Improvements in or relating to energy generation

Abstract

A turbine rotor has two blades 7, 8 connected to each other at a rotor hub with a connection which allows independent turning of each of the first and second blades about a pitch axis. The rotor blades’ centre of effort of thrust may be offset from the pitch axis to induce a ‘nose-down’ pitching moment of the blades. The connection between the blades may be resilient 22 so that blades individually pitch ‘nose-down’ to alleviate excessive thrust loads. Because the blades pitch individually, they can respond to the load acting on each blade and so adapt to non-uniform flow. The blades may also passively flip so that they can be used in bi-directional e.g. tidal currents.

Description

This invention relates to improvements in energy generation using horizontal axis turbines and in particular to a reversible turbine rotor blade with passive movement about a pitch axis. WO2009 / 031887 discloses a device for converting kinetic energy of flowing water into kinetic energy of a rotatable rotor shaft, wherein the device is adapted to be effective in a first flow direction and a second flow direction substantially opposite to the first, the device comprising two rotor blades each connected to a blade shaft coupled to the rotor shaft for transfer of a torque relative to the rotor shaft, each of the blades being rigidly connected to the respective blade shafts, the blade shafts being mounted for passively rotating about their own axis relative to the rotor shaft, the blade shafts also being coupled to each other for rotation in the same direction and the rotor blades each have an asymmetrical cross-section. Thus, there is always a flow against the asymmetrical rotor blades from the same direction relative to the blades during the energy conversion process, so that the profile of the rotor blades can be optimised to a certain degree for the flow direction. Alternative means of thrust reduction in the prior art is to actively (rather than passively) rotate the turbine blades to face the leading edge of the turbine blade(s) into the oncoming flow thereby reducing the thrust upon the blade(s) when the flow speed increases. Actively controlled pitch systems, however, require power transmission across a rotating interface, sensors, actuators, and control systems. A huge problem results in that active pitch systems are too expensive and can be unreliable. Owing to the inability for pitch to reduce load in a fixed pitch device, fixed pitch turbines either have a smaller rotor diameter than turbines with variable pitch blades (to keep the loads within limits at high flow velocity, which results in lower power extraction for a given flow speed as power scales with the square of the rotor diameter) or they have a lower cut-out flow velocity, i.e. the flow velocity of the fluid above which the turbine cannot operate without breaching, for example, thrust force limits which could compromise the structural integrity of the machine. This avoids high loads, which results in lost energy yield, exacerbated by the fact that the power in tidal currents scales with the cube of flow velocity. Beneficially, the turbine could generate more power for lower cost if the turbine could remain close to its maximum potential power output despite natural variations in flow, for more of the time because such variations in flow are linked to fatigue loads which can be mitigated with blade pitching systems. Power can be harnessed from the movement of a fluid by inducing rotation of one or more turbine rotor blades. A desirable property of such rotor blades designed for power production is the ability to produce a force tangential to the rotor plane that induces torque which is converted into power. An unwanted property of the rotor blades is, for example, the thrust force created in the direction of the fluid flow, particularly in flowing water environments, or any other parameter that varies as a function of flow speed. Such unwanted properties are a driver for high cost as it requires high strength in supporting structures and in the turbine drive train. Therefore, the maximum thrust allowed for a given strength capacity limits the size of the rotor blade(s) mounted on the turbine, as well as the maximum flow speed in which the turbine can operate. This, however, also limits the power output and energy yield of the turbine. Thrust forces are just one example of mechanical or electro-mechanical loads associated with turbine rotor blades, other examples being power, torque, and voltage. According to one aspect of the present invention, there is provided apparatus comprising a first rotor blade, a second rotor blade, each rotor blade having respective first end regions connected together at a rotor hub, and a load alleviation mechanism of said first and second blades allowing independent turning of each of the first and second blades about a pitch axis. According to a second aspect of the present invention, there is provided a method of alleviating load upon a first rotor blade and a second rotor blade connected to each other at a rotor hub and allowing independent turning of each of the first and second blades about a pitch axis. Owing to these aspects, more power can be generated at lower fluid flow velocities, and the flow velocity at which the turbine must stop generating power is increased, resulting in a greater energy yield. Advantageously, a resilient device is located at the respective first end regions between the first and second rotor blades, which forms part of the load alleviation mechanism. Alternatively, the load alleviation mechanism may be formed by the material of which the rotor blades are made, being able to flex, allowing the blade cross-section to translate and rotate about the pitch axis. The connectable first and second rotor blades enables a load such as the thrust force upon them to be alleviated or reduced when the flow speed is high (around 1m / s to around 6m / s depending upon the particular site and could be as high as around 10m / s), by means of a passive pitch mechanism formed by the resilient connection between the first and second rotor blades. The benefit of such a resilient connection is that larger turbine blades can be installed and thereby capture more of the available energy when the flow speed is low (around 0.2m / s to 3m / s depending upon the site). When the flow speed is high, the passive pitch mechanism acts to alleviate or reduce loads such as thrust force to compensate for the use of a larger rotor blade. Moreover, when the flow speed is high, the action of the passive pitch mechanism and the resilient device reduces loads such as thrust below what they would be with fixed pitch rotor blades. This keeps loads such as thrust within allowable limits, which allows the generation of power to continue when otherwise it would be necessary to stop the turbine. Additionally, the passive pitch mechanism will also help to alleviate fluctuating or cyclical loads due to turbulence, waves, flow misalignment or varying flow speed at different depths within the water column. When the flow velocity, and therefore the thrust force, is relatively low, there is very little blade pitch movement and therefore the thrust reducing effect (and power reducing effect) is minimised. The load alleviation mechanism serves to alleviate or reduce loads such as thrust force without the need for power transmission across a rotating interface, sensors, 3 actuators, or control system, as would be needed for an active pitching mechanism, making the passive pitch system cheaper and more reliable than such active systems. Advantageously, a rotor blade centre of effort of thrust is behind the pitch axis of the first and second rotor blades such that the load induces a leading edge- or nose-down pitch moment upon the rotor blades. The location of the centre of effort can be beneficially adjusted by design using, for example, blade sweep, blade rake, and blade aerofoil section shape techniques. Preferably, each rotor blade is pivotally mounted to resiliently rotate about the pitch axis. Preferably, the resilient device is a torsionally compliant fixing and passive pitch rotation about the pitch axis is restrained by the torsionally compliant fixing, such as at least one spring device, for example. The amount of rotation about the pitch axis may be limited by one or more physical stops arranged at the interface between the first and second rotor blades. The amount of rotation about the pitch axis may be controlled by the response of the resilient device. This may be designed such that a linear or non-linear response is achieved (for example, where the loads generated at low flow speeds cause low deflections and loads generated at high flow speeds cause high deflections). The resilient device may be preloaded such that no deflection occurs below a critical flow speed. It will be appreciated that the more load such as thrust force acting on the rotor blades the more leading edge- or nose-down pitching moment is created, and more pitch angle change thereby partially alleviates the thrust force. Natural feedback between load and pitch is formed in which the load is reduced more at higher flow speeds and is reduced less at lower flow speeds thereby achieving the benefit of power generation at a wider range of flow speeds for a given load threshold. A major advantage of the resilient connection between the first and second rotor blades is that the blades are configured on a rotatable hub, as per the arrangement described in WO2009 / 031887. This allows the blades to rotate together about a common axis in order to stay in equilibrium in terms of hydrodynamic forces (meaning the pitch angle on each blade is substantially the same, and the load exerted on each blade is substantially the same). This prevents unwanted out-of-plane bending on the rotor bearings, and unwanted vibration. If the first and second rotor blades were separately mounted to the rotor hub using separate resilient devices, then the behaviour of the respective resilient devices may change differently over time resulting in the rotor blades becoming hydrodynamically unbalanced. The resilient coupling between the first and second rotor blades prevents any such unbalancing from occurring, and the coupling is configured to enable rotation about the pitch axis and provide moment connection of the first rotor blade to the second rotor blade. Such a connection is advantageously achieved, for example, by means of a blade root cylindrical stub or plug at the first end region of the first rotor blade inserted into or on to a corresponding opening or socket in the first end region of the second rotor blade, the connection being aligned with the common pitch axis. A bearing surface, for example, could thereby be formed between the outer surface of the cylindrical stub of the first rotor blade and the inner cylindrical face of the opening in the second rotor blade. Alternatively, the root of each blade could be connected to a common blade hub, each having separate bearings mounted on the outer edges of the hub, where torsional flexibility would exist between the root of each blade and the common blade hub to enable passive pitch when experiencing a load. The load alleviating mechanism may comprise resilience in the material of which the rotor blades are made. The rotor blades may be arranged to flex and thereby turn about the pitch axis under a load. The rotor hub within which the first and second rotor blades are connectable preferably comprises a cylindrical bearing tube extending in a rotor plane with the inner face of the hub bearing tube capturing the first and second rotor blade connection via an outer cylindrical bearing surface of the cylindrical opening of the second rotor blade. The hub bearing surface enables free rotation of the blade pair about the pitch axis within the rotor hub. Advantageously, as with the arrangement described in WO2009 / 031887, the configuration of the first and second rotor blades also permits flipping of the blades in order to face flow direction in two substantially opposite directions. Flow direction reversal is commonly found in tidal environments owing to the occurrence of flood and ebb tides. The flipping-around mechanism of the rotor blades enables self-pitch alignment of the blades preventing large hydrodynamic misalignment problems in a change of flow condition. A further benefit of the present invention is that the self-pitch alignment capability of the first and second rotor blades helps to alleviate imbalanced forces on the blades which might otherwise occur owing to waves, flow shear, yaw misalignment and turbulence. The passive pitch mechanism could equally apply to unidirectional fluid flows, such as those in non-tidal regions of rivers. In order that the present invention can be clearly and completely disclosed, reference will now be made, by way of example only, to the accompanying drawings, in which:- Figure 1 shows a diagrammatic elevational view of a known two-bladed turbine mounted to a supporting structure, Figure 2 shows a diagrammatic view similar to Figure 1 (without the support structure) of a two-bladed turbine having a first rotor blade resiliently connected to a second rotor blade, Figure 3a shows a partial cross-section of the resilient coupling of Figure 2, with the axis of pitching movement shown with a dotted line, Figure 3b shows a cross-section of the resilient coupling of Figure 3a in a plane normal to the axis of pitching movement, Figure 4a shows a cross-section of the resilient coupling in a plane normal to the axis of pitching movement, Figure 4b shows an isometric perspective of the rotor blades during a flood tide, with relatively low flow speed, Figures 5a and 5b show views similar to Figures 4a and 4b during a flood tide, with relatively high flow speed, Figures 6 and 7 show similar views to those of Figures 4b and 5b but during an ebb tide, with relatively low flow speed and with relatively high speed respectively, Figures 8a and 8b show diagrammatic sectional views of a second embodiment of the resilient coupling during a flood tide, with relatively low flow speed, Figures 9a and 9b show diagrammatic sectional views of the second embodiment of the resilient coupling during a flood tide, with relatively high flow speed, Figure 10a shows a cross-section of the resilient coupling of the second embodiment, Figure 10b shows an isometric perspective of the rotor blades with the resilient coupling of the second embodiment during a flood tide, with relatively low flow speed, Figures 11a and 11b show views similar to Figures 10a and 10b, but of the second embodiment but during a flood tide, with relatively high flow speed, Figures 12a and 12b show views similar to Figures 3a and 3b respectively, but of a third embodiment of the resilient coupling, Figures 13a and 13b show views similar to Figures 4a and 4b respectively, but of the third embodiment of the resilient coupling, Figures 14a and 14b show views similar to Figures 5a and 5b respectively, but of the third embodiment of the resilient coupling, Figures 15a and 15b show views similar to Figures 10a and 10b respectively, but of a fourth embodiment of the resilient coupling, Figures 16a and 16b show views similar to Figures 11a and 11b respectively, but of the fourth embodiment of the resilient coupling, Figure 17a is a partial side elevational view of a turbine blade whose blade bodies flex to resiliently turn about a pitch axis, Figure 17b shows an isometric perspective of the rotor blade pair of Figure 17a during a flood tide, with relatively low flow speed, Figures 18a and 18b are similar views to Figures 17a and 17b respectively during a flood tide, with relatively high flow speed, Figure 19 is a line graph of th rust forces upon a turbine blade as a function of fluid flow speed for the two different types of turbine, and Figure 20 is a line graph of power generation as a function of fluid flow speed for the two different types of turbine. Referring to Figure 1, a known turbine with first and second rotor blades with flipping functionality comprises a support structure 1 fixed to the seabed by any suitable conventional technique and a turbine fixation means 2 of the turbine support structure configured prevent movement or rotation of the turbine, the structure 1 being located in a bi-directional water flow environment in which the arrow 3 shows direction of a thrust load during a flood tide and arrow 4 shows direction of a thrust load in an ebb tide. Arrows X and Y indicate the direction of fluid flow during a flood tide and an ebb tide respectively. A rotor hub 6 and a rotor shaft 6a 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 being connected to a blade shaft 9 located between the ends of the respective first end regions of the rotor blades 7 and 8. In Figure 1, the turbine is shown as fixed with an alignment to face the rotor blades 7 and 8 substantially into the direction of the flood tide X exerting the thrust load 3. When the tide changes direction to the ebb tide Y, the turbine alignment remains unchanged and the ebb flow Y is temporarily substantially aligned with the reverse side of the rotor blade faces. The first blade 7 is immovably fixed to the blade shaft 9 and the second blade 8 is also immovably fixed to the blade shaft 9 such that these three components 7, 8 and 9 rotate and pitch bodily as one so that the alignment changes to face the rotor blades 7 and 8 substantially into the direction of the ebb tide Y under the action of the fluid motion on the blades 7 and 8, thereby enabling a flipping-around function of the rotor blades to realign and face the reversed flow direction. Thereafter, once the flipping-around function has occurred, the pitch alignment of the rotor blades 7 and 8 is held substantially fixed by forces of the fluid on the rotor blades such that when a pitch disturbance occurs, a nose-up pitching motion and hence a thrust increase on the first blade 7 occurs together with a corresponding nose-down pitch and a thrust reduction in the second blade 8 owing to the unified movement of the rotor blades 7 and 8 and the blade shaft 9. This produces an imbalance of thrust forces at respective centres of effort 14 and 15 of the rotor blades 7 and 8 which act to restore pitch orientation to counter as much as possible the pitch disturbance. This mechanism holds the intended orientation of the rotor blades 7 and 8 to the oncoming flow. The blade shaft 9 is rotatably fixed inside the rotor hub 6 about an axis of pitch movement 10, bearings 11 enabling free rotation of the pair of blades 7 and 8 in the pitch-wise direction. When the flood tide X changes direction to the ebb tide direction Y, the rotor blades 7 and 8 are able to turn about the axis 10 and thereby flip-around to face the changed flow direction. The shape of the rotor blades 7 and 8 is such that this flipping- around occurs passively under the action of the fluid flow. The advantage of rotor blades that change orientation with changes in the direction of flow is in saving of costs and improving reliability. This benefit can be utilised when tide is highly directional, and the ebb tide direction Y is close to 180 degrees compared to the flood tide X. Arrows A and B indicate the perpendicular distance between the respective blade’s centre of effort 14 and 15 and the axis of pitching movement. Referring to Figures 2, 3a and 3b and in accordance with the present invention, within the rotor hub sleeve 6’, a first end region of the first rotor blade T is connected to a first end region of the second rotor blade 8’ by way of a resilient device 22 in the form of, for example, a torsional coupling. The first and second rotor blades T and 8’ are connected by way of a male blade root cylindrical stub connector part 7a at the first end region of the first rotor blade 7’ being inserted into a corresponding female cylindrical opening or socket part 8a in the first end region of the second rotor blade 8’, the connection being aligned with a pitch axis 16 of the rotor blades 7’ and 8’. In a similar manner to that of Figure 1, bearings 11’ enable the free rotation of the pair of blades 7’ and 8’ in the pitch-wise direction owing to the cylindrical opening or socket part 8a extending through the rotor hub sleeve 6’. Further bearings 18, 19, 20 and 21 enable resilient rotation of the first blade 7’ relative to the second blade 8’. These further bearings 18, 19, 20, 21 preferably also perform the function of preventing the rotor blades 7’ and 8’ form sliding apart axially. The resilient device 22 is a means of providing the function of a torsion spring to hold the blades in substantially neutral alignment (i.e. alignment unaltered from the design condition suited for low flow velocity operation (for maximised power without any thrust shedding)) under low thrust conditions and to correspondingly flex to enable independent leading edge- or nosedown pitch of the first blade 7’ relative to the second blade 8’ under the action of high load / thrust conditions thereby providing a load alleviation mechanism the function of which is to passively allow load alleviation. The hub bearings 11’ ensure that realignment and pitch equilibrium occurs. The torsional coupling 22 is located at an interface between the first and second rotor blades 7’ and 8’, the coupling being configured to fixedly support bending moments across the joint from the first rotor blade 7’, via bearings 18, 19, 20 and 21 to the second rotor blade 8’. Torsional resilience between the two rotor blades 7’ and 8’ could also be achieved using alternative embodiments (described below). For example, a rubber connector could be placed between the first and second rotor blades, which shears to provide torsional resilience. A damping means, in the form of, for example, a preloaded and dampened-torsion spring may be provided to hold the orientation alignment of the rotor blades 7’ and 8’ with some degree of torsional resilience such that rotation about the pitch axis occurs under the action of pitching moment. The preloading means may be incorporated to prevent pitch movement below a threshold load or flow speed, while damping means may be incorporated to slow the rate of pitch movement for example by friction, viscous effects, or other means. Such damping may be required to prevent excessive movement or reduce responsiveness. When the thrust force is relatively small, the force on the centres of effort 14’ and 15’ (see Figure 4b) is correspondingly relatively small and the nosedown pitch of the rotor blades 7’ and 8’ is not changed substantially, if at all. When, however, the flow speed is relatively large, the thrust force is correspondingly relatively large. The thrust force acting on the centres of effort 14’ and 15’ have 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’ and rotation occurs at the coupling to enable the first rotor blade 7’ to pitch nose-down independently and relative to the second rotor blade 8’ providing passive pitch reduction to alleviate the thrust load, i.e. when the load is bigger, the reaction of the resilient device 22 is correspondingly bigger to alleviate the experienced load. In this way, the rotation of the first rotor blade 7’ is independent and in the opposite direction of the second rotor blade 8’. The rotor blades 7’ and 8’ also have the ability to flip-around with tide direction change. The degree of movement of the first rotor blade 7’ relative to the second rotor blade 8’ by way of the resilient device 22 may be limited by physical stops formed by longitudinal projections 7c of the male blade root cylindrical stub connector part 7a and corresponding longitudinal projections 8c of the cylindrical opening or socket part 8a, either of which may comprise a means of cushioning 23 to reduce the impact force between the two rotor blades in the event of a rapid change of load. The longitudinal projections 7c are located in channels formed between the longitidinal projections 8c, the width of the channels defining the amount of movement about the pitch axis by the rotor blades 7’ and 8’, The resilient device 22 is preferably located between opposing surfaces of the longitudinal projections 7c and 8c. The coupling of the rotor blades comprising the resilient device 22 and the bearings 18, 19,20 and 21 allows rotational movement about the pitch axis, but restrains rotational movement about any other axis and translational movement in any direction. It is possible to arrange for zero pitch change below a threshold flow velocity or a threshold load if the resilient device is pre-loaded against the physical stop. The resilient coupling may also be achieved by a suitable hydraulic coupling. The turbine fixation means 2 (see Figure 1) is configured to hold the turbine stationary, but the turbine of Figure 2 could equally, alternatively, be mounted on a yawing turret of a support structure to be located in a flow environment where the direction of flow is variable beyond bi-directional. The reference numeral 5 and the associated dot-dash lines in Figure 1 indicate additional rotor blade length permitted owing to the load alleviation mechanism of the first and second rotor blades 7’ and 8’, enabling greater power generation at a given flow speed. Referring to Figures 4a and 4b, the rotor blades 7’ and 8’ are diagrammatically shown as operating during a flood tide, with relatively low flow speed, as indicated by the arrows 26. The rotor blades 7’ and 8’ comprise at the respective first ends the connecting parts 7a and 8a and respective blade bodies 7b and 8b. The blade bodies 7b and 8b have respective leading edges 12’ and 13’ and are mounted to the connecting parts 7a and 8a at an angle such that the longitudinal axes of the blade bodies are neither coincident with nor parallel to the pitch axis 16 which is coincident with the axis of pitch movement about the hub bearings 11’. The centres of effort 14’ and 15’ are thereby located off-set from the pitch axis 16 and thus have respective moment arms 24 and 25 about the pitch axis 16 when thrust force acts upon the blade bodies. The moment 27 on the first rotor blade 7’ opposes the moment 28 on the second rotor blade 8’ and rotation occurs at the coupling interface to enable the first rotor blade T to pitch leading-edge or nose-down relative to the second rotor blade 8’ (see Figures 5a and 5b) to alleviate thrust load across both rotor blades. Referring to Figures 5a and 5b, the rotor blades 7’ and 8’ are diagrammatically shown as operating during a flood tide, but with relatively high flow speed, again as indicated by the arrows 26. The moment on the first rotor blade 7’ opposes the moment on the second rotor blade 8’ and rotation occurs at the coupling to enable the first rotor blade 7’ to pitch independently, in a leading-edge or nose-down direction 27 (anticlockwise as shown in Figure 5b) about the pitch axis 16 (compared to the position of Figure 4b), and the second rotor blade 8’ to pitch in a leading-edge or nose-down direction 28 (clockwise as shown in Figure 5b) about the pitch axis 16 (compared to the position of Figure 4b), to alleviate thrust load across both rotor blades, and compressing the resilient device 22 between each of the longitudinal projections 7c and 8c. Figures 6 and 7 show similar passive pitching movements as in Figures 4b and 5b, but with the rotor blades 7’ and 8’ flipped-around and facing an ebb tide at relatively low (Figure 6) and relatively high (Figure 7) flow speeds. Referring to Figures 8a to 11b, a second embodiment of the resilient coupling in which, within the rotor hub sleeve 6”, the first end region of the first rotor blade 7” is connected to the first end region of the second rotor blade 8” by way of a hub root part 30 and at least one resilient device 22 in the form of, in a similar manner to that of Figures 2 to 7, a torsional coupling. The hub root part 30 is formed with recesses 32 at opposite end regions arranged to receive projections 34 extending outwardly from the respective first end regions of the first and second rotor blades 7” and 8” and corresponding resilient devices 22 attached between opposing surfaces of the projections 34 and a wall surface bounding the recesses 32. These abutting surfaces of the recesses 32 and the projections 34 form the physical stops 23 and may also include the cushioning means 23. As with the arrangement in Figures 2 to 7, the connection of the first and second rotor blades is aligned with the pitch axis 16. In a similar manner to that of the first embodiment described above with reference to Figures 2 to 7, the bearings 11” enable the free rotation of the pair of blades 7” and 8” in the pitch-wise direction. The resilient devices 22 are a means of providing the function of a torsion spring to hold the blades substantially in an alignment unaltered from the design condition suited for low flow speed operation (for maximised power without any thrust shedding) under low thrust conditions and to correspondingly turn about the pitch axis 16 to enable independent leading edge- or nose-down pitch of the first blade 7” relative to the second blade 8” under the action of high thrust load conditions. The hub bearings 11” ensure that re-alignment and pitch equilibrium occurs. The torsional coupling 22 is located at an interface between the first and second rotor blades 7” and 8”, the coupling being configured to fixedly support bending moments across the joint from the first rotor blade 7”, by way of the hub root part 30 to the second rotor blade 8”. A dampened-torsion spring may be provided to hold the orientation alignment of the rotor blades 7” and 8” with some degree of torsional resilience such that rotation about the pitch axis occurs under the action of pitching moment. When the thrust force load is relatively small (see Figure 10b) the force on the centres of effort 14” and 15” is correspondingly relatively small and the leading edge-12” and 13” or nose-down pitch of the rotor blades 7” and 8” is not changed substantially. When, however, the flow speed is relatively large (see Figure 11b), the thrust force load is correspondingly relatively large. The thrust force acting on the centres of effort 14” and 15” have 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” and rotation occurs at the coupling to enable the first rotor blade 7” to pitch nose-down independently and relative to the second rotor blade 8” to alleviate thrust load across both rotor blades. As already mentioned, the rotor blades 7” and 8” also have the ability to flip-around with tide direction change. The degree of movement of the first rotor blade 7” relative to the second rotor blade 8” by way of the resilient device 22 can be limited by the physical stops 23 arranged at the interface between the rotor blades. The further bearings 18, 19, 20 and 21 enable resilient rotation of the first blade 7” relative to the second blade 8” by way of the hub root part 30. Referring to Figures 12a to 14, a third embodiment of the connection between the first and second rotor blades 7”’ and 8”’ is substantially the same arrangement as shown in Figures 2 to 7, but the torsional resilience between the two rotor blades 7”’ and 8”’ is achieved using a shearing rubber connector 29 placed between the first and second rotor blades, which may be shaped to provide some mechanical engagement or interference with adjacent components and shears to provide torsional resilience. Referring to Figures 15a to 16b, a fourth embodiment of the connection between the first and second rotor blades 7”” and 8”” comprises a resilient coupling 32 extending through the rotor hub 6”” and rotor hub part 30”’ to bridgingly connect the first end regions of the rotor blades 7”” and 8””. When the thrust force load is relatively small (see Figure 15b) the force on the centres of effort 14”” and 15”” is correspondingly relatively small and the leading edge-12”” and 13”” or nose-down pitch of the rotor blades 7”” and 8”” is not changed substantially. When, however, the flow speed is relatively large (see Figure 16b), the thrust force load is correspondingly relatively large. The thrust force acting on the centres of effort 14”” and 15”” have 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”” and rotation occurs at the coupling to enable the first rotor blade 7”” to pitch nose-down independently and relative to the second rotor blade 8”” to alleviate thrust load across both rotor blades. Referring to Figures 17a to 18b, the first and second rotor blades 70 and 80 are made from a composite material which exhibits bend-twist coupling (i.e. an applied bending moment due to thrust force results in twist deflection about the pitch axis). Figures 17a and 17b show that when the thrust force load is relatively small the force exerted on the rotor blades is correspondingly relatively small and the leading edge-120 and 130 or nose-down pitch of the rotor blades 70 and 80 is not changed substantially. When, however, the flow speed is relatively large (see Figures 18a and 18b), the thrust force load is correspondingly relatively large. The thrust force acting on the bodies of the rotor blades enables twisting of the body of the first rotor blade 70 to cause the cross-sectional profile of the first blade 70, outboard of the root, to pitch nose-down independently and relative to the second rotor blade 80 to alleviate thrust load across both rotor blades. Referring to Figures 19 and 20, line graphs represent behaviour of different pitch mechanisms (fixed pitch and passive pitch) in terms of thrust force (Figure 19) and power generation (Figure 20) over a range of fluid flow velocities. Each turbine shown has the same cut-in flow velocity, namely the flow velocity when the rotor blades begin to rotate and generate power at a low thrust force and each turbine has a cut-out flow velocity where at least one load on the turbine, such as thrust force in the instances portrayed by Figures 19 and 20, is high enough that the turbine has to be shut down and stopped. Figures 19 and 20 show typical variation of thrust and power with flow speed for a fixed pitch turbine rotor, as well as an example of what could be expected from a passive pitch rotor in order to illustrate the benefit of the present invention for maximizing turbine performance by increasing rotor size and / or increasing cut-outflow velocity. Figure 19 shows the relative thrust force that could be expected to act on each turbine rotor over a range of fluid flow velocities. This supports Figure 20 which shows the relative performance that could typically be achieved by turbines with fixed pitch and passive pitch blades, in terms of power generation over the same range of fluid flow velocities. Each turbine shown in Figures 19 and 20 also has a cut-out flow velocity above which the turbine cannot operate. For the sake of simple comparison, it has been assumed that the cut-out flow velocity for both the fixed and passive pitch turbine rotors is limited by the thrust force acting on the rotor, which could compromise the structural integrity of the machine. It is possible that other means of control could reduce the thrust force for a turbine with passive blade pitch above this speed, allowing for an increased cutout flow velocity. This functionality is, however, not reflected in Figures 19 and 20 for the sake of simplicity. Referring specifically to Figure 19, the line graph indicates the relative thrust force that would be exerted at each flow velocity on a turbine rotor with a passive pitch mechanism 38, such as that of the current invention, compared to a fixed pitch turbine rotor with the same rated power 36. For the turbine with the fixed pitch mechanism 36, the thrust force increases proportional to the square of the flow velocity until the maximum or rated power is reached, at which point the rotor would have to rotate faster, resulting in an increase in the rate at which thrust force increases with flow speed until the maximum allowable thrust force is exerted upon the rotor, at which point the cut-out flow velocity is reached and the turbine has to be stopped. The turbine with the passive pitch mechanism 38 shows an initial increase in thrust force at a greater rate than the turbine with fixed pitch blades 36, owing to the larger rotor diameter that could be installed. As the flow speed increases, the present invention allows the rotor blades to pitch leading edge down, resulting in a reduction in the rate at which thrust force increases, resulting in an S-shaped thrust curve. The current invention would alleviate the thrust exerted on the rotor blades with the passive pitch mechanism at high flow velocities so that the maximum allowable thrust limit is not expected to be reached until a greater flow velocity than that at which the same limit is reached for the fixed pitch turbine rotor 36. This means that the cut-out flow velocity is expected to be higher for the rotor with the passive pitch mechanism 38, as per the present invention. Referring specifically to Figure 20, the line graph indicates the additional power that can be generated at each flow speed when using the turbine rotor with a passive pitch mechanism 38, such as that of the current invention, compared to the fixed pitch turbine rotor 36 with the same rated power. With the fixed pitch turbine 36, power generation increases proportional to the cube of the flow velocity until the rated power for the turbine is achieved at which point the rotor speed must increase in order to maintain power output as flow velocity increases, until the cut-out flow velocity is reached. The turbine with the passive pitch mechanism 38 shows an initial increase in power at a greater rate than the turbine with fixed pitch blades 36, owing to the larger rotor diameter that could be installed. As the flow speed increases, the turbine with the passive pitch mechanism 38 causes the rotor blades to pitch leading edge down, resulting in an alleviation in the rate at which thrust force increases, resulting in an S-shaped power curve. The power from the turbine with a passive pitch mechanism 38 would then continue to increase at a very low rate as flow velocity increased until the cut-out flow velocity is reached. The shaded area on the graph shows the additional power captured from the fluid flow comparing the turbine with the fixed pitch 36 and the turbine with the passive pitch mechanism 38. The passive pitch enabled by the present invention enables the turbine to remain 5 closer to its maximum power for more of the time because it reduces loads such as thrust more when the fluid flow speed is high than it does when the fluid flow speed is low, thereby producing a smoothing effect on thrust loads, thus providing more power for less cost.

Claims

1. Apparatus comprising a first rotor blade, a second rotor blade, each rotor blade having respective first end regions connected together at a rotor hub, and a load alleviation mechanism of said first and second blades allowing independent turning of each of the first and second blades about a pitch axis.

2. Apparatus according to claim 1, and further comprising a resilient device located between the respective first end regions3. Apparatus according to claim 2, wherein the resilient device is located at an interface between the first and second rotor blades.

4. Apparatus according to any preceding claim, wherein the connection between the first and second rotor blades forms a passive pitch mechanism.

5. Apparatus according to claim 4 as appended to claim 3, wherein the resilient connection between the first and second rotor blades serves to alleviate a thrust load acting upon the first and second rotor blades.

6. Apparatus according to any preceding claim, wherein a rotor blade centre of effort of thrust is off-set from a pitch axis of the first and second rotor blades such that the thrust load induces a leading edge- or nose-down pitch moment upon the rotor blades.

7. Apparatus according to claim 2 or 3, or any one of claims 4 to 6 as appended to claim 2, wherein the resilient device is a torsionally compliant fixing.

8. Apparatus according to claim 7 as appended to claim 4, wherein the passive pitch mechanism is restrained by the torsionally compliant fixing.

9. Apparatus according to claim 2 or any one of claims 3 to 9 as appended to claim 2, and further comprising one or more physical stops serving to limit the amount of turning about the pitch axis.10.Apparatus according to any preceding claim, wherein the connection between the first and second rotor blades is formed by a blade root cylindrical stub part at the first end region of the first rotor blade being insertable into a corresponding cylindrical opening or socket part in the first end region of the second rotor blade.

11. Apparatus according to claim 10, wherein the connection between blade root cylindrical stub part and the cylindrical opening or socket part is aligned with the pitch axis.

12. Apparatus according to claim 10 or 11, and further comprising a bearing surface between the first rotor blade and the second rotor blade.13.Apparatus according to any one of claims 10 to 12, wherein the rotor hub includes a cylindrical bearing tube or sleeve extending in a rotor plane with the inner face of the hub bearing tube or sleeve capturing the first and second rotor blade connection by way of an outer cylindrical bearing surface of the cylindrical opening or socket part of the second rotor blade.14.Apparatus according to any one of claims 1 to 9, wherein the connection between the first and second rotor blades is formed by a hub root part within a rotor hub sleeve, the first end region of the first rotor blade being connected to the first end region of the second rotor blade by way of the root hub part and at least one resilient device.15.Apparatus according to claim 14, wherein the hub root part is formed with recesses at opposite end regions arranged to receive projections extending outwardly from the respective first end regions of the first and second rotor blades and corresponding resilient devices attached between opposing surfaces of the projections and a wall surface bounding the recesses.

16. Apparatus according to any one of claims 1 to 10, wherein torsional resilience between the two rotor blades is achieved using a shearing rubber connector placed between the first and second rotor blades.1 / .Apparatus according to any one of claims 1 to 9, wherein the connection between the first and second rotor blades comprises a resilient coupling extending through the rotor hub and a rotor hub part to bridgingly connect the first end regions of the rotor blades.

18. Apparatus according to any preceding claim, wherein the rotor hub enables free rotation of the first and second rotor blades about the pitch axis.

19. Apparatus according to any preceding claim, wherein the first and second rotor blades are configured to flip-around in order to face flow direction in two substantially opposite directions.20.Apparatus according to claim 6 as appended to claim 1, wherein the load alleviation mechanism is formed by the material of which the first and second rotor blades are made, being able to flex, allowing the blade cross-section to translate and rotate about the pitch axis.

21. A method of alleviating load upon a first rotor blade and a second rotor blade connected to each other at a rotor hub and allowing independent turning of each of the first and second blades about a pitch axis.

22. A method according to claim 21, and further comprising connecting a first rotor blade to a second rotor blade at a rotor hub and thereby forming an interface between respective first end regions of the respective first and second rotor blades and locating a resilient device at the interface between the first and second rotor blades.

23. A method according to claim 22, wherein the connection between the first and second rotor blades forms a passive pitching mechanism.

24. A method according to claim 22 or 23, wherein the resilient connection between the first and second rotor blades serves to reduce a thrust force acting upon the first and second rotor blades.

25. A method according to any one of claims 21 to 24, wherein a rotor blade centre of effort of thrust is off-set from a pitch axis of the first and second rotor blades such that the thrust induces a leading edge- or nose-down pitch moment upon the rotor blades.26.A method according to any one of claims 22 to 25, and further comprising limiting the amount of rotation about a pitch axis with one or more physical stops arranged at the interface between the first and second rotor blades.27.A method according to any one of claims 22 to 26, wherein the connection between the first and second rotor blades is formed by a blade root cylindrical stub part at the first end region of the first rotor blade being inserted into a corresponding cylindrical opening or socket part in the first end region of the second rotor blade.

28. A method according to any one of claims 23 to 27, wherein when the flow speed is relatively high, the passive pitch mechanism acts to reduce the thrust force upon the first and second rotor blades.

29. A method according to any one of claims 21 to 28, wherein the first and second rotor blades are configured to maintain equilibrium in terms of hydrodynamic forces.30.A method according to any one of claims 21 to 29, and further comprising natural feedback between thrust and pitch in which the thrust is reduced more at relatively higher flow speeds and is reduced less at relatively lower flowspeeds, thereby achieving power generation at a wider range of flow speeds for a given thrust threshold.

Images