US20140234114A1

US20140234114A1 - Wind turbine blade with asymmetrical spar caps

Info

Publication number: US20140234114A1
Application number: US13/769,908
Authority: US
Inventors: Karsten Schibsbye
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2013-02-19
Filing date: 2013-02-19
Publication date: 2014-08-21
Also published as: WO2014127925A1

Abstract

A wind turbine blade (10) with asymmetric spar caps (36,38). The blade includes a pressure side spar cap (36) having pressure side fibers (56) having a pressure side fiber diameter (54), the pressure side fibers configured to resist a first flap deflection (20) in a first direction via tensile strength; and a suction side spar cap (38) having suction side fibers (50) having a suction side fiber diameter (52), the suction side fibers configured to resist the first flap deflection via a compressive strength. At a radial cross section, the suction side spar cap exhibits a greater compressive strength and the pressure side spar cap, for example, by the suction side fibers having a different compressive strength than the pressure side fibers.

Description

FIELD OF THE INVENTION

The present invention relates to wind turbine blades. In particular, the invention relates to spar caps having improved resistance to flap deflection.

BACKGROUND OF THE INVENTION

Wind turbine blades rotate about a rotor hub of a wind turbine as a result of aerodynamic forces created by relative wind passing over the airfoil surfaces of the blade. The airfoil surfaces include a pressure side and a suction side. Some of the relative wind encounters the pressure side and imparts force normal to the pressure side via a momentum of the relative wind. Some of the remaining relative wind traverses the suction side of the blade and increases in velocity as it does so. A velocity difference between the increased velocity on the suction side and a velocity of air on the pressure side creates a suction force normal to the suction side. The pressure side force and the suction side force combine to form a net aerodynamic force having an aerodynamic force direction that is the same as or close to the directions of the suction side and pressure side forces.
Each point of a rotating wind turbine blade experiencing no aerodynamic forces would rotate in a respective theoretical plane of rotation. However, the wind turbine blade is not perfectly rigid and as a result the blade tends to deflect in a flap wise direction, which may be the same or similar to the aerodynamic force direction. The amount of deflection of each point on the blade from that point's location in the respective theoretical plane of rotation increases from a base of the blade to a tip of the blade. This occurs because the base of the blade is fixed to the rotor hub, while the deflections cumulate in the radially outward direction.
As technology advances, lengths of the blades increase. As the lengths of the blades increase, the amount of flap deflection also increases. However, too much flap deflection may result in the blade contacting a tower that supports the wind turbine. Consequently, flap deflection must be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a perspective schematic view of a wind turbine blade.

FIG. 2 is a cross section along A-A of one embodiment of the wind turbine blade of FIG. 1.

FIG. 3 is a cross section along A-A of an alternate embodiment of the wind turbine blade of FIG. 1.

FIG. 4 is a side view of the wind turbine blade of FIG. 1.

FIGS. 5-11 are cross sections of various exemplary embodiments of spar caps of the wind turbine blade of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present inventor has recognized that a blade's resistance to flap deflection can be improved by incorporating a relatively more compression-resistant spar cap in the suction side of a wind turbine blade, and a relatively more tension resistant spar cap in the pressure side of the wind turbine blade.
As can be seen in FIG. 1, a turbine blade 10 may incorporate a structural spar having a spar cap 12 to reinforce each side of the blade 10, and a spar web (not shown) spanning between and holding the spar caps in a spaced apart relationship. The spar cap 12 may be a member that extends from a base 14 of the blade 10 toward a tip 16 of the blade 10. The spar cap 12 may be incorporated into or otherwise secured to a skin (shell) 18 that forms an aerodynamic shape of the blade 10.
As can be seen in FIG. 2 the spar (not shown) acts to resist flap deflection 20 of the blade 10 from a plane of rotation 22 in a first direction that traverses the plane of rotation 22 from a pressure side 24 of the blade toward a suction side 26 of the blade. FIG. 3 is a cross section of the blade 10 of FIG. 1, taken along line A-A and showing the pressure side 24 of the blade 10 having a pressure side skin 28, the suction side 26 of the blade 10 having a suction side skin 30, and one configuration of the spar 32 having a spar web 34, a pressure side spar cap 36, and a suction side spar cap 38. Flap deflection 20 may occur as shown, and the flap deflection 20 shown would put the pressure side spar cap 36 in tension and the suction side spar cap 38 in compression. The spar web 34 serves to hold the pressure side spar cap 36 and the suction side spar cap 38 in a spaced-apart relationship. This increases the resistance of the spar 32, which acts like an “I-beam”, to flap deflection 20. FIG. 4 shows a cross section of the blade 10 of FIG. 1, taken along line A-A, showing another configuration of a spar 40 having two spar webs 42 connecting ends of the pressure side spar cap 44 and the suction side spar cap 46. Any blade configuration having a structural support meant to resist flap deflection, and having a portion subject to tension and a portion subject to compression during the flap deflection, would be suitable for the structure disclosed herein.
Historically, the spar and spar caps were designed to provide adequate strength for the blade so that it would simply withstand operating stresses, such as centrifugal force, and spars have been symmetrical from pressure to suction side. As the blades have lengthened, the increased flap deflection of a blade tip has increased to a point where the blade tip could collide with the tower during operation, even in a blade that is structurally sound in terms of strength. As a result, stiffness is becoming a limiting design criteria, and spars and spar caps are being designed that are stronger in tension than a minimum required to resist operational forces such as centrifugal forces in order to provide the stiffness required to prevent collision with the tower. The present inventor has innovatively taken advantage of this relationship to develop an improved overall blade design as described more fully below.
Conventional turbine blade spars have historically been designed to be symmetrical using reinforcing fibers throughout the spar caps and spar web having fibers of one diameter and one material type. As used herein, a fiber diameter is an average diameter of the fibers in the spar cap, as individual fibers may vary in diameter due to manufacturing tolerances. In certain instances the average fiber diameter has been limited to not greater than 20 micrometers due to industry standards established by Germanischer Lloyd in cooperation with the Wind Energy Committee as of Jul. 1, 2010. Other diameters were only permitted upon verification of a safe design. Consequently, until now, the designs have resulted in pressure side spar caps having comparable cross sections to suction side spar cap cross sections at any given radial location. In other words, the pressure side spar caps and the suction side spar caps exhibited the same tensile strength and compressive strength at a given cross section.
For any given blade 10, both the pressure side spar cap 36 and the suction side spar cap 38 may vary in shape and orientation from the base 14 of the blade 10 to the tip 16 of the blade 10. However, for any given radial location, in conventional blades a cross section of the pressure side spar cap 36 and the suction side spar cap 38 have been comparable in terms of compressive strength exhibited. The inventor proposes to change this such that for any given radial cross section, the suction side spar cap 38 has a greater compressive strength than does the pressure side spar cap 36. For a given set of rigidity requirements, this will allow for a lighter suction side spar cap than prior art designs
During flap deflection the pressure side is in tension and the suction side is in compression. Reinforcing fibers used in the spar caps have a compressive strength that may be comparable to the tensile strength, but the compressive strength is often not realized because the fibers themselves tend to buckle in compression before realizing their full compressive strength. When in a spar cap, the fibers are held in alignment by matrix material and therefore buckling is hindered, and thus the compressive strength of the reinforcing fibers contributes significantly to a compressive strength of the spar cap.
Since the pressure side fibers are in tension during flap deflection, buckling is not an issue, and they will be much more likely to reach their full tensile strength before breaking. However, the present inventor has recognized that the ability of the matrix material to hold the compression side fibers in alignment is limited, and as a result, the compression side fibers are likely to buckle before reaching their full compressive strength, and before the pressure side fibers reach their full tensile strength. Consequently, the suction side spar cap is more likely to fail than the pressure side spar cap. The present inventor exploits this fact by making an improvement in the compressive strength of the suction side an important design goal.
This invention presents an innovative strategy for improving the resistance to flap deflection based on tailoring the suction side spar cap to improve its compressive strength. Such an approach, where the pressure side spar cap 36 and the suction side spar cap 38 are asymmetric, is contrary to the prior art turbine blades. Several ways to improve the compressive strength of the suction side spar cap 38 exist. Those ways can be grouped into fiber-related improvements, non fiber-related improvements, and any combination thereof.
Fiber related improvements acknowledge the fact that the reinforcing fibers have a greater compressive strength than the matrix material, but virtually no resistance to buckling without the matrix material. In turn, however, the matrix material can offer a certain resistance to fiber buckling. A fiber with a greater compressive strength will tend to buckle at a higher compressive load, and so for a given matrix material, the spar cap using fiber with the greater compressive strength will be able to withstand a greater compressive load before buckling. Thus, while the matrix material may not be able to hold the stronger fibers in alignment until they reach their full compressive strength, it will hold the stronger fibers in alignment until the suction side spar cap 38 reaches a greater compressive load than would a suction side spar cap 38 having fibers with a lower compressive strength.
One way to increase a compressive strength of the fiber, and therefore the spar cap having the stronger fiber, is to increase a diameter of the fiber. For example, a single fiber from one roving, (a roving is a large number of roughly parallel fibers bundled together, twisted or untwisted), having a diameter of approximately 18 micrometers, may have an E-modulus of approximately 79.0 GPa. The E-modulus is associated with the compressive strength of the fiber. A fiber with a diameter of 24 micrometers may have an E-modulus of approximately 89.0 GPa. Thus, an increase of 6 micrometers in diameter may represent a 1.2% increase in the E-modulus, and an associated increase in the compressive strength. Greater increases in the diameter may represent greater increases in the E-modulus and the associated compressive strength.
FIGS. 5-11 show a cross section of the pressure side spar cap 36 compared to a cross section of the suction side spar cap 38 at any given radial location. FIG. 5 shows a cross section of an exemplary embodiment of the suction side spar cap 38 where suction side fibers 50 have a greater diameter 52 than a diameter 54 of pressure side fibers 56. In this exemplary embodiment all of the suction side fibers 50 have the same diameter 52 and all of the pressure side fibers 56 have the same diameter 54. In conventional blades all fiber diameters may be 20 micrometers. In the exemplary embodiment of FIG. 5, the pressure side fibers diameter 54 may still be 20 micrometers, but the suction side diameter 52 may be any size larger than 20 micrometers. In an exemplary embodiment the suction side diameter 52 may fall within a range of 25 micrometers to 34 micrometers, inclusive. The suction side diameter 52 may range much higher as necessary, up to and including 100 micrometers. Final diameters will be determined when considering all factors for each application.
FIG. 6 shows a cross section of another exemplary embodiment of the suction side spar cap 38 where the suction side fibers 50 include plural different diameters.
Each fiber having a distinct diameter may be considered a different fiber type. Therefore, the suction side spar cap 38 may have a plurality of fiber types. A first fiber type 60 may have a first diameter 62 and a second fiber type 64 may have a second fiber diameter 66. There may be any number of fiber types in both the pressure side spar cap 36 and the suction side spar cap 38, so long as a mixture of types of fibers in the suction side spar cap 38 yields a greater compressive strength than a mixture of types of fibers in the pressure side spar cap 36. For example, the first diameter 62 may be the same as the pressure side fiber diameter 54 and the second diameter 66 may be greater than the pressure side fiber diameter 54. Alternately, both the first diameter 62 and the second diameter 66 may be greater than the pressure side fiber diameter 54. It is also conceivable that the first diameter 62 could be smaller than the pressure side fiber diameter 54 and the second diameter 66 may be so much greater than the pressure side fiber diameter 54 as to yield an overall greater compressive strength of the suction side spar cap 38. Any mixture of diameters is possible so long as the cross section of the suction side spar cap 38 ends up having a greater compressive strength that the cross section of the pressure side spar cap at the same radial location.
Another way to increase a compressive strength of a fiber is to change a composition of the fiber to a composition stronger in compressive strength. For example, a carbon fiber has a greater compressive strength than a glass fiber. FIG. 7 shows a cross section of another exemplary embodiment of the spar caps 36, 38 where the suction side fibers 50 include a stronger composition than do the pressure side fibers 56. In this exemplary embodiment all of the suction side fibers 50 have a same composition 68 as each other and all of the pressure side fibers 56 have the same composition as each other. For example, the suction side fibers 50 may be carbon fibers, which are stronger than glass fibers, while the pressure side fibers 56 may be glass fibers. Another type of fiber may include aramide fibers. Any composition may be used in such an exemplary embodiment, so long as the composition of the suction side fibers 50 has a greater compressive strength than the composition of the pressure side fibers 56.
FIG. 8 shows a cross section of another exemplary embodiment of the spar caps 36, 38 where the suction side fibers 50 include plural different compositions. Each fiber having a distinct composition may be considered a different fiber type. Therefore, the suction side spar cap 38 may have a plurality of fiber types. A first fiber type 70 may have a first composition 72 and a second fiber type 74 may have a second composition 76. There may be any number of fiber types in both the pressure side spar cap 36 and the suction side spar cap 38, so long as a mixture of types of fibers in the suction side spar cap 38 yields a greater compressive strength than a mixture of types of fibers in the pressure side spar cap 36. For example, the first composition 72 may be the same as a composition 78 the pressure side fibers 56 and the second composition 76 may be a composition having a greater compressive strength. Alternately, both the first composition 72 and the second composition 76 may have a greater compressive strength than the composition 78 of the pressure side fibers 56. It is also conceivable that the first composition 72 may have a weaker compressive strength and the second composition 76 may be so much greater than the composition 78 the pressure side fibers 56 as to yield an overall greater compressive strength of the suction side spar cap 38. Any mixture of compositions is possible so long as the cross section of the suction side spar cap 38 ends up having a greater compressive strength that the cross section of the pressure side spar cap at the same radial location.
FIG. 9 shows a cross section of another exemplary embodiment of the spar caps 36, 38 having a plurality of fiber types, where each fiber type has a distinct combination of fiber diameter and fiber composition. The suction side spar cap 38 may have plural fiber types. There may be, for example, the first diameter 62, the second diameter 64, and a third diameter 80. There may be the first composition 72, and the second composition 76. A first fiber type 90 may have the first diameter 62 and the first composition 72. A second fiber type 92 may have the first diameter 62 and the second composition 76. A third fiber type 94 may have the second diameter 64 and the first composition 72. A fourth fiber type 96 may have the second diameter 64 and the second composition 76. A fifth fiber type 98 may have the third diameter 80 and the first composition 72. A sixth fiber type 100 may have the third diameter 80 and the second composition 76. The number of fiber types is unlimited. The suction side spar cap 38 and the pressure side spar cap 36 each can include any mixture of fiber types, so long as the cross section of the suction side spar cap 38 ends up having a greater compressive strength that the cross section of the pressure side spar cap at the same radial location.
FIG. 10 shows a cross section of another exemplary embodiment of the spar caps 36, 38 where a matrix material 110 in the suction side spar cap 38 has a compressive strength greater than a compressive strength of matrix material 112 in the pressure side spar cap 36. Increasing the compressive strength of the matrix material itself will contribute to the compressive strength of the suction side spar cap 38. The matrix material 110 in the suction side spar cap 38 may be the only difference between the pressure side spar cap 36 and the suction side spar cap 38. Alternately, adding matrix material 110 in the suction side spar cap 38 having the greater compressive strength may be done in conjunction with any other technique described herein.
FIG. 11 shows a cross section of another exemplary embodiment of the spar caps 36, 38 where a cross sectional area 120 of the suction side spar cap 38 is greater than a cross sectional area 122 of the pressure side spar cap 36 at the same radial location. Increasing the cross sectional area 120 of the suction side spar cap 38 will necessarily result in a suction side spar cap 38 with a greater compressive strength. The increased cross sectional area 120 may be the only difference between the pressure side spar cap 36 and the suction side spar cap 38. Alternately, increasing the cross sectional area 120 of the suction side spar cap 38 may be done in conjunction with any other technique described herein.
From the foregoing it is apparent that the inventor has broken with convention in order to tailor the design of the suction side spar cap to better meet the load conditions specific to the suction side of the blade. This individualized tailoring provides a suction side spar cap with a reduced weight, a greater compressive strength, or a combination of both when compared to the pressure side spar cap, and when compared to prior art suction side spar caps of similar stiffness requirements. This greater improved design allows for a lighter blade design to achieve a similar compressive strength, and the lighter blade may reduce forces and increase the life of the blade. It therefore represents an improvement in the art.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.

Claims

The invention claimed is:

1. A wind turbine blade comprising:

a pressure side spar cap comprising pressure side fibers comprising a pressure side fiber diameter, the pressure side fibers configured to resist a first flap deflection of the blade in a first direction via tensile strength; and

a suction side spar cap comprising suction side fibers comprising a suction side fiber diameter, the suction side fibers configured to resist the first flap deflection via a compressive strength;

wherein at a radial cross section the suction side fibers comprise a different compressive strength than the pressure side fibers.

2. The wind turbine blade of claim 1, wherein the suction side fibers exhibit a greater compressive strength than the pressure side fibers.

3. The wind turbine blade of claim 1, wherein the pressure side fibers comprise fibers of a first diameter, the suction side fibers comprise fibers of a second diameter, and wherein the second diameter is greater than the first diameter.

4. The wind turbine blade of claim 3, wherein the second diameter is greater than 20 micrometers.

5. The wind turbine blade of claim 3, wherein the second diameter falls within a range from 25 micrometers to 34 micrometers, inclusive.

6. The wind turbine blade of claim 1, wherein the pressure side fibers comprise a first mixture of fiber types, each type comprising a distinct diameter, and wherein the suction side fibers comprise a second mixture of fiber types different than the first mixture of fiber types.

7. The wind turbine blade of claim 1, wherein the pressure side fibers comprise fibers of a first composition and the suction side fibers comprise fibers of a second composition different than the first composition.

8. The wind turbine blade of claim 1, wherein the first fiber composition comprises glass fiber, and wherein the second fiber composition comprises carbon fiber.

9. The wind turbine blade of claim 1, wherein the pressure side fibers comprise a first mixture of fiber types, each type having a distinct composition, and wherein the suction side fibers comprise a second mixture of fiber types different than the first mixture of fiber types.

10. The wind turbine blade of claim 1, wherein the suction side fibers and the pressure side fibers comprise plural fiber types, wherein each of the plural fiber types comprises a distinct combination of diameter and composition, wherein the pressure side fibers comprise a first mixture of fiber types, and wherein the suction side fibers comprise a second mixture of fiber types different than the first mixture of fiber types.

11. A wind turbine blade, comprising:

a spar, comprising:

a pressure side spar cap comprising pressure side fibers configured to resist a first flap deflection in a first direction;

a suction side spar cap comprising suction side fibers configured to resist the first flap deflection;

a spar web configured to hold the pressure side spar cap and the suction side spar cap in a spaced-apart relationship;

a pressure side shell comprising the pressure side spar cap; and

a suction side shell comprising the suction side spar cap,

wherein the pressure side fibers and the suction side fibers comprise at least one of a plurality of different diameters and a plurality of different compositions, and

wherein at a given radial cross section the suction side fibers comprise a greater compressive strength than the pressure side fibers.

12. The wind turbine blade of claim 11, wherein a diameter of the pressure side fibers is less than or equal to 20 micrometers, and a diameter of the suction side fibers are greater than 20 micrometers.

13. The wind turbine blade of claim 11, wherein the pressure side spar cap comprises glass fibers and the suction side spar cap comprises carbon fibers.

14. The wind turbine blade of claim 11, wherein the pressure side spar cap comprises a first mixture of carbon fibers and glass fibers and the suction side spar cap comprises a comprises a second, different mixture of carbon fibers and glass fibers.

15. A wind turbine blade, comprising:

a pressure side spar cap comprising pressure side fibers; and

a suction side spar cap comprising suction side fibers;

wherein for a given radial location along the blade, cross sections of the pressure side spar cap and the suction side spar cap are asymmetric; and

wherein the suction side cross section comprises a greater compressive strength than the pressure side cross section.

16. The wind turbine blade of claim 15, wherein the suction side cross section comprises a larger area than the pressure side cross section.

17. The wind turbine blade of claim 15, wherein the suction side fibers comprise a larger diameter than the pressure side fibers.

18. The wind turbine blade of claim 15, wherein a composition of the suction side fibers comprises a greater compressive strength than a composition of the pressure side fibers.

19. The wind turbine blade of claim 15, wherein matrix material in the suction side cross section comprises a greater compressive strength than matrix material in the pressure side cross section.