Improvements in or relating to energy generation

Abstract

The apparatus and method include alleviating loads on first and second rotor blades connected to one another at a rotor hub and enabling independent rotation of each of the first and second blades about a pitch axis. The apparatus includes first and second rotor blades, each having a respective first end region connected to one another at a rotor hub, and a load alleviation mechanism for the first and second blades that enables independent rotation of each of the first and second blades about the pitch axis, the load alleviation mechanism preventing rotation of each of the first and second blades about the pitch axis until a threshold load is reached.

Description

[Technical Field]

[0001] The present invention relates to improvements in energy generation using horizontal axis turbines, and in particular to reversible turbine rotor blades that passively move about a pitch axis. [Background technology]

[0002] Patent Document 1 discloses an apparatus for converting the kinetic energy of flowing water into the kinetic energy of a rotatable rotor shaft, the apparatus being adapted to be effective in a first flow direction and a second flow direction substantially opposite the first flow direction, the apparatus comprising two rotor blades each connected to a blade shaft coupled to the rotor shaft for transmitting torque to the rotor shaft, each of the blades being rigidly connected to its respective blade shaft, the blade shafts being mounted to passively rotate about their own axes relative to the rotor shaft, the blade shafts also being coupled to each other to rotate in the same direction, and the rotor blades each having an asymmetric cross section. Therefore, since there is always a flow from the same direction to the asymmetric rotor blades during the energy conversion process, the profile of the rotor blades can be optimized to some extent with respect to the flow direction.

[0003] Prior art alternatives for thrust reduction include actively (rather than passively) rotating turbine blades to orient their leading edges into the oncoming flow, thereby reducing thrust on the blades as the flow speed increases. However, actively controlled pitch systems require power transmission across rotating interfaces, sensors, actuators, and control systems. As a result, major problems arise in that active pitch systems are too expensive and unreliable.

[0004] Because fixed-pitch devices cannot reduce load with pitch, fixed-pitch turbines either have a smaller rotor diameter than turbines with variable-pitch blades (to keep load within limits at high flow speeds, resulting in lower power extracted at a given flow speed as power scales with the square of the rotor diameter), or have a lower cut-out flow speed (i.e., the fluid flow speed above which the turbine cannot operate without damage, e.g., thrust limit that could compromise the structural integrity of the machine). This avoids high loads that would lead to loss of energy yield. This loss is further exacerbated by the fact that the power of tidal currents is proportional to the cube of the flow speed.

[0005] Beneficially, if a turbine can maintain near its maximum potential power output for a longer period of time despite natural flow fluctuations, the turbine can generate more power at a lower cost because such flow fluctuations are associated with fatigue loads that can be mitigated by a blade pitching system.

[0006] Electrical power can be obtained from the movement of a fluid by rotating one or more turbine rotor blades. A desirable characteristic of such rotor blades designed for power generation is their ability to generate a force tangential to the rotor plane, inducing torque and converting it into electrical power. Undesirable characteristics of rotor blades include, for example, thrust in the direction of fluid flow, especially in flowing water environments, and other parameters that vary with flow velocity. These undesirable characteristics result in high costs due to the high strength required for the support structure and turbine drive train.

[0007] Therefore, the maximum thrust allowed for a given strength capacity limits the size of the rotor blades attached to the turbine and the maximum flow speed at which the turbine can operate. However, this also limits the turbine's power output and energy yield. Thrust is just one example of a mechanical or electromechanical load associated with a turbine rotor blade; other examples include power, torque, and voltage. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2009 / 031887 Summary of the Invention

[0009] According to one aspect of the present invention, there is provided an apparatus comprising: first and second rotor blades, each rotor blade having a respective first end region connected to one another at a rotor hub; and a load relief mechanism for the first and second blades that enables independent rotation of each of the first and second blades about a pitch axis, the load relief mechanism preventing rotation of each of the first and second blades about the pitch axis until a threshold load is reached.

[0010] According to a second aspect of the present invention, there is provided a method for relieving loads on first and second rotor blades connected to one another at a rotor hub to enable independent rotation of each of the first and second blades about a pitch axis, wherein rotation of each of the first and second blades about the pitch axis is prevented until a threshold load is reached.

[0011] These aspects allow more power to be generated at lower fluid flow velocities, increasing the flow velocity at which the turbine must stop generating power, resulting in a greater energy yield.

[0012] Advantageously, the resilient device is located in each first end region between the first and second rotor blades and forms part of the load relief mechanism. Alternatively, the load relief mechanism may be formed by the material from which the rotor blades are made and is capable of bending, allowing the blade cross section to translate and rotate about the pitch axis.

[0013] The connectable first and second rotor blades allow a passive pitch mechanism formed by the elastic connection between the first and second rotor blades to mitigate or reduce thrust and other loads on them when current velocities are high (approximately 1 m / s to approximately 6 m / s, depending on the particular location, and possibly as high as approximately 10 m / s). The advantage of such an elastic connection is that larger turbine blades can be installed, thereby capturing more of the available energy when current velocities are low (approximately 0.2 m / s to 3 m / s, depending on the location). When current velocities are high, the passive pitch mechanism acts to mitigate or reduce thrust and other loads to compensate for the use of larger rotor blades. Furthermore, when current velocities are high, the action of the passive pitch mechanism and elastic device reduces thrust and other loads below what would be the case with fixed-pitch rotor blades. This keeps thrust and other loads within an acceptable range, allowing power generation to continue when the turbine would otherwise need to be shut down.

[0014] Additionally, the passive pitch mechanism also helps to mitigate fluctuating or cyclic loads due to turbulence, waves, flow misalignments, or varying current velocities at different depths in the water column. When the current velocity, and therefore the thrust, is relatively low, there is little blade pitch motion, and therefore the thrust reduction effect (and power reduction effect) is minimized.

[0015] The load alleviation mechanism serves to relieve or reduce loads such as thrust without requiring power transfer across rotary interfaces, sensors, actuators, or control systems as required for active pitching mechanisms, making passive pitch systems less expensive and more reliable than such active systems.

[0016] Advantageously, the rotor blade center of effort of thrust is aft of the pitch axis of the first and second rotor blades, resulting in loads inducing leading edge or nose-down pitching moments on the rotor blades. The location of the center of effort can be advantageously adjusted by design using, for example, blade sweep, blade rake, and blade aerofoil section shape techniques.

[0017] Preferably, each rotor blade is pivotally mounted for resiliently rotating about a pitch axis.

[0018] Preferably, the resilient device is a torsionally compliant fixture, and the passive pitch rotation about the pitch axis is constrained by the torsionally compliant fixture, such as at least one spring device. The amount of rotation about the pitch axis may be limited by one or more physical stops located at the interface between the first rotor blade and the second rotor blade.

[0019] The resilient device may take the form of a suspension unit disposed between the first and second rotor blades forming a resilient connection, and preferably made from a rubber material or the like.

[0020] The amount of rotation about the pitch axis can be controlled by the response of the elastic device, which may be designed to achieve a linear or non-linear response (e.g., loads occurring at low flow velocities result in small deflections, and loads occurring at high flow velocities result in large deflections). The elastic device may be preloaded so that no deflection occurs below a critical flow velocity.

[0021] It will be appreciated that the greater the load, such as thrust, acting on the rotor blades, the more leading edge or nose-down pitching moment will be generated, and therefore the more pitch angle change will partially relieve the thrust. A natural feedback between load and pitch is created, with the load being reduced more at higher flow speeds and less at lower flow speeds, thereby benefiting from achieving power generation over a wider range of flow speeds for a given load threshold.

[0022] The primary advantage of the elastic connection between the first and second rotor blades is that the blades are configured on a rotatable hub according to the arrangement described in U.S. Patent No. 6,277,949. This allows the blades to rotate together about a common axis to remain balanced with respect to hydrodynamic forces (meaning that the pitch angle on each blade is substantially the same, and the loads applied to each blade are substantially the same). This prevents undesirable out-of-plane bending and undesirable vibration on the rotor bearings. If the first and second rotor blades were separately attached to the rotor hub using separate elastic devices, the behavior of each elastic device would change differently over time, potentially causing the rotor blades to become hydrodynamically unbalanced. The elastic coupling between the first and second rotor blades prevents such unbalance from occurring; the coupling is configured to allow rotation about the pitch axis and provide a moment connection of the first rotor blade to the second rotor blade. Such a connection is advantageously achieved, for example, by a blade root cylindrical stub or plug at the first end region of the first rotor blade that is inserted into or over a corresponding opening or socket in the first end region of the second rotor blade, the connection being aligned with the common pitch axis. This may, for example, provide a bearing surface between the outer surface of the cylindrical stub of the first rotor blade and the cylindrical inner surface of the opening of the second rotor blade. Alternatively, each blade root may be connected to a common blade hub, each with a separate bearing attached to the outer edge of the hub, such that torsional flexibility exists between each blade root and the common blade hub, allowing passive pitch under load.

[0023] The load relief mechanism may comprise a resilience in the material from which the rotor blade is made. The rotor blade may be arranged to flex under the load, thereby rotating about the pitch axis.

[0024] The rotor hub to which the first and second rotor blades are connectable preferably includes a cylindrical bearing tube extending in the rotor plane, the inner surface of the hub bearing tube capturing the connection of the first and second rotor blades via an outer cylindrical bearing surface of the cylindrical opening of the second rotor blade, which allows free rotation of the blade pair within the rotor hub about the pitch axis.

[0025] Advantageously, the configuration of the first and second rotor blades, similar to the configuration described in the '661 patent, also allows the blades to be reversed to face substantially opposite current directions. Reversals in current direction are typically found in tidal environments due to the occurrence of flood and ebb tides. The rotor blade reversal mechanism allows for self-pitch alignment of the blades, preventing significant hydrodynamic misalignment issues during changing flow conditions. A further advantage of the present invention is that the self-pitch alignment capability of the first and second rotor blades helps reduce unbalanced forces on the blades that may arise due to waves, current shear, misalignment, and turbulence.

[0026] The passive pitch mechanism may also be applied to unidirectional fluid flow, such as in non-tidal regions of a river. [Brief explanation of the drawings]

[0027] In order to clearly and completely disclose the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:

[0028] [Figure 1] 1 shows a schematic elevation view of a known two-blade turbine mounted on a support structure; [Figure 2] 2 shows a schematic view similar to FIG. 1 of a two-blade turbine (without support structure) with a first rotor blade resiliently connected to a second rotor blade; [Figure 3a] 3 shows a partial cross-sectional view of the elastic coupling of FIG. 2, with the axis of pitching motion indicated by a dotted line. [Figure 3b] 3b shows a cross section of the elastic coupling of FIG. 3a in a plane perpendicular to the axis of the pitching movement. [Figure 3c] 3b, but with multiple magnetic devices between the rotor blades providing an initial torque that holds the first and second rotor blades in an initial position. [Figure 3d] 3c shows a graphical plot illustrating the relationship between the elastic connection between the first and second rotor blades and the pitching moment exerted by the plurality of magnetic devices of FIG. 3c for a range of pitch angles. [Figure 4a] 1 shows a cross-sectional view of the elastic coupling in a plane perpendicular to the axis of pitching motion. [Figure 4b] 1 shows an isometric perspective view of a rotor blade during high tide with relatively low current speeds. [Figure 5a] A similar view to Figure 4a is shown during flood tide, when the flow velocity is relatively high. [Figure 5b] A similar view to Figure 4b is shown during flood tide, when the flow velocity is relatively high. [Figure 6] A similar view to Figure 4b is shown, but during an ebb tide with relatively high current velocities. [Figure 7] A similar view to Figure 5b is shown, but during an ebb tide with a relatively high current velocity. [Figure 8a] 10 shows a schematic cross-sectional view of a second embodiment of a resilient coupling during high tide with relatively low flow velocity. [Figure 8b] 10 shows a schematic cross-sectional view of a second embodiment of a resilient coupling during high tide with relatively low flow velocity. [Figure 9a] 10 shows a schematic cross-sectional view of a second embodiment of a resilient coupling during high tide with relatively high flow velocity. [Figure 9b] 10 shows a schematic cross-sectional view of a second embodiment of a resilient coupling during high tide with relatively high flow velocity. [Figure 10a] 10 shows a cross-sectional view of a resilient coupling according to a second embodiment. [Figure 10b] FIG. 10 shows an isometric perspective view of a rotor blade with a resilient coupling of the second embodiment during high tide with relatively low current speeds. [Figure 11a] A view similar to Figure 10a is shown, but for the second embodiment, during a relatively high current. [Figure 11b] A view similar to Figure 10b is shown, but for the second embodiment, during a flood tide with a relatively high flow velocity. [Figure 12a] 3a shows a view similar to FIG. 3a, but of a third embodiment of the elastic coupling; [Figure 12b] 3b shows a view similar to FIG. 3b, but of a third embodiment of the elastic coupling. [Figure 13a] 4a shows a view similar to FIG. 4a, but of a third embodiment of the elastic coupling; [Figure 13b] 4b shows a view similar to FIG. 4b, but of a third embodiment of the elastic coupling. [Figure 14a] 5a shows a view similar to FIG. 5a, but of a third embodiment of the elastic coupling; [Figure 14b] 5b shows a view similar to FIG. 5b, but of a third embodiment of the elastic coupling. [Figure 15a] 10a shows a view similar to FIG. 10a, but showing a fourth embodiment of the resilient coupling. [Figure 15b] 10b shows a view similar to FIG. 10b, but of a fourth embodiment of the resilient coupling. [Figure 16a] 11 shows a view similar to FIG. 11a, but showing a fourth embodiment of the elastic coupling. [Figure 16b] 11b shows a view similar to FIG. 11b, but showing a fourth embodiment of the elastic coupling. [Figure 17a] FIG. 1 is a partial side view of a turbine blade in which the blade body is flexed to elastically rotate about a pitch axis. [Figure 17b] 17b shows an isometric perspective view of the rotor blade pair of FIG. 17a in a relatively low current flowing in a flood tide. [Figure 18a] Similar to Figure 17a, but during a flood tide with relatively high current velocities. [Figure 18b] Similar to Figure 17b, but during a flood tide with relatively high current velocities. [Figure 19] 1 is a line graph of thrust on a turbine blade as a function of fluid flow velocity for two different types of turbines. [Figure 20] 1 is a line graph of power generation as a function of fluid flow rate for two different types of turbines. DETAILED DESCRIPTION OF THE INVENTION

[0029] Referring to FIG. 1 , a known turbine having first and second rotor blades with reversible function includes a support structure 1 fixed to the seabed by any suitable conventional technique, and a turbine fixing means 2 of the turbine support structure configured to prevent the turbine from moving or rotating. The structure 1 is located in a bidirectional water flow environment, with arrow 3 indicating the direction of thrust load during tidal currents and arrow 4 indicating the direction of thrust load during ebb tides. Arrows X and Y indicate the directions of fluid flow during flood tide and ebb tide, respectively. A rotor hub 6 and a rotor shaft 6 are immovably fixed together, and the rotor hub 6 is arranged as a tube / sleeve to receive a first end region of a first rotor blade 7 having a leading edge 12 and a first end region of a second rotor blade 8 having a leading edge 13. Both the first and second rotor blades are connected to a blade shaft 9 located between the ends of the first end regions of the rotor blades 7 and 8.

[0030] In Figure 1, the turbine is shown aligned and fixed to urge rotor blades 7 and 8 substantially toward the direction of flood tide X, which exerts thrust load 3. When the tide turns to ebb current Y, the turbine alignment remains unchanged and the flow of ebb current Y temporarily aligns substantially with the underside of the rotor blade faces. A first blade 7 is fixedly fixed to blade shaft 9 and a second blade 8 is also fixedly fixed to blade shaft 9, and these three components 7, 8 and 9 rotate and pitch together to change alignment and urge rotor blades 7 and 8 substantially toward ebb current Y under the action of fluid motion on blades 7 and 8, thereby allowing the flip-around function of the rotor blades to realign and face the reverse current direction. Thereafter, once the flip-around function occurs, the pitch alignment of the rotor blades 7 and 8 is held substantially fixed by fluid forces on the rotor blades, such that when a pitch disturbance occurs, the coordinated movement of the rotor blades 7, 8 and blade shaft 9 produces a nose-up pitching motion and associated thrust increase of the first blade 7, and a corresponding nose-down pitch and thrust decrease of the second blade 8. This creates a thrust imbalance at the centers of forces 14 and 15 of the rotor blades 7 and 8, respectively, which acts to restore the pitch orientation to counter the pitch disturbance as much as possible. This mechanism maintains the intended orientation of the rotor blades 7 and 8 relative to the oncoming flow. The blade shaft 9 is rotatably fixed within the rotor hub 6 about a pitch movement axis 10, and bearings 11 allow the pair of blades 7, 8 to freely rotate in the pitch direction. When the flood tide X shifts to the ebb tide Y, the rotor blades 7 and 8 rotate about axis 10, thereby flipping to face the changed flow direction. The shape of the rotor blades 7 and 8 is such that this reversal occurs passively under the action of the fluid flow. The advantage of rotor blades that change direction with the change in flow direction is that they save costs and improve reliability. This advantage can be exploited when there is a strong tidal direction, with the ebb tide direction Y being close to 180 degrees to the flood tide direction X.Arrows A and B indicate the vertical distance between the centers of action 14 and 15 of the respective blades and the axis of pitching motion.

[0031] Referring to Figures 2, 3a, and 3b, according to the present invention, within the rotor hub sleeve 6', a first end region of a first rotor blade 7' is connected to a first end region of a second rotor blade 8' by a resilient device 22, e.g., in the form of a torsional coupling. The first and second rotor blades 7' and 8' are connected by inserting a male blade root cylindrical stub connector portion 7a in the first end region of the first rotor blade 7' into a corresponding female cylindrical opening or socket portion 8a in the first end region of the second rotor blade 8', with this connection aligned with the pitch axis 16 of the rotor blades 7' and 8'. As in Figure 1, a bearing 11' allows free rotation of the pair of blades 7' and 8' in the pitch direction through the cylindrical opening or socket portion 8a that extends through the rotor hub sleeve 6'. Additional bearings 18, 19, 20, and 21 allow resilient rotation of the first blade 7' relative to the second blade 8'. These further bearings 18, 19, 20, 21 preferably also function to prevent rotor blades 7' and 8' from sliding apart axially. Resilient device 22 is a means for maintaining the blades in a substantially neutral alignment (i.e., an alignment not altered from the design condition suitable for low-flow speed operation (for maximum power without any thrust shedding)) under low thrust conditions, and correspondingly flexes to provide the function of a torsion spring to allow independent leading edge or nose-down pitch of first blade 7' relative to second blade 8' under high load / thrust conditions, thereby providing a load relief mechanism whose function is to passively allow load relief. Hub bearing 11' ensures that realignment and pitch balance occur.

[0032] A torsional coupling 22 is located at the interface between the first rotor blade 7' and the second rotor blade 8', and the coupling is configured to rigidly support bending moments across the joint from the first rotor blade 7' to the second rotor blade 8' via bearings 18, 19, 20, and 21. Torsional resilience between the two rotor blades 7' and 8' can also be achieved using alternative embodiments (described below). For example, a rubber connector can be disposed between the first and second rotor blades, which shears to provide torsional resilience. Damping means, for example, in the form of a preloaded, damped torsion spring, may be provided to maintain the orientation alignment of the rotor blades 7' and 8' with a degree of torsional resilience, thereby causing rotation about the pitch axis under the action of a pitching moment. Preloading means may be incorporated to prevent pitching motion below a threshold load or flow velocity, and damping means may be incorporated to slow the rate of pitching motion, for example, by friction, viscous effects, or other means. Such damping may be required to prevent excessive movement or to reduce responsiveness. When thrust is relatively small, the forces on the centers of action 14' and 15' (see FIG. 4b) are correspondingly small, and the nose-down pitch of rotor blades 7' and 8' does not change substantially, if at all. However, when flow speed is relatively large, thrust is correspondingly large. The thrust acting on centers of action 14' and 15' has a moment arm about pitch axis 16. The moment on the first rotor blade 7' opposes the moment on the second rotor blade 8', and a rotation occurs at the coupling, allowing the first rotor blade 7' to independently pitch nose-down relative to the second rotor blade 8'. Passive pitch reduction reduces the thrust load; i.e., the greater the load, the correspondingly greater the reaction of elastic device 22, reducing the load experienced. In this manner, the rotation of the first rotor blade 7' is independent and opposite that of the second rotor blade 8'. The rotor blades 7' and 8' also have the ability to reverse direction in response to changes in current direction.

[0033] The rotational speed about the pitch axis 16 can be reduced by a damper installed parallel to the elastic device 22. This damper can be a rotary damper, and the torque generated is a function of the rotational speed of the blade root about the pitch axis. Alternatively, a linear damper can be connected between the roots of the first and second rotor blades 7' and 8' or between the roots of each blade and the rotor hub 6. The damper thus provides a force proportional to the speed at which the connection points between the first and second rotor blades 7' and 8' move relative to one another. This force acts radially from the pitch axis 16, and the damper generates a torque about the pitch axis. The damper therefore slows the pitch response to changes in tidal flow, helping to extract as much power as possible from the tidal flow.

[0034] In addition to, or instead of, preloading the resilient device 22, one or more magnetic devices may be attached to the roots of the first and second rotor blades 7' and 8' to provide an initial torque that holds the first and second rotor blades 7' and 8' in their initial position; this torque must be overcome before the blades 7' and 8' begin to pitch nose-down. The advantage of using one or more magnetic devices for this purpose is that the force required to separate two magnetically attracted objects decreases as the distance between the magnetic devices increases. Thus, the magnetic devices can help hold the rotor blades 7' and 8' in their optimal initial position at low flow speeds, but once a sufficient load is applied to overcome the torque of magnetic attraction so that the blades begin to pitch in opposite directions, the force from the one or more magnetic devices rapidly decays. Thus, the one or more magnetic devices do not prevent the rotor blades 7' and 8' from pitching sufficiently to reduce the load generated by the rotor at high flow speeds.

[0035] 3c shows one possible arrangement of multiple magnetic devices in the form of magnets 31, 32, 33, and 34 arranged around the radial space between the root portions of the first and second rotor blades 7' and 8' and alternately connected to the first rotor blade 7' and the second rotor blade 8' with resilient devices 22 therebetween. The attractive force between the north pole element of one magnet 31 connected to the root portion of the first blade and the south pole element of magnet 34 connected to the root portion of the second blade decays rapidly as the first rotor blade 7' rotates clockwise relative to the second rotor blade 8'.

[0036] FIG. 3d shows a graph illustrating the relationship between the pitching moment exerted by the elastic device 22 and the magnets 31, 32, 33, and 34 shown in FIG. 3c for a range of pitch angles of the first blade 7' relative to the second blade 8'. The solid line in FIG. 3d shows the sum of the pitching moment of the elastic device 22 combined with the pitching moment of the magnets. The advantage of using magnets 31, 32, 33, and 34 can be seen by comparing the combined behavior with the behavior of the elastic device 22 alone, shown by the dashed-dotted line in FIG. 3d. Because the pitching moment that must be overcome to initiate pitch and exceed zero degrees increases, the first and second rotor blades are held at an optimal pitch angle that allows them to capture more power until the flow velocity increases. The pitching moment required to hold the first and second rotor blades at pitch angles at the end of the shown range increases very little, meaning the rotor blades can still pitch sufficiently, reducing loads on the structure in high-speed flows.

[0037] The extent of movement of the first rotor blade 7' relative to the second rotor blade 8' by the elastic device 22 may be limited by a physical stop formed by the longitudinal protrusion 7c of the male blade root cylindrical stub connector portion 7a and the corresponding longitudinal protrusion 8c of the cylindrical opening or socket portion 8a. Both may be equipped with damping means 23 to reduce impact forces between the two rotor blades in the event of a sudden change in load. The longitudinal protrusions 7c are located within channels formed between the longitudinal protrusions 8c, and the width of the channels defines the amount of movement around the pitch axis by the rotor blades 7' and 8'. The elastic device 22 is preferably disposed between the opposing surfaces of the longitudinal protrusions 7c and 8c.

[0038] The rotor blade coupling, comprising elastic device 22 and bearings 18, 19, 20 and 21, allows rotational movement about the pitch axis but constrains rotational movement about any other axis and translational movement in any direction.

[0039] If the elastic device is preloaded against a physical stop, it will be possible for the pitch change to be zero below a threshold flow rate or load.

[0040] The resilient coupling may also be achieved by a suitable hydraulic coupling.

[0041] Although the turbine fixing means 2 (see FIG. 1) is configured to hold the turbine stationary, the turbine of FIG. 2 may also alternatively be mounted to a yawing turret on a support structure so as to be located in a flow environment where the flow direction is variable beyond bidirectional.

[0042] Reference numeral 5 and associated dashed lines in FIG. 1 indicate the additional rotor blade length allowed by the load-relief mechanism of the first and second rotor blades 7a(TM) and 8a(TM), which allows for greater power generation at a given flow speed.

[0043] 4a and 4b, rotor blades 7' and 8' are shown diagrammatically operating in a tidal current at a relatively low current speed, as indicated by arrow 26. Rotor blades 7' and 8' have connectors 7a and 8a and blade bodies 7b and 8b at first ends. Blade bodies 7b and 8b have respective leading edges 12' and 13' and are attached to connectors 7a and 8a at an angle such that the longitudinal axes of the blade bodies are not coincident with or parallel to pitch axis 16, which coincides with the axis of pitch motion about hub bearing 11'. As a result, centers of action 14' and 15' are offset from pitch axis 16 and therefore have respective moment arms 24 and 25 about pitch axis 16 when thrust acts on the blade bodies. The moment T1 on the first rotor blade 7' opposes the moment 28 on the second rotor blade 8', causing rotation at the coupling interface, which allows the first rotor blade 7' to move leading edge or nose down relative to the second rotor blade 8' (see Figures 5a and 5b) and reduce the thrust load across both rotor blades.

[0044] 5a and 5b, rotor blades 7' and 8' are shown diagrammatically operating at relatively high current speeds, also during high tide, as indicated by arrow 26. The moment on the first rotor blade 7' opposes the moment on the second rotor blade 8', causing rotation at the coupling, which allows the first rotor blade 7' to independently pitch in a leading edge or nose-down direction T1 (counterclockwise as shown in FIG. 5b) about pitch axis 16 (compared to the position in FIG. 4b) and the second rotor blade 8' to pitch in a leading edge or nose-down direction 28 (clockwise as shown in FIG. 5b) about pitch axis 16 (compared to the position in FIG. 4b), reducing the thrust load across both rotor blades and compressing the resilient device 22 between each of the longitudinal protrusions 7c and 8c.

[0045] Figures 6 and 7 show passive pitching motions similar to Figures 4b and 5b, but with rotor blades 7' and 8' inverted and facing the ebb tide at relatively low current speeds (Figure 6) and relatively high current speeds (Figure 7).

[0046] 8a-11b, a second embodiment of a resilient coupling is shown in which a first end region of a first rotor blade 7'' is connected to a first end region of a second rotor blade 8'' within a rotor hub sleeve 6'' by a hub root 30 and at least one resilient device 22 in the form of a torsional coupling, similar to that of FIGS. 2-7. The hub root 30 is formed with recesses 32 at opposing end regions positioned to receive protrusions 34 extending outward from the first end regions of each of the first and second rotor blades 7'' and 8'' and corresponding resilient devices 22 mounted between opposing surfaces of the protrusions 34 and the wall surfaces that bound the recesses 32. These abutting surfaces of the recesses 32 and the protrusions 34 form physical stops 23 and may include damping means 23. As with the configuration of FIGS. 2-7, the connection of the first and second rotor blades is aligned with the pitch axis 16.

[0047] Similar to the first embodiment described above with reference to Figures 2-7, bearing 11'' allows free rotation in pitch of pair of blades 7'' and 8''. Resilient device 22 provides a torsion spring function, which, under low thrust conditions, keeps the blades in a configuration substantially unchanged from the design condition suitable for low flow speed operation (maximum power without thrust degradation) and rotates accordingly about pitch axis 16, while under high thrust load conditions, allows independent leading edge or nose down pitch of first blade 7'' relative to second blade 8''. Hub bearing 11'' ensures that realignment and pitch balance occur.

[0048] The torsional coupling 22 is disposed at the interface between the first rotor blade 7'' and the second rotor blade 8'', and the coupling is configured to rigidly support bending moments across the joint from the first rotor blade 7'' through the hub root 30 to the second rotor blade 8''.

[0049] A damped torsion spring may be provided to maintain the orientation alignment of rotor blades 7'' and 8'' with some degree of torsional resilience as they rotate about the pitch axis under the action of a pitching moment.

[0050] When the thrust load is relatively small (see FIG. 10b), the thrust acting on the centers of action 14" and 15" is correspondingly relatively small, and the leading edges 12" and 13" or nose-down pitch of the rotor blades 7" and 8" does not change substantially. However, when the current speed is relatively large (see FIG. 11b), the thrust load is correspondingly large. The thrust acting on the centers of action 14" and 15" has a moment arm about the pitch axis 16. The moment on the first rotor blade 7" opposes the moment on the second rotor blade 8", causing rotation at the coupling, which allows the first rotor blade 7" to independently pitch nose-down relative to the second rotor blade 8", reducing the thrust load across both rotor blades. As noted above, the rotor blades 7" and 8" also have the ability to reverse direction with changes in current direction.

[0051] The angle of movement of the first rotor blade 7'' relative to the second rotor blade 8'' via the resilient device 22 can be limited by a physical stop 23 located at the interface between the rotor blades.

[0052] Further bearings 18, 19, 20 and 21 allow the first blade 7'' to rotate resiliently relative to the second blade 8'' via the hub root 30.

[0053] Referring to Figures 12a to 14, a third embodiment of the connection between a first rotor blade 7''' and a second rotor blade 8''' is substantially the same as the configuration shown in Figures 2 to 7, except that the torsional resilience between the two rotor blades 7''' and 8''' is achieved using a shear rubber connector 29 positioned between the first rotor blade and the second rotor blade, which connector may be shaped to provide some mechanical engagement or interference with adjacent components and shear to provide the torsional resilience.

[0054] 15a-16b, a fourth embodiment of the connection between the first rotor blade 7'''' and the second rotor blade 8'''' includes a resilient coupling 32 extending through the rotor hub 6'''' and the rotor hub portion 30''' and bridging the first end region of the rotor blade 7'''' and the second rotor blade 8''''. When the thrust load is relatively small (see FIG. 15b), the thrust on the centers of action 14'''' and 15'''' is correspondingly relatively small, and the leading edges 12'''' and 13'''' or nose-down pitch of the rotor blades 7'''' and 8'''' remains substantially unchanged. However, when the flow velocity is relatively large (see FIG. 16b), the thrust load is correspondingly large. The thrust acting on the centers of action 14'''' and 15'''' has a moment arm about the pitch axis 16. The moment on the first rotor blade 7'''' opposes the moment on the second rotor blade 8'''', causing rotation at the coupling, allowing the first rotor blade 7'''' to independently pitch nose down relative to the second rotor blade 8'''', reducing the thrust load across both rotor blades.

[0055] Referring to Figures 17a-18b, the first and second rotor blades 70 and 80 are fabricated from composite materials that exhibit flexural-torsional coupling (i.e., a bending moment applied by thrust results in a torsional deflection about the pitch axis). Figures 17a and 17b show that when thrust loads are relatively small, the forces acting on the rotor blades are correspondingly small, and the leading edges 120 and 130 or nose-down pitch of the rotor blades 70 and 80 do not change substantially. However, when the flow velocity is relatively large (see Figures 18a and 18b), the thrust loads are correspondingly large. Thrust acting on the rotor blade bodies twists the body of the first rotor blade 70, causing the cross-sectional shape of the first blade 70 to pitch down independently at the outside of its root relative to the second rotor blade 80, reducing the thrust load across both rotor blades.

[0056] 19 and 20, line graphs depict the behavior of different pitch mechanisms (fixed pitch and passive pitch) over a range of fluid flow speeds in terms of thrust (FIG. 19) and power generation (FIG. 20). Each turbine shown has the same cut-in flow speed, i.e., the flow speed at which the rotor blades begin to rotate and generate power at low thrust, and each turbine has a cut-out flow speed where at least one load on the turbine is high enough, such as the thrust in the example depicted by FIGS. 19 and 20, to cause the turbine to shut down and stop.

[0057] Figures 19 and 20 show typical variations in thrust and power with flow speed for a fixed-pitch turbine rotor and an example of what may be expected from a passive-pitch rotor, illustrating the benefits of the present invention for maximizing turbine performance by increasing rotor size and / or increasing cutout flow speed. Figure 19 shows the relative thrust that may be expected to act on each turbine rotor over a range of fluid flow speeds. This supports Figure 20, which shows the relative performance that may typically be achieved by turbines with fixed-pitch and passive-pitch blades in terms of power generation over the same range of fluid flow speeds.

[0058] Each turbine shown in Figures 19 and 20 also has a cutout flow speed, above which the turbine cannot operate. For ease of comparison, it is assumed that the cutout flow speed for both fixed pitch and passive pitch turbine rotors is limited by thrust forces acting on the rotors, which could compromise the structural integrity of the machine. Other control measures may reduce thrust for turbines with passive blade pitch above this speed, allowing for an increase in the cutout flow speed. However, this feature is not reflected in Figures 19 and 20 for simplicity.

[0059] Referring specifically to FIG. 19 , a line graph illustrates the relative thrust exerted at various flow speeds on a turbine rotor with a passive pitch mechanism 38, such as that of the present invention, compared to a fixed-pitch turbine rotor with the same rated power 36. For a turbine with a fixed pitch mechanism 36, thrust increases proportionally to the square of the flow speed until a maximum or rated power is reached, at which point the rotor's rotational speed increases, and the rate of thrust increase with increasing flow speed also increases. At this point, the cutout flow speed is reached and the turbine must be shut down. A turbine with a passive pitch mechanism 38 exhibits an initial increase in thrust at a greater speed than a turbine with fixed-pitch blades 36 due to the larger rotor diameter that can be installed. As the flow speed increases, the present invention allows the leading edges of the rotor blades to pitch down, resulting in a slower rate of thrust increase and an S-shaped thrust curve. Because the present invention reduces the thrust exerted on the rotor blades by the passive pitch mechanism at high flow speeds, the maximum allowable thrust limit is not expected to be reached until a flow speed greater than the flow speed at which the same limit is reached for a fixed-pitch turbine rotor 36. This means that a higher cutout flow velocity is expected for a rotor with a passive pitch mechanism 38 in accordance with the present invention.

[0060] Referring specifically to FIG. 20 , a line graph shows the additional power that can be generated at each flow speed when using a turbine rotor with a passive pitch mechanism 38, such as that of the present invention, compared to a fixed-pitch turbine rotor 36 with the same rated power. With a fixed-pitch turbine 36, power generation increases proportionally to the cube of the flow speed until the turbine's rated power is achieved, at which point rotor speed must increase to maintain power as the flow speed increases until cutout flow speed is reached. The turbine with the passive pitch mechanism 38 exhibits an initial increase in power at a greater rate than the turbine with fixed-pitch blades 36 due to the larger rotor diameter that can be installed. As the flow speed increases, the turbine with the passive pitch mechanism 38 pitches down, resulting in a relaxation of the rate at which thrust increases, resulting in an S-shaped power curve. Power from the turbine with the passive pitch mechanism 38 will then continue to increase at a much slower rate as the flow speed increases until cutout flow speed is reached. The shaded area on the graph shows the additional power captured from the fluid flow compared to a turbine with a fixed pitch mechanism 36 and a turbine with a passive pitch mechanism 38.

[0061] The passive pitch enabled by the present invention reduces loads such as thrust at high fluid flow rates more than at low fluid flow rates, providing a smoothing effect on thrust loads, allowing the turbine to remain near maximum power for more of the time, thus providing more power at less cost.

Claims

1. A device comprising a first rotor blade and a second rotor blade, each having a first end region connected to the other at a rotor hub, and a load reduction mechanism for the first and second rotor blades that enables the independent rotation of the first and second rotor blades around a pitch axis, wherein the load reduction mechanism prevents the first and second rotor blades from rotating around the pitch axis until a threshold load is reached.

2. The apparatus according to claim 1, further comprising an elastic device located between each of the first end regions.

3. The apparatus according to claim 2, wherein the elastic device is disposed at the interface between the first rotor blade and the second rotor blade.

4. The apparatus according to claim 1, wherein the connection between each of the first end regions in the rotor hub forms a passive pitch mechanism.

5. The apparatus according to claim 4, further comprising an elastic device located between each of the first end regions, wherein the elastic device is positioned at the interface between the first rotor blade and the second rotor blade, and the elastic connection between the first rotor blade and the second rotor blade serves to reduce the thrust load acting on the first and second rotor blades.

6. The apparatus according to claim 5, wherein the center of action of the thrust of the rotor blade is offset from the pitch axis of the first and second rotor blades, and as a result, the thrust load induces a leading edge or nose-down pitching moment in the rotor blade.

7. The apparatus according to claim 2, wherein the elastic device is a torsion-compliant fastener.

8. The apparatus according to claim 7, wherein the connection between each of the first end regions in the rotor hub forms a passive pitch mechanism, and the passive pitch mechanism is restrained by the torsion-compliant fastener.

9. The apparatus according to claim 2, further comprising one or more physical stopping units that serve to limit the amount of rotation around the pitch axis.

10. The apparatus according to claim 1, wherein the connection between the first rotor blade and the second rotor blade is formed by the blade root cylindrical stub portion in the first end region of the first rotor blade being insertable into a corresponding cylindrical opening or socket portion in the first end region of the second rotor blade.

11. The apparatus according to claim 10, wherein the connection between the cylindrical stub portion at the base of the blade and the cylindrical opening or socket portion is aligned with the pitch axis.

12. The apparatus according to claim 10, further comprising a bearing surface between the first rotor blade and the second rotor blade.

13. The apparatus according to claim 10, wherein the rotor hub includes a cylindrical hub bearing tube or sleeve extending into the rotor plane, and the inner surface of the hub bearing tube or sleeve captures the connection between the first and second rotor blades by the outer cylindrical bearing surface of the cylindrical opening or socket portion of the second rotor blade.

14. The apparatus according to claim 1, wherein the connection between the first rotor blade and the second rotor blade is formed by a hub root within a rotor hub sleeve, and the first end region of the first rotor blade is connected to the first end region of the second rotor blade by the hub root and at least one elastic device.

15. The apparatus according to claim 14, wherein the hub root is formed to have recesses in opposing end regions, the recesses being arranged to receive projections extending outward from the first end regions of each of the first and second rotor blades, and corresponding elastic devices mounted between the opposing surfaces of the projections and a wall surface defining the boundary of the recesses.

16. The apparatus according to claim 1, wherein the torsional elasticity between the two rotor blades is achieved using a shear rubber connector positioned between the first rotor blade and the second rotor blade.

17. The apparatus according to claim 1, wherein the connection between the first rotor blade and the second rotor blade extends through the rotor hub and the rotor hub portion to bridge the first end region of the rotor blade.

18. The apparatus according to claim 1, wherein the rotor hub enables the free rotation of the first and second rotor blades around the pitch axis.

19. The apparatus according to claim 1, wherein the first and second rotor blades are configured to reverse so that they face two substantially opposite directions in the direction of fluid flow.

20. The apparatus according to claim 2, further comprising a damper installed parallel to the elastic device.

21. The apparatus according to claim 1, further comprising an elastic device located between each of the first end regions, and one or more magnetic devices attached to the first end regions of the first and second rotor blades.

22. The apparatus according to claim 21, wherein a plurality of magnetic devices are arranged around the radial space between the first end regions of the first and second rotor blades and are alternately connected to the first rotor blade and the second rotor blade with the elastic device in between.

23. The apparatus according to claim 1, wherein the load reduction mechanism is formed from the material on which the first and second rotor blades are manufactured, is bendable, and allows the blade cross section to move and rotate around the pitch axis.

24. A method for reducing the load on a first rotor blade and a second rotor blade connected to each other by a rotor hub, and for enabling independent rotation of the first and second rotor blades around a pitch axis, wherein the rotation of each of the first and second rotor blades around the pitch axis is prevented until a threshold load is reached.

25. The method according to claim 24, further comprising connecting a first rotor blade to a second rotor blade at a rotor hub, thereby forming an interface between the respective first end regions of the first and second rotor blades, and arranging an elastic device at the interface between the first and second rotor blades.

26. The method according to claim 25, wherein the connection between the first rotor blade and the second rotor blade forms a passive pitch mechanism.

27. The method according to claim 25, wherein the elastic connection between the first rotor blade and the second rotor blade serves to reduce the thrust acting on the first and second rotor blades.

28. The method according to claim 24, wherein the center of action of the thrust of the rotor blade is offset from the pitch axis of the first and second rotor blades, and as a result, the thrust induces a leading edge or nose-down pitching moment in the rotor blade.

29. The method according to claim 25, further comprising limiting the amount of rotation around the pitch axis using one or more physical stopping parts disposed at the interface between the first rotor blade and the second rotor blade.

30. The method according to claim 25, wherein the connection between the first rotor blade and the second rotor blade is formed by inserting the blade root cylindrical stub portion in the first end region of the first rotor blade into the corresponding cylindrical opening or socket portion in the first end region of the second rotor blade.

31. The method according to claim 26, wherein when the flow velocity is relatively high, the passive pitch mechanism acts to reduce the thrust on the first and second rotor blades.

32. The method according to claim 24, wherein the first and second rotor blades are configured to maintain equilibrium with respect to hydrodynamic forces.

33. The method according to claim 24, further comprising natural feedback between thrust and pitch, wherein the thrust is reduced more significantly at relatively high flow velocities and less significantly at relatively low flow velocities, thereby achieving power generation over a wider range of flow velocities with respect to a given thrust threshold.

34. The method according to claim 25, further comprising damping with some torsional elasticity to maintain the orientation alignment of the first and second rotor blades such that rotation about the pitch axis occurs under the action of a pitching moment.

35. The method according to claim 25, further comprising magnetically connecting the first end regions of the first and second rotor blades.

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