Abstract:
A turbine for extracting power from a flowing fluid comprises a blade for interaction with the fluid, the blade being rotatable both about a primary axis and a blade axis. The blade axis is proximate to the blade, substantially parallel to the primary axis and spaced therefrom. Rotation actuating means is provided to rotate the blade about the blade axis in dependence upon the rotation of the blade about the primary axis such that the rotation angle of the blade about the blade axis is a function of the rotation angle of the blade about the primary axis. The rotation of the blade about the blade axis is in the same direction of rotation as the blade axis about the primary axis.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the extraction of power from a flowing fluid by means of a bladed turbine. Such a turbine may be used to generate electrical energy from fluid flow such as river water flow, tidal water flow or from wind. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various devices used in the extraction of energy are known in the art using two basic mechanisms: these are variously denoted by; i) “drag” or “momentum transfer” or “momentum reversal” and ii) by “lift”. Known devices typically use one or other of these mechanisms. Momentum transfer systems rely on the fluid flow pushing against a vane, paddle or blade so that the vane is pushed in the same direction as the flow. A well-known example of a momentum transfer type device is the Pelton Wheel. However, during a full rotation cycle the cups produce significant drag on the return half of the cycle and therefore the efficiency is reduced. For this reason, such designs have not found favour for wind generation schemes, although their simplicity makes them ideal for use as anemometers for example where power efficiency is not an important consideration. 
         [0003]    Another conventional means of transferring energy to a moving part is by use of the lift mechanism, such as the horizontal propeller blade turbine. With lift-type devices, the blade is impelled in a direction perpendicular to the direction of flow. 
         [0004]    In the extraction of energy from a fluid or gas medium, a key parameter of importance and means of comparison of various methods is the efficiency with which energy is extracted. Efficiency is compared over a common area over which a mechanism intercepts the fluid. The efficiency in this case is defined as the ratio of the extracted power from the fluid flow over a defined area compared to the power available over the same area. The available power is proportional to the cube of the mean flow velocity and it can be shown that there is a fundamental limit to the amount of power per unit area that can be extracted from any medium flow. The efficiency used to compare different mechanisms is therefore defined as the power generated compared to that which would be theoretically available under the optimum load condition over an intercept area A. 
         [0005]    In considering the force F perpendicular to the flow direction produced by flow against an inclined plate, the lift coefficient C 1  is shown in  FIG. 1   a  with dependence on incident flow angle, according to 
         [0000]    
       
         
           
             F 
             = 
             
               
                 1 
                 2 
               
               · 
               
                 C 
                 l 
               
               · 
               m 
               · 
               
                 v 
                 2 
               
             
           
         
       
     
         [0006]    The maximum lift coefficient is about 1 and at large angles it approximates a sinusoidal function. At an angle of 90 degrees where the flow, designated by z in  FIG. 1   a , is parallel to the plate normal, the lift force is zero. The drag force defined as the force in the direction of flow is determined by the drag coefficient C d  according to 
         [0000]    
       
         
           
             F 
             = 
             
               
                 1 
                 2 
               
               · 
               
                 C 
                 d 
               
               · 
               m 
               · 
               
                 v 
                 2 
               
             
           
         
       
     
         [0007]      FIG. 1   b  shows that the drag force is a maximum when the plate normal is aligned to the flow direction and varies approximately sinusoidally with incident angle, being zero when the plate surface normal is perpendicular to the flow direction. The maximum drag force coefficient of about 2 is about twice that of the lift coefficient for a thin plate. Thus, for power conversion, it is advantageous to include drag or momentum reversal effects, since the maximum efficiency factor is high. For a mechanism which utilises drag such as the Pelton wheel, the drag force is reversed over half the rotation and thus considerably reduces the efficiency. In lift-based designs the efficiency factor is optimised for a particular flow velocity by means of the blade incidence angle, therefore at low velocities and high velocities the efficiency is low. Consequently external means may have to be introduced to start the turbine particularly at low flow velocities. Vertical turbine types such as Savonius and Darrieus turbines are examples with low efficiencies at low flow velocities. Both the conventional horizontal and vertical rotating aerofoil blade turbines rely on lift. It is defined that Lift forces act perpendicularly to the flow direction as in an aeroplane wing, whereas drag forces act in the direction of flow. The lift force depends on the attack (incidence) angle of the aerofoil blade, its area and its cross-section geometry. The section geometries are well understood and are characterised in the NACA numbered catalogue for their aerodynamic properties. They are typified by angles of attack between zero degrees to about twenty degrees of incidence to the flow, beyond which the wing is liable to stall. As above, the lift force can be expressed in terms of the lift coefficient C 1 : 
         [0000]    
       
         
           
             L 
             = 
             
               
                 1 
                 2 
               
                
               
                 
                   C 
                   l 
                 
                 · 
                 ρ 
                 · 
                 
                   V 
                   2 
                 
                 · 
                 A 
               
             
           
         
       
     
         [0000]    where A is the aerofoil area, V is the flow velocity relative to the foil and ρ is the density of the flowing medium. Similarly, the drag force D can be expressed in terms of a drag coefficient C d : 
         [0000]    
       
         
           
             D 
             = 
             
               
                 1 
                 2 
               
                
               
                 
                   C 
                   d 
                 
                 · 
                 ρ 
                 · 
                 
                   V 
                   2 
                 
                 · 
                 A 
               
             
           
         
       
     
         [0008]    Although drag mechanisms are not generally used to advantage in aerodynamics, for situations where the flow velocity is to be deliberately reduced, it is clear that from measurement of various plates that the drag mechanism can be about twice as efficient as the lift mechanism for producing utilisable force. 
         [0009]    It is known to provide a vertical mill-type turbine in which each or several blades is rotatable about its respective axis, and also at a fixed distance (radius) about a common central axis, all these axes being parallel. Such devices include those disclosed in GB-A-2373028, JP-A-2004353637, EP-A1-1457672, BE-A-1013928, DE-A1-10123544 and FR-A1-2845428, GB 2241 747A, GB 2263 735A Such known turbines provide a simple linear relationship between the angle of each blade and the angle of rotation of that blade about the common central axis, to provide a blade rotation profile such as shown in graph  FIG. 2   a , denoted by the central line labelled  1 , in which each blade turns through π radians during one complete revolution about the central axis. The linear relationship between the respective rotations is shown by the line  1  joining the origin (0,0) with points  3  and  4 . In other words, after a complete revolution about the central axis, each blade will be in the same position as it was before, but rotated through 180°. The lift and drag forces on a blade and the angular definitions are shown in  FIG. 2   b.    
         [0010]    In addition to the strictly linear relationship between the rotation of the blade around its own axis and the rotation of the blade axis about a common central axis as cited, there is also defined by Goodden (GB 2241 747A, GB 2263 735A) that the blades may rotate counter directionally to the rotation around the common axis. It is also known that oscillation of the blade about its axis rather than complete rotation about its axis can be used to cause rotation of the central axis in a flowing medium. This is exemplified by Doering U.S. Pat. No. 5,324,164, Hamel U.S. Pat. No. 4,260,328, Unyushiyou JP55057672, Raymo EP0046122, Williams U.S. Pat. No. 4,618,312, Fork U.S. Pat. No. 4,380,417. However, these turbines suffer from inefficiency as the drag and lift contributions are not maximised, and cannot take into account factors such as blade interaction as discussed above. 
         [0011]    The present considerations of full rotation of the blade in the same direction of the rotation about the common central axis therefore fall outside of the above. 
       SUMMARY OF THE INVENTION 
       [0012]    According to a first aspect of the present invention, there is provided a turbine for extracting power from a flowing fluid, comprising: 
         [0000]    a blade or blades for interaction with the fluid, the blades being rotatable both about a primary axis and a blade axis, the blade axis being proximate to the blade and substantially parallel to the primary axis and spaced therefrom, and rotation actuating means to rotate the blade about the blade axis in dependence upon the rotation of the blade about the primary axis such that the rotation direction of the blade about the blade axis is the same as that of the rotation direction of the blade about the primary axis, wherein the blade rotation angle about its axis can depart from a linear relationship with respect to the rotation angle about the common central axis but is within an angle of 45 degrees (π/4) of the linear relationship defined where the rotation angle of the blade about its axis is half the rotation angle of the blade axis about the common central axis. 
         [0013]    The turbine preferably comprises a plurality of blades, each blade being rotatable about both the primary axis and a respective blade axis proximate the respective blade, which axis is parallel to, and spaced from, the primary axis, and wherein the rotation actuating means rotates each blade about its respective blade axis in dependence upon the rotation of each blade about the primary axis. Some means of achieving the relationship between the two rotation angles θ and Φ as shown in the hatched domain  6  in  FIG. 2  are discussed below. 
         [0014]    The present invention provides a turbine that addresses the above problems in providing maximum efficiency by combining lift and momentum reversal such that and in contrast to Goodden, the rotation of the blade is limited to rotation in the same rotation direction around its axis as about the common central axis. The invention provides a turbine with a rotating spindle motivated by a number of vanes which can be rotated or inclined to a fluid flow and which are enabled to utilise specific amounts of both lift and momentum change mechanisms in the appropriate position with respect to the flow direction. 
         [0015]    An embodiment of a three-bladed design is shown in  FIG. 3   a  and  FIG. 3   b , where the fluid flow direction z is presented vertically to the page as shown and the mill blades turn in the clockwise direction. The blade  7  on the left ( FIG. 3   a ) provides maximum drag or momentum change effect, while the drag effect of the other two blades is much reduced so that there is greater net force acting on the left of the mill than the right, so causing rotation. There will also be a lift force contribution dependent on the blade angle φ at intermediate positions. Since the degree to which lift and momentum reversal effects are dependent on the absolute flow velocities, rotation speed, viscosity and other parameters, it is required to be able to vary the dependence of the relative rotations of the blades about their axes with respect to rotation about the common central axis. The angular domain over which this will be useful is therefore defined in this application to be within and limited to a region surrounding the defined linear dependence as shown in  FIG. 2  by the shaded region  6  extending over an angular region about the central line connecting (0,0) to (2π,4π). 
         [0016]    For periodically rotating blades or vanes, the available dynamic variables which can be optimised as a function of blade rotation angle around a central axis are: 
         [0000]    i) blade radial distance R(θ) from a central axis from which the angle is measured,
 
ii) instantaneous angular velocity v=dθ/dt where v has period 2π,
 
iii) blade angle, φ(θ), and
 
iv) blade area, A(θ).
 
         [0017]    These variables are shown in  FIG. 5 . The prior art does not specify variation of the above factors other than a linear (proportional) dependence of the blade angle with respect to the central rotation angle, i.e. Φ=θ/2. The present invention encompasses and considers the above variable φ(θ) dependent according to the above constraints on the central rotation angle θ as a means to providing optimisation on energy extraction efficiency under different environmental and material flow conditions. 
         [0018]    Within the present scope of mechanisms utilising both lift and momentum reversal, there are additional parameters which can be varied to optimise the power extraction efficiency and provide a more efficient mechanism for power conversion than those turbines which only utilise lift where the fluid flow direction is near perpendicular to the vane or blade surface normal. These include:— 
         [0000]    i) the ability of a given area of cross-section to intercept the fastest stream flow where the stream velocity is not uniform. For tidal water flow for example where the flow velocity varies with depth above the bed this may be achieved by arranging the axis of rotation to be horizontal and such that the larger flow rate occurs for the maximum momentum reversal blade orientation and lower flow occurs for the returning blade;
 
ii) increasing the horizontal length of the blade in order to achieve a large area where the depth within the flow stream is small. In this case the length to diameter ratio of the blade can be large;
 
iii) the ability to minimise the effects of vortex shedding and turbulence by the provision of plates at the ends of the blades which maintain the flow direction over the plates and additionally serve to protect any gearing mechanism designed to rotate the blades in (i) above;
 
iv) the ability to optimise any interactive effects between blades and to aid efficiency by defining the relative dimensions and geometry of the blades as well as defining the interdependence of blade rotation and central axis rotation.
 
v) the ability to redirect flow to and from the turbine thus increasing the effective capture area or increasing the effective flow velocity. It is well known that this may be achieved by the provision of guiding blades parallel to the turbine axis and which are placed ahead or behind the turbine to funnel flow into and out of the turbine.
 
         [0019]    In accordance with a second aspect of the present invention, there is provided a turbine for extracting power from a flowing fluid, comprising: 
         [0000]    a blade for interaction with the fluid, the blade being rotatable both about a primary axis and a blade axis, the blade axis being proximate to the blade and substantially parallel to the primary axis and spaced therefrom, a primary spindle substantially co-axial with the primary axis, a blade spindle substantially co-axial with the blade axis, the blade being mounted on the blade spindle, and a cap mounted at an end of the primary spindle such that the cap is rotatable about the primary axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIGS. 1   a,b  shows the conventional lift ( 1   a ) and drag or momentum reversal ( 1   b ) functions for a flat plate inclined to a flowing medium, where the lift force is defined in a direction perpendicular to the flow direction and drag (momentum reversal) is defined in a direction parallel to the flow direction. 
           [0021]      FIG. 2  describes the angular relationship between the rotation of the blade (vane) (φ) about its own axis and the rotation (θ) of the blade axis about a common central axis where the allowed blade angles according to the present invention are shown by the shaded region  3  about the line  1  which defines the relationship between (φ) and (θ) where dφ=dθ/2 and where dφ and dθ are incremental changes in angle. 
           [0022]      FIGS. 3   a  and  b  show an embodiment of the turbine showing blades  7  supported and rotated around axis  12  which is fixed into plates  10  at either end. The plates  10  are fixed to a common central axis  9  which rotates and transfers power to a load. Gear train  11 ,  13  and  14  are eccentric or non-circular in order to achieve the angular paths described. 
           [0023]      FIG. 4  shows an embodiment of the turbine where the turbine rotor is held within a rotation reference frame which also provides mechanical support to ends of the rotor and where rotation angles of the blades are fixed by a chain or belt over non-circular or eccentric gears. 
           [0024]      FIG. 5  illustrates the operating variables: blade angle φ(θ), radial velocity of blade about central axis v(θ), radius of blade axis from central axis R(θ), blade area A(θ) denoted as dependent on the central rotation angle θ. This application considers only the variable φ(θ). 
           [0025]      FIG. 6   a  shows an embodiment of a gear train referred to in  FIG. 3  such as to produce an optimised angular locus of the blade about the central axis shown in  FIG. 6   b , where the loci of blade angles falls within the domain referred to in  FIG. 2 . The gear train shows eccentric non-circular gears  15  and  16  which rotation is transferred by means of intermediate circular gears  17 ,  18 ,  19 .  FIG. 6   b  illustrates the angular path between the blade φ(θ) about the common central axis θ for specific gear forms  15  and  16  shown in  FIG. 6   a.    
           [0026]      FIG. 7  shows an embodiment of the gear train incorporating eccentric gears and non-circular gears mounted on the rotation plate  10  by which means rotation of the blade is defined within the domain shown in  FIG. 2 . 
           [0027]      FIG. 8   a  shows a typical power performance surface dependent on blade rotation angle φ(θ) and the central rotation angle θ. Here it is apparent that the surface has a maximum power path in progressing from the origin (0,0) to the point P where the blade has completed a full rotation about the central axis. The purpose of the invention is to operate the device close to the maximum power path shown as path B ( FIG. 8   b ) in this example. 
           [0028]      FIG. 9  shows an embodiment of a method of achieving a path described by  FIG. 6   b  and within the domain shown in  FIG. 2  by use of a toothed pulley and belt system as an alternative to a gear train, where the pulleys are circular and eccentrically mounted or non-circular or a combination of both. 
           [0029]      FIG. 10  shows the end-plate disposed to support and protect the gearing or other mechanical means for controlling the blade rotations as shown in  FIG. 7  and for providing boundaries for flow at the ends of the blades. 
           [0030]      FIG. 11  shows an alternative blade drive and control method in accordance with the first aspect of the present invention whereby each blade is driven by a motor independently using a rotation sensor and controller. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Referring to  FIGS. 3   a  and  3   b , the turbine blade  7  is fixed by its spindle  12  to top and bottom plates  10  where the axis is located by holes in the plates through which the spindle passes. A gear  11  is fixed to the spindle  12 . A gear set  13  is allowed to freely rotate about a spindle  8  fixed to the bottom plate  10 . This gear set meshes with gear  11 . Gear  14  is fixed to a cylindrical outer spindle  15  which is also fixed to a base  16 . Gear set  13  also meshes with gear  14  which may be eccentrically mounted or non-circular. Gear set  13  comprises either one or two gears fixed on the same spindle and rotate together such that a top gear meshes with gear  11  and a bottom gear meshes with gear  14  or vice versa. The two gears comprising set  13  may be different diameters or one or both may be eccentrically mounted or non-circular. The gear set comprising  11 ,  13  and  14  acts such that when the spindle  12  is rotated about the central axis  9 , the blade rotates about its axis  12  within the domain described in  FIG. 2 . Power is accessed via the rotation of the central spindle  9  within the cylinder  15  whereby the cylinder  15  attached to the base  16  acts as a rotation reference. The rotation reference is also illustrated by the framework  16  shown in  FIG. 4 . This framework also serves to support the rotor assembly. In this example the blade angle control is by means of a chain and gear or belt and toothed pulley. A further embodiment of the rotation mechanism is shown in  FIG. 7  where the non-circular gear train is seen in section. The gear  11  and meshing gear  13   b  are circular and have ratio 2:1. The gear  13   a  and meshing  14  have nominally unity ratio (the same number of teeth about the circumference) but they are either eccentric or non-circular. The gear  14  is fixed to the tube  15  and thereby to the frame  16  which does not rotate. The solutions to the non-circular gear geometry conditions imposed by meshing with fixed axis separation are well known (Toshiyuki U.S. Pat. No. 5,251,507 for example). These combinations of gears allow a strictly 2:1 angular ratio at the points  3  and  4  referring to  FIG. 2 , but allow variation in the rotation path away from the 2:1 ratio at other rotation angles within the hatched domain defined in  FIG. 2 . In  FIG. 7  the gear mechanism supported by plates  10  and  10   b  is contained within the plates  10  and  10   b  and can be isolated with an end cap  10   c  which is fixed to plates  10  and  10   b  such that the assembly  10 , 10   b , 10   c  forms an enclosure which can be sealed around the complete mechanism. The caps  20  serve to provide hermetic seals to stopped spindles  8  and  12 . The grommets  21  serve to seal the spindles  12  and  15 . Casing  10 , 10   b  and  10   c  are affixed to the central spindle  9  and they rotate together. Spindle  9  rotates within the reference tube  15  and frame  16 . The rotation actuating means may comprise a gear train. In this case, the gear train may comprise at least one gear which is eccentrically mounted for rotation, and/or at least one substantially non-circular gear. 
         [0032]      FIG. 6  shows another embodiment in accordance with the invention for achieving this path by means of a gear train for mechanically driving rotation of a blade from rotation of the primary axis comprising non-circular and eccentrically-mounted gears. Two end gears  15  and  16 , for connection to a blade spindle and the primary spindle respectively, are designed to give net rotation ratio of 2:1 over 2n, i.e. two complete revolutions of gear  16  results in one complete revolution of gear  15 . Since the separation between the two gears  15 ,  16  is forced to be variable by virtue of the non-circularity, the rotation is transferred between the two without changing the angular dependence by means of satellite circular gear train  17 ,  18 ,  19 . There may be an alternative number of gears comprising the gear train to transfer the rotation from  16  to  15 . This allows freedom of separation between  15  and  16  without altering the rotation dependence. The circular gears  17 ,  18 ,  19  are held in a moving assembly  20  which is free to slide from side to side along a linear bearing in a carrier plate  21 . In the figure, gears  15  and  16  are eccentrically-mounted for rotation, where gears  15  and  16  are also non-circular. Of course, many other combinations of non-circular and eccentrically-mounted gears may be used to provide non-linear profiles. The non-circular gear  16  is held fixed at the common axis around which the arm  21  revolves together with and on which is mounted the eccentrically mounted gear  15 . The rotation of the vane is fixed to the rotation of this gear  15 . Rotation velocity of  15  is determined by the ratio of contact radius between the two gears  15  and  16 . The circular gears  17 ,  18  and  19  are free to move along the axis between the gears  15  and  16  and are held by the track  20 . As with the design shown in  FIG. 7 , the arrangement also preserves the correct rotation direction as defined in  FIG. 2 . The profiles of gears  15  and  16  will determine the rotation profile between the blade about its own axis and rotation of the blade axis about the common central axis. It is also a condition that the number of peripheral teeth on gear  15  is twice the number on gear  16  to give a nominal full rotation ratio of 2:1. The solid line of  FIG. 6   b  shows the rotation ratio for the gears shown  15  and  16  which deviates from the condition φ=θ/2. 
         [0033]      FIG. 8   a, b  shows the power as a function of both angle around the central spindle and angle of the blade. There is a path on the power surface which represents the greatest total power in going from the origin (0, 0) to point P one complete cycle of rotation around the central spindle. This is shown for a specific ratio of the blade angular rotation tangential velocity around the primary spindle  9  and the flow velocity. In this case the total power over a complete rotation cycle is given by the integrated power over the path shown and the non-linear path is obtained by use of either of the gear arrangements described in  FIG. 6   a  or  7  or  9 . The paths A and B show the linear dependence of phi and theta for a conventional circular gear or chain system and the path b) shows how the power path may be optimised through 360 degrees of rotation around the central axis. It is important that the blade is rotated to an angle whereby it does not experience a large counter (negative) torque acting against the rotation of the central axis. In  FIGS. 8   a, b  there are shown two positions which require energy to rotate (negative power) at phi=zero and phi=Pi, and where theta=Pi. In this position the blade area is facing the flow direction but rotating against the flow. For a constant rotation ratio (phi/theta) the path relating phi and theta is linear (path A) but part of the path dips into the regions of negative power referred to above. The maximum power is achieved by maintaining the highest path from theta=0 to theta=2Pi. It is clear that to avoid dropping into the lowest power regions, the path B has to be bent around these dips as shown in  FIG. 8   b , i.e. the path has to have a non-linear profile as previously described. 
         [0034]    In another embodiment, the rotation actuating means may comprise a toothed belt mounted on toothed pulleys. At least one pulley may be eccentrically mounted for rotation, and/or is substantially non-circular. This design may provide a simpler mechanism for achieving the desired non-linear rotation profile. An exemplary method is shown in  FIG. 9  where the angular path is similar to that shown in  FIG. 5   b . The toothed belt may also have an idler gear  25  designed to maintain the belt tension throughout the rotation where the idler gear is free to move against a spring force. 
         [0035]    A further embodiment is that the rotation actuating means comprises a motor connected for rotating the/or each blade about its blade axis and a controller for controlling driving of the motor as shown in  FIG. 10 . The controller may determine the rotation angle of the blade about the primary and or blade axis from an angular sensor. The controller may use a fixed algorithm to control driving of the motor. Alternatively, the controller may control the driving of the motor in dependence on at least one of the power output of the turbine and the speed of fluid flow proximate the turbine. This driving arrangement allows time variable driving of the blades, e.g. to take into account varying flow conditions and more complex non-linear rotation relationships than are possible with mechanical rotation transfer means. 
         [0036]    In a particular embodiment, the cross-section of the/or each blade in the plane orthogonal to the blade axis has four-fold mirror symmetry about its axis. In another embodiment the cross-section of the or each blade in the plane orthogonal to the blade axis has 180° rotational symmetry such that there is a concave section on one side of the blade and a convex section on the other side. This design similar to a Pelton wheel would assist in rotation of the blade. The blade forms are such that the extremities from the axis are narrower than the central thickness. The central part is necessarily of greater thickness than other parts of the blade for reasons of mechanical strength. As the blade rotates about its blade axis one complete revolution for every two revolutions of rotation about the primary axis, it is natural that the blade geometry should possess 180 degree rotational symmetry. Additionally, the edge of the blade is thinned so that, when positioned with the narrowest section to the flow direction (i.e. edge-on), the flow is split without turbulence and with least resistance to the flow. The best overall section geometry may be selected according to the intended application or specific flow operating conditions. 
         [0037]    Preferably, the turbine comprises a primary spindle substantially co-axial with the primary axis and a blade spindle substantially co-axial with the blade axis, the blade being mounted on the blade spindle. Advantageously, the turbine may comprise a cap mounted at an end of the primary spindle such that the cap is rotatable about the primary axis, and wherein the cap houses the rotation actuating means. An additional cap may be mounted at the other end or at each end of the primary spindle as shown in  FIG. 9  wherein the or each cap shape may be adapted to guide fluid flow toward the turbine and or each cap may be adapted to minimise the occurrence of turbulence and vortices within the fluid flow. This also provides robust protection of the rotation actuating mechanism and aids guidance of flow into the turbine, as well as reducing vortex shedding. 
         [0038]    Another method, not shown, of linking the rotations between the blade and the central axis in a non-linear way is by means of a drive shaft connecting non-linear gears in a gearbox. In this embodiment there is a gear-box located close to the central shaft and incorporating the gear fixed to the static central axis  9 . A transmission shaft rotates from the central gear-box to another gear box located close to the each blade and meshes with a gear fixed to the blade such that they co-rotate. There are then separate transmission shafts to each blade from the central hub. 
         [0039]      FIG. 11  shows an embodiment for controlling the rotation of the blade with respect to its rotation around the primary axis using a motor drive where the motor rotation is controlled for each blade and can be programmed to rotate the blade along any angular path. The rotation of the motor drive is determined by an angle sensor  26  measuring rotation of primary spindle  9  and a sensor  27  measuring rotation of blade spindle  12 . A controller  29  activates a motor  28  depending on an error signal between the two optical sensors  26  &amp;  27  which read the angle from a digitised bar code on drums  25  fixed to respective spindles  9  and  12 , and an algorithm representing the required phi-theta path. This provides a very flexible method of controlling the rotation of the blades. This method can also compensate for changes in path due to changes in flow velocity in order to achieve maximum power. The controller  29  can receive sensing signals from the rotation sensors  26  via lines  30  and  31  and also from a generator  34  via line  33 . The controller  29  sends the required control signal to the rotation motor  28  via a line  32 . The control of the blade angles can be predetermined by a fixed algorithm relating the angle of the blades to the angle of rotation around the primary axis, or the blade angles can be adjusted such that the algorithm is also sensitive to the output power via line  33  from the power generator  34 . The generator is geared to the primary spindle  9  via gears  36  and  38 . The controller may also receive a signal from a separate flow velocity sensor  37  which can be situated in front of, by the side of or behind the turbine. If the controller receives a signal related to the output power of the turbine generator, it can be arranged such that the algorithm controlling the blades will iteratively find the optimised blade angle by searching for the blade angle which produces the greatest mean power over a number of rotations. Adaptive Dynamic Control such as this may maintain close to optimum power conditions under all other circumstances such as changes in flow rates and changes in flow direction. 
         [0040]    A simpler system can be implemented whereby, if the interdependence of a number of parameters such as flow rate, blade angle, and the dependence of the efficiency of the turbine on these is known, then a static algorithm can be used such that the controller does not rely on measurement of the generator output power or the rotation position. In this case there is no control feedback since the efficiency for any condition is assumed to be predictable. Additionally, due to varying flow conditions, blade-blade interactions or for any other reason the static algorithm may not be assumed to be optimal and the output load and rotation positions of the blades may not be optimally known for all conditions. It is suggested that search algorithms may be designed to find the optimum rotation paths for blades by monitoring the output power and adjusting the rotation paths and/or load or other variable parameters in order to maximise the output power.