Patent Publication Number: US-9845787-B2

Title: Efficient wind turbine blades, wind turbine blade structures, and associated systems and methods of manufacture, assembly and use

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/951,727, filed Jul. 26, 2013, entitled “EFFICIENT WIND TURBINE BLADES, WIND TURBINE BLADE STRUCTURES, AND ASSOCIATED SYSTEMS AND METHODS OF MANUFACTURE, ASSEMBLY AND USE,” which is a continuation of U.S. patent application Ser. No. 13/154,384, filed Jun. 6, 2011, entitled “EFFICIENT WIND TURBINE BLADES, WIND TURBINE BLADE STRUCTURES, AND ASSOCIATED SYSTEMS AND METHODS OF MANUFACTURE, ASSEMBLY AND USE,” which is a continuation of International Application Serial No. PCT/US2009/066875, filed Dec. 4, 2009, entitled EFFICIENT WIND TURBINE BLADES, WIND TURBINE BLADE STRUCTURES, AND ASSOCIATED SYSTEMS AND METHODS OF MANUFACTURE, ASSEMBLY AND USE,” which claims priority to the following U.S. Provisional Patent Applications, each of which is incorporated herein in its entirety by reference: 61/120,338, filed Dec. 5, 2008; 61/220,187, filed Jun. 24, 2009; and 61/271,179, filed Jul. 17, 2009. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed generally to efficient wind turbine blades and wind turbine blade structures, including lightweight, segmented and/or otherwise modular wind turbine blades, and associated systems and methods of manufacture, assembly, and use. 
     BACKGROUND 
     As fossil fuels become scarcer and more expensive to extract and process, energy producers and users are becoming increasingly interested in other forms of energy. One such energy form that has recently seen a resurgence is wind energy. Wind energy is typically harvested by placing a multitude of wind turbines in geographical areas that tend to experience steady, moderate winds. Modern wind turbines typically include an electric generator connected to one or more wind-driven turbine blades, which rotate about a vertical axis or a horizontal axis. 
     In general, larger (e.g., longer) wind turbine blades produce energy more efficiently than do short blades. Accordingly, there is a desire in the wind turbine blade industry to make blades as long as possible. However, long blades create several challenges. For example, long blades are heavy and therefore have a significant amount of inertia, which can reduce the efficiency with which the blades produce energy, particularly at low wind conditions. In addition, long blades are difficult to manufacture and in many cases are also difficult to transport. Accordingly, a need remains for large, efficient, lightweight wind turbine blades, and suitable methods for transporting and assembling such blades. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially schematic, isometric illustration of a wind turbine system having blades configured in accordance with an embodiment of the disclosure. 
         FIG. 2A  is a partially schematic, side elevation view of a wind turbine blade having a hybrid truss/non-truss structure in accordance with an embodiment of the disclosure. 
         FIG. 2B  is an enlarged illustration of a portion of the wind turbine blade shown in  FIG. 2A . 
         FIGS. 2C-2F  are schematic cross-sectional illustrations of wind turbine blade portions having truss structures in accordance with several embodiments of the disclosure. 
         FIG. 3  is a partially schematic, isometric illustration of a portion of a wind turbine blade having three spars that form part of a truss structure in accordance with an embodiment of the disclosure. 
         FIG. 4  is a partially schematic, isometric illustration of a portion of a wind turbine blade having a non-truss internal structure in accordance with an embodiment of the disclosure. 
         FIG. 5A  is a partially schematic, isometric illustration of an internal portion of a wind turbine blade having truss attachment members configured in accordance with an embodiment of the disclosure. 
         FIGS. 5B-5C  are enlarged isometric illustrations of a truss attachment member configured in accordance with an embodiment of the disclosure. 
         FIGS. 5D-5F  illustrate several views of an internal portion of a wind turbine blade having a truss structure secured at least in part with truss attachment members configured in accordance embodiments of the disclosure. 
         FIG. 6A  is a partially schematic, side elevation view of a spar having multiple portions, each with layers that terminate at staggered positions to form a non-monotonically varying bond line. 
         FIG. 6B  is an illustration of an embodiment of the structure shown in  FIG. 6A  with clamps positioned to prevent or limit delamination in accordance with an embodiment of the disclosure. 
         FIG. 6C  is an enlarged illustration of a portion of the spar shown in  FIG. 6B . 
         FIGS. 6D-6G  are partially schematic illustrations of spars having joints configured in accordance with further embodiments of the disclosure. 
         FIG. 7A  is a partially schematic, isometric illustration of a spar having layers that fan out at a hub attachment region in accordance with an embodiment of the disclosure. 
         FIG. 7B  is a partially schematic, isometric illustration of a spar connected to fan-shaped transition plates at a hub attachment region in accordance with another embodiment of the disclosure. 
         FIG. 8A  is a partially schematic, side elevation view of a wind turbine blade structure subassembly configured in accordance with an embodiment of the disclosure, and  FIG. 8B  is an enlarged, partially schematic end view of a rib from the subassembly of  FIG. 8A . 
         FIGS. 9A-9C  are partially schematic, not-to-scale isometric views of inboard, midboard, and outboard spar portions configured in accordance with embodiments of the disclosure. 
         FIGS. 9D and 9E  include partially schematic, cut-away side elevation views of the inboard and midboard spar portions of  FIGS. 9A and 9B , respectively, and  FIG. 9F  is a partially schematic, side elevation view of a joint between adjacent end portions of the inboard spar portion and the midboard spar portion of  FIGS. 9A and 9B , configured in accordance with several embodiments of the disclosure. 
         FIGS. 10A and 10C-10E  are a series of partially schematic, side elevation views of a portion of a blade subassembly illustrating various stages in a method of manufacturing a blade spar in accordance with an embodiment of the disclosure, and  FIG. 10B  is an enlarged end view of a portion of a representative rib illustrating another stage in the method of blade manufacture. 
         FIGS. 11A-11C  are an enlarged isometric view of a portion of a wind turbine blade structure, an end view of a representative rib, and an isometric view of the wind turbine blade structure, respectively, illustrating various aspects of a spar manufactured in accordance with an embodiment of the disclosure. 
         FIG. 12A  is an isometric view of a compressing apparatus configured in accordance with an embodiment of the disclosure, and  FIG. 12B  is a partially exploded isometric view of the compressing apparatus of  FIG. 12A . 
         FIGS. 13A and 13B  are enlarged isometric views of opposing end portions of a first tool portion of the compressing apparatus of  FIGS. 12A and 12B . 
         FIG. 14A  is an isometric view of a second tool portion of the compressing apparatus of  FIGS. 12A and 12B , and  FIG. 14B  is a partially exploded isometric view of the second tool portion of  FIG. 14A . 
         FIG. 15  is an enlarged, cross-sectional end view of a laminated blade spar being compressed by the compressing apparatus of  FIGS. 12A and 12B  during an adhesive curing cycle in accordance with an embodiment of the disclosure. 
         FIG. 16  is a partially schematic isometric view of a lay-up tool illustrating various stages in a method of manufacturing a wind turbine blade spar in accordance with another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed generally to efficient wind turbine blades, wind turbine blade spars and other structures, and associated systems and methods of manufacture, assembly, and use. Several details describing structures and/or processes that are well-known and often associated with wind turbine blades are not set forth in the following description to avoid unnecessarily obscuring the description of the various embodiments of the disclosure. Moreover, although the following disclosure sets forth several embodiments, several other embodiments can have different configurations or different components than those described in this section. In particular, other embodiments may have additional elements or may lack one or more of the elements described below with reference to  FIGS. 1-16 . In  FIGS. 1-16 , many of the elements are not drawn to scale for purposes of clarity and/or illustration. 
       FIG. 1  is a partially schematic, isometric illustration of an overall system  100  that includes a wind turbine  103  having blades  110  configured in accordance with an embodiment of the disclosure. The wind turbine  103  includes a tower  101  (a portion of which is shown in  FIG. 1 ), a housing or nacelle  102  carried at the top of the tower  101 , and a generator  104  positioned within the housing  102 . The generator  104  is connected to a shaft or spindle having a hub  105  that projects outside the housing  102 . The blades  110  each include a hub attachment portion  112  at which the blades  110  are connected to the hub  105 , and a tip  121  positioned radially or longitudinally outwardly from the hub  105 . In an embodiment shown in  FIG. 1 , the wind turbine  103  includes three blades connected to a horizontally-oriented shaft. Accordingly, each blade  110  is subjected to cyclically varying loads as it rotates between the 12:00, 3:00, 6:00 and 9:00 positions, because the effect of gravity is different at each position. In other embodiments, the wind turbine  103  can include other numbers of blades connected to a horizontally-oriented shaft, or the wind turbine  103  can have a shaft with a vertical or other orientation. In any of these embodiments, the blades  110  can have structures configured in accordance with the arrangements described in further detail below with reference to  FIGS. 2A-16 . 
       FIG. 2A  is a partially schematic, partially cut-away illustration of one of the blades  110  shown in  FIG. 1 . The blade  110  extends outwardly in a radial or longitudinal direction from an inner region  113  that includes the hub attachment portion  112 , to an outer region  114  that includes the tip  121 . The hub attachment portion  112  can include one or more hub attachment elements, e.g., a ring with a bolt circle, one or more bearings, fasteners, and/or other elements. The internal structure of the blade  110  can be different at the inner region  113  than at the outer region  114 . For example, the inner region  113  can include a truss structure  140  formed from a plurality of longitudinally or radially extending beams or spars  170 , chordwise extending ribs  142 , and truss members  143  connected among the spars  170  and the ribs  142 . The truss structure  140  can be surrounded by a skin  115  (most of which is removed in  FIG. 2A ) that presents a smooth, aerodynamic surface to the wind during operation. The outer region  114  can include a non-truss structure, which will be described in further detail later with reference to  FIG. 4 . As used herein, the term “truss structure” refers generally to a load-bearing structure that includes generally straight, slender members forming closed shapes or units (e.g., triangular units). The term “non-truss structure” refers generally to a load-bearing structure having an arrangement that does not rely on, or does not primarily rely on, straight slender members forming closed-shape units for strength. Such structures may include, for example, monocoque and semi-monocoque structures. Accordingly, the skin  115  of the inner region  113  is generally non-load bearing, and the skin  115  at the outer region  114  is load bearing. 
     In a particular aspect of an embodiment shown in  FIG. 2A , the blade  110  includes three segments  116 , shown as a first segment  116   a , a second segment  116   b , and a third segment  116   c . The first and second segments  116   a ,  116   b  can each have the truss structure  140  described above, and the third segment  116   c  can have a non-truss structure. Accordingly, the blade  110  can have a truss structure for the inner two-thirds of its span, and a non-truss structure for the outer one-third. In other embodiments, these values can be different, depending, for example, on the size, shape and/or other characteristics of the blade  110 . For example, in one embodiment, the truss structure  140  extends outwardly over a majority of the span or length of the blade  110 , but by an amount less than or greater than two-thirds of the length. The segments  116  can be manufactured individually and then connected to each other at a manufacturing facility, or at an end user installation site. For example, the segments  116  can each be sized to fit in a 53-foot or other suitably sized container for shipment. In other embodiments, one or more of the segments (e.g., the first segment  116   a  and the second segment  116   b ) can be built entirely at the installation site. 
     In still further embodiments, the blade  110  can include other numbers of segments  116  (e.g., two or more segments). In any of these embodiments, individual segments  116  can include ribs  142 , truss members  143 , and portions of the spars  170  that extend for the length of the segment  116 . The segments  116  can be joined to each other by joining adjacent spar portions, e.g., as discussed later with reference to  FIGS. 6A-6C and 8A-16 . For example, the first segment  116   a  can include one or more first spar segments that are joined to corresponding second spar segments of the second segment  116   b . The resulting joined spars can extend along corresponding generally smooth, continuous longitudinal axes. In any of these embodiments, the skin  115  can be laid up on the truss structure  140  with or without forming a joint at the interface between adjacent segments  116 . For example, the spar portions can be joined at a location between two neighboring ribs  142 , and a relatively small panel of skin  115  can be laid over the spar joint and the two neighboring ribs  142 . The neighboring ribs  142  can be spaced apart by about one meter in one embodiment, and by other values in other embodiments. Larger panels of the skin  115  can be laid inboard and outboard of the small panel. In another embodiment, the skin  115  can have no spanwise joints and can be laid up as a continuous element. In any of these embodiments, the skin  115  can be attached (e.g., adhesively bonded or ultrasonically bonded) to the ribs  142  alone, or to the ribs  142  and the spars  170 . In any of these embodiments, the truss structure  140  can serve as primary structure for carrying shear and bending loads in the blade  110 . 
       FIG. 2B  is a partially schematic, isometric illustration of a portion of the blade  110  shown in  FIG. 2A , taken at a location where the internal structure of the blade  110  is a truss structure  140 . Accordingly, the truss structure  140  can include multiple spars  170  (four are shown in  FIG. 2B ) attached to spaced-apart ribs  142 . Truss members  143  can be connected between neighboring spars  170 , for example, using techniques described later with reference to  FIGS. 5A-5F . 
       FIGS. 2C-2F  are schematic, cross-sectional illustrations of blades  110  having truss arrangements configured in accordance with a variety of embodiments.  FIG. 2C  illustrates a blade  110  having four spars  170  positioned in a generally rectangular arrangement.  FIG. 2D  illustrates a blade  110  having six spars  170 , including four spars  170  positioned in a generally rectangular arrangement, and two additional spars  170 , one positioned forward of the generally rectangular arrangement, and one positioned aft of the generally rectangular arrangement.  FIG. 2E  illustrates a blade  110  having four spars  170  positioned in a generally diamond-shaped arrangement, and  FIG. 2F  illustrates a blade  110  having three spars  170  positioned in a triangular arrangement. In other embodiments, the blade  110  can include spars  170  having other arrangements. 
       FIG. 3  is an isometric illustration of an internal portion of a blade  110  having a truss structure  140  that includes a triangular arrangement of spars  170 , generally similar to that shown in  FIG. 2F . The blade  110  extends in a longitudinal radial, or spanwise direction along a spanwise axis S, and extends fore and aft along a transverse chordwise axis C. Accordingly, the blade  110  can have a forward leading edge region  117  with a leading edge  117   a  and an aft trailing edge region  118  with a trailing edge  118   a . The thickness of the blade  110  can be measured relative to a thickness axis T transverse to both the spanwise axis S and the chordwise axis C. 
     In a particular embodiment shown in  FIG. 3 , the blade  110  can include three spars  170 , including a first spar  170   a  and a second spar  170   b , both positioned at the leading edge region  117  and/or toward the leading edge  117   a  and spaced apart from each other along the thickness axis T. The blade  110  can further include a third spar  170   c  positioned at the trailing edge region  118  and/or toward the trailing edge  118   a  and spaced apart from both the first spar  170   a  and the second spar  170   b  along the chordwise axis C. Each of the spars  170   a - 170   c  is attached to a plurality of ribs  142  (one of which is visible in  FIG. 3 ) which are in turn spaced apart from each other along the spanwise axis S. Each of the spars  170   a - c  can have a generally rectangular cross-section. The forward spars  170   a ,  170  can have a chordwise dimension greater than a thickness dimension, and the aft spar  170   c  can have a thickness dimension greater than a chordwise dimension. The third spar  170   c  can extend over a majority of the thickness dimension of the blade  110  and in a particular embodiment, can extend over the entirety or nearly the entirety of the thickness dimension. For example, the third spar  170   c  can have a dimension in the thickness direction that is about the same as the dimension of the rib  142  in the thickness direction. 
     One feature of the arrangement shown in  FIG. 3  is that it can include a single spar (the third spar  170   c ) at the trailing edge region  118 . For example, the truss structure  140  can include only three longitudinally extending spars  170  at any given longitudinal location, with only one of the spars  170  at the trailing edge region  118 . This arrangement can allow the third spar  170   c  to be positioned a greater chordwise distance away from the first and second spars  170   a ,  170   b  than some arrangements that include four spars (e.g., the arrangement shown in  FIGS. 2B-2C ). By spacing the third spar  170   c  further away from the first and second spars  170   a ,  170   b , the ability of the truss structure  140  to handle large loads in the chordwise direction C is enhanced. This can be particularly important for wind turbine blades mounted to a horizontal shaft because such blades are subjected to significant gravity loads in the chordwise direction C when the blades are at the 3:00 and 9:00 positions described above with reference to  FIG. 1 . Accordingly, it is expected that this arrangement may be lighter and/or better able to withstand significant loads in the chordwise direction C than at least some arrangements having four spars. At the same time, it is expected that this arrangement will be simpler, lighter and/or less costly than arrangements that include more than four spars e.g., the arrangement described above with reference to  FIG. 2D . 
     The internal structural components described above can be manufactured from suitable composite and/or non-composite materials. For example, the spars  170  can be formed from a laminate of layers that each include unidirectional fiberglass, carbon fibers, and/or other fibers in a matrix of suitable thermoset and/or thermoplastic resins. The fibers can be oriented generally parallel to the spanwise axis S over most of the length of the blade  110 , and can have other orientations at specific locations, as described further below with reference to  FIGS. 6A-7A . In other embodiments, composite spars can also be fabricated by infusion, prepreg, pultrusion, or press molding. In still further embodiments, the spars  170  can be formed from metallic materials, including machined, forged or cast alloys, metallic laminates, sandwich structures, as well as metal/composite hybrids (e.g., composite facesheets with metallic core, e.g., honeycomb core), etc. The truss members  143  can be formed from aluminum (e.g., 2024-T6 aluminum) or another suitable metal, composite, or other material. The ribs  142  can be formed from a composite of fiberglass and foam or balsa, e.g., a balsa core sandwiched between fiberglass faceplates. In other embodiments, the ribs  142  can be formed from fiberglass alone, without a foam or balsa core, or the ribs  142  can be formed with other techniques and/or components. For example, the ribs  142  can have a corrugated or beaded construction. The ribs  142  can be formed from a single panel, or two spaced apart panels, with no core structure between the two panels. The ribs  142  can also be made from metal; from composite materials such as fiberglass, carbon fibers, and/or other fibers in a matrix of thermoset and/or thermoplastic; and/or from (unreinforced) plastic materials (e.g., resin without fibers). For example, composite ribs can be fabricated by wet lamination, infusion, prepreg, sprayed chopped fiber, press molding, vacuum forming, and/or other suitable mass production techniques. 
       FIG. 4  is a partially schematic illustration of a portion of the wind turbine blade  110  located at the outer region  114  described above with reference to  FIG. 2A . In this embodiment, the internal structure of the wind turbine blade  110  at the outer region  114  is not a truss structure. For example, the structure can instead include a relatively thin web  119  oriented generally parallel to the thickness axis T and extending along the spanwise axis S. The web  119  can be connected to or formed integrally with flanges  120  extending in the chordwise direction C. Spanwise-extending spars  470   a ,  470   b  are attached to each of the flanges  120  and are in turn connected to a skin  115 , a portion of which is shown in  FIG. 4A . In one embodiment, the structure can include spaced-apart ribs  142  positioned in the trailing edge region  118 . In other embodiments, such ribs  142  can extend into the leading edge region  117  as well. The skin  115  can be formed from a fiberglass-balsa-fiberglass sandwich, or a fiberglass-foam-fiberglass sandwich. In other embodiments, the skin  115  can be formed from composite materials fabricated by wet lamination, infusion, prepreg, sprayed chopped fiber, press molding, vacuum forming, and/or other mass production techniques. The skin  115  can have the same construction in both the outer region  114  shown in  FIG. 4 , and the inner region  113  shown in  FIG. 3 . The ribs  142  can have a similar construction. The web  119  and flanges  120  can be formed from fiberglass, e.g., unidirectional fiberglass. In other embodiments, any of the foregoing components can be formed from other suitable materials. The spars  470   a ,  470   b  located in the outer region  114  can be bonded to corresponding spars at the inner region  113  ( FIG. 2A ) using a variety of techniques including, but not limited to, those described later with reference to  FIGS. 6A-6C and 8A-16 . In any of these embodiments, the spars  470   a ,  470   b  located in the outer region  114  can extend along the same generally smooth, continuous longitudinal axes as the counterpart spars in the inner region  113  to efficiently transfer loads from the outer region  114  to the inner region  113 . 
     One feature of the arrangement described above with reference to  FIGS. 2A-4  is that the blade  110  can include both truss and non-truss internal structures. An advantage of this arrangement is that it can be more structurally efficient than a design that includes either a truss structure alone or a non-truss structure alone. For example, the truss structure can be used at the inner region  113  (e.g., near the hub) where bending loads are higher than they are near the tip  111 , and where the blade  110  is relatively thick. At the outer region  114 , the non-truss structure can be easier to integrate into this relatively thin portion of the blade  110 . The non-truss structure in this region is also expected to be more structurally efficient than a truss structure, which tends to lose efficiency when the aspect ratio of the closed shapes formed by the truss members becomes large. 
       FIG. 5A  is a partially schematic, isometric illustration of a portion of a representative truss structure  140  configured in accordance with a particular embodiment of the disclosure. In this embodiment, the truss structure  140  includes three spars  170 , identified as a first spar  170   a , a second spar  170   b  and a third spar  170   c . In other embodiments, the truss structure  140  can have other numbers and/or arrangements of spars  170 . In any of these embodiments, the truss structure  140  can include truss members  143  and ribs  142 , in addition to the spars  170 . Truss attachment members  150  can connect the truss members  143  to the spars  170 . For example, truss members  143  can include a first attachment feature  151   a  (e.g., a first mounting hole) that is aligned with a second attachment feature  151   b  (e.g., a second corresponding mounting hole) carried by the truss attachment member  150 . When the two attachment features  151   a ,  151   b  include corresponding holes, they can be connected via an additional fastening member  157 , e.g., a rivet or threaded fastener. In other embodiments, the attachment features  151   a ,  151   b  can be connected directly to each other, for example, if one feature includes an expanding prong and the other includes a corresponding hole. 
       FIG. 5B  illustrates a representative portion of the truss structure  140  described above with reference to  FIG. 5A . As shown in  FIG. 5B , a representative truss attachment member  150  is positioned along the second spar  170   b  so as to receive and attach to multiple truss members  143 . Each of the truss members  143  can include a slot  145  which receives a flange-shaped truss attachment portion  154  of the truss attachment member  150 . In this embodiment, the attachment features  151   a ,  151   b  include corresponding holes  158   a ,  158   b  that are connected with the fastening members  157  described above with reference to  FIG. 5A . 
       FIG. 5C  is an enlarged isometric illustration of one of the truss attachment members  150  shown in  FIGS. 5A-5B . In this embodiment, the truss attachment member  150  includes a spar attachment portion  152  (e.g. having a channel  153  in which the corresponding spar  170  is positioned), and one or more truss attachment portions  154  (two are shown in  FIG. 5B ). The truss attachment portions  154  can have a flat, flange-type shape in which the second attachment features  151   b  (e.g., the mounting holes  158   b ) are positioned. In a particular embodiment shown in  FIG. 5B , the truss attachment member  150  is formed from two complementary components or pieces: a first component or piece  156   a  and second component or piece  156   b . The first piece  156   a  includes two first flange portions  155   a , and the second piece  156   b  includes two second flange portions  155   b . When the two pieces  156   a ,  156   b  are placed together, the first flange portions  155   a  mate with corresponding second flange portions  155   b  to form two flange pairs, each of which forms one of the truss attachment portions  154 . Accordingly, each first flange portion  155   a  can be in surface-to-surface contact with the corresponding second flange portion. The first and second portions  155   a ,  155   b  can have aligned mounting holes configured to receive a corresponding fastener. The two pieces  156   a ,  156   b  also form the channel  153 . In a particular aspect of this embodiment, the first piece  156   a  and the second piece  156   b  are sized so that, when placed together, the resulting channel  153  is slightly smaller than the cross section of the spar around which it is placed. Accordingly, when the two pieces  156   a ,  156   b  are forced toward each other, the truss attachment member  150  can be clamped around the corresponding spar, thus securing the truss attachment member  150  in position. For example, when second attachment feature  151   b  includes a mounting hole, the manufacturer can pass a fastener  157  through the mounting hole to both attach the truss attachment member  150  to the corresponding truss member  143  ( FIG. 5A ), and also clamp the truss attachment member  150  around the corresponding spar  170  ( FIG. 5A ). 
     In other embodiments, the truss attachment members  150  can be formed using other techniques. For example, the truss attachment members  150  can be extruded, molded, cast, or machined. In any of these embodiments, the truss attachment member  150  can be formed from a light-weight material, e.g. a metal such as aluminum or steel, or a suitable composite. In other embodiments, the truss attachment members  150  can be formed from other materials that readily accommodate the attachment features  151   b . The truss attachment members  150  can be secured to the corresponding spars using the clamping technique described above, and/or other techniques, including but not limited to adhesive bonding or co-curing. 
     The truss attachment members  150  can have other shapes and/or configurations in other embodiments. For example, the spar attachment portion  152  need not extend around the entire circumference of the corresponding spar  170 , but can instead extend around only a portion of the spar  170 . In some embodiments for which an adhesive joint between the truss attachment member  150  and the spar  170  provides sufficient strength, the truss attachment member  150  can have only a relatively small surface contacting the spar  170 . The truss attachment member can include other numbers of truss attachment portions  154 , e.g., only one truss attachment portion  154 , or more than two truss attachment portions  154 . 
     In still further embodiments, the truss attachment members  150  can be formed from other materials. For example, the truss attachment members  150  can be formed from a composite material. In a particular example, the truss attachment member  150  is formed by wrapping strands (e.g., plies of strands) around the spar  170 , and overlapping the ends of the strands (or plies) to form one or more flanges. The strands are attached to the spar  170  with an adhesive, or via a co-curing process. The corresponding truss member  143  attached to the truss attachment member  150  can have a slot  145  that receives the flange and is secured to the flange with an adhesive. 
     One feature of an embodiment of the truss attachment member  150  described above with reference to  FIGS. 5A-5C  is that it does not require holes in the spar  170  to provide an attachment between the spar  170  and the corresponding truss members  143 . Instead, the truss attachment member  150  can be clamped or otherwise secured to the spar  170  and the holes can be located in the truss attachment member  150  rather than in the spar  170 . This arrangement can be particularly beneficial when the spar  170  includes composite materials, as it is typically more difficult to form mounting holes in such materials, and/or such holes may be more likely to initiate propagating fractures and/or create stress concentrations in the spar  170 . 
       FIGS. 5D-5F  illustrate other views of the truss structure  140  described above with reference to  FIG. 5A .  FIG. 5D  is a side view of a portion of the truss structure  140 , illustrating a representative rib  142 . The rib  142  includes a web  146  and a flange  147  extending around the web  146 . The web  146  can include one or more cut-outs  148  (three are shown in  FIG. 5D ) that accommodate the corresponding spars  170   a - 170   c . In a particular embodiment shown in  FIG. 5D , the cut-out  148  accommodating the third spar  170   c  can extend entirely through the thickness of the rib  142 . As a result, a trailing edge portion  141  of the rib  142  is discontinuous from the rest of the web  146  of rib  142 . Accordingly, the flange  147  of the rib  142  can secure the trailing edge portion  141  of the rib  142  to the rest of the rib  142 . 
       FIG. 5E  is a view of the truss structure  140  from a position forward of and above the leading edge region  117 , and  FIG. 5F  is a view of the truss structure  140  from a position above the trailing edge region  118 . As is shown in both  FIGS. 5E and 5F , the truss members can include first truss members  143   a  and second truss members  143   b . The first truss members  143   a  can be positioned adjacent to the web  146  of a corresponding rib  142 , and can be joined to the web  146 , in particular, via an adhesive or other bonding technique. Accordingly, the first truss members  143   a  in combination with the truss attachment members  150  can secure the ribs  142  to the spars  170   a - 170   c . The second truss members  143   b  can extend transversely (e.g., diagonally) between neighboring ribs  142  and/or spars  170  to increase the overall strength and stiffness of the truss structure  140 . 
       FIG. 6A  is a partially schematic, side elevation view of a joint between two portions  171  of a spar  170 . The two portions can include a first portion  171   a  and a second portion  171   b , and the joint can be formed along a non-monotonically varying (e.g., zig-zagging) bond line  176 . Such a bond line  176  is expected to produce a stronger bond between the first and second portions  171   a ,  171   b  than is a straight or diagonal bond line. The first and second portions  171   a ,  171   b  may each form part of a different neighboring segment of the overall spar  170 , as described above with reference to  FIG. 2A . For example, the first portion  171   a  can be part of the first segment  116   a  shown in  FIG. 2A , and the second portion  171   b  can be part of the second segment  116   b.    
     The first portion  171   a  can include multiple, stacked, laminated first layers  172   a , and the second portion  171   b  can include multiple, stacked, laminated second layers  172   b . In a particular embodiment, the layers  172   a ,  172   b  can be formed from a unidirectional fiber material (e.g., fiberglass or a carbon fiber) and a corresponding resin. Each of the layers  172   a ,  172   b  can be formed from a single ply or multiple plies (e.g., six plies). The layers  172   a ,  172   b  can be prepreg layers, hand lay-ups, pultrusions, or can be formed using other techniques, e.g., vacuum-assisted transfer molding techniques. The first layers  172   a  terminate at first terminations  173   a , and the second layers  172   b  terminate at second terminations  173   b . Neighboring terminations  173   a ,  173   b  located at different positions along the thickness axis T can be staggered relative to each other to create the zig-zag bond line  176 . This arrangement produces projections  174  and corresponding recesses  175  into which the projections  174  fit. In a particular aspect of this embodiment, each layer has a termination that is staggered relative to its neighbor, except where the bond line  176  changes direction. At such points, two adjacent layers can be terminated at the same location and bonded to each other, to prevent a single layer from being subjected to increased stress levels. 
     During a representative manufacturing process, each of the first layers  172   a  are stacked, bonded and cured, as are each of the second layers  172   b , while the two portions  171   a ,  171   b  are positioned apart from each other. The layers  172 ,  172   b  are pre-cut before stacking so that when stacked, they form the recesses  175  and projections  174 . After the two portions  171   a ,  171   b  have been cured, the recesses  175  and/or projections  174  can be coated and/or filled with an adhesive. The two portions  171   a ,  171   b  are then brought toward each other so that projections  174  of each portion are received in corresponding recesses  175  of the other. The joint region can then be bonded and cured. 
       FIG. 6B  is an illustration of a spar  170  having a bond line  176  generally similar to that described above with reference to  FIG. 6A . As is also shown in  FIG. 6B , the spar  170  can include one or more clamps or straps  177  that are positioned at or near the bond line  176 . The clamps  177  can be positioned to prevent or halt delamination that might result between any of the layers in the composite spar  170 . For example, as shown in  FIG. 6C , if a potential delamination  178  begins between two layers  172   a , the compressive force provided by the clamp  177  can prevent the delamination  178  from spreading further in a spanwise direction. The clamp  177  can be positioned where it is expected that the potential risk of delamination is high, e.g., at or near the termination  173  of the outermost layers  172   a ,  172   b  shown in  FIG. 6B . In other embodiments, the function provided by the clamps  177  can be provided by other structures. For example, the truss attachment members  150  described above can perform this function, in addition to providing attachment sites for the truss members. 
       FIGS. 6D-6G  are a series of partially schematic, side elevation views of spars  670   a - 670   d , respectively, illustrating various joints that can be formed between adjacent spar portions  671  in accordance with other embodiments of the disclosure. The spars  670  can be at least generally similar in structure and function to the spar  170  described in detail above. For example, as shown in  FIG. 6D , the spar  670   a  can include a first portion  671   a  having multiple, stacked, laminated first layers  672   a , and a second portion  671   b  having multiple, stacked, laminated second layers  672   b . In addition, the first portion  671   a  can be joined to the second portion  671   b  along a bond line  676   a  that is non-monotonically varying (e.g., zigzagging) along the thickness axis T. In this particular embodiment, however, the first layers  672   a  and the second layers  672   b  have first terminations  673   a  and second terminations  673   b , respectively, that are not parallel to the chordwise axis C. That is, the terminations  673  are beveled or slanted relative to the chordwise axis C. The bevels can have the same direction and extent for each layer, or these characteristics can vary from one layer to the next. For example, as shown in  FIGS. 6D and 6E  in dashed lines, the layer below the topmost layer can be beveled in the opposite direction as the topmost layer. Bevels in neighboring layers can be positioned directly above and below each other, as shown in  FIGS. 6D and 6E , or the bevels in neighboring layers can be offset in a spanwise direction so as not to overlay each other. 
     Referring next to  FIG. 6E , the spar  670   b  can be at least generally similar in structure and function to the spar  670   a  described in detail above. For example, the spar  670   b  can include a first portion  671   c  having multiple, stacked, laminated first layers  672   a , and a second portion  671   d  having multiple, stacked, laminated second layers  672   b . In this particular embodiment, however, the first layers  672   a  have first terminations  673   c  that form a projection  674   a , and the second layers  672   b  have second terminations  673   d  that form a recess  675   a . The projection  674   a  is received in the recess  675   a  to form a bond line  676   b  that is non-monotonically varying along both the thickness axis T and the chordwise axis C. 
     Referring next to  FIG. 6F , the spar  670   c  is at least generally similar in structure and function to the spar  670   a  described in detail above. In this particular embodiment, however, the first layers  672   a  include first terminations  673   e , and the second layers  672   b  include second terminations  673   f , that form alternating projections  674   b  and recesses  675   b  along the chordwise axis C. This results in a bond line  676   c  that is non-monotonically varying along the chordwise axis C but not along the thickness axis T. 
     Referring next to  FIG. 6G , in this particular embodiment the first layers  672   a  include first terminations  673   g , and the second layers  672   b  include terminations  673   h , that form alternating projections  674   c  and recesses  675   c  along the chordwise axis C, and alternating projections  674   d  and recesses  675   d  along the thickness axis T. As the foregoing discussion illustrates, there are a wide variety of non-monotonically varying, staggered, zigzagging, overlapping, and/or other bond lines that can be used to efficiently and strongly join spar portions together in accordance with the present disclosure. Accordingly, the present disclosure is not limited to bond lines having any particular configuration. 
     One feature of embodiments described above with reference to  FIGS. 6A-6G  is that they can include spar portions connected to each other along a bond line that has a zig-zag shape, or otherwise varies in a non-monotonic manner. An expected advantage of this arrangement is that the bond line will be stronger than a simple vertical or diagonal bond line. In addition, it is expected that forming the bond line can be simplified because it does not require the use of a significant number of additional fastening elements, and can instead employ a bonding technique generally similar to the technique used to bond the individual layers of the two portions. Still further, the bond between the spar portions may be formed with no heating, or only local heating, which avoids the need to heat the entire blade. The foregoing characteristics can in turn facilitate the ease with which a manufacturer and/or installer forms a large wind turbine blade that is initially in multiple segments (e.g., the segments  116  described above with reference to  FIG. 2A ), which are then joined to each other, for example, at an installation site. Further details of suitable manufacturing techniques are described later with reference to  FIGS. 8A-16 . 
     In other embodiments, the spar  170  can include other configurations and/or materials. For example, selected plies can be formed from metal or carbon fiber rather than glass fiber. The plies need not all have the same thickness. Accordingly, the dimensions and materials selected for each ply can be selected to produce a desired strength, stiffness, fatigue resistance and cost. 
       FIG. 7A  is a partially schematic illustration of a hub attachment portion  112  configured in accordance with an embodiment of the disclosure. For purposes of illustration,  FIG. 7A  illustrates only the hub attachment portion  112 , and in particular, the transition between the longitudinally extending spars  170  and a hub attachment element, e.g., a circumferentially extending hub attachment ring  180 . The ring  180  can include a non-composite structure, e.g., a metallic element, and can have a relatively short spanwise direction as shown in  FIG. 7A , or a longer spanwise dimension in other embodiments. The ring  180  or the hub attachment portion  112  can be circumferentially continuous, or formed from multiple sections arranged circumferentially. For example, the hub attachment portion  112  can include one circumferential section for each spar  170 , with each section connected to a continuous ring  180 . Other hub attachment elements that may be included in the hub attachment region  112  are not shown in  FIG. 7A . The hub attachment portion  112  can include a transition to four spars  170  (as shown in  FIG. 7A ) or other numbers of spars  170  (e.g., three spars  170 , as shown in  FIG. 3 ). 
     Each of the spars  170  can include a laminate composite of layers  172 , and each of the layers  172  can in turn include multiple plies. For example, in a particular embodiment, each of the spars  170  can include a laminate of fifteen layers  172 , each having a total of six plies, for a total of ninety plies. Each of the plies can have fibers that are oriented unidirectionally, for example, in alignment with the spar axis S. Accordingly, such fibers have a 0° deviation from the spar axis S. The layers  172  can be stacked one upon the other, each with fibers oriented at 0° relative to the spar axis S, and can be cut so as to have the shape shown in  FIG. 7A . The number of plies oriented at 0° relative to the spar axis S can be reduced in a direction extending toward the ring  180 . For example, the number of such plies can be reduced from ninety at the right side of  FIG. 7A  (where the spars  170  have a generally fixed, rectangular cross-sectional shape) to twenty at the ring  180  on the left side of  FIG. 7A  (where the structure has thinner, arcuate shape). The seventy deleted layers  172  can be terminated in a staggered fashion so that the overall thickness of the structure is gradually reduced from right to left 
     As the 0° orientation layers  172  are dropped off, the manufacturer can add layers that are oriented at other angles relative to the spar axis S. For example, the manufacturer can add layers having fibers oriented at +45° and −45° relative to the spar axis S. In a particular embodiment, twenty to thirty such plies can be added, so that the total number of plies at the ring  180  is between forty and fifty, as compared with ninety plies at the right side of  FIG. 7A . By adding the +45°/−45° oriented plies to the structure at the hub attachment portion  112 , the load carried by the spars  170  can be spread out in a circumferential direction and distributed in a more uniform fashion at the ring  180 . To further enhance this effect, the load path can be “steered” by providing a different number of +45° plies as compared with −45° plies. This arrangement can accordingly reduce or eliminate the likelihood that individual bolts passing through bolt holes  182  in the ring  180  will experience significantly higher loads than other bolts located at different circumferential positions. As a result, this arrangement is expected to not only provide a smooth transition from the airfoil-shaped cross section of the blade  110  to the circular cross-section shape at the hub attachment portion  112 , but is also expected to more evenly distribute the loads than do existing structures. 
       FIG. 7B  is another illustration of a hub attachment portion  112  in which the spar  170  includes layers  172  of unidirectionally extending fibers, aligned with the spar axis S. In this embodiment, individual layers  172  terminate at terminations  173 . One or more termination elements  179  (e.g., plates), each having a curved, fan-type shape, can be butted up against the spar  170 , and can include recesses that receive the terminated layers  172 . In a particular embodiment shown in  FIG. 7B , this arrangement includes three transition elements  179 , two of which are visible in  FIG. 7B . The two visible transition elements  179  each accommodate multiple layers  172  (e.g., four or more layers  172 ). A gap  183  between the two transition elements  179  receives a third transition element (not shown in  FIG. 7B  for purposes of clarity) that in turn receives the remaining layers  172 . Each of the transition elements  179  can then be attached to the ring  180 , which is in turn connected to a pitch bearing  181 . The pitch bearing  181  is used to vary the pitch of the wind turbine blade  110  in use. Each of the transition elements  179  can have a generally arcuate cross-sectional shape where it connects to the ring  180 , and a generally flat, rectangular or rectilinear cross-sectional shape at its furthest point from the ring  180 , where it connects to the spar  170 . 
     In other embodiments, the transition region between the hub attachment ring  180  or other attachment feature, and the rest of the blade  110  can have other arrangements. For example, the general arrangement of fan-shaped plies or plies in combination with transition elements can be applied to other blade structures that may not include the spars described above. In another example, the arrangement of +45°/−45° plies described above can be used to “steer” loads (e.g., to more evenly distribute loading at the boltholes  182 ) in blades  110  that do not include the spars  170 , or in blades  110  that include spars or other structures arranged differently than is described above. 
       FIG. 8A  is a partially schematic, side elevation view of a manufacturing assembly  801  of the turbine blade  110  configured in accordance with an embodiment of the disclosure, and  FIG. 8B  is an enlarged end view taken along line  8 B- 8 B in  FIG. 8A  illustrating a representative rib  142  supported by a tool stanchion  802 . Referring to  FIGS. 8A and 8B  together, the manufacturing assembly  801  includes a plurality of ribs  142  supported by individual tool stanchions  802  at the appropriate spanwise locations. As discussed above, the turbine blade  110  includes an inboard or first blade segment  116   a , a midboard or second blade segment  116   b , and an outboard or third blade segment  116   c . In the illustrated embodiment, the second spar  170   b  (e.g., the lower or “pressure” spar) has been assembled onto the ribs  142 . The spar  170   b  includes an inboard or first spar portion  871   a , a midboard or second spar portion  871   b , and an outboard or third spar portion  871   c.    
     Referring next to  FIG. 8B , as explained above with reference to  FIG. 5D , the ribs  142  include a plurality of cutouts  148  configured to receive corresponding truss attachment members  150 . More particularly, in the illustrated embodiment the representative rib  142  includes a first cutout  148   a  configured to receive the first spar  170   a  (e.g., the suction spar; not shown in  FIG. 8A or 8B ), a second cutout  148   b  configured to receive the second spar  170   b  (e.g., the pressure spar), and a third cutout  148   c  configured to receive the third spar  170   c  (e.g., the aft spar; also not shown in  FIG. 8A or 8B ). As described in greater detail below, in various embodiments one or more of the spars  170  can be manufactured by laminating a plurality of prefabricated composite layers or “pultrusions” together in position on the manufacturing assembly  801 . Further details of these embodiments are described in greater detail below with respect to  FIG. 9A-16 . 
       FIGS. 9A-9C  are a series of partially schematic, enlarged isometric views of the inboard spar portion  871   a , the midboard spar portion  871   b , and the outboard spar portion  871   c  configured in accordance with embodiments of the disclosure. Referring first to  FIG. 9A , in the illustrated embodiment the spar  170   b  can be manufactured from a plurality of layers  972  (identified individually as layers  972   a - o ) that are bonded or otherwise laminated together in place on the manufacturing assembly  801  ( FIG. 8A ). In particular embodiments, the layers  972  can include prefabricated composite materials, such as pultrusions or “planks” of pultruded composite materials. As is known, composite pultrusion is a manufacturing process that creates fiber-reinforced polymer or resin products having relatively consistent shape, strength and resilience characteristics. In a typical pultruding process, the reinforcement material (e.g., unidirectional fibers, tows, roving, tape etc. of glass fibers, aramid fibers, carbon fibers, graphite fibers, Kevlar fibers, and/or other material) is drawn through a resin bath (e.g., a liquid thermosetting resin bath of epoxy resin, vinylester resin, polyester resin, plastic). The wet, fibrous element is then pulled through a heated steel die, in which accurate temperature control cures the resin and shapes the material into the desired profile. The pultrusions can then be cut to the desired length for use. Strength, color and other characteristics can be designed into the profile by changes in the resin mixture, reinforcement materials, die profiles, and/or other manufacturing parameters. 
     In the illustrated embodiment, the layers  972  can be formed from pultruded planks having generally rectangular cross sections. In one embodiment, for example, the layers  972  can have cross-sectional widths of from about 2 inches to about 12 inches, or from about 4 inches to about 10 inches, and cross-sectional thicknesses of from about 0.10 inch to about 0.5 inch, or about 0.25 inch. In other embodiments, the layers  972  can have other shapes and sizes. In particular embodiments, the layers  972  can be provided by Creative Pultrusions, Inc., of 214 Industrial Lane, Alum Bank, Pa. 15521. In other embodiments, the layers  972  can be comprised of other types of pultruded materials as well as other types of composite materials including both prefabricated and hand-laid composite materials. In yet other embodiments, the methods of manufacturing turbine blade spars described herein can be implemented using other types of laminated materials. Such materials can include, for example, wood (e.g., balsa wood, plywood, etc.), metals (e.g., aluminum, titanium, etc.) as well as combinations of wood, metals, composites, etc. 
     Referring still to  FIG. 9A , the inboard spar portion  871   a  includes an inboard end portion  979   a  and an outboard end portion  979   b . Each of the end portions includes a staggered arrangement of layers  972 . For example, with reference to the outboard end portion  979   b , each of the layers  972  includes a corresponding termination  973  (identified individually as terminations  973   a - o ) which is staggered relative to adjacent terminations  973  to form projections  974  and corresponding recesses  975 . In addition, in various embodiments the layers  972  can be tapered toward the terminations  973  at the end portions  979 . As described in greater detail below, this arrangement of alternating projections  974  and recesses  975  facilitates joining the first spar portion  871   a  to the second spar portion  871   b  in a very efficient overlapping joint with a zigzag bond line. 
     Referring next to  FIG. 9B , the second spar portion  871   b  is also comprised of a plurality of layers  972  having terminations  973  that are staggered to create an alternating arrangement of projections  974  and corresponding recesses  975 . Like the first spar portion  871   a , the second spar portion  871   b  includes an inboard end portion  979   c  and an outboard end portion  979   d . As illustrated in  FIG. 9B , however, the second spar portion  871   b  becomes thinner (i.e., it tapers in thickness) toward the outboard end portion  979   d . In the illustrated embodiment, this is accomplished by successive termination of the outer layers  972  as they extend outwardly from the inboard end portion  979   c . This gradual tapering of the spar  170   b  can be done to reduce weight and/or tailor the strength of the spar  170   b  for the reduced structural loads that occur toward the tip of the turbine blade  110 . 
     Referring next to  FIG. 9C , the third spar portion  871   c  includes an inboard end portion  979   c  and a corresponding outboard end portion  979   f . As this view illustrates, the spar  170   b  continues to taper toward the outboard end portion  979   f  by terminating various layers  972  as they approach the end portion  979   f.    
       FIGS. 9D and 9E  include partially schematic, enlarged side views illustrating additional details of the first spar portion  871   a  and the second spar portion  871   b  configured in accordance with an embodiment of the disclosure. In addition, these Figures also illustrate various features of the end portions of some of the layers  972 . As shown in  FIG. 9D , the outboard end portion  979   b  of the first spar portion  871   a  includes a plurality of alternating projections  974  and corresponding recesses  975  formed by the staggered terminations  973  of the respective layers  972 . As this view further illustrates, the end portions of the layers  972  can be gradually tapered toward the termination  973  to further facilitate and shape the projections  974 /recesses  975  into gradually transitioning recesses/projections. For example, in the illustrated embodiment, the last 2 to 6 inches, or about the last 4 inches of each layer  972  can have a double-sided taper (if, e.g., an inner layer  972 ) or a single-sided taper (if, e.g., an outer layer  972 ) to a termination  973  of from about 0.0 inch to about 0.07 inch, or about 0.04 inch. 
     Referring next to  FIG. 9E , the inboard end portion  979   c  of the second spar portion  871   b  includes a plurality of projections  974  configured to fit into corresponding recesses  975  of the outboard end portion  979   b  of the first spar portion  871   a . Similarly, the inboard end portion  979   c  also includes a plurality of recesses  975  configured to receive corresponding projections  974  of the outboard end portion  979   b  of the first spar portion  871   a . For example, during manufacture of the spar  170   b , the first projection  974   a  on the outboard end portion  979   b  of the first spar portion  871   a  is fit into the corresponding first recess  975   a  on the inboard end portion  979   c  of the second spar portion  871   b . Although the respective end portions  979  are fit together in this manner during assembly of the spar  170   b  on the manufacturing assembly  801  of  FIG. 8A , the mating end portions  979  are not actually bonded together at this time, so that the blade sections  116  ( FIG. 8A ) can be separated after manufacture and individually transported to the installation site. 
     As shown in  FIG. 9F , when the outboard end portion  979   b  of the first spar portion  871   a  is ultimately joined to the inboard end portion  979   c  of the second spar portion  871   b  at the installation site, the alternating projections  974  and recesses  975  create an overlapping or a zigzag bond line  976 . As is known to those of ordinary skill in the art, this is a very efficient structural joint, and can avoid or at least reduce the need for further structural reinforcement of the joint between the first spar portion  871   a  and the second spar portion  871   b.    
       FIGS. 10A and 10C-10E  are a series of partially schematic side elevation views of a portion of the manufacturing assembly  801  of  FIG. 8A , illustrating various stages in a method of manufacturing the spar  170   b  in situ on the truss structure of the turbine blade  110  in accordance with an embodiment of the disclosure.  FIG. 10B  is an enlarged end view taken along line  10 B- 10 B in  FIG. 10A , further illustrating aspects of this spar manufacturing method. Referring first to  FIGS. 10A and 10B  together, the ribs  142  have been secured to their corresponding tool stanchions  802 , and a plurality of truss members  143  have been installed (at least temporarily) between corresponding truss attachment members  150 . Each truss attachment member  150  of the illustrated embodiment includes a first piece  1056   a  and a mating second piece  1056   b . As shown in  FIG. 10A , only the first piece  1056   a  is attached to the truss structure during build-up of the spar  170   b . As discussed in more detail below, after all of the spar layers  772  have been properly arranged on the first piece  1056   a  of the truss attachment member  150 , the second piece  1056   b  is fit into position and secured to the first piece  1056   a.    
     Referring next to  FIG. 10C , the individual spar layers  772  are sequentially placed into position on the first piece  1056   a  of the truss attachment member  150  of each rib  142 . As the spar layers  772  are placed on top of each other, the terminations  773  are positioned as shown in  FIGS. 7A-7E  to produce the desired spar profile. A layer of adhesive (e.g., epoxy adhesive, thermosetting resin adhesive, etc.) can be applied to one or both of the mating surfaces of adjacent layers  772 . The spar layers  772  can be temporarily held in position during the stacking process with clamps  1002  (e.g., C-clamps and/or other suitable clamps known in the art). 
     Referring next to  FIG. 10D , once all of the layers  772  have been properly arranged on the first pieces  1056   a  of the truss attachment members  150 , the layers  772  can be compressed during the adhesive curing cycle using a suitable clamping tool, such as the compressing apparatus  1090  described in greater detail below. More particularly, a plurality of the compressing apparatuses  1090  can be positioned on the spar portion  871  between the ribs  142  to compress the layers  972  together during the curing process. The compressing apparatus  1090  is described in greater detail below with reference to  FIGS. 12A-15 . 
     Referring next to  FIG. 10E , once the adhesive between the layers  972  has cured, the second pieces  1056   b  of each of the truss attachment members  150  can be installed on the truss structure and joined to the corresponding first pieces  1056   a  with threaded fasteners and/or other suitable methods. In one embodiment, adhesive can be applied between the mating surfaces of the first piece  1056   a  and the spar portion  871 , and/or the second piece  1056   b  and the spar portion  871 , to bond the spar portion  871  to the respective truss attachment members  150 . In other embodiments, such adhesive can be omitted. 
       FIG. 11A  is an enlarged isometric view of a portion of the truss structure of the turbine blade  110 , and  FIG. 11B  is an end view of a representative rib  142  illustrating aspects of the installed spars  170 . In one embodiment, the second piece  1056   b  of the truss attachment member  150  can be mated to the first piece  1056   a  by sliding the second piece  1056   b  sideways into the cutout  148 . For this procedure, the end portions of the truss members  143  can be temporarily detached from corresponding truss attachment portions  1154  of the truss attachment member  150 . Once both pieces  1056  of the truss attachment member  150  are in their respective positions, the end portions of the truss members  143  can be rejoined to the truss attachment portions  1154 . In one embodiment, the end portions of the truss members  143  and the corresponding truss attachment portions  1154  can be pilot drilled undersize, and then drilled full size during final assembly. Moreover, the end portions of the truss numbers  143  can be attached to the truss attachment portions  1154  by fasteners  859  that are frozen before installation in the corresponding fastener holes so that they expand to a press fit after installation. In other embodiments, the truss members  143  can be attached to the truss attachment members  150  using other suitable methods known in the art. 
       FIG. 11C  is a partially schematic isometric view of a portion of the manufacturing assembly  801  after the spar  170   b  has been fully assembled and installed on the truss structure of the turbine blade  110 . Referring to  FIGS. 11A and 11C  together, although the mating end portions  979  of the second spar portion  871   b  and the third spar portion  871   c  are assembled in place to ensure that they will fit neatly together during final assembly, the end portions  979  are not bonded during truss manufacture. This enables the second blade section  116   b  and the third blade section  116   c  to be separated from each other at the manufacturing facility for transportation to the installation site. Accordingly, in the illustrated embodiment the end portions  979  of the spar portions  871  are not bonded together during the manufacturing process, but instead form separation joints  1120  where the spars  170  will be joined together when the turbine blade  110  is assembled on site. In one embodiment, the spars can be joined together on site using the systems and methods described in detail in U.S. Provisional Patent Application No. 61/180,816, filed May 22, 2009 and incorporated herein in its entirety by reference. The blade segments can be transported to the site using systems and methods described in detail in U.S. Provisional Patent Application No. 61/180,812, filed May 22, 2009 and incorporated herein in its entirety by reference. 
       FIG. 12A  is an isometric view of the compressing apparatus  890  configured in accordance with an embodiment of the disclosure, and  FIG. 12B  is a partially exploded isometric view of the compressing apparatus  1090 . Referring to  FIGS. 12A and 12B  together, the compressing apparatus  1090  includes a first tool portion  1250   a  and a second tool portion  1250   b . In the illustrated embodiment, the tool portions  1250  are mirror images of each other, or are at least very similar to each other. Each tool portion  1250  includes a support plate  1254  and opposing side flanges  1256  (identified individually as a first side flange  1256   a  and a second side flange  1256   b ) extending therefrom. As described in greater detail below, the tool portions  1250  are configured to fit together in a clamshell arrangement around a portion of the laminated spar  170  to compact and compress the spar layers (e.g., the layers  772 ) together while the adhesive between the layers cures. More particularly, each of the tool portions  1250  includes one or more expandable members  1258  configured to expand inwardly from the support plate  1254  to thereby compress the corresponding spar section during the curing process. In the illustrated embodiment, the first side flange  1256   a  is somewhat wider than the second side flange  1256   b , so that the mating flanges  1256  can overlap and be temporarily held together with fasteners  1252  (e.g., threaded fasteners, such as bolts, screws, etc.) during the compressing and curing process. Each tool portion  1250  can also include a first end portion  1261  and an opposing second end portion  1262 . Handles  1253  can be provided on the end portions  1261  and  1262  to facilitate manual placement, installation and/or removal of the tool portions  1250 . The tool portions  1250  can be manufactured from various materials having sufficient strength, stiffness, and manufacturing characteristics. For example, in one embodiment the tool portions  1250  can be formed from aluminum that is machined, welded, or otherwise formed to the desired shape. In other embodiments, the tool portions  1250  can be fabricated from other suitable metals including steel, brass, etc., as well as suitable non-metallic materials such as composite materials. 
       FIG. 13A  is an exploded isometric view of the first end portion  1261  of the first tool portion  1250   a , and  FIG. 13B  is an enlarged isometric view of the second end portion  1262 . Referring first to  FIG. 13A , each tool portion  1250  includes a manifold  1360  for filling and unfilling the expandable members  1258  with a fluid (e.g., compressed air). In the illustrated embodiment, a conduit  1368  (identified individually as conduits  1368   a - c ) extends between each expandable member  1258  and a fill/drain fitting  1366 . The fill/drain fitting  1366  can include a threaded orifice  1370  or other feature (e.g., a high-pressure air coupling) configured to receive a corresponding fitting for flowing fluid into the respective expandable members  1258  through the conduits  1368 . In one embodiment, for example, the expandable members  1258  can be filled with compressed air to inflate the expandable members  1258  and thereby compress the layers of the spar  170  together during the curing cycle. In other embodiments, the expandable members  1258  can be filled with other types of gas or liquids (e.g., water, oil, etc.) to inflate the expandable members  1258  and compress the spar layers together. 
     The proximal end portions of the expandable members  1258  can include an end closure  1364  to seal the expandable member  1258  and maintain pressure. In the illustrated embodiment, the end closures  1364  can include two or more plates that sandwich the end portion of the expandable member  1258  therebetween to prevent leakage. In other embodiments, other structures and systems can be used to seal the proximal end portions of the expandable members  1258 . As shown in  FIG. 13B , the distal end portions of the expandable members  1258  can be closed off and sealed with a suitable end closure plate  1365  that is fastened to the support plate  1254  with a plurality of fasteners  1352 . In other embodiments, the end portions of the expandable members  1258  can be secured to the tool portion  1250  and/or closed off and sealed using other suitable means. 
       FIG. 14A  is an enlarged isometric view of the second tool portion  1250   b , and  FIG. 14B  is a partially exploded isometric view of the second tool portion  1250   b . With reference to  FIG. 14B , each of the expandable members  1258  can include a flexible tubular structure comprised of an outer layer  1430  and an inner layer  1432 . The outer layer  1430  can include a suitable material to provide strength to the expandable member  1258 , and the inner layer  1432  can include a suitable material for sealing the expandable member  1258 . For example, the inner sealing layer  1432  can include a rubber liner, and the outer layer  1430  can include woven nylon, fiberglass, etc. Accordingly, in one embodiment the expandable member  1258  can include a structure that is at least generally similar in structure and function to a fire hose. In other embodiments, the expandable members  1258  can include other materials and have other structures. 
       FIG. 15  is an enlarged end view taken substantially along line  15 - 15  in  FIG. 10D  illustrating use of the compressing apparatus  1090  in accordance with an embodiment of the disclosure. In this view, the spar layers  972  have been appropriately positioned on the truss substructure, with bonding adhesive between the layers. The first tool portion  1250   a  has been positioned on one side of the spar  170 , and the second tool portion  1250   b  has been positioned on the other side. Each first flange  1256   a  of each tool portion  1250  overlaps the corresponding second flange  1256   b  of the opposing tool portion  1250 . Once the two tool portions  1250  have been properly positioned, the tool portions  1250  are temporarily attached with the fasteners  1252 . A pressure source (e.g. a source of compressed air) is then attached to the manifold  1360  on each tool portion  1250 , and the expandable members  1258  are inflated to a sufficient pressure. As they expand, the expandable members  1258  provide an even, distributed pressure over the laminated spar  170 . The pressure can be modulated as required to provide a desired level of compaction and compression during the curing process. Moreover, a suitable vacuum bag or other thin film protective layer can be wrapped around the spar  170  to avoid getting adhesive on the compressing apparatus  1090 . After the spar  170  has suitably cured, the compressing apparatus  1090  can be disassembled by relieving the pressure in the expandable members  1258  and removing the fasteners  1252 . 
     The methods and systems described in detail above can be used to assemble a wind turbine blade spar in situ on a manufacturing subassembly in accordance with embodiments of the disclosure. More particularly, several embodiments of the disclosure have been described in detail above for manufacturing laminated spars using pultruded composite materials, such as pultruded composite “planks.” There are a number of advantages associated with some of these embodiments. These advantages can include, for example, lower cost and lower weight wind turbine blades as compared to conventional manufacturing techniques. Moreover, use of pultrusions can reduce dimensional variations in the finished parts. 
     In certain embodiments, other turbine blade structures, such as outer skins, ribs, truss members, etc. can be formed from pultruded composite materials. For example, in one embodiment skins can be formed from one or more pultruded composite members (e.g., sheets) that are laminated together. In other embodiments, truss members can be formed from composite pultrusions. Accordingly, the methods and systems disclosed herein for forming turbine blade structures from pultruded materials are not limited to use with turbine blade spars or spar caps, but can be used to form other turbine blade structures. 
     In other embodiments, however, turbine blade spars and/or other blade structures, such as the spars  170  described herein, can be manufactured from pultruded composite materials using a suitable production tool.  FIG. 16 , for example, illustrates a tool  1610  having a mold surface  1612  with an appropriate contour for the spar  170   b . To manufacture the spar  170   b  on the tool  1610 , the layers  972  (e.g., pultruded planks) are sequentially positioned on the mold surface  1612 . Tooling pins  1614  and/or other locaters can be used to accurately position the layers  972 . The layers  972  can be precut to the appropriate lengths so that when arranged on the tool surface  1612 , the respective end portions  979  form the desired zigzagging joint or overlapping fingers. Although no adhesive is used between the mating end portions  979  at this time, each layer  972  is covered with adhesive prior to installation on the tool  1610 . After all the layers  972  have been placed on the tool surface  1612 , the lay up can be vacuum-bagged to extract the air from the laminate and compress the layers  972  together. The spar can be cured at room temperature, or heat can be applied via an autoclave or other means if desired for the particular adhesive used. 
     From the foregoing, it will be appreciated that specific embodiments have been described herein for purposes of illustration, but that the invention maybe include other embodiments as well. For example, features described above with reference to  FIG. 7A  in the context of four spanwise extending spars can be applied to wind turbine blades having other numbers of spars, including three spars. In addition, the truss structures described above can have arrangements other than those specifically shown in the Figures. The attachments between spars, ribs, and truss members can have arrangements other than those described above. Certain aspects of the disclosure described in the context of particular embodiments may be combined or eliminated in other embodiments. Further, while advantages associated with certain embodiments have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the disclosure. Accordingly, the invention can include other embodiments not explicitly shown or described above. Therefore, the invention is not limited, except as by the appended claims.