Abstract:
A vertical axis wind turbine ( 1 ) comprises three vertically extending sails ( 8 ) where each sail ( 8 ) comprises a strip ( 80 ) of substantially constant width. The opposite ends of each sail ( 8 ) are longitudinally twisted to have a pitch angle of approximately 90 degrees. The turbine ( 1 ) further comprises a vertically extending central core ( 7 ) and a vertically extending opening ( 9 ) between each sail ( 9 ) and the core ( 7 ). Also disclosed is an improvement in a vertical axis wind turbine ( 91 ) having at least one main blade ( 94 ) each of which has a longitudinal extent and a longitudinally extending radially outermost edge ( 924 ). The improvement comprises a longitudinally extending auxiliary blade ( 944 ) spaced from the main blade ( 94 ) to define a venturi inducing gap ( 99 ) between the main blade ( 94 ) and the auxiliary blade ( 944 ) whereby the turbine ( 91 ) has a zone of influence which extends radially beyond the maximum radial extent of the blades.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of PCT/AU2005/001621 filed Oct. 19, 2005 which in turn claims priority from Australian Applications 2004906051 filed Oct. 20, 2004, 2005900746 filed Feb. 17, 2005 and 2005902305, filed May 6, 2005 disclosures of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to wind turbines and, in particular, to vertical axis wind turbines. 
     BACKGROUND ART 
     It is known from the present applicant&#39;s earlier international patent application published under No. WO 03/058061, to provide a vertical axis wind turbine with three blades or sails spaced at an angle of substantially 120° which is able to provide a substantially constant torque output. In particular, the leading surface of the sails develops lift (in a manner similar to the headsail of a yacht) whilst the trailing surface of the blades develops drag (in the manner of the mainsail of a yacht sailing before the wind). 
     The present invention also relates to turbines including the Savonius turbine, the vertical axis turbine of WO 03/058061, and to the vertical axis wind turbine disclosed in Australian Patent Application No. 2004 906 051 (lodged 20 Oct. 2004 and presently at the priority date of the present application). In the two last mentioned specifications, the turbine has one or more stages and each stage has preferably three blades each of which functions in a manner somewhat akin to a sail so that an aerodynamic force is generated by the blades which rotates the turbine, irrespective of whether the blades are moving in the direction from which the wind is blowing or are moving into the direction from which the wind is blowing. This is explained in more detail in the abovementioned PCT specification. 
     In general, the larger the radial extent of the blades of the turbines, the greater the power able to be generated by the turbine. However, as the radial extent of the blades increases, so do the mechanical loads on the blades, the cost of construction, and like factors. 
     OBJECT OF THE INVENTION 
     The present invention has been devised in order to provide a turbine which is easy to fabricate and which provides various aerodynamic advantages. It is an object of preferred embodiments of the invention to provide a turbine of increased power for a given radial dimension. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is disclosed a vertical axis wind turbine comprising three vertically extending sails, each said sail comprising a strip of substantially constant width, opposite ends of each said sail being longitudinally twisted to have a pitch angle of approximately 90°, and said turbine further comprising a vertically extending central core and a vertically extending opening between each sail and the core. 
     In accordance with another aspect of the present invention there is disclosed in a vertical axis wind turbine having at least one main blade each of which has a longitudinal extent and a longitudinally extending radially outermost edge, the improvement comprising a longitudinally extending auxiliary blade spaced from said main blade and extending radially beyond said outermost edge to define a venturi inducing passage between said main blade and said auxiliary blade whereby said turbine has a zone of influence which extends radially beyond the maximum radial extent of said blades. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic perspective view of a two stage wind turbine in accordance with a first preferred embodiment of the present invention, 
         FIG. 2  is a side elevation of a planar untwisted strip from which a single sail is formed, 
         FIG. 3  is computer generated perspective view showing a single one of the sails of one stage of the turbine of  FIG. 1 , 
         FIG. 4  is a view similar to  FIG. 3  but showing all three of the sails of the single stage, 
         FIG. 5  is a schematic plan view of the three sails in a single stage of a second preferred embodiment, 
         FIG. 6  is a view similar to  FIG. 5  but showing the centres from which the curved upper edges and lower edges of the sails are generated, 
         FIG. 7  is a further plan view of the sails showing the projected area of the sails of the second embodiment, 
         FIG. 8  is a side elevation of a multi stage vertical axis wind turbine of the prior art (being a reproduction of  FIG. 13  of the abovementioned PCT specification), 
         FIG. 9  is a schematic perspective view of a blade of a wind turbine and having an auxiliary blade in accordance with a third preferred embodiment of the present invention, 
         FIG. 10  is a plan view of a vertical axis wind turbine incorporating auxiliary blades as illustrated in  FIG. 9 , 
         FIG. 11  is a partial plan view illustrating a pivoted auxiliary blade of a fourth preferred embodiment, and 
         FIG. 12  is a plan view of one blade only of the fourth preferred embodiment, 
         FIG. 13  is a plan view of one blade only of a turbine of a further embodiment, and 
         FIG. 14  repeats  FIG. 10  but for the further embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As seen in  FIG. 1 , the turbine  1  of the first preferred embodiment comprises two vertically arranged stages  2  and  3  which rotate about a vertical axis  4 . The upper stage  2  is substantially identical to the lower stage  3  but is rotated by substantially 60° about the vertical axis  4 . 
     Each stage  2 ,  3  has a pair of spaced apart substantially horizontal discs  6  and a central cylindrical core  7  ( FIG. 3 ) which is co-axial with the vertical axis  4 . In addition, each stage  2 ,  3  has three blades or sails  8  set at an angle of 120° around the vertical axis  4  and which are best illustrated in  FIGS. 3 and 4 . 
     With reference to  FIGS. 2 and 3 , it will be seen that each sail  8  is fabricated from a relatively thin planar strip  80  of approximately constant width and which is not rectangular. As seen in  FIG. 2  each edge of the strip  80  is curved when the strip  80  is in its planar condition. The strip  80  is able to be twisted about its longitudinal extent and also curved or warped into the desired shape for the sail  8 . In particular, each end of the strip  80  is curved to lie in a quarter ellipse (that is a portion of an ellipse extending between one major axis and one minor axis). 
     These two elliptical curves are then twisted so that the major axis of each part ellipse is substantially perpendicular to the major axis of the other part ellipse. This imparts a longitudinal twist to the sail  8  with a pitch angle of 90°. Each planar strip  80  includes a plurality of expansion elements  84 . The expansion elements  84  are cut or otherwise disposed through the strip  80  to allow for movement of the strip  80  in the plane thereof in response to twisting the strip  80 . 
     The expansion elements  84  shown in  FIG. 2  are in the form of shaped slots extending through the strip  80 . Once the strip as being twisted into position as described above, the expansion elements  84  can be filled in. 
       FIG. 3  illustrates a single sail  8  in its final position with the expansion elements  84  filled in. It follows from the above that the curved surface of each sail  8  is not a simple geometric shape such as a cylindrical surface. 
     As best seen in  FIG. 4 , each of the sails  8  extends between a lower disc  6  and an upper disc  6  but does not meet with the central core  7 . Thus there is a vertically extending gap or opening  9  between each of the sails  8  and the central core  7 . 
     The existence of the opening  9  between the sails  8  and the central core  7  constitutes a substantial difference between the arrangements of the present invention and the abovementioned prior art specification. In particular, the opening  9  permits air impinging on one sail  8  to be directed onto another so as to increase the rotational force created by wind flowing onto the stages  2 ,  3 . 
     The discs  6  are not essential and can be dispensed with if desired. However, the discs  6  do provide additional structural rigidity for the turbine. 
     Wind tunnel testing of a prototype indicates good power output and, in particular, as relatively high efficiency which is a substantially linear function of wind speed with efficiency increasing with increasing wind speed. 
     Turning now to  FIGS. 5-7 , the second embodiment is illustrated which has three sails  81 . The top edge  18  and bottom edge  28  of each of the sails  81  is illustrated in plan. Each of the top edge  18  and bottom edge  28  constitutes an arc of a circle (rather than an ellipse as in the first embodiment) which is drawn from a corresponding rotational centre  19 ,  29 . The positions of the centres  19  and  29  relative to the vertical axis  4  are as illustrated in  FIG. 6 . 
       FIG. 8  illustrates a prior art vertical axis wind turbine  91  mounted atop a tower  92 . The turbine  91  has two stages but in principle any number of stages from one upwards can be utilized. Each stage has three blades  94  which are aerodynamically shaped so as to be driven by the wind, irrespective of the wind direction. The greater the radial extent of the blades  94 , the greater the power of the wind turbine. 
     In  FIG. 9  there is shown a schematic perspective view of a blade of a wind turbine having an auxiliary blade in accordance with the third preferred embodiment. A single (main) blade  94  is schematically illustrated which extends radially from a central core  96 . Not indicated in  FIG. 9 , but illustrated and explained in the two abovementioned patent applications, there is preferably a 90° twist between the upper and lower edges of the blade  94 . As indicated by broken lines in  FIG. 9  there may or may not be a slot  97  between the radially inner portion  914  of the main blade  94  and the central core  96 . The benefits of the present invention are obtained irrespective of whether such a slot  97  is present or not. 
     The radially outer edge  924  of the main blade  94  conventionally determines the radial extent of the turbine. However, in the embodiment illustrated in  FIG. 10  an auxiliary blade  944  is provided which is spaced from the radial outer edge  924  and preferably extends radially beyond the radial outer edge  924 . The auxiliary blade  944  is inwards of the main blade  94 . 
     Turning now to  FIG. 10 , three main blades  94  each with an auxiliary blade  944  as illustrated in  FIG. 9  are illustrated in  FIG. 10  together with an indication of wind direction. The blades  94  and  944  at the two o&#39;clock position in  FIG. 10  are pointing directly into the wind and thus play no role this particular instant. The main blade  94  and auxiliary blade  944  at the five o&#39;clock position in  FIG. 10  are functioning in the manner of a sail which is sailing down wind in that the main blade  94  and auxiliary blade  944  are both catching the wind and being driven before the wind which is indicated by arrow A in  FIG. 10 . 
     However, the passage of the air past the gap  99  between the main blade  94  and the auxiliary blade  944  generates a venturi effect which draws additional air flows as indicated by arrows B and C in  FIG. 10  onto the rearward face of the main blade  94 . 
     Similarly, the wind incident on the leading face of the main blade  94  shown at the nine o&#39;clock position in  FIG. 10  generates lift on that main blade as it passes over the curved advancing surface. In addition, this movement of air past the auxiliary blade  944  also draws a flow indicated by arrow E in  FIG. 10  as a consequence of venturi effect into the gap  99  between the main blade  94  and the auxiliary blade  944 . This additional flow E also increases the forces applying to the main blade  94 . 
     It is particularly desirable there be the slot  97  between each of the main blades  94  and the central core  96  so that the combined flow A, B and C indicated in  FIG. 10  can pass through that slot  97  and thereby contribute to the flow E also illustrated in  FIG. 10 . 
     It will be apparent from  FIG. 10  that the actual radius of the turbine is that indicated as R 1  in  FIG. 10 , however, because of the venturi effect of the auxiliary blades  944 , the effective radius of the turbine is increased as indicated by dotted lines to an effective radius of R 2 . This means that the power of the turbine is much increased over what one would expect of a turbine of diameter R 1  formed from main blades  4  only. 
     Turning now to  FIG. 11 , a modified arrangement according to the fourth preferred embodiment is indicated in which the auxiliary blades  944  are pivoted as indicated by hinge  947 . In addition, adjacent the radially inner edge of the auxiliary blade  944  is positioned a spring  949  which resiliently biases the auxiliary blade  944 . The spring  949  is schematically illustrated in  FIG. 11  and can be either a tension spring or a compression spring depending upon the direction in which it is desired to pivot the auxiliary blade  944  with increasing wind velocity. Alternatively, the spring(s)  949  can be replaced by actuator(s) such as a pneumatic ram or an electro mechanical device which can be automatically controlled. 
     Turning now to  FIG. 12 , one blade only of a three blade stage of a two stage turbine is shown in plan view. The main blade  94  is twisted through 90° from its upper edge relative to the lower edge as explained in the abovementioned patent applications. The auxiliary blade  944  follows this twist and the slot  97  (marked AIR FLOW GAP in  FIG. 12 ) is also provided. In this embodiment the auxiliary blade  944  is fixed. The three blades are arranged with equal 120° spacing about the cylindrical central core  96 . 
     Finally, turning to  FIG. 13 , one blade of a turbine of a further embodiment is schematically illustrated in plan view of a top edge of the blade. The main blade central core  96  and slot  97  are as before. There are a number of connectors  917  of short longitudinal extent which connect the main blade  94  to the central core  96 , and one of these is illustrated in  FIG. 13 . 
     In particular in  FIG. 13 , the auxiliary blade  144  is illustrated and is located outwards of the main blade  94  (in contra distinction to the auxiliary blades  944  of  FIGS. 9-12  which are located inwards of the corresponding main blade  94 ). Again a venturi effect is achieved by the auxiliary blade  144  and this is illustrated in  FIG. 14  which is analogous to  FIG. 10 . 
     In  FIG. 13  the auxiliary blade  144  is pivoted at  145  and is movable by means of an electro-mechanical actuator  146 . The blade  144  is shown in three possible positions in  FIG. 13 , one without dots, a second position with closely spaced dots and a third position with widely spaced dots. 
     Thus, the auxiliary blades  944 ,  144  can be positioned either radially inwardly or outwardly of the edge  924  of the main blade  94 , if desired. Also the gap  99  whilst preferably of substantially uniform width can vary (for example by means of a tapering width). 
     The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the aerodynamic arts, can be made thereto without departing from the scope of the present invention. For example, the leading face of the main blade  94  can be polished whilst the trailing face of the main blade  94  can be roughened to thereby enhance the forces acting on the main blade  94 . 
     The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “including” or “having” and not in the exclusive sense of “consisting only of”.