Abstract:
A wind turbine for converting wind energy into electric energy using a primary and secondary pivotable wind rotor. The rotors of the wind turbine have a horizontal and a vertical axis. The secondary rotor rotates independently from the wind, enabling a continuous production of electric energy, which is advantageous in that the turbine overcomes drawbacks in intermittent wind speeds.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is the 35 U.S.C. §371 national stage of PCT Application No. PCT/EP2012/069184, filed Sep. 28, 2012, which is herein incorporated by reference in its entirety and which also claims priority to, and the benefit of, U.S. Provisional Application Ser. No. 61/548,067, filed Sep. 30, 2011; and which also claims priority to, and the benefit of, Italian Patent Application No. RM2011A000516, filed Oct. 17, 2011, each of which are hereby incorporated by reference herein in their entirety. 
    
    
     BRIEF SUMMARY OF THE INVENTION 
     The present description refers to the technical field of the production of electric energy and in particular it concerns a wind turbine having horizontal axis. 
     Renewable energy sources are increasingly widely used for the production of electric energy. In the field of renewable energy, there is currently particular interest in the conversion of wind energy into electric energy. Such conversion takes place by means of suitable electromechanical machines, called wind turbines, capable of transforming the kinetic energy of the wind into electric energy ready to be entered into an electric network. It is possible to distinguish wind turbines of two different types, in particular wind turbines with vertical axis and wind turbines with horizontal axis. 
     Wind turbines with horizontal axis, currently more common than those with vertical axis, generally comprise a vertical support structure, an orientable nacelle pivotably hinged to the top of the vertical support structure, a wind rotor comprising a group of blades fixed to a hub, a rotary shaft connected to the hub and an electric alternator housed inside the nacelle and adapted to convert the rotational mechanical energy of the shaft into electric energy. The wind rotor through the effect of the wind intercepted by the group of blades is such as to rotate around a generally horizontal axis, or slightly inclined to an exactly horizontal axis, to set the rotary shaft in rotation. 
     In the production of electric energy, for wind turbines with horizontal axis there are generally two operating thresholds, minimum and maximum, respectively, linked to the wind speed. Indeed, below a minimum wind speed, for example if such a speed is below 3 m/s, the wind rotor remains stationary, or is held immobile, and in such a condition the wind turbine does not deliver electric energy. Moreover, if the wind speed exceeds a maximum threshold, for example if it is above 25 m/s, for safety reasons and to avoid damaging the wind turbine it is foreseen to forcibly lock the wind rotor. Also in this case no electric energy is delivered. Therefore, as a function of the wind speed, generally highly variable, wind turbines, in terms of the production of electric energy, have generally intermittent operation. On the other hand, the electronic management system of the wind turbine, generally arranged at the base of the vertical support, is generally always kept fed with power and in an operating state, and therefore if the wind rotor is immobile because the wind speed is outside of the operating range between the aforementioned minimum and maximum threshold, it is foreseen to draw electric energy from the network instead of supplying electric energy. The starting operations of the wind rotor generally also involve drawing energy from the electric network. 
     The intermittent operation described above represents probably the greatest drawback of a wind turbine with horizontal axis. 
     The purpose of the present description is to provide a wind turbine that is such as to at least partially avoid the drawback described above with reference to turbines of the prior art. 
     Such a purpose is achieved through a wind turbine as defined in general in claim  1 . Preferred and advantageous embodiments of the aforementioned wind turbine are defined in the attached dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become clearer from the following detailed description of a particular embodiment given as an example and, therefore, in no way limiting, in reference to the attached drawings, in which: 
         FIG. 1  shows an elevated side view of an embodiment of a wind turbine with horizontal axis comprising a nacelle, a primary wind rotor and a secondary wind rotor; 
         FIG. 2  shows an elevated side view of the nacelle and of the secondary wind rotor of the turbine of  FIG. 1 ; 
         FIG. 3  shows a plan view from above of the nacelle and of the secondary wind rotor of the turbine of  FIG. 1 ; and 
         FIG. 4  shows a perspective view of the nacelle and of the secondary wind rotor of the turbine of  FIG. 1 . 
     
    
    
     In the figures, elements that are the same or similar will be indicated with the same reference numerals. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the attached figures, a non-limiting embodiment of a wind turbine with horizontal axis is shown, globally indicated with  1 . 
     In accordance with an embodiment, without for this reason introducing any limitation, the wind turbine  1  is a so-called mini-wind turbine since it is able to develop an electric power below 200 kW, for example equal to about 50-60 kW. 
     The wind turbine  1  comprises a support tower  30  that in the example represented is fixed to a steelwork support base  32  and it is secured to it through a plurality of cables  31 , for example made of steel. The support base  32  is for example suitable for being buried so that an upper face thereof is flush with the level of the ground. 
     The wind turbine  1  also comprises a nacelle  2 , comprising a head portion  11  and a tail portion  12 . The nacelle  2  is fixed to the top of the support tower  30  and it is for example pivotably hinged to it, so as to be able to be oriented in a controllable manner, for example through a servomotor, not shown in the figures. In accordance with a preferred embodiment, the nacelle  2  comprises a lower base portion  20  and an upper shell  22 , for example shaped like a dome, fixed to the lower base portion  20 . Between the upper shell  22  and the lower base portion  20  a housing space is defined suitable for housing some of the mechanical, electrical and electromechanical components of the wind turbine  1 . 
     The wind turbine  1  comprises a primary wind rotor  3  pivotable with respect to the nacelle  2  around a primary rotational axis A 1  and comprising a primary group of blades  4 , a fastening hub  5  for said blades  4  projecting from the head portion  11  of the nacelle  2  and a shaft, not shown in the figures, operatively connected to the hub  5  and adapted for being rotatably moved by the primary wind rotor  3 . 
     In accordance with an embodiment, the primary group of blades  4  has just two blades  4 . 
     The primary rotational axis A 1  is a horizontal axis. This means that such an axis A 1  can be exactly horizontal or, like in the example represented, slightly inclined with respect to an exactly horizontal axis, for example inclined by about 5°. 
     The wind turbine  1  also comprises at least one primary electric generator comprising at least one primary electric stator integral with the nacelle  2  and a primary electric rotor integral with the shaft or operatively connected to it. The aforementioned components are not visible in the attached figures since they are housed inside the nacelle  2 . 
     The primary electric generator is such as to convert wind energy intercepted by the primary group of blades  4  into electric energy, and in particular into alternating current electric energy. 
     In accordance with an embodiment, the primary electric generator is a synchronous permanent magnet generator capable of developing a unitary nominal power of about 50 kW. 
     In accordance with a further embodiment, the primary electric generator includes two generators arranged aligned, for example each capable of developing a unitary nominal power of about 27 kW. 
     The wind turbine  1  also comprises an electrical box  35 , comprising an AC/DC/AC converter for the connection of the wind turbine  1  to an electric network into which the electric energy produced by the primary electric generator is inserted. From such an electric network the electric energy necessary for the operation of the wind turbine  1  is also optionally taken, for example to supply power to the electrical box  35 , to feed power to the actuators comprised in the wind turbine, for example for the actuator foreseen to controllably orient the nacelle in order to maximise/optimise the production of electric energy. 
     The AC/DC/AC converter is for example a static converter in back to back configuration and it is such as to make the alternating current electric energy supplied by the primary wind turbine compatible with the characteristics imposed by the electric network. In the electrical box  35  it is optionally possible to house the control electronics of the wind turbine  1 , intended to manage/control the operation of the wind turbine  1  itself and optionally to remotely collect and transmit status information of the wind turbine  1 . In a per se known way, the electrical box  35  is operatively connected to the nacelle  2  through electrical through cables through the support tower  30 . 
     The wind turbine  1  also comprises an auxiliary wind rotor  15  pivotably hinged to the tail portion  12  and comprising a secondary group of blades  25  pivotable around a secondary rotation axis A 2  perpendicular to the primary rotational axis A 1 . In accordance with a preferred embodiment, the secondary axis A 2  is arranged, with respect to the level of a surface for installing the wind turbine  1 , at a greater height with respect to the primary axis A 1 . According to a further embodiment, as is clearly shown in  FIGS. 1-3 , the secondary rotation axis A 2  is an horizontal axis. 
     In the particular example represented, the secondary wind rotor comprises, without for this reason introducing any limitation, a group made up of ten blades  25 . In accordance with an embodiment, such blades  25  are concave on one side and convex on the opposite side and are essentially spoon shaped. In the particular example represented in the figures, the blades  25  of the secondary group of blades also have a shape that in plan is essentially similar to a droplet or to a petal. The aforementioned blades  25  project from a cylindrical central hub  27  to which they are stably fixed. 
     In accordance with an embodiment, the tail portion  12  of the nacelle  2  comprises a fork  6  having two support arms  16 . For example, the fork  6  is fixed to the lower base portion  20  of the nacelle  2 . The auxiliary wind rotor  15 , and in particular its cylindrical central hub  27 , is pivotably hinged to the fork  6  being fixed between said support arms  16 . In accordance with an advantageous embodiment, each of the support arms  16  of the fork  6  is bent so as to have an elbow  26 , foreseen to decentre the secondary wind rotor with respect to the nacelle  2 . For example, such an elbow is such that each of the arms  26  is bent by an angle α equal to about 30°. 
     In accordance with a further embodiment, the secondary wind rotor  15  comprises a secondary electric rotor, for example contained inside the cylindrical central hub  27  and not visible in the figures. The wind turbine  1  comprises a secondary electric generator comprising the aforementioned secondary electric rotor and a secondary electric stator integral with the tail portion  12  of the nacelle  2  and not visible in the figures. The secondary electric stator is relatively inside the aforementioned secondary electric rotor, therefore arranged inside the cylindrical central hub  27 , and it is adapted to cooperate with the secondary electric rotor to convert the wind energy intercepted by the secondary wind rotor into electric energy. For example, the secondary electric generator is a permanent magnet generator. In accordance with an embodiment, such a secondary electric generator is capable of supplying electric energy with a nominal power of about 1 kW. 
     In accordance with an embodiment, the secondary electric generator is connected to the electric network, for example through the electrical box  35  itself. In this case, the AC/DC/AC converter comprised in the electrical box  35  has a first inlet adapted for receiving alternating current electric energy produced by the rotation of the primary wind rotor  3 , and therefore by the primary electric generator, and it has an auxiliary inlet adapted for receiving alternating current electric energy produced by the rotation of the secondary wind rotor  15  and therefore by the secondary electric generator. 
     With reference to  FIG. 1 , in accordance with an embodiment the nacelle  2  is orientable and when it is oriented so that the main axis A 1  is oriented along a prevailing direction of the wind W, the secondary axis A 2  is arranged with respect to the nacelle  2  at a height such that the nacelle  2  is such as to partially shield the secondary wind rotor  15  from the wind in an asymmetric manner with respect to the secondary axis A 2 . For example, it is possible, as represented in the attached figures, to foresee for the secondary wind rotor  15  to be arranged at a height such that half, or about half, of said secondary wind rotor  15  projects upwards above the nacelle  2 , or rather above the upper shell  22  of the nacelle  2 . 
     In operation, the system under the supervision of the control electronics operates so that the primary wind rotor  3 , like in wind turbines of the state of the art, can only rotate if the characteristics of the wind satisfy predetermined conditions, with particular reference to the minimum and maximum operating thresholds described above. The secondary wind rotor  15 , also called tail rotor, on the other hand having a smaller mass and involving less safety problems with respect to the primary wind rotor  3 , can be made to rotate independently from the characteristics of the wind, determining a continuous production of electric energy that at least counterbalances or partially compensates the electric energy taken from the network for the operation of the wind turbine  1  itself, with particular reference for example to the energy necessary for the orientation of the nacelle  2  or to the inrush energy necessary to set the blades  5  of the primary wind rotor  3  in motion, etc. Moreover, advantageously, the tail rotor  15  makes it possible to have the impression that in any case the wind turbine  1  is a machine that is always working and operative, even when the primary wind rotor  3  is stationary and this certainly contributes to improving opinion and the community with regard to this type of machines, at times criticised precisely because they are immobile and have a negative impact in terms of appearance and fitting into the landscape. 
     From the description that has just been made it is possible to understand how a wind turbine of the type described above achieves the predetermined purposes and therefore is capable of overcoming or at least minimising the drawbacks described above with reference to wind turbines of the prior art. 
     Of course, a man skilled in the art can bring numerous modifications and variants to the wind turbine described above, in order to satisfy contingent and specific requirements, all of which are in any case covered by the scope of protection of the invention, as defined by the following claims.