Cross Flow Twist Turbine

Abstract

A cross flow turbine having 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. The blades have a degree of torsional flexibility that make them twistable about the longitudinal blade axis to reduce the aerodynamic efficiency of the blades to control the rotational speed of the turbine. The twist of the blades can be actively controlled by means of a spring, other mechanical actuator or motor.

Description

The invention relates to a turbine and in particular, but not exclusively to a turbine of the form where the operating fluid moves substantially across the axis of rotation of the machine.

Wind turbines, and in particular horizontal axis wind turbines (HAWTs) are commonly used to harness the kinetic energy of wind to produce electricity. HAWTs can be seen in many places across the country, mounted on large towers to catch the faster winds that blow at such heights.

HAWTs have a rotor shaft and a generator mounted atop such towers, with (usually) three large turbine blades designed to convert a perpendicular airflow into rotational motion. The rotation of the rotor shaft generates electricity by means of the generator. Such turbines have high tip speed ratios, high efficiency and low torque ripple which increases reliability.

As the towers on which the HAWTs are mounted generate turbulence, the turbine itself will most often be positioned upwind of the tower. For a change in wind direction from, say, NE to SW, this would require a 180° rotation of the turbine to resume. Some small turbines make use of a wind vane to align the turbine with the wind. Other large turbines have wind direction sensors and motors to rotate the turbines automatically and optimise efficiency.

One drawback of realigning a turbine by rotation is that gyroscopic forces act on the blades as they rotate and the whole turbine turns. This causes twisting forces to be exerted on the turbine which can result in fatigue and eventually damage to components of the turbine.

Savonius type wind turbines operate on a vertical axis, but are generally less efficient than lift producing turbines. Savonius type wind turbines are similar to anemometers, being that they have two or three scoops arranged to catch the wind. The main benefit of such turbines is that they require little maintenance, and are much cheaper than similarly sized HAWTs. Additionally, there is no need to direct the turbine as they can operate with any cross flowing wind. However, Savonius turbines are inefficient as there is always a surface which is subject to some amount of drag. Hence Savonius turbines are known as drag type systems.

Darrieus wind turbines (also known as “eggbeater” turbines) are another example of vertical axis turbines. One of the benefits of vertical axis turbines is that the generator (which may be bulky and/or heavy) can be located at the base of the turbine or on the ground. As with Savonius type wind turbines, there is no requirement to point Darrieus wind turbines into the wind. This is particularly advantageous for situations where the turbine is located in built up areas where nearby buildings cause increased wind turbulence.

Other advantages over HAWTs are that the blades have no tips or ends, and therefore there is no tip noise, turbulence or drag on blade ends. Additionally, the troposkien shape that the blades naturally assume mean that there is no bending force on the rope or ropes therein, only tensile forces distributed along the length of the rope(s).

Darrieus wind turbine devices have been in existence since 1931. In that time very few significant advances have been made on the initial design. Commercially exploitable Darrieus turbines have been difficult to produce for a number of reasons.

In general, they are low efficiency, which significantly limits their potential applications and their commercial viability. Also, large thrust loadings on the main bearings of such turbines means that bearing selection is critical.

The long blades of Darrieus turbines have many natural frequencies of vibration which must be avoided during operation. Some turbines have two or three rotational speeds that must be gone through quickly to reach operating speed. Several modes may fall within the operational band and thus a control system should be used to avoid these modes.

When this type of machine is used in a variable speed fluid such as atmospheric wind flows there can be a problem controlling the rotational speed in high wind conditions. This is particularly problematic in efficient “lifting” type machines, such as HAWTs or Darrieus type turbines, where destructive rotational speeds can be reached.

Therefore, another important consideration is that the rotational speed of the turbine must be limited in high wind conditions and various attempts have been made to do so.

UK Patent Application 2,216,606 A in the name Jeronimidis et al discloses blades for use with turbines with a horizontal or vertical axis of rotation. The blades exhibit an anisotropy which causes them to bend or stretch as the rotational speed increases. The bending and/or stretching affect the rotational speed of the blades as the angle of attack is changed and the load on the blades is altered.

U.S. Pat. No. 4,500,257 discloses a braking system for a vertical axis wind turbine in which a block is slidably located on a blade. A solenoid releases the block at a desired time and the block moves up the blade towards its outermost point under centripetal force. The reduced aerodynamic efficiency reduces the rotational speed.

French Patent Application 2 583 823 shows a vertical axis wind turbine which has a drum or disk brake to implement a mechanical braking system when the rotation of the turbine reaches a threshold speed.

Such drag devices and mechanical brakes have been proposed to limit rotational speeds in horizontal and vertical axis turbines. Drag devices can be unreliable, and need to be maintained. Mechanical brakes are cumbersome and result in wear and tear on the system. Such methods of limiting rotation may also impact on the smoothness of power output from the turbine.

An object of this invention is to provide aerodynamic limiting of the upper rotational speed of a turbine.

A further object of this invention is to provide aerodynamic limiting of the upper rotational speed of a turbine by twisting.

In accordance with a first aspect of the invention there is provided 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.

Twisting the one or more aerofoil blades out of optimal lift conditions limits the speed of rotation of the turbine by reducing forward driving forces and increasing drag forces.

Preferably, the one or more blades are provided with a rotatable connector to allow the blade to twist about the longitudinal blade axis.

Preferably, the rotatable connector couples the one or more aerofoil blades to the central axis at one end of the blade.

Optionally, the rotatable connector couples the one or more aerofoil blades to the central axis at both ends of the one or more blades.

Optionally, 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.

Optionally, rotation of the rotatable connector is driven by tension in the one or more blades caused by centripetal force.

Preferably, the rotatable connector is provided with a rotation inhibiting means that prevents rotation below a predetermined centripetal force threshold.

Preferably, the rotation inhibiting means comprises a torsion spring wound against a rotation stop which holds the one or more blades in place.

Optionally, 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.

Optionally, the rotation inhibiting means comprises two triangular sections of stiff material with flexible links therebetween, said links forming a Z shape.

Preferably, the one or more aerofoil blades are configured to twist in a predetermined direction when a tension threshold is reached.

Preferably, the one or more aerofoil blades are configured to twist in a first direction to feather turbine rotation.

Optionally, the one or more aerofoil blades are configured to twist in a second direction to stall turbine rotation.

Optionally, rotation of the rotatable connector is driven by an actuator.

Preferably, the actuator operates at a predetermined threshold of central axis rotational velocity.

Preferably, the actuator is powered.

Preferably, the actuator is manually controllable.

Optionally, the actuator is automatically controllable.

Preferably, the torsional flexibility of the one or more aerofoil blades are set at a predetermined level.

Preferably, the torsional flexibility of the one or more blades can be engineered such that the degree of twist causes a proportional degree of twist at the mid-point between the ends of the one or more blades.

Preferably, said level is set such that substantially 180° of twist at one end of the one or more blades causes substantially 90° of at the mid-point between the ends.

This will effectively stop the driving force on the one or more blades.

Optionally, said level is set such that substantially 180° of twist at one end of the one or more blades causes 120° of twist at the mid-point between the ends.

Optionally, said level is set such that substantially 180° of twist at one end of the one or more blades causes 60° of twist at the mid-point between the ends.

Typically, the speed of rotation of the turbine will be controlled by a lesser rotation at the one or more blade ends as any rotation will affect the aerodynamic properties of the one or more blades and increase drag.

Once the speed of rotation of the turbine had reduced to or below an acceptable operating level, the one or more blade ends will return to their original position for optimum blade aerodynamics.

Preferably, the one or more aerofoil blades are capable of adopting a troposkien shape during rotation about the central axis.

Preferably, the one or more aerofoil blades comprise one or more flexible ropes enclosed by an aerofoil shaped profile.

Preferably, the aerofoil shaped profile contains a packing material to mechanically fix the aerofoil shaped profile to the one or more ropes.

Preferably, the cross flow turbine further comprises connection means provided at an end of the one or more blades which is 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.

By releasing one end of the one or more aerofoil blades to fly out, the forward driving force of the one or more blades is reduced.

Excess tension in the one or more blades due to centripetal forces caused by excess rotational speed can cause the one or more blade ends to be released.

This feature provides the present invention with a fail safe mechanism operable in extreme weather conditions.

Preferably, the one or more aerofoil blades are flexible.

Preferably, the connection means is releasably connectable by means of a clamp.

Preferably, the cross flow turbine comprises a plurality of aerofoil blades each of which are releasably connectable and wherein release of all blades occurs upon reaching said predetermined speed of rotation threshold.

Preferably, said blades are released substantially simultaneously.

Preferably, a single mechanism is used to release all of the blades.

When the blade ends are released they swing out under centripetal forces. The resulting increase in diameter produces an increase in angular inertia which immediately slows the turbine. Further slowing then occurs due to the adverse aerodynamic geometry of the blades when held at one end only.

In accordance with a second aspect of the invention there is provided 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.

By releasing one end of one or more aerofoil blades to fly out, the forward driving force of the one or more blades is reduced.

Excess tension in the one or more blades due to centripetal forces caused by excess rotational speed can cause the one or more blade ends to be released.

This feature provides the present invention with a fail safe mechanism operable in extreme weather conditions.

Preferably, the one or more aerofoil blades are flexible.

Preferably, the connection means is releasably connectable by means of a clamp.

Preferably, the cross flow turbine comprises a plurality of aerofoil blades each of which are releasably connectable and wherein release of all blades occurs upon reaching said predetermined speed of rotation threshold.

Preferably, said blades are released substantially simultaneously.

Preferably, a single mechanism is used to release all of the blades.

When the blade ends are released they swing out under centripetal forces. The resulting increase in diameter produces an increase in angular inertia which immediately slows the turbine. Further slowing then occurs due to the adverse aerodynamic geometry of the blades when held at one end only.

The invention will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 shows a view of a twist type turbine perpendicular to the axis of turbine rotation;

FIG. 2 shows a view along the axis of turbine rotation;

FIG. 3 (Detail A) shows a representation of a rotatable end fixing for a blade;

FIG. 4 shows a representation of a hub twisting configuration;

FIG. 5 shows the hub twisting configuration in more detail;

FIG. 6 shows a representation of an alternative hub twisting configuration;

FIG. 7 shows the alternative hub twisting configuration in more detail;

FIG. 8 shows a representation of a twisting mechanism located at both ends (hubs) of the blade;

FIG. 9 shows a representation of a twisting mechanism located at the centre of the blade;

FIGS. 10 (a) to (f) show cross-sectional representations of proposed blade configurations;

FIG. 11 shows a view of a release type turbine perpendicular to the axis of turbine rotation;

FIG. 12 shows a view along the axis of turbine rotation; and

FIG. 13 (Detail A) shows a representation of a releasable end fixing for a blade.

The embodiments that will be discussed herein are intended to twist turbine blades out of optimum lift conditions, incorporating either stall or feathering conditions. The aim is to limit the rotational speed of the turbine, for example in high wind conditions. Twisting of the blades may occur naturally at a particular centripetal force corresponding to perhaps a maximum desired rotational speed.

Twisting to stall involves twisting in such a direction as to increase the angle of attack sufficiently to induce aerodynamic stall. Twisting to feather involves twisting in the opposite direction, inducing feathering by decreasing the angle of attack. Stalling may cause excessive vibration of the blades to occur. Feathering does not produce such vibration problems, however a much larger degree of twist is required.

As shown in FIG. 1 three aerofoil blades 2 are fixed at each end to hubs (4 and 5) mounted on a rotating shaft 3. The shaft will normally be mounted in bearings not shown and connected to a driven load such as an electrical generator.

Each aerofoil blade 2 is made to be strong in tension but semi flexible in bending.

In this example each blade is held firmly at one hub 4 end. The other end of the blade is held in a rotating section 1. In this example the rotation is induced by tension force in the blade due to centripetal forces on the blade as it rotates. The rotating section may be adjusted so that no rotation occurs until a threshold force is reached so that the blade stays in its preset (as shown) start up position until this point.

An example of one form of rotatable connector will be described, with reference to FIGS. 4 and 5. A short length of ball screw 6 is attached to the blade end 7. The matching recirculating ball nut 8 is attached to the hub. A torsion spring 9 is axially aligned along the ball screw 6 axis and attached at one end to the hub 5 and at the other end to the blade end 7. The torsion spring 9 is wound against a rotation stop which holds the blade end in the normal angular position and is wound up enough to prevent the ball screw 6 turning until the design rpm has been reached for speed control to start. When this speed is exceeded the ball screw 6 turns as the tension on the blade 2 creates enough force along the helical slope of the screw 6 to overcome the torsion spring 9 preload. As the blade 2 is twisted by this action the net forward aerodynamic forces on the blade 2 are reduced preventing further increase in rpm. Preferably all three blade ends act in this way to preserve balance.

Another example of the rotatable connector is illustrated in FIGS. 6 and 7. The blade 2 comprises 2 ropes (10 and 11), which run the length of the blade 2. One rope 10 is bolted to the hub 5 so as to provide a fixed pivot point. The other rope 11 is connected to a spring 12 or other damper such that when the threshold speed is exceeded, similarly to the abovementioned example, the spring tension is overcome and the blade 2 is able to twist, with the bolted rope 10 acting as a pivot for said twisting.

The decision on which side is bolted and which side is connected to the spring will depend on whether a stalling or a feathering effect is desired.

Another example of the rotational mechanism is a short spiral arrangement of spring lengths at the end of the blades such that each individual spring section is fixed at a helical angle to the hub at one end and the blade at the other end, all forming a circularly displaced group. When tension is applied by the blade centripetal force the extension of the springs produces a rotational effect on the blade.

Another example of the rotational mechanism is an arrangement of two triangular sections of stiff material with flexible links between arranged such that the axis of the links form a Z shape. Each of the three link sections is folded in opposition to the adjacent such that when one end link of the “Z” is attached to the blade and the other end link of the “Z” is attached to the hub, when the blade is moved away from the hub the folds open and the blade rotates with respect to the hub.

The spring retaining force can be by a separate attached spring or by making the links themselves of a spring material.

FIGS. 8 and 9 show different ways in which the rotatable connector twisting mechanism may be deployed. FIG. 8 shows the rotatable connector located at both hubs (4 and 5). FIG. 9 shows an alternative configuration where the rotatable connector is located at the midpoint 13 of the blade 2. Twisting at the midpoint 13 of the blade 2 may serve to reduce the extent of displacement required when compared to twisting at the hubs (4 and 5). Any of the twisting mechanisms herein discussed may be suitable for locating at either hub, or indeed at the midpoint of the blades.

FIG. 10, (a) to (f), show various configurations of blade that may be adopted. (a) shows a blade consisting of a single rope 14 inserted in an aerofoil shaped cross-section rubber body 15. (b) comprises a double rope 16 for added tensile strength. Such a blade may also be used with the twisting mechanism of FIGS. 6 and 7. Multiple ropes or wires 17 may also be used for tensile strength and also to control the extent and conformity of the twist. Similarly, a double loop rope 18 might offer increased tensile strength while still be suitable for the twist mechanism employed in FIGS. 4 and 5. 2 double loop ropes 19 offers an analogous configuration for the embodiment of FIGS. 6 and 7. A yet further alternative embodiment utilises a hollow body 20 with a filler 21. The hollow body is preferably of a fibre material to carry tensile loads, e.g. in the troposkien shape during operation.

The cross-section may be varied towards the hubs in order to smooth out the variation in forward thrust depending on position along the axis of rotation.

An embodiment of the present invention which incorporates means for releasing one or more blades is illustrated in FIG. 11. This embodiment of the invention provides a fail safe mechanism and will prevent rotation of the turbine in extremely high winds. This mechanism can be incorporated in a turbine containing means for twisting the blade in accordance with the present invention. Three aerofoil blades 102 are fixed at each end to hubs (104 and 105) mounted on a rotating shaft (103). The shaft will normally be mounted in bearings (not shown) and connected to a driven load such as an electrical generator.

Each aerofoil blade is made to be strong in tension but semi flexible in bending.

Each blade is held firmly at one end to hub 104. The other end of the blade is held in a releasing clamp 101. Blade release from the clamp is induced by tension force in the blade due to centripetal forces on the blade as it rotates. This is calibrated to occur if other speed limiting systems such as generator loading have failed and emergency overspeed protection is needed.

To maintain rotational balance in the turbine all the clamps are linked such that when one releases all the others are released.

One example of a releasing mechanism is to hold the blade ends in a slot which keeps them in the correct orientation. All the blades are prevented from pulling out of the slot by a loop of wire or cord of known breaking strength which is looped in turn through a hole or pin in each blade. If the rotational speed of the turbine reaches overspeed condition the loop breaks and all the blades are released from the slots.

Another example of a releasing mechanism is to hold all the releasable blade ends in a slot formed by the gap between two hub sections. Each blade has a “detent” at its end that engages with a protrusion in one hub “half” to hold it in position. The force to keep the blades engaged is provided by a common spring or weight acting substantially along the axis of the turbine shaft. The moving hub half is able to rock slightly to apply equal force to all blade clamps. If the blade centripetal tension increases enough to pull the blade from one node of the clamp the resulting void allows the clamp to tilt and release the other blades.

It is clearly advantageous to release all the blades simultaneously.

It is envisaged that a combination of the twist-type turbine and the release-type turbine would provide a solution with inherent speed limiting means and an emergency means for stopping the turbine if a threshold release speed was reached.

The present invention provides many advantages suitable for domestic implementation of turbines. Turbulent airflows, such as are common in domestic environs, may be harnessed by vertical axis turbines. Additionally, the safety aspects of the invention, namely the velocity limiting system and the emergency release that can be effected by the release system, make the invention advantageous over HAWTs for domestic use. There is also the significant advantage of reduced vibration compared to small scale HAWTs and previous Darrieus-type turbines.

Improvements and modifications may be incorporated herein without deviating from the scope of the invention. For example, the invention has been exemplified by application to wind turbines. It is proposed that the invention could be employed in other fluid mediums such as water. Additionally, the twisting mechanism may be implemented by motors or any other suitable control device.

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 claim 2 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 claim 1 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 claim 2 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 claim 2 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 claim 1 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 claim 1 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 claim 1 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 claim 2 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 wherein, the actuator is powered.

18. A turbine as claimed in claim 15 wherein, the actuator is manually controllable.

19. A turbine as claimed in claim 15 wherein, the actuator is automatically controllable.

20. A turbine as claimed in claim 1 wherein, the torsional flexibility of the one or more aerofoil blades are set at a predetermined level.

21. A turbine as claimed in claim 1 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 claim 1 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 claim 1 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 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 claim 26 wherein the one or more aerofoil blades are flexible.

30. A turbine as claimed in claim 26 wherein the connection means is releasably connectable by means of a clamp.

31. A turbine as claimed in claim 26 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 claim 26 wherein the one or more blades are released substantially simultaneously.

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

34. A turbine as claimed in claim 26 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.