GB2460021A

GB2460021A - Rotor blades for underwater turbines

Info

Publication number: GB2460021A
Application number: GB0808313A
Authority: GB
Inventors: Angus Fleming
Original assignee: Aviation Enterprises Ltd
Current assignee: Aviation Enterprises Ltd
Priority date: 2008-05-08
Filing date: 2008-05-08
Publication date: 2009-11-18
Anticipated expiration: 2028-05-08
Also published as: GB2471031A; GB2471031B; GB0808313D0; GB2460021B; GB201012668D0

Abstract

A blade 14 for an underwater turbine has an inner void and a foam material 30, 32, located therein. The blade may comprise first and second skins between which the foam material extends. The foam material may substantially fill the inner void. Alternatively the foam material may only partially fill the void. In some arrangements, the foam material may extend only partially from the blade tip to the blade root to leave a space which may be divided into regions by one or more bulkheads, the regions having means for enabling them to be flooded or drained. In other arrangements, the foam material may be in the form of ribs separated by gaps.

Description

ROTOR BLADES

The present invention relates to rotor blades, and, in particular, to rotor blades for use in underwater tidal power generation installations.

Background of the Invention

There is increasing interest in the use of underwater power generating equipment that makes use of the energy of tidal flows. Such equipment is secured to the bed of a body of water, such as a sea, estuary or river, and makes use of a rotary generator to generate electricity. The generator is driven by a number of rotor blades placed in the water flow. An example of such a tidal power generating installation is illustrated in Figure 1 of the accompanying drawings.

In the example shown in Figure 1, the installation 1 is located on a bed 2 of a body of water 3. A generating unit 4 is mounted on an underwater support structure 5 which is fixed to the bed 2. The generating unit 4 includes a rotary generator and associated equipment for generating electricity. The generator is driven by a rotor 6 carried on an input shaft of the generator. The rotor 6 has a plurality of rotor blades 14.

Figures 2 and 3 illustrate respective cross sectional views of a rotor blade 14 which comprises a root 16 by which the rotor blade is attached to the rotor of the generator. A spar extends from the root 16 to a tip 18 of the blade. The blade has a leading edge 22 and a trailing edge 24, and the shape of the blade is defined by first and second skins 26 and 28.

The skins 26 and 28 are generally of a composite fibre/resin material, and are moulded to the correct shape. The skins 26 and 28 are supported by the spar 20 which provides the blade 14 with additional strength.

One technique that can be used to generate more electricity from a given flow is to use larger diameter rotor blades. However, large rotor blades can be restrictively heavy if constructed using traditional techniques. It has, therefore, been proposed that hollow composite blades be used, in which a shaping skeleton is covered by a thin skin to provide the required blade shape. However, it has been discovered that the significant pressure differences experienced by the underwater rotor blade leads to significant flexing of the blade skin, and hence to significant problems with failure due to fatigue of the blade material.

One possible solution to this problem has been proposed in United Kingdom Patent No.2394515, in which the void inside the hollow blade is completely filled with water in order to equalise the internal and external hydrostatic pressure experienced by the rotor blade.

However, this does not in itself help to alleviate the suction problem caused by hydrodynamic pressure.

However, completely flooding the blade as suggested in the prior art creates a number of additional problems. For example, filling the blade interior with water can lead to build up of marine growth, which is undesirable.

A more significant problem is associated with the pressure generated by the filling water itself. As the rotor blade rotates, centrifugal forces acting on the water in the blade cause highly undesirable increases in internal pressure inside the blade. Such increases in internal pressure can increase the likelihood of failure of the blade.

The centrifugal loads on the fluid inside the blades vary with radius, as a "station" pressure, and with rotational speed. In addition, the loads also accumulate as a head -i.e. the centrifugal pressure built up inside at a root element is added to that on the next outboard element and so on to the tip. This is a serious problem as it is an internal pressure which tends to inflate the structure. Such inflation makes the process of designing a blade structure much more difficult.

Summary of the Invention

According to one aspect of the present invention, there is provided a blade for an underwater turbine, the blade comprising first and second skins which define an outer shape of the blade having a root portion, a tip portion, and leading and trailing edges that extend between the root and tip portions, the first and second skins defining an inner void of the blade; and a foam material located in the inner void of the blade, wherein the foam material substantially extends between the first and second skins, and substantially extends from the leading edge to the trailing edge of the blade.

In one example embodiment, the foam material substantially fills the inner void of the blade.

In another example embodiment, the foam material partially fills the inner void of the blade.

In yet another example embodiment, the foam material extends only partially from the tip portion towards the root portion of the blade, such that a region of the inner void at the root portion of the blade lacks the foam material.

The foam material may be arranged as a series of spaced apart ribs, each of the ribs extending substantially from the leading edge to the trailing edge of the blade. Such ribs in the series may have a constant predetermined width, or may have variable predetermined widths. Such ribs may be spaced apart by a constant amount, or may be spaced apart by a variable amount.

The series of ribs may extend only partially from the tip portion towards the root portion of the blade, such that a region of the inner void at the root portion of the blade lacks the foam material. Such an example may also comprise means for flooding and draining the region of the inner void at the root portion of the blade. In addition, such an example may also include at least one water tight bulkhead located in the region of the inner void at the root portion of the blade, the bulkhead serving to divide the region into a plurality of sub-regions, each sub-region having associated means for flooding and draining that sub-region.

Blades embodying the present invention may also include a hollow spar which extends from the root portion to the tip portion of the blade, through the inner void. The spar may be at least partially filled with a foam material, and such foam material may be arranged as a series of spaced apart ribs.

According to another aspect of the present invention, there is provided a blade for an underwater turbine, the blade having an inner void, and a foam material located in the inner void, which foam material partially fills the inner void.

Bnef Description of the Drawings

Figure 1 illustrates an underwater tidal power generation installation; Figure 2 illustrates a first cross sectional view of a rotor blade for use in the installation of Figure 1; Figure 3 illustrates a second cross sectional view of the blade of Figure 2; Figure 4 illustrates a cross sectional view of a rotor blade embodying one aspect of the present invention; Figure 5 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 6 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 7 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 8 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 9 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 10 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 11 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 12 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; Figure 13 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention; and Figure 14 illustrates a cross sectional view of a rotor blade embodying another aspect of the present invention.

Detailed Description of the Preferred Embodiments

As mentioned above, Figures 2 and 3 illustrate a blade 14 suitable for use with an underwater power generating installation. The blade is of a spar design in which a spar 20 extends from a root 16 of the blade to a tip region 18 of the blade 14. In the example shown in Figure 3, the spar 20 is hollow, having a cavity 21 that extends along its length. An alternative possibility is for the spar to be in the form of an I-beam that runs the length of the blade. The blade 14 is completed by first and second skins 26 and 28 which extend from the leading edge 22 to the trailing edge 24 of the blade 14. The skins 26 and 28 define an inner void 29 inside the blade 14.

Figure 4 shows a cross sectional view of a first blade embodying the principles of the present invention. The example shown in Figure 4 has the same basic construction as that of the blade as shown in Figures 2 and 3. In the example shown in Figure 4, the void 21 inside the spar 20 is filled, from the root 16 to the tip 18, with a substantially incompressible foam material 30. The foam material 30 is bonded to inner surfaces of the spar 20 using an appropriate bonding agent. The foam material 30 serves to prevent the spar 20 deforming under changes in external pressure as the rotor rotates in the current flow.

The foam material must have sufficient compressive strength and shear strength. The foam material ideally has a closed cell structure with low water absorption characteristics.

The bonding agent must be compatible with the adherands (i.e. the foam material and the composite skin material), and must exhibit adequate shear strength, toughness, and water tolerance.

A foam material is chosen as the filling material, because such a material is able to combine the strength and incompressibility required for improving the blade design, with a relatively low weight.

A second blade design embodying the principles of the present invention is illustrated in cross sectional view in Figure 5. Once again, the basic structure is the same as the blade of figures 2 and 3. In the example of Figure 5, the void 29 of the blade 14 is filled with a substantially incompressible foam material 32. The foam material 32 serves to prevent the skins 26 and 28 of the blade 14 deforming under changes in external pressure as the rotor rotates in the current flow. The foam material extends from the leading edge to the trailing edge of the blade, and from the root to the tip of the blade, such that the inner void of the blade is completely filled with the foam material.

As will be readily apparent, it is possible to combine the examples of Figures 4 and 5, such that the void 21 of the spar 20, and the void 29 defined by the skins 26 and 28 are both fully filled with foam material 30 and 32. Such an example is shown in Figure 6.

Although the examples shown in Figures 4, 5 and 6 overcome the drawbacks and problems of the prior art very well, the costs associated with the complete filling of the blade voids can be high. Accordingly, it is desirable to provide solutions in line with the principles of the present invention, which are potentially more cost efficient, whilst overcoming the problems

of the prior art.

A first example of such a blade construction is shown in Figure 7. In this example, the void 21 of the spar 20 has a foam filling. Instead of a complete foam filling, the example of Figure 7 has a series of foam ribs 34 which extend across the spar 20. The ribs 34 are spaced apart by gaps 35. The relative dimensions of the ribs 34 and the gaps 35 are determined such that the amount of foam material used is minimised, whilst the support for the spar 20 and the skins 26 and 28 is maintained at a desired level.

Using a discontinuous foam filling also has the significant advantage that the bonding agent used to bond the foam to the spar 20 can be more effective. This is because suitable pressure can be applied to the surface of the spar 20 in order that the bonding agent is suitably squeezed between the spar 20 and the foam ribs 34. The gaps 35 allow excess bonding agent to flow out of the joint area, which in turn serves to strengthen the joint. All of the ribs 34 can be of the same size, and spaced apart by equal amounts. Alternatively, the sizes of the ribs 34 can vary along the length of the spar, as can the sizes of the gaps 35.

For example, the ribs may be 50mm in width and the gaps 8mm in width. In another example, the ribs may be 50mm in width, and the gaps varying from 8mm at the tip region to 50mm at the root region. It will be readily appreciated that any rib width and gap width combination can be used in accordance with the principles of the present invention.

As an alternative to using spaced apart ribs, grooves or slots can be machined into at least one solid foam block to allow the adhesive to flow. Such a technique has the advantage that the foam core can be pre-formed.

Use of a number of foam material ribs can be applied to the main void of the blade defined by the skins 26 and 28, as is illustrated in Figure 8. In this example, ribs 36 extend across the void of the blade, and are separated by gaps 37. Once again, the relative dimensions of the ribs 36 and the gaps 37 are determined by the need to provide sufficient support for the skins 26 and 28, whilst minimising, or at least lowering, the amount of foam material being used. For example, the ribs may be 50mm in width and the gaps 8mm in width. In another example, the ribs may be 50mm in width, and the gaps varying from 8mm at the tip region to 50mm at the root region. It will be readily appreciated that any rib width and gap width combination can be used in accordance with the principles of the present invention.

Figure 9 illustrates an example of a blade 14 in which the spar 20 has a portion towards the tip 18 which is fully filled with a foam material 30, and a portion towards the root 16 which includes spaced apart ribs 34 of foam material. Similarly, Figure 10 illustrates a blade have a region of the main void 29 towards the tip of the blade 14 fully filled with foam material 32, and a region towards the root 16 of the blade 14 filled using spaced apart foam ribs 36. The use of the combination of spaced apart foam ribs and fully filled regions gives the blade designer more variation when considering the balance between strength and amount of foam material to be used.

Since the tip region of the blade 14 experiences greater changes in static pressure than the root 16, due to greater difference in water depth as the rotor blade turns, greater strength and support is generally needed at the tip region 18.

An alternative solution is then simply to fill only a tip region the main void 29 of the blade 14, as illustrated in Figure 11. In the Figure 11 example, the tip region of the blade 14 is completely filled with foam material 32. As an alternative, the tip region can be provided with foam ribs 36 spaced apart by gaps 37, as shown in Figure 12. In the examples in Figures 11 and 12, the root region 38 of the blade 14 is left hollow, since this region experiences less static pressure difference as the rotor turns. As will be readily appreciated, the spars 20 of the examples of Figures 11 and 12 could also be fully or partially filled with foam material.

The tip region of the blade can be chosen in dependence upon the design of blade and its intended use. In one particular example, the tip region may extend 45% of the blade length from the tip.

Figure 13 shows an example blade 14 that uses a combination of the previously described techniques. The spar 20 is partially fully filled with foam material 30, and partially filled with ribs 34 and gaps 35. A tip region of the main void 29 is filled with foam material ribs 36 and associated gaps 37. In addition, the root region 38 of the blade 14 is designed to be flooded when the rotor blade 14 is placed under water. In order to enable the void 38 to fill and drain, holes 40 are provided in one or more locations in one or both of the skins 26 and 28.

The Figure 13 example makes use of foam material in the tip region of the main void 29 and in the spar 20 in order to strengthen the blade, and provide resistance to the pressure differences as the rotor turns. The root region 38 is filled with water in order to provide resistance in that region to fatigue. The use of water filling in a smaller region close to the root means that the problems of the prior art do not affect such a blade, because the amount of water is reduces, and so the pressure head created when the rotor turns is smaller.

In order to reduce the possible effects of the centrifugal pressure associated with flooding the main void of the blade, the root region 38 can be subdivided into any number of smaller cavities. Such a design is illustrated in Figure 14, in which the region 38 is divided into separate cavities 41 to 46, each of which is provided with at least one drain and fill hole 40.

It will be readily appreciated that the region 38 can be subdivided into any number of cavities. The cavities are separated by watertight bulkheads 48, which prevent water moving between cavities. This serves to reduce the effects of pressure caused by rotation of the blade 14. The example shown in Figure 14 also incorporates a watertight bulkhead at the root 16, in order that water is not able to transfer from the region 38 into the root itself.

It will be readily appreciated that the blades shown in Figures 4 to 14 are examples of blades that embody the principles of the present invention, and that any combinations of features can be used within the principles of the present invention. For example, the size and spacing of the ribs of foam material can be chosen in dependence on the size and intended use of the blade, and the amount of the inner void 29 filled by the foam material can also vary.

It will also be readily appreciated that the principles of the present invention can be applied to any type of blade construction that features an inner void. For example, the principles set out above can be applied to a blade construction that does not make use of a spar, but that rather uses two skins to form the blade profile and to provide the necessary strength.

Claims

CLAIMS: 1. A blade for an underwater turbine, the blade comprising: first and second skins which define an outer shape of the blade having a root portion, a tip portion, and leading and trailing edges that extend between the root and tip portions, the first and second skins defining an inner void of the blade; and a foam material located in the inner void of the blade, wherein the foam material substantially extends between the first and second skins, and substantially extends from the leading edge to the trailing edge of the blade.
2. A blade as claimed in claim 1, wherein the foam material substantially fills the inner void of the blade.
3. A blade as claimed in claim 1, wherein the foam material partially fills the inner void of the blade.
4. A blade as claimed in claim 1, wherein the foam material extends only partially from the tip portion towards the root portion of the blade, such that a region of the inner void at the root portion of the blade lacks the foam material.
5. A blade as claimed in claim 1, wherein the foam material is arranged as a series of spaced apart ribs, each of the ribs extending substantially from the leading edge to the trailing edge of the blade.
6. A blade as claimed in claim 5, wherein the ribs in the series have a constant predetermined width.
7. A blade as claimed in claim 5, wherein the ribs in the series have respective predetermined widths.
8. A blade as claimed in claim 5, 6 or 7, wherein the ribs are spaced apart by a constant amount.
9. A blade as claimed in claim 5, 6 or 7, wherein the ribs are spaced apart by a variable amount.
10. A blade as claimed in any one of claims 5 to 9, wherein the series of ribs extends only partially from the tip portion towards the root portion of the blade, such that a region of the inner void at the root portion of the blade lacks the foam material.
11. A blade as claimed in claim 10, comprising means for flooding and draining the region of the inner void at the root portion of the blade.
12. A blade as claimed in claim 11, comprising at least one water tight bulkhead located in the region of the inner void at the root portion of the blade, the bulkhead serving to divide the region into a plurality of sub-regions, each sub-region having associated means for flooding and draining that sub-region.
13. A blade as claimed in claim 1, wherein the foam material is provided by at least one block of foam material into which a series of slots are provided, the block having a first and second surfaces arranged to be adjacent inner surfaces of the first and second skins respectively, the slots being arranged to extend into the block away from the first and second surfaces.
14. A blade as claimed in any one of the preceding claims, further comprising a hollow spar which extends from the root portion to the tip portion of the blade, through the inner void.
15. A blade as claimed in claim 14, wherein the spar is at least partially filled with a foam material.
16. A blade as claimed in claim 14, wherein the foam material is arranged as a series of spaced apart ribs.
17. A blade as claimed in claim 14, wherein the foam material is provided by at least one block of foam material into which a series of slots are provided, the block having a first and second surfaces arranged to be adjacent respective inner surfaces of the spar, the slots being arranged to extend into the block away from the first and second surfaces
18. A blade for an underwater turbine, the blade having an inner void, and a foam material located in the inner void, which foam material partially fills the inner void.
19. A blade for an underwater turbine substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.Amended claims have been filed as follows:-CLAIMS: 1. A blade for an underwater turbine, the blade comprising: first and second skins which define an outer shape of the blade having a root portion, a tip portion, and leading and trailing edges that extend between the root and tip portions, the first and second skins further defining an inner void of the blade; and a foam material located in the inner void of the blade, and substantially extending between the first and second skins, and from the leading edge to the trailing edge of the blade, the foam material extending only partially from the tip portion towards the root portion of the blade, such that a region of the inner void at the root portion of the blade lacks the foam material.2. A blade as claimed in claim 1, further comprising means for flooding and draining the Co region of the inner void at the root portion of the blade.3. A blade as claimed in claim 1 or 2, wherein the foam material is arranged as a series of spaced apart ribs, each of the ribs extending substantially from the leading edge to the trailing edge of the blade.4. A blade as claimed in claim 3, wherein the ribs in the series have a constant predetermined width.5. A blade as claimed in claim 3, wherein the ribs in the series have respective predetermined widths.6. A blade as claimed in any one of claims 3 to 5, wherein the ribs are spaced apart by a constant amount.7. A blade as claimed in any one of claims 3 to 5, wherein the ribs are spaced apart by a variable amount.8. A blade as claimed in any one of the preceding claims, comprising at least one water tight bulkhead located in the region of the inner void at the root portion of the blade, the bulkhead serving to divide the region into a plurality of sub-regions, each sub-region having associated means for flooding and draining that sub-region.9. A blade for an underwater turbine, the blade comprising: CO 10 first and second skins which define an outer shape of the blade having a root portion, a tip portion, and leading and trailing edges that extend between the root and tip portions, the first and second skins further defining an inner void of the blade; andCOa foam material located in the inner void of the blade, and substantially extending between the first and second skins, and from the leading edge to the trailing edge of the blade, the foam material arranged as a series of spaced apart ribs, each of the ribs extending substantially from the leading edge to the trailing edge of the blade.10. A blade as claimed in claim 9, wherein the ribs in the series have a constant predetermined width.11. A blade as claimed in claim 9, wherein the ribs in the series have respective predetermined widths. X1712. A blade as claimed in claim 9, 10 or 11, wherein the ribs are spaced apart by a constant amount.13. A blade as claimed in claim 9, 10 or 11, wherein the ribs are spaced apart by a variable amount.14. A blade as claimed in any one of the preceding claims, further comprising a hollow spar which extends from the root portion to the tip portion of the blade, through the inner void.15. A blade as claimed in claim 14, wherein the spar is at least partially filled with a foam material.CO 10 16. A blade as claimed in claim 15, wherein the foam material is arranged as a series of spaced apart ribs.CO 17. A blade for an underwater turbine substantially as hereinbefore described with reference to, and as shown in, Figures 4 to 14 of the accompanying drawings.