Patent Application
20140234114
The present invention relates to wind turbine blades. In particular, the invention relates to spar caps having improved resistance to flap deflection.
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.
The invention is explained in the following description in view of the drawings that show:
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.
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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.
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.
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.
