NL2014140B1

NL2014140B1 - A wind turbine blade.

Info

Publication number: NL2014140B1
Application number: NL2014140A
Authority: NL
Inventors: Van Breugel Sjef; Ellam Darren
Original assignee: Pontis Group Holding B V
Priority date: 2014-10-05
Filing date: 2015-01-15
Publication date: 2016-10-04
Also published as: DK3002452T3

Abstract

The invention relates to a wind turbine blade comprising an aerodynamic shell that extends between a leading edge and a trail ing edge, said shell being comprised of a windward shell and a leeward shell, said shell comprising composite sandwich panels to support localised loading and a spar system to provide local ri gidity to the blade and to transfer the accumulated load to the blade root. The blade comprises: [a] an edge spar at either lead ing edge and trailing edge of the blade; and [b] from said edge spar at either leading edge and trailing edge of the blade, an edge component, having it's leeward surface aligned with and adja cent to said blade's leeward shell, and it's windward surface, aligned with and adjacent to said blade's windward shell, so as to provide a continuous aerodynamic shell; - said edge component hav ing a lower rigidity than the blade's main structure.

Description

A wind turbine blade

The present invention relates to a wind turbine blade according to the preamble of claim 1. More in particular, the present invention relates to edge design of a wind turbine blade that is tolerant to damage and is prevented from buckling due to extreme loads. The term "edge design" relates to the design concept applied at the trailing edge (TE) and/or leading edge (LE).

It is known in the art to have a wind turbine blade designed such that it is less prone to buckling failure under extreme loads. In the art, the term "extreme load" refers to loads that may only be encountered once in a lifetime, which is typically 20 to 25 years. For example, international patent application W02011/088834 describes a wind turbine blade that has been designed to provide benignly buckling of the trailing edge when the blade is subjected to extreme reverse edgewise loading so as to prevent blade failure.

According to the present invention, blade buckling is defined as an instability that leads to a sudden failure of the blade due to high compressive stress caused by blade bending. This is most likely to be instigated locally through the mechanism of trailing edge buckling (also known as knife edge buckling), shell panel buckling, or spar (either spar cap or spar web) buckling.

Blade buckling strength is an increasingly difficult requirement to meet considering the demand and trend for larger blade sizes. To achieve a certified blade design, international guideline requirements need to be satisfied. Adding additional material and structure to solely meet buckling design margins leads to a less cost effective blade design. As such, in the art of blade design, buckling can often be a significant design and cost driver.

Tendency of most blade manufacturers is to offer a wind turbine blade that will not buckle at all, either benignly. Reason for this is that buckling is likely to destroy the wind turbine blade at least partially. The presently available blades need to be taken out of service and replaced when such failure occurs, at significant cost. Worst still the whole turbine may need to be taken out of service and replaced.

The invention aims at providing an improved blade design that is less prone to buckling failure due to extreme loads and/or edge damage. The invention especially aims at providing a blade design in which buckling is not a limiting design driver, allowing for a more optimised blade design.

The invention further aims at providing a blade design that allows the blade to continue safe operation even if some edge wear or damage has occurred.

So as to obtain at least one of the above mentioned aims, the invention provides a wind turbine blade comprising the features as indicated in claim 1. This wind turbine blade has the advantage that edge buckling under extreme loads is not a design driver, as edge buckling is designed not to occur.

Special preference is given to a blade, wherein said edge spar is embodied for supporting and carrying loads, acting upon the blade's edge, down the blade length towards the root. Since the edge spar forms part of the main section of the wind turbine blade, any forces acting upon it are transferred in the direction of the blade root. As known in the art, the root is the part of the blade that is connected to the hub of the wind turbine. Furthermore, the edge spar is designed such that buckling will not occur. The edge spar provides additional strength though the fact that the main shells are short in length.

It is preferred in the blade according to the invention that said edge component is comprised of any of a foam material, a honeycomb material or a combination thereof, each providing a lightweight yet sufficiently dimensionally stable material to be used in the present invention.

It is furthermore preferred that said edge component is comprised of a low elastic modulus material, providing the advantage that it easily withstands damage due to loads, acting upon the blade's edge.

It is especially preferred if said edge component has a relatively low rigidity, and wherein the main structure and spar system have a relatively high rigidity for carrying the main blade loads, since such has shown to provide a blade that is much less prone to buckling of the edge component.

The invention further relates to a wind turbine blade wherein said edge component comprises low structural stiffness relative to the said main blade section. Said main blade section, embodies main spar and edge spar that comprise of high stiffness and strength structural materials. This way, all load that is exerted on the blade during use, is carried to the blade root by the main spar and edge spars, whereas the edge component need withstand local loading only. In essence, the main edge load carrying structure is moved further inwards (towards the pitch axis) than is typical in state of the art, and the immediate blade edge profiles are replaced with a light weight structure parts with strength to support only the local aerodynamic loads.

In the art, moving the main edge load carrying material towards the pitch axis is actually a less mass efficient way of obtaining the required blade edge stiffness. However, it can be shown with the present invention, when considering that buckling strengths previously difficult to achieve are now attainable, this compromise results in no net gain in mass, in fact a saving is expected. The reason for this is considering the continued trend towards longer blade lengths, in which ever increasingly difficult to attain edge load strengths, are now obtainable with the present invention. Since blade mass is a significant turbine design and cost drive, such a synergistic advantage leads to more efficient energy generation.

An additional advantage is found in the ease with which damaged or worn parts can be replaced, for example after prolonged operation in extreme weather conditions, since only a modular edge component needs to be replaced. Furthermore, since the edge components are not designed to add to the blade's constructional strength, the wind turbine does not need to be taken out of service when some ware or damage on either LE or TE occurs. Operation can be safely continued. Such is an advantage over the wind turbines according to the state of the art.

Further advantages for a wind turbine blade according to the present invention, over that of state of the art, is that smaller size parts are manufactured and glued or otherwise mutually connected. This essentially involves combining the main blade section with a LE and/or a TE component. This allows the option if preferred to assemble the blade on site which would aid transportation. Additionally, manufacturing the main blade section using a mould of smaller size would lead to cost savings. Furthermore the edge components, that are the most aerodynamically critical, are now separate smaller pieces, that could now be manufactured more precisely than following generic state of the art methods.

It is preferred that, at the position of the edge component, at least one of said windward shell and said leeward shell are bent towards one another, i.e. such that said bent end or ends, respectively, do not form part of the aerodynamic shell, and such that said bent end or ends, respectively, form said edge spar.

This embodiment provides the advantage that a separate rear spar does not need to be connected to said shells. The core material of said edge spar may consist of a high stiffness to carry bending loads, the skins of said edge spar would carry the shear loads.

According to a further embodiment, for providing a contact surface for connecting the edge component to the blade's main body, it is preferred that, at a position of the edge component, at least one of said windward shell and said leeward shell are bent towards one another, forming a contact surface for connecting said edge component.

Preferably, in the wind turbine blade according to the present invention, at the position of the edge component, additional bends of the main shell structure, in the chord direction form a ridge, and wherein said edge component comprises a recess for receiving said ridge. This way, by locking up said ridges inside said recess, a connection of said windward shell, leeward shell and edge component is obtained.

Said edge component may for example be connected to said edge spar by means of gluing, clipping or any other means. If preferred, connection of said edge component to said edge spar can be by means of a removable connection, but a permanent connection will also suffice. For example, at said ridge which mutually receives said recess, a clip on connection or other removable con- nection can be used. A further connecting means for thoroughly connecting said edge component is preferably provided, so as to manufacture a wind turbine blade having an aerodynamic shell smoothly running from LE to TE, both at the windward side and the leeward side.

It is preferred that said edge component comprises a closed surface, providing a smooth and substantially airtight surface layer. Especially when said edge component is comprised of a foam material, such smooth and airtight surface is highly preferred. Preferably this closed surface is a material skin that would give sufficient torsional stiffness to the edge component to support aerodynamic loads.

Hereafter, the invention will be further described by means of a drawing. The drawing shows in:

Fig. 1 a cross-section view of a wind turbine blade according to the state of the art,

Fig. 2 a cross-section view of a wind turbine blade according to the invention,

Fig. 3 and 4 a method of manufacturing a wind turbine blade according to an embodiment of the present invention, and

Fig. 5 and 6 an impression of TE buckling in a blade according to the state of the art and according to the invention, respectively .

The same and similar parts and features have been denoted by the same reference numerals in the figures. However, for ease of understanding the figures, not all parts that are required for a practical embodiment have been shown in the figures.

Fig. 1 shows a simplified perspective cross-section of a wind turbine blade 1. As a matter of fact, Fig. 1 components have not been depicted in proportion. The blade 1 has a windward side 2 and a leeward side 3. Both sides 2, 3 extend between a leading edge LE 4 and a trailing edge TE 5. Said sides extending between said LE 4 and TE 5 are referred to as windward shell 6, leeward shell 7 and are both often manufactured as separate complete pieces. Said shells 6, 7 form an aerodynamic profile, running substantially smoothly between LE 4 and TE 5. Said shells are largely composed of composite sandwich panels 8, 9, 10, 11 which are lightweight yet can withstand localised loading. Windward shell 6 and leeward shell 7 are coupled through an adhesive 12 at LE 14 and an adhesive 13 at TE 5.

As known in the art, the blade 1 uses a spar system 14 which functions as the main structural member withstanding and transferring load to the blade root. Said spar also functions as a reinforcing member for supporting windward shell 6 and leeward shell 7. As well as the main spar 14, additional spars (i.e. TE spar 15) can be used, and is more typical on larger blade sections. Said spars (i.e. 14, 15) can be subdivided into separate components, spar caps 16 and spar webs 17 (also known as a shear webs). Said spar cap 16 is primarily designed to support loads caused by bending, and is usually constructed from unidirectional glass or carbon composite material. Said spar web 17 is primarily designed to transfer shear load, and is usually a glass composite sandwich panel construction.

The TE component 18 starts from the TE 5 and is of unstipulated length (but could be considered about 10% of chord length). This TE component 18 and TE panels 10, 11 are prone to buckling under compressive edgewise loads of blade 1. To prevent this, strains are reduced by increasing overall blade edgewise stiffness. This can be achieved by utilizing a TE windward stiffener 19, and TE leeward stiffener 20. The material of said TE stiffeners 19, 20 is usually unidirectional glass or carbon composite.

Due to the construction method according to the state of the art, when blade buckling occurs, failure of either said LE bonding 12 or more usually TE bonding 13, between said shells 6, 7, often leads to catastrophic failure of the blade 1.

According to an embodiment of the state of the art, the edge stiffness of said blade 1 is kept sufficiently high, so as to prevent buckling as much as possible. According to another state of the art where a rear spar web 15 has been applied, especially WO2011/088834, stiffness of constructional parts 18 is such that some benign buckling is allowed under certain loads.

Fig. 2 shows a simplified perspective cross-section of a wind turbine blade 1 according to the present invention. As a matter of fact, components in Fig. 2 have not been depicted in proportion.

The LE edge component 21 and/or TE edge component 22 is manufactured such that its structural stiffness and thus rigidity is low relative to that of the main blade section 23. Said edge part 21, 22 is able to support all aerodynamic loads and blade strains, under nominal operating conditions of wind turbine blade 1, such as to provide and maintain the required aerodynamic profile within a required tolerance.

According to a further embodiment, said edge component is comprised of low elastic modulus material. As such, a preferred material for manufacturing the edge component would be foam or honeycomb material or a combination thereof. For example, an elastic modulus of the order of 100 to 1000 MPa (N/m2) is anticipated to be preferred.

It is preferred that said edge component comprises a closed surface 26, as indicated in Fig. 3 and Fig. 4. Additionally, this closed surface 26 is preferably a thin skin adding some torsional stiffness to the edge component.

Said edge part 21, 22 is connected to the main blade part 23 at edge spar 24, 25 respectively. Said edge spar 24, 25 provides a stiff constructional box to the main blade section 23. Since the edge spars 24, 25 are an integral part of the main blade shell, any forces acting upon it are distributed and transferred in the direction of the blade root.

Said edge spar 24, 25 is to be constructed of sufficiently stiff material to maintain the required edgewise stiffness of blade 1. As a matter of fact, edge components 21, 22 are designed such that they do not add any significant edgewise stiffness to the blade. The core material of edge spar 24, 25 would include unidirectional glass or carbon composite to add the blades overall edgewise stiffness. Whereas the outer skin surface material of edge spar 24, 25 would be a biaxial glass composite, as a matter of fact it would be of similar layup of that used to make composite panels 8, 9, 10, 11. The said edge spar is designed rigidly enough such that it will not buckle locally.

Since the shell panels 8, 9, 10, 11 can be relatively short compared to state of the art, the thickness of which can be smaller while still maintaining required panel strength. This will help to decrease the total weight of the wind turbine blade according to the present invention.

Fig. 3 and Fig. 4 show a preferred way of manufacturing the wind turbine blade 1 according to the present invention. As a matter of fact, Fig. 3 and Fig. 4 components have not been depicted in proportion. The method shown in Fig. 3 and Fig. 4 is for the TE 22 but, as a matter of fact, the concept applies to the LE 21 also. In the embodiment shown, at position 27, 28 on the main section edge, parts 29, 30 of windward shell 10 and leeward shell 11, respectively are bent out of aerodynamic plane towards each other. Said bent shape may be manufactured directly, but some small slope (say 5 degrees) can be maintained to aid de-moulding. A further bend directed towards trailing edge 5, at the ends of said parts 29, 30 forms ridges 31, 32.

When connecting the ends of parts 29, 30, for example by means of an adhesive 33 that connects ridges 31, 32, windward shell 6 and leeward shell 7 are connected. This connection forms a rib 34 on said edge spar 24, 25. Said rib 34 adds to the strength of the said edge spar 24, 25. Said parts 29, 30 hence form TE edge spar 25. Edge component 21, 22 is preferably connected to said edge spar 24, 25 by means of a non-permanent connection, and inserting mutually connected ridges 31, 32 into a recess 35 provided in said TE part 22.

Recess 35 may be dimensioned such that rib (also called ridge) 34 is inserted substantially in close fitting, which eases alignment of edge part 21, 22 with respect to the main part 23 of blade 1.

To add to connection strength, maintain a smooth aerodynamic surface, and to aid quick removal of edge part 21, 22, a windward strip 36 and leeward strip 37 is envisaged as shown in Fig. 3.

This strip may be connected by means of bolts or the like, or could even be a strong adhesive tape.

Due to the inherent low structural strength of the edge component compared to that of the main structure, it has been shown that edgewise buckling (Fig. 5) as in current state of the art does not occur in the present invention. Instead panel buckling (Fig. 6) at a higher load occurs. In the present invention under extreme load the edge component is designed such that it can lose its aerodynamic shape. The edge component thus does not buckle, either benignly, or otherwise. Instead it conforms to a shape though elastic deformation to that dictated by the main blade structure .

The invention is not limited to the embodiments as mentioned above and as shown in the drawings. The invention is limited by the claims only.

The invention also relates to all combinations of features described here independently of each other.

Claims

1. A wind turbine blade (1) comprising an aerodynamic shell that extends between a leading edge (4) and a trailing edge (5), wherein said shell is comprised of a windward shell (6) and a leeward shell (7), said shell (6,7) comprising composite sandwich panels (8,9,10,11) to support localised loading and a spar system (14,15) to provide local rigidity to the blade and to transfer the accumulated load to the blade root, characterized in that said wind turbine blade is comprised of [a] an edge spar (24,25) at either leading edge (4) and trailing edge (5) of the blade, and [b] from said edge spar at either leading edge (4) and trailing edge (5) of the blade, an edge component (21,22), having it's leeward surface aligned with and adjacent to said blade's leeward shell, and its windward surface, aligned with and adjacent to said blade's windward shell, so as to provide a continuous aerodynamic shell; said edge component (21,22) having a lower rigidity than the blade's main structure (23).

2. A blade according to claim 1, wherein said blade edge spar is embodied for supporting and carrying loads down the blade length towards the blade root.

3. A blade according to claims 1 and 2, wherein said edge component is comprised of any of a foam material, a honeycomb material or a combination thereof.

4. A blade according to any of the preceding claims, wherein said edge component is comprised of a low elastic modulus material.

5. A blade according to any of the preceding claims, wherein said edge component has a relatively low rigidity, and wherein the main structure and spar system have a relatively high rigidity for car rying the main blade loads.

6. A blade according to any of the preceding claims, wherein at the position of the edge component, at least one of said windward shell and said leeward shell are bent towards one another.

7. A blade according to any of the preceding claims, wherein at a position of the edge component, at least one of said windward shell and said leeward shell are bent towards one another, forming a contact surface for connecting said edge component.

8. A blade according to any of the preceding claims, wherein at the position of the edge component, additional bends of the main shell structure, in the chord direction form a ridge, and wherein said edge component comprises a recess for receiving said ridge.

9. A blade according to any of the preceding claims, wherein said edge component is connected to said edge spar by means of gluing, clipping or any other suitable method.

10. A blade according to any of the preceding claims, said edge component is a closed surface.