This invention relates to a method of manufacturing a turbine blade, in particular for a water current turbine.

Water current turbines, such as tidal current turbines for use in estuaries or open water, are exposed to extremely high loading due the higher density of water as compared with air, so typically suffer far higher loads than equivalent sized wind turbines. The depth of water available in areas of high current flow where the water current turbines are installed may limit the maximum possible blade length, so there are many factors to be taken into account for efficient bladed design. The blade design needs to minimise drag close to the root, yet still have sufficient strength to withstand the forces experienced at this point, if it is to be efficient enough to produce energy. Tidal blade roots are designed to be as thin as possible, but there may still be reductions in efficiency resulting from the way in which the blade connects to the hub.

EP1633624 describes a two part blade construction, in which a holder with spaced recesses receives bushings which are fixed in place by layers of fibre matt.

WO2011/015666 describes a number of alternatives in which an annular strap moulding, bonded to the outside surfaces of the root, holds in place against the base of the root, a plurality of threaded inserts for bolts.

In accordance with a first aspect of the present invention a method of manufacturing an integral turbine blade spar comprises forming a male mould with root end cavities spaced about one end of the mould; applying a first plurality of layers of resin impregnated fibres to the mould, forming fibre lined recesses over the root end cavities; fitting a plurality of bushes to a former, spaced at intervals corresponding to the recesses; fitting the former to the male mould with the bushes aligned with and in contact with the recesses; applying a second plurality of layers of resin impregnated fibres over the first layers and the bushes; curing the formed product; removing the cured product from the mould; and disconnecting the former from the bushes.

The integral turbine blade spar is able to withstand higher shear forces and the embedded bushes enable the root end to be thinner than is possible with a spar having adhesive, or scarf laminated joints. Preferably, the second plurality of layers form gaps between protuberances where the bushes are positioned, the method further comprising providing inserts on the outermost fibre layer in the gaps.

Preferably, the method further comprises vacuum bagging the formed product before curing.

This applies pressure through the inserts to the laminated layers, improving the quality of the resulting laminate

The winding of the layers of fibre laminate may cover all regions of the mould for each layer, but preferably, the method further comprises applying layers of fibre impregnated resin to a transition region and spar box region of the mould after embedding the bushes in the root end.

Preferably, the method further comprises applying spar caps in the spar box region.

Preferably, the method further comprises applying further layers of fibre impregnated resin in the spar box region over the spar caps.

Preferably, the layers of fibre impregnated resin are applied by winding one or more fibre impregnated tapes around the male mould.

Tapes have a width less than the length of the spar, so multiple windings are required to cover the full length of the mould, therefore, preferably a minimum overlap width for adjacent tapes is set.

Preferably, the bush comprises one of a steel alloy bush.

Preferably, the resin impregnated fibres comprise one of glass or carbon fibres. In accordance with a second aspect of the present invention, a water current turbine blade comprises an integral turbine blade spar manufactured by a method according to any preceding claim; and a turbine blade fairing.

The present invention provides an integrated spar root end with embedded bushings which has the advantages of a single part spar construction to deal with the high loading suffered by water current turbines, as well as the robustness of embedded bushings for joining the spar root end to the hub.

An example of a water current turbine blade and method of manufacture in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a male mould design for a method of manufacturing a blade spar according to the present invention;

Figure 2 illustrates the structure of a spar and root for manufacturing a turbine blade according to the present invention;

Figure 3 illustrates a first winding stage in the method of the present invention;

Figure 4 shows a typical bush and a section through it;

Figure 5 illustrates placement of bushes in a circular flange in the method of the present invention;

Figure 6 shows the process of applying windings over the bushes of Fig.5; Figure 7 illustrates placement of strips in the method of the present invention;

Figure 8 is a cross section through an example of a spar root manufactured according to the method of the present invention;

Figure 9 shows a cross section of an embedded bush in the spar root of Fig.8; and,

Figure 10 is a flow diagram of a method of manufacturing a blade spar root in accordance with the present invention.

A turbine blade comprises an internal structure to give support and a blade fairing, or blade shell, to give the desired hydrodynamic properties. A joint- less internal structure of the blade is preferred to maintain structural integrity in the case of extreme load. Normal thrust for a 1MW tidal turbine blade is 2.6 times higher than for a similar power in a wind turbine blade. To form a turbine, the blades are joined to a turbine hub via a root of the blade. A typical water current turbine blade has a blade length in the region of 8m to 12m from its root to its tip to generate a nominal power of 1.OMW to 1.5MW, although shorter or longer blade lengths may be used according to the local conditions, with a corresponding change in the nominal power achieved. The choice of blade length is dependent upon the water depth at the site at which it will be installed. This limitation means that the blade must be as efficient as possible close to the root in order to produce energy and as a result tidal blade roots need to be as thin as possible, with as efficient as possible a blade connection to the hub.

A number of different mechanisms have been used to join the blade to the hub. One option is the use of T-bolts which comprise a longitudinal hole along the root with bolts screwed into nuts inserted into radial holes. However, this does not result in the most effective load transfer between the bolts and the composite fibres and a lot of glass and carbon is required in the manufacture of the product, with the production costs being relatively high because of the thick laminate required.

Another option which provides better transfer of the load from the blade to the root is to use fully bonded insert bushings in the root, with internal threads for mounting bolts on the hub to screw into, allowing releasable attachment to the hub. The production of blades with fully bonded insert bushings in the root may be done by embedding inserts in between layers of fibre when manufacturing the blade casing in a half female mould, or by drilling holes in the root structure after this has been formed and bonding inserts into the holes with adhesive at a final machining station on the production line. However, this method also results in a lot of material being wasted and the process is not efficient. Special equipment is required to drill the holes and bond the inserts and the risk of damage is high if the process is not carried out correctly. Furthermore, if an integral spar, where the internal structure of the blade is in one piece without joints, is desired, then this method is not suitable because by manufacturing the blade in two halves in half female moulds, one half blade has to be integrated with another half blade to create the spar.

Wind turbine blades are most commonly manufactured in female moulds because this allows the blade manufacturing process to be an infusion process whereby the resin is mixed with fibre to create a structural laminate. However, the structure that has been created in two separate moulds has to be combined and for that adhesive joints or scarf laminated joints are required. With tidal, or water current, blades, thick laminates are required in the structure and the blade thickness is small, making it very difficult to integrate any joint in the spar in such a way that potentially very high loads, in particular spar web shear loads, can be supported. The high loading of tidal blades and the extensive laminates required in the structure, for a blade which is relatively thin, increase the risk that a spar structure with joints would be unable to support the loads on the blade.

The present invention overcomes the water turbine specific problems by manufacturing an integral spar with bushings embedded in the laminate using a male mould.

An example of a method of manufacturing a turbine blade according to the present invention is illustrated in Fig. 10. In a first step 50, a male mould 20 is manufactured with the required shape, including protrusions 21 and recesses 22 formed into a root end of the mould to act as a former for where bushes will be added at a later stage. The protrusions and recesses are part of the male mould skin and are not demoulded with the finished product. A former, for example a circular flange 23, is fitted to the root end of the male mould and has adapter plate holes 24 at a spacing corresponding to the spacing of the recesses 22 on the male mould, with appropriate tolerances to assemble the blade afterwards in the turbine.

As can be seen in Fig.2, an integral spar 10 comprises a root 11 , a transition region 12 and a spar box 13. In this example, a root end 14 of the root 11 is cylindrical with a circular cross section - the recesses 22 are not shown in this. The shape of the spar tapers through a transition end 15 of the root into the transition region 12 to a box section of the spar box 13. The spar box section remains substantially constant in width, defined by its spar caps, to its tip 16 and decreases in thickness, defined by its shear web, as required by the blade fluid dynamics.

The first stage 51 of laminate winding comprises winding a plurality of layers

25 of pre-preg, fibre impregnated with resin and partially cured, as shown in Fig.3. The fibres may be glass or carbon fibres and the resin may be epoxy, but any suitable combination can be used. The windings to form the spar cover the mould over the root end, transition region and spar box region. The windings follow the protrusions and recesses of the mould surface at the root end and the outermost layer is just clear of the adapter plate holes 24. The next step is to incorporate the bushings. A typical bushing is illustrated in more detail in Fig.4. Bush 26 comprises a body which is substantially cylindrical along most of its length, tapering towards one end and typically made from steel alloy. The non-tapered end is hollowed out along most of the length of the cylindrical part 27 and a bush thread 28 is formed in the inner surface of the hollowed part 27 of the bush, in a region remote from the open end of the hollowed out part. A section 17 through the hollow end is shown. In the method of the present invention, a plurality of bushes 26 are bolted 52 onto the circular flange 23 using the adapter plate holes. The flange 23 and bushes 26 are fixed in place relative to the mould recesses 22, with the bushes in contact with the outermost layer of the first stage laminate in the recesses, as shown in Fig.5.

In the next winding stage, the bushes are embedded into the blade structure by laminating over the bushes as shown in Fig.6. A plurality of layers of pre-preg 29 are provided, the layers following the contours of the recesses 21, protrusions 22 and bushes 26 in the root end 14. The windings may be continued down the mould 20, beginning the process of forming the spar in the transition region and spar box region at this stage, or else the process of winding the root 11 and embedding the bushes 26 is completed before the laminate in the spar transition region and spar box region is begun. The tape width is generally not sufficient to cover the whole length of the bushes 26 with one tape, so multiple windings are used, each with a specified overlap between them, such that the resulting wound material is sufficient to cover the full length of the bushes.

When sufficient layers of pre-preg 29 have been wound 53 over the bushes to give the required product thickness, then strips 30 are inserted 54 in gaps 31 formed between raised parts 18 of the root end containing the embedded bushes 26. These strips 30 improve the quality of the laminate at a later vacuum bag stage 56 by putting pressure on the wound laminate between the bushes. The strips may be foam, fibre glass pultrusion products, epoxy resin, or other materials with properties which allow them to be shaped to fit the gap, yet still transfer pressure when vacuum bagged.

The spar as a whole is manufactured with a combined winding process and longitudinal material placement which is also wrapped in the winding process. As mentioned above the root end of the spar, where the bushes are embedded, is only made by winding, without any spar caps extending to this section, so the root end is completed before longitudinal material is placed in the spar caps. Once the root end has been completed, longitudinal stacked material is placed on the mould to form the main spar caps, the material layers being cut with the correct shape and placed longitudinally over the cap faces. Pre-preg layers are then wound over the longitudinal caps to complete 55 the lamination process.

Once all of the fibre layers have been wound on the mould, the laminate is vacuum bagged 56 to apply pressure and so compact the laminate before and during the curing process. The material is cured, for example in an oven or an autoclave.

During the curing cycle, the resin contained in the pre-preg fibre tape flows at a certain temperature. In this curing process under the vacuum bag, all the air is removed continuously and the resin fills all voids in the laminate, creating a compact and solid structure integrating all the embedded elements, the bushes, the foam strips and the spar caps. After curing, the circular flange 23 is used to remove 57 the spar product from the spar mould 20. Once the spar is released from the mould, the bushes are unbolted 58 from the circular flange 23 and the product is finished. The embedded bushes allow the blade to be connected with the bearing securely.

The product of the present invention is a one piece spar, with bushes embedded in the laminate, requiring a reduced amount of material to manufacture, as compared with prior art designs discussed above the root due to the better load transfer characteristics of the design. As this design transfers load more effectively than prior art constructions, it is then possible to design a blade with a reduced root diameter and thinner laminates, relative to that required for a spar made on a female mould, thereby improving the efficiency of the blade.