Patent Publication Number: US-2023133500-A1

Title: Method for manufacturing a wind turbine blade and mold for manufacturing a wind turbine blade

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/EP2021/055237, having a filing date of Mar. 3, 2021, which claims priority to EP Application No. 20166192.3, having a filing date of Mar. 27, 2020, the entire contents both of which are hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The following relates to a method for manufacturing a wind turbine blade and to a mold for manufacturing the wind turbine blade. 
     BACKGROUND 
     One way to produce more power using a wind turbine under given wind conditions is to increase the size of the blades. However, the manufacture of wind turbine blades is becoming increasingly difficult for increasing blade sizes. 
     Currently, many wind turbine blades are made by pre-manufacturing parts of the blade separately, such as a pressure-side shell and a suction-side shell, and gluing the parts to each other. The parts are, for example, pre-manufactured by infusing composite materials such as glass fibers with a resin and curing the resin. However, the gluing process has many disadvantages. It is, for example, difficult to achieve a sufficient strength and robustness of the glueline. 
     In another method, disclosed in EP 1 310 351 A1, the blade is manufactured by packing composite materials for the entire blade, or for a lengthwise blade section, on a mandrel (mold core) and by infusing resin in a vacuum-assisted manner and curing the resin. Thereby, glue joints are avoided. 
     However, with increasing blade sizes and, thus, increasing blade cross-sections it becomes difficult to raise the resin to the top of the blade by vacuum-assisted infusion. The gravitational force in combination with the density of the resin used for the infusion defines the maximum height the resin can be raised by the use of vacuum alone. Therefore, manufacturing large wind turbine blades having for example a diameter at the root section of larger than about 5.5 meter by vacuum-assisted infusion is challenging. 
     SUMMARY 
     An aspect relates to provide an improved method for manufacturing a wind turbine blade. 
     Accordingly, a method for manufacturing a wind turbine blade is provided. The method comprises the steps of:
         arranging an upper mold on a lower mold, wherein a dry fiber lay-up is arranged in the upper mold or in the upper mold and the lower mold,   applying vacuum to a space between the upper and lower molds, and   infusing the dry fiber lay-up in the upper and/or lower molds with resin, wherein the resin is at least partially provided through at least one upper resin inlet arranged at an upper portion of the upper mold.       

     Having the at least one upper resin inlet of the upper mold allows to improve the vacuum-assisted resin infusion. For example, resin provided through the at least one upper resin inlet has to be raised by a smaller height to reach the top of the mold. Further, an inlet pressure of the resin can be allowed to be higher during an initial phase of the infusion process. For example, a resin flow of the resin provided through the at least one upper resin inlet is supported in a downward direction by the gravitational force. Therefore, dry fiber lay-up in the upper and lower molds can be better wetted with resin, even for large blade cross-sections. Thus, dry areas can be better prevented in the manufactured blade. In addition, a time needed for infusing the entire blade may be reduced. 
     The wind turbine blade is part of a rotor of a wind turbine. The wind turbine is an apparatus to convert the wind&#39;s kinetic energy into electrical energy. The wind turbine comprises, for example, the rotor having one or more of the blades connected each to a hub, a nacelle including a generator, and a tower holding, at its top end, the nacelle. The tower of the wind turbine may be connected via a transition piece to a foundation of the wind turbine, such as a monopile in the seabed. 
     The wind turbine blade, e.g., its root section, is fixedly or rotatably connected to the hub. Apart from the (cylindrical) root section, the wind turbine blade is formed aerodynamically. The wind turbine blade comprises, for example, a pressure side (upwind side) and a suction side (downwind side). The pressure side and the suction side are connected with each other at a leading edge and a trailing edge. The pressure and suctions sides and the leading and trailing edges define an airfoil of the wind turbine blade. 
     The wind turbine blade is, in particular, manufactured by arranging a dry fiber lay-up in the lower mold and in the upper mold, infusing resin into the dry fiber lay-up by a vacuum-assisted resin infusion process, and curing the resin. 
     The wind turbine blade may be manufactured in one piece. In this case, dry fiber lay-up for the entire blade is arranged in the lower mold and in the upper mold and infused with resin and cured. 
     Alternatively, the wind turbine blade may be manufactured by pre-manufacturing lengthwise blade sections (e.g., an inboard blade section and an outboard blade section) and connecting the lengthwise blade sections with each other to form the full blade. In this case, dry fiber lay-up for a first lengthwise blade section is arranged in the lower mold and in the upper mold and infused with resin and cured. Then, dry fiber lay-up for a further lengthwise blade section is arranged in the lower mold (or a further lower mold) and in the upper mold (or a further upper mold) and infused with resin and cured. After pre-manufacturing two or more lengthwise blade sections, the lengthwise blade sections are connected to each other by any suitable means to form the full blade. 
     The fibers are, for example, arranged in the lower and upper molds in a dry condition, i.e. as fibers without a resin. The fibers may also be (partially) pre-impregnated fibers (prepreg). The fibers include, for example, glass fibers, carbon fibers, aramid fibers and/or natural fibers. 
     The fiber lay-up may include a fiber core material, such as wood, balsa, PET foam and/or PVC foam. The core material may be sandwiched between layers of fibers such that a fiber-reinforced resin laminate with a core structure is obtained. 
     Further, reinforcement beams may be arranged in the mold and may be connected to the fiber lay-up by the resin infusion and curing process. The reinforcement beams may include a leading-edge beam, a trailing-edge beam, a suction-side beam and/or a pressure-side beam. 
     Further, one or more webs may be arranged in the mold and may be connected to the fiber lay-up by the resin infusion and curing process. The one or more webs include, for example, a shear web connecting the blade shells of the pressure side and the suction side in the interior cavity of the manufactured blade. The shear web provides shear strength to the blade. 
     There may also be arranged pre-casted elements along with the dry fibers. For example, the reinforcement beams and/or the webs may be pre-casted elements. 
     A mold for manufacturing the wind turbine blade, or a lengthwise section of the wind turbine blade, comprises the lower mold and the upper mold. A fiber lay-up for forming a lower shell of the blade (e.g., a suction side of the blade), or a lengthwise section of the lower shell, may be arranged in the lower mold, and a fiber lay-up for forming an upper shell of the blade (e.g., a pressure side of the blade), or a lengthwise section of the upper shell, may be arranged in the upper mold. 
     The upper mold is arranged on the lower mold, thus forming a cavity between them. The mold may also comprise a mold core in the mold cavity. 
     The dry fibers may be arranged in the upper mold before arranging the upper mold on the lower mold. In this case, arranging the upper mold on the lower mold means arranging the upper mold having the dry fiber lay-up on the lower mold. 
     Alternatively, the dry fiber lay-up (e.g., for forming the upper blade shell) may be arranged on top of a mold core arranged in the lower mold before arranging the upper mold on the lower mold. In this case, arranging the upper mold on the lower mold means arranging the upper mold without the dry fiber lay-up on the mold core with the dry fiber lay-up. At the end of arranging the upper mold on the lower mold, the dry fiber lay-up is arranged in the upper mold core. 
     The fiber lay-up in the lower and upper molds may be covered by a vacuum bag, and a vacuum may be generated in a space covered by the vacuum bag, e.g. in a space between the lower and upper molds and the vacuum bag. The resin is then provided through the at least one upper resin inlet and infused into said space due to the generated vacuum, thus wetting the fibers. 
     The resin includes, for example, thermosets, thermoplastics, epoxy, polyurethane, vinyl ester and/or polyester. 
     The resin is, for example, cured by applying heat. The result is a fiber-reinforced resin laminate. 
     The at least one upper resin inlet is, in particular, at least one elongated fluid supply line with one or more openings through which the resin is infused into the fiber lay-up. The at least one upper resin inlet is, for example, at least one upper resin inlet channel. The at least one upper resin inlet channel comprises an opening along its length at its top portion, through which opening the resin is infused into the fiber lay-up. Such (a) channel(s) may be rigid or may be flexible. The at least one upper resin inlet channel is, in particular, arranged along a longitudinal direction of the upper mold. The longitudinal direction of the upper mold defines a longitudinal direction of the manufactured blade. 
     The at least one upper resin inlet channel includes, for example, two or more upper resin inlet channels. The two or more upper resin inlet channels may, for example, be arranged parallel to each other. The at least one upper resin inlet channel includes, for example, two, four or another even number of upper resin inlet channels. The two, four or another even number of upper resin inlet channels are arranged, for example, symmetrically with respect to a vertical plane intersecting a longitudinal axis of the mold/blade. 
     The at least one upper resin inlet being arranged at an upper portion of the upper mold means, in particular, that the at least one upper resin inlet is arranged in a volume defined by the upper portion of the upper mold. 
     The at least one upper resin inlet being arranged at an upper portion of the upper mold includes cases in which the at least one upper resin inlet is arranged directly at the upper mold as well as cases in which another element, such as the dry fiber lay-up, is arranged between the at least one upper resin inlet and the upper mold. 
     The at least one upper resin inlet is, for example, arranged between the upper mold and the dry fiber lay-up in the upper mold. Alternatively, the at least one upper resin inlet is arranged at an inner side of the dry fiber lay-up in the upper mold, wherein the inner side is facing away from the upper mold. 
     The upper portion of the upper mold includes, in particular, a top portion of the upper mold or a middle portion of the upper mold. Thus, the at least one upper resin inlet is, for example, arranged at a top portion of the upper mold. In another example, the at least one upper resin inlet is arranged half-way between the top portion of the upper mold and a bottom portion of the upper mold. The at least one upper resin inlet is, for example, arranged half-way between an upper end of the upper mold and a lower end of the upper mold. 
     According to an embodiment, at least one further resin inlet is arranged at the upper or lower mold and at a different height than the at least one upper resin inlet. Furthermore, the dry fiber lay-up in the upper and lower molds is infused, in addition to the resin being provided through the at least one upper resin inlet, with resin being provided through the at least one further resin inlet. 
     Having the at least one further resin inlet arranged at a different height than the at least one upper resin inlet allows to provide resin at two different heights. Therefore, an upward flow of resin and/or a download flow of resin through the fiber lay-up can be supported by that resin inlet arranged at a larger/smaller height, respectively. 
     In particular, the at least one further resin inlet is arranged at a different height, with respect to a bottom portion or lower end of the lower mold, than the at least one upper resin inlet. In particular, the at least one further resin inlet is arranged at a different height, with respect to a floor level of a manufacturing site, than the at least one upper resin inlet. 
     The at least one upper resin inlet is arranged, for example, at a larger height than the at least one further resin inlet. 
     The at least one further resin inlet is, for example, arranged in a connection region between the upper and lower molds. 
     The at least one further resin inlet being arranged at the upper or lower mold includes cases in which the at least one further resin inlet is arranged directly at the upper/lower mold as well as cases in which another element, such as the dry fiber lay-up, is arranged between the at least one further resin inlet and the upper/lower mold. 
     The at least one further resin inlet is, in particular, at least one further elongated fluid supply line with openings through which the resin is infused into the fiber lay-up. The at least one further resin inlet is, for example, at least one further resin inlet channel with an opening along its length through which resin is provided. Such (a) further channel(s) may be rigid or may be flexible. The at least one further resin inlet channel is, in particular, arranged along the longitudinal direction of the upper mold/the manufactured blade. 
     According to a further embodiment, the method comprises the step of detecting a position of the resin during its flow through the dry fiber lay-up in the upper and lower molds. 
     Detecting the position of the resin, e.g., a flow front of the resin, during its flow through the dry fiber lay-up, allows to better control the infusion process. For example, the manner how resin is provided through a specific resin inlet, such as an amount of resin, an inlet pressure of resin and/or a timing can be better controlled. 
     The position of the resin is detected by one or more sensors. The one or more sensors are, for example, arranged in the internal structure of the upper and/or lower molds, e.g., within one or more boreholes of the upper and/or lower molds. Alternatively or in addition, the one or more sensors may also be arranged at an inner surface of the mold. The one or more sensors may be connected, wired or wireless, with a computing unit. The one or more sensors may, for example, be RFID sensors (radio-frequency identification sensor). 
     The one or more sensors may, for example, be gossip suction ports placed in the mold. Gossip suction ports comprise small suction ports with a transparent hose arranged in one or more through holes of the mold. Resin becoming visible within the transparent hose indicates the arrival of resin at the gossip suction port. 
     The one or more sensors may, for example, be pressure sensors, thermo sensors and/or dielectric sensors in or at the mold. The one or more sensors may, for example, include inspection glasses in the mold and/or a semitransparent mold. 
     The one or more sensors may, for example, also include line sensors detecting a position of the resin along a line. The line sensors may, for example, be optical line sensors and/or dielectric line sensors. 
     According to a further embodiment, the position of the resin is detected at the at least one upper resin inlet, at the at least one further resin inlet and/or at a lower or bottom portion of the lower mold. 
     Thus, an arrival and/or passing of the resin flowing through the dry fiber lay-up in the upper and lower molds can be detected at the at least one upper/further resin inlet and/or at a lower or bottom portion of the lower mold. 
     For example, detecting the position of the resin at the at least one upper resin inlet allows to notice the arrival and/or passing of the resin at the least one upper resin inlet and to control the providing of resin through this at least one upper resin inlet. Further, detecting the position of resin at the at least one further resin inlet allows to notice the arrival and/or passing of the resin at the least one further resin inlet and control the providing of resin through this at least one further inlet. 
     Detecting the position of the resin at the at least one upper resin inlet includes detecting the position of the resin, with respect to a flow direction of the resin, behind the at least one upper resin inlet. 
     Detecting the position of the resin at the at least one further resin inlet includes detecting the position of the resin, with respect to a flow direction of the resin, behind the at least one further resin inlet. 
     Additionally or alternatively, the position of the resin may also be detected at any other position of the upper and lower molds and/or the fiber lay-up. 
     According to a further embodiment, the infusion of the dry fiber lay-up in the upper and lower molds is started by providing resin through the at least one upper resin inlet. Further, the infusion of the dry fiber lay-up in the upper and lower molds is continued, after providing resin through the at least one upper resin inlet, by providing resin through the at least one further resin inlet. 
     Starting the resin infusion through the at least one upper resin inlet allows to wet the dry fiber lay-up in the upper and lower molds predominantly by a downward resin flow (top-down-resin flow). This reduces or avoids the need for the resin to be raised in height. Furthermore, the flow of resin provided through the at least one upper resin inlet is supported by additional resin provided through the at least one further resin inlet. Thus, the dry fiber lay-up can be better infused with resin, even for very large blade sizes. 
     According to a further embodiment, the infusion of the dry fiber lay-up in the upper and lower molds is started by providing resin through the at least one further resin inlet. Further, the infusion of the dry fiber lay-up in the upper and lower molds is continued, after providing resin through the at least one further resin inlet, by providing resin through the at least one upper resin inlet. 
     Starting the resin infusion through the at least one further resin inlet allows to wet the dry fiber lay-up in the upper mold predominantly by an upward resin flow. Furthermore, the flow of resin provided through the at least one further resin inlet is supported by additional resin provided through the at least one upper resin inlet. 
     According to a further embodiment, if it was detected that the resin provided through the one of the at least one upper resin inlet and the at least one further resin inlet has flown to the other one of the at least one upper resin inlet and the at least one further resin inlet, resin is provided through the other one of the at least one upper resin inlet and the at least one further resin inlet. 
     Detecting that the resin has flown to the other one of the at least one upper resin inlet and the at least one further resin inlet includes that it was detected that the resin has passed the other one of the at least one upper resin inlet and the at least one further resin inlet. 
     By providing resin through the other one of the at least one upper resin inlet and the at least one further resin inlet only when the flow of resin has reached and/or passed this inlet, air pockets can be avoided. In particular, air pockets between converging flow fronts from the at least one upper resin inlet and the at least one further resin inlet can be avoided. Thus, it can be better prevented that dry areas remain in the manufactured blade. 
     According to a further embodiment, an inlet pressure of the resin being provided through the at least one upper resin inlet and/or the at least one further resin inlet is controlled to a pressure below atmospheric pressure. 
     Providing the resin with an inlet pressure below atmospheric pressure allows a better compaction of the fiber lay-up. 
     According to a further embodiment, an inlet pressure of the resin being provided through the at least one upper resin inlet and/or the at least one further resin inlet is controlled such that a pressure inside the space between the upper and lower molds is controlled to a pressure below atmospheric pressure. 
     For example, a pressure inside the space between the upper and lower molds and the vacuum bag is controlled to a pressure below atmospheric pressure. 
     In particular, a pressure inside the entire space between the upper and lower molds, for example the entire space between the upper and lower molds and the vacuum bag, is controlled to a pressure below atmospheric pressure. 
     Providing the resin with an inlet pressure such that the space comprising the dry fiber lay-up and into which the resin is drawn in is kept below atmospheric pressure allows a better compaction of the fiber lay-up, even in the lower and/or bottom portion of the lower mold. 
     According to a further embodiment, the inlet pressure of the resin being provided through the at least one upper resin inlet and/or the at least one further resin inlet is reduced during a flow of the resin through the dry fiber lay-up in the upper and lower molds. 
     Reducing the resin inlet pressure during a flow of the resin through the dry fiber lay-up in the upper and lower molds allows to compensate for an increase of a pressure inside the space between the upper and lower molds (the space between the upper and lower molds and the vacuum bag) during the resin&#39;s downward flow. 
     According to a further embodiment, the method comprises the step of extracting excess resin from a lower or bottom portion of the upper mold, from the lower mold and/or from a lower or bottom portion of the lower mold. 
     Extracting excess resin allows to prevent an overabundance of resin (resin lake) in a lower part of the mold. 
     According to a further embodiment, the at least one upper resin inlet is arranged exclusively at a lengthwise section of the upper mold configured for manufacturing an inboard blade section of the wind turbine blade. 
     In particular, the at least one upper resin inlet has an elongated channel-like structure and is arranged along the longitudinal direction of the mold exclusively at a lengthwise section of the upper mold configured for manufacturing an inboard blade section of the wind turbine blade. 
     Thus, (additional) upper resin inlets are applied only where the manufactured blade cross-section has a circular or close to circular cross-section, and, therefore, a large height. In other words, additional upper resin inlets are avoided for manufacturing an outboard blade section, where the blade cross-section has an airfoil shape, and, therefore, a smaller height. 
     The inboard blade section comprises, for example, a root section of the manufactured blade. 
     In particular, in a case in which the entire wind turbine blade is manufactured (in one piece or in lengthwise sections) by the use of one single upper mold and one single lower mold, a lengthwise section of the upper mold configured to manufacture an inboard blade section comprises the at least one upper resin inlet, whereas a lengthwise section of the upper mold configured to manufacture an outboard blade section does not comprise any upper resin inlets. 
     In an alternative case, in which there are two or more upper molds as well as two or more lower molds, a first one of the upper molds configured to manufacture an inboard blade section comprises the at least one upper resin inlet, whereas one or more further upper molds configured to manufacture one or more further outboards sections, respectively, do not comprise any upper resin inlets. 
     According to a further embodiment, the at least one upper resin inlet is arranged at a height above a lower end of the upper mold corresponding, as seen in cross-section, to an angle larger than 15 degrees, larger than 30 degrees, larger than 40 degrees, larger than 45 degrees, larger than 60 degrees, larger than 75 degrees and/or larger than 80 degrees. The angle being defined between a horizontal plane including the lower end of the upper mold and a plane intersecting a longitudinal axis of the manufactured blade and the at least one upper inlet. 
     The at least one upper resin inlet is, for example, arranged at a top portion of the upper mold (e.g., angle being larger than 75 and/or 90 degrees). In another example, the at least one upper resin inlet is arranged half-way between the top portion of the upper mold and a bottom portion of the upper mold (e.g., angle ranging between 30 and 60 degrees). 
     According to a further aspect, a mold for manufacturing a wind turbine blade is proposed. The mold comprises a lower mold and an upper mold. Further, at least one upper resin inlet is arranged at an upper portion of the upper mold. 
     The embodiments and features described with reference to the method of embodiments of the present invention apply mutatis mutandis to the mold of embodiments of the present invention. 
     Further possible implementations or alternative solutions of embodiments of the invention also encompass combinations—that are not explicitly mentioned herein—of features described above or below with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG.  1    shows a wind turbine according to an embodiment; 
         FIG.  2    shows a cross-section view of a mold for manufacturing a wind turbine blade of the wind turbine of  FIG.  1   , according to a first embodiment; 
         FIG.  3    shows a view similar as  FIG.  2    but with resin partly infused into a fiber lay-up arranged in the mold; 
         FIG.  4    shows a view similar as  FIG.  2    but with an apparatus for extracting excess resin; 
         FIG.  5    shows a perspective view of a variant of the mold of  FIG.  2   ; 
         FIG.  6    shows a perspective view of a variant of the mold of  FIG.  2   ; 
         FIG.  7    shows a flowchart illustrating a method for manufacturing the wind turbine blade of the wind turbine of  FIG.  1    using the mold of  FIG.  2   , according to a first embodiment; 
         FIG.  8    shows a cross-section view of a mold for manufacturing a wind turbine blade of the wind turbine of  FIG.  1   , according to a second embodiment; 
         FIG.  9    shows a flowchart illustrating a method for manufacturing the wind turbine blade of the wind turbine of  FIG.  1    using the mold of  FIG.  8   , according to a second embodiment; and 
         FIG.  10    illustrates, in cross-section, a geometrical arrangement of resin inlets of the mold of  FIG.  2    according to the first embodiment and of the mold of  FIG.  8    according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the Figures, like reference numerals designate like or functionally equivalent elements, unless otherwise indicated. 
       FIG.  1    shows a wind turbine  1  according to an embodiment. The wind turbine  1  comprises a rotor  2  having one or more blades  3  connected to a hub  4 . The hub  4  is connected to a generator (not shown) arranged inside a nacelle  5 . During operation of the wind turbine  1 , the blades  3  are driven by wind to rotate and the wind&#39;s kinetic energy is converted into electrical energy by the generator in the nacelle  5 . The nacelle  5  is arranged at the upper end of a tower  6  of the wind turbine  1 . The tower  6  is erected on a foundation  7  such as a monopile or tripile. The foundation  7  is connected to and/or driven into the ground or seabed. 
     In the following an improved method for manufacturing a wind turbine blade  3  according to a first embodiment is described with respect to  FIGS.  2  to  7   . 
       FIG.  2    shows a mold  8  in cross-section for manufacturing one of the wind turbine blades  3  of  FIG.  1   . The mold  8  comprises a lower mold  9  and an upper mold  10 . 
     In step S 1  of the method, a dry fiber lay-up  11  is arranged in the upper and lower molds  9 ,  10 . The fiber lay-up  11  may also comprise a core material and/or reinforcement beams (not shown). Furthermore, a shear web  12  is arranged in the mold  8 . 
     Furthermore, the fiber lay-up  11  is covered with one or more vacuum bags  13 . 
     In step S 2  of the method, the upper mold  10  is arranged on the lower mold  9 .  FIG.  2    shows the mold  8  in a closed state in which the upper mold  10  is already arranged on the lower mold  9 . 
     It is noted that the mold  8  may further comprise one or more mold cores  14 . In the example of  FIG.  2    two mold cores  14  are shown. The mold cores  14  allow an improved arrangement of the dry fiber lay-up  11  in the mold  8 , particularly in the upper mold  10 , and of the shear web  12 . Further, the upper mold  10  can be more easily arranged on the lower mold  9 . In the case of having one or more mold cores  14 , each of them is, for example, covered by one of the vacuum bags  14 . 
     In step S 3  of the method, a vacuum is generated in a space  15  covered by the vacuum bag  13 . The space  15  is, in particular a space between the upper and lower molds  10 ,  9  and the one or more vacuum bags  13 . The space  15  may include an area around the shear web  12 . 
     The upper mold  10  comprises at least one upper resin inlet channel  16 . In the example shown in  FIG.  2   , there are two upper resin inlet channels  16  arranged between the dry fiber lay-up  11  and the vacuum bag  13 . The two upper resin inlet channels  16  are arranged on the mold cores  14  covered by the vacuum bags  13 . Further, the two upper resin inlet channels  16  in  FIG.  2    are arranged symmetrically with respect to a vertical center line V of the mold  8 . 
     Each of the upper resin inlet channels  16  has an elongated structure arranged along a longitudinal direction L 1  of the mold  8  (the longitudinal direction L 1  is illustrated in  FIG.  5   ). Further, each of the upper resin inlet channels  16  comprises an opening at its top along its length through which resin  17  can be provided and infused into the fiber lay-up  11 . 
     In step S 4  of the method, a supply of resin  17  to the at least one upper resin inlet channel  16  is started, and resin  17  is provided through the opening of the at least one upper resin inlet channel  16  ( FIG.  3   ). 
     The resin provided through the openings of the two upper resin inlet channels  16  is delivered with an inlet pressure below atmospheric pressure. The inlet pressure is, for example, slightly below atmospheric pressure. The inlet pressure has, for example, a value in the range of 0.6 to 0.95 bar, in particular in the range of 0.8 to 0.95 bar, even more particularly a value of about 0.9 bar. 
     In step S 5  of the method, the resin  17  provided through the openings of the upper resin inlet channels  16  is infused into the fiber lay-up  11  due to the vacuum generated in the space  15 . In particular, the resin  17  provided through the openings of the upper resin inlet channels  16  is infusing the fiber layup  11  in the upper mold  10 . 
     The mold  8  comprises at least one further resin inlet channel  18 ,  19 . In the example shown in  FIGS.  2 - 4   , the mold  8  comprises two further resin inlet channels  18 ,  19  arranged at a connection region between the upper mold  10  and the lower mold  9 . The further resin inlet channel  18  is, for example, a trailing-edge resin inlet channel. The further resin inlet channel  19  is, for example, a leading-edge resin inlet channel. Each of the further resin inlet channels  18 ,  19  has an elongated structure comprising an opening along its length. 
     The two further resin inlet channels  18 ,  19  in the example of  FIG.  2 - 4    are arranged between the upper/lower mold  10 ,  9  and the dry fiber lay-up  11 . In another example, the at least one further resin inlet channel  18 ,  19  may be arranged between the dry fiber lay-up  11  and the vacuum bag  13 . In even another example, there may be arranged at least one further resin inlet channel  18 ,  19  between the upper/lower mold  10 ,  9  and the dry fiber lay-up  11  (e.g., as shown in  FIGS.  2 - 4   ), and there may be in addition arranged at least one further resin inlet channel between the dry fiber lay-up  11  and the vacuum bag  13 . 
     In step S 6  of the method, the position of the resin  17  ( FIG.  3   ) is detected by means of one or more sensors  20 ,  21  at the at least one further resin inlet  18 ,  19 . 
     The sensors  20 ,  21  in the example of  FIGS.  2 - 4    are arranged within the lower mold  9 . The lower mold  9  comprises several boreholes  22 ,  23  having each an opening towards an inner surface  24  of the lower mold  9 . Each of the sensors  20 ,  21  is arranged in one of the boreholes  22 ,  23  such that the respective sensor  20 ,  21  is flush with the inner surface  24  of the lower mold  9 . The sensors  20 ,  21  could also be configured and arranged differently, for example as stickers at the inner surface  24  (i.e. without boreholes in the mold  8 ). 
     The sensors  20 ,  21  are, for example, pressure sensors. The sensors  20 ,  21  could also be other sensors capable of detecting the resin  17 . 
     The sensor  20  is arranged below the further resin inlet channel  18  to detect if the resin  17  provided through the openings of the upper resin inlet channels  16  has passed the further resin inlet channel  18 . 
     The sensor  21  is arranged below the further resin inlet channel  19  to detect if the resin  17  provided through the upper resin inlet channels  16  has passed the further resin inlet channel  19 . 
     In step S 7  of the method, resin  17  is provided through the openings of the further resin inlet channels  18  and  19 , if it was detected by means of the sensors  20 ,  21  that a flow front of the resin  17  provided through the upper resin inlet channels  16  has reached and passed the further resin inlet channels  18  and  19 . 
     In detail, a supply of resin  17  to the further resin inlet channels  18  and  19  is started in step S 7  and the resin  17  is provided through the openings of the further resin inlet channels  18 ,  19  with an inlet pressure below atmospheric pressure. The resin provided through the further resin inlet channels  18 ,  19  is, for example, delivered with an inlet pressure slightly below atmospheric pressure. The inlet pressure has, for example, a value in the range of 0.6 to 0.95 bar, in particular in the range of 0.8 to 0.95 bar, even more particularly a value of about 0.9 bar. 
     Furthermore, a supply of resin  17  to the upper resin inlet channels  16  may be closed in step S 7 . 
     In step S 8  of the method, the fiber layup  11  in the lower mold  9  is infused with resin  17  provided through the upper resin inlet channels  16  and the further resin inlet channels  18 ,  19 . 
     In step S 9  of the method, the inlet pressure of the resin  17  provided through the further resin inlet channels  18 ,  19  is reduced. The inlet pressure of the resin  17  may be reduced continuously or may be reduced stepwise. The inlet pressure of the resin  17  is reduced in order to keep the pressure in the entire space  15  (chamber  15 ) below atmospheric pressure. In particular, the inlet pressure of the resin  17  is reduced in order compensate for an increase of the pressure in the space  15  during the downward flow of the resin  17 . 
     Step S 9  may be carried out throughout the whole infusion process (e.g., in S 5  to S 10 ), whenever an adjustment of the pressure within the space  15  is necessary to keep the pressure in the space  15  below atmospheric pressure. 
     In step S 10  of the method, a position of the resin  17  is detected by means of one or more sensors  25  at a bottom portion  26  of the lower mold  9 . 
     In the example of  FIGS.  2 - 4   , there are shown two sensors  25  arranged within boreholes  27  in the lower mold  9 . The sensors  25  are, for example, pressure sensors, but could be also other sensors capable of detecting the position of the resin  17 . 
     In step S 11  of the method, an overabundance of resin  17  in a lower portion  28  and/or in the bottom portion  26  of the lower mold  9  is handled by extracting excess resin  17 ″ from the lower portion  28  and/or from the bottom portion  26  ( FIG.  4   ). The excess resin  17 ″ is, for example, extracted by means of a through hole  31  within the lower mold  9 , a hose  32 , a pump  33 , and an overflow container  34 . 
     It is noted that in  FIG.  4    the resin  17  infused already into the fiber lay-up  11  is not shown for clarity. 
     Step  11  of the method may be carried out throughout the whole infusion process (e.g., in S 8  to S 10 ), whenever an overabundance of resin  17  occurs in a lower part of the mold  8 . 
     In step S 12  of the method, a vacuum is applied to the top inlet channels  16  in order to keep the resin  17  in an upper portion  29  and/or a top portion  30  of the upper mold  10 . 
     Step S 12  may be carried out simultaneously with any of steps S 7  to S 11 . In particular, step  12  may be carried out at the end of the infusion process and/or after finishing the infusion process (e.g., at the end of S 8 , after S 8 , S 9 , S 10  and/or S 11 ). 
       FIG.  5    shows a variant of the at least one upper resin inlet channel  16  of  FIGS.  2 - 4   . Shown in  FIG.  5    is a perspective view of the lower mold  9  and the two mold cores  14  of the mold  8 . The upper mold  10  is not shown in  FIG.  5    for illustration purposes. In the example of  FIG.  5   , the at least one upper resin inlet channel  116  comprises four upper resin inlet channels  116  arranged on the mold cores  14 , parallel to each other and along a longitudinal direction L 1  of the mold  8 . The longitudinal direction L 1  of the mold  8  defines a longitudinal direction of the blade manufactured by use of the mold  8 . Resin is supplied through a hose  42  via a branch  43  and two manifolds  35  to the four upper resin inlet channels  116 . 
     An insert I in  FIG.  5    shows an extended view of the branch  43 , the manifolds  35  and the upper resin inlet channels  116 . 
     In the example of  FIG.  5   , there are shown one branch  43 , two manifolds  35  and one hose  42 . However, in another example, there may be two hoses, two branches and no manifold. In this case, each of the two hoses is connected fluidly with one of the two branches. Further, each of the two branches is fluidly connected with two of the four upper resin inlet channels. 
     In the example of  FIG.  5   , there are shown four upper resin inlet channels  116 . However, in another example, there may be a different (even or uneven) number of upper resin inlet channels. 
     Furthermore, any suitable number of upper resin inlet channels  116 , hoses  42 , branches  43  and manifolds  35  can be used. 
     The method according to the first embodiment may be applied for manufacturing of the entire wind turbine blade  3  ( FIG.  1   ). In this case the described upper resin inlet channels  16  (or  116 ) are arranged along the entire upper mold  10 . Alternatively, as shown in  FIG.  6   , the upper mold  10  may comprise the described upper resin inlet channels  16  (or  116 ) only in a lengthwise section C 1  of the upper mold  10  configured for manufacturing an inboard blade section of the wind turbine blade  3 . The section C 1  corresponds, for example, to one third of the entire length L 3  of the mold  8 . The fiber lay-up  11 ,  11 ′ in the mold  8  in the section C 1  is, for example, infused with resin as described in steps S 1  to S 12 . The fiber lay-up  11 ,  11 ′ in the mold  8  in the section C 2  is, for example, infused with resin by providing resin only through the inlet channels  18  and  19 . 
     In the following an improved method for manufacturing a wind turbine blade  3  according to a second embodiment is described with respect to  FIGS.  8  and  9   . 
     The mold  8 ′ used in the method according to the second embodiment is similar to the mold  8  used in the method according to the first embodiment apart from a different arrangement of upper resin inlet channels  37 ,  38  and of sensors  39 ,  40 . 
     As shown in  FIG.  8   , the upper mold  10 ′ of the mold  8 ′ according to the second embodiment comprises in addition to or instead of upper resin inlet channels  16 ′ upper resin inlet channels  37  and  38 . The upper resin inlet channels  37  and  38  are arranged half way between a top portion  30 ′ of the upper mold  10 ′and a lower end  41 ′ of the upper mold  10 ′. Furthermore, the upper resin inlet channels  37  and  38  are, for example, arranged between the dry fiber lay-up  11  and the vacuum bag  13 . 
     Furthermore, the upper mold  10 ′ according to the second embodiment comprises the sensors  39  and  40  above the upper resin inlet channels  37  and  38 , respectively. 
     Steps S 1 ′, S 2 ′ and S 3 ′ of the method according to the second embodiment are similar as steps S 1 , S 2 , and S 3  of the method according to the first embodiment. Therefore, a description thereof will be omitted. 
     In step S 4 ′ of the method according to the second embodiment, a supply of resin to the at least one further resin inlet channel, i.e. in the example of  FIG.  8    to the two further resin inlet channels  18 ′ and  19 ′, is started. Furthermore, resin (not shown in  FIG.  8   ) is provided through openings of the further resin inlet channels  18 ′ and  19 ′. The resin is provided through the openings of the further resin inlet channels  18 ′ and  19 ′ with an inlet pressure below atmospheric pressure. The inlet pressure has, for example, a similar value or range as the inlet pressure of the resin  17  provided through the upper resin inlet channels  16  in the first embodiment. 
     In step S 5 ′ of the method according to the second embodiment, the resin provided through the further resin inlet channels  18 ′ and  19 ′ is infused into the fiber layup  11 ′ due to the vacuum generated in the space  15 ′. In particular, the resin provided through the further resin inlet channels  18 ′ and  19 ′ is flowing upwards wetting the fiber layup  11 ′ in the upper mold  10 ′. Furthermore, the resin provided through the further resin inlet channels  18 ′ and  19 ′ is also flowing downwards wetting the fiber layup  11 ′ in the lower mold  9 ′. 
     In step S 6 ′ of the method according to the second embodiment, a position of the resin provided through the further resin inlet channels  18 ′ and  19 ′ and flowing upwards is detected by means of the sensors  39  and  40 . 
     In particular, the sensor  39  is arranged above the upper resin inlet channel  37 . By means of the sensor  39  it can be detected if the resin provided through the further resin inlet channel  18 ′ and flowing upwards has passed the upper resin inlet channel  37 . Furthermore, by means of the sensor  40  it can be detected if the resin provided through the further resin inlet channel  19 ′ and flowing upwards has passed the upper resin inlet channel  38 . 
     In step S 7 ′ of the method according to the second embodiment, resin is provided through the upper resin inlet channels  37  and  38 , if it was detected by means of the sensors  39 ,  40  that a flow front of the resin provided through the further resin inlet channels  18 ′,  19 ′ has reached and passed the upper resin inlet channels  37  and  38 , respectively. 
     In detail, a supply of resin to the upper resin inlet channels  37  and  38  is started in step S 7 ′ and resin (not shown) is provided through openings of the upper resin inlet channels  37  and  38 . The resin provided through the upper resin inlet channels  37 ,  38  is delivered with an inlet pressure below atmospheric pressure. The inlet pressure has, for example, a similar value or range as the inlet pressure of the resin  17  provided through the upper resin inlet channels  16  in the first embodiment. 
     In step S 8 ′ of the method according to the second embodiment, the fiber layup  11 ′ in the upper mold  10 ′ is infused with resin provided through the further resin inlet channels  18 ′ and  19 ′ and through the upper resin inlet channels  37  and  38 . 
     The resin provided through the upper resin inlet channels  37  and  38  supports an upward flow of resin through the fiber layup  11 ′ in the upper mold  10 ′. Thus, the fiber layup  11 ′ can be better wetted even in an upper portion  29 ′ and/or a top portion  30 ′ of the upper mold  10 ′. Therefore, spots of fiber layup  11 ′ remaining dry, i.e. without resin, in the manufactured blade  3  can be better prevented, even for very large blades having a large cross section. 
     The upper mold  10 ′ according to the second embodiment may comprise in addition to the upper resin inlet channels  37  and  38  also upper resin inlet channels  16 ′, as shown in  FIG.  8   . Resin provided through the optional upper resin inlet channels  16 ′ can further support the infusion of the fiber layup  11 ′ in the upper portion  29 ′ and in the top portion  30 ′ of the upper mold  10 ′. 
     The mold  8 ′ according to the second embodiment may also comprise further sensors in addition to the sensors  39 ,  40 . For example, the mold  8 ′ may comprise sensors  25 ′ similar to the sensors  25  of the first embodiment. 
       FIG.  10    illustrates, in cross-section, a geometrical arrangement of the at least one upper inlet  16  and the at least one further inlet  18 ,  19  according to the first embodiment. Furthermore,  FIG.  10    illustrates, in cross-section, the geometrical arrangement of the at least one upper inlet  37 ,  38 , the optional at least one upper inlet  16 ′ and the at least one further inlet  18 ′,  19 ′ according to the second embodiment. 
     The at least one upper inlet  16  according to the first embodiment is arranged at the upper portion  29  of the upper mold  10 . In the example shown in  FIGS.  2 - 4  and  10   , the at least one upper inlet  16  comprises two upper resin inlet channels  16 . The upper resin inlet channels  16  are both arranged at the top portion  30  of the upper mold  10 . In particular, each of the upper resin inlet channels  16  is, as seen in the cross-section view of  FIGS.  2 - 4  and  10   , arranged at a height H 1  above a lower end  41  of the upper mold  10 . The height H 1  corresponds to an angle α 1  larger than 80 degrees in the example of  FIGS.  2 - 4  and  10   . The angle α 1  is, in particular, defined between a horizontal plane H and a plane A. The horizontal plane H includes the lower end  41  of the upper mold  10  and/or a dividing line between the lower and upper molds  9 ,  10 . The plane A is, as seen in cross-section, intersecting a longitudinal axis L 2  of the manufactured blade and the at least one upper inlet  16 . 
     The at least one upper inlet  37 ,  38  according to the second embodiment is arranged at the upper portion  29 ′ of the upper mold  10 ′. In the example shown in  FIGS.  8  and  10   , the at least one upper inlet  37 ,  38  comprises two upper resin inlet channels  37  and  38 . Each of the upper resin inlet channels  37 ,  38  is, as seen in the cross-section view of  FIGS.  8  and  10   , arranged at a height H 2  above a lower end  41 ′ of the upper mold  10 ′. The height H 2  corresponds to an angle α 2  of about 45 degrees in the example of  FIGS.  8  and  10   . The angle α 2  is, in particular, defined between a horizontal plane H including the lower end  41 ′ of the upper mold  10 ′ and a plane B. The plane B is, as seen in cross-section, intersecting a longitudinal axis L 2  of the manufactured blade and the at least one upper inlet  37 ,  38 . 
     The mold  8 ′ according to the second embodiment may optionally also comprise the upper resin inlet channels  16 ′, as shown in  FIGS.  8  and  10   . In the example shown in  FIGS.  8  and  10   , the at least one upper resin inlet  16 ′ comprises two upper resin inlet channels  16 ′ configured and arranged similar as the upper resin inlet channels  16  according to the first embodiment. 
     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.