Patent Publication Number: US-2019176445-A1

Title: Methods for Manufacturing Spar Caps for Wind Turbine Rotor Blades Using Thermoplastic-Based Composite Plates

Description:
FIELD OF THE INVENTION 
     The present subject matter relates generally to spar caps for wind turbine rotor blades and, more particularly, to a method for manufacturing a spar cap using thermoplastic-based composite plates. 
     BACKGROUND OF THE INVENTION 
     Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid. 
     Wind turbine rotor blades typically include an outer body skin or shell formed from a composite laminate material. In general, the body shell is relatively lightweight and has structural properties (e.g., stiffness, buckling resistance and strength) which are not configured to withstand the bending moments and other loads exerted on the rotor blade during operation. In addition, wind turbine rotor blades are becoming increasingly longer in order to produce more power. As a result, the blades must be stiffer and thus heavier so as to mitigate loads on the rotor. 
     To increase the stiffness, buckling resistance and strength of the rotor blade, the body shell is typically reinforced using one or more structural components (e.g. opposing spar caps with a shear web configured therebetween) that engage the inner surfaces of the shell. The spar caps are typically constructed from laminate composites (e.g., glass fiber laminate composites and/or carbon fiber laminate composites) that include dry or non-cured fabric plies that are laid up within the blade mold and subsequently infused with resin. Such materials, however, can be difficult to control during the manufacturing process and/or are often defect prone and/or highly labor intensive due to handling of the non-cured fabrics and the challenges of infusing large laminated structures. 
     As such, recent attempts have been made to form spar caps from pre-fabricated, pre-formed composites that can be produced in thicker sections, and are typically less susceptible to defects. However, the use of these thicker, pre-formed composites also presents unique challenges during the blade manufacturing process. For example, such composites often present challenges with respect to coupling or bonding adjacent composite structures together to form the spar cap. 
     Accordingly, an improved method for manufacturing a spar cap using thermoplastic-based composite plates that allows for adjacent plates to be easily and efficiently secured to one another would be welcomed in the technology. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In one aspect, the present subject matter is directed to a method for manufacturing a spar cap for a wind turbine rotor blade. The method may generally include stacking a plurality of plates together to form a plate assembly, wherein each of the plates is formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. The method may also include positioning the plate assembly relative to a mold defining a mold surface, wherein the mold surface is shaped so as to correspond to at least one blade parameter of the wind turbine rotor blade. In addition, the method may include applying pressure to the plate assembly via the mold such that at least a portion of the plate assembly conforms to the shape of the mold surface. 
     In another aspect, the present subject matter is directed to a system for manufacturing a spar cap for a wind turbine rotor blade. The system may include a plate assembly including a plurality of plates positioned one on top of another, wherein each of the plates is formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. In addition, the system may include a mold defining a mold surface, wherein the mold surface is shaped so as to correspond to at least one blade parameter of the wind turbine rotor blade. The mold may be configured to apply pressure to the plate assembly such that at least a portion of the plate assembly conforms to the shape of the mold surface. 
     In a further aspect, the present subject matter is directed to a method for manufacturing a spar cap for a wind turbine rotor blade. The method may generally include stacking a first plate on top of a second plate, wherein each of the plates is formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. The method may also include transporting the first and second plates past a heated roller to heat the thermoplastic resin material of the plates and pressing the first and second plates together such that the thermoplastic resin material of the first plate is welded to the thermoplastic resin material of the second plate to form a first plate assembly. 
     In an additional aspect, the present subject matter is directed to a system for manufacturing a spar cap for a wind turbine rotor blade. The system may include a first plate stacked on top of a second plate, wherein each of the plates is formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. In addition, the system may include a heated roller configured to heat the thermoplastic resin material of the plates as the plates are transported past the heated roller. Moreover, the heated roller may be configured to press the first and second plates together such that the thermoplastic resin material of the first plate is welded to the thermoplastic resin material of the second plate to form a first plate assembly. 
     In yet another aspect, the present subject matter is directed to a method for manufacturing a spar cap for a wind turbine rotor blade. The method may generally include assembling a plurality of plates one on top of another, wherein each of the plates is formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. The method may also include positioning the plates relative to a heated pressing device and pressing the plates together via the heated pressing device as heat is being transferred from the heated pressing device to the thermoplastic resin material contained within each of the plates such that the plates are welded to one another to form a plate assembly. 
     In a further aspect, the present subject matter is directed to a system for manufacturing a spar cap for a wind turbine rotor blade. The system may include an assembly of plates stacked one on top of another, wherein each of the plates is formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. In addition, the system may include a heated pressing device configured to press the plates together as heat is being transferred from the heated pressing device to the thermoplastic resin material contained within each of the plates such that the plates are welded to one another to form a plate assembly. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  illustrates a side view of one embodiment of a wind turbine in accordance with aspects of the present subject matter; 
         FIG. 2  illustrates a perspective view of one embodiment of a rotor blade suitable for use within the wind turbine shown in  FIG. 1  in accordance with aspects of the present subject matter; 
         FIG. 3  illustrates a cross-sectional view of the rotor blade shown in  FIG. 2  taken about line  3 - 3 ; 
         FIG. 4  illustrates a close-up view of a portion of the rotor blade shown in  FIG. 3 , particularly illustrating a spar cap of the rotor blade formed from an assembly of pre-formed composite plates; 
         FIG. 5  illustrates a perspective view of a portion of one of the composite plates shown in  FIG. 4 ; 
         FIG. 6  illustrates a simplified view of one embodiment of a system for manufacturing a spar cap from pre-formed, composite plates in accordance with aspects of the present subject matter, particularly illustrating a stack of plates being transported across a heated roller; 
         FIG. 7  illustrates a close-up view of a portion of the system shown in  FIG. 6 , particularly illustrating a welded joint being formed between adjacent composite plates as the plates are heated and compressed by the heated roller; 
         FIG. 8  illustrates another embodiment of the system shown in  FIG. 6 , particularly illustrating the stack of plates being transported between opposed heated rollers; 
         FIG. 9  illustrates another simplified view of the system shown in  FIG. 6 , particularly illustrating an additional plate being added to a previously formed welded plate assembly to build-up the thickness of the spar cap being manufactured; 
         FIG. 10  illustrates a similar close-up view to that shown in  FIG. 7 , particularly illustrating an additional layer(s) of thermoplastic resin material positioned between the adjacent plates; 
         FIG. 11  illustrates a simplified view of another embodiment of a system for manufacturing a spar cap from pre-formed, composite plates in accordance with aspects of the present subject matter, particularly illustrating a stack of plates being inserted between opposed mold portions of a mold; 
         FIG. 12  illustrates a close-up, chordwise view of a portion of the system shown in  FIG. 11 , particularly illustrating the stack of plates positioned between the upper and lower mold portions of the mold; 
         FIG. 13  illustrates another embodiment of the system shown in  FIG. 6 , particularly illustrating a spanwise section of the stack of plates being welded together using a heated pressing device; 
         FIG. 14  illustrates another view of the embodiment of the system shown in  FIG. 13 , particularly illustrating the stack of plates after it has been moved relative to the heated pressing device to allow another spanwise section of the plates to be welded together; 
         FIG. 15  illustrates another simplified view a stack of plates being inserted between upper and lower mold portions of a mold, particularly illustrating the plates being initially clamped together at their ends via one or more clamping devices; and 
         FIG. 16  illustrates another view of the stack of plates shown in  FIG. 15  after the mold has been closed, particularly illustrating the clamping devices being released to allow the plates to pull inwardly as the mold closes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     In general, the present subject matter is directed to methods for manufacturing spar caps for wind turbine rotor blades using thermoplastic-based, pre-formed composite plates, such as pultruded plates. Specifically, in several embodiments, a spar cap for a rotor blade may be formed by welding a plurality of composite plates together, with each individual plate being formed from a continuous fiber-reinforced composite including a plurality of fibers surrounded by a thermoplastic resin material. For example, in one embodiment, a stack of two or more plates may be transported past a heated roller that applies heat and pressure to the stack in order to weld the plates to one another. In another embodiment, the stack of plates may be welded together in spanwise sections using a heated press or any other suitable device(s) configured to apply heat and pressure to a given spanwise section of the stacked plates. In a further embodiment, the stack of plates may be positioned within a heated mold configured to apply heat and pressure to the stack so as to weld the plates together. 
     Additionally, in a particular embodiment, the stack of plates may be shaped so as to conform to a portion of the aerodynamic shape of the rotor blade within which the resulting spar cap will be installed. Such shaping of the plate stack may be performed while the individual plates are being welded together or after the plates have been welded together. For instance, in one embodiment, a stack of pre-formed composite plates may be inserted within a mold including at least one mold surface that is shaped so as to match the shape of the portion of the rotor blade shell along which the resulting spar cap will extend. As such, when pressure is applied to the stack of plates via the mold, an outer surface(s) of the stack positioned adjacent to the mold surface(s) may be molded to the shape of the rotor blade, such as by molding the outer surface(s) to match the chordwise curvature, the spanwise curvature and/or the twist of the rotor blade. Accordingly, when the resulting spar cap is subsequently being installed within the rotor blade, the outer surface(s) of the spar cap may match the corresponding inner surface(s) of the rotor blade shell, thereby allowing the spar cap to conform to the blade shape without requiring that the spar cap be pre-stressed and/or manufactured in a manner that reduces its structural integrity. 
     It should be appreciated that, as described herein, thermoplastic materials generally encompass a plastic material(s) or polymer(s) that is reversible in nature. For example, thermoplastic materials typically become pliable or moldable when heated to a certain temperature and return to a more rigid state upon cooling. Further, thermoplastic materials may include amorphous thermoplastic materials and/or semi-crystalline thermoplastic materials. For example, some amorphous thermoplastic materials may generally include, but are not limited to, styrenes, vinyls, cellulosics, polyesters, acrylics, polysulphones, and/or imides. More specifically, exemplary amorphous thermoplastic materials may include polystyrene, acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA), glycolised polyethylene terephthalate (PET-G), polycarbonate, polyvinyl acetate, amorphous polyamide, polyvinyl chlorides (PVC), polyvinylidene chloride, polyurethane, or any other suitable amorphous thermoplastic material. In addition, exemplary semi-crystalline thermoplastic materials may generally include, but are not limited to polyolefins, polyamides, fluropolymer, ethyl-methyl acrylate, polyesters, polycarbonates, and/or acetals. More specifically, exemplary semi-crystalline thermoplastic materials may include polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polypropylene, polyphenyl sulfide, polyethylene, polyamide (nylon), polyetherketone, or any other suitable semi-crystalline thermoplastic material. Further, thermoset materials generally encompass a plastic material(s) or polymer(s) that is non-reversible in nature. For example, thermoset materials, once cured, cannot be easily remolded or returned to a liquid state. As such, after initial forming, thermoset materials are generally resistant to heat, corrosion, and/or creep. Example thermoset materials may generally include, but are not limited to, some polyesters, some polyurethanes, esters, epoxies, or any other suitable thermoset material. 
     Referring now to the drawings,  FIG. 1  illustrates a side view of one embodiment of a wind turbine  10 . As shown, the wind turbine  10  generally includes a tower  12  extending from a support surface  14  (e.g., the ground, a concrete pad or any other suitable support surface). In addition, the wind turbine  10  may also include a nacelle  16  mounted on the tower  12  and a rotor  18  coupled to the nacelle  16 . The rotor  18  includes a rotatable hub  20  and at least one rotor blade  22  coupled to and extending outwardly from the hub  20 . For example, in the illustrated embodiment, the rotor  18  includes three rotor blades  22 . However, in an alternative embodiment, the rotor  18  may include more or less than three rotor blades  22 . Each rotor blade  22  may be spaced about the hub  20  to facilitate rotating the rotor  18  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub  20  may be rotatably coupled to an electric generator (not shown) positioned within the nacelle  16  to permit electrical energy to be produced. 
     Referring now to  FIGS. 2 and 3 , one embodiment of a rotor blade  22  suitable for use within the wind turbine  10  shown in  FIG. 1  is illustrated in accordance with aspects of the present subject matter. Specifically,  FIG. 2  illustrates a perspective view of the rotor blade  22 . Additionally,  FIG. 3  illustrates a cross-sectional view of the rotor blade  22  taken about line  3 - 3  shown in  FIG. 2 . 
     As shown, the rotor blade  22  generally includes a blade root  24  configured to be mounted or otherwise secured to the hub  20  ( FIG. 1 ) of the wind turbine  10  and a blade tip  26  disposed opposite the blade root  24 . Additionally, the rotor blade  22  may include a body shell  28  configured to extend between the blade root  24  and the blade tip  26  along a longitudinal axis  30  of the blade  22 . The body shell  28  may generally serve as the outer casing/covering of the rotor blade  22  and may define a substantially aerodynamic profile, such as by defining a symmetrical or cambered airfoil-shaped cross-section. For example, as shown in  FIG. 3 , the body shell  28  may define a pressure side  32  and a suction side  34  extending between leading and trailing edges  36 ,  38  of the rotor blade  22 . Further, the rotor blade  22  may also have a span  40  defining the total length between the blade root  24  and the blade tip  26  and a chord  42  defining the total length between the leading edge  36  and the trailing edge  38 . As is generally understood, the chord  42  may generally vary in length with respect to the span  40  as the rotor blade  22  extends from the blade root  24  to the blade tip  26 . 
     In several embodiments, the body shell  28  of the rotor blade  22  may be formed from a plurality of shell components or sections. For example, in one embodiment, the body shell  28  may be manufactured from a first shell half or section generally defining the pressure side  32  of the rotor blade  22  and a second shell half or section generally defining the suction side  34  of the rotor blade  22 , with such shell sections being secured to one another at the leading and trailing edges  36 ,  38  of the blade  22 . Alternatively, the body shell  28  may be formed from any other suitable number and/or arrangement of shell sections. For instance, in one embodiment, the body shell  28  may be segmented along the longitudinal axis  30  of the rotor blade  22 , with each spanwise segment being formed from one or more shell sections. 
     Additionally, the body shell  28  may generally be formed from any suitable material. For instance, in one embodiment, the body shell  28  may be formed entirely from a laminate composite material, such as a carbon fiber reinforced laminate composite or a glass fiber reinforced laminate composite. Alternatively, one or more portions of the body shell  28  may be configured as a layered construction and may include a core material, formed from a lightweight material such as wood (e.g., balsa), foam (e.g., extruded polystyrene foam) or a combination of such materials, disposed between layers of laminate composite material. 
     Referring particularly to  FIG. 3 , the rotor blade  22  may also include one or more longitudinally extending structural components configured to provide increased stiffness, buckling resistance and/or strength to the blade  22 . For example, the rotor blade  22  may include a pair of longitudinally extending spar caps  44 ,  46  configured to be engaged against opposing inner surfaces  48 ,  50  of the pressure and suction sides  32 ,  34  of the rotor blade  22 , respectively. Additionally, one or more shear webs  52  may be disposed between the spar caps  44 ,  46  so as to form a beam-like configuration. The spar caps  44 ,  46  may generally be designed to control the bending stresses and/or other loads acting on the rotor blade  22  in a generally spanwise direction (a direction parallel to the span  40  of the rotor blade  22 ) during operation of a wind turbine  10 . Similarly, the spar caps  44 ,  46  may also be designed to withstand the spanwise compression occurring during operation of the wind turbine  10 . 
     Referring now to  FIG. 4 , a close-up, cross-sectional view of one of the spar caps  46  shown in  FIG. 3  is illustrated in accordance with aspects of the present subject matter, particularly illustrating the spar cap  46  being constructed or formed from a plurality of pre-formed composite plates  100 . In addition,  FIG. 5  illustrates a more detailed, cross-sectional view of a portion of one of the pre-formed composite plates  100  shown in  FIG. 4 . 
     In several embodiments, each pre-formed plate  100  may correspond to a pultruded plate. In such embodiments, one or more fiber materials  102  (e.g., glass or carbon fibers) may be formed during the manufacturing process to form each individual pultruded plate. For example, the fibers  102  may be impregnated with at least one resin material  104  using any suitable means. As indicated above, the resin material  104  may correspond to a thermoplastic resin material. The impregnated fibers  102  may then be pulled through a heated stationary die or any other suitable device to form each plate  100 . The individually formed plates  100  may then be assembled or joined together to form the resulting spar cap  46 . For instance, as shown in  FIG. 4 , each pre-formed plate  100  may form a single layer  106  of the spar cap  46 . As will be described below with reference to  FIGS. 6-12 , the individual layers  106  may then be stacked one on top of the other and welded together to form the spar cap  46 . The resulting pre-fabricated spar cap  46  may then be installed and/or assembled within the rotor blade  22 , such as by integrating the spar cap  46  into the body shell  28  or otherwise securing the spar cap  46  to the body shell  28  along the pressure or suction side  32 ,  34  of the blade  22 . 
     As particularly shown in  FIG. 5 , the fibers  102  included within each plate  100  may generally be oriented in a common fiber direction  108 . In several embodiments, the fiber direction  108  may extend parallel to the longitudinal or spanwise direction of the rotor blade  22 . As such, the fibers  102  contained within each plate  100  used to form the spar cap  46  may generally extend continuously in the spanwise direction along the length of the spar cap  46  between the blade root  24  and the blade tip  26 . In addition, each plate  100  may also include fibers  102  oriented in any other suitable direction. 
     Referring now to  FIGS. 6 and 7 , one embodiment of a system  200  for manufacturing a spar cap from pre-formed, composite plates  100  is illustrated in accordance with aspects of the present subject matter. One embodiment of a related method for manufacturing a spar cap from pre-formed, composite plates  100  will also be described with reference to  FIGS. 6 and 7 . 
     In several embodiments, two or more pre-formed, composite plates  100  may be initially stacked one on top of the other. For example, as shown in  FIG. 6 , a first plate  100 A may be stacked on top of a second plate  100 B (as indicated by the dashed lines in  FIG. 6 ). In such an embodiment, the first and second plates  100 A,  100 B may each be formed from a thermoplastic-based fiber-reinforced composite. For instance, each plate  100 A,  100 B may correspond to a pultruded plate including a plurality of continuous fibers (e.g., the fibers  102  shown in  FIG. 5 ) surrounded by a thermoplastic resin material (e.g., the resin material  104  shown in  FIG. 5 ). 
     Additionally, as shown in  FIGS. 6 and 7 , the stack of plates  100 A,  100 B may then be transported in a processing direction (e.g., as indicated by arrow  202  shown in  FIGS. 6 and 7 ) past a heated roller  204 . In general, the heated roller  204  may be configured to heat the thermoplastic resin material of the first and second plates  100 A,  100 B as the plates  100 A,  100 B are passed by the roller  204 . For example, the heated roller  204  may include or be coupled to a heat source  206 , such as an electric heating device, that is configured to heat the roller  204  to a specified temperature. Thus, as the plates  100 A,  100 B are moved past the heated roller  204 , heat may be transferred from the roller  204  to the plates  100 A,  100 B. 
     In addition, the heated roller  204  may also be configured to press the first and second plates  100 A,  100 B together. For instance, in the illustrated embodiment, the heated roller  204  may be configured to apply a downward force against the first and second plates (e.g., as indicated by arrow  208  in  FIG. 7 ) such that the plates  100 A,  100 B are compressed between the outer circumference of the roller  204  and an opposed surface  210  of an adjacent component  212  (e.g., a support platform configured to vertically support the plates  100 A,  100 B). By simultaneously heating and compressing the first and second plates  100 A,  100 B, the heated thermoplastic resin material of the first plate  100 A may be welded to the heated thermoplastic resin material of the second plate  100 B. Thus, as shown in  FIG. 7 , a welded plate assembly  214  may be formed that includes a welded joint  216  at the interface defined between the first and second plates  100 A,  100 B. 
     It should be appreciated that the particular temperature at which the thermoplastic resin material of the first and second plates  100 A,  100 B should be heated to allow the plates  100 A,  100 B to be welded together may generally vary depending on the resin chemistry of the thermoplastic resin material being utilized. However, in general, for an amorphous thermoplastic resin material, the thermoplastic resin material may be heated to a temperature at or above the glass transition temperature associated with such amorphous thermoplastic resin material to allow the plates  100 A,  100 B to be welded together. Similarly, for a semi-crystalline thermoplastic resin material, the thermoplastic resin material may be heated to a temperature at or above the melting temperature of such semi-crystalline thermoplastic resin material to allow the plates  100 A,  100 B to be welded together. 
     It should also be appreciated that, as an alternative to the single heated roller  204  shown in  FIGS. 6 and 7 , the stacked plates  100 A,  100 B may be configured to be transported between opposed, heated rollers. For instance, as shown in  FIG. 8 , the stacked plates  100 A,  100 B may be passed between a first heated roller  204 A positioned directly above the plates  100 A,  100 B and a second heated roller  204 B positioned directly below the plates  100 A,  100 B. In such an embodiment, the first heated roller  204 A may be configured to apply a downward force against the stacked plates  100 A,  100 B (as indicated by arrow  220  in  FIG. 8 ) while the second heated roller  204 B may be configured to apply an upward force against the stacked plates  100 A,  100 B (as indicated by arrow  222  in  FIG. 8 ). As such, the plates  100 A,  100 B may be compressed between the rollers  204 A,  204 B as heat is transferred from the rollers  204 A,  204 B to the plates  100 A,  100 B to allow such plates  100 A,  100 B to be thermoplastically welded together. 
     Additionally, it should be appreciated that, in alternative embodiments, the system  200  may include a separate heat source configured to serve as the primary means for heating the thermoplastic resin material or as a secondary heating means in addition to the heated roller(s)  204 . For instance, in one embodiment, a heating device may be positioned directly upstream of the roller(s)  204  to heat the thermoplastic resin material of the plates  100 A,  100 B prior to the plates  100 A,  100 B transported past the roller(s)  204 . In such an embodiment, the roller(s)  204  may correspond to a heated or a non-heated roller(s) configured to apply pressure to the stacked plates  100 A,  100 B. 
     Moreover, the above-described method for welding pre-formed composite plates  100  to one another may be repeated with one or more additional plates to build-up the thickness of the spar cap being manufactured. For instance, as shown in  FIG. 9 , a third plate  100 C may be assembled relative to the welded plate assembly  214  previously formed from the first and second plates  100 A,  100 B, such as by stacking the third plate  100 C on top of the plate assembly  214  (as indicated by the dashed lines in  FIG. 9 ) or by stacking the plate assembly  214  on top of the third plate  100 C. Regardless, the resulting plate stack may be transported past the heated roller(s)  204  such that the thermoplastic resin material contained within the third plate  100 C and the plate assembly  214  is heated as such components are being pressed together via the roller(s)  204 . As such, the thermoplastic resin material of the third plate  100 C may be welded to the thermoplastic resin material of the welded plate assembly  214  to form a second welded plate assembly having a thickness  230  that is greater than a thickness  232  of the previously formed plate assembly  214 . Thereafter, if desired, one or more additional plates  100 , such as a fourth plate, a fifth plate, etc., may be similarly welded onto the resulting plate assembly to further increase the overall thickness of the spar cap being manufactured. 
     As indicated above, in several embodiments, the pre-formed, composite plates may be configured to be stacked directly one on top of another when welding the plates together to manufacture a spar cap. Alternatively, one or more additional layers of thermoplastic resin material may be provided between each adjacent pair of plates. For example,  FIG. 10  illustrates an additional layer(s)  250  of thermoplastic resin material positioned between the first and second plates  100 A,  100 B described above with reference to  FIGS. 6 and 7 . In such an embodiment, the additional thermoplastic material may serve as donor material for the welding process. Specifically, when the plates  100 A,  100 B are transported past the heated roller(s)  204 , both the thermoplastic resin material contained within the plates  100 A,  100 B and the additional layer(s)  250  of thermoplastic resin material may be heated and pressed together to form the welded joint  216  between the first and second plates  100 A,  100 B. 
     It should be appreciated that, in the embodiment shown in  FIG. 10 , the additional layer(s)  250  of thermoplastic resin material may consist entirely of resin material. Alternatively, the additional layer(s)  250  may be fiber-reinforced and, thus, may include a plurality of fibers (e.g., carbon and/or glass fibers) surrounded by the thermoplastic resin material. In such an embodiment, it may be desirable for the additional layer(s)  250  to have a fiber-weight fraction that is less than the fiber-weight fractions of the first and second plates  100 A,  100 B. As such, the additional layer(s)  250  may have a higher concentration of thermoplastic resin material than the plates  100 A,  100 B. As used herein, the term “fiber-weight fraction” generally refers to the percentage of fibers by weight contained within a given volume of a fiber-reinforced composite. For instance, to calculate the fiber-weight fraction of the additional layer(s)  250 , the weight of all of the fibers contained within the additional layer(s)  250  may be divided by the total weight of the additional layer(s) (i.e., the weight of both the fibers and the thermoplastic resin material), with the resulting value being multiplied by 100 to obtain the percentage. 
     Additionally, it should be appreciated that, as an alternative to the roller(s)  204 , the system  200  may include any other suitable device(s) configured to weld the stacked plates  100 A,  100 B together. For instance, in several embodiments, the system  200  may include a heated pressing device that is configured to apply heat and pressure along a spanwise section of the stacked plates  100 A,  100 B to weld the plates  100 A,  100 B together across such spanwise section. The stacked plates  100 A,  100 B may then be moved relative to the heated pressing device to allow a different spanwise section of the plates  100 A,  100 B to be welded together. 
     For example,  FIGS. 13 and 14  illustrate one embodiment of the disclosed system  200  in which a heated pressing device  260  is used to weld the stacked plates  100 A,  100 B together. As shown in  FIG. 13 , the heated pressing device  260  may be configured to apply a compressive force against a spanwise section  264  of the stacked plates  100 A,  100 B. For example, the heated pressing device  260  may be movable between an opened position (shown in solid lines in  FIGS. 13 and 14 ), at which the stacked plates  100 A,  100 B may be moved relative to the device  260 , and a closed position (shown in dashed lines in  FIGS. 13 and 14 ), at which the heated pressing device  260  applies a compressive force against the stacked plates  100 A,  100 B so as to press the plates  100 A,  100 B together. In addition, the heated pressing device  260  may include or be coupled to a heat source  262 , such as an electric heating device, that is configured to heat the device  260  to a specified temperature. Thus, when the heated pressing device  260  is moved to its closed positon in order to compress the plates  100 A,  100 B, heat may be transferred from the device  260  to the plates  100 A,  100 B. Similar to the embodiments described above, by simultaneously heating and compressing the first and second plates  100 A,  100 B, the heated thermoplastic resin material of the first plate  100 A may be welded to the heated thermoplastic resin material of the second plate  100 B. Thus, as shown in  FIG. 14 , a welded plate assembly  214  may be formed that includes a welded joint  216  at the interface defined between the first and second plates  100 A,  100 B. 
     Using the heated pressing device  260 , the stacked plates  100 A,  100 B may be welded together incrementally along their spanwise length. Specifically, after a given spanwise section  264  of the stacked plates  100 A,  100 B are initially welded together (e.g., as shown in  FIG. 13 ), the heated pressing device  260  may be moved to its opened position to allow the plates  100 A,  100 B to be moved relative to the device  260 . For example, as shown in  FIG. 14 , the plates  100 A,  100 B may be moved relative to the device  260  a distance corresponding to the length of the spanwise section  264  previously welded to allow the next spanwise section  264  of the plates  100 A,  100 B to be welded together. Thereafter, the heated pressing device  260  may be moved to its closed position to weld the plates  100 A,  100 B together across the next spanwise section  264 . 
     It should be appreciated that the heated pressing device  260  may generally correspond to any suitable device(s), mechanism(s) and/or component(s) that is configured to apply a combination of heat and pressure to a given spanwise section  264  of the stacked plates  100 A,  100 B. For example, in one embodiment, the heated pressing device  260  may correspond to a heated press. In another embodiment, the heated pressing device  260  may correspond to an inflatable, heated bladder that, when inflated, applies heat and pressure to the stacked plates  100 A,  100 B. 
     As shown in  FIGS. 13 and 14 , the heated pressing device  260  is configured to compress the stacked plates  100 A,  100 B against an opposed surface  210  of an adjacent component  212  (e.g., a support platform configured to vertically support the plates  100 A,  100 B). However, it should be appreciated that, in alternative embodiments, the heated pressing device  260  may be configured to apply heat and pressure along both sides of the stacked plates  100 A,  100 B. For instance, in an embodiment in which the heated pressing device  260  corresponds to a heated press, the device  260  may include a first press portion positioned directly above the plates  100 A,  100 B and a second press portion positioned directly below the plates  100 A,  100 B. In such an embodiment, the stacked plates  100 A,  100 B may be compressed between the first and second press portions to allow the plates  100 A,  100 B to be thermoplastically welded together along the spanwise section  264 . 
     Referring now to  FIGS. 11 and 12 , another embodiment of a system  300  for manufacturing a spar cap from pre-formed, composite plates is illustrated in accordance with aspects of the present subject matter. Additionally, one embodiment of a related method for manufacturing a spar cap from pre-formed, composite plates  100  will also be described with reference to  FIGS. 11 and 12 . 
     As shown, unlike the embodiment described above, the pre-formed, composite plates  100  may be configured to be welded together using a mold  302 . Specifically, in several embodiments, a plurality of pre-formed, composite plates  100  may be assembled together and positioned within the mold  302 , such as by stacking the plates  100  one on top of the other and properly positioning the stacked plates  100  relative to the mold  302 . For instance, as shown in  FIGS. 11 and 12 , the mold  302  includes an upper mold portion  304  and a lower mold portion  306 . In several embodiments, the mold  302  may correspond to a heated mold, with each mold portion  304 ,  306  including or being coupled to a heat source  308 , such as an electric heating device, that is configured to heat the mold portion  304 ,  306  to a specified temperature. As such, the stacked plates  100  may be configured to be positioned between the upper and lower mold portions  304 ,  306 . Thereafter, the mold  302  may be closed around the stacked plates  100  so as to simultaneously compress and heat the plates  100 . For instance, the plates  100  may be compressed between the upper and lower mold portions  304 ,  306  as heat is being transferred from the mold portions  304 ,  306  to the thermoplastic resin material contained within each of the plates  100 . A resulting welded plate assembly may then be formed from the compressed/heated plates  100 , with a welded joint being defined between each pair of adjacent plates  100 . The welded plate assembly may then be installed as a spar cap within a wind turbine rotor blade. 
     It should be appreciated that, in other embodiments, the mold  302  may only include an upper mold portion or a lower mold portion. In such an embodiment, a stationary component (e.g., a support platform) may be positioned on the opposed side of the mold  302  to allow the plates  100  to be compressed together. 
     Additionally, it should be appreciated that, similar to the embodiment described above, the plates  100  may be configured to be stacked directly one on top of another. Alternatively, one or more additional layers  250  ( FIG. 10 ) of thermoplastic resin material may be provided between each adjacent pair of plates  100 . In such an embodiment, the plates  100  positioned within the heated mold  302  may be stacked in an alternating arrangement such that each adjacent pair of plates  100  is separated from one another by the additional layer(s)  250  of thermoplastic resin material. 
     It should also be appreciated that, in several embodiments, the stacked plates  100  may be configured to be pre-heated prior to being positioned within the mold  302 . In such embodiments, the mold  302  may correspond to a heated mold, a non-heated mold or a cooled mold. For instance, in one embodiment, the plates  100  may be initially placed within an oven or may be pre-heated used any other suitable device to allow the thermoplastic resin material contained within the plates  100  to be heated to a temperature above the welding temperature for the material (e.g., the glass transition temperature for an amorphous thermoplastic resin material or the melting temperature for a semi-crystalline thermoplastic resin material). In such instance, the mold  302  may not be configured to transfer heat to the stacked plates  100  and, in fact, may be configured to reduce the temperature of the plates  100  as they are compressed within the mold  302 . In another embodiment, the plates  100  may be pre-heated to allow the thermoplastic resin material contained within the plates  100  to be heated to a temperature at or just below the welding temperature for the material. In such instance, the mold  302  may need to be configured to transfer heat to the stacked plates  100  to allow the plates  100  to be welded together. Regardless, once pre-heated, the stacked plates  100  may then be moved to the mold  302  to allow the plates  100  to be welded together and/or formed. In doing so, it is been found that the fibers contained within the plates  100  provide sufficient structural support to allow the pre-heated assembly to be transported to the mold  302 . 
     Additionally, it should be appreciated that, when pre-heating the stacked plates  100 , it may be desirable to securely clamp the plates  100  together using one or more clamping devices. However, when subsequently compressing the plates  100  within the mold  302 , the clamping device(s) should be released or removed to allow the plates  100  to move relative to one another as the plates  100  are compressed. For instance, the clamping device(s) may correspond to a releasable clamping device(s) that is configured to allow the plates  100  to pull out of the device(s) when the mold  302  is closed and the plates  100  are compressed. Alternatively, the clamping device(s) may correspond to an electronically controlled clamping device(s) that is configured to automatically release the stacked plates  100  when the mold  302  is closed. 
     For example,  FIGS. 15 and 16  illustrate simplified views of one example of a stack of plates  100  being clamped together and then released as the mold is closed down onto the plates  100 . Specifically, as shown in  FIG. 15 , one or more first clamping devices  382  may be installed along a first side  384  of the stack of plates  100  and one or more second clamping devices  386  may be installed along a second side  388  of the stack of plates  100 . For instance, as indicated above, the clamping devices  382 ,  386  may be installed in order to clamp the plates  100  together as they are being pre-heated. Upon pre-heated, the clamped stack of plates  100  may then be transferred to the mold  302 . As shown in  FIG. 16 , when the mold  302  is closed down onto the stack of plates  100 , the clamping devices  382 ,  386  may release the plates  100  to allow the material to pull in around its edges. For example, as shown in  FIG. 16 , when the mold  302  is closed and the plates  100  are released from the clamping devices  382 ,  386 , the sides  384 ,  388  of the plates  100  may be pulled inwardly (as indicated by arrows  390  in  FIG. 16 ). 
     Referring back to  FIGS. 11 and 12 , in several embodiments, at least one surface of the mold  302  may be shaped so as to correspond to at least one blade parameter that varies along a span of a wind turbine rotor blade. In such embodiments, when the pultruded plates  100  are pressed together within the mold  302 , at least a portion of the resulting welded plate assembly may conform to the shape of the mold surface. Thereafter, when the plate assembly is installed as a spar cap within a corresponding rotor blade, the plate assembly may generally conform to the aerodynamic profile of the portion of the blade shell along which the spar cap extends. 
     For instance, as shown in the illustrated embodiment, a mold surface  310  of the upper mold portion  304  may be configured to define a profile in a chordwise direction (indicated by arrow  312  in  FIGS. 11 and 12 ) and/or a spanwise direction (indicated by arrow  314   FIG. 11 ) that matches or substantially matches the chordwise profile and/or the spanwise profile of the rotor blade  22  within which the spar cap being manufactured is to be installed. Specifically, as shown in  FIG. 12 , the mold surface  310  may be configured to define a chordwise curvature along its length that generally corresponds to the chordwise curvature of the portion of the blade shell  28  along which the spar cap will be installed (e.g., the chordwise curvature along a curved section  316  of the blade shell  28  shown in  FIG. 4 ). Similarly, the mold surface  310  may be configured to define a spanwise curvature along its length that generally corresponds to the spanwise curvature of the portion of the blade shell  28  along which the spar cap will be installed. For instance, as shown in  FIG. 11 , the mold surface  310  may be curved along its length to accommodate any pre-bend  318  defined along the span  40  of the rotor blade  22 . 
     The mold surface  310  may also be configured to be shaped so as to match any other suitable blade parameter(s) of the rotor blade  22  within which the spar cap will be installed. For instance, in addition to the chordwise curvature and/or the spanwise curvature, the mold surface  310  may be shaped so as to accommodate any twist within the rotor blade  22 . Specifically, as is generally understood, a rotor blade  22  may be twisted along its longitudinal axis  30  such that a twist angle  320  of the rotor blade  22  generally varies as the blade extends between its blade root  24  and its blade tip  26 . In such an embodiment, the mold surface  310  may be similarly twisted to match the spanwise profile of the rotor blade  22 . For instance, as shown in  FIG. 12 , the mold surface  310  may be twisted or rotated about a reference plane  322  in order to match the twist angle  320  of the corresponding rotor blade  22 . 
     It should be appreciated that, when the mold  302  includes upper lower mold portions  304 ,  306 , both the upper mold portion  304  and the lower mold portion  306  may include mold surfaces shaped so as to match one or more blade parameters of the rotor blade. For instance, as shown in  FIG. 11 , a mold surface  311  of the lower mold portion  306  may also be shaped along its length so as to accommodate the spanwise curvature of the rotor blade  22 . 
     It should also be appreciated that, in one embodiment, the stacked plates  100  may be shaped simultaneously with welding the plates  100  together. For instance, the plates  100  may simply be stacked together prior to being inserted into the shaped mold. Alternatively, the plates  100  may be welded together prior to being inserted into the shaped mold, such as by welding the plates  100  together using a heated roller(s)  204 , a heated pressing device  260  and/or any other suitable means. In such an embodiment, the welded plate assembly may then be inserted into the shaped mold to allow one or more portions of the welded plate assembly to be shaped so as to conform to the shape of the rotor blade  22 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.