Abstract:
A multiple generator wind turbine employs a single blade arrangement to drive multiple generators. The multiple generators are preferably substantially tubular and can all be mounted on one side of the turbine support structure or can be divided, preferably symmetrically, on opposite sides of the support structure. Preferably, a single drive blade arrangement drives a rotor of a first generator and a shaft connects the first generator to a rotor of a second generator. Additionally, a clutch can be placed in the drive train between two generators to allow turbine operation at lower speeds. The substantially tubular nature of the turbine allows easy access by humans to the interior of the wind turbine and provides ready air flow through the wind turbine to the hub and blades for cooling of equipment therein and/or deicing of the blades.

Description:
PRIORITY CLAIM 
       [0001]    This application is a national stage application of PCT/IT2006/000870, filed Dec. 22, 2006, the entire contents of which is incorporated herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a wind power generator or turbine. More particularly, embodiments relate to a large-scale wind powered machine including two or more power generators and that accommodates humans within the workings for easy access and maintenance while providing efficient cooling of components and/or de-icing of blades. Embodiments are particularly suited to electrical power generation via wind power. 
         [0003]    Wind powered machines, particularly large scale electrical generators, include blades mounted on a hub attached to a rotor that rotates when wind passes over the blades. The rotation of the rotor is then used to drive machinery, such as pumps or electrical generators. In the case of electrical generators, the rotor will typically carry conductor windings/coils or magnetic field generators that face magnetic field generators or conductor windings/coils, respectively, on a stator such that there is relative motion between the coils and the magnetic field generators, producing electricity. The magnetic field generators are typically field windings that are electromagnets powered by the electrical generator once it begins producing electricity, but that require electricity from a battery or the like before the electrical generator produces electricity. 
       BACKGROUND 
       [0004]    Large scale wind powered electrical generators are becoming more common, particularly in onshore and offshore wind farm applications. In such large scale generators, a tower supports a nacelle housing the stator, which supports the rotor, which supports the hub and blades. Equipment required for controlling the generator, including controls for the blades and other machinery, can be housed in the tower, the nacelle, and/or in cavities within the stator and/or the rotor. As suggested by this description, such wind machines typically include a single rotor and a single stator. In the power generation industry, there is a constant demand for more power production and/or higher efficiency in power production. A difficulty associated with meeting these demands with single generator arrangements is that a high-power generator can be quite heavy, impeding assembly of the wind machine. As generators are built to produce more power, the quantity of magnets and coils must increase by increasing diameter of the generator, allowing more magnets and coils to be installed, increasing the length of the generator, allowing longer magnets and coils to be used, or both. Increases in diameter and length present transportation and support-structure related problems in that the roads on which components will be transported can only handle so large an object and the structures involved in supporting a long object can be more complicated and expensive. Additionally, such high-power generators tend to be more difficult to drive than lower-power generators, requiring higher initial operating wind speed and/or larger blades. 
         [0005]    Some prior art wind machines attempt to overcome these difficulties by employing more than one rotor, more than one stator, or more than one of both rotor and stator. For example, U.S. Published Applications Nos. 2006/0066110 and 2006/0071575 disclose wind turbines including at least one double-sided stator and at least one double-sided rotor. The stator and the rotor are concentrically arranged so that the rotor has both inner and outer magnetic sides that rotate with respect to respective faces of the stator. While the rotor and stator of this arrangement are both horizontally axially arranged, the support arrangement of this arrangement requires one single bearing and one double bearing to support the rotor on the stator. This arrangement does increase power output for a given diameter wind power generator, it does not overcome the weight issue described above in that the double-sided rotor and double-sided stator are both still single components. In fact, this arrangement might even worsen the weight issue since there is more material on each of the rotor and stator. 
         [0006]    U.S. Pat. No. 6,278,197 discloses another wind power generator that employs a horizontally axially arranged first rotor and replaces the usual stator with a concentric, contra-rotating second rotor that is also horizontally axially arranged. The first rotor rotates opposite to the second rotor, thereby increasing power output by effectively increasing the speed of rotation of the rotor. While this is an interesting solution to the problem of obtaining more power from a given diameter generator, it introduces undesirable complexities in the support and wind harnessing structures of the device. 
         [0007]    U.S. Pat. Nos. 6,285,090 and 7,042,109, as well as PCT Application No. WO 01/06623 A1, disclose wind turbines each employing a double sided rotor within a double sided stator. Unlike embodiments and the devices discussed above, the rotor and stator are radially arranged, presenting disc-like faces to each other rather than the annuli and/or cylinders of embodiments and the devices above, though the &#39;109 patent includes an annular embodiment. The structure is analogous to those above in that each side of the inner disc carries magnets while each face of the outer discs carries windings/coils, or vice versa. Multiple discs can be employed to create multiple generators within the turbine. Because of the disc configuration, the length increase associated with the power increase seen by adding a disc set is reduced as compared to an annular configuration. However, while possibly increasing power output for a given turbine diameter, weight is still an issue. Additionally, because the power generating components extend vertically in generators employing discoid rotors and stators, an increase in disc diameter is required for an increase in power output. 
         [0008]    U.S. Pat. No. 6,504,260 discloses another disc-configured wind turbine, but in which contra-rotating rotors are powered by respective sets of blades. This contra-rotating arrangement differs from that of the U.S. Pat. No. 6,278,197 in that each rotor has a respective stators instead of having oppositely-rotating rotors. Thus, the turbine has two independent power generation and collection arrangements mounted on opposite sides of a support tower substantially symmetrically. While this allows the use of two smaller generators to create a high power wind turbine, the use of completely independent drive and power collection systems introduces undesirable cost and complexity into the device. Additionally, because the power generating components extend vertically in generators employing discoid rotors and stators, an increase in disc diameter is required for an increase in power output. 
       SUMMARY 
       [0009]    The various embodiments of the present invention avoid the shortcomings of conventional wind power generators by providing a multiple generator wind turbine with a simpler structure, yielding higher power output for a given turbine diameter while keeping component diameter, weight, length, and cost down. Additionally, embodiments employ a largely hollow construction in which a maximum of ventilation possibilities is available for cooling and/or de-icing. In addition, embodiments afford a large degree of accessibility to the various components of the generator while providing a high level of structural stiffness. Embodiments further allow for the use of standard components, particularly in embodiments in which modular arrangements are employed, which can result in easier manufacture, assembly, and production. By virtue of the mounting of generators on opposite sides of the support structure according to embodiments, optimization of loads on the wind turbine can be realized. 
         [0010]    In a preferred embodiment, the wind power generator is a multipolar, gearless, synchronous generator that extends substantially horizontally and is largely hollow by virtue of the use of modular coaxial tubular stator and rotor elements. For additional simplification, embodiments employ permanent magnets on one of stator and rotor, and windings/coils on the other of stator and rotor. In a first arrangement, a single set of blades is mounted on a side of a supporting structure. A first rotor is mounted on the blade side of the turbine, while a second rotor is mounted on the opposite side of the turbine in substantially symmetric arrangement. Respective stators are mounted concentrically with the rotors to enable power generation when the rotors rotate relative to their respective stators. A shaft connects the two rotors, and the two are driven by the single set of blades. For example, the blade-side rotor can be driven by the blades and connected to the shaft, which drives the opposite rotor. In embodiments, the shaft includes two half shafts extending from the rotors toward the center of the turbine. The end of one half shaft is inserted into the end of the other and connected to form the shaft. 
         [0011]    In a second arrangement according to embodiments, multiple generators are coaxially arranged between the turbine blades and the support structure of the turbine. Thus, the turbine blades drive a first rotor that is connected to and drives the second rotor, each having a respective concentric stator. Embodiments of course allow use of more than two rotors. The first rotor serves simultaneously as a shaft that can be supported by bearings and as a structure for anchoring power generation elements. Advantageously, the first and second arrangements can be used together such that multiple generators can be mounted on either side of the support structure of the turbine, the two generator clusters being connected by a shaft or other suitable connector between the two generators. Thus, a first rotor can drive a second rotor in a first cluster on the blade side of the turbine and a third rotor can drive a fourth rotor in a second cluster on the opposite side of the turbine. 
         [0012]    A third arrangement employs a double-sided rotor within two concentric stators in a fashion similar to the annular arrangement discussed above. The double-sided rotor includes an annular portion with inner and outer surfaces extending substantially horizontally. One set of rotor elements is mounted on the inner surface and another set of rotor elements is mounted on the outer surface, each set of rotor elements facing a corresponding set of stator elements. The inner stator elements are preferably mounted on an outer surface of an annulus arranged within the double-sided rotor, while the outer stator elements are preferably mounted on the inner surface of the housing of the turbine. 
         [0013]    A fourth arrangement employs multiple generators with double-sided rotors and so is effectively a combination of the first and second arrangements described above. As with the second arrangement, the rotor of a first generator connected to the blades is connected to a second generator, such as to the rotor of the second generator, to drive the second generator. 
         [0014]    As should be apparent, embodiments can use an annular generator of the first arrangement with a concentric generator of the third arrangement. The order in which they are arranged will depend on the particular requirements of the wind turbine in which they are to be installed. Thus, in some situations, the rotor of the simple annular generator could be connected to the blades and drive the rotor of the concentrically arranged generator, but in others, the double-sided rotor of the concentrically arranged generator will be connected to the blades and drive the rotor of the simple annular generator. 
         [0015]    In any arrangement, and in the combination, a clutch can be placed between a respective pair of generators to allow removal of the downstream generator(s) from the drive train. This allows the turbine to operate in a lower-power mode in which a lower wind speed is required for power generation, then, if demand or wind speed increases, reengage the downstream generator(s) to increase power output. Conversely, if the turbine is operating with all generators engaged, the clutch(es) can be disengaged when the wind drops below the minimum speed for operation with all generators, allowing power generation at lower wind speeds. Preferably, the clutch is automated mechanically or electrically so that rotational speed causes engagement. For example, a centrifugal clutch could be used so that the downstream generator(s) would be off line until the first rotor reached a predetermined speed, at which point the clutch would gradually engage the next rotor to bring the next rotor up to speed. 
         [0016]    The generator of embodiments is the integrating component of the supporting structure, and the loads are transferred directly from the hub onto the rotor shaft of the generator. The tubular rotor element transfers the loads into the tubular stator body by way of one bearing in each generator of the electrical machine. 
         [0017]    The largely hollow structure of embodiments provides several advantages over the structures of the prior art. For example, housing electrical and electronic subsystems inside the nacelle affords excellent protection from lightning since the structure employs the principle of the Faraday cage. In addition, because the tubular structure is configured to accommodate the passage of adult humans, it permits easy access to the front portion of the nacelle and to the hub, which facilitates maintenance and repair work on other subsystems of the wind power generator. This also allows one to mount the hub from the inside. 
         [0018]    The substantially hollow structure also facilitates use of the heat given off by equipment, such as power electronics, housed in the tower, as well as heat released by the generator itself. The heat can promote the chimney effect to guide warm air into the hub and from there into and through the rotor blades. The warm air can thus be used as a particularly efficient de-icing system in cooler times of the year, and provides a cooling effect for equipment in the generator as cooler air is drawn into and passes through the hollow structure. No external energy needs to be supplied during operation to heat the rotor blades. Thus, the heat given off by the generator and by the power electronics themselves is put to use in a simple fashion. 
         [0019]    Additional cooling benefits are derived from the hollow structure since the components that produce heat are moved to the periphery of the generator. More specifically, the generator of embodiments places the windings on the inner periphery of the generator housing. Heat produced by the windings during electricity generation is easily conducted to the outer surface of the generator. By adding cooling fins on the outer surface according to embodiments, the heat can be transferred from the generator to the air stream passing over the generator during electricity production. The cooling fins preferably project transversely from the outer surface and are substantially equally spaced apart. While the fins extend longitudinally along the outer surface, they can also have a sweep or profile that takes into account disturbances in the air stream introduced by motion of the blades and/or the fins themselves to enhance effectiveness. 
         [0020]    In embodiments, each generator has permanent magnets on an outer body and has windings/coils on an interior body. This yields a machine having a stator unit on the inside and a rotor on the outside. The magnets are preferably attached to the inner surface of the rotor in this arrangement, and the windings to the outer surface of the rotor shaft. The advantages of such a solution are a greater specific output, the possibility of using the total heat released by the generator for the de-icing system, and a simplification of the positioning of the power cables required to conduct the electric current from the generator to the tower. 
         [0021]    Preferably, each rotor is supported via a single bearing, preferably of the tapered roller type. The single bearing arrangement provides simplification of the generator mounting structure since only one-side need accommodate a bearing. The single bearing arrangement also eliminates hazardous eddy currents in the generator that form temporary circuits between the stator wall, the rotor wall, and roller bodies of the bearings disposed at the ends of the active portion (windings/coils) of the two bearing arrangement. Further, the single bearing arrangement simplifies adjustment processes of the bearing since the tapered rollers must be pre-stressed; embodiments with two bearings, one at each end of the generator, present design problems with respect to the construction tolerances and thermal deformation. The single bearing arrangement requires only one system of seals and lubrication concentrated in the front region of the generator. And the bearing typology used in the single bearing arrangement offers a high degree of rolling precision since pre-stressing the rollers substantially eliminates play in the bearing, as well as providing a low rolling resistance that increases generator productivity and efficiency. 
         [0022]    Additional features and advantages are described in, and will be apparent from, the following Detailed Description and the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Additional features and details are contained in the claims and in the description of a power generator actuated by wind, in its preferred embodiments as illustrated in the accompanying drawings, in which: 
           [0024]      FIG. 1  shows a sectional view along a vertical axial plane of a multiple power generator wind turbine in which the generators are on opposed sides of a wind turbine support structure in accordance with embodiments. 
           [0025]      FIG. 2  shows a sectional view along a vertical axial plane of a multiple power generator wind turbine in which the generators are in series on one side of a wind turbine support structure in accordance with embodiments. 
           [0026]      FIG. 3  shows a sectional view along a vertical axial plane of a multiple power generator wind turbine in which the generators are concentric in accordance with embodiments. 
           [0027]      FIG. 4  shows a sectional view along a vertical plane of a multiple power generator wind turbine in accordance with embodiments that is similar to that of  FIG. 1 , but using multiple generators on each side of the support structure similar to the wind turbine shown in  FIG. 2 . 
           [0028]      FIG. 5  shows a sectional view along a vertical plane of a multiple power generator wind turbine in accordance with embodiments that is similar to that of  FIGS. 2 and 3 , employing a multiple concentric generator of  FIG. 3  on each side of the support structure similar to the wind turbine shown in  FIG. 2 . 
           [0029]      FIG. 6  shows a sectional view along a vertical plane of a multiple power generator wind turbine in accordance with embodiments that is similar to that of  FIGS. 2 and 3 , employing multiple concentric generators of  FIG. 3  in series on each side of the support structure similar to the wind turbine shown in  FIG. 2 . 
           [0030]      FIG. 7  shows a sectional view along a vertical plane of a multiple power generator wind turbine in accordance with embodiments that is similar to that of  FIGS. 1 and 3 , employing multiple concentric generators of  FIG. 3  in series on one side of the support structure similar to the wind turbine shown in  FIG. 1 . 
           [0031]      FIG. 8  shows a sectional view along a vertical plane of a multiple power generator wind turbine in accordance with embodiments that is similar to that of  FIG. 7 , but employing a more modular form. 
           [0032]      FIG. 9  shows a multiple power generator wind turbine as in  FIG. 1 , but also including at least one clutch in accordance with embodiments. 
           [0033]      FIG. 10  shows a multiple power generator wind turbine as in  FIG. 2 , but also including at least one clutch in accordance with embodiments. 
           [0034]      FIG. 11  shows a multiple power generator wind turbine as in  FIG. 4 , but also including at least one clutch in accordance with embodiments. 
           [0035]      FIG. 12  shows a multiple power generator wind turbine as in  FIG. 5 , but also including at least one clutch in accordance with embodiments. 
           [0036]      FIG. 13  shows a multiple power generator wind turbine as in  FIG. 6 , but also including at least one clutch in accordance with embodiments. 
           [0037]      FIG. 14  shows a multiple power generator wind turbine as in  FIG. 7 , but also including at least one clutch in accordance with embodiments. 
           [0038]      FIG. 15  shows a multiple power generator wind turbine as in  FIG. 8 , but also including at least one clutch in accordance with embodiments. 
           [0039]      FIG. 16  shows a sectional view along a vertical plane of a multiple power generator wind turbine in accordance with embodiments that combines different types of generators. 
           [0040]      FIG. 17  shows a multiple power generator wind turbine as in  FIG. 16 , but also including at least one clutch in accordance with embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    In  FIG. 1  a multiple power generator wind turbine is generally indicated by the reference number  1 . A support structure  2  of the wind turbine  1  includes a connecting structure  3  that rests atop a support tower  4 , preferably with a rotatable connection  5  allowing the single drive blade arrangement  6  to face the direction from which wind blows. The blade arrangement  6  includes a plurality of blades and drives two generators  110 ,  120 . As shown in  FIG. 1 , the generators  110 ,  120  can be arranged with one generator  110  on a blade side of the support structure  2  and another generator  120  on the opposite side of the support structure. The housings  111 ,  121  of the generators  110 ,  120  each preferably carry a plurality of circumferentially-distributed cooling fins  112 ,  122  that draw heat away from the generators  110 ,  120 , releasing the heat into the slipstream as air passes over the fins  112 ,  122 . The blade side generator  110  includes a rotor  113  connected to the drive blade arrangement  6 , which rotates the rotor  113  within its housing  111  and within a stator  114  attached to the connecting structure  3 . Preferably, the housings  111 ,  121  are the outer surfaces of the stators  114 ,  124 , which stators are a principal source of heat within the generators  110 ,  120 . The rotor  113  of the first generator  110  in embodiments is mechanically connected to the rotor  121  of the second generator, thereby providing drive to the second generator  120 . Each rotor  113 ,  123  is supported by a bearing  7  that can be mounted in a respective stator  114 ,  124  to allow rotation of the rotor  113 ,  123 . Preferably, the rotor  113  of the first generator  110  is selectively mechanically connected to the rotor  123  of the second generator via a clutch, thereby allowing operation of the turbine  1  with only one generator producing power when wind speed is too low to drive both generators. 
         [0042]    In an alternative embodiment seen in  FIG. 2 , the multiple power generator wind turbine  1  again includes two generators  110 ,  120 , but they are both on one side of the supporting structure  2 . Thus, the drive blade arrangement  6  is connected to the rotor  113  of the first generator  110 , which is connected to the second rotor  123  via a relatively short connector  230 , such as a short tube. In this arrangement, the two housings  111 ,  121  can be combined into a single housing  200 , and the fins  112 ,  122  can be combined to form one longer plurality of circumferentially-distributed cooling fins  210  extending from the housing  200 . 
         [0043]    As seen in  FIG. 3 , in another alternative embodiment, the first and second generators  110 ,  120  can be concentrically arranged by using a double sided rotor  310 , one side of which, such as the inner side  311 , carries the first rotor  113 , and the other side of which, such as the outer side  312 , carries the second rotor  123 . The double sided rotor  310  rotates within a double stator  320  with the first rotor  113  facing the first stator  114  on the outer surface of an inner portion  321  of the double stator  320  and the second rotor  123  facing the second stator  124  on the inner surface of an outer portion  322  of the double stator  320 . The blade arrangement  6  thus drives the double sided rotor  310  within the double stator  320  to produce power. 
         [0044]    The additional alternative embodiment of a wind turbine  1  shown in  FIG. 4  employs two generators  110 ,  120  arranged on opposite sides of the supporting structure  2  as in  FIG. 1 , but each generator  110 ,  120  itself includes multiple generators, all driven by the single blade arrangement  6 . For convenience, the first and second generators  110 ,  120 , will be called first and second generator clusters with respect to  FIGS. 4-7 . On the blade side of the supporting structure  2 , the first generator cluster  110  includes at least two generators  410 ,  420  arranged in series as in  FIG. 2 , while the second generator cluster  120  on the opposite side of the supporting structure  2  includes at least two generators  430 ,  440  similarly arranged. The first rotor  413  is driven by the blades  6  to rotate within its stator  414 , the first rotor being connected to the second rotor via a relatively short shaft  415 . The second rotor  423  of the first cluster  110  is connected to the main shaft  130 , which is mechanically connected to the first rotor  433  of the second generator cluster  120 , providing drive to the second cluster  120 . The first rotor  433  of the second cluster  120  rotates within its respective stator  434  and is connected to the second rotor  443  of the second cluster  120  via a relatively short shaft  435 . The housings of the generators  410 ,  420  of the first cluster  110  and the generators  430 ,  440  of the second cluster can be merged into a single housing  450  on each side of the supporting structure  2  as in the turbine shown in  FIG. 2 . Likewise, the fins can be combined into a single set of longer, circumferentially-distributed fins  460  on each housing  450 . 
         [0045]    The wind turbine  1  as shown in  FIG. 5  in another embodiment again employs two generator clusters  110 ,  120  arranged on opposite sides of the supporting structure  2  and connected by a main shaft  130  as in  FIGS. 1 and 4 , but the generators of each cluster are concentric as in  FIG. 3 . On the blade side of the supporting structure  2 , the first cluster  110  includes at least two generators  510 ,  520  arranged concentrically as in  FIG. 3 , while the second cluster  120  on the opposite side of the supporting structure  2  includes at least two generators  530 ,  540  similarly arranged. The first double sided rotor  550 , one side of which, such as the inner side  551 , carries the first rotor  513 , and the other side of which, such as the outer side  552 , carries the second rotor  523 . The double sided rotor  550  rotates within a double stator  560  with the first rotor  513  facing the first stator  514  on the outer surface of an inner portion  561  of the double stator  560  and the second rotor  523  facing the second stator  524  on the inner surface of an outer portion  562  of the double stator  560 . The first rotor  513  is driven by the blades  6  to rotate within its stator  514 , the first rotor  513  being connected to the first rotor  533  of the second cluster  120  via the main shaft  130 . 
         [0046]      FIG. 6  shows a wind turbine according to another embodiment that combines the concentric multiple cluster arrangement of  FIG. 5  with the serial arrangement of  FIG. 2 . The blade arrangement  6  drives the first rotor, which drives the second rotor, which is mechanically connected to the second cluster via the main shaft  130 . In the second cluster  120 , a first rotor is connected to the main shaft  130  and the second rotor. 
         [0047]      FIG. 7  shows a wind turbine that combines the serial multiple cluster arrangement of  FIG. 2  with the concentric multiple generator of  FIG. 3 . Thus, drive blades  6  drive a first double rotor  71  within a first double stator  72 , the first double rotor being mechanically connected to a second double rotor  73  within a respective double stator  74 . 
         [0048]      FIG. 8  shows a wind turbine very similar to that shown in  FIG. 2 , but which employs flanges  86  between generators to create a modular arrangement. As in the arrangement shown in  FIG. 2 , two generators  81 ,  82  are both on one side of the supporting structure  2 . Thus, the drive blade arrangement  6  is connected to the rotor  813  of the first generator  81 , which includes a relatively short connector  815 , such as a short tube, that terminates in a flange  816 . The flange  816  is connected to a corresponding flange  826  on a second connector  825  of the second generator  82 . Thus, the first rotor  813  is connected to the second rotor  823  via connectors  815 ,  825 , and flanges  816 ,  826 . In this arrangement, the two housings  811 ,  821  preferably also include corresponding flanges  817 ,  827 . As shown, the two generators  81 ,  82  are effectively modules. The modules can rely on the single bearing  7  of the first generator  81 , though additional bearings could be employed if necessary. The fins  112 ,  122  of  FIG. 2  can be combined to form one longer plurality of circumferentially-distributed cooling fins  83  extending from the housing  80 , or can simply be left separate and aligned when the modules are assembled. As should be clear, the modular arrangement shown in  FIG. 8  can be employed in other arrangements, such as those shown in  FIGS. 1-7 , to allow modular construction of wind turbines including multiple generators and/or generator clusters. 
         [0049]    As should be apparent, while one or two generators are shown in each cluster in  FIGS. 1-7 , more generators could be combined in each embodiment as desired within each cluster or as additional clusters. In all embodiments, one or more clutches can be included between various of the generators to enable variable power output of the wind turbine and operation of the wind turbine at lower speeds than would be required if all generators were operating at the same time. Some examples of arrangements that can be employed are shown in  FIGS. 9-16 .  FIG. 9  shows the arrangement of  FIG. 1 , but with a clutch  910  schematically illustrated in the path between the first and second rotors. While the clutch is shown in the main shaft, it should be apparent that it could be in one of the rotors, between one of the rotors and the shaft, or embedded within the joint of the two half-shafts of the main shaft. Any suitable type of clutch can be used. For a clutch between the half shafts, a centrifugal clutch can be particularly advantageous. In operation, the first generator would operate for all wind speeds over the minimum speed required to drive just the first generator. When the wind speed is below a minimum for using both generators, the clutch is not engaged and only the first generator is used. When the wind speed reaches a minimum for using both generators, the clutch is engaged to bring the second generator on line.  FIGS. 12 ,  14 , and  15  show clutched versions of  FIGS. 5 ,  7 , and  8  that operate in a manner similar to the clutched version of  FIG. 1  shown in  FIG. 9 . 
         [0050]      FIG. 10  shows the arrangement of  FIG. 2 , but with a clutch  1010  schematically illustrated between the first and second rotors. It should be apparent that the clutch could be in any suitable location between the two rotors, and that any suitable type of clutch can be used. In operation, the first generator would operate for all wind speeds over the minimum speed required to drive just the first generator. When the wind speed is below a minimum for using both generators, the clutch is not engaged and only the first generator is used. When the wind speed reaches a minimum for using both generators, the clutch is engaged to bring the second generator on line. 
         [0051]      FIG. 11  shows the arrangement of  FIG. 4 , but with clutches  1110 ,  1120 ,  1130  schematically illustrated between the first and second rotors  1110 , in the path between the first and second generator clusters  1120 , and between the third and fourth rotors  1130 . Three clutches are shown, but not all are necessarily required. They are included for exemplary purposes. Any one, any two, or all three clutches could be used, and additional clutches could be used as appropriate. It should be apparent that each clutch could be in any suitable location between, and that any suitable type of clutch can be used. For a clutch between the half shafts, a centrifugal clutch can be particularly advantageous. In operation with the three clutches shown, the first generator would operate for all wind speeds over the minimum speed required to drive just the first generator. When the wind speed is below a minimum for using both generators in the first cluster, the clutch  1110  is not engaged and only the first generator is used. When the wind speed reaches a minimum for using both generators in the first cluster, the clutch  1110  between the first and second rotors is engaged to bring the second generator on line. When the wind speed reaches a higher speed required to drive the first cluster and one of the generators from the second cluster, the clutch  1120  between the clusters can be engaged. And when a still higher wind speed required to drive all generators, the third clutch  1130  can be engaged.  FIG. 13 , showing a clutched version of  FIG. 6 , can operate in a very similar manner. 
         [0052]      FIG. 16  is illustrative of the ability to mix different types of generators in the multiple generator turbine of embodiments. The particular example shown combines the simple annular generator of  FIG. 1  on the left with the double-sided concentric generator of  FIG. 3  on the right.  FIG. 17  illustrates that clutches can be used in the mixed generator turbines of embodiments. The combination shown in  FIGS. 16 and 17  is an example of a combination that could be made. It should be apparent that other combinations of generator types, even within clusters, are within the scope of the invention. 
         [0053]    It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, it should be noted that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.