minimising the bending moments to be carried by the hub connection, as illustrated in Figures 25 and 26.

The cost of maintenance and repair of submerged tidal turbines is high, for example much greater than for onshore wind turbines. The preferred embodiments of the present invention provide an advantage that the constructions can eliminate the use of highly-loaded adhesive joints in an underwater environment, which enhances the reliability of the tidal turbine incorporating the tidal turbine blade. The preferred embodiments of the present invention can also provide significant labour cost savings, efficiency and reliability improvements compared to other methods, particularly for an axial-flow turbine construction.

Various other modifications to the tidal turbine blades of the present invention, and their manufacturing process, will be apparent to those skilled in the art.

Claims (5)

1. CLAIMS1. A spar for a water-driven turbine blade, the spar comprising a substantially planar stack of first sheets secured together, each first sheet comprising fibre reinforced resin composite material, wherein opposed edges of the first sheets define opposed edges of the stack, the edges of the stack defining a mounting portion for mounting the spar to a turbine member and at least one elongate blade supporting portion.
2. 2. A spar according to claim 1, wherein the at least one elongate blade supporting portion includes unidirectional fibres substantially aligned along a longitudinal axis of the elongate blade supporting portion.
3. 3. A spar according to claim 2, wherein the unidirectional fibres are aligned at an angle of no more than +1-20 degrees to the longitudinal axis of the elongate blade supporting portion, 4. A spar according to claim 2 or claim 3, wherein the mounting portion includes fibres inclined to the longitudinal axis.5. A spar according to any foregoing claim, wherein each first sheet comprises a plurality of laminated fibrous plies.6. A spar according to claim 5, wherein at least some of the plies have unidirectional fibres and at least some of the plies have multiaxial, optionally biaxial, fibres.7. A spar according to any foregoing claim, wherein at least one of the first sheets comprises a first fibre material at the mounting portion and a second fibre material at the blade supporting portion.8. A spar according to any foregoing claim, wherein at least one of the first sheets comprises a first fibre orientation at the mounting portion and a second fibre orientation at the blade supporting portion.9. A spar according to any foregoing claim, wherein the mounting portion comprises a root mounting portion for mounting an end of the spar to a turbine member.10. A spar according to claim 9, wherein the root mounting portion includes mechanical fittings for fitting the spar to the turbine member.11. A spar according to claim 10, wherein the mechanical fittings include an elongate nut having a plurality of spaced threaded holes for receiving a plurality of bolts for fitting the spar to the turbine member, the elongate nut extending through a plurality of the first sheets of the spar.12. A spar according to any one of claims 9 to 11, wherein the root mounting portion includes an outwardly directed flange in the plane of the spar, the flange being adapted to be bolted to the turbine member.13. A spar according to any one of claims 9 to 12, wherein the root mounting portion includes a central cavity located between opposed ends of the root mounting portion.14. A spar according to claim 9, wherein the root mounting portion includes a pair of longitudinally spaced annular bearing surfaces surrounding the spar.15. A spar according to claim 14, wherein a first endmost annular bearing surface has a smaller diameter than a second innermost annular bearing surface.16. A spar according to any one of claims I to 8, wherein the spar includes two opposite elongate blade supporting portions on opposed sides of a central mounting portion for mounting a central part of the spar to a turbine member.17. A spar according to claim 16, wherein the mounting portion includes a pair of longitudinally spaced annular bearing surfaces each surrounding the spar.18. A spar according to any foregoing claim, wherein edges of adjacent first sheets are shaped to provide a continuously curved edge profile on opposed edges of the stack.19. A spar according to any foregoing claim, further comprising at least one hole extending at least partially though the stack of first sheets.20. A spar according to claim 19, wherein the at least one hole is for aligning the first sheets of the stack and/or for receiving bolts for bohing together the first sheets of the stack.21. A spar according to any foregoing claim, further comprising a circumferential wrapping of niultiaxial fibre reinforced composite material.22. A spar according to any foregoing claim, ftirther comprising at least one edge portion extending outwardly from a face of the stack of first sheets, the edge portion comprising a stack of second sheets, each second sheet comprising fibre reinforced resin composite material, 23. A spar according to claim 22, wherein each second sheet comprises a plurality of laminated fibrous plies.24. A spar according to claim 23, wherein at least some of the plies of the second sheets have unidirectional fibres and at least some of the plies of the second sheets have multiaxial, optionally biaxial, fibres.25, A spar according to any one of claims 22 to 24, wherein at least one of the second sheets comprises a first fibre material at a mounting portion of the edge portion and a second fibre material at a blade supporting portion of the edge portion.26. A spar according to any one of claims 22 to 25, wherein at least one of the second sheets comprises a first fibre orientation at a mounting portion of the edge portion and a second fibre orientation at a blade supporting portion of the edge portion.27. A water-driven turbine blade including a spar according to any foregoing claim, the turbine blade including opposite first and second surfaces each defining a respective load bearing blade surface, and the spar being located between the first and second surfaces, the first sheets of the stack lying in a plane extending between the first and second surfaces.28. A water-driven turbine blade according to claim 27, wherein the opposed edges of the stack are attached to opposed inner faces of the first and second surfaces.29. A water-driven turbine blade according to claim 27, wherein the opposed edges of the stack each form a respective part of the first and second surfaces.30. A water-driven turbine blade according to any one of claims 27 to 29, wherein the first sheets of the stack lie in a plane extending substantially orthogonally to a chordal direction of the turbine blade.31. A water-driven turbine blade according to any one of claims 27 to 30, which is adapted for use in an axial flow turbine, in which plural radial turbine blades are fixed to a rotatable hub which is caused to rotate about an axis by water movement over the turbine blade surfaces.32. A water-driven turbine blade according to any one of claims 27 to 30, which is adapted for use in a cross flow turbine, in which the turbine blade is fixed to a support and adapted to rotate substantially about a longitudinal axis of the turbine blade by water movement over the turbine blade surfaces, optionally the rotation being in an oscillating motion.33. A water-driven turbine blade according to any one of claims 27 to 32 which is adapted for use in a tidal turbine.34. A method of manufacturing a spar for a water-driven turbine blade, the method comprising the steps of: a. laying up a plurality of plies of fibre reinforced resin composite material to form a substantially planar laminate thereof; b. curing the resin to form a cured sheet from the laminate; c. cuffing the sheet to form opposed edges which define a part of a mounting portion for mounting the spar to a turbine member and a part of at least one elongate blade supporting portion; and d. assembling together a substantially planar stack of the cut sheets to form a spar, the opposed edges of the sheets being arranged to form opposed edges of the stack, the edges of the stack defining the mounting portion for mounting the spar to a turbine member and the at least one elongate blade supporting portion.35. A method according to claim 34, wherein the at least one elongate blade supporting portion includes unidirectional fibres substantially aligned along a longitudinal axis of the elongate blade supporting portion.36. A method according to claim 35, wherein the unidirectional fibres are aligned at an angle of no more than +1-20 degrees to the longitudinal axis of the elongate blade supporting portion.37. A method according to claim 35 or claim 36, wherein the mounting portion includes fibres inclined to the longitudinal axis.38. A method according to any one of claims 34 to 37, wherein the mounting portion is formed from a plurality of first plies and the blade supporting portion is formed from a plurality of second plies, the first and second plies being interconnected at adjacent edges.39. A method according to claim 38, wherein the first plies and the second plies are interconnected by a staggered buff joint at the adjacent edges.40. A method according to claim 38 or claim 39, wherein steps (a) and (b) are carried out to produce two sheets simultaneously, each of two parts of spaced mounting portions being formed from a respective laminate of first plies and each of two parts of blade supporting portions being formed from a common laminate of second plies, the laminate of the second plies being located between, and interconnected at adjacent edges to, a respective laminate of the first plies.41. A method according to any one of claims 38 to 40 wherein the first and second plies each have a respective different shape and dimensions substantially corresponding respectively to the shape and dimensions of the mounting portion and the blade supporting portion formed therefrom.42. A method according to any one of claims 38 to 41, wherein at least some of the plies have unidirectional fibres and at least some of the plies have multiaxial, optionally biaxial, fibres.43. A method according to any one of claims 38 to 42, wherein at least one of the plies comprises a first fibre material at the mounting portion and a second fibre material at the blade supporting portion.44. A method according to any one of claims 34 to 43, wherein at least one of the plies comprises a first fibre orientation at the mounting portion and a second fibre orientation at the blade supporting portion.45. A method according to any one of claims 34 to 44, wherein the mounting portion comprises a root mounting portion for mounting an enlarged end of the spar to a turbine member.46. A method according to claim 45, wherein in cuffing step (c) shaped regions for receiving root fittings are cut in the root mounting portion of the sheet.47. A method according to claim 45, wherein the root mounting portion includes a pair of longitudinally spaced annular bearing surfaces each surrounding the spar.48. A method according to any one of claims 34 to 44, wherein the spar is shaped to include two opposite elongate blade supporting portions on opposed sides of a central mounting portion for mounting a central part of the spar to a turbine member.49. A method according to claim 48, wherein the central mounting portion includes a pair of longitudinally spaced annular bearing surfaces each surrounding the spar.50. A method according to any one of claims 34 to 49, wherein edges of adjacent sheets are shaped to provide a continuously curved edge profile on opposed edges of the stack.51. A method according to any one of claims 34 to 50, further comprising at least one hole extending at least partially though the stack of first sheets.52. A method according to claim 51, wherein the at least one hole is for aligning the first sheets of the stack and/or for receiving bolts for bolting together the first sheets of the stack, 53. A method according to any one of claims 34 to 52, further comprising, after step (d) circumferentially wrapping multiaxial fibre reinforced composite material around the spar.54. A method of manufacturing a water-driven turbine blade, the method comprising manufacturing a spar according to any one of claims 34 to 53, and assembling a blade shell around the spar.55. A method according to claim 54, wherein the opposed edges of the stack are attached to opposed inner faces of the blade shell.56. A method according to claim 54, wherein the opposed edges of the stack each form a respective part of the blade surfaces and the spar is fitted to the blade shell with the opposed edges of the stack being exposed in openings of the blade shell.Amendments to the claims have been filed as followsCLAIMS1. A spar for a water-driven turbine blade, the spar comprising a substantially planar stack of first sheets secured together, each first sheet comprising fibre reinforced resin composite material, wherein opposed edges of the first sheets define opposed edges of the stack, the edges of the stack defining a mounting portion for mounting the spar to a turbine member and at least one elongate blade supporting portion, the first sheets extending in a longitudinal direction of the elongate blade supporting portion.2. A spar according to claim 1, wherein the at least one elongate blade supporting portion includes unidirectional fibres substantially aligned along a longitudinal axis of the elongate blade supporting portion.3. A spar according to claim 1, wherein the at least one elongate blade supporting portion includes unidirectional fibres aligned at an angle of no more than +1-20 degrees to a longitudinal axis of the elongate blade supporting portion.
4. 4. A spar according to claim 2 or claim 3, wherein the mounting portion includes fibres inclined to the longitudinal axis.
5. 5. A spar according to any foregoing claim, wherein each first sheet comprises a plurality of laminated fibrous plies. Co6. A spar according to claim 5, wherein at least some of the plies have unidirectional fibres and at least some of the plies have multiaxial fibres.7. A spar according to any foregoing claim, wherein at least one of the first sheets comprises a first fibre material at the mounting portion and a second fibre material at the blade supporting portion.8, A spar according to any foregoing claim, wherein at least one of the first sheets comprises a first fibre orientation at the mounting portion and a second fibre orientation at the blade supporting portion.9. A spar according to any foregoing claim, wherein the mounting portion comprises a root mounting portion for mounting an end of the spar to a turbine member.10. A spar according to claim 9, wherein the root mounting portion includes mechanical fittings for fitting the spar to the turbine member.11, A spar according to claim 10, wherein the mechanical fittings include an elongate nut having a plurality of spaced threaded holes for receiving a plurality of bolts for fitting the spar to the turbine member, the elongate nut extending through a plurality of the first sheets of the spar.12. A spar according to any one of claims 9 to 11, wherein the root mounting portion includes an outwardly directed flange in the plane of the spar, the flange being adapted to be bolted to the turbine member.13. A spar according to any one of claims 9 to 12, wherein the root mounting portion includes a central cavity located between opposed ends of the root mounting portion.14. A spar according to claim 9, wherein the root mounting portion includes a pair of longitudinally spaced annular bearing surfaces surrounding the spar.15. A spar according to claim 14, wherein a first endmost annular bearing surface has a smaller diameter than a second innermost annular bearing surface.16. A spar according to any one of claims 1 to 8, wherein the spar includes two opposite elongate blade supporting portions on opposed sides of a central mounting portion for mounting a central part of the spar to a turbine member.v" 17. A spar according to claim 16, wherein the mounting portion includes a pair of v" longitudinally spaced annular bearing surfaces each surrounding the spar. P 18. A spar according to any foregoing claim, wherein edges of adjacent first sheets are shaped to provide a continuously curved edge profile on opposed edges of the stack.C'sj 19. A spar according to any foregoing claim, further comprising at least one hole extending at least partially though the stack of first sheets.20. A spar according to claim 19, wherein the at least one hole is for aligning the first sheets of the stack and/or for receiving bolts for bolting together the first sheets of the stack.21. A spar according to any foregoing claim, further comprising a circumferential wrapping of multiaxial fibre reinforced composite material, 22. A spar according to any foregoing claim, further comprising at least one edge portion extending outwardly from a face of the stack of first sheets, the edge portion comprising a stack of second sheets, each second sheet comprising fibre reinforced resin composite material.23. A spar according to claim 22, wherein each second sheet comprises a plurality of laminated fibrous plies.24. A spar according to claim 23, wherein at least some of the plies of the second sheets have unidirectional fibres and at least some of the plies of the second sheets have multiaxial, optionally biaxial, fibres.25. A spar according to any one of claims 22 to 24, wherein at least one of the second sheets comprises a first fibre material at a mounting portion of the edge portion and a second fibre material at a blade supporting portion of the edge portion.26. A spar according to any one of claims 22 to 25, wherein at least one of the second sheets comprises a first fibre orientation at a mounting portion of the edge portion and a second fibre orientation at a blade supporting portion of the edge portion.27. A water-driven turbine blade including a spar according to any foregoing claim, the turbine blade including opposite first and second surfaces each defining a respective load bearing blade surface, and the spar being located between the first and second surfaces, the first sheets of the stack lying in a plane extending between the first and second surfaces.28. A water-driven turbine blade according to claim 27, wherein the opposed edges of L the stack are attached to opposed inner faces of the first and second surfaces. r 29. A water-driven turbine blade according to claim 27, wherein the opposcd edges of the stack each form a respective part of the first and second surfaces.30. A water-driven turbine blade according to any one of claims 27 to 29, wherein the first sheets of the stack lie in a plane extending substantially orthogonally to a chordal direction of the turbine blade.31. An axial flow turbine including a water-driven turbine blade according to any one of claims 27 to 30, the axial flow turbine including plural radial turbine blades fixed to a rotatable hub which is caused to rotate about an axis by water movement over the turbine blade surfaces.32. A cross flow turbine including a water-driven turbine blade according to any one of claims 27 to 30, the cross flow turbine including the turbine blade fixed to a support for rotation substantially about a longitudinal axis of the turbine blade by water movement over the turbine blade surfaces.33. A tidal turbine including a water-driven turbine blade according to any one of claims 27to32.34. A method of manufacturing a spar for a water-driven turbine blade, the method comprising the steps of a. laying up a plurality of plies of fibre reinforced resin composite material to form a substantially planar laminate thereof; b. curing the resin to form a cured sheet from the laminate; c. cutting the sheet to form opposed edges which define a part of a mounting portion for mounting the spar to a turbine member and a part of at least one elongate blade supporting portion; and d. assembling together a substantially planar stack of the cut sheets to form a spar the opposed edges of the sheets being arranged to form opposed edges of the stack, the edges of the stack defining the mounting portion for mounting the spar to a turbine member and the at least one elongate blade supporting portion the sheets extending in a longitudinal direction of the elongate bTade supporting portion.35. A method according to claim 34, wherein the at least one elongate blade supporting portion includes unidirectional fibres substantially aligned along a longitudinal axis of the elongate blade supporting portion.36. A method according to claim 34, wherein the at least one elongate blade supporting portion includes unidirectional fibres aligned at an angle of no more than +/-20 degrees to a longitudinal axis of the elongate blade supporting portion.37. A method according to claim 35 or claim 36, wherein the mounting portion includes fibres inclined to the longitudinal axis.38, A method according to any one of claims 34 to 37, wherein the mounting portion is formed from a plurality of first plies and the blade supporting portion is formed from a plurality of second plies, the first and second plies being interconnected at adjacent edges.39. A method according to claim 38, wherein the first plies and the second plies are interconnected by a staggered buff joint at the adjacent edges.40. A method according to claim 38 or claim 39, wherein steps (a) and (b) are carried out to produce two sheets simultaneously, each of two parts of spaced mounting portions being formed from a respective laminate of first plies and each of two parts of blade supporting portions being formed from a common laminate of second plies, the laminate of the second plies being located between, and interconnected at adjacent edges to, a respective laminate of the first plies.41. A method according to any one of claims 38 to 40 wherein the first and second plies each have a respective different shape and dimensions substantially corresponding respectively to the shape and dimensions of the mounting portion and the blade supporting portion formed therefrom, 42. A method according to any one of claims 38 to 41, wherein at least some of the plies have unidirectional fibres and at least some of the plies have multiaxial, optionally biaxial, fibres.43. A method according to any one of claims 38 to 42, wherein at least one of the plies comprises a first fibre material at the mounting portion and a second fibre material at the blade supporting portion.44. A method according to any one of claims 34 to 43, wherein at least one of the plies comprises a first fibre orientation at the mounting portion and a second fibre orientation at the blade supporting portion.r 45. A method according to any one of claims 34 to 44, wherein the mounting portion comprises a root mounting portion for mounting an end of the spar to a turbine member.CD 46. A method according to claim 45, wherein in cutting step (c) shaped regions for receiving root fittings are cut in the root mounting portion of the sheet.47. A method according to claim 45, wherein the root mounting portion includes a pair of longitudinally spaced annular bearing surfaces each surrounding the spar.48, A method according to any one of claims 34 to 44, wherein the spar is shaped to include two opposite elongate blade supporting portions on opposed sides of a central mounting portion for mounting a central part of the spar to a turbine member.49. A method according to claim 48, wherein the central mounting portion includes a pair of longitudinally spaced annular bearing surfaces each surrounding the spar.50. A method according to any one of claims 34 to 49, wherein edges of adjacent sheets are shaped to provide a continuously curved edge profile on opposed edges of the stack.51. A method according to any one of claims 34 to 50, further comprising at least one hole extending at least partially though the stack of first sheets.52. A method according to claim 51, wherein the at least one hole is for aligning the first sheets of the stack and/or for receiving bolts for bolting together the first sheets of the stack.53. A method according to any one of claims 34 to 52, further comprising, after step (d) circumfercntially wrapping multiaxial fibre reinforced composite material around the spar.54. A method of manufacturing a water-driven turbine blade, the method comprising manufacturing a spar according to any one of claims 34 to 53, and assembling a blade shell around the spar.55. A method according to claim 54, wherein the opposed edges of the stack are attached to opposed inner faces of the blade shell.56. A method according to claim 54, wherein the opposed edges of the stack each form a respective part of the blade surfaces and the spar is fitted to the blade shell with the opposed edges of the stack being exposed in openings of the blade shell.F