Patent Publication Number: US-2016230740-A1

Title: Vertical axis wind turbine rotor

Description:
TECHNICAL SECTOR 
     The invention relates to a vertical axis wind turbine rotor. More precisely, but not exclusively, the invention relates to a wind turbine rotor belonging to the category of small-size or mini wind turbines, with power outputs of between 10 and 100 kW. 
     PRIOR ART 
     The most important characteristics of surface areas used for wind power generation are the speed and direction of the wind and the duration and intensity of atmospheric events during the seasons of the year. 
     In general the number of recorded land sites where it is possible to install large-size wind turbines in optimum conditions, namely such that there is an advantage in economic terms, is relatively small. Some wind turbines, moreover, are sophisticated and costly machines and the number of sites where it is economically convenient to install this type of machine is very limited. Large-size wind turbines, moreover, give rise to problems of environmental impact and therefore cannot be installed in any location. 
     On the other hand, the installation of small-size wind turbines, owing also to the low production and operating cost, is economically advantageous in a geographically larger area. In particular, the installation of wind turbines, with a power output of between 10 and 100 kW, i.e. turbines belonging to the category of so-called mini wind turbines, allows an economically advantageous production of electric energy in zones where the use of more complex and larger-size turbines is not possible. 
     One of the problems which must be solved with regard to the design of a wind turbine belonging to the aforementioned class of mini wind turbines is that of causing the turbine to start when the wind speed is low. Experience and the known constructional designs which have been tested hitherto by manufacturers have shown in fact how it is extremely difficult, if not impossible, to start up a wind turbine of this category, both when the wind speed is low and when the wind direction is not optimum. 
     The present invention proposes overcoming the drawbacks of the prior art by providing a vertical axis wind turbine rotor which may be installed in a large number of sites with satisfactory results in economic terms. 
     Another object of the invention is to provide a wind rotor of the aforementioned type, which may be designed with dimensions such that it may be associated with wind turbines with power outputs of between 10 and 100 kW. 
     A further object of the invention is to provide a wind rotor which may be produced industrially at a low cost and which has low management and maintenance costs. 
     DESCRIPTION OF THE INVENTION 
     These and other objects are achieved with a wind rotor as claimed in the attached claims. 
     In a preferred embodiment of the invention, the wind rotor comprises a plurality of parallel rotor blades. The blades are arranged along the generatrices of a vertical cylinder. The blades are also connected to a central shaft passing along the axis of the cylinder. The connection between the blades and the shaft is performed by means of a radial-arm structure. The rotor is preferably made of metal, for example steel. The shaft of the rotor can be associated, using means known to the person skilled in the art, with a machine for converting the movement into electric energy, for example a dynamo or an alternator. According to the invention, each rotor blade of said plurality of blades comprises three elongated bodies arranged in cascade and having the same wing profile. Said elongated bodies are moreover arranged vertical and parallel to each other and to the central shaft. The elongated bodies are moreover preferably arranged in series with each other and staggered. 
     Still according to the invention, the wind rotor may be associated with a wind turbine comprising a support structure which is stably fixed to the ground and inside which a machine for converting the movement into electric energy is housed. The wind rotor is associated with the generator by means of the central shaft, which is stably connected to the vertical rotating shaft of the generator. The thrust of the wind causes rotation of the wind rotor and the rotatable shaft of the wind turbine, with the consequent production of energy by means of the machine for converting the movement into electric energy. 
     Still in accordance with the invention, the rotor comprises preferably from seven to fifteen fixed-pitch rotor blades. Each of said blades in turn comprises three elongated bodies arranged in cascade and having the same wing profile. According to the invention preferably the angular distance between the blades in the wind rotor is substantially equal to their overall transverse width. Advantageously, according to the preferred embodiment of the invention, a cascade effect corresponding to the fluid-dynamic condition which ensures a high energy conversion efficiency, in any direction and with any wind strength, is generated. 
     The wind rotor according to the invention advantageously envisages a very high elongation ratio of the aerodynamic profile used. In particular, according to the invention, said elongation ratio, which is defined by the ratio between the overall length of the profile of the elongated bodies and the transverse chord of the wing profile of the elongated body is greater than or equal to 30. 
     The series arrangement of three identical wing profiles staggered at an average distance of between 0.4 and 0.8 times the length of the chord moreover advantageously produces the desired fluid-dynamic effect, known as the cascade effect. 
     The deviation of the wind flow generates a vortical circulation which advantageously allows the transfer of a significant amount of movement to the rotor blades. The vortical nature of the air which circulates between the three wing profiles which form each rotor blade generates a resultant force tangential to the axis of the wind rotor which causes rotation thereof with a reasonable torque, even in the case of low wind speed values. 
     As a result of the geometric configuration of the blades and the characteristic design parameters, which are the result of numerous tests carried out systematically for a wide range of speeds and different flow directions, it has been possible to achieve high efficiency values for the wind rotor. 
     The absence of moving parts and directional mechanisms, as is instead required in the case of blades of conventional rotors with a directable pitch angle, results in the rotor according to the invention being extremely reliable. Moreover, the rotor according to the invention, essentially does not require maintenance, since the flow deviation of the wind is achieved statically. 
     The advantage derived from the static condition of the blades consists not only in the greater operating simplicity of the wind machine, but also in the significant reduction of industrial manufacturing costs and the greater degree of standardization, thus making this type of wind machine more competitive than the other types of generators. 
     The wind rotor according to the invention is advantageously capable of being started also at very low wind speeds, for example of the order 1 m/s and therefore is substantially self-starting. 
    
    
     
       SHORT DESCRIPTION OF THE FIGURES 
       A number of preferred embodiments of the invention will be described below with reference to the accompanying figures in which: 
         FIG. 1  is a perspective view of the rotor according to the invention associated with a wind turbine; 
         FIG. 2  is a perspective view of a blade of the rotor according to  FIG. 1 , separate from the rotor; 
         FIG. 3  is a view, on a larger scale, of a portion of the blade shown in  FIG. 2 ; 
         FIG. 4  is a view, on a larger scale, of a portion of the rotor blade shown in  FIG. 3  fixed to the rotor; 
         FIGS. 5A and 5B  are diagrammatic cross-sectional views of a blade and an elongated body of the rotor according to the invention, respectively; 
         FIG. 6  is a schematic graph of the average curvature of the profile of the elongated bodies of the rotor according to the invention. 
     
    
    
     DESCRIPTION OF A NUMBER OF PREFERRED EMBODIMENTS OF THE INVENTION 
     With reference to  FIG. 1 , this shows the rotor  11  of a vertical axis wind turbine according to a preferred embodiment of the invention. The rotor  11  comprises a plurality of rotor blades  13 . The blades  13  are arranged parallel along the generatrices of a vertical cylinder defined by the rotor  11 . The blades  13  are connected by means of a radial-arm structure  15  to a central shaft  17  passing along the axis of the cylinder. The central shaft  17  is associated with the rotating shaft  19  of a machine  21  for converting the movement into electric energy. The machine  21  is housed inside a support structure  23  which is able to be stably fixed to the ground. According to the invention each rotor blade  13  of said plurality of blades comprises three elongated bodies  25  arranged in cascade, vertical and parallel to each other and to the central shaft. 
     With reference also to  FIGS. 2 and 3 , the elongated bodies  25  are preferably formed as hollow metal profiles. Moreover, according to invention, the elongated bodies  25  have the same wing profile. The three elongated bodies  25  which define each rotor blade  13  are held together by pairs of plates  27 . The plates  27  are preferably flat and are arranged transverse to the elongated bodies  25 . The plates  27  comprise openings  29  for receiving the elongated bodies  25 . Three pairs of parallel and equidistant plates  27  are preferably provided for each blade. Owing to the plates  27  the elongated bodies  25  are kept parallel at the desired distance from each other. Furthermore, the plates  27  allow the blade  13  to be connected to the structure  15  of the rotor  11 . The plates  27  are provided with openings  31   a  and holes  31   b  for fixing the plates  27  to the radial-arm structure  15 . 
     As can be seen more clearly from  FIG. 4 , the radial-arm structure  15  comprises a plurality of tie-rods  33  which connect the plates  27  to the central shaft  17  of the rotor  11 . At least one of these tie-rods is provided for each pair of plates  27 , said tie-rod engaging between the two plates  27  with a vertical bracket  35  joined to a coupling piece  37 . In the example shown, the tie-rods  33  are two in number per pair of upper plates  27  and one for the other two pairs of plates  27 . The vertical bracket  35  is engaged between the plates  27  via the openings  31   a.  The holes  31   b  in the plates  27  allow fixing of the vertical bracket  35  by means of bolts  39  passing through said holes  31   b . Reinforcing tie-rods  41  connect the coupling piece  37  of one blade  13  to that of the adjacent blade  13  in both the angular directions. 
       FIGS. 5A and 5B  show the cross-section of the cascade of wing profiles of the elongated bodies  25  which form the single rotor blade  13 . According to the invention the transverse chord C of the wing profile of the elongated body  25  and the pitch H of the cascade are chosen so that the ratio H/C is between 0.2 and 1.2 and preferably equal to 0.4. 
     Furthermore, still according to the invention, the cascade angle α is preferably between 15° and 60° and even more preferably is chosen close to 30°. 
     Still according to the invention, the overall blade chord C T , equal to the distance between the centre of rotation O of the shaft  17  and the point M of connection of the blades  13  to the structure  15 , the number of blades N P  and the diameter D R  of the rotor are chosen so that the ratio D R /(N P ×C T ) is between 0.5 and 1.0. With this configuration advantageously it is possible to obtain a good efficiency of the wind turbine provided with the rotor  11 . The best results are obtained, however, for values of said ratio equal to about 0.7. 
     According to the invention, the trailing edge profile of the elongated body  24  is flat so as to emphasize the cascade effect and minimize the overall wake resistance of the blade  13 . This configuration furthermore improves significantly the fluid-dynamic efficiency of the wind rotor  11  according to the invention. 
     Still with reference to  FIGS. 5A and 5B , the wing profile of each elongated body  25  is chosen so that the ratio C U /C P  between the length C U  of the thin part  25   a  and the length C P  of the profiled part  25   b  of the wing profile of the elongated body  25  is between 0.1 and 0.5 and preferably equal to about 0.4. 
     The ratio λ=V P /V between the peripheral speed V P  of the blades (V P =C T ×Q, where Ω is the angular velocity of the rotor) and that V of the wind in the direction of the arrow F, which ensures optimum performances and the greatest power of the rotating shaft, is obtained for λ ranging between 0.7 and 1.1. This value is also a function of the instantaneous turbulence index of the wind. 
     With reference to  FIG. 6 , still according to the invention, the average curvature of the wing profile of the elongated bodies is considerable and determines the geometric thin-blade condition with a high directional variation between the entry angle of the flow α 1  and the exit angle α 2 . 
     The wind rotor as described and illustrated may be subject to numerous variations and modifications which fall within the same inventive principle.