WO2013075718A1

WO2013075718A1 - A wind turbine blade

Info

Publication number: WO2013075718A1
Application number: PCT/DK2012/050423
Authority: WO
Inventors: Paul Hibbard
Original assignee: Vestas Wind Systems A/S
Priority date: 2011-11-24
Filing date: 2012-11-19
Publication date: 2013-05-30

Abstract

A wind turbine blade, in particularly, a sectional wind turbine blade comprising a blade body extending along a longitudinal axis between a root and a tip thereof, the blade body being divided transversely to its longitudinal axis into a first blade portion proximate to the root and a second blade portion proximate to the tip, each blade portion having upper and lower blade surfaces which are opposite to each other and spaced from each other in a thickness direction of the blade body, and the first and second blade portions being connected to each other at a connection portion wherein the first and second blade portions comprise first and second flange portions, respectively, facing each other at the connection portion, wherein the first flange portion comprises a first upper airfoil portion and a first lower airfoil portion opposite to and spaced from the first upper airfoil portion in the thickness direction, which both extending from the first blade portion towards the second blade portion, wherein the second flange portion comprises a second upper airfoil portion and a second lower airfoil portion opposite to and spaced from the second upper airfoil portion in the thickness direction, both extending from the second blade portion towards the first blade portion, and wherein the first and second upper airfoil portions are connected to each other and the first and second lower airfoil portions are connected to each other to thereby form the connection portion.

Description

A WIND TURBINE BLADE Field of Invention

[0001] The present invention relates to a wind turbine blade, particularly, a sectional wind turbine blade.

Background

[0002] Wind turbines have been conventionally used to convert wind energy to electrical energy. Typically, a wind turbine has three blades and each blade can be as long as 55 metres or more and weigh up to 15 tonnes.

[0003] Due to the length and weight of a blade, huge production facilities are required to fabricate a single continuous piece of blade and extensive transportation means are needed to move the single blade from the production facilities to the installation site. As such, it is foreseeable that it would be easier and more economical to fabricate and transport a blade if the blade is fabricated by sections or known as commonly known as sectional blades have it assembled and installed on site.

[0004] When in operation, apart from its own weight, the blades are subjected to considerable amount of forces from the strong wind. As a blade is anchored only at one end of the blade, there is a considerable amount of moment exerted onto the blade due to the aerodynamic thrust loads exerted along the length of the blade thus causing flapwise loading on the blade. Flap- wise load is a force applied on the blade in the direction substantially perpendicular to the blade chord (a straight line between the leading edge and trailing edge of the blade) and blade length of the wind turbine blades. In addition, there are also edgewise loads in the crosswise direction (leading edge to trailing edge) resulting in additional bending stress on the blade.

[0005] Under the considerable amount of stress and shear, a blade having a plurality of sections and joints is highly susceptible to failure at the joints. There have been various attempts in designing a sectional blade that is able to withstand, apart from its own weight, the load or forces on the blade. One of the attempts was to increase the cross-section of the blade, thereby increasing the dimensions of the joints and thus increasing the amount of load that could be borne by the joints. An increase in the thickness of the blade would require an accompanying increase in the chord length of the blade so that the thickness to chord ratio of the blade is maintained in order not to disrupt the aerodynamic profile of the blade. However, increasing the cross-section of the blade inevitably increases the material used on the blade and thus increases the weight of the blade and this may not be aerodynamically or economically efficient and may outweigh the benefits of using sectional blades.

[0006] The present invention aims to provide a sectional wind turbine blade that is able to withstand a relatively high amount of load or stress at the joints and yet avoiding or minimizing the disadvantages discussed above.

Summary of the Invention

[0007] According to the present invention, a wind turbine blade having a first and second blade portions is provided.

[0008] The wind turbine blade basically comprises a blade body extending along a longitudinal axis between a root and a tip thereof. The blade body being divided transversely to its longitudinal axis into a first blade portion proximate to the root and a second blade portion proximate to the tip, each blade portion having upper and lower blade surfaces which are opposite to each other and spaced from each other in a thickness direction of the blade body, and the first and second blade portions being connected to each other at a connection portion. The first and second blade portions comprise first and second flange portions, respectively, facing each other at the connection portion. The first flange portion comprises a first upper airfoil portion and a first lower airfoil portion opposite to and spaced from the first upper airfoil portion in the thickness direction, which both extending from the first blade portion towards the second blade portion. The second flange portion comprises a second upper airfoil portion and a second lower airfoil portion opposite to and spaced from the second upper airfoil portion in the thickness direction, both extending from the second blade portion towards the first blade portion. The first and second upper airfoil portions are connected to each other and the first and second lower airfoil portions are connected to each other to thereby form the connection portion. The spacing between the upper airfoil portions and the lower airfoil portions may allow the connection portion to withstand higher bending moment and thus bending stress thus increasing the ability to withstand a relatively high amount of load or stress at the joints.

Brief Description of the Drawings

[0009] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0010] Fig. 1 shows a common setup of a wind turbine.

[0011] Fig. 2 shows a perspective view of an embodiment of the present invention; [0012] Fig. 3 shows a perspective view of a first blade portion of the embodiment in Fig. 2;

[0013] Fig. 4 shows a perspective view of a second blade portion of the embodiment in Fig. 2;

[0014] Fig. 5 shows a partial elevation view of the embodiment in Fig. 2;

[0015] Fig. 6 shows a closed-up sectional view of the connection portion of the embodiment in Fig. 5 having a coupling element;

[0016] Fig. 7 shows an elevation view of the blade body in Fig. 5 having a web;

[0017] Fig. 8 shows an elevation view of the blade body in Fig. 7 with another embodiment of the web;

[0018] Fig. 9 shows a perspective view of another embodiment of the present invention;

[0019] Fig. 10 shows a perspective view of a partial first blade portion of the embodiment in Fig. 9;

[0020] Fig. 11 shows a perspective view of a second blade portion of the embodiment in Fig. 9;

[0021] Fig. 12 shows a partial elevation view of the embodiment in Fig. 9 having a web;

[0022] Fig. 13 shows a closed-up elevation view of first and second airfoil portion of the embodiment in Figs. 1 or 9 having a hammerhead insert;

[0023] Fig. 14 shows a closed-up sectional view of T-bolt system for the embodiment in Figs. 1 or 9; [0024] Fig. 15 shows a top view of the T-bolt system in a first and second airfoil portions of the embodiment in Figs, lor 9 with the inserted partially removed to show the stud bolt; and

[0025] Fig. 16 shows a perspective view of a cut up blade portion of the embodiment in Figs. 1 or 9 having the airfoil portions extending from a spar.

Description

[0026] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0027] Fig. 1 shows a common setup of a wind turbine 10 in which embodiments may be used. The wind turbine 10 is mounted on a base 12. The wind turbine 10 includes a tower 14 having a number of tower sections. A wind turbine nacelle 16 is placed on top of the tower 14. A wind turbine rotor, connected to the nacelle 16, includes a hub 18 and at least one rotor blade or wind turbine blade, e.g. three wind turbine blades 20. The wind turbine blades 20 are connected to the hub 18 which in turn is connected to the nacelle 16 through a low speed shaft which extends out of the front of the nacelle 16.

[0028] Fig. 2 shows exemplary embodiment of the present invention. As shown in Fig. 2, the wind turbine blade 20 has a blade body 100 of an elongated profile. The blade body 100 has a root 102 at one end of the blade body 100, the root 102 being connectable to the hub 18 of the wind turbine 10 (not shown in Fig. 2), and a tip 104 at the other end of the blade body 100, the tip 104 being a free end of the blade body 100. The blade body 100 extends between the root 102 and the tip 104 and has a longitudinal axis 106 that passes through the blade body 100 from the root 102 to the tip 104. The blade body 100 has an airfoil cross-sectional area, perpendicular to the longitudinal axis 106 of the blade body 100 and the cross-sectional area proximate the root 102 is larger than the cross-sectional area proximate the tip 104 and from the root 102 to the tip 104, the cross-sectional area of the blade body 100 reduces along the longitudinal axis 106 except at a connection portion 118 where the cross-sectional area may increase as shown later. The blade body 100 has a leading edge 108 (edge contacting the wind first) extending substantially from the root 102 to the tip 104 and a trailing edge 110 (edge where the wind leaves the blade) extending substantially from the root 102 to the tip 104 opposite the leading edge 108 in a span-wise direction of the blade body 100.

[0029] As shown in Fig. 2, the blade body 100 is divided transversely to the longitudinal axis 106 along a dividing line 112 into a first blade portion 114 which is proximate to the root 102 and a second blade portion 116 which is proximate to the tip 104. The dividing line 112 cuts across the blade body 100 from the leading edge 108 to the trailing edge 110. The first blade portion 114 and the second blade portion 116 are connected to each other at the connection portion 118 along the dividing line 112 and the first blade portion 114 is arranged such that it faces the second blade portion 116 at the connection portion 118.

[0030] As shown in Figs. 3 and 4, each of the first blade portion 114 and the second blade portion 116 has first and second upper blade surfaces 120, 124 and first and second lower blade surfaces 122,126 which are opposite to each other and are spaced from each other in a thickness direction of the blade portions 114, 116. The first blade portion 114 and second blade portion 116 has a first flange portion 130 (see Fig. 3) and a second flange portion 140 (see Fig. 4), respectively. Each of the flange portions 130, 140 extends in the direction of the longitudinal axis 106 from its respective blade portions 114, 116 towards its first and second connection portion 118 A, 118B respectively. The first blade portion 114 and the second blade portion 116, each has a spar 128 (see Fig. 16) for providing structural strength and integrity to the blade portions 114, 116, the spar 128 is positioned between the upper blade surfaces 120, 124 and lower blade surfaces 122, 126 and extends from one end, i.e. root 102 or tip 104, of the blade portions 114, 116 to its flange portion 130, 140. The first upper blade surface 120 and the first lower blade surface 122 of the first blade portion 114 extend in the direction of the longitudinal axis 106 from the root 102 to the first flange portion 130. Similarly, the upper blade surface 124 and lower blade surface 126 of the second blade portion 116 extend in the direction of the longitudinal axis 106 from the tip 104 to the second flange portion 140. The blade surfaces 120, 122, 124, 126 extend transversely from the leading edge 108 to the trailing edge 110 of the blade portions 114, 116 respectively.

[0031] As shown in Fig. 3, the first flange portion 130 includes a first upper airfoil portion 132 and a first lower airfoil portion 134 opposite to and spaced from the first upper airfoil portion 132 in the thickness direction and both first airfoil portion 132 and second airfoil portion 134 extend from the first blade portion 114 towards the second blade portion 116. More clearly shown in Fig. 5, the first upper airfoil portion 132 and the first lower airfoil portion 134 branch out from the first blade portion 114 in opposite obliquely direction away from each other and arches towards the connection portion 118. Due to this arrangement, the leading edge 108 (see Fig. 3) and trailing edge 110 along the first blade portion 114 split at a junction where the first upper and lower airfoil portion 132, 134 branch out and the edges 108, 110 follow along the direction of the first upper and lower airfoil portion 132, 134 and end at a first upper connection face 136 and a first lower connection face 138 respectively. As seen in Fig. 5, the first upper and lower connection face 136, 138 may be substantially on the same plane which passes through the dividing line 112 and both the first upper and lower connection faces 136, 138 are spaced in the thickness direction from each other.

[0032] The first upper and lower connection faces 136, 138 of the first upper and lower airfoil portions 132, 134 respectively, may have an airfoil cross-sectional area as shown in Fig. 3. With an airfoil cross-sectional area, each of the first upper and lower airfoil portions 132, 134 has an upper surface or a suction surface and a lower surface or a pressure surface. The relationship between a suction surface and the pressure surface is that the static pressure on the suction surface is lower than the static pressure on the pressure surface thereby causing a lift or force to turn the blade. The first upper and lower airfoil portions 132, 134 are arranged in the same orientation such that the pressure surface of the first upper airfoil portion 132 faces the pressure surface 120 of the first lower airfoil portion 134 as shown in Fig. 3.

[0033] As shown in Fig. 4, the design of the second flange portion 140 is similar to the first flange portion 130. The second flange portion 140 includes a second upper airfoil portion 142 and a second lower airfoil portion 144 opposite to and spaced from the second upper airfoil portion 142 in the thickness direction, both the second upper airfoil portion 142 and second lower airfoil portion 144 extend from the second blade portion 116 towards the first blade portion 114. As shown in Fig. 5, the second upper airfoil portion 142 and the second lower airfoil portion 144 branch out from the second blade portion 116 in opposite obliquely direction away from each other and arch towards the connection portion 118. The leading edge 108 (see Fig. 4) and trailing edge 110 along the second blade portion 116 splits at a junction where the second upper and lower airfoil portions 142, 144 branch out and the edges 108, 110 follow along the direction of the second upper and lower airfoil portion 142, 144 and end at a second upper connection face 146 and a second lower connection face 148 respectively. Although, not shown in the figures, the splitting of the edges 108, 110 from the first and second blade portion 114, 116 respectively at the junction does not change the profile of the blade body 100 when the blade body 100 is viewed from the top. In other words, the profile of the blade body 100 when viewed from the top of the blade body 100 is maintained (e.g. no bends or stepped profile) despite the splitting of the edges 108 and 110. As shown in Fig. 5, the second upper and lower connection face 146, 148 may be substantially on the same plane which passes through the dividing line 112 and substantially perpendicular to the longitudinal axis 106 and the second upper and lower connection faces 146, 148 are spaced in the thickness direction from each other. The spacing between second upper and lower connection faces 146, 148 would preferably be equal to the spacing between the first upper and lower connection faces 136, 138 so that the first and second upper airfoil portions 132, 142 are connected to each other and the first and second lower airfoil portions 134, 144 are connected to each other to form the connection portion 108 at dividing line 112. The spacing between the upper connection faces 136, 146 and lower connection faces 138, 148 forms a through opening 180 between the upper airfoil portions 132, 142 to allow air to flow through and the through opening 180 extends in a direction substantially transverse to the longitudinal axis 106 of the blade body. As shown in Fig. 5, the first upper airfoil portion 132, 142 bulge from the first upper blade surface 120, 124 of the first and second blade portion 130, 140 in the thickness direction. Similarly, the first and second lower airfoil portions 134, 144 bulge from the first and second lower blade surfaces 122, 126 of the first and second blade portion 130, 140 in the thickness direction and the through opening 180 is formed between the bulges.

[0034] As shown in Fig. 4, the second upper and lower connection faces 146, 148 of the second upper and lower airfoil portions 142, 144 may have an airfoil cross-sectional area. With an airfoil cross-sectional area, each of the second upper and lower airfoil portions 142, 144 has an upper surface or suction surface and a lower surface or a pressure surface. The second upper and lower airfoil portions 142, 144 are arranged in the same orientation such that the pressure surface of the second upper airfoil portion 142 faces the suction surface of the second lower airfoil portion 144.

[0035] As shown in Figs. 3 and 4, the first airfoil portions 132, 134 and second airfoil portions 142, 144 include a plurality of through holes 150 in the first airfoil portions 132, 134 and second airfoil portions 142, 144 through the respective connection faces 136, 138, 146, 148 for retaining inserts 160 (not shown in Figs. 3 and 4). Each of the plurality of through holes 150 are aligned in the direction of the longitudinal axis 106 and penetrates the first airfoil portions 132, 134 and second airfoil portions 142, 144 through and substantially perpendicular to the respective first and second connection faces 136, 138, 146, 148 and exits via the suction surface or pressure surface of the first and second airfoil portions 132, 134, 142, 144 respectively. The plurality of through holes 150 may preferably be aligned along a camber line 152 of the airfoil shaped first connection surfaces 136, 138, 146, 148. The camber line is a line on the airfoil cross-section, connecting all points that are equidistant from the suction and pressure surface. However, the plurality of through holes 150 may not need to be along the camber line 152 of the first and second airfoil portions 132, 134, 142, 144 and may be along or parallel to the chord of the airfoil profile depending on the design requirement of the blade. Although it is shown in Figs. 3 and 4 that there are five through holes 150 on the connection faces 136, 138, 146, 148, the number of through holes 150 on each airfoil portion may vary according to the design variables, e.g. weight, bending moment, of the wind turbine blade 20. [0036] The first and second airfoil portions 132, 134, 142, 144 may further comprise inserts 170 in the first and second airfoil portions at the connection portion 118, the inserts 170 in the first upper and lower airfoil portions 132, 134 face the inserts 170 in the second upper and lower airfoil portions 142, 144 and are in the direction parallel to the longitudinal axis 106. The inserts 170 are preferably in a circular tubular cross-sectional area to allow coupling elements 154 (see Fig. 6) to go through. The inserts 170 are located in the plurality of holes 150. As an example, Fig. 6 shows a partial sectional view of the connection portion 118 where an insert 170 is shown to be fixed in the first upper airfoil portion 132 and the second upper airfoil portion 142. As shown in Fig. 6, an end of the insert 170 in the first upper airfoil portion 132 is proximate to or flushed with the first upper connection face 136 of the first upper airfoil portion 132. Similarly, an end of the insert 170 in the second upper airfoil portion 142 is proximate or flushed with the second upper connection face 146. Preferably, the inserts 170 are fixed to the holes 150 by bonding agent, e.g. epoxy, or by interference fit. Although only the connection between the first upper airfoil portion 132 and second upper airfoil portion 142 is shown in Fig. 6, the same connection is applicable between the first lower airfoil portion 134 and second lower airfoil portion 144.

[0037] Referring to Fig. 5, to connect first flange portion 130 to the second flange portion 140, the first and second upper connection faces 136, 146 are brought into contact with each other by placing the first flange portion 130 of the first blade portion 114 to face and be in contact with the second flange portion 140 of the second blade portion 116 and align the plurality of through holes 150 in the first upper and lower airfoil portions 132, 134 with the plurality of through holes 150 in the second upper and lower airfoil portions 142, 144.

Thereafter, as shown in Fig. 6, coupling elements 154, e.g. bolt and nut, are inserted through the holes of the inserts 170 and coupled together to connect the first blade portion 114 to the second blade portion 116 as shown in Fig. 5.

[0038] Fig. 7 shows an elevation view of the blade body 100 having the first flange portion 130 and second flange portion 140 connected at the connection portion 118. As shown in Fig. 7, the blade body 100 includes a web 160 of a plate-like structure having an upper edge portion 162 attached to at least one of the upper airfoil portions 132, 142 and a lower edge portion 164 attached to at least one of the lower airfoil portions 134, 144. In the embodiment of Fig. 7, the upper edge portion 162 of the web 160 is preferably mounted between the first upper airfoil portion 132 and second upper airfoil portion 142 and the lower edge portion 164 of the web 160 is mounted between the first lower airfoil portion 134 and the second lower airfoil portion 144 such that the web 160 is along a plane substantially perpendicular or radial to the longitudinal axis 106 of the blade body 100 and extends between the leading edge 108 and trailing edge 110 (not shown in Fig. 7) to provide structural stability against flap-wise loading on the first and second flange portions 130, 140 of the blade body 100 in order to prevent collapse of the flange portions 130, 140 under the flap- wise load. Flap- wise load is a force applied on the blade body 100 in the direction substantially perpendicular to the plane of rotation of the wind turbine blade 20. The web 160 may have an aero-dynamic cross-sectional area or rounded edges along the windward edge and leeward edge of the web 160 so as to minimise disturbances to the airflow along the surfaces between the windward and leeward edges of the web 160. The web 160 may have a plurality of web holes 166 at the upper edge portion 162 and lower edge portion 164 of the web 160 for the penetration of the coupling elements 154. The plurality of web holes 166 at the upper edge portion 160 may be aligned to coincide with the plurality of holes 150 on the first and second upper connection faces 136, 146 and lower plurality of holes 150 at the lower edge portion 164 may be aligned to coincide with the plurality of through holes 150 on the lower connection faces 138, 148 so that coupling elements 154 used to connect the first and second blade portions 114, 116 at the connection portion 118 are inserted through the plurality of web holes 166 in the web 160 and secure the web 160 in position.

[0039] Alternatively, as shown in Fig. 8, the web 160 may be mounted between the upper airfoil portions 132, 142 and lower airfoil portions 134, 144. The web 160 may be of an I- shape cross-sectional area profile between the upper airfoil portions 132, 142 and lower airfoil portions 134, 144 such that the upper edge portion 162 of the web 160 can be mounted to the pressure surface of the upper airfoil portions 134, 144 and the lower edge portion 164 of the web 160 can be mounted to the suction surface of the lower airfoil portion 134, 144. The mounting of the web 160 to the airfoil portions 132, 134, 142, 144 may be by bolts or by adhesion. As it may be seen from the embodiment of Fig. 8 that the web 160 can be mounted anywhere in the through opening 180 other than along the dividing line 112. Besides having plate-like structure, the web 160 may consist of strip-like structure, e.g. rib, extending between the upper airfoil portions 132, 134 and lower airfoil portions 142, 144. In addition, there may be a plurality of webs 160 mounted between the upper airfoil portions 132, 134 and lower airfoil portions 142, 144 spaced from one another along the direction of the longitudinal axis 106.

[0040] Fig. 9 shows another exemplary embodiment of the present invention. Like features in the present embodiment has like reference numerals (e.g. 218 and 118) in the first embodiment.

[0041] Fig. 9 shows a blade body 200 as shown in the first embodiment in Fig. 2. The blade body 200 comprises the first blade portion 214 and the second blade portion 216 as in the blade body 100 in the first embodiment. Similar to the first embodiment in Fig. 2, and as shown in Fig. 9, the first blade portion 214 includes the first flange portion 230 and the second blade portion 216 includes the second flange portion 240. The first flange portion 230 includes the first upper airfoil portion 232, the first lower airfoil portion 234 and a first middle airfoil portion 282. The first middle airfoil portion 282 extends from the first blade portion 214 towards the second blade portion 216 and is between the first upper airfoil portion 232 and first lower airfoil portion 234 and spaced therefrom in the thickness direction. The second flange portion 216 includes the second upper airfoil portion 242, the second lower airfoil portion 244 and a second middle airfoil portion 292 which extends from the second blade portion 216 towards the first blade portion 214 and is between the second upper airfoil portion 242 and second lower airfoil portion 244 and spaced therefrom in the thickness direction. The first and second middle airfoil portions 282, 292 are connected to each other to thereby further form the connection portion 218. Similar to the first embodiment, the first upper and lower airfoil portions 232, 234 have a plurality of through holes 250 substantially parallel to the direction of the longitudinal axis 206 along the connection faces 236, 238 (hidden) respectively. Further, as shown in Fig. 10, the first middle airfoil portion 282 may comprise at least one shear pin hole 296 along and on the first middle connection face 284 parallel to the direction of the longitudinal axis 206 for receiving a shear pin 298 (see shear pin in Fig. 11) which will be explained in detail later. In Fig. 11 , two shear pin holes 296 are shown but the number of shear pin holes 296 may vary according to the design of the blade 20.

[0042] As shown in Fig. 10, the first middle airfoil portion 282 may extend along the longitudinal axis 206 from the first blade portion 214 and end at a first middle connection face 284. The first middle connection face 284 may be in the same plane which passes through the first upper connection face 236 and first lower connection face 238. As shown in Fig. 11, the second middle airfoil portion 292 may extend along the longitudinal axis 206 from the second blade portion 216 and end at a second middle connection face 294 and the second middle connection face 294 may be in the same plane which passes through the second upper connection face 246 and second lower connection face 248. Similar to the first embodiment, the second upper and lower airfoil portions 242, 244 have a plurality of through holes 250 substantially parallel to the direction of the longitudinal axis 206 and along the connection faces 246, 248 respectively. Further, the second middle airfoil portion 292 may comprise at least one shear pin hole 296 (hidden) along and on the second middle connection face 294 parallel to the direction of the longitudinal axis 206 for receiving the shear pin 298. The shear pin holes 296 is located along the second middle connection faces 246, 248 to coincide with the shear pin holes 296 along the first connection faces 236, 238 (see Fig. 10) so as to allow the insertion of the shear pin 298 into the shear pin holes 296 in both the first and second middle airfoil portions 282, 292 accordingly (see Fig. 12). The shear pin 298 may be detachably inserted and/or permanently fixed into the shear pin hole 296. In Fig. 11, two shear pins 298 are shown although the number of shear pin holes 296 may vary according to design requirement. As shown in Fig. 9, the first and second middle airfoil portions 282, 292 may be part of the first and second blade portions 214, 216 such that the surfaces of blade portions 214, 216 are flushed or continuous with the surfaces of the first and second middle airfoil portion 282, 292. In other words, the middle airfoil portions 282, 292 do not bulge from the surfaces of the blade portions 214, 216. However, the surfaces may not be flushed or continuous.

[0043] Referring to Fig. 12, the web 260 is shown to be connected between the first flange portion 230 and the second flange portion 240 as mentioned previously. The web 260 in the present embodiment includes at least one web hole 266 through the web 260 in a middle portion 268 between the upper edge portion 162 and lower edge portion 164 and coincides with the location of the shear pin hole 296 on the first middle connection face 284 which coincides with the location of the shear pin hole 296 on the second middle connection face 294. The web 260 may be of an I-shaped cross-sectional area profile between the upper airfoil portions 132, 142 and lower airfoil portions 134, 144 and mounted between the airfoil portions 134, 144 as described earlier. The web 260 may also be ribs mounted between the upper and lower airfoil portions 132, 134, 142, 144.

[0044] Similar to the first embodiment and as shown in Fig. 12, the web 260 may be mounted between the first flange portion 230 and the second flange portion 240. The upper edge portion 262 is mounted between the first upper airfoil portion 232 and second upper airfoil portion 242 such that web holes 266 through the upper edge portion 262 of the web 260 as aligned longitudinally with the plurality of through holes 250 on the first and second upper airfoil portions 232, 242. Once the web holes 266 on the upper edge portion 262 are aligned with the plurality of through holes 250 in the first and second airfoil portions 232, 242, the web holes 266 in the middle portion 268 between the upper and lower edge portions 262, 264 would be automatically aligned with the shear pin holes 296 in the first and second middle airfoil portion 282, 292 and the web holes 266 in the lower edge portion 264 of the web 260 would be aligned with the plurality of through holes 250 in the first and second lower airfoil portions 234, 244. To couple the first flange portion 230, web 260 and second flange portion 240 together, coupling elements 254, e.g. bolts and nuts, are used to connect the first upper airfoil portion 232 to the second upper airfoil portion 242 and the first lower airfoil portion 234 to the second lower airfoil portion 244 with the web 260 in between. The first and second middle airfoil portions 282, 292 are held together abutting the middle portion 268 of the web 260 by the coupling of the upper and lower airfoil portions 232, 234, 242, 244 wherein the shear pin or pins 298 are held in position within the shear pin holes 266 in the first and second middle airfoil portions 282, 292 and through the web 260. The shear pin 298 prevents excessive movement of the web 260 with respect to the first and second middle airfoil portions 282, 292 in a direction perpendicular to the longitudinal axis 206, i.e.

shearing, so as to provide further structural stability at the connection portion 218.

[0045] As shown in Fig. 6, the coupling elements 154, 254 may be applied to circular tubular inserts 170. Alternatively, the inserts 170 may consist of hammerhead inserts as shown in Fig. 13 wherein one end, usually a threaded tapered end, of each of the hammerhead inserts is screwed into the airfoil portions, e.g. first and second upper airfoil portions 132, 142 as shown in Fig. 13, and the other end has a radial flange with through holes substantially parallel to the longitudinal axis and spaced apart from each other around the flange in a circumferential direction. To connect the first and second flange portions 130, 140 together, the flanges on the hammerhead on both the first and second flange portions 130, 140 are brought together and face each other and are flanges are connected to each other using bolts and nuts.

[0046] Alternatively, T-bolt system may be used as shown in Fig. 14 to connect the first blade portion 114 to the second blade portion 116. The T-bolt system comprises an insert with a threaded bore being inserted or screwed into e.g. the first upper airfoil portion 132 at a distance behind the first upper connection face 136 and another insert having a through bore being inserted or screwed into the second upper airfoil portion 142 at a distance behind the second upper connection face 146. A stud bolt having threaded ends is inserted into the first upper airfoil portion 132 with one of the threaded ends screwed into the insert with threaded bore so that the stud bolt is rigidly attached to the first upper airfoil portion 132. To connect the first upper airfoil portion 132 to the second upper airfoil portion 142, the other threaded end is inserted into the second upper airfoil portion 142 and through the insert with through bore. The second upper airfoil portion 142 has an opening on its surface as shown in Fig. 15 whereby the other threaded end of the stud bolt can be seen through the opening after the stud bolt is fully inserted into the second upper airfoil portion 142. To secure the first and second upper airfoil portions 132,142, a nut is inserted through the opening to be screwed onto the other threaded end of the stud bolt so as to tighten the joint between the first and second upper airfoil portions 132,142.The same arrangement can be made to the first and second lower airfoil portions. When the first and second flange portions 130, 140 are connected together, fairings or covers may be used to conceal the coupling elements as well as to provide a continuous surface between the surfaces of the first and second airfoil portions 132,142. As it can be understood by a skilled person that the above mentioned inserts 170, e.g. hammerhead inserts, are applicable to any of the embodiments in the present invention.

[0047] As shown in all the figures, in particular, Fig. 9, the first and second upper airfoil portions 132, 142 and the first and second lower airfoil portions 134, 144 may be adhered to the first and second blade portions 130, 140 respectively by bonding agents, e.g. epoxy, or by connecting elements, e.g. bolt and screws. Alternatively, as shown in Fig. 16, blade portion 130 comprises a spar 128 as mentioned earlier and the upper airfoil portion 132 and the lower airfoil portion 134 of the blade portions 130 may be extended from the spar 128 of the blade portions 130. In this way, airfoil portions may be added to an existing conventional sectional blade body by adhering airfoil portions to the blade surface or installing airfoil portions to the spar of the blade portions to strengthen the joints. The same arrangement is applicable to the second blade portion 140 and for all the embodiments described herein.

[0048] At the connection portion 118, 218, the bulging and "splitting" of the blade body into two or three or more airfoil profiles in the thickness direction allows an increase in the sectional thickness at the connection portion 118, 218 as compared to a blade body having only one airfoil profile without bulging. By increasing the sectional thickness, the capacity of the connection portion 118, 218 to resist bending moment is increased. Although the sectional thickness is increased, the splitting of the blade body into two or three thin sections minimizes the detriment to aerodynamic performance of the blade due to the increased sectional thickness as compared to a similarly thick blade body having a bulge without the splitting and the through openings in the blade body. By introducing a bulge on both sides of the blade body, the use of coupling elements 154, 254, e.g. bolts and nuts, is thus possible to fasten the blade portions 114, 116, 214, 216 together due to the tilted surface of the bulge with respect to the longitudinal axis at the connection portion 118, 218. At the same time, tilt in the surface of the bulge of the airfoil portions allows the coupling elements 154, 254 to be accessed for fitting or inspection. The use and inspection of the coupling elements 154, 254 would not have been possible along a normally flat surfaced blade body. The increase in the sectional thickness thus the increase in the sectional area at the connection portion 118, 218 as compared to a slender and thin blade body having a smaller sectional height or area. In other words, the blade body 100 is able to withstand a higher loading as the connection portion 118, 218 is able to withstand a higher bending moment and stress as compared to the joint of a conventional sectional blade.

[0049] More importantly, for a blade body having two, three or more airfoil portions subjected to bending moment, the increase in distance between the upper airfoil portion 132, 142 or lower airfoil portion 134, 144 and the neutral axis of the blade body reduces the stress experienced in the coupling elements 154, 254, e.g. bolts, as the stress experienced in the bolt is inversely proportional to the distance between the airfoil portion and the neutral axis. The neutral axis being a longitudinal axis along the length of the blade body where there are no longitudinal stresses or strains. [0050] Alternatively, the width of the wind turbine blade 20 may be widened in a substantially width- wise direction at the connection portion 118 of a joint to allow a higher loading at the joint. This may be achieved by widening the upper and/or lower airfoil portions 132,134,142,144,232,234,242,244 and/or the middle airfoil portions 282, 292 at the connection portion 118. The connection portion 118 of the airfoil portions may be widened to an extent that the profile of the blade body 100 is augmented at the joints when viewed from the top of the blade 20. In another word, the leading edge 108 and/or trailing edge 110 at the connection portion 118 budges in a substantially width- wise direction from leading edge 108 and/or trailing edge 110 of the blade portions 114,116,214,216.

[0051] Although the embodiments described herein provides for the blade body 100 having a first and second blade portions 114, 116, 214, 216, the present invention is also applicable to a blade body having a plurality of blade portions e.g. three, four, five or more.

[0052] As mentioned above, fairings may be used on the embodiments to conceal the coupling elements 154, 254 as well as providing a flushed and/or continuous surface between the flange portions 130, 140. Also, fairings may be used to cover corners and bends from the bulging of the airfoil portions 132, 134, 142, 144 from the blade portions 114, 116 to provide a smooth and well faired geometry in order to minimize drag and noise.

[0053] The airfoils in the embodiments may be made from carbon or fibreglass materials. The bonding agents, e.g. epoxy, may be polyurethane based bonding agents. The coupling elements may be made from anti-corrosive materials. All the materials used are commonly known in the field and to the person skilled in the art.

[0054] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A wind turbine blade comprising: a blade body extending along a longitudinal axis between a root and a tip thereof, the blade body being divided transversely to its longitudinal axis into a first blade portion proximate to the root and a second blade portion proximate to the tip, each blade portion having upper and lower blade surfaces which are opposite to each other and spaced from each other in a thickness direction of the blade body, and the first and second blade portions being connected to each other at a connection portion, wherein the first and second blade portions comprise first and second flange portions, respectively, facing each other at the connection portion, wherein the first flange portion comprises a first upper airfoil portion and a first lower airfoil portion opposite to and spaced from the first upper airfoil portion in the thickness direction, which both extending from the first blade portion towards the second blade portion, wherein the second flange portion comprises a second upper airfoil portion and a second lower airfoil portion opposite to and spaced from the second upper airfoil portion in the thickness direction, both extending from the second blade portion towards the first blade portion, and wherein the first and second upper airfoil portions are connected to each other and the first and second lower airfoil portions are connected to each other to thereby form the connection portion.

2. The blade according to claim 1 , wherein the first flange portion further comprises a first middle airfoil portion extending from the first blade portion towards the second blade portion and between the first upper and first lower airfoil portions and spaced therefrom in the thickness direction, the second flange portion further comprises a second middle airfoil portion extending from the second blade portion towards the first blade portion and between the second upper and lower airfoil portions and spaced therefrom in the thickness direction, wherein the first and second middle airfoil portions are connected to each other to thereby further form the connection portion.

3. The blade according to claim 1 or 2, wherein a through opening is formed between the upper airfoil portions and the second lower airfoil portions, the through opening extending in a direction substantially transverse to the longitudinal axis of the blade body.

4. The blade according to any one of claims 1 to 3 further comprising a web having an upper edge portion attached to at least one of the upper airfoil portions and a lower edge portion attached to at least one of the lower airfoil portions.

5. The blade according to claim 4, wherein the upper edge portion of the web is mounted between the first and second upper airfoil portions and the lower edge portion of the web mounted between the first and second lower airfoil portions respectively.

6. The blade according to claims 4 and 5 further comprising a shear pin extending in the direction of the longitudinal axis and through the web, the shear pin having one end inserted into the first middle airfoil portion, and another end inserted into the second middle airfoil portion.

7. The blade according to any one of claims 4 to 6, wherein the web includes a plate-like structure, or a strip-like structure having at least one strip.

8. The blade according to any one of the preceding claims, wherein the upper airfoil portions bulge from the respective upper blade surface in the thickness direction and the lower airfoil portions bulge from the respective lower blade surface in the thickness direction.

9. The blade according to any one of claims 4 to 6, wherein the middle airfoil portions do not bulge from the respective upper or lower blade surfaces so as to maintain a flushed surface with the respective blade surfaces.

10. The blade according to any one of the preceding claims, wherein the first and second airfoil portions further comprise inserts in the first and second airfoil portions at the respective connection portions, the inserts in one of first and second airfoil portions facing the inserts in the other of the first and second airfoil portions and are in the direction parallel to the longitudinal axis, for allowing connection between the first and second flange portions.

11. The blade according to any one of the preceding claims, wherein the first and second flange portions are connected to each other by a plurality of coupling elements.

12. The blade according to claim 11, wherein the coupling elements include bolts.

13. The blade according to any one of the preceding claims, wherein the first and second upper airfoil portions and the first and second lower airfoil portions are adhered to the first and second blade portions respectively.

14. The blade according to any one of the preceding claims, wherein each of the blade portions comprise a spar, and the upper airfoil portions and the lower airfoil portions of the respective blade portions are extended from the spar of the respective blade portions.

15. A wind turbine comprising a plurality of blades as claims in any one of claims 1 to 14.