WO1989009336A1

WO1989009336A1 - Improvements in or relating to structures containing anisotropic material

Info

Publication number: WO1989009336A1
Application number: PCT/GB1989/000314
Authority: WO
Inventors: George Jeronimidis; Nikos Michael Karaolis; Peter James Musgrove
Original assignee: George Jeronimidis; Nikos Michael Karaolis; Peter James Musgrove
Priority date: 1988-03-23
Filing date: 1989-03-23
Publication date: 1989-10-05
Also published as: GB8906720D0; GB2216606A

Abstract

An elongated structure such as a blade or fin, and especially the blade of a wind turbine, comprises an outer part shaped so as to generate useful thrust when set in motion relative to surrounding fluid at an appropriate angle of attack, and a reinforcing inner part. At least one of these parts includes material of anisotropic character, whereby elongation or bending of the length of the structure causes the outer part to twist about its length, so varying the angle of attack. As wind speed increases, and with it the risk of overloading or damaging the blade or turbine, this angular variation progressively moderates the forces upon the blades and so avoids the overloading.

Description

IMPROVEMENTS IN OR RELATING TO STRUCTURES CONTAINING ANISOTROPIC MATERIAL This invention relates to structures containing anisotropic material. By anisotropic material, we mean material that exhibits different physical properties or actions in different directions. The invention applies particularly to structures in which the anisotropic materials are in sheet-like form, the sheets exhibiting greater extensibility in one of their surface directions than in the direction at right angles.

The invention applies especially to blades or other elongated structures, shaped so as to be capable of generating useful thrust when in motion relative to surrounding fluid at an appropriate angle of attack. Such blades may acquire energy and so generate useful force because moving fluid flows over them, as is the case for example with wind turbines and other wind-energy devices. Alternatively, as with a propeller, such a blade may be rotated under power and transfer energy to the surrounding fluid which then exerts useful force.

It has been proposed, for instance in the published

Proceedings of a Workshop on "Use of Composite Materials for Wind

Turbines", run jointly by the British Wind Energy Association and the UK Department of the Environment at Harwell, UK in

November 1987, that an isotropic material could be used for wind turbine blades. Such blades can be mounted in various ways known in the art, but are typically either vertically-mounted, by which we mean mounted to rotate about a vertical axis, or horizontally mounted so that they rotate about a horizontal axis. In those

Proceedings it is appreciated that as wind turbine blades rotate at increasing speed, they are subjected to bending and elongating forces which increase with that speed. For a horizontally-mounted blade the bending force results from the wind which strikes the blade in a 'direction parallel to the axis of rotation and tends to bend it lengthwise so that its tip lies downwind of its root. The elongating force is ^*a centrifugal force caused by the rotation of the blade, and tends to stretch the blade radially and thus increase the blade radius. A vertically-mounted blade is typically in the form of a long straight blade mounted with its length vertical and with the blade centre fixed to one end of a horizontal arm which rotates about a vertical axis displaced from the blade and intersecting the arm. In use such a blade is subjected to bending by centrifugal force which tends to distort the blade so that its tips lie at a greater radius than its centre. For such a blade, the bending effect is of a greater order than any elongation effect resulting from the same rotation.

In the Proceedings it is appreciated that if the blade is made of anisotropic material, then the bending or elongation of the blade just described can result also in twist of the blade about its lengthwise axis, with the result that the angle of attack between.the blade and the surrounding fluid - air in the case of a wind turbine - may change. This change may be put to useful effect; as the blade bends or lengthens, indicating a tendency to overload or structural failure, the progressive change in the angle of attack may be used to counteract this tendency, for instance by diminishing the load upon the blade and so the resultant bending, or elongation.

The present invention arises from appreciating two things in particular. Firstly that there are limitations on making such structures as wind turbine blades entirely from anisotropic materials. The shape of such blades is complex and closely specified, and if such a blade is made wholly from anisotropic material and is subjected in use to bending or elongation, that change of shape is certain to result not only in useful twist but also in detrimental other changes to the blade shape, such as buckling. Secondly, that by the appropriate inclusion of anisotropic material in a structure such as a wind turbine blade, twist and useful resultant change to the angle of attack can be induced other than by lengthwise bending, or by elongation resulting from the force of the wind, or from forces generated by the rotation of the blade.

The present invention is defined by the claims, the contents of which are to be read as included within the disclosure of the specification, and the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which :-

Figures 1 to 5 are transverse sections through five different wind turbine blades; Figure 6 is a plan view of the blade of Figure 1;

Figure 7 is a cut-away elevation of a hollow cylindrical reinforcing spar;

Figure 8 is a cut-away perspective view of another hollow cylindrical reinforcing spar; Figures 9 and 10 show the effects of bending and elongation respectively upon cylindrical spars containing anisotropic material ;

Figures 11 and 12 show a vertical-axis wind turbine in elevation and plan view respectively; Figures 13 and 14 illustrate the effects of bending and elongation upon a blade of the turbine of Figures 11 and 12;

Figures 15 and 16 are views taken in the directions of arrows XV and XVI in Figures 13 and 14 respectively, and

Figures 17 and 18 respectively illustrate the effects of bending and elongation upon blades of a horizontal-axis wind turbine.

Figure 1 is a transverse section through a wind turbine blade comprising two parts. The outer part 1 is of generally aerofoil section and the hollow inner part 2 is a reinforcing spar which presents a long axis 3 and is of circular section. Parts 1 and 2 are attached together by glued joints 4 and 5 so that the effect of the attachment extends over the full length of part 2, which in turn will typically extend for the full span length of the blade from root 25 to tip 26, as shown in the plan view of Figure 6. In the blade of Figure 2 a spar 6, roughly oval in cross-section, replaces the cylindrical spar 2 of Figure 1. The leading edge of spar 6 registers with the leading edge 7 of the blade itself, and the section of spar 6 registers exactly with the internal section of the surface skin 8 of the blade in which it is located. Behind the spar 6, a secondary spar or insert 9, roughly triangular in section, defines the shape of the trailing part of the blade. In the blade of Figure 3 an upper outer skin 10 and lower outer skin 11 meet at a leading edge joint 12 and a trailing edge joint 13, the joints being reinforced by formers 14 and 15 respectively. The formers 14 and 15 both extend for substantially the whole span length of the blade 1, like the longitudinal spars 2, 6 and 9 of Figures 1 and 2, but are less effective than those spars to control the aerofoil shape of the blade when subjected to loads in use. To provide this control, the blade of Figure 3 includes transverse ribs 18 located at intervals along the length of the blade; these ribs connect the formers 14 and 15 so as to maintain the chord of the blade, and the upper and lower edges 16 and 17 of each rib 18 are attached to the inner surfaces of skins 10 and 11 respectively. The blade 20 of Figure 4 comprises a filler interior .21, which extends for the full span of the blade and is surrounded by a skin 22. Filler 21 is typically of polyurethane foam, skin 22 is typically of plastics reinforced with glass fibre (GRP) and upper and lower spars 23 and 24, typically also of GRP, are bonded to the skin 22. Spars 23 and 24 typically extend the full span length of the blade and promote the requisite bending stiffness like the spars 2, 6 and 9 of Figures 1 and 2. Figure 5 shows a simplified version of the blade of Figure 4, intended typically for small-scale blades, in which the spars 23 and 24 are omitted and the mass of filler 21 by itself provides all the bending stiffness that the blade requires.

According to the invention either the outer part or the inner part, or both parts, of the blades shown by way of example in Figures 1 to 6 include material of an overall anisotropic character relative to the length of the blade. In Figure 1 either the outer part 1 or the spar 2 could have this character. In the blade of Figure 2 the character could be present in any or all of the skin 8, the forward spar 6 and the rearward spar 9. In Figure 3 the character could be present in the upper and lower skins 10 and 11, and in Figure 4 it could be present in the skin 22 and/or the upper spar 23 and/or the lower spar 24. In Figure 5 the anisotropy must be present in the skin 22.

Figure 7 shows a hollow GRP cylinder 30 of circular section and formed about an axis 31, which could in principle be a suitable unit for use as the spar 2 of Figure 1, where that spar is intended to undergo useful twist as a consequence of elongation. Cylinder 30 comprises three layers 32-34 of sheet-form GRP laminated together. As is shown for layer 32, a single web of aligned glass fibres 35 could be embedded in each layer, these fibres being laid helically at an angle A to the axis 31. More typically, as indicated for layers 33 and 34, each of the laminated sheets comprises two webs of aligned fibres 37, 38, either woven together or laid one over the other within the sheet, with the fibres of the two webs 37, 38 making angles of B and C with axis 31 respectively. Provided angle A is other than zero or a right angle, or the mean of angles B and C is other than zero or a right angle when webs 37, 38 are similar in all respects other than the angle at which they are laid, then layers 32-34 will inherently have an anisotropic character, with the result that if as shown in Figure 10 such a hollow cylinder 30 is open but supported at its root 40, is closed at its tip 45, and is connected by way of a conduit 46 to a source of pressurised fluid 47 by way of a valve 48, and if that valve is opened so as to fill the interior of the cylinder with pressurised fluid so as to elongate the cylinder length from an original value of L to a new value of L+D, then the axis 31 will remain straight. However the use within the cylinder wall of an isotropic material as shown in Figure 7, in which the fibres which give the material its anisotropic character are wound helically, will cause an imaginary line 42, drawn on the surface of the cylinder parallel to axis 31 before elongation, to change from straight to helical. Figure 8 diagrammatically shows an arrangement of anisotropic material within a hollow cylinder 67 which leads to a different effect from the helical arrangement of Figure 7. In Figure 8 the material is incorporated within, or is applied to, the hollow cylindrical structure in two regions 61 and 62, lying respectively to opposite sides of a mid-plane 60 which includes the cylinder axis 31. The two separate sections of the anisotropic material are arranged so that their aligned fibre or other constituents 63 and 64 are arranged in mirror-image fashion to each other on opposite sides of the mid-plane 60. That is to say if the elements 63 in region 61 are disposed at an angle of +F relative to the axis as seen by a viewer progressing continuously around the circumference of the cylinder in a clockwise or anticlockwise direction, those elements 64 in the opposite region 62 are arranged at .an angle of -F as seen by the same viewer. It is also within the scope of the invention, as shown in part of Figure 7, that the two sections of anisotropic material need not between them encompass the axis 31 of the structure entirely, but could be confined to discrete axially- extending strips 65 and- 66 lying on opposite sides of the mid-plane 60, the remainder of the structure being of isotropic character. As well as being incorporated within the total structure during its fabrication, it is also possible that the necessary anisotropic property of this invention could be applied to an otherwise isotropic structure by bonding or otherwise attaching anisotropic material, for instance in the form of strips 65, 66 attached to the surface of an otherwise isotropic cylinder after fabrication. It is further within the scope of the invention that an appropriate material could be confined to region 61 and that region 62 of the structure could be of isotropic character, or that strip 65 could be present but strip 66 omitted: again the result would be a structure having an overall lengthwise anisotropy due to the different structural characters existing to either side of the mid-plane 60. Let us assume now that a hollow cylindrical cylinder 67, as just described with reference to Figure 8, 1s anchored at its root 40 and subjected to a bending load directed as indicated by arrow 41, as shown in Figure 9. Provided the direction of load 41 does not lie exactly in mid-plane 60, not only will the axis 31 bend but an imaginary line 42 drawn on the surface of the cylinder, as in Figure 10, can take up the helical configuration shown in Figure 9 when bending is applied.

Figures 11 to 14 show the potential practical effect of these changes for a vertical-axis wind turbine (VAWT). All these VAWT's comprise a central mast 50 supporting a hub 51 from which radiate arms 52 on which the blades 53 are carried. At rest, the blades are set at an angle of attack E. When the blades 53 contain anisotropic material arranged in the "mirror image" fashion of Figure 8 and are subjected in use by centrifugal action to an externally-generated bending force as in Figure 9, the result is as shown, in simplified and schematic elevation, in Figure 13. Figure 15 is a similarly simplified and schematic view taken in the direction of the arrow XV in Figure 13, and shows how the twist, caused by the centrifugal action and the resultant bending, may be used to diminish the angle of attack from an unchanged E, where blade 53 is attached to arm 52, to a diminished E_] at the tip 54 of the blade, thereby providing the means whereby as the wind strength increases towards the level at which the VAWT and/or associated components might become overloaded or fail, the turbine blades automatically change shape so that blade bending and rotor speed both increase no further. Figures 14 and 16 are comparable with Figures 13 and 15 respectively, but instead of indicating the bending effect which the blades 53 will still undergo in use, they illustrate the lengthening and consequent twisting effect that the blades would undergo if they were hollow, closed-ended blades with anisotropic material arranged helically as in Figures 7 and 10, and if the valve 48 were to respond to a wind-speed sensor 49, which opens the valve and pressurises the interior of the blades 53 whenever the wind speed approaches a level at which the turbine and/or its components might be overloaded or damaged. The blade twist induced by lengthening the blade again diminishes the angle of attack from an unchanged E where the blade meets the arm 52 to a diminished E^ at the tip 54, with the same automatic safety effect as before.

It is of course within the scope of the invention that the anisotropic material could be arranged, in appropriate cases, so that the twist caused by bending or elongation results in an increase of the angle of attack, and thus an approach to a stall condition, rather than a decrease and an approach to a "feathered" condition.

The same beneficial twisting of a blade-like structure, comprising material laid in "mirror image" fashion as shown in Figure 8, and resulting from an externally-applied bending force such as force 41 of Figure 9, is obviously applicable not only to the blades of wind turbines but also to other blade-like structures which move through surrounding masses of fluid, for instance the blades of comparable hydrodynamic rotary mechanisms and also to the fins, keels and the like of sailing craft, whether fixed or of centerboard, dagger plate or other moveable type.

Figures 17 and 18 are comparable with Figures 13 and 14 but illustrate the effects of in-use bending and elongation upon the blades of a horizontal-axis wind turbine (HAWT), instead of a VAWT. In Figure 17 the blades 70 are shown bending, under the force exerted upon them by the incident wind 71 which lies parallel to the axis of rotation 72. This bending causes the blades 70 to distort so that their tips 73 lie downwind of their roots 74. If the blades contain material of bend-responsive anisotropic type, for instance of the "mirror image" type illustrated in Figure 8, then such bending of the blades can result in useful twist and consequent change of the angle of attack, comparable to that shown in Figure 15. Figure 18 illustrates radial elongation of the blades in use. Solid blades could undergo such elongation as a result of centrifugal force, and if the blades were hollow, as illustrated in Figure 10, the elongation could be augmented by using source 47 and associated equipment to pressurise the hollow Interior of the blade as described with reference to that Figure. If the blade contains anisotropic material arranged in an elongation-sensitive way, as described for instance with reference to Figure 7, then such elongation of the blades by internal pressurisatlon and/or centrifugal force can result in twist and in useful change of attack comparable to what is shown in Figure 16. Figures 7 and 8 show elongated structures in which the anisotropic material is plastics-based, and in which the fibrous elements responsible for the anisotropic behaviour are arranged in a particular way relative to the axis 31 of the structure. Anisotropic behaviour can of course result from the use of other materials. For example, the grain in veneers or other thin sheets of wood can create comparable effects to the webs of aligned fibres 1n Figures 7 and 8, and spars such as items 2, 6, 9, 23 and 24 of Figures 1 to 4 could typically be fabricated from bonded wood laminates instead of from bonded sheets of GRP or other plastics-based compositions.

It will have been apparent that in a typical structure according to this invention, the directions of the fibre or other aligned constituents are such that for the structure as a whole there is overall asymmetry relative to the long axis of the structure. The invention includes structures in which that overall asymmetry results from anisotropic material confined to only part of the length of the structure, the remainder being of isotropic character, for instance. It is also possible that material exhibiting one type of anisotropic behaviour (e.g. that of Figures 7 and 10) could be used in one part of the length of a complete structure, and material exhibiting another, type (e.g. that of Figures 8 and 9) in another part. Also, the anisotropic material or materials may contain elements that are not continuous and precisely parallel, like the fibres 35, 37, 38 of Figure 7, but shorter elements that have undergone an orientation so that they all lie generally parallel to a predetermined direction. Generally the angle (A, B, C in Figure 7; +F, -F in Figure 8) which those elements make with the long axis of the structure will be approximately constant around the circumference of the structure from root to tip, although 1n some designs it could be desirable for this angle to be varied along the lengthwise axis, so that close to the root (or to the point of attachment to the rotor, e.g. the joints between 52/53 in Figures 11 and 12) the value of the angle will be different from the value towards the tip. Tests suggest that where the anisotropic material comprises two overlaid layers of fibre or other reinforcement, as in webs 37 and 38 in Figure 7 for instance, maximum beneficial twist resulting from bending and/or stretching of the structure is achieved when angles B and C are respectively about 20° and 70°.

In one tested construction, a tubular spar (similar to items 2, 30 in Figures 1 and 7) for a wind turbine blade was a cylinder with a mean diameter of 150mm corresponding to a mean blade chord (27, Figure 6) of about 1 metre. The wall of this cylindrical spar was made from eight laminae of glass-reinforced polyester resin, and in each lamina the glass fibres were predominantly unidirectional, as in web 35 in Figure 7. Four laminae each of thickness 0.5mm were laid with their fibre directions at an angle of 60° to the spanwise direction, that is to say a direction parallel to axis 31. The other four laminae each of thickness 0.25mm were laid with their fibre directions at an angle of -8° to the same direction. The spar was made by filament winding, each 0.5mm thick lamina at a 60° orientation being followed by an 0.25mm thick lamina laid at -8° orientation. The spar was fabricated so that it was impervious to gas and was closed at its tip end (45, Figure 10). At the root end (40) it was connected, as shown in Figure 10, not only to pressure source 47 and associated parts in the way that has already been described, but also by way of a second conduit 80 via a second sensor-operated valve 81 to the open air. When valve 81 is opened in response to a signal from the sensor 49, the gas within the spar is vented to atmosphere, so reducing the twist formerly induced by pressurisation from source 47.

Claims

1. An elongated structure comprising a first and hollow elongated outer part and at least one second and reinforcing part within and attached to the first part, in which the first part is so shaped as to be capable of generating useful thrust when set in motion relative to surrounding fluid at an appropriate angle of attack, and in which at least one of the first and second parts includes material of an overall anisotropic character relative to the long axis of the structure, whereby elongation or bending of the length dimension causes a twisting of the structure about the same dimension, whereby varying the angle of attack.

2. A structure according to Claim 1 in which at least one second and reinforcing part is elongated and arranged lengthwise within the elongated outer part.

3. A structure according to Claim 1 in which the anisotropic effect results from the use of a material, constituents of which are arranged in physical alignment.

4. A structure according to Claim 3 in which that alignment follows a helical path relative to the axis of the respective part.

5. A structure according to Claim 3 in which the alignment makes the same angle with the long axis of the respective part at any cross section through that part, but the sign of the angle changes between one half of that cross section and the other.

6. A structure according to Claim 5 in which the anisotropic material is confined to only a fraction of the periphery of the respective part..

7. A structure according to Claim 3 in which the anisotropic material includes fibre-reinforced matrix material, and in which the anisotropy results from the effective alignment of the fibres within the matrix.

8. A structure according to Claim 7 in which the fibre is glass fibre and the matrix is polyester resin.

9. A structure according to Claim 3 in which the anisotropic material includes wood, and the anisotropy results from the alignment of the grain of the wood.

10. A structure according to Claim 9 in which the anisotropic material comprises wood veneers laminated together.

11. A structure according to Claim 2 in which the anisotropic character is present substantially in an elongated inner reinforcing part only.

12. A structure according to Claim 11 in which the said inner reinforcing part is in the form of a closed-ended hollow and fluid-tight cylinder, the interior of which is connectable to a source of pressurised fluid, whereby when so connected the cylinder elongates axially, thereby inducing twist of the inner part about its long axis and altering the angle of attack of the attached outer part.

13. A structure according to Claim 11 in the form of a blade for a fluid turbine, in which the outer part presents the necessary fluid-dynamic shape and the attached inner part both acts as reinforcement for the outer part and presents the means of attachment for the blade to a hub.

14. A structure according to Claim 11 in the form of a blade for a vertical-axis wind turbine.

15. A structure according to Claim 13 in the form of a blade for a horizontal-axis wind turbine.

16. A structure according to Claim 11 in the form of a keel, centre plate or the like for a sailing craft.

17. A structure according to Claim 3 in which the part of the structure which exhibits the anisotropic behaviour is itself composite, comprising an isotropic core and an anisotropic outer layer.

18. A structure according to Claim 1, substantially as described with reference to the accompanying drawings.