Abstract:
The drive device is provided for inclusion in a windmill with two counter-rotating screws. The shafts of the screws are each connected to a spindle of an epicyclical gearbox, the third spindle providing the energy for the user. The device further comprises a brake device simultaneously acting on the two shafts of the screws. In particular, said device permits the installation of a second screw on a windmill mast designed for a single screw on a windmill mast designed for a single screw, thus greatly increasing the power supplied.

Description:
The content of Application PCT/CH2003/000625, filed Sep. 16, 2003, in Switzerland is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention concerns an electrical generator&#39;s drive device, or that of any other mechanism that requires energy from a wind turbine provided with two counter-rotative propellers. 
     PRIOR ART 
     To this date, it is still common practice to commercialize windmills equipped with only one set of blades. Theoretically, the aerodynamic efficiency yielded by a single propeller cannot exceed 59.6% (Betz, A., “Wind-Energie und Ihre Ausnützung durch Windmuelen”, van den Hoeck &amp; Ruprecht, Goettingen, 1926). In practice, the output is actually even weaker (between 40 and 45% in the best conditions). 
     An attractive solution for higher global output consists in a mechanism including two counter-rotative sets of blades. 
     Several documents offer methods or drive devices with two propellers. Schönball (U.S. Pat. No. 3,974,396/1976) was one of the first to describe a mechanism composed of one set of blades driving a rotor, and another one driving the stator of the generator. Moreover, he provided the possibility of combining the stators and rotors to regulate the generator&#39;s electric excitation, thereby affecting the speed at which the blades rotate. As far as the cinematic chain was concerned, this device included a mechanical link between the two sets of blades. 
     Another famous patent, U.S. Pat. No. 5,506,453/1996, by McCombs, develops a system comprising two sets of blades that propel the rotor and stator directly, similarly to what is mentioned above. Thanks to a mechanism that enables to tilt the blades, the amount of revolutions yielded by the system can be regulated. 
     Both Appa patents U.S. Pat. Nos. 6,127,739/2000 and 6,278,197B1/2001 are based on a concept close to McCombs&#39;, as far as the drive of the generator is concerned. What distinguishes them from the previous ones, however, is how the blades are built; following the Appa model, the blades comprise longitudinal canals through which air is drawn in towards the center of the rotation, and then expelled at their tips by centrifugal force. The direction of the air outflow is tangential to the circumference described by the tips of the blades. Such a design increases the propeller&#39;s aerodynamic power. 
     Generally, the various mechanisms described above are inconveniently linked to the generator through a slow transmission shaft. Consequently, for any given power, the generator&#39;s dimensions and mass will be considerably larger, therefore calling for highly resistant supporting poles. 
     SUMMARY OF THE INVENTION 
     One of the first goals of the present invention is to propose a drive device for an improved windmill composed of two counter-rotative sets of blades. First of all, its efficiency will be better than the output yielded by single propeller constructions. Secondly, the improved windmill will be able to connect to a smaller generator, comparable in size to the one generally required by a single propeller windmill. 
     Thus the new device is designed to suit already existing infrastructures: the second set of blades will be mounted either right in front of or right behind the first propeller. 
     A close study of the annex illustration exposes further advantages of the invention: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  indicates a general view of one side of the improved windmill, 
         FIG. 2  displays a longitudinal section of a embodiment of drive device, built according to the invention, 
         FIG. 3  provides a longitudinal section of another embodiment of drive device, built according to the invention, 
         FIG. 4  shows various relative settings of both propellers, viewed laterally, 
         FIG. 5  is a frontal view of various propeller shapes, and 
         FIG. 6  presents a transversal section of the braking system designed for the drive device proposed by the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  exposes a preferential display of counter-rotative propellers  10  and  11 , both mounted windward and in front of tubular mast  9 . The present invention involves setting the drive device and the generator inside nacelle  8 . 
     A first embodiment of the drive device is indicated in  FIG. 2 . The first set of blades  10  and its hub  26  are supported by shaft  12 , itself piloted by bearings  22  and  23  located inside hollow shaft  13 . The second set of blades  11  is supported by hollow shaft  13 , itself piloted by bearings  14  and  16 . These two bearings  14  and  16 , as well as portion  21   a  of the main armature of nacelle  8 , are interdependent. Shafts  12  and  13  are coupled with an epicycloidal multiplier, wherein shaft  12  is coupled with train of planetary wheels  17 , which in turn is linked to the epicycloidal multiplier; hollow shaft  13  is coupled with crown wheel  18  of the epicycloidal multiplier. The solar wheel activated by the epicycloidal multiplier is coupled with shaft  19 , which in turn drives generator  20  through coupling  24 . Since propellers  10  and  11  are counter-rotative, crown wheel  18  and train of planetary wheels  17  turn counter-rotatively as well. Power is therefore transmitted from shafts  12  and  13  to the generator shaft  19  through planetary gearings  27 ; these gearings are linked to train of planetary wheels  17  incorporated in the epicycloidal multiplier. 
       FIG. 3  provides a second embodiment of the drive device proposed by the invention. In this case, the epicycloidal multiplier is directly implanted in the hub of one of the propellers. Propeller  10  is piloted by bearings  22  and  23  and is linked to shaft  12  through train of planetary wheels  17 , while the train of wheels itself is implanted between the two bearings. Propeller  11  is directly mounted onto crown wheel  18 , which in turn is connected to shaft  13  operated by bearings  14  and  16 . Shaft  19  is linked on one side to the solar wheel, and on the other, to generator  20  through coupling  24 . 
     Ultimately, both drive models function in similar ways. 
     The speed at which shaft  19  operates generator  20 , as well as its drive couple are proportional to respectively the rotation speed and the couple yielded by propellers  10  and  11 , respectively mounted on shafts  12  and  13 . Propeller  10  and shaft  19  rotate in the same direction. 
     It will therefore be preferable that both propellers turn at the same speed. 
     Appropriate aerodynamic profiles cause the sets of blades to spin counter-rotatively. As far as the specific regulation of revolutions generated by the propellers is concerned, it may be achieved through already familiar solutions. 
     Since both sets of blades stand in the same air flow, measures may be taken to have them rotate at the same speed.  FIGS. 4 and 5  indicate various solutions in the propellers&#39; disposition and shape: these various aerodynamic adjustments will affect the speed and the couple yielded by each set of blades. 
     A first solution consists in using two identical propellers  10  and  11 , given that both have the same exterior diameter and number of blades; likewise, their rotation planes should be parallel, as indicated in  FIGS. 2 and 3 . 
       FIG. 4  provides a second alternative, in which the rotation planes are not parallel. In this example, the first propeller  100  rotates on a conical surface, the point of which is directed towards nacelle  8 . In another case, the point might aim in the opposite direction of the nacelle. Angle α, enclosed between the rotation surface and the plane perpendicular to the propeller&#39;s rotation plane, is generally below 10°. Preferably, angle α should be below 5°, indeed better still below 30. In a third instance, either propeller  11  alone rotates on a conical surface, or both propellers rotate on two conical surfaces. In the latter case, it is not absolutely necessary that both angles α be identical; one could conceive having two conical surfaces with points directed towards one another, or turned away from each other. 
       FIG. 4  exemplifies yet another alternative: the exterior diameter of the first propeller—represented here by its lower blade—may be different from that of the second propeller, preferably smaller. Such a difference in diameter may be applied to a pair of propellers rotating on two parallel planes, as well as to a pair of propellers of which one at least turns on a conical surface. 
       FIG. 5  provides various blade shapes: propeller  10 ,  11  or  100  may bear blades  101 ,  102  and  103 . Blade  101  is absolutely conventional: its axis is rectilinear and perpendicular to the rotation axis. Blade  102  has a curved axis. Likewise, blade  103  has a curved axis, but it is smaller in length to the blades on the second propeller. These blade shapes may be built on a propeller rotating either on a plane perpendicular to its rotation axis, or on a conical surface. All blades of any given propeller will obviously have the same shape and dimensions; however, blade shapes and dimensions may vary between two propellers of a same windmill. Alternative shapes and configurations can be envisaged to harmonize the speed and couple between both propellers. 
     Therefore, the choice of blade shapes and dimensions will depend on the wind regime the wind turbine undergoes; the ultimate goal will be to harmonize the couple and rotation speed between the two propellers. 
     Generally, windmills include an inbuilt braking device for their propellers. A windmill assembled with one of the drive devices described above may incorporate an ordinary braking mechanism. However, the following example is especially appropriate for the windmill model demonstrated above:  FIGS. 2 and 3  outline such a braking system, represented in  15  and developed in more detail in  FIG. 6 . 
     Following the first embodiment of drive device indicated in  FIG. 2 , braking mechanism  15  is designed to act simultaneously on coaxial shafts  12  and  13 , each bearing a propeller. The braking mechanism comprises two half-drums  31   a  and  31   b  built around hollow shaft  13  and acting on a plurality of tappets  30  located in the radial openings  130  arranged around shaft  13 . As implied in the plan, tappets  30  rotate as the wind turbine operates, driven by shaft  13 . Though the plan does not indicate them here, configurations permitting a loose working between tappets  30  and shaft  12 , as well as between tappets  30  and half-drums  31   a  and  31   b , do exist: unnecessary heating and friction loss are therefore avoided. 
     Activation devices such as hydraulic, pneumatic or electro-mechanical jacks ( 32   a ), or mechanical devices such as cam systems ( 32   b ), can be operated to draw half-drums  31   a  and  31   b  closer to each other: consequently, these will press against shaft  13  to slow it down, as well as press against tappets  30  that will, in turn, slow down shaft  12 . Locking systems outlined in  34   a  and  34   b  prevent half-drums  31  and  31   b  from rotating at locations  33   a ,  33   b ,  35   a  and  35   b  of the frame. 
     In this case, reaction due to the braking pressure applied by tappets  30  on shaft  12  is used by spacings  130  incorporated in shaft  13 . The final reaction on frames  35   a  and  35   b  of the nacelle will correspond to the difference between these two couples. This reaction will therefore be weaker than that of a single propeller braking device. 
     The amount of tappets  30  depends on technical parameters; preferably however, the tappets must be of pair number. 
     Instead of two half-drums  31   a  and  31   b , an alternative pressure device on the tappets of shaft  13  consists in elaborating a ribbon braking system. 
     Following the second embodiment of drive device, indicated in  FIG. 3 , the braking system will be designed to act on shafts  13  and  19  simultaneously, as described above. 
     The above description mentions that the wind turbine produces energy for a generator; it is quite obvious that the drive device put forward by the present invention is applicable to all windmills producing energy through torque for any type of industry fit to receive it. 
     A first advantage of the present invention is to propose a device that substantially enhances the aerodynamic efficiency of a wind turbine. This is exemplified by the possibility of extracting more energy out of a land or sea surface allocated to an wind farm. 
     Another advantage of the proposed invention is to work out a solution whereby power increase depends on higher relative rotation speed, and not on higher torque of the cinematic chain. Therefore, the dimensions of both the drive device and its generator remain similar to those of a single propeller drive device. Moreover, the multiplying coefficient of the speed multiplier can be divided by two (i.e. the previous multiplication, 20-30, now ranges between 10 and 15). This may be achieved by a simple planetary, one-level multiplier. 
     A further advantage consists in keeping the mechanical reaction of the drive device on the mast (tower) at an acceptable level, despite the power increase. Indeed, while the windward reaction increases, the reaction due to the torque of one of the propellers is absorbed by the other propeller; thus the system achieves almost total balance within the perpendicular plane facing the air flow. Another propeller can therefore simply be added to the wind turbine, originally built with a single propeller and a mast designed to support only one set of blades. 
     Yet another advantage is the braking system provided for the two propellers: it is indeed designed in such a way that the braking torque engendered by one propeller is compensated by the braking torque of the other. Thus the strain on the nacelle&#39;s mast is lighter than in the case of a single propeller wind turbine. 
     Still a further advantage consists in having the multiplier rotate around its own central axis, thereby enhancing the thermal exchange with the surrounding air. As a result, the cooling system may either be considerably reduced, or completely removed.