A Precured Fibrous Strip for a Load-Carrying Structure for a Wind Turbine Blade

Abstract

A precured fibrous composite strip for a load-carrying structure for a wind turbine blade has a first longitudinal end and a second longitudinal end, a first side and a second side with a width defined as the distance between the first side and the second side, and an upper surface and a lower surface with a thickness defined as the distance between the upper surface and the lower surface. The strip includes a taper region with a taper length at the first longitudinal end. The taper region tapers in thickness towards the first longitudinal end. The taper region includes a first taper section proximal to the first longitudinal end and having a first average taper angle, a third taper section distal to the first longitudinal end and having a third average taper angle, and a second taper section between the first taper section and the third taper section.

Claims

1-24. (canceled)

25. A precured fibrous composite strip for a load-carrying structure, such as a spar cap, for a wind turbine blade, wherein the strip has a first longitudinal end and a second longitudinal end; a first side and a second side with a width defined as the distance between the first side and the second side; and an upper surface and a lower surface with a thickness defined as the distance between the upper surface and the lower surface; and wherein the strip comprises a taper region with a taper length at the first longitudinal end, wherein the taper region tapers in thickness towards the first longitudinal end and comprises: a first taper section proximal to the first longitudinal end and having a first average taper angle; a third taper section distal to the first longitudinal end and having a third average taper angle; and a second taper section between the first taper section and the third taper section and having a second average taper angle, wherein the second average taper angle is larger than both the first average taper angle and the third average taper angle.

26. The precured fibrous strip according to claim 5, wherein the taper section has a blunt face at the first longitudinal end, wherein the blunt face has an end step thickness.

27. The precured fibrous strip according to claim 26, wherein the end step thickness is between 0.01 mm and 0.3 mm.

28. The precured fibrous strip according to claim 25, wherein the taper region has a substantially S-shaped profile in the longitudinal direction of the strip.

29. The precured fibrous strip according to claim 28, wherein the taper region comprises: a first curvature region proximal to the first longitudinal end and having an outer radius of curvature, wherein the first curvature region has an intersection length, and a second curvature region distal to the first longitudinal end and having an inner radius of curvature.

30. The precured fibrous strip according to claim 26, wherein the thickness of the strip is defined as t.sub.plank, the taper length is defined as L.sub.chamfer, the outer radius of curvature is defined as R.sub.1, the inner radius of curvature is defined as R.sub.2, the end step thickness is defined as t.sub.es, the longitudinal direction is defined as z, and the thickness direction is defined as y, wherein the first curvature region approximately has a first profile defined by:
z=R.sub.1 cos (q.sub.1) and y=t.sub.es−R.sub.1 (sin (q.sub.1)−1) for q.sub.1=[p/2;q.sub.1.sup.IS]; and wherein the second curvature region approximately has a second profile defined by:
z=L.sub.chamfer−R.sub.2 cos (q.sub.2) and y=t.sub.plank−R.sub.2(1−sin (q.sub.2)) for q.sub.2=[q.sub.2.sup.IS;p/2] wherein q.sub.1.sup.IS is the value of variable q.sub.1 at an intersection between the first curvature region and the second curvature region; and q.sub.2.sup.IS is the value of variable q.sub.2 at the intersection between the first curvature region and the second curvature region.

31. The precured fibrous strip according to claim 30, wherein a profile of the taper section deviates at most 5%.

32. The precured fibrous strip according to claim 25, wherein the taper region has a thickness to length ratio in the range 1:10 to 1:200.

33. The precured fibrous strip according to claim 25, wherein the strip is a pultruded element and comprises unidirectionally oriented reinforcement fibres, such as glass fibres or carbon fibres, oriented in the longitudinal direction.

34. The precured fibrous strip according to claim 25, wherein the thickness of the strip is between 1 mm and 10 mm.

35. The precured fibrous strip according to claim 25, wherein the width of the strip is between 30 mm and 300 mm.

36. The precured fibrous strip according to claim 25, wherein the length of the strips is between 100 mm and 100 m.

37. A spar cap for a wind turbine blade comprising a precured fibrous strip according to claim 25.

38. The spar cap of claim 36, wherein the spar cap comprises a plurality of stacked precured fibrous strips and comprising at least one precured fibrous strip according to claim 1.

39. A wind turbine blade comprising a spar cap according to claim 37.

40. A method of manufacturing a spar cap for a wind turbine blade, the method comprising: providing a plurality of precured fibrous strips including at least one precured fibrous strip according to claim 25, stacking the plurality of precured fibrous strips such that interface regions are formed between adjacent precured fibrous strips, supplying resin to the plurality of precured fibrous strips and causing the resin to fill the interface regions between adjacent strips, and curing the resin in order to form the spar cap.

41. The method according to claim 40, wherein the plurality of precured strips are stacked in an array.

42. The method according to claim 41, wherein the method comprises the step of arranging flow-promoting material between at least some of the stacked precured fibrous strips.

43. The method according to claim 41, wherein the method comprises the step of draping a fibre layer over the plurality of stacked precured fibrous strips before the step of supplying resin.

44. A wind turbine blade spar cap manufactured according to claim 43.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0053] The invention is explained in detail below with reference to embodiments shown in the drawings, in which

[0054] FIG. 1 shows a wind turbine,

[0055] FIG. 2 shows a schematic view of a wind turbine blade,

[0056] FIG. 3 shows a schematic view of a wind turbine blade shell,

[0057] FIG. 4 shows a side view of a spar cap,

[0058] FIGS. 5a and 5b show embodiments for a cross-section of a spar cap,

[0059] FIG. 6 shows a taper region of a precured fibrous strip,

[0060] FIG. 7 shows various profiles for the taper region of a precured fibrous strip, and

[0061] FIG. 8 shows steps in a manufacturing method.

DETAILED DESCRIPTION OF THE INVENTION

[0062] In the following, a number of exemplary embodiments are described in order to understand the invention.

[0063] FIG. 1 illustrates a conventional modern upwind wind turbine according to the so-called “Danish concept” with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 nearest the hub and a blade tip 14 farthest from the hub 8. The rotor has a radius denoted R.

[0064] FIG. 2 shows a schematic view of a first embodiment of a wind turbine blade 10 disclosure. The wind turbine blade 10 has the shape of a conventional wind turbine blade and comprises a root region 30 closest to the hub, a profiled or an airfoil region 34 farthest away from the hub and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 comprises a leading 35 edge 18 facing the direction of rotation of the blade 10 when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.

[0065] The airfoil region 34 (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region 30 due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade 10 to the hub. The diameter (or the chord) of the root region 30 may be constant along the entire root area 30. The transition region 32 has a transitional profile gradually changing from the circular or elliptical shape of the root region 30 to the airfoil profile of the airfoil region 34. The chord length of the transition region 32 typically increases with increasing distance r from the hub. The airfoil region 34 has an airfoil profile with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10. The width of the chord decreases with increasing distance r from the hub.

[0066] A shoulder 39 of the blade 10 is defined as the position, where the blade 10 has its largest chord length. The shoulder 39 is typically provided at the boundary between the transition region 32 and the airfoil region 34.

[0067] It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub.

[0068] The blade is typically made from a pressure side shell part 36 and a suction side shell part 38 that are glued to each other along bond lines at the leading edge 18 and the trailing edge 20 of the blade.

[0069] In the following, the invention is explained with respect to the manufacture of the pressure side shell part 36 or suction side shell part 38.

[0070] FIG. 3 shows a perspective view of a blade shell part, here illustrated with the suction side shell part 38, which is provided with a load-carrying structure, which forms a spar cap 40 or main laminate. The spar cap 40 can be integrated into the blade shell or it can be a separate spar cap that is attached, e.g. by adhesion, to the blade shell 38. The spar cap 40 may be part of a separate spar structure. However, it is also possible to provide a blade with spar caps provided at both the pressure side shell part 36 and the suction side shell part 38, with one or more shear webs attached between 35 the spar caps.

[0071] FIG. 4 shows a side view of a manufactured spar cap 40. The spar cap is made from a plurality of precured fibrous strips 50 that extends in the longitudinal direction of the spar cap 40. The precured fibrous strips 50 may be stacked on top of each other as shown in FIG. 4. In a cross-sectional view, the strips 50 may be stacked right on top of each other as shown in FIG. 5a, or they may be displaced between layers, e.g. as shown in FIG. 5b with partially overlapping strips.

[0072] The precured fibrous strips 50 preferably comprise unidirectionally oriented reinforcement fibres, such as glass fibres or carbon fibres, oriented in the longitudinal direction. Further, the strips are preferably pultruded elements.

[0073] The pultruded strips 50 each have a first longitudinal end 51 and a second longitudinal end 52, a first side 53 and a second side 54 with a width defined as the distance between the first side 53 and the second side 54, and an upper surface 55 and a lower surface 56 with a thickness defined as the distance between the upper surface 55 and the lower surface 56. The upper surface 55 and lower surface 56 are defined in relation to how the pultruded strips are laid up, and the upper surface 55 will typically be arranged towards the inner surface of the spar cap 50, whereas the lower surface 56 will typically face towards the outer surface of the spar cap 50, as seen in relation to the blade shell 38. The strips each comprise a taper region 60 with a taper length at the first longitudinal end 51 and may further comprise a second taper region 70 at the second longitudinal end 52.

[0074] The taper region 60 advantageously has a thickness to length ratio in the range 1:10 to 1:200, preferably in the range 1:20 to 1:150, and more preferably in the range 1:50 to 1:125, e.g. around 1:100. The thickness of the strip 50 may advantageously be between 1 mm and 10 mm, preferably between 3 mm and 8 mm, and more preferably between 4 and 7 mm, e.g. around 5 mm. the width of the strip is between 30 mm and 300 mm, preferably between 50 mm and 200 mm, e.g. around 100 mm. The length of the strips 50 may advantageously be at least 100 mm and be up to the length of the blade, e.g. up to 100 m. In the future, even long strips may be used.

[0075] In addition, flow-promoting material 80, such as a fibre layer or fibre veil, may be arranged between precured fibrous strips 50 in order to promote resin flow during the infusion process. In the shown embodiment, the flow promoting material 80 is arranged in layers between layers of precured fibrous strips 50. However, flow-promoting material may also be arranged between neighbouring strips 50.

[0076] Further, the precured fibrous strips may be arranged between an inner skin layer 82 comprising one or more fibre layers, and an outer skin layer comprising one or more fibre layers, as shown in FIG. 4.

[0077] FIG. 6 shows the taper region 60 of a precured fibrous strip 50 in more detail. As shown in the figure, the taper region tapers in thickness towards the first longitudinal end 51 and comprises a first taper section 61 proximal to the first longitudinal end and having a first average taper angle, a third taper section distal 63 to the first longitudinal end 51 and having a third average taper angle; and a second taper section 62 between the first taper section 61 and the third taper section 63 and having a second average taper angle. The second average taper angle is larger than both the first average taper angle and the third average taper angle. Accordingly, it is seen that the precured fibrous composite strip 50 comprises a section 61 with a shallow taper angle near the start of the taper region, a middle section 62 with a larger taper angle, and a section 63 with a shallow taper angle near the longitudinal end 51 of the strip 50. Thereby, a smooth transition is obtained in the longitudinal direction, which is particular advantageous, when stacking such strips on top of each other in a load-carrying structure or where a fibre material is arranged over the strips 50. The smooth transition lowers the formation of resin rich area and the formation of wrinkles in the fibre material that is draped over the strips 50.

[0078] Further, as shown in FIG. 6, the taper region 60 has a blunt face 64 at the first longitudinal end 51, wherein the blunt face 64 has an end step thickness t.sub.es. By leaving the end face slightly blunt, the end of the precured fibrous strip 50 is even less likely to break off or form cracks. The end step thickness t.sub.es may be between 0.01 mm and 0.3 mm, preferably between 0.05 mm and 0.2 mm, e.g. around 0.1 mm.

[0079] In a preferred embodiment, the taper region 60 has a substantially S-shaped profile as seen in the longitudinal direction of the strip 50. In practice, this may be obtained by dividing the taper region 60 into a first curvature region 65 proximal to the first longitudinal end 51 and having an outer radius R.sub.1 of curvature, wherein the first curvature region 65 has an intersection length L.sub.is, and a second curvature region 66 distal to the first longitudinal end and having an inner radius R.sub.2 of curvature. This provides a particular simple way of providing a substantially S-shaped profile.

[0080] The S-shaped profile can be described using a single variable. The model for the profile of the taper region 60 has been compared to precured fibrous strips with a straight taper section to demonstrate their potential in optimising an allowable crack length at the first longitudinal end 51.

[0081] The principle of the model is that a S-shaped profile can be described by two circles with different radii. The S-shaped profile is illustrated in FIG. 6. A coordinate system with the y-axis oriented in the thickness direction of the strip 50 and a z-axis oriented in the longitudinal direction is shown in the lower left corner.

[0082] The profile of the first curvature region 65 and the second curvature region 66 follows two circles as shown in FIG. 6. The centres of the two circles are fixed in the z-direction at z=0 and z=L.sub.chamfer respectively, while the y-coordinate is governed by the radii of the two circles, R.sub.1 and R.sub.2. The end step thickness is denoted t.sub.es, and the thickness of the strip 50 is denoted t.sub.plank. The intersection between the two circles has the global coordinate (y, z)=(H.sub.IS, L.sub.IS), where subscript “IS” denoted the intersection. H.sub.IS may be determined from the following equation:

[00001]$H_{\mathrm{IS}}=\frac{L_{\mathrm{IS}}{t}_{\mathrm{plank}}}{L_{\mathrm{chamfer}}}.$

[0083] The S-shaped profile may be determined from the following system of equations:

R.sub.1 cos (θ.sub.1.sup.IS)=L.sub.IS  (1)

R.sub.1(1−sin(θ.sub.1.sup.IS))=H.sub.IS−t.sub.es  (2)

R.sub.2 cos(θ.sub.2.sup.IS)=L.sub.chamfer−L.sub.IS  (3)

t.sub.plank−R.sub.2(1−sin(θ.sub.2.sup.IS))=H.sub.IS  (4)

[0084] The above equations may be split in to two sets of equations, (1 and 2) and (3 and 4), with two unknowns, (R.sub.1 and θ.sub.1) and (R.sub.2 and θ.sub.2) respectively.

[0085] The solutions to the four equations are:

[00002]$\begin{array}{cc}{R}_1=\frac{{\left(H_{\mathrm{IS}}-t_{es}\right)}^2+L_{\mathrm{IS}} ^2}{2\left(H_{\mathrm{IS}}-L_{\mathrm{IS}}\right)}& \left(5\right)\end{array}$$\begin{array}{cc}{\theta }_1^{\mathrm{IS}}={\cos }^{-1}\left(\frac{L_{\mathrm{IS}}}{H_{\mathrm{IS}}}\right)& \left(6\right)\end{array}$$\begin{array}{cc}{R}_2=\frac{{\left(t_{plank}-H_{\mathrm{IS}}\right)}^2+{\left(L_{\mathrm{chamfer}}-L_{\mathrm{IS}}\right)}^2}{2{\left(t_{plank}-H_{\mathrm{IS}}\right)}^2}& \left(7\right)\end{array}$$\begin{array}{cc}{\theta }_2^{\mathrm{IS}}={\cos }^{-1}\left(\frac{L_{\mathrm{chamfer}}-L_{\mathrm{IS}}}{R_2}\right)& \left(8\right)\end{array}$

[0086] For a given chamfer length, L.sub.chamfer, plank thickness, t.sub.plank, and end step thickness, t.sub.es, the chamfer profile may be described using a single variable, viz. the z-coordinate for the intersection length, L.sub.IS. Typically, the thickness, t.sub.plank is fixed and the end step thickness, t.sub.es, is predefined, leaving the single variable to be the non-dimensional parameter, L.sub.IS/L.sub.chamfer.

[0087] The S-shaped profile is described by the first curvature region 65 and the second curvature region as shown in the below table:

TABLE-US-00001 z y Range First curvature region 65: 0 ≤ z ≤ L.sub.IS R.sub.1 cos(θ.sub.1) t.sub.es − R.sub.1(sin(θ.sub.1) − 1) [00003]${\theta }_1:\frac{\pi }{2}.fwdarw.{\theta }_1^{\mathrm{IS}}$ Second curvature region 66: L.sub.IS ≤ z ≤ L.sub.chamfer L.sub.chamfer − R.sub.2 cos(θ.sub.2) t.sub.plank − R.sub.2(1 − sin(θ.sub.2)) [00004]${\theta }_2:{\theta }_2^{\mathrm{IS}}.fwdarw.\frac{\pi }{2}$

[0088] In a preferred embodiment, the profile of the taper region 60 deviates at most 5%, preferably at most 3%, and more preferably at most 2% from the above model. In other words, the profile has a tolerance of at most 5%, at most 3%, or at most 2% of the defined profile.

[0089] Examples of different profiles for the taper region 60 with different values of L.sub.IS/L.sub.chamfer are presented in FIG. 7 and compared to a profile with a straight chamfer.

[0090] The different profiles have been compared to profiles with a straight chamfer using J. W. Hutchinson's model for evaluating an interface crack between two elastic layers to evaluate an Energy Release Rate (ERR) as described in the article “Interface crack between two elastic layers,” published in International Journal of Fracture 43 in 1988. Applying Hutchinson's model to the problems of plank drop at the end of a strip and describing the height of the chamfered profile as a function of the crack length, the ERR may be calculated for different crack lengths along the interface between strips to determine a critical crack length. For a strip with an S-shaped taper region 60, the critical crack length is improved approximately with a factor of 6 compared to the critical crack length of a strip with a straight chamfer for L.sub.IS/L.sub.chamfer=0.4. For higher values of this ratio, the critical crack length is improved even further.

[0091] In the following, a method of manufacturing a spar cap for a wind turbine blade according to the present disclosure is described. The method comprises the steps shown in FIG. 8.

[0092] In a first step 90, a plurality of precured fibrous strips including at least one precured fibrous strip 50 as described above is provided. In a second step 91, the plurality of precured fibrous strips are stacked such that interface regions are formed between adjacent precured fibrous strips. In an optional third step 93, flow-promoting material is arranged between at least some of the stacked precured fibrous strips. In a fourth step 94, resin is supplied to the plurality of precured fibrous strips and causing the resin to fill the interface regions between adjacent strips. In a fifth step 95, the resin is cured in order to form the spar cap.

[0093] The plurality of precured strips may be stacked in an array. Further, the manufacturing method may additionally comprise the step of draping a fibre layer over the plurality of stacked precured fibrous strips before the step of supplying resin.

LIST OF REFERENCE NUMERALS

[0094] 2 wind turbine [0095] 4 tower [0096] 6 nacelle [0097] 8 hub [0098] 10 blade [0099] 14 blade tip [0100] 16 blade root [0101] 18 leading edge [0102] 20 trailing edge [0103] 22 pitch axis [0104] 30 root region [0105] 32 transition region [0106] 34 airfoil region [0107] 36 pressure side shell [0108] 38 suction side shell [0109] 39 shoulder [0110] 40 spar cap [0111] 50 precured fibrous strips/pultruded elements [0112] 51 first longitudinal end [0113] 52 second longitudinal end [0114] 53 first side [0115] 54 second side [0116] 55 upper surface [0117] 56 lower surface [0118] 60 taper region/first taper region [0119] 61 first taper section [0120] 62 second taper section [0121] 63 third taper section [0122] 64 blunt face [0123] 65 first curvature region [0124] 66 second curvature region [0125] 70 taper region/second taper region [0126] 80 flow promoting material [0127] 82 inner skin layer(s) [0128] 84 outer skin layer(s) [0129] 90-94 Method steps