WO2005100782A1

WO2005100782A1 - Vertical axis wind turbine

Info

Publication number: WO2005100782A1
Application number: PCT/GB2005/001510
Authority: WO
Inventors: John Edward Mullenger
Original assignee: Posh Power Limited
Priority date: 2004-04-19
Filing date: 2005-04-19
Publication date: 2005-10-27
Also published as: GB0408645D0

Abstract

A wind turbine has two closely juxtaposed rotors (22, 23) mounted for rotation about a common vertical shaft (21), the two rotors being similar but mirror images of each other. Each rotor (22, 23) has a set of vanes (25) extending generally radially so that the vane of one rotor co-operates with a corresponding vane of the other rotor. A coupling (Figure 18) interconnects the two rotors so that the relative axial or rotational position of one rotor (22) with respect to the other (23) may be adjusted, thereby adjusting the degree of co-operation of the vanes of the two rotors and so adjusting the efficiency of the turbine in extracting energy from the wind.

Description

VERTICAL AXIS WIND TURBINE

This invention relates to a wind machine. In particular, though not exclusively, this invention relates to a vertical-axis wind turbine adapted to extract energy from the wind, for example for driving an electricity generator or pumping water. Wind machines in the form of windmills of various kinds have been known for very many years, for the purpose of extracting energy from the wind. Typically, such windmills have a rotor provided with a number of sails or blades (both of which are referred to hereinafter simply as sails) mounted for rotation about a generally horizontal axis, such that the rotor will rotate when there is a sufficient wind strength to drive the load coupled to the rotor. With a horizontal axis windmill, some kind of wind-sensing mechanism must be provided to allow the sails to face into the wind and this significantly increases the complexity of the windmill. In an attempt to address the above problem, it is known to have a vertical axis windmill where the rotor is arranged with its axis vertical and is provided with sails or other kinds of wind collectors to extract energy from the wind. With a vertical axis windmill, no adjustment of the rotor orientation is required, which leads to a significant reduction in complexity. Despite this, there are still problems associated with providing a mounting mast of sufficient strength for carrying the rotor, as the rotor cannot be turned away from the wind if the wind strength increases too much. With all kinds of windmill, measures must be taken in order to de-power the rotor when the wind strength increases above design limits, in order to prevent catastrophic failure of the structure. In the case of a horizontal axis windmill, the pitch of the sails may be changed or the sails may be moved to a position where they are more or less parallel to the wind direction so as no longer to be rotated by the wind. With a vertical axis windmill, other measures must be taken since one aspect of the rotor will always be facing the direction of maximum wind strength, in view of the symmetry of the rotor. For example, a powerful braking system may be provided for a vertical axis rotor, in order to prevent rotation when the wind strength rises above a preset value. There is a further problem with a vertical axis turbine having a rotor with a number of sails or equivalent wind collectors extending generally radially outwardly from a central hub. It is found that the number of individual sails which may be provided is severely restricted - and typically to only three - else any one sail will be masked from the wind by the next following sail on the rotor. As such, the efficiency of a vertical axis windmill may be severely restricted as compared to the maximum energy which could be extracted. It is a principal aim of the present invention to address at least some of the above problems associated with vertical axis windmills and so provide a structure which is able to operate reliably and with good efficiency, for a wide range of wind strengths. According to this invention, there is provided a wind turbine having two closely juxtaposed rotors mounted for rotation about a common vertical axis, the two rotors being similar but mirror images of each other and each having a set of vanes extending generally radially so that each vane of one rotor cooperates with a corresponding vane of the other rotor thereby to extract energy from air flowing past the turbine at substantially 90° to the common axis of the rotors, adjustment means being provided to control the efficiency of the turbine which adjustment means operates by adjusting the position of one rotor with respect to the other, thereby adjusting the degree of co-operation of the vanes of one rotor with respect to those of the other. It will be appreciated that this invention allows for the de-powering of a vertical axis wind turbine having a pair of rotors arranged one above the other, by adjusting the relative position of one rotor with respect to the other. This may be achieved by performing relative rotational adjustment between the rotors, or relative axial adjustment of the rotors. In the normal setting of the rotors, the lower surface of the upper rotor may be closely adjacent to the upper surface of the lower rotor and the vanes of the two rotors should be in alignment with each other. Adjustment is then effected either by turning one rotor - and typically through not more than ±45° - with respect to the other, or by lifting the upper rotor with respect to the lower, or vice versa. In a self-regulating form of the invention where there is rotational adjustment of the rotors, the adjustment means includes a resilient coupling interconnecting the two rotors, whereby the phase of one rotor may be changed with respect to the other against the action of the resilient bias provided by the coupling. Depending upon the wind loading of the overall turbine, there will tend to be automatic regulation of the wind spill as one rotor moves with respect to the other under the action of the wind load, while both rotors are rotating about their common axis to drive a load. Another possibility is to provide separate loads for each rotor, and to adjust the coupling between the loads in order to effect the adjustment of the relative alignment of the rotors. In the case of electrical power generation where the loads comprise generators, this may be achieved electrically. In an alternative form of this invention, power means are provided to effect adjustment of the rotors. Such power means may continuously monitor the power withdrawn from the rotors, for example by powering a load, as well as the wind strength, the power means then operating to control the relative positions of the two rotors. In a preferred form of this invention, the vanes of the rotors are oppositely arranged whereby each vane on one rotor when aligned with the corresponding vane on the other rotor defines a V-shaped wind collector. That collector may be disposed at least partially if not wholly within a pair of concentrator panels, also arranged one on each rotor respectively so as to define a V-shape. Thus, there will be a generally V-shaped space between each pair of vanes and the associated pair of concentrator panels through which the wind may be funnelled during operation of the turbine. However, by effecting relative adjustment of the rotors in order to de-power the turbine, that V-shaped space will be opened, so allowing wind to spill from the turbine and from the V-shaped space in particular, so reducing the power being extracted from the wind. By way of example only, one specific embodiment of wind turbine and certain variations thereof all constructed and arranged in accordance with this invention will now be described in detail, reference being made to the accompanying drawings in which:- Figure 1 is an isometric view of the embodiment of wind turbine, from one side and slightly from above; Figure 2A is an isometric view of the upper rotor of the turbine of Figure 1 , from one side and above; Figure 2B is an isometric view of the lower rotor of the turbine of Figure 1 , again from one side and above; Figure 3 is an isometric view from one side and above of the upper rotor of Figure 2A, but with the covering removed for clarity; Figure 4 is a plan view of the rotor of Figure 3; Figure 5 is a plan view on a vane as used in each rotor, together with sections taken on lines A-A, B-B, C-C and D-D, as marked on Figure 5; Figure 6 is a plan view on a concentrator as used in each rotor, together with sections taken on lines A-A, B-B, C-C and D-D, as marked on Figure 6; Figures 7A and 7B respectively show the entrance and exit apertures to the vane and concentrator arrangement as used in the rotor; Figure 8 is a diagrammatic plan view of the rotor, showing airflow paths therethrough; Figures 9A to 9D respectively show the airflow paths within four zones of the rotor; Figure 10 illustrates the separation of the upper rotor from the lower rotor in order to de-power the turbine; Figures 11 A and 11B diagrammatically illustrate a vane and concentrator assembly for the upper and lower rotors, respectively when in the normal position and when separated for de-powering; . Figure 12 corresponds to Figure 1 , but illustrates the upper rotor separated from the lower rotor for de-powering; Figure 13 illustrates the rotation of the upper rotor relative to the lower rotor in order to de-power the turbine; Figures 14A and 14B diagrammatically illustrate a vane and concentrator assembly for the upper and lower rotors, respectively when in the normal position and when adjusted rotationally for de-powering; Figure 15 corresponds to Figure 1 but illustrates the upper rotor rotated relatively with respect to the lower rotor for de-powering; Figure 16 diagrammatically illustrates spring-loading of the upper rotor in order to provide automatic relative adjustment as shown in Figures 13 to 15, for de-powering the turbine; Figure 17 diagrammatically illustrates an electrical load control arrangement for providing relative rotation between the rotors as shown in Figures 13 to 15; and Figure 18 diagrammatically illustrates an alternative coupling arrangement for the two rotors of a different form of turbine. Figure 1 diagrammatically illustrates a wind turbine as an embodiment of this invention. This turbine comprises a mast 20 extending vertically and having a shaft 21 at its upper end. As shown in further detail in Figures 2A, 2B, 3 and 4, rotatably mounted on the shaft are upper and lower rotors 22,23, each constructed from a plurality of frame members 24 suitably joined together for example by welding. An impervious covering material is furnished over certain of those members, in order to provide, for each rotor, four vanes 25 and four concentrator panels 26, referred to hereinafter simply as "concentrators". The lower rotor 23 is a mirror image of the upper rotor 22 such that when the two rotors are fitted together as shown in Figure 1 , there are four pairs of vanes 25 and four pairs of concentrators 26. Each pair of concentrators 26 defines a generally V-shaped space therebetween and located within that space is a corresponding pair of vanes 25. As shown in Figure 1 , the trailing edge 27 of each concentrator 26 is common with the trailing edge of the corresponding vane 25 lying within the space defined by the pair of concentrators. Also as shown in Figure 1 , the overall turbine has four essentially identical assemblies of pairs of vanes 25 and concentrators 26, equi-spaced around the axis of the shaft 21. As will be appreciated, wind may be incident upon the turbine from any generally horizontal direction and the turbine will start to turn, as the wind impinges on the vanes 25 facing the airflow. The arrangement of the vanes and concentrators is such that the effect of that wind is enhanced as compared to a simple vane or other wind-collector turbine, such that the power output of the turbine may be much increased for a given turbine size and speed of rotation. The arrangement of the turbine will now be described in further detail. Each rotor 22,23 includes a central frame section 28 of a cuboid shape, extending along the centre of which is a bearing tube 29 arranged for location on the shaft 21. In the case of the upper plane of the upper rotor 22 and the lower plane of the lower rotor 23, the frame members defining the edges of the square end face of the frame section 28 are extended laterally so as to define the trailing edges 27 of the vanes 25 and concentrators 26. In the case of the lower plane of the upper rotor 22 and the upper plane of the lower rotor 23, the frame members defining diagonals of the square end face of the frame section 28 are extended radially so as to define, respectively, each outer corner 30 of the leading edges 31 of the concentrators 26. The inner corner 32 of each concentrator leading edge 31 is defined by other frame members connected back to the central frame section 28. The outer edge of each concentrator is defined by an outer frame member 33 extending generally peripherally of the frame. The frame members defining the edges of the square end face of the central frame section 28 at the lower plane of the upper rotor 22 and at the upper plane of the lower rotor 23 are extended laterally so as to define in conjunction with respective other frame members the leading inner corners 34 (Figure 3) of the vanes 25. One of those frame members defines the leading edge 36 of the respective vane 25 and the outer corner 37 thereof is at the outer end of that frame member, to which the outer side members 38 of the vanes are connected. As will be appreciated, the upper rotor and the lower rotor are symmetrical but are mirror images of each other. Each provides four essentially identical sections, each having a vane 25 and a concentrator 26. Then, when the two rotors are fitted together as shown in Figure 1 , the leading edges of the vanes and concentrators may be aligned so defining the V-shaped pairs of vanes and concentrators. Though not shown in the drawings, the square end face at the upper end of the central frame section 28 of the upper rotor 22 the lower end face at the lower end of the central frame section 28 of the lower rotor 22 may both be covered with an impervious sheet in order to constrain the air flows within the rotor when in use, as will be described below. The covering material furnished on the frame members to provide the vane and concentrator surfaces may be a metallic sheet, such as an aluminium alloy, a plastics material or even a fabric, suitably treated. Figure 5 shows in plan view one vane 25, together with horizontal cross-sections through that vane, taken on lines A-A, B-B, C-C and D-D, marked on the vane shown in Figure 5. As can be seen, at any given section line, the vane has an essentially linear cross-section but the angle of incidence to the horizontal of that cross- section varies, from a maximum at the outer end of the vane to a minimum at the inner end of the vane. As such, the vane will take up a curved profile. Further, and again as best appreciated from Figure 5, the length of the vane in the rotational direction E of the rotor increases from a minimum at the outer end of the vane to a maximum at the inner end of the vane. Figure 6 shows in plan view one concentrator 26 together with horizontal cross-sections through that concentrator, taken on lines A-A, B-B, C-C and D- D, marked on the concentrator shown in Figure 6. As can be seen, at any given section line, the concentrator has an essentially linear cross-section and the angle of incidence to the horizontal of that cross-section is constant. Thus, the concentrator is essentially planar. Further, and as best appreciated from Figure 6, the length of the concentrator in the rotational direction E of the rotor is constant. The arrangement of vanes and concentrators as described above gives rise to a generally V-shaped space between a pair of concentrators and a pair of vanes, disposed respectively on the upper and lower rotors when fitted together in alignment with each other. The entrance to that generally V-shaped space at the outer periphery of the turbine is of a relatively large cross-sectional area as shown in Figure 7A. By contrast, at the inner end of that space, the exit therefrom is of relatively small cross-sectional area, as shown in Figure 7B. Figures 8 and 9A to 9D show airflow paths through the turbine when the upper and lower rotors 22,23 are fitted together, in alignment with each other, and the turbine is subjected to incident wind. For the purpose of this discussion, it is presumed the wind is incident upon the rotor generally parallel to the trailing edges 27 of a pair of vanes 25 and a pair of concentrators 26, as shown in Figure 8. For convenience, the pair of vanes and the pair of concentrators upon which the wind is first incident is referred to as Zone A of the turbine and each successive pair in the direction of rotation F of the turbine is referred to Zone B, Zone C and Zone D. Further, in this description, the term "behind" is used to refer to the trailing side of a vane, concentrator or edge and "ahead" is used to refer to the leading side of a vane, concentrator or edge, having regard to the direction of rotation F of the turbine. As shown in Figure 8, the incident wind is identified by arrows 1. Some of that wind is collected behind the pair of vanes 25 in Zone A to impart drive to the rotor, whereas more of that wind enters into the reducing generally V- shaped space ahead of the pair of vanes 25 in Zone A but behind the leading edge of the pair of concentrators 26. As described above, the area of that V- shaped space reduces in the generally inward direction and so the speed of the air entering the space increases the deeper that air moves into that space, so applying drive to the turbine in the direction of arrow F. Ahead of the leading edge 31 of the pair of concentrators, the wind is also incident on the trailing surfaces behind the vanes 25 of the pair thereof in Zone B, so applying drive to the rotor in the direction of arrow F. On striking those trailing surfaces, some of the wind will be spilled outwardly but much will be turned inwardly to run along the trailing surfaces of those vanes, as shown by arrow 4. The air which has entered the V-shaped space (arrows 2) will leave that space as shown by arrow 3 but at a significantly higher speed than the wind entering the space. That air is then incident on the trailing surfaces behind the vanes of Zone B, imparting more drive on the turbine in the direction of arrow F. That air mixes with the air which has been turned inwardly to run along the trailing surfaces of the vanes, as mentioned above. That mixed air continues along the trailing surfaces of the vanes of Zone B (arrow 4) and then beyond the vanes 25 of Zone B (arrow 5) to be incident upon the trailing surfaces of vanes 25 of Zone C, imparting further drive on the turbine in the direction of arrow F. As the air is incident upon those surfaces more or less at right-angles, some of the air will be turned to run along the trailing surfaces of those vanes towards Zone D (arrow 6), whereas other air will be turned outwardly as shown by arrow 7. However, all of the air shown by arrow 5 imparts drive to the vanes of Zone C, so assisting turning of the rotor in the direction of arrow F. Finally, the air which is turned towards Zone D (arrow 6) is incident upon the trailing surfaces of the vanes 25 of Zone D imparting yet further drive to the turbine, to assist turning it in the direction of arrow F. That air finally leaves the turbine as shown by the extended arrow 6, in the outward direction. Figures 9A to 9D show the airflow paths in the various zones as described above. In Figure 9A, the increase in the speed of the air within the V-shaped space of Zone A is illustrated and in Figure 9B there is shown the air impinging on the trailing surfaces of the vanes 25. As the V-shaped space between the pairs of vanes and pairs of concentrators in Zone B are more or less at right-angles to the wind direction, there is essentially stagnant air within that space. Figure 9C shows the air of arrow 5 incident upon the trailing surfaces of the vanes of Zone C with some of the air turned outwardly to leave the turbine (arrow 7) and other air turned towards Zone D (arrow 6). Finally, Figure 9D shows Zone D, where the leading edge 31 of the concentrators faces the wind direction. As such, there will be a low pressure region behind the vanes in Zone D; this increases the extraction of the air from Zone C (arrow 6). The turbine arrangement described above is intended to rotate relatively slowly, in order to allow time for the airflows to develop as explained above. For example, a typical turbine of this invention might have a diameter of several metres and possibly even as much as twenty metres, and may rotate as slowly as 20 rpm. Thus, the turbine may operate effectively at relatively low wind speeds, but then there could arise a problem if the wind speed should rise significantly so endangering the structure. To allow for this, measures are taken for de-powering the turbine, in strong wind conditions. Figures 10 to 2 illustrate one possible way in which the turbine may be de-powered. As shown in Figure 10, the upper rotor 22 may be separated from the lower rotor 23 by a mechanism arranged to lift the upper rotor, so producing a gap d between the two rotors. Figure 11A shows the driving of the turbine when in its normal condition with the two rotors immediately adjacent to each other and in alignment, Figure 11A corresponding to Figure 9B showing Zone B. Figure 11 B shows the rotor separated; as can be seen, there is a clear path 42 for air between the rotors. By virtue of that airflow through the rotors, air is also encouraged to flow through the V-shaped space between the rotors into the path 42, so removing the airflow of arrows 2 (Figures 8 and 9A). As such, the air will pass cleanly through the rotor. Of course, there may be a measure of control over the de-powering of the rotor by controlling separation; in slight over-wind conditions, the upper rotor may be lifted by a relatively small amount but in strong over-wind conditions, complete de-powering may be achieved by lifting the upper rotor through a significant distance. Figures 13 to 15 show an alternative technique for de-powering the turbine. Here, the upper rotor 22 is rotated relative to the lower rotor 23 through a controlled angle (arrow G), again so as to provide an airflow path 43 through the turbine (Figure 14B). As before, this path allows air to flow out of the V- shaped space, without that air being directed on to the next zone to add to the drive. Control over the degree of de-powering of the turbine by controlling the angular rotation of the upper rotor with respect to the lower rotor. Figures 16 and 17 diagrammatically show two mechanisms which may be used to control the relative rotation of the upper rotor with respect to the lower rotor, in order to achieve de-powering of the turbine in strong wind conditions. In both of these arrangements, one rotor is secured to shaft 21 , whereas the other rotor is rotatably supported on the shaft 21 , in order that the relative angular orientation of the rotors might be adjusted. In these figures, like parts with those of the previous figures are given like reference numbers and will not be described again here. Referring initially to Figure 16, the upper end 50 of shaft 21 carries a pair of arms 51 which are connected to frame members 53 of the upper rotor 22 by means of springs 52, extending generally in the circular direction. For convenience, those springs are shown connected to certain frame members of the upper rotor but the upper rotor may be provided with mounting points specifically adapted for receiving the springs. Instead of helical coil springs as shown, pneumatic ram springs or some other resilient arrangement could be employed. Below the mast 20, the shaft 21 is drivingly connected to an electrical generator 54 by means of a step-up toothed belt drive 55, the generator 54 being mounted on a support 56 for the mast 20. The support is bolted or otherwise secured to the ground or some other structure such as the roof of a building in a suitable manner, having regard to the anticipated loads. As the wind speed increases, so too will the rate of rotation of the turbine and in turn the power output of the generator 54. With increasing power extracted from the wind, the upper rotor will tend to move in advance of the lower rotor, so starting to generate the wind de-powering effect as described above with reference to Figures 13 to 15. Once the wind strength has increased above a pre-set value, de-powering will start to occur and the stronger the wind, the greater will be the de-powering as the upper rotor turns against the spring bias through a greater extent. Though not shown, a damper may be provided between the upper and lower rotors, to prevent the upper rotor cycling with respect to the lower rotor and to assist in maintaining stability of the feedback loop. In the arrangement of Figure 17, the upper rotor 22 is secured to the shaft 21 and the lower rotor 23 is rotatably mounted thereon. The shaft drives a generator 54 as described above with reference to Figure 16 but the generator 54 is here driven by the upper rotor 22, rather than the lower rotor 23.

A second generator 57 is secured to the mast 20 and is driven by a step-up toothed belt drive 58 from the lower rotor 23. A limit stop arrangement is provided to limit the maximum angular rotation of one rotor with respect to the other. In Figure 17, this stop arrangement is shown diagrammatically by a pair of abutments 59 on the lower rotor and a projecting peg 60 on the upper rotor, engageable with the abutments 59 at the limits of relative angular rotation, of approximately ±45°. As with the previous arrangement, a damper may be provided between the two rotors in order to prevent cycling, when in operation. In normal operation the electrical outputs of the generators 54,57 will be the same as both generators will be driven at the same speed and they are connected in series to a common load L. However, the balance between the two generators may be varied by a switch SW1 , arranged to connect across a selected generator 54 or 57 an additional load LB. If for example the load is connected across the output of generator 57, a greater torque will be required to rotate generator 57 than generator 54 and so the lower rotor will lag behind the upper rotor until the limit stops are engaged. In this condition, de-powering of the overall assembly is taking place as the upper rotor will have moved in advance of the lower rotor, by a maximum of 45° as defined by the limit stops. In practice, switch SW1 will be electronically or computer controlled, with the load LB switched in and out of circuit at an appropriate frequency to control the relative movement of one rotor with respect to the other about the axis of rotation. In this way, precise control of the de-powering of the turbine can be achieved. Figure 18 illustrates a different coupling arrangement for the two rotors of an alternative design of turbine of this invention. The details of the rotors are not here illustrated but the rotors have respective hubs 65,66 carried on a common shaft 67, the lower hub 66 being rigidly connected to that shaft but the upper hub being free to rotate about the shaft. A side wall of the upper hub 65 has been cut away to show the internal details of the coupling between the upper hub and the shaft. The shaft 67 is directly connected to the lower hub 66, the shaft 21 extending upwardly into the upper hub 65 wherein the shaft extends through a lower bearing (not shown) in a lower end plate of the upper hub and a further bearing 68 carried on a plate 69 within the upper hub. A pair of crank arms 70 is secured to the shaft 67 within the upper hub, a connecting rod 71 being pivoted to those arms and extending out of the hub and through the base of a U-shaped bracket 72. A pair of arms 73 is attached to the lower plate of the upper hub 65 and to plate 69, so as to project radially from the hub. The U-shaped bracket 72 is pivoted to the radially outer ends of those arms 73, by a spindle 74. A helical compression spring 75 is provided on the connecting rod 71, between the base of the U-shaped bracket 72 and a washer 76 retained between the arms 73 by a nut 77 threaded to the end portion of the connecting rod. Adjustment of the nut 77 allows pre-compression of the spring 75 to a required extent. Normally, the upper and lower hubs 65,66 are in alignment and the spring is uncompressed as far as nut 77 allows. In operation, as the wind force on the rotor increases, the upper hub may turn with respect to the lower hub, so reducing the efficiency of the rotor, as a whole. The turning of the upper hub is against the force provided by the spring 75 and the relative rotation of the upper hub with respect to the lower hub is automatically balanced dynamically by the wind force acting against the spring force. Figure 18 shows the coupling mechanism displaced from this normal setting, through approximately 45°. The spring 75 is fully compressed and so further displacement of the upper hub with respect to the lower hub is not possible and thus this is the limiting maximum displacement. The spring exerts a restoring force on the upper hub, so as to bring the upper hub back into alignment with the lower hub when the wind force reduces.

Claims

1. A wind turbine having two closely juxtaposed rotors mounted for rotation about a common vertical axis, the two rotors being similar but mirror images of each other and each having a set of vanes extending generally radially so that each vane of one rotor co-operates with a corresponding vane of the other rotor thereby to extract energy from air flowing past the turbine at substantially 90° to the common axis of the rotors, adjustment means being provided to control the efficiency of the turbine which adjustment means operates by adjusting the position of one rotor with respect to the other, thereby adjusting the degree of co-operation of the vanes of one rotor with respect to those of the other.

2. A wind turbine as claimed in claim 1 , wherein the adjustment means operates to shift the phase of one rotor with respect to the other, about the common axis of the rotors.

3. A wind turbine as claimed in claim 2, wherein limiting means are provided to limit the range of adjusting movement of one rotor with respect to the other.

4. A wind turbine as claimed in claim 3, wherein the limiting means limits the range of adjusting movement of one rotor with respect to the other about the common axis to substantially ±45°.

5. A wind turbine as claimed in any of claims 2 to 4, wherein the adjustment means includes a resilient coupling interconnecting the two rotors, whereby increasing the phase shift of one rotor with respect to the other is performed against the resilient bias provided by said coupling.

6. A wind turbine as claimed in claim 5, wherein the resilient coupling biases the rotors to a setting of maximum co-operation where the two rotors are aligned with one another, one rotor moving with respect to the other away from that setting with increased energy in the wind flow.

7. A wind turbine as claimed in claim 5 or claim 6, wherein the two rotors are mounted on a common shaft with one rotor directly connected thereto and the other rotor being indirectly connected to the shaft through a spring mechanism.

8. A wind turbine as claimed in any of claims 2 to 4, wherein electrical control means is provided to perform adjusting movement of one rotor with respect to the other.

9. A wind turbine as claimed in claim 1 , wherein the adjustment means operates to shift one rotor with respect to the other along the common axis of the two rotors.

10. A wind turbine as claimed in any of the preceding claims, wherein a damper interconnects the two rotors.

11. A wind turbine as claimed in any of claims 1 to 8, wherein the two rotors are separately connected to individual loads to be driven thereby, the adjustment means controlling at least one of the loads thereby to adjust the phase of the rotors.

12. A wind turbine as claimed in claim 11 , wherein said loads comprise electrical generators and the adjustment means controls the loading of the generators thereby to effect a phase shift in the rotors.

13. A wind turbine as claimed in any of the preceding claims, wherein the vanes of the rotors are oppositely arranged whereby each vane on one rotor when aligned with the corresponding vane on the other rotor defines a V- shaped wind collector.

14. A wind turbine as claimed in claim 13, wherein each vane has a leading edge and a trailing edge with respect to the normal direction of the rotor, the leading edge of each vane of one rotor being disposed closely adjacent to the leading edge of the corresponding vane of the other rotor for the maximum efficiency setting of the adjustment means.

15. A wind turbine as claimed in any of the preceding claims, wherein there is associated with each vane of each rotor an associated concentrator panel arranged to direct wind impinging on the turbine circumferentially around the rotors to enhance the efficiency of the turbine.

16. A wind turbine as claimed in claim 15, wherein the space between a vane and its associated concentrator panel on one rotor and between the corresponding vane and associated concentrator panel on the other rotor is generally V-shaped and of reducing cross-sectional area in the radially inward direction of the rotors.