Patent Publication Number: US-2012045332-A1

Title: Vertical axis wind turbine

Description:
This invention relates to a vertical axis wind turbine. The invention is applicable for use in generating electricity. 
     Wind turbines have increasingly been used to convert wind energy to electrical energy. The turbines can either be horizontal axis turbines, which need to face directly into the wind in order to best achieve rotation of the turbine; or vertical axis turbines which have the advantage that they will rotate regardless of the direction of the wind. The present invention relates to a vertical axis wind turbine which may be used to generate electricity. 
     Previous disclosures of vertical axis wind turbines include FR2494781. This document describes a vertical axis wind turbine that has three horizontal radial arms. At the end of each arm a vertical vane is mounted. These vertical vanes can pivot and have a limit on their rotational motion. The pivot point on the vertical vane is near one end of the vane. The pivot motion of the vane is used to allow the vane to self-orientate based on the direction of wind flow. This self-orientation is limited by limit of rotational motion. Due to this limit on the rotation, the vane presents a surface to catch the wind during part of the precession of the turbine. During another part of the precession the vane is self-orientating, the placement of the pivot point near one end of the vane means that maximum turning force is generated on the vane at this point. The inventor of the present invention has identified that generating such a large turning force can be a disadvantage in wind turbines of this type. Such a large force can cause a considerable shock to the radial arm when the vane reaches the limit on rotation, which can require the arm and the vane to be reinforced, increasing weight and reducing efficiency. 
     Another disclosure of a vertical axis wind engine is presented in US2003/0235498. This document discusses the difference between drag-based vertical axis wind engines and lift-based vertical axis wind engines. Drag-based being engines use flat panels, such as those described in FR2494781, for the vanes that catch the wind. Lift-based engines use airfoils as the vanes. US2003/0235498 teaches that there has been a progression from drag-based, flat panel wind engines to the use of airfoils in a lift-based system. This document describes a configuration of the airfoils in such a lift-based system. It discusses that the airfoils need to be balanced such that the pivot point is at the average centre of pressure/lift. Furthermore, lift-based turbines require relatively complex vanes, which can increase cost and weight. 
     It would be desirable to produce a simple but efficient vertical axis wind turbine that addresses one or more of the problems discussed herein. 
     According to a first aspect of the invention there is provided a turbine head comprising a rotatable member; a plurality of laminar blades each including a surface, the blades being rotatably mounted to the rotatable member such that the blades can each rotate relative to the rotatable member from a first position to a second position, the angle between the first and second positions being less than 180 degrees; wherein each blade is rotatably mounted to the rotatable member about an axis that is near the centre of the blade such that, if a force is applied to the blade in a direction that is perpendicular to the surface, the difference in force between the force exerted on a portion of the surface on one side of the axis and the force exerted on a portion of the surface extending to the other side of the axis is sufficient to cause the blade to rotate from the first position towards the second position. 
     The surface area of the portion of the surface of the blade on one side of the axis may be greater than the surface area of the surface on the other side of the axis. 
     Each blade may be rotatably mounted at a point between the centre of the blade and a point one-quarter of the length of the blade in from the end of the blade. Each blade may rotatably mounted at a point one-sixth of the length of the blade from the centre of the blade. 
     The surface of each blade may have a concave profile. Each blade may comprise a blade frame and a blade covering, the blade frame may form the outer edges of the blade and the covering may form the surface of each blade. The blade covering may comprise a flexible material. 
     The angle between the first and second positions may be between seventy five and one hundred and five degrees. 
     The turbine head may further comprise a first stop means that limits the degree of rotation of the blade and, optionally, a second stop means to further limit the degree of rotation of the blade. The stop means may be a projection extending from the rotatable member; a tether between a fixed point and a point on the blade; or a part of the rotatable member or any other suitable means for limiting rotation of the blade. 
     The stop means may limit the degree of rotation of the blade about the mounting point to an angle between approximately seventy five and approximately one hundred and five degrees. Preferably, the degree of rotation of the blade about the mounting point is limited to ninety degrees where the first position is coplanar with the body and the second position is perpendicular to the body. 
     The turbine head may be provided with a shock absorber to minimise the impact imparted by the blade upon the stop means when they come into contact. 
     When the first or second stop means comprises the projection extending from the rotatable member or a part of the rotatable member the shock absorber may be a surface made from a resilient material situated on the projection, the part of the rotatable member or part of the blade coming into contact with the projection or the part of the rotatable member. 
     Alternatively, when the first or second stop means comprises a tether between a fixed point and a point on the blade wherein the shock absorber comprises the tether made from a resilient material. 
     The resilient material may be rubber, a synthetic equivalent of rubber or any other material which has elastomeric properties. 
     Alternatively, the shock absorber may comprise a resilient spring mechanism located on the blade or the rotatable member. 
     Optionally, the head includes means to fix the blade at a point of its rotation. The rotatable member may also include a plurality of turbine vanes and at least one of each of the plurality of blades may be mounted to a respective one of the turbine vanes. The means to fix the blade may comprise means to fix the blade at a direction that is perpendicular to the vane or means to fix the blade comprises means to hold the blade in a direction that is coplanar to the vane or means to fix the blade in either direction. 
     The blade may be mounted at an asymmetric point along the width of the blade or at a point such that the surface area of the portion of the surface area of the blade on one side of the mounting point is greater than the surface area of the surface extending the other side of the mounting point. 
     Advantageously, the blade is made from any one of: rigid plastics material, sail cloth, carbon fibre and metal. The metal may be, for example steel or aluminium. 
     The blade may also have a planar shape or a shape including a scoop at one end of the blade. 
     The turbine vanes may include an elongate portion and a frame portion at one end of the elongate portion, the blade being mounted within the frame portion. Alternatively the turbine vanes may include a pair of arms the blade being mounted at one point to one arm of the pair of arms and at another point to the other arm of the pair of arms. 
     According to another aspect of the present invention there is provided a turbine head for a vertical axis wind turbine as described above. 
     According to another aspect of the present invention there is provided a vertical axis wind turbine including a turbine head rotatably mounted upon a support, the turbine head as described above. 
     The vertical axis wind turbine may be provided with a braking means to limit the speed of rotation of the turbine head about the support. This can prevent the head rotating so fast about the support that damage occurs to any part of the vertical axis wind turbine. 
    
    
     
       Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
         FIG. 1  is a side view of a vertical axis wind turbine. 
         FIG. 2  is a bird&#39;s eye view of the vertical axis wind turbine. 
         FIG. 3  is a bird&#39;s eye view of the vertical axis wind turbine having rotated 30 degrees from the position illustrated in  FIG. 2 . 
         FIG. 4  is a bird&#39;s eye view of the vertical axis wind turbine having rotated 45 degrees from the position illustrated in  FIG. 2 . 
         FIG. 5  is a bird&#39;s eye view of the vertical axis wind turbine having rotated 60 degrees from the position illustrated in  FIG. 2 . 
         FIG. 6  is a bird&#39;s eye view of the vertical axis wind turbine having rotated 90 degrees from the position illustrated in  FIG. 2 . 
         FIGS. 7 to 9  show alternative blade, arm and mounting configurations. 
         FIG. 10  is a side view of a vertical axis wind turbine including the alternative blade and arm configuration of  FIG. 7 . 
         FIG. 11  is a side view of the end of an arm of a vertical axis wind turbine including a blade frame and covering. 
         FIG. 12  is a side view of the end of an arm of another vertical axis wind turbine including a blade frame and covering. 
     
    
    
       FIGS. 1 and 2  illustrate a wind turbine  10  including a turbine head  11  and a support  16 . The turbine head  11  comprises a rotatable member  12 , including four arms  14 , rotatably mounted on the support  16 . Each of the arms  14  is provided with a blade  18 ,  20 ,  22 ,  24  at the end of the arm  14  furthest from the support  16 . The blade  18 ,  20 ,  22 ,  24  is attached to the arm  14  at a mounting point  26  about which the blade  18 ,  20 ,  22 ,  24  can pivot. The axis for the pivot for each blade  18 ,  20 ,  22 ,  24  can be in a substantially vertical orientation. 
     The mounting point is located along the length of the blade  18 ,  24  such that the distance between one end of the blade  18 ,  24  and the mounting point is greater than the distance between the other end of the blade  18 ,  24  and the mounting point. In the turbine head  11  of the present embodiment, when the arm  14  and the blade  18 ,  24  are coplanar the length of the blade extending between the mounting point and the support  16  is greater than the length of the blade  18 ,  24  extending past the end of the arm  14 . The difference in length between the length of blade extending between the mounting point and the support and the length of blade extending past the end of the arm is selected such that the turbine head operates in an efficient manner. Such configuration of the difference in length is discussed further below. 
     Rotatably mounting the blade upon the rotatable member in such a way means that when the blade experiences a fluid flow acting against it in a first direction the blade may be caused to rotate about that mounting by inequality of the forces on the blade either side of the site at which it is mounted to the rotatable member so that it tends to become parallel to the direction of the force, unless rotation of the blade about that mounting is limited by some means. This means that the blade may be moved by the fluid flow between a first position and a second position due to the action of the fluid flow on the blade. Conveniently, in the first position the blade is generally radial to the axis of rotation of the rotatable member. Conveniently in the second position the blade is generally tangential to the axis of rotation of the rotatable member. Conveniently, if the rotatable member rotates in a first sense (clockwise or anti-clockwise) the blade is restrained from moving further than the second position in that sense of rotation and from moving further than the first position in the opposite sense of rotation. In this configuration, the blade can be influenced by fluid flow acting upon it in a direction having a component perpendicular to the axis of rotation of the rotatable member to act in the following way: 
     1. When the blade is upstream of the axis of rotation of the rotatable member it can be caused to adopt the first position, and since it is restrained from further motion away from the second position it can impart a torque on the rotatable member until it approaches a position downstream of the axis of rotation.
 
2. When the blade is downstream of the axis of rotation of the rotatable member it can be caused to move toward the second position, and since it is restrained from further motion away from the first position it can impart a torque on the rotatable member until it approaches a position in which it is broadly parallel with the flow.
 
3. As the blade is moved further towards a position upstream of the axis of rotation of the rotatable member it can be caused to move toward the first position, remaining broadly parallel with the flow, and since it is broadly parallel with the flow it can impart substantially no torque on the rotatable member at this part of the cycle. Preferably the first and second positions are separated by between 110 and 70 degrees, and most preferably by about 90 degrees of rotation about the blade mounting. Preferably each blade is mounted in like fashion.
 
     The action of the turbine head  11  in the presence of wind will now be described with reference to  FIGS. 2 to 6 . When the wind blows in a direction as illustrated in  FIG. 2  a force is exerted on the blade  20  which is perpendicular to the wind direction. 
     As the part of the blade  20  which extends from the mounting point  26  towards the support  16  is greater than the part of the blade  20  extending past the end of the arm  14 , the overall force experienced by the blade  20  is a rotational force about the mounting point  26  in an anti-clockwise direction. Thus, rotational movement of the blade about the mounting point is prevented by the arm  14  which partially overlaps with the blade  20 . 
     The overall force about the support  16  due to the action of the wind on the blade  20  and the arm  14  is a clockwise rotational force about the support  16 . 
     Blade  24 , which is at 90° to the direction of the wind due to the force of the wind acting on it causing it to pivot about the in a clockwise direction about the mounting point  26 . This is because the length of the blade  24  extending between the mounting point  26  and the support  16  is greater than the length of the blade  24  extending past the end of the arm  14 . Hence, a greater force is exerted by the wind on the portion of the blade  24  extending between the mounting point  26  and the support  16  than on the portion of the blade  24  extending past the end of the arm and the blade  24  rotates in a clockwise direction. 
     A stop member  28  is provided to prevent the blade  24  rotating more than ninety degrees. The rotation of the blade  24  to a position parallel with the wind direction means that the blade  24  has a minimal amount of force exerted on it by the wind. Therefore, the amount of anti-clockwise rotational force about the support from the wind acting on the blade  24  and its associated arm is less than the clockwise rotational force exerted by blade arm  20  and its associated arm. 
     Blades  18  and  22  and their associated arms are parallel to the direction of the wind and therefore exert no rotational force about the support. Thus, an overall clockwise rotational force is experienced by the turbine head  11  resulting in a clockwise rotation of the arms and blades. 
       FIG. 3  illustrates the positions of the blades  18 ,  20 ,  22 ,  24  of the wind turbine  10  when the turbine head  11  has rotated approximately 30 degrees in a clockwise direction. In this position, the wind is exerting a force on blade  18 . Rotation of the blade  18  about the mounting point  26  is prevented by the arm  14  contacting the part of the blade  24  which extends between the mounting point  26  and the support for the same reasons as those given with reference to blade  20  when describing  FIG. 2 . Thus, blade  18  and its associated arm  14  experience a clockwise rotational force about the support  16 . 
     Blade  20  still experiences an anticlockwise force about the mounting point  26  but rotation of blade  20  is prevented by the arm  14  as described with reference to  FIG. 2 . Thus, the combination of the blade  20  and its associated arm  14  experiences an overall clockwise rotational force about the support  16  for the same reasons given with reference to blade  20  and  FIG. 2 . 
     As the turbine head  11  moves from the position in  FIG. 2  to that in  FIG. 3  blade  22  rotates in a clockwise direction about the mounting point  26 . This is because the surface area of the part of the blade  22  which extends from the mounting point  26  towards the support  16  is greater than the surface area of the part of the blade  22  that extends past the end of the arm  14 . Thus, the blade  22  experiences a larger force on the part of the blade  22  which extends the greater distance from the mounting point  26  than the part of the blade which extends the lesser distance from the mounting point  26 . Hence, the overall force experienced by the blade  22  is clockwise and the blade  22  rotates in a clockwise direction about the mounting point  26 . 
     As detailed above, the blade  22  experiences a larger force on the part of the blade  22  which extends the greater distance from the mounting point  26  than the lesser force on part of the blade which extends the lesser distance from the mounting point  26 . However, the difference between the larger force and the lesser force on the parts of the blade  22  only needs to be sufficient to cause the rotating force about the mounting point  26 . Therefore the mounting point  26  can be located as close to the centre point of the blade  22  as possible whilst still providing the required difference in force needed to rotate the blade  22 . The exact position of the mounting point  26  along the length of blade  22  will be dependent on, for example, the frictional forces that need to be overcome in the rotational mounting. Ideally, the mounting is located no less than one-quarter of the length of the blade  22  from the end of the blade. 
     In an exemplary configuration each blade is 600 mm long by 600 mm wide. The rotational mounting is located 100 mm from the centre line of the blade, or alternatively 200 mm from one edge. In such a configuration the rotational mounting is one-sixth of the length of the blade away from the centre line of the blade. Such a configuration gives sufficient force to move the blade from one position to the other, whilst avoiding a large shock when operating at operational speed. The centre point of the blade is determined by reference to its area exposed to the fluid flow, not its mass. 
     Clockwise rotation of the blade  22  due to the force of the wind is limited by a stop member  28 . The stop member  28  is a face formed by an extension from the arm  14 . As can be seen in  FIG. 3 , the stop member prevents rotation of the blade  22  when it is still experiencing an overall clockwise force due to the wind. Preferably, the shape of the blade  22  and its associated arm  14  is such that the clockwise force on the blade  22  is greater than the anti-clockwise force on the arm  14 . Thus, the overall force experienced by the blade  22  and its associated arm  14  is in a clockwise direction. By minimizing or substantially minimizing the rotational force experienced on blade  22  during the transition from the position in  FIG. 2  to the position in  FIG. 3 , the shock experienced when the blade reaches the stop  28  can also be reduced. Such a reduction in this shock will provide for a smoother rotation of the turbine head  11 . The reduction will also reduce the chances of damage due to the blade  22  swinging around with too much speed. Indeed, it has been observed that when the turbine head  11  approaches operational rotational speed, the blade  22  no longer contacts, or contacts very lightly, with the stop  28  when undergoing the rotation described above. With reference to  FIGS. 2 and 3 , this can be explained by stop  28  travelling away from blade  22  as blade  22  rotates. Therefore increasing the distance that blade  22  needs to rotate such that blade  22  does not contact stop  28  before approaching the position of blade  22  indicated in  FIG. 6 . 
     Thus, in the turbine illustrated in the figures, the blades can rotate about a common primary axis  16 , and each blade can rotate about a respective secondary axis  26 . The secondary axes themselves rotate about axis  16  as the head of the turbine rotates. By locating the axes  26  so that when each blades rotate about the respective axis  26  the forces that exert a moment acting on it about that axis are almost balanced, the moment will be small and the blade will rotate more slowly and less vigorously about that axis than it would if the forces were highly unbalanced. Since the rotation is relatively slow, the head will have rotated somewhat before the blade hits its end-stop or other limiting means, with the result that the end-stop is moving in somewhat the same direction as the proximate part of the blade at the time the two come together; this can reduce the impact between the two. Also, since the rotation is relatively non-vigorous, the force at that impact can be relatively small. 
     Blade  24  experiences an overall clockwise rotational force about its mounting point  26 . This is for the same reasons as given in the description of  FIG. 2  with reference to blade  24 . The clockwise rotational force causes the blade  24  to pivot about the mounting point  26  until there is the minimum amount of force exerted upon it, i.e. until it is parallel with the direction of the wind as illustrated in  FIG. 3 . 
     As the total clockwise rotational force exerted on the turbine head  11  about the support is greater than the anticlockwise rotational force, the turbine head  11  continues to rotate about the support  16  in a clockwise direction. 
     When the turbine head  11  has rotated 45 degrees, as illustrated in  FIG. 4 , the wind continues to exert a clockwise rotational force on blades  18  and  20  and their associated arms  14  for the same reasons given with reference to  FIG. 3 . 
     Blade  22  continues to experience an overall clockwise force. As discussed with reference to  FIG. 2 , clockwise rotation of blade  22  is prevented by stop member  28  and an overall clockwise rotational force is exerted on blade  22  and its associated arm  14  about the support. Blade  24  continues to rotate in an anti-clockwise direction to remain parallel to the direction of the wind. This is due to the action of the force of the wind on blade  24 , as discussed with reference to  FIG. 2 . 
     Thus, as the total clockwise rotational force exerted on the turbine head  11  about the support is greater than the anticlockwise rotational force, the turbine head  11  continues to rotate about the support in a clockwise direction. 
     When turbine head  11  has rotated 60 degrees, as illustrated in  FIG. 5 , the wind continues to exert a clockwise rotational force on blades  18  and  20  and their associated arms, as discussed with reference to  FIG. 3 . 
     Blade  22  continues to experience a clockwise force. Clockwise rotation of blade  22  is prevented by stop member  28  and an overall clockwise rotational force is exerted on blade  22  and its associated arm  14  about the support for the same reasons given with reference to blade  22  in the discussion of  FIG. 3 . The force of the wind acting on blade  24  continues to cause blade  24  to rotate in an anticlockwise direction to remain oriented parallel to the direction of the wind as discussed with reference blade  24  in the description of  FIG. 3 . 
     Thus, as the total clockwise rotational force exerted on the turbine head  11  about the support is greater than the anticlockwise rotational force, the turbine head  11  continues to rotate about the support  16  in a clockwise direction. 
     When the turbine head  11  has rotated 90 degrees, to the position illustrated in  FIG. 6 , blade  18  is in the position of blade  20  in  FIG. 2 , blade  20  is in the position of blade  22  in  FIG. 2 , blade  22  is in the position of blade  24  in  FIG. 2  and blade  24  is in the position of blade  18  in  FIG. 2 . Thus, it can be seen that the forces exerted upon blades  18 ,  20 ,  22  and  24  in  FIG. 6  are the same as those exerted on blades  20 ,  22 ,  24  and  18  respectively in  FIG. 2 . Thus the turbine head  11  continues to experience an overall clockwise rotational force about the support. 
     The turbine head  11  may be connected in any suitable way to a device for generating electricity. Any device capable of generating electricity from rotational movement may be used. For example, the head of the turbine may be connected to a drive shaft which rotates facilitating the generation of electricity. 
     As will be understood by the skilled person the wind turbine may be provided with any number of blades. Advantageously, the wind turbine is provided with a minimum of 3 blades. Such a configuration allows for the self-starting of the wind turbine. Additionally, only a subset of the arms may be provided with pivotable  20  blades. Most preferably the turbine has only three blades, which are equally spaced apart radially. 
     In order to be most effective, the rotating mass of the wind turbine, particularly the arms and blades, should be as light as possible. This makes it easier for the turbine to start to rotate in a light wind, reduces shock on the structure as the blades hit their stops, minimises centrifugal effects on the structure which may otherwise require the structure to be reinforced to cope with high wind speeds, and allows the turbine to recover energy more efficiently in conditions when the wind speed is not constant and so the head is desired to accelerate and decelerate in sensitive response to changes in the wind in order to best convert the wind energy into torque on the head. In such a configuration, the wind turbine is suitable for producing the torque required to drive an electricity generating system in wind speeds of greater than 5 ms −1  or greater than 10 ms −1 , and preferably in wind speeds near 5 ms −1 . In such a configuration, the wind turbine may be capable of generating, for instance, 3 to 4 Nm of torque at a wind speed of 5 ms −1 . Testing on turbines of the type described herein suggests that they may be especially advantageous for generation of electricity in such modest wind speeds. 
     The support may be any conventional type of support. For example, the support may be a pole upon which the rotatable member is mounted. 
     The skilled person will understand that the blades may be of any suitable shape so that rotation of the blades about their mounting on the arm can be caused by the force of the wind acting on the blades. Additionally but with reference to the above discussion on the position of the mount, the mounting point of the blades on the arm may be at any suitable position such that when a blade is perpendicular to the direction of the wind the force on the blade extending on one side of the mounting point is greater than the force on the other side of the mounting point. 
     Examples of possible shapes and mounting configurations of the blades and arms are illustrated in  FIGS. 7 to 9  and  11  to  12 . In  FIG. 7  the arm  14  comprises an elongate portion  30  from the support (not shown) and a frame portion  32 . The blade  34  comprises a planar sheet which is mounted within the frame portion  32  at two mounting points  36  about which the blade  34  can pivot. The mounting points  36  are located closer to one end of the frame portion  32  than the other so that when wind blows into or out of the page the blade  34  experiences a greater force on one side of the mounting point  36  than the other  32 , causing the blade  34  to rotate until it comes into contact with a stop member  38 ,  40 . 
     In  FIG. 7  the blade overlaps with the end of the frame portion  32  closest to the elongate portion  30  to limit movement of the blade  34 . Additionally, the mounting points have extensions  40  which prevent the blade from rotating more than ninety degrees from the plane of the frame portion  32 . Optionally, the extensions  40  may connect to form a single stop member that extends between the mounting points  36 . 
     Although  FIG. 7  illustrates the mounting points  36  over half the length of the frame portion  32  away from the elongate portion  30  the skilled person will understand that the mounting points  36  may be situated any suitable distance from the elongate portion to effect the rotational movement of the blade  34  in response to the wind. Additionally, the stop members  38  and  40  may be located at any suitable position with the blade  34  overlapping the frame portion  32  at any suitable position and an extension from the frame portion  34  being from any suitable position. 
     In  FIG. 8  the arm comprises two arms  42  with the blade  44  having separate mounting points  46  on each of the arms  46 . The mounting points  46  may be located closer to one end of the blade  44  than the other so that when a wind blows into or out of the page the blade  44  experiences a greater force on one side of the mounting points  46  than the other, causing the blade  44  to rotate until it comes into contact with a stop member. In this instance the arms form may form a first stop member and extensions, such as those described with reference to  FIG. 7  may form second stop members. 
       FIG. 9  illustrates an alternative blade shape mounted between the two arm configuration  15  described with reference to  FIG. 8 . 
       FIG. 10  illustrates the alternative blade shape and arm configuration of  FIG. 7  mounted upon a vertical axis wind turbine. 
       FIG. 11  shows a blade  56  mounted on an arm  70 . The arm terminates in a bifurcated structure having stanchions  52 ,  54  which extend to points above and below the blade. The blade is mounted to the stanchions by bearings  46 . The blade comprises a frame  57  and a web of flexible impermeable material  58  which is attached at its periphery to the frame  57 . The upper edges of the frame  57  are straight and parallel with each other. The lateral edges of the frame  57  are curved. 
     In alternative configuration, each arm has a blade mounted above and below the arm. Such a configuration doubles the surface area of the turbine without increasing the number of arms required. This configuration also allows for a doubling of the number of blades without the consequence of the blades further shielding each other, which would be the case if more blades were deployed on a single side of the arms. Such a structure with blades and arms of a design broadly analogous to those of  FIG. 11  is shown in  FIG. 12 . The structure  60  comprises an arm  71  having blades  62 ,  64  mounted above and below it. 
     As discussed previously but with reference to the above discussion on the position of the mount, the mounting point of a blade may be at any suitable position such that, when a blade is perpendicular to the direction of the wind, the force on the blade extending on one side of the mounting point is greater than the force on the other side of the mounting point. 
     Optionally, the blade may be mounted such that it pivots about a substantially horizontal axis. A blade mounted in this way rotates between a position where a face of the blade is situated so that the wind is directed into the face of the blade and a position where the wind is directed across the face of the blade. 
     A blade may also be provided with biasing means to encourage it to return to a position where its face is situated such that the direction of the wind is into the face of the blade. 
     Each blade is of laminar form. Each blade can thus take the form of a sheet, optionally with other features such as a strengthening frame or rib. Most conveniently the blade comprises a frame with a sheet of material attached across it. The blade may be planar or substantially planar. Preferably the blade is configured so as to be of a non-aerofoil shape. The blades can be substantially flat and planar. In this configuration the two sides of the blade coplanar with the axis of rotation are parallel, or substantially parallel, with each other. Additionally, the blade may be any suitable shape enabling them to carry out the functionality described above other examples of suitable shapes not shown in the Figures include a blade with a scoop on one side of the mounting point to increase the force felt on that side of the mounting point. The blades may also be rigid or flexible in shape and may be made from any suitable material. Examples of material include, plastic, material such as sail cloth, carbon fibre or metal. The metal may be, for example aluminium or steel. 
     Advantageously, the blades may also be constructed with a blade frame and a covering. The covering including materials such as sail cloth, kite-material such as rip-stop nylon, or other flexible material. The face of the blade will then be able to change shape when a force is applied to it, and therefore have a concave shape. Such a concave shape for the blade could provide better ability to catch the wind. Furthermore, the covering can be made of a relatively inexpensive material. This provides the advantage that the covering can be made to automatically release if the wind conditions are adverse, for example during gale force winds. The covering can then be replaced without the whole wind turbine being damaged by the adverse conditions. 
     Thus, in one embodiment, one or more of the blades comprises a peripheral structure defining a frame having an aperture therein and a web of flexible material disposed over the aperture and attached to the frame. The flexible material is preferably impermeable so that the wind can act on it. In this way, when the wind acts on the flexible material it can cause it to adopt a non-planar shape, for example by bowing into the aperture. This non-planar shape can enhance efficiency by catching the wind and/or by acting as a sail. One convenient way to achieve such a structure is for the frame to be formed of one or more thin struts which circumscribe the aperture and for the flexible material to be attached to that frame by adhesive, staples or another fixing means. 
     The stop member  28 ,  36 ,  46  may be a face formed on an extension from the arm  14  as shown in  FIGS. 2 to 6 . Alternatively, any of the stop members illustrated in and described with reference to  FIGS. 7 and 8  may be used. The skilled person will also understand that any other suitable means of restraining the movement of the blades may be used. For example, alternative stop members include a tether connecting the blade to a fixed point. 
     Additionally, the stop member may not limit the rotation of the blade to 90 degrees but may be maximised with the opposing rotational forces minimised. This angle will be less than 180 degrees. 
     Optionally, the mounting point of each blade may be a spring loaded mechanism to prevent sudden movement of the blades impacting on the rotational movement of the blade and arms. Alternatively, the stop member  28  may be configured to prevent sudden movement of the blades impacting on the rotational movement of the blade and arms. For example, the stop member  28  may be made from a resilient material such as rubber. 
     Optionally, the vertical axis wind turbine may be provided with a centrifugally operated declutching device. This device may comprise a means to fix a blade in a position so that it acts to slow the rotation of the turbine head about the support. Any suitable means for fixing the position of the blade may be used. Alternatively, the device may comprise braking means to apply a braking force to decelerate the turbine head. Any suitable means for decelerating the rotation of the turbine head about the support may be used. 
     The device may be activated when the centrifugal force experienced at a point on the vane is greater than a threshold amount. Alternatively, the device may be activated when the rotational velocity of the turbine head is greater than a threshold amount. When the device is means to fix a blade and the rotational velocity or centrifugal force is greater than the threshold amount the device may cause one blade to be fixed; then, if the speed of centrifugal force becomes greater than the threshold amount for the device a second time the device may cause a second blade to be fixed. 
     Preferably, the blades are freed from their fixed position, or the braking force is reduced, when the centrifugal force, or rotational velocity falls below a second threshold amount. 
     The turbine may be coupled so as to drive a consumer of torque such as an electricity generator or a water pump, or to a mechanism that converts the turbine&#39;s rotating motion into motion of another form, such as reciprocating motion. 
     The presence of such a device is useful as it prevents the rotational velocity of the turbine head increasing to a point where damage may occur to one or more of the blades, the turbine head, the support or the means used to generate electrical energy from the rotational movement of the turbine head.