AU2005283996A1

AU2005283996A1 - Cross flow wind turbine

Info

Publication number: AU2005283996A1
Application number: AU2005283996A
Authority: AU
Inventors: Gordon Proven
Original assignee: Proven Energy Ltd
Current assignee: Proven Energy Ltd
Priority date: 2004-09-13
Filing date: 2005-09-13
Publication date: 2006-03-23
Also published as: WO2006030190A2; WO2006030190A3; US20080075595A1; EP1805415A2; CA2580094A1; GB0703442D0; GB2431698B; GB2431698A; WO2006030190A8

Description

WO 2006/030190 PCT/GB2005/003517 1 Cross Flow Twist Turbine 2 3 The invention relates to a turbine and in particular, but 4 not exclusively to a turbine of the form where the 5 operating fluid moves substantially across the axis of 6 rotation of the machine. 7 8 Wind turbines, and in particular horizontal axis wind 9 turbines (HAWTs) are commonly used to harness the kinetic 10 energy of wind to produce electricity. HAWTs can be seen 11 in many places across the country, mounted on large 12 towers to catch the faster winds that blow at such 13 heights. 14 15 HAWTs have a rotor shaft and a generator mounted atop 16 such towers, with (usually) three large turbine blades 17 designed to convert a perpendicular airflow into 18 rotational motion. The rotation of the rotor shaft 19 generates electricity by means of the generator. Such 20 turbines have high tip speed ratios, high efficiency and 21 low torque ripple which increases reliability. 22 WO 2006/030190 PCT/GB2005/003517 2 1 As the towers on which the HAWTs are mounted generate 2 turbulence, the turbine itself will most often be 3 positioned upwind of the tower. For a change in wind 4 direction from, say, NE to SW, this would require a 1800 5 rotation of the turbine to resume. Some small turbines 6 make use of a wind vane to align the turbine with the 7 wind. Other large turbines have wind direction sensors 8 and motors to rotate the turbines automatically and 9 optimise efficiency. 10 11 One drawback of realigning a turbine by rotation is that 12 gyroscopic forces act on the blades as they rotate and 13 the whole turbine turns. This causes twisting forces to 14 be exerted on the turbine which can result in fatigue and 15 eventually damage to components of the turbine. 16 17 Savonius type wind turbines operate on a vertical axis, 18 but are generally less efficient than lift producing 19 turbines. Savonius type wind turbines are similar to 20 anemometers, being that they have two or three scoops 21 arranged to catch the wind. The main benefit of such 22 turbines is that they require little maintenance, and are 23 much cheaper than similarly sized HAWTs. Additionally, 24 there is no need to direct the turbine as they can 25 operate with any cross flowing wind. However, Savonius 26 turbines are inefficient as there is always a surface 27 which is subject to some amount of drag. Hence Savonius 28 turbines are known as drag type systems. 29 30 Darrieus wind turbines (also known as "eggbeater" 31 turbines) are another example of vertical axis turbines. 32 One of the benefits of vertical axis turbines is that the 33 generator (which may be bulky and/or heavy) can be WO 2006/030190 PCT/GB2005/003517 3 1 located at the base of the turbine or on the ground. As 2 with Savonius type wind turbines, there is no requirement 3 to point Darrieus wind turbines into the wind. This is 4 particularly advantageous for situations where the 5 turbine is located in built up areas where nearby 6 buildings cause increased wind turbulence. 7 8 Other advantages over HAWTs are that the blades have no 9 tips or ends, and therefore there is no tip noise, 10 turbulence or drag on blade ends. Additionally, the 11 troposkien shape that the blades naturally assume mean 12 that there is no bending force on the rope or ropes 13 therein, only tensile forces distributed along the length 14 of the rope(s). 15 16 Darrieus wind turbine devices have been in existence 17 since 1931. in that time very few significant advances 18 have been made on the initial design. Commercially 19 exploitable Darrieus turbines have been difficult to 20 produce for a number of reasons. 21 22 In general, they are low efficiency, which significantly 23 limits their potential applications and their commercial 24 viability. Also, large thrust loadings on the main 25 bearings of such turbines means that bearing selection is 26 critical. 27 28 The long blades of Darrieus turbines have many natural 29 frequencies of vibration which must be avoided during 30 operation. Some turbines have two or three rotational 31 speeds that must be gone through quickly to reach 32 operating speed. Several modes may fall within the WO 2006/030190 PCT/GB2005/003517 4 1 operational band and thus a control system should be used 2 to avoid these modes. 3 4 When this type of machine is used in a variable speed 5 fluid such as atmospheric wind flows there can be a 6 problem controlling the rotational speed in high wind 7 conditions. This is particularly problematic in efficient 8 "lifting" type machines, such as HAWTs or Darrieus type 9 turbines, where destructive rotational speeds can be 10 reached. 11 12 Therefore, another important consideration is that the 13 rotational speed of the turbine must be limited in high 14 wind conditions and various attempts have been made to do 15 so. 16 17 UK Patent Application 2,216,606 A in the name Jeronimidis 18 et al discloses blades for use with turbines with a 19 horizontal or vertical axis of rotation. The blades 20 exhibit an anisotropy which causes them to bend or 21 stretch as the rotational speed increases. The bending 22 and/or stretching affect the rotational speed of the 23 blades as the angle of attack is changed and the load on 24 the blades is altered. 25 26 US Patent 4,500,257 discloses a braking system for a 27 vertical axis wind turbine in which a block is slidably 28 located on a blade. A solenoid releases the block at a 29 desired time and the block moves up the blade towards its 30 outermost point under centripetal force. The reduced 31 aerodynamic efficiency reduces the rotational speed. 32 WO 2006/030190 PCT/GB2005/003517 5 1 French Patent Application 2 583 823 shows a vertical axis 2 wind turbine which has a drum or disk brake to implement 3 a mechanical braking system when the rotation of the 4 turbine reaches a threshold speed. 5 6 Such drag devices and mechanical brakes have been 7 proposed to limit rotational speeds in horizontal and 8 vertical axis turbines. Drag devices can be unreliable, 9 and need to be maintained. mechanical brakes are 10 cumbersome and result in wear and tear on the system. 11 Such methods of limiting rotation may also impact on the 12 smoothness of power output from the turbine. 13 14 An object of this invention is to provide aerodynamic 15 limiting of the upper rotational speed of a turbine. 16 17 A further object of this invention is to provide 18 aerodynamic limiting of the upper rotational speed of a 19 turbine by twisting. 20 21 In accordance with a first aspect of the invention there 22 is provided a cross flow turbine comprising: 23 one or more aerofoil blades rotatably mounted about a 24 central axis and connected to said axis at or near each 25 end of the one or more blades 26 wherein said one or more blades are provided with a 27 degree of torsional flexibility such that they are 28 twistable about a longitudinal blade axis to reduce the 29 aerodynamic efficiency of the one or more blades to 30 control the rotational speed of the turbine. 31 32 Twisting the one or more aerofoil blades out of optimal 33 lift conditions limits the speed of rotation of the WO 2006/030190 PCT/GB2005/003517 6 1 turbine by reducing forward driving forces and increasing 2 drag forces. 3 4 Preferably, the one or more blades are provided with a 5 rotatable connector to allow the blade to twist about the 6 longitudinal blade axis. 7 8 Preferably, the rotatable connector couples the one or 9 more aerofoil blades to the central axis at one end of 10 the blade. 11 12 Optionally, the rotatable connector couples the one or 13 more aerofoil blades to the central axis at both ends of 14 the one or more blades. 15 16 Optionally, the rotatable connector is positioned a 17 distance along the longitudinal axis of the one or more 18 aerofoil blades to couple two sections of the one or more 19 aerofoil blades. 20 21 Optionally, rotation of the rotatable connector is driven 22 by tension in the one or more blades caused by 23 centripetal force. 24 25 Preferably, the rotatable connector is provided with a 26 rotation inhibiting means that prevents rotation below a 27 predetermined centripetal force threshold. 28 29 Preferably, the rotation inhibiting means comprises a 30 torsion spring wound against a rotation stop which holds 31 the one or more blades in place. 32 WO 2006/030190 PCT/GB2005/003517 7 1 Optionally, the rotation inhibiting means comprises one 2 or more springs fixed at a helical angle to the central 3 axis of rotation and to the one or more blades at the 4 other end. 5 6 Optionally, the rotation inhibiting means comprises two 7 triangular sections of stiff material with flexible links 8 therebetween, said links forming a Z shape. 9 10 Preferably, the one or more aerofoil blades are 11 configured to twist in a predetermined direction when a 12 tension threshold is reached. 13 14 Preferably, the one or more aerofoil blades are 15 configured to twist in a first direction to feather 16 turbine rotation. 17 18 Optionally, the one or more aerofoil blades are 19 configured to twist in a second direction to stall 20 turbine rotation. 21 22 Optionally, rotation of the rotatable connector is driven 23 by an actuator. 24 25 Preferably, the actuator operates at a predetermined 26 threshold of central axis rotational velocity. 27 28 Preferably, the actuator is powered. 29 30 Preferably, the actuator is manually controllable. 31 32 Optionally, the actuator is automatically controllable. 33 WO 2006/030190 PCT/GB2005/003517 8 1 Preferably, the torsional flexibility of the one or more 2 aerofoil blades are set at a predetermined level. 3 4 Preferably, the torsional flexibility of the one or more 5 blades can be engineered such that the degree of twist 6 causes a proportional degree of twist at the mid-point 7 between the ends of the one or more blades. 8 9 Preferably, said level is set such that substantially 10 1800 of twist at one end of the one or more blades causes 11 substantially 900 of at the mid-point between the ends. 12 13 This will effectively stop the driving force on the one 14 or more blades. 15 16 optionally, said level is set such that substantially 17 1800 of twist at one end of the one or more blades causes 18 1200 of twist at the mid-point between the ends. 19 20 optionally, said level is set such that substantially 21 1800 of twist at one end of the one or more blades causes 22 600 of twist at the mid-point between the ends. 23 24 Typically, the speed of rotation of the turbine will be 25 controlled by a lesser rotation at the one or more blade 26 ends as any rotation will affect the aerodynamic 27 properties of the one or more blades and increase drag. 28 29 Once the speed of rotation of the turbine had reduced to 30 or below an acceptable operating level, the one or more 31 blade ends will return to their original position for 32 optimum blade aerodynamics. 33 WO 2006/030190 PCT/GB2005/003517 9 1 Preferably, the one or more aerofoil blades are capable 2 of adopting a troposkien shape during rotation about the 3 central axis. 4 5 Preferably, the one or more aerofoil blades comprise one 6 or more flexible ropes enclosed by an aerofoil shaped 7 profile. 8 9 Preferably, the aerofoil shaped profile contains a 10 packing material to mechanically fix the aerofoil shaped 11 profile to the one or more ropes. 12 13 Preferably, the cross flow turbine further comprises 14 connection means provided at an end of the one or more 15 blades which is releasably connectable to the central 16 axis such that when speed of rotation of the turbine 17 about the central axis increases to or over a 18 predetermined threshold level the one or more blades are 19 released. 20 21 By releasing one end of the one or more aerofoil blades 22 to fly out, the forward driving force of the one or more 23 blades is reduced. 24 25 Excess tension in the one or more blades due to 26 centripetal forces caused by excess rotational speed can 27 cause the one or more blade ends to be released. 28 29 This feature provides the present invention with a fail 30 safe mechanism operable in extreme weather conditions. 31 32 Preferably, the one or more aerofoil blades are flexible. 33 WO 2006/030190 PCT/GB2005/003517 10 1 Preferably, the connection means is releasably 2 connectable by means of a clamp. 3 4 Preferably, the cross flow turbine comprises a plurality 5 of aerofoil blades each of which are releasably 6 connectable and wherein release of all blades occurs upon 7 reaching said predetermined speed of rotation threshold. 8 9 Preferably, said blades are released substantially 10 simultaneously. 11 12 Preferably, a single mechanism is used to release all of 13 the blades. 14 15 When the blade ends are released they swing out under 16 centripetal forces. The resulting increase in diameter 17 produces an increase in angular inertia which immediately 18 slows the turbine. Further slowing then occurs due to 19 the adverse aerodynamic geometry of the blades when held 20 at one end only. 21 22 In accordance with a second aspect of the invention there 23 is provided a cross flow turbine comprising: 24 one or more aerofoil blades rotatably mounted about a 25 central axis and connected to the central axis at or near 26 each end of the one or more blades by connection means 27 wherein 28 the connection means provided at one end of the one or 29 more blades is releasably connectable and is released 30 when speed of rotation of the turbine about the central 31 axis increases to or over a predetermined threshold 32 level. 33 WO 2006/030190 PCT/GB2005/003517 11, 1 By releasing one end of one or more aerofoil blades to 2 fly out, the forward driving force of the one or more 3 blades is reduced. 4 5 Excess tension in the one or more blades due to 6 centripetal forces caused by excess rotational speed can 7 cause the one or more blade ends to be released. 8 9 This feature provides the present invention with a fail 10 safe mechanism operable in extreme weather conditions. 11 12 Preferably, the one or more aerofoil blades are flexible. 13 14 Preferably, the connection means is releasably 15 connectable by means of a clamp. 16 17 Preferably, the cross flow turbine comprises a plurality 18 of aerofoil blades each of which are releasably 19 connectable and wherein release of all blades occurs upon 20 reaching said predetermined speed of rotation threshold. 21 22 Preferably, said blades are released substantially 23 simultaneously. 24 25 Preferably, a single mechanism is used to release all of 26 the blades. 27 28 When the blade ends are released they swing out under 29 centripetal forces. The resulting increase in diameter 30 produces an increase in angular inertia which immediately 31 slows the turbine. Further slowing then occurs due to 32 the adverse aerodynamic geometry of the blades when held 33 at one end only.

WO 2006/030190 PCT/GB2005/003517 12 1 2 The invention will now be described by way of example 3 only with reference to the accompanying drawings in 4 which: 5 6 Fig. 1 shows a view of a twist type turbine perpendicular 7 to the axis of turbine rotation; 8 9 Fig. 2 shows a view along the axis of turbine rotation; 10 11 Fig. 3 (Detail A) shows a representation of a rotatable 12 end fixing for a blade; 13 14 Fig. 4 shows a representation of a hub twisting 15 configuration; 16 17 Fig. 5 shows the hub twisting configuration in more 18 detail; 19 20 Fig. 6 shows a representation of an alternative hub 21 twisting configuration; 22 23 Fig. 7 shows the alternative hub twisting configuration 24 in more detail; 25 26 Fig. 8 shows a representation of a twisting mechanism 27 located at both ends (hubs) of the blade; 28 29 Fig. 9 shows a representation of a twisting mechanism 30 located at the centre of the blade; 31 32 Figs. 10 (a) to (f) show cross-sectional representations 33 of proposed blade configurations; WO 2006/030190 PCT/GB2005/003517 13 1 2 Fig. 11 shows a view of a release type turbine 3 perpendicular to the axis of turbine rotation; 4 5 Fig. 12 shows a view along the axis of turbine rotation; 6 and 7 8 Fig. 13 (Detail A) shows a representation of a releasable 9 end fixing for a blade. 10 11 The embodiments that will be discussed herein are 12 intended to twist turbine blades out of optimum lift 13 conditions, incorporating either stall or feathering 14 conditions. The aim is to limit the rotational speed of 15 the turbine, for example in high wind conditions. 16 Twisting of the blades may occur naturally at a 17 particular centripetal force corresponding to perhaps a 18 maximum desired rotational speed. 19 20 Twisting to stall involves twisting in such a direction 21 as to increase the angle of attack sufficiently to induce 22 aerodynamic stall. Twisting to feather involves twisting 23 in the opposite direction, inducing feathering by 24 decreasing the angle of attack. Stalling may cause 25 excessive vibration of the blades to occur. Feathering 26 does not produce such vibration problems, however a much 27 larger degree of twist is required. 28 29 As shown in Fig. 1 three aerofoil blades 2 are fixed at 30 each end to hubs (4 and 5) mounted on a rotating shaft 3. 31 The shaft will normally be mounted in bearings not shown 32 and connected to a driven load such as an electrical 33 generator.

WO 2006/030190 PCT/GB2005/003517 14 1 2 Each aerofoil blade 2 is made to be strong in tension but 3 semi flexible in bending. 4 5 In this example each blade is held firmly at one hub 4 6 end. The other end of the blade is held in a rotating 7 section 1. In this example the rotation is induced by 8 tension force in the blade due to centripetal forces on 9 the blade as it rotates. The rotating section may be 10 adjusted so that no rotation occurs until a threshold 11 force is reached so that the blade stays in its preset 12 (as shown) start up position until this point. 13 14 An example of one form of rotatable connector will be 15 described, with reference to Figures 4 and 5. A short 16 length of ball screw 6 is attached to the blade end 7. 17 The matching recirculating ball nut 8 is attached to the 18 hub. A torsion spring 9 is axially aligned along the ball 19 screw 6 axis and attached at one end to the hub 5 and at 20 the other end to the blade end 7. The torsion spring 9 21 is wound against a rotation stop which holds the blade 22 end in the normal angular position and is wound up enough 23 to prevent the ball screw 6 turning until the design rpm 24 has been reached for speed control to start. When this 25 speed is exceeded the ball screw 6 turns as the tension 26 on the blade 2 creates enough force along the helical 27 slope of the screw 6 to overcome the torsion spring 9 28 preload. As the blade 2 is twisted by this action the 29 net forward aerodynamic forces on the blade 2 are reduced 30 preventing further increase in rpm. Preferably all three 31 blade ends act in this way to preserve balance. 32 WO 2006/030190 PCT/GB2005/003517 15 1 Another example of the rotatable connector is illustrated 2 in Figures 6 and 7. The blade 2 comprises 2 ropes (10 3 and 11), which run the length of the blade 2. One rope 4 10 is bolted to the hub 5 so as to provide a fixed pivot 5 point. The other rope 11 is connected to a spring 12 or 6 other damper such that when the threshold speed is 7 exceeded, similarly to the abovementioned example, the 8 spring tension is overcome and the blade 2 is able to 9 twist, with the bolted rope 10 acting as a pivot for said 10 twisting. 11 12 The decision on which side is bolted and which side is 13 connected to the spring will depend on whether a stalling 14 or a feathering effect is desired. 15 16 Another example of the rotational mechanism is a short 17 spiral arrangement of spring lengths at the end of the 18 blades such that each individual spring section is fixed 19 at a helical angle to the hub at one end and the blade at 20 the other end, all forming a circularly displaced group. 21 When tension is applied by the blade centripetal force 22 the extension of the springs produces a rotational effect 23 on the blade. 24 25 Another example of the rotational mechanism is an 26 arrangement of two triangular sections of stiff material 27 with flexible links between arranged such that the axis 28 of the links form a Z shape. Each of the three link 29 sections is folded in opposition to the adjacent such 30 that when one end link of the "Z" is attached to the 31 blade and the other end link of the "Z" is attached to 32 the hub, when the blade is moved away from the hub the 33 folds open and the blade rotates with respect to the hub.

WO 2006/030190 PCT/GB2005/003517 16 1 The spring retaining force can be by a separate attached 2 spring or by making the links themselves of a spring 3 material. 4 5 Figures 8 and 9 show different ways in which the 6 rotatable connector twisting mechanism may be deployed. 7 Figure 8 shows the rotatable connector located at both 8 hubs (4 and 5). Figure 9 shows an alternative 9 configuration where the rotatable connector is located at 10 the midpoint 13 of the blade 2. Twisting at the midpoint 11 13 of the blade 2 may serve to reduce the extent of 12 displacement required when compared to twisting at the 13 hubs (4 and 5). Any of the twisting mechanisms herein 14 discussed may be suitable for locating at either hub, or 15 indeed at the midpoint of the blades. 16 17 Figure 10, (a) to (f), show various configurations of 18 blade that may be adopted. (a) shows a blade consisting 19 of a single rope 14 inserted in an aerofoil shaped cross 20 section rubber body 15. (b) comprises a double rope 16 21 for added tensile strength. Such a blade may also be 22 used with the twisting mechanism of Figures 6 and 7. 23 Multiple ropes or wires 17 may also be used for tensile 24 strength and also to control the extent and conformity of 25 the twist. Similarly, a double loop rope 18 might offer 26 increased tensile strength while still be suitable for 27 the twist mechanism employed in Figures 4 and 5. 2 28 double loop ropes 19 offers an analogous configuration 29 for the embodiment of Figures 6 and 7. A yet further 30 alternative embodiment utilises a hollow body 20 with a 31 filler 21. The hollow body is preferably of a fibre 32 material to carry tensile loads, e.g. in the troposkien 33 shape during operation.

WO 2006/030190 PCT/GB2005/003517 17 1 2 The cross-section may be varied towards the hubs in order 3 to smooth out the variation in forward thrust depending 4 on position along the axis of rotation. 5 6 An embodiment of the present invention which incorporates 7 means for releasing one or more blades is illustrated in 8 Figure 11. This embodiment of the invention provides a 9 fail safe mechanism and will prevent rotation of the 10 turbine in extremely high winds. This mechanism can be 11 incorporated in a turbine containing means for twisting 12 the blade in accordance with the present invention. Three 13 aerofoil blades 102 are fixed at each end to hubs (104 14 and 105) mounted on a rotating shaft (103). The shaft 15 will normally be mounted in bearings (not shown) and 16 connected to a driven load such as an electrical 17 generator. 18 19 Each aerofoil blade is made to be strong in tension but 20 semi flexible in bending. 21 22 Each blade is held firmly at one end to hub 104. The 23 other end of the blade is held in a releasing clamp 101. 24 Blade release from the clamp is induced by tension force 25 in the blade due to centripetal forces on the blade as it 26 rotates. This is calibrated to occur if other speed 27 limiting systems such as generator loading have failed 28 and emergency overspeed protection is needed. 29 30 To maintain rotational balance in the turbine all the 31 clamps are linked such that when one releases all the 32 others are released. 33 WO 2006/030190 PCT/GB2005/003517 18 1 One example of a releasing mechanism is to hold the blade 2 ends in a slot which keeps them in the correct 3 orientation. All the blades are prevented from pulling 4 out of the slot by a loop of wire or cord of known 5 breaking strength which is looped in turn through a hole 6 or pin in each blade. If the rotational speed of the 7 turbine reaches overspeed condition the loop breaks and 8 all the blades are released from the slots. 9 10 Another example of a releasing mechanism is to hold all 11 the releasable blade ends in a slot formed by the gap 12 between two hub sections. Each blade has a "detent" at 13 its end that engages with a protrusion in one hub "half" 14 to hold it in position. The force to keep the blades 15 engaged is provided by a common spring or weight acting 16 substantially along the axis of the turbine shaft. The 17 moving hub half is able to rock slightly to apply equal 18 force to all blade clamps. If the blade centripetal 19 tension increases enough to pull the blade from one node 20 of the clamp the resulting void allows the clamp to tilt 21 and release the other blades. 22 23 It is clearly advantageous to release all the blades 24 simultaneously. 25 26 It is envisaged that a combination of the twist-type 27 turbine and the release-type turbine would provide a 28 solution with inherent speed limiting means and an 29 emergency means for stopping the turbine if a threshold 30 release speed was reached. 31 32 The present invention provides many advantages suitable 33 for domestic implementation of turbines. Turbulent WO 2006/030190 PCT/GB2005/003517 19 1 airflows, such as are common in domestic environs, may be 2 harnessed by vertical axis turbines. Additionally, the 3 safety aspects of the invention, namely the velocity 4 limiting system and the emergency release that can be 5 effected by the release system, make the invention 6 advantageous over HAWTs for domestic use. There is also 7 the significant advantage of reduced vibration compared 8 to small scale HAWTs and previous Darrieus-type turbines. 9 10 Improvements and modifications may be incorporated herein 11 without deviating from the scope of the invention. For 12 example, the invention has been exemplified by 13 application to wind turbines. It is proposed that the 14 invention could be employed in other fluid mediums such 15 as water. Additionally, the twisting mechanism may be 16 implemented by motors or any other suitable control 17 device.

Claims

CLAIMS :

1. A cross flow turbine comprising: one or more aerofoil blades rotatably mounted about a central axis and connected to said axis at or near each end of the one or more blades wherein said one or more blades are provided with a degree of torsional flexibility such that they are twistable about a longitudinal blade axis to reduce the aerodynamic efficiency of the one or more blades to control the rotational speed of the turbine.

2. A turbine as claimed in claim 1 wherein, the one or more blades are provided with a rotatable connector to allow the one or more aerofoil blades to twist about the longitudinal blade axis.

3. A turbine as claimed in claim 2 wherein, the rotatable connector couples the one or more aerofoil blades to the central axis at one end of the one or more blades.

4. A turbine as claimed in claim 2 wherein, the rotatable connector couples the one or more aerofoil blades to the central axis at both ends of the one or more blades.

5. A turbine as claimed in claims 2 to 4 wherein, the rotatable connector is positioned a distance along the longitudinal axis of the one or more aerofoil blades to couple two sections of the one or more aerofoil blades.

6. A turbine as claimed in any preceding claim wherein, rotation of the one or more aerofoil blades is driven by tension in the one or more blades caused by centripetal force.

7. A turbine as claimed in any of claims 2 to 6 wherein, rotation of the rotatable connector is driven by tension in the one or more blades caused by centripetal force.

8. A turbine as claimed in any of claims 2 to 7 wherein the rotatable connector is provided with a rotation inhibiting means that prevents rotation below a predetermined centripetal force threshold.

9. A turbine as claimed in claim 8 wherein, the rotation inhibiting means comprises a torsion spring wound against a rotation stop which holds the one or more blades in place.

10. A turbine as claimed in claim 8 wherein, the rotation inhibiting means comprises one or more springs fixed at a helical angle to the central axis of rotation and to the one or more blades at the other end.

11. A turbine as claimed in claim 8 wherein, the rotation inhibiting means comprises two triangular sections of stiff material with flexible links therebetween, said links forming a Z shape.

12. A turbine as claimed in any preceding claim wherein, the one or more aerofoil blades are configured to twist in a predetermined direction when a tension threshold is reached.

13. A turbine as claimed in any preceding claim wherein, the one or more aerofoil blades are configured to twist in a first direction to feather turbine rotation.

14. A turbine as claimed in any preceding claim wherein, the one or more aerofoil blades are configured to twist in a second direction to stall turbine rotation.

15. A turbine as claimed in any of claims 2 to 14 wherein, rotation of the rotatable connector is driven by an actuator.

16. A turbine as claimed in claim 15 wherein, the actuator operates at a predetermined threshold of central axis rotational velocity.

17. A turbine as claimed in claim 15 or claim 16 wherein, the actuator is powered.

18. A turbine as claimed in any of claims 15 to 17 wherein, the actuator is manually controllable.

19. A turbine as claimed in any of claims 15 to 18 wherein, the actuator is automatically controllable.

20. A turbine as claimed in any preceding claim wherein, the torsional flexibility of the one or more aerofoil blades are set at a predetermined level.

21. A turbine as claimed in any preceding claim wherein, the one or more aerofoil blades are capable of adopting a troposkien shape during rotation about the central axis.

22. A turbine as claimed in any preceding claim wherein the one or more aerofoil blades comprise one or more flexible ropes enclosed by an aerofoil shaped profile.

23. A turbine as claimed in claim 22 wherein, the aerofoil shaped profile contains a packing material to mechanically fix the aerofoil shaped profile to the one or more ropes .

24. A turbine as claimed in any preceding claim further comprising connection means provided at an end of the one or more blades which are releasably connectable to the central axis such that when speed of rotation of the turbine about the central axis increases to or over a predetermined threshold level the one or more blades are released.

25. A turbine as claimed in claim 24 wherein, the connection means is releasably connectable by means of a clamp.

26. A cross flow turbine comprising: one or more aerofoil blades rotatably mounted about a central axis and connected to the central axis at or near each end of the one or more blades by connection means wherein the connection means provided at one end of the one or more blades is releasably connectable and is released when speed of rotation of the turbine about the central axis increases to or over a predetermined threshold level.

27. A turbine as claimed in claim 26 wherein releasing one end of the one or more aerofoil blades causes the forward driving force of the one or more blades to be reduced.

28. A turbine as claimed in claim 26 or 27 wherein excess tension in the one or more blades due to centripetal forces caused by excess rotational speed causes the one or more blade ends to be released.

29. A turbine as claimed in any of claims 26 to 28 wherein the one or more aerofoil blades are flexible.

30. A turbine as claimed in any of claims 26 to 29 wherein the connection means is releasably connectable by means of a clamp.

31. A turbine as claimed in any of claims 26 to 30 wherein each of the one or more aerofoil blades are releasably connectable and wherein release of the one or more blades occurs upon reaching said predetermined speed of rotation threshold.

32. A turbine as claimed in any of claims 26 to 31 wherein the one or more blades are released substantially simultaneously.

33. A turbine as claimed in any of claims 26 to 32 wherein a single mechanism is used to release all of the one or more blades.

34. A turbine as claimed in any of claims 26 to 33 wherein when the one or more blades are released they swing out under centripetal forces.

35. A turbine as claimed in claim 34 wherein release of the one or more blades immediately slows the turbine.

36. A turbine as claimed in claim 35 wherein further slowing then occurs due to the adverse aerodynamic geometry of the one or more blades when held at one end only.