Patent Application
20160369770
The invention relates generally to wind turbines, and more particularly to a wind turbine blade having a pre-bend that reduces instabilities at above-rated wind speeds, a wind turbine having such a wind turbine blade, and a method of making a pre-bent wind turbine blade.
Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor having a plurality of blades and supported in the nacelle by means of a shaft. The shaft couples the rotor either directly or indirectly with a generator, which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator.
In recent years, wind power has become a more attractive alternative energy source and the number of wind turbines, wind farms, etc. has significantly increased, both on land and off-shore. Additionally, the size of wind turbines has also significantly increased, with modern wind turbine blades extending between 50 and 70 meters in length, and is expected to further increase in the future. The increased length in the wind turbine blades has introduced a number of interesting design considerations for wind turbine designers and manufacturers. For example, with increasing blade length, the rearward deflection of the blade under aerodynamic loading also increases, with the maximum deflection typically occurring at the tip end of the blade. However, the as the tip thereof) closer to the wind turbine tower as the blade rotates past the tower. Should the deflection of the blade become significant, such as during unexpected high wind conditions, high turbulence conditions, or the like, it may be possible for the blade to come into contact with the tower. A tower strike may damage the tower or the blade such that repair or replacement procedures must be implemented to make the wind turbine operational once again. These procedures are expensive and may result in the wind turbine being inoperable and off-line for an extended period of time, costing the wind turbine owner valuable production time. Accordingly, the clearance between the blade and the tower is a constraint taken into consideration during the design phase of the wind turbine in order to prevent or reduce the likelihood of a potential tower strike during operation.
One approach for maintaining an acceptable clearance between the blade and the tower during operation under perhaps unexpected and harsh conditions is to limit or reduce the amount of blade deflection under aerodynamic load. This may be achieved, for example, by generally making the blades stiffer. In a conventional approach, wind turbine blades are typically made stiffer by simply adding more material during the manufacture of the wind turbine blade (e.g., adding more fiber material and resin in the manufacture of the blade). This approach, however, has a number of drawbacks. For example, as one might imagine, the amount of material to make a 50-70 meter cantilevered blade sufficiently stiff under aerodynamic load is not insignificant. Thus, the material costs, such as for a sufficient amount of fiber material and resin, for manufacturing sufficiently stiff wind turbine blades are relatively high. Additionally, the added material increases the overall weight of the wind turbine blade, which not only potentially negatively impacts its aerodynamic performance, but also makes storage, transportation, and assembly of the wind turbine blade more expensive and complex.
Another approach used to achieve increased clearance between the tower and the blade during operation is to use a pre-bent wind turbine blade. Pre-bent wind turbine blades are generally known, such as from EP 1019631. The broad concept of the pre-bent blade is to move away from an overly stiff blade design and allow for a certain amount of flexibility or deflection in the blade under load. By specifically designing some level of flexibility into the blade, the blade may be less stiff, and consequently the amount of material used in making the blade may be significantly reduced. Furthermore, to address the issue of clearance between the blade and tower, the blade may be designed with a pre-bend such that in its natural, non-operative state (undeflected shape), the blade curves or bends in the upwind direction away from the tower.
By way of example, a standard pre-bent blade 10 is illustrated in
In any event, during aerodynamic loading of the standard pre-bent blades 10, the blades 10 defect in the downwind direction, as illustrated by arrow W, but maintain an increased clearance from the tower as compared to an aerodynamically-loaded straight wind turbine blade. Additionally, with pre-bent blades, the downwind deflection of the blades during use is generally associated with a slight increase in the swept area of the wind turbine rotor. This is in contrast with straight blades, wherein the downwind deflection of the blades during use is generally associated with a decrease in the swept area of the blades.
While standard pre-bent blades, such as that illustrated in
Currently, the primary approaches configured to alleviate the instability are to increase the torsional stiffness of the blade, such that the blade does not twist so easily with edgewise vibrations, or reduce the pre-bend in the blade so as to reduce the bend-twist coupling associated with the blade. Similar to the above, however, in order to increase the torsional stiffness of the blade, more material must be used to manufacture the blade, thus negating the material cost savings associated with pre-bent blades, and increasing the overall weight of the blade. Moreover, reducing the pre-bend moves the blade design back toward the straight blade, and may suffer from a lack of sufficient clearance between the tower and blade upon aerodynamic loading of the blade.
Accordingly, there is a need in the wind turbine industry for a pre-bent wind turbine blade that does not suffer from the instabilities of standard pre-bent blades in above-rated wind conditions, and also provides a sufficient clearance between the blade and tower during aerodynamic loading of the blade.
A wind turbine blade for a wind turbine includes an elongate body having a root end, a tip end, a leading edge, a trailing edge, a pressure side and a suction side, wherein the elongate body defines a longitudinal axis extending along a length of the elongate body from the root end to the tip end. The elongate body includes a pre-bend formed therein configured such that when the wind turbine blade is mounted on the wind turbine and orientated in an operative position, the blade generally curves in an upwind direction such that the tip end of the blade is positioned more upwind than the root end of the blade. The curve that defines the longitudinal axis of the elongate body has an inflection point between the root end and the tip end of the blade. In one embodiment, the inflection point is located along a central portion of the blade. More particularly, the inflection point may be located between about 30% and about 70% of the blade length. Even more particularly, the inflection point may be located between about 30% and about 50% of the blade length.
In another embodiment, a wind turbine blade for a wind turbine includes an elongate body having a root end, a tip end, a leading edge, a trailing edge, a pressure side and a suction side, wherein the elongate body defines a longitudinal axis extending along a length of the elongate body from the root end to the tip end. The elongate body includes a pre-bend formed therein configured such that when the wind turbine blade is mounted on the wind turbine and orientated in an operative position, the blade generally curves in an upwind direction such that the tip end of the blade is positioned more upwind than the root end of the blade. The longitudinal axis of the elongate body includes a first portion having a center of curvature generally on the upwind side of the blade, and a second portion having a center of curvature generally on the downwind side of the blade. In one embodiment, the first portion is adjacent the root end of the blade and the second portion is adjacent the tip end of the blade.
In yet another embodiment, a wind turbine blade for a wind turbine includes an elongate body having a root end, a tip end, a leading edge, a trailing edge, a pressure side and a suction side, wherein the elongate body defines a longitudinal axis extending along a length of the elongate body from the root end to the tip end. The elongate body includes a pre-bend formed therein configured such that when the wind turbine blade is mounted on the wind turbine and orientated in an operative position, the blade generally curves in an upwind direction such that the tip end of the blade is positioned more upwind than the root end of the blade. The wind turbine blade is characterized by a first edgewise mode shape having a twist component along the length of the blade, wherein a maximum in the twist component occurs at an interior portion of the blade between the root end and the tip end of the blade. In one embodiment, the maximum in the twist component of the first edgewise mode shape may be located along a central portion of the blade. More particularly, the maximum twist component of the first edgewise mode shape may be located between about 40% and about 60% of the blade length.
In these various embodiments, the longitudinal axis of the elongate body may be defined according to the following equation:
where x is the distance along the blade from the root end 34; A is the pre-bend amplitude at the tip end of the blade; L is the length of the blade from the root end; i is the location of an inflection point; and s is the spread of the curve. The pre-bend amplitude may be between about 3% and about 7% of the blade length. Moreover, the spread s may be between about 10% and about 60% of the blade length.
In still another embodiment, a wind turbine includes a tower, a nacelle disposed adjacent a top of the tower, and a rotor including a hub and at least one wind turbine blade extending from the hub. The wind turbine blade may be in accordance with any of the wind turbine blades described above.
A method of making a wind turbine blade comprises forming an outer shell of the wind turbine blade so as to define a root end, a tip end, a leading edge, a trailing edge, a pressure side, a suction side, and a longitudinal axis extending along a length of the blade, wherein the outer shell includes a pre-bend configured such that when the wind turbine blade is mounted on the wind turbine and orientated in an operative position, the blade generally curves in an upwind direction such that the tip end of the blade is positioned more upwind than the root end of the blade. The curve that defines the longitudinal axis of the elongate body has an inflection point between the root end and the tip end of the blade. In one embodiment, the outer shell may be formed by molding a first shell half in a first mold half, molding a second shell half in a second mold half wherein the first and second mold halves are contoured so as to form the inflection point, and coupling the first and second shell halves to form the outer shell of the wind turbine blade.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.
With reference to
The rotor 26 of the wind turbine 20, which is represented as a horizontal-axis wind turbine, serves as the prime mover for the electromechanical system. Wind exceeding a minimum level will activate the rotor 26 and cause rotation in a plane substantially perpendicular to the wind direction. The rotor 26 of wind turbine 20 includes a central hub 28 and at least one blade 30 that projects outwardly from the central hub 28 at locations circumferentially distributed thereabout. In the representative embodiment, the rotor 26 includes three blades 30, but the number may vary. The blades 30 are configured to interact with the passing air flow to produce lift that causes the central hub 28 to spin about a longitudinal axis.
The wind turbine 20 may be included among a collection of similar wind turbines belonging to a wind farm or wind park that serves as a power generating plant connected by transmission lines with a power grid, such as a three-phase alternating current (AC) power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities. Under normal circumstances, the electrical power is supplied from the generator to the power grid as known to a person having ordinary skill in the art.
With reference to
In an exemplary embodiment, the blade 30 includes a pre-bend such that when the blade 30 is mounted onto the wind turbine 20 and oriented in a normal, operative position (but aerodynamically unloaded), the blade 30 is configured to curve or bend in the upwind direction, similar to standard pre-bent blades, such that the tip end 36 of the blade 30 is positioned more upwind than the root end 34 of the blade 30. However, the shape of the pre-bend in blade 30 differs from that of the standard pre-bent blade, such as blade 10 illustrated in
As noted above, this concept of inflection point I and change in the sign of curvature may be expressed in other ways. By way of example, in the reference frame shown in
While the serpent configuration of the pre-bent blade 30 may have several forms, in one embodiment of the invention, the pre-bent shape of the blade 30, as provided by the longitudinal axis 50, for example, may generally be defined according to the parameterized equation:
where x is the distance along the blade from the root end 34; A is the pre-bend amplitude at the tip end 36 of the blade; L is the length of the blade from the root end 34; i is the location of the inflection point I; and s is the spread or softness of the curve. In various embodiments, the amplitude A of the pre-bend may be related to the blade length L and be between about 3% and about 7% of the blade length L. As noted above, the inflection point I may be located in the central portion of the blade, such as between about 30% and about 70% of the blade length L, and more particularly, between about 30% and about 50% of the blade length L. Lastly, the softness of the curve may be between about 10% and about 60% of the blade length L. In one particular embodiment, a pre-bent blade 30 may be defined by the equation above with the amplitude A at about 3.5 m; the length of the blade at about 58 m; the inflection point location i at about 36 m; and the softness s at about 20 m. These values and the above ranges are exemplary and it should be recognized that other values may be selected depending on the particular application. In this regard, the equation above may be subject to an optimization process to determine a desired shape depending of the circumstances of the particular application.
Without being limited to any particular theory, it is believed that by providing an inflection point I in the interior of the blade 30, the net effect is to shift the twist component of the bent-twist coupling toward the root end 34 of the blade 30 and away from the tip end 36, which is most vulnerable to the instability. In this regard,
In contrast to this, the standard pre-bent blade has a small twist component of the first edgewise mode shape near the root end of the blade, but starts increasing in a substantially linear manner such that the twist component is at a maximum value at the tip end of the blade. As noted above, the inventors believe the bend-twist coupling is the mechanism behind the instability and the propensity of the blade to twist with bend is relatively high at the tip end of standard pre-bent blade. This may explain, for example, why the instability manifests itself at the tip end of the blade in above-rated wind conditions.
As illustrated in
Due to this significant reduction in the twist component near the tip end 36 of the blade 10, the inventors believe that the propensity for the blade 10 to exhibit an instability in above-rated wind conditions has been significantly decreased. Thus, by the serpent design as described herein, the blade 10 has improved stability characteristics in above-rated wind conditions and provides sufficient clearance with the tower so as to reduce the likelihood of a tower strike. In this regard,
In contrast to this, the standard pre-bent blade has a positive minimum damping ratio over a range of wind speeds less than about the rated wind speed, but has a negative minimum damping ratio for wind speeds generally greater than the rated wind speed. Thus, as discussed above, the standard pre-bent blade is considered unstable in above-rated wind conditions. As illustrated in
In accordance with an aspect of the invention, the wind turbine blade 30 may be formed through a molding process, as is generally known in the art. In this regard, a molding apparatus (not shown) may include two shell halves corresponding to, for example, the lower shell half 38 and the upper shell half 42 of the outer shell 32 of the wind turbine blade 30. Each of the mold shell halves may be formed or contoured to provide the desired pre-bend in the blade 30. Fiber and resin material may be introduced into the mold halves in the usual manner to form the blade shell halves 38, 42. When the blade shell halves 38, 42 are at least partially cured, the mold halves 38, 42 may be mated together to form the wind turbine blade 30 having the desired pre-bend configuration.
While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the inventors to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA201470087 | Feb 2014 | DK | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DK2014/050404 | 11/27/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61911811 | Dec 2013 | US |
