Wind turbine blade with carbon fibre tip
The invention relates to a wind turbine blade (14) of fibre-reinforced polymer. The blade (14) is divided into an inner end portion (15) including the blade root and made substantially from fibre glass-reinforced polymer, and an outer end portion (17) including the blade tip and made substantially from carbon fibre-reinforced polymer.
The invention relates to a wind turbine blade according to the preamble of claim 1.
Wind turbine blades are typically made by means of two blade shell halves of fibre-reinforced polymer. When moulded the two halves are glued together along the edges and via two bracings, which prior thereto have been glued to the inner face of one the blade shell halves. The other blade shell half is then arranged on top of bracings and glued thereto and along the edges.
The blade shell halves per se are typically made by vacuum infusion, in which evenly distributed fibres, rovings, which are fibre bundles, bands of rovings or mats which may be felt mats of single-fibres or woven mats of fibre rovings, are layered in a mould part and cover by a vacuum bag. By creating vacuum (typically 80-90%) in the cavity between the inner face of the mould part and the vacuum bag resin is sucked into and fills the cavity containing the fibre material. In order to obtain the optimum distribution of resin, so-called distribution layers and distribution channels are often used between the vacuum bag and the fibre material.
The used polymer is typically polyester or epoxy, and the fibre reinforcement is usually based on fibre glass. It is, however, also known to use carbon fibres which are stiffer than glass fibres, but have a smaller elongation at breakage than glass fibres. The carbon fibres may be added to obtain a higher degree of stiffness and/or a lower weight. It is thus possible to let a portion of the fibre reinforcement be formed of carbon fibres to reduce the weight of the blade without the blade loosing too much of its stiffness. Carbon fibres are, however, encumbered by the drawback of being significantly more expensive than glass fibres, which is one of the reasons why wind turbine blades of carbon fibre-reinforced polymer are not widely used.BACKGROUND ART
From WO 00/14405 it is known to reinforce a wind turbine blade of fibre glass polymer with longitudinal bands of carbon fibre-reinforced polymer.
U.S. Pat. No. 6,287,122 discloses the manufacture of elongated composite products, wherein a variation in the stiffness of the product along its length is obtained by altering the fibre content or the angle orientation of braided fibres.
U.S. Pat. No. 5,520,532 discloses a mould part of fibre-reinforced polymer of a varying stiffness, said stiffness being obtained by varying the number of fibre mat layers.
U.S. Pat. No. 4,077,740 discloses a helicopter rotor blade of a fibre composite material, the stiffness of the blade varying when seen in longitudinal direction. This feature is obtained by varying the fibre orientation so as to obtain an enhanced vibration dampening.
The dead load of modern fibreglass blades constitutes a problem in that a high dead load moment requires a high fatigue resistance in the edgewise direction of the blade. This problem increases with the length of the blades.BRIEF DESCRIPTION OF THE INVENTION
The object of the invention is to solve the above problem in a simple and inexpensive manner.
According to the invention the object is obtained in that the blade is divided into an inner end portion including the blade root and made substantially from fibre glass-reinforced polymer and an outer end portion including the blade tip and made substantially from carbon fibre-reinforced polymer. The weight is thus reduced in the outermost part, whereby the dead load moment is minimised. Less material and/or a smaller cross section is thus required at the innermost portion of the blade and the load on the turbine hub is reduced. The outermost portion of the blade may furthermore be provided with an increased stiffness, whereby the risk of the blade deflecting so heavily that the blade tip hits the turbine tower is reduced. Such a wind turbine blade is more inexpensive to produce than a blade made solely of carbon-fibre-reinforced polymer.
At a certain degree of stiffness, the dead load may be reduced by using carbon fibres in the outer end portion, whereby the dynamic loads on the blade shell and the blade root may also be reduced, said parts being particularly sensitive to dynamic loads.
By changing the carbon fibre content in the outer end portion or the length thereof, the stiffness as well as the natural frequencies may be varied. The stiffness and the natural frequencies may thus be optimised to the specific conditions.
A comparatively stiff outer end portion and a comparatively less stiff inner end portion result in an advantageous deflection shape as regards aerodynamic damping, the damping depending on the integrated deflection along the blade during a vibration. An increased aerodynamic damping is advantageous in that the aerodynamic load thus is reduced.
Compared to a blade made solely of fibre glass-reinforced polymer or a blade made solely of carbon-fibre-reinforced polymer, a blade according to the invention renders an optimum stiffness to costs ratio.
According to an embodiment the outer end portion may constitute between 25% and 50% of the entire length of the blade.
The outer end portion may, however, constitute 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or even 90% of the blade's length.
According to a preferred embodiment the outer end portion opposite the blade tip may include a transition zone in which the carbon fibres are gradually replaced by glass fibres. As a result an abrupt change in the blade stiffness in the transitional area between the carbon fibres and the glass fibres is avoided. At heavy dynamic or static stresses, an abrupt transition between the carbon fibres and the glass fibres causes a concentration of stress, the carbon fibres typically being 3-4 times stiffer than the glass fibres. This may involve a risk of destroying the blade. By using such a transition zone a heavy stress concentration is avoided at the boundary surface between the carbon fibres and the glass fibres.
According to an embodiment the length of the transition zone may be between 0.5 and 1 metre. A length of up to 10 metres or of even more than 10 metres may, however, also be preferred.
According to the invention the two types of fibres may be distributed such in the polymer matrix that carbon fibres or carbon fibre bundles of varying lengths extend from a first end of the transition zone, and glass fibres or glass fibre bundles extend from the opposite end of the transition zone, whereby a particularly smooth transition in stiffness is obtained.
According to another embodiment the transition zone may be formed of a laminate of several fibre layers, in which each fibre layer has a boundary surface at a position in the longitudinal direction, the fibre layer including carbon fibres on one side of the boundary surface and glass fibres on the other side of the boundary face, the boundary surfaces of each fibre layer being displaced in relation to each other in the longitudinal direction of the blade. As a result a gradual change in stiffness in the transition zone is obtained in a particularly simple manner.
According to an embodiment the boundary surfaces may be serrated in a sectional view parallel to the fibre layers. An even smoother transition in the stiffness is thus obtained in the transition zone. The tips of the serrated boundary surfaces may be displaced in relation to each other in the transverse direction of the blade. As a result an additional smooth variation in the stiffness is obtained in the transition zone.BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in greater detail below by means of various embodiments of the invention in diagrammatical views in the drawings, in which
A transition zone may be provided in the blade during the fibre lay-up per se in the mould parts. It is, however, also possible to use prefabricated transitional laminates produced according to the principles shown in
If an existing wind turbine is to be provided with longer blades, this may be obtained by replacing the outermost portion of the blade by a transition zone including one or more transitional laminates and a carbon fibre tip. The weight of the blade is not or only slightly increased compared to the original blades made completely from fibreglass-reinforced polymer. Optionally completely new blades may be made for an existing wind turbine or the outermost portion blades may be cut off and replaced by a carbon fibre tip with or without a transition zone.
The advantages according to the invention are particularly obtained by making the load-bearing portions of the outer end portion substantially from carbon fibre-reinforced polymer. The load-bearing portions include the main laminates in form of longitudinal fibre-reinforced polymer bands provided in the areas of the suction and pressure sides of the blade shell being furthest from the centre of the blade cross section. The laminates reinforcing the blade in edgewise direction at the leading and trailing edges of the blade may also advantageously be made of carbon fibre-reinforced polymer in the outer end portion of the blade.
The main laminates may advantageously be provided as hybrid mats in which evenly distributed rovings or bundles of either glass fibres or carbon fibres are distributed over the cross-sectional area.
For lightning reasons it may be advantageous to make the outermost portion of the blade tip entirely out of fibre glass so as to ensure that strokes of lightning hit a purpose-built lightning receptor and not the electrically conducting carbon fibre material.
Tests have shown that the outermost portions of the carbon fibres in the transition zone may break at deflection of the transition zone, but this is not an entirely undesirable effect, as it contributes to a further smoothing of the stiffness transition. The frequency of broken fibres may thus be high but not critical, as they are surrounded by more compliant glass fibres. However, the broken fibres still contribute to reducing the deflection and thus the breakage of additional fibres. The gradual and even transition between the properties of the composite material, which is based on glass fibres and carbon fibres, is thus obtained by means of two factors. The first factor is the distribution of stiff and compliant fibres to obtain a smooth transition from the stiff to the compliant area. The second factor is the non-critical breakage, which further smoothens the transition. An additional not shown embodiment of a wind turbine blade according to the invention may be obtained by means of a so-called spray-up process. In this process a spray gun is used for the polymer material and a mixture of chopped fibres of the two types are ejected into a resin stream and sprayed into the mould. By varying the mix ratio during the spray-up process, the intended transition zone may be obtained.
The elongation at breakage for glass fibres is typically about 4.8%, while it typically ranges between 0.3% and 1.4% for carbon fibres. Young's Modulus of glass fibres is about 73,000 MPa, while Young's Modulus of carbon fibres (means modulus) typically is about 245,000 MPa. Carbon fibres are typically 3-4 times stiffer than glass fibres. The density of glass is about 2.54 g/cm3, while the density of carbon is about 1.75 g/cm3.
1. Wind turbine blade (14) of fibre-reinforced polymer characterised in that it is divided into an inner end portion (15) including the blade root and made substantially from fibre glass-reinforced polymer, and an outer end portion (17) including the blade tip and made substantially from carbon fibre-reinforced polymer.
2. Wind turbine blade (14) according to claim 1, characterised in that the outer end portion (17) constitutes between 25 and 50% of the entire length of the blade (14).
3. Wind turbine (14) according to claim 1, characterised in that the outer end portion (17) opposite the blade tip includes a transition zone (16) in which the carbon fibres are gradually replaced by glass fibres.
4. Wind turbine blade (14) according to claim 3, wherein the length of the transition zone (16) is between 0.5 and 1 metre.
5. Wind turbine blade (14) according to claim 3, characterised in that the two types of fibres are distributed such in the polymer matrix that carbon fibres or carbon fibre bundles (1) with varying lengths extend from a first end of the transition zone (II) and glass fibres or glass fibre bundles (2) extend from the opposite end of the transition zone (II).
6. Wind turbine blade (14) according to claim 3, characterised in that the transition zone (II) is formed of a laminate of several fibre layers (6,7), in which each fibre layer has a boundary surface (10) at a position in the longitudinal direction, the fibre layer including carbon fibres (6) on one side of the boundary surface and glass fibres (7) on the other side of the boundary face, the boundary surfaces (10) of each fibre layer being displaced in relation to each other in the longitudinal direction of the blade (14).
7. Wind turbine blade according to claim 6, wherein the boundary surfaces (11) are serrated in a sectional view parallel to the fibre layers (3, 4, 5).
8. Wind turbine blade according to claim 7, wherein the tips (12) of the serrated boundary surfaces (11) are displaced in relation to each other in the transverse direction of the blade (14).