Passive pitch system

Abstract

A passive pitch system for a tidal turbine comprises a rotor hub with a blade about to rotate about a pitch axis, and with means 22, 26 to apply a moment about the pitch axis to the blade root. An ad

Description

The present invention relates to a passive pitch system for controlling the pitch angle of the blades of a rotor for a tidal turbine. BACKGROUND OF THE INVENTION Conventional tidal turbines have actuator operated 'pitch systems', that control the pitch angle of the blades during power generation. The purpose of such a pitch system is to control the loads on the turbine to within acceptable levels. The amount of lift or drag generated by a turbine blade is dependent on the flow speed and the angle that flow approaches the blade (i.e. the 'angle of attack' or AoA), so a pitch system can be used to control the blade loading by controlling the blade angle. Since the loading through the turbine is dominated by the rotor loading, the effectiveness of a pitch system can make a big difference to the cost of the tidal turbine. Most commonly, the pitch system is used to control the turbine power output to a controllable level up to the rated power. However, it can also be used to control other loads such as rotor thrust or blade bending moment. Most pitch systems are controlled 'actively'. Loads that need to be controlled are measured by sensors, and the readings from these are fed into a control system, where an algorithm is used to calculate the required pitch angle to bring the loads to the desired level. A controller then commands actuators (e.g. motors, hydraulic cylinders) to drive the blades to the required pitch angle. Active pitch systems are operable to control the pitch angle of the blades to control the output power of a turbine. These active systems are very good at controlling mean loads in response to low frequency changes in the mean flow. They are also good at controlling loads due to some local flow transients from waves and large-scale turbulence which are relatively slow to materialise and can be detected and responded to by the control system. However, they are very poor at responding to short-term local flow transients, which have a very significant effect on the turbine loads. These effects cannot be easily detected in advance and occur so quickly that practically sized systems have insufficient power to pitch the blades quickly enough to significantly mitigate the load fluctuation. There is therefore a need for an improved system that allows the control of turbine blade pitch, particularly in response to short-term local flow transients. SUMMARY OF THE INVENTION Aspects of the present invention are set out in the attached claims. According to an aspect of the present invention, there is provided a passive pitch system for a tidal turbine, the passive pitch system comprising a rotor having: a) a rotor hub for rotatable engagement with a mechanical power transmission system of a tidal turbine and rotatable about a rotor axis, the rotor hub having a blade receiving portion for engagement with a blade; b) a blade comprising: (i) an elongate blade body having a first end and a second end opposing the first end, a blade inboard portion extending from the first end towards the second end, and a blade outboard portion extending from the second end toward the first end, wherein the elongate body defines a leading edge extending between the first and second ends and a trailing edge extending between the first and second ends and opposing the leading edge; (ii) a blade root portion located at the first end of the blade in fixed engagement with the elongate blade body and configured for rotatable engagement with the rotor blade receiving portion about a pitch axis; c) a passive pitch mechanism comprising: (i) a moment application element comprising a body having: a. a first engagement portion for engagement with the blade root portion; and b. a second engagement portion opposing the first engagement portion; (ii) an adjustable portion comprising: a. a force application element having a rigid elongate body having a first end portion in engagement with the second engagement portion of the moment application element and a second end portion opposing the first end portion; b. a resilient biasing means with a first end in engagement with the first end portion of the force application element and a second end opposing the first end, the resilient biasing means extending from the first end portion of the force application element towards the second end portion of the force application element; c. a stop member mounted on the force application element and located distal to the first end portion of the force application element; and d) an adjustment element comprising an adjustment element engagement portion defining an axis X, the adjustment element engagement portion being located distal to the moment application element, herein the resilient biasing means has a predetermined preload in dependence upon the distance between the adjustable element engagement portion and the first end of the resilient biasing means, wherein when the hydrodynamic moment acting at the blade about the pitch axis exceeds the moment applied to the blade about the pitch axis by the resilient biasing means, the blade is rotatable about the pitch axis and the force application element moves relative to the adjustment element engagement portion to move the stop member from a first configuration where it abuts the adjustment element engagement portion to a second configuration where the stop member is moved away from the adjustable element engagement portion. In one embodiment, the passive pitch system further comprises: an adjustment element actuator in engagement with the adjustment element; and a controller in communication with the adjustment element actuator and operable to drive the adjustment element actuator to move the adjustment element relative to the rotor hub to move the moment application element relative to the rotor hub blade receiving portion and adjust the pitch angle of the blade. In a further embodiment, the passive pitch system further comprises a stop member actuator in engagement with the stop member, wherein the controller is operable to drive the stop element actuator to move the stop member along the force application element and adjust the distance between the stop member and the first end portion of the force application element, and wherein the controller is operable to drive both the adjustment element actuator and the stop member actuator to adjust the load within the resilient biasing means without changing the pitch angle of the blade when the position of the second engagement portion of the moment application element remains unchanged. The adjustable portion may further comprise an abutment member in moveable engagement with the force application element and abutting the second end of the resilient biasing means, and, for example, a gear located between the abutment member and the adjustment element engagement portion, and wherein the passive pitch system may further comprise a gear actuator operable, under command from the controller, to move the abutment member along the force application element to move the second end of the resilient biasing means relative to axis X to adjust the preload within the resilient biasing means. In a further embodiment, the passive pitch system comprises a plurality of blades as previously described, each blade having a respective passive pitch mechanism and adjustment element. Each passive pitch mechanism may further comprise an adjustment element actuator in engagement with a respective adjustment element; and wherein the passive pitch system further comprises a controller in communication with each respective adjustment element actuator and operable to drive each respective adjustment element actuator to move the respective adjustment element relative to the rotor hub and move the respective moment application element relative to the respective rotor hub blade receiving portion and adjust the pitch angle of each respective blade when the stop member is in the first configuration. In one embodiment, each passive pitch mechanism further comprises a stop member actuator in engagement with the respective stop member of that passive pitch mechanism, wherein the controller is operable to drive each stop element actuator to move the respective stop member along the respective force application element and adjust the distance between the stop member and the first end portion of the respective force application element, and wherein the controller is operable to drive both the adjustment element actuators and the stop member actuators to adjust the load within each resilient biasing means without changing the pitch angle of each respective blade when the position of the second engagement portion of the respective moment application element of that passive pitch mechanism remains unchanged. In one embodiment, the adjustable portion of each passive pitch mechanism further comprises a. an abutment member in moveable engagement with the respective force application element and abutting the second end of the respective resilient biasing means, and b. an actuator operable, under command from the controller, to move the abutment member along the respective force application element to move the second end of the respective resilient biasing means relative to axis X to adjust the preload within the resilient biasing means. In a further embodiment, the passive pitch system comprises a plurality of blades, each blade having a respective passive pitch mechanism, the passive pitch system having a single adjustment element, wherein the adjustment element comprises a plurality of adjustment element engagement portions, each adjustment engagement element portion being in engagement with the force application element of a respective blade, and abutting the second end of the respective resilient biasing means of that blade, the passive pitch system further comprising: an actuator in engagement with the adjustment element; and a controller in communication with the adjustment element actuator and operable to drive the actuator to move the adjustment element such that each respective adjustment element engagement portion moves relative to the respective rotor hub blade receiving portion and adjusts the pitch angle of each respective blade by the same amount. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A and IB illustrate a rotor for a tidal turbine; Figure IC is a perspective view of a blade for a tidal turbine; Figure ID is a view from one end of the blade of Figure IC; and Figure IE is a view from the opposing end of the blade of figure IC; Figure 2A shows the blade and passive pitch mechanism of a first embodiment of a passive pitch system in accordance with the present invention located within the rotor hub; Figure 2B shows the arrangement of figure 2A with the hub omitted; Figure 2C shows a single lever and rod arrangement of figure 2B; and Figure 2D shows the arrangement of figure 2C with the rod and spring shown in cross-section; Figure 3A shows the blade and passive pitch mechanism of a further embodiment of a passive pitch system in accordance with the present invention located within the rotor hub; Figure 3B shows the arrangement of figure 3A with the hub omitted; Figure 3C shows an enlarged view of the lever and rod arrangement of Figure 3B; and Figure 3D shows the arrangement of Figure 3C with the rod shown in cross-section; Figure 4A shows the blade and passive pitch mechanism of a further embodiment of a passive pitch system in accordance with the present invention located within the rotor hub and including a pitch adjustment ring; Figure 4B shows the arrangement of figure 4A with the hub omitted; Figure 4C shows a lever and rod arrangement of figure 4B; and Figure 4D shows the arrangement of Figure 4C with the rod shown in cross-section; Figures 5A and 5B are respective views from opposing ends of the blade of the embodiment of Figure 4A showing the adjustment element, i.e. ring in a first position and the blade pitch at a first pitch angle; and Figures 5C and 5D are respective views from opposing ends of the blade of Figure 4A showing the adjustment element i.e. ring in a second position and the blade pitch at a second pitch angle; Figure 6A shows the blade and passive pitch mechanism of a further embodiment of a passive pitch system in accordance with the present invention located within the rotor hub; Figure 6B shows the arrangement of figure 6A with the hub omitted; Figure 6C shows an enlarged view of the lever and rod arrangement of Figure 6B; and Figure 6D shows the arrangement of Figure 6C with the rod shown in cross-section; Figure 7A shows the blade and passive pitch mechanism of a further embodiment of a passive pitch system in accordance with the present invention located within the rotor hub and including a pitch adjustment ring; Figure 7B shows the arrangement of figure 7A with the hub omitted; Figure 7C shows a lever and rod arrangement of figure 7B; and Figure 7D shows the arrangement of Figure 7C with the rod shown in cross-section; Figures 8A to 8C show graphic representations of the hydrodynamic moment versus pitch angle for a range of nominal flow speeds; and Figures 9A and 9B show the spring and passive pitch mechanism of a further embodiment of a passive pitch system in accordance with the present invention, in which spring preload adjustment can be achieved by changing the length of the spring by moving its end adjacent to the lever. Figure 9A is a cross-sectional view through the spring of Figure 9B. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1A and IB show a rotor 10 for a tidal turbine. The rotor 10 comprises a rotor hub 12 with a plurality of blade receiving portions 14, each of which is engaged with a respective blade 16. The direction of flow of water is shown by arrow F. The hydrodynamic forces from the water flowing past the blades 16 causes a moment about the rotor axis R, thereby causing the rotor to rotate about rotor axis R. This kinetic energy is converted to electrical energy by an electrical generator with which the rotor 10 is engaged. Each blade 16 has an inboard portion 18i extending from the end of the blade 16 adjacent to the rotor hub 12, and an outboard portion 18o extending from the opposing end of the blade 16 that is distal to the rotor hub 12. Each blade 16 has a blade root 20 at the inboard portion 18i (see figure IC). The blade root 20 of each blade 16 is engaged with a respective blade receiving portion 14 and the blade 16 can be rotated relative to the rotor hub 12 about a pitch axis P. The blade also has a leading edge 11 and a trailing edge 13 which extend along the longitudinal length of the blade. Further each blade has two faces, a pressure face 15 on one side of the blade 16, and a suction face 17 on the opposing side to the pressure face 15. The pressure face 15 and suction face 17 meet at the leading and trailing edges 11, 13 (see figure 1 E). The relative rotation of each blade 16 with the rotor hub 12 is typically achieved by locating bearings between each rotor blade receiving portion 14 and respective blade root 20. Figure 1A and IB also illustrates the orientation of the rotor hub 12 relative to the direction of tidal flow F and the leading and trailing edges 11, 13 of each blade 16. In use, the hydrodynamic forces from the water flow (shown by arrow F) past the blades 16 causes a moment about the rotor axis (shown by arrow A in figure IB), causing the rotor hub 12 to rotate. These moments are reacted by an electrical generator to convert the mechanical energy generated at the rotor hub 12 into electrical energy. It is important that the amount of power and loads being applied to the turbine are controlled in order to avoid overloading the turbine subsystems including, but not limited to, generators, gearboxes and electrical cables. This is achieved by rotating the blades 16 about the pitch axis P to control the orientation of the blades 16 relative to the direction of flow F. At flow speeds below rated flow speed, the turbine will have its blades set to what is known as 'fine', where the pitch angle [3 = zero, so that the blades will capture the maximum amount of energy from the flow. Once the flow exceeds the rated flow speed i.e. the flow speed at which the turbine is operating at rated power whilst the blade pitch angle, [3 = zero, the pitch angle of the blades 16 must be increased to limit the power being generated to that of the rated power of the turbine. The higher the flow speed, the greater the pitch angle required to limit the power to rated power. In active pitch control systems, the orientation of the blades 16 relative to the rotor hub 12 is controlled by an actuation system which sets the blade position in dependence upon the flow speed and the rated power of the turbine. Such actuation systems involve the use of actuators and mechanisms such as hydraulic rams, levers, motors, sensors and gearboxes, all of which are expensive to install, complicated to maintain, and provide multiple potential failure points. However, flow speed through the rotor area is not a constant and varies in dependence upon the tidal sequences, the wave dynamics, turbulence, water depth and influences from the turbine operation and its mounting structure within the local environment. Conventionally, an active pitch control system will respond to these variations in order to smooth out the power produced by the turbine. However, in practice, active control systems are slow to respond to flow fluctuations, which can result in the turbine operating at greater than rated power for short periods of time, thus requiring a greater capacity and therefore having a higher production cost. Passive pitch system Figure 2A shows the blade and passive pitch mechanism of a first embodiment of a rotor of a passive pitch system in accordance with the present invention in engagement with the rotor hub 12. Further, although the described embodiments include a lever as the element for applying moment in the system, it is to be appreciated that any alternative arrangement known to the skilled person and suitable for the function of applying moment as described below in relation to the lever, can be used as an alternative to, or in addition to, such a lever. For example, the moment application element could comprise a lever with a pin joint, frictional joint, or a cam joint; a gear; a toothed or plane pully; a sprocket for a chain, or any other element suitable to transmit a moment to the blade. However, for the purposes of clarity, the embodiments described below refer to a lever as the moment application element. Further, the described embodiments employ a rod as the element for applying force to the lever. However, it is to be understood that any alternative arrangement, known to the skilled person, and suitable for achieving the desired function, could be used as an alternative to, or in addition to, the rod described below. For example, the force application element could comprise a gear, chain, toothed belt, cam, cam follower, or any other element suitable to apply a force to the moment application element. However, for the purposes of clarity, the embodiments below refer to a rod as the force application element. Blade 16 is held in position relative to the rotor hub 12 by bearings that are fixed to the blade receiving portions 14 of the rotor hub 12 and which allow the blade 16 to rotate about pitch axis P to change the orientation of the blade 16 relative to the rotor hub. The rotor hub 12 is fixed to a shaft (not shown) that supports it and drives a gearbox and generator to convert kinetic energy generated at the rotor 10 to electrical energy. Rotor hub 12 has multiple blades 16 engaged with respective blade receiving portions 14 of the rotor hub 12, each defining a separate pitch axis P. Each lever 22 is engaged with each respective blade 16 such that movement of each respective blade about the associated pitch axis P results in movement of the engaged lever 22 relative to the rotor hub 12. Figure 2B shows the positions of the blades 16 of figure 2A with the rotor hub omitted to further clarify the positions of the blades 16 and levers 22 within the rotor 10. For the purposes of clarity, the following description relates to one of the blades 16 in engagement with the rotor hub 12, but it is to be appreciated that it is applicable to each blade 16. The blade and passive pitch mechanism includes a blade 16, a lever 22 and a resilient biasing means in the form of a coil spring 30. In the embodiment shown, the lever 22 is attached to the blade root portion 20. In the embodiment shown, the resilient biasing means is a coil spring 30. However, it is to be appreciated that any other suitable resilient biasing means may be used in addition to, as an alternative to the coil spring 30 including, but not limited to a leaf spring, a hydraulic ram with accumulator, a pneumatic ram with accumulator, or the like. Lever 22 has a first engagement portion for fixed engagement with the blade root portion 20; and a second engagement portion 26 opposing the first engagement portion for pivotable engagement with rod 31. Rod 31 has a rigid elongate body with first end portion 34 in pivotable engagement with second engagement portion 26 of lever 22; and a second end portion 36 opposing the first end portion 34. Rod 31 extends in a direction away from lever 22. Spring 30 comprises a first end 38 in engagement with the first end portion 34 of rod 31 and a second end 40 opposing first end 38. Spring 30 extends around rod 31 and extends along rod 31 in a direction towards the second end portion 36 of rod 31. A stop member 41 is mounted on rod 31 in fixed engagement with rod 31 and located distal to the first end portion 34 of rod 31. An adjustment element 42 is engaged with rod 31. Adjustment element 42 includes an adjustment element engagement portion 50 located adjacent rod 31 and between the stop member 41 and the second end 40 of spring 30. Adjustment element engagement portion 50 in the embodiment of figures 2A to 2D has a fixed location with respect to the rotor hub and provides an abutment for stop member 41. As can be seen in figures 2C and 2D, adjustment element engagement portion 50 defines axis X which extends transverse to the elongate axis of rod 31. The location of the adjustment element engagement portion 50 is selected to set the desired distance between the adjustment element engagement portion 50 and the first end portion 38 of the spring 30 when the adjustment element engagement portion 50 abuts the stop member 41. In this arrangement, the adjustment element engagement portion 50 abuts the second end 40 of spring 30 distal to lever 22 and spring 30 has the desired predetermined preload. Thus, as tidal flow F passes over the hydrodynamic control faces of the blade 16 i.e. pressure face 15 and suction face 17, the flow F applies a pressure distribution that results in a net moment about the pitch axis P. In addition, the spring 30 applies a force to the lever 22 which applies moment to the blade 16 about the pitch axis P. When the flow speed is zero, all the force from spring 30 is reacted by the stop member 41 and the blade 16 will be at zero pitch angle i.e. the resting state where the blade orientation is fully determined by the preload of the spring 30. In an ideal system, once the speed of the flow F reaches the rated speed i.e. the flow speed at which the turbine is operating at rated power, the hydrodynamic moment at the blade 16 equals the moment produced by the preload of the spring 30 and is in the opposite direction to that moment. As the flow speed increases, the hydrodynamic moment acting at the blade 16 also increases. When the hydrodynamic moment acting at the blade 16 about the pitch axis P exceeds the moment applied to blade 16 about the pitch axis by spring 30, blade 16 rotates about the pitch axis P, rod 31 moves relative to the adjustment element engagement portion 50 thereby pushing the stop member 41 from a first configuration (the stop position) in which it abuts the adjustment element engagement portion 50 to a second configuration (the non-stop position) in which stop member 41 is pushed away from the adjustment element engagement portion 50 in a direction away from the first end 38 of spring 30, thereby reducing the distance between the adjustment element engagement portion 50 and the first end 38 of the spring 30 and increasing the load through spring 30. The increased load through the spring 30 changes the moment about the pitch axis, causing the blade 16 to come to a new equilibrium position, where the applied moment from the lever 22 equals the hydrodynamic moment. Thus, the passive pitch system is operable to influence the blade pitch angle when the speed of the flow F exceeds the rated speed. Using a preloaded spring to apply a moment to the blade enables the pitch angle to autonomously and rapidly change, dependent on the flow, to the required pitch angles, thereby reducing the transient loading on the turbine. Spring preload adjustment The ability to adjust the preload during operation is important, as demonstrated by figures 8A to 8C, which show graphic representations of the hydrodynamic moment versus pitch angle for a range of nominal flow speeds. The dots show the required pitch angles for a set power production at these flows. As the graph in figure 8A shows, a constant stiffness spring (see the straight dashed line) is appropriately sized for the operation of the system when the flow speeds are at 3.4 or above. At flow speeds lower than this the force from the spring produces a moment that is lower than the hydraulic moment at the blade. As a result, the pitch angle of the blades will start to increase at flows which are lower than desired and will therefore start to shed power. This results in an undesirable loss of yield. To address this, it is desirable to be able to adjust the spring preload such that: • at lower speeds, the spring preload can be increased (see the straight solid line in the graph of figure 8B); and • as the flow increases, the spring preload could be reduced (see the straight solid line in the graph of figure 8C). Once the spring preload has been reduced to correspond to the dashed line shown in the graphs, it can be retained at that setting until the flow speed reduces sufficiently that the system will again start to increase the spring preload until it reaches the level shown by the straight solid line in the graph of figure 8B. Thus, as demonstrated in figures 8A to 8C, the ability to adjust the spring preload during operation increases the yield. Actuator driven adjustment of pitch angle Although the above-described system is a passive pitch system, it may be desirable to be able to influence the adjustment of the pitch angle |3, for example to feather the blades under command from a turbine controller (not shown). Feathering of the blades is when the blade 16 is rotated about the pitch axis P to turn the blades to circa [3=90° to reduce the rotor torque to zero. This is done to stop the rotation of the rotor 10, for example, as an emergency measure after a system fault to bring the system to rest. In the embodiment of figures 3A to 3D, adjustment element 42 further includes a pivoted support in the form of an adjustment element engagement portion 50 defining an axis X. The second end 40 of spring 30 bears against adjustment element engagement portion 50 during use. An actuator (not shown) is provided in engagement with adjustment element engagement portion 50 and operable on command from the turbine controller (not shown) to move the position of the adjustment element engagement portion 50 and thus axis X with respect to the pitch axis. Movement of axis X away from the pitch axis causes the pitch angle to increase when the stop member 41 is at the stop position, thus allowing the blades to be set to feather. The actuator (not shown) may comprise any suitable actuator known to the skilled person and includes, but is not limited to, a hydraulic or pneumatic ram or screwjack, or the like. Although there is a separate actuator for each blade, typically during use, the blade pitch angles are adjusted in unison. However, it is to be noted that having a pitch angle control system which allows independent control of the pitch angle of each blade will mean that the rotor can still be brought to rest even if one of the actuators failed. As described above, the actuator-driven pitch angle adjustment may be applied to each blade 16 separately using multiple actuators under common controller command, as shown in figures 3A to 3D. However, an alternative embodiment is shown in figures 4A to 4D in which the adjustment element 42 comprises a ring 60 provided with a plurality of adjustment element engagement portions 50, each adjustment element engagement portion 50 abutting the second end 40 of respective spring 30 of each respective blade and passive pitch mechanism. Ring 60 is engaged with an actuator (not shown) which can be controlled, under command from the turbine controller, to move the ring 60 in the direction of the rotor axis R. If it becomes desirable to control the pitch angle by actuator under command from the turbine controller, for example to move the blades to feather, the turbine controller can operate the actuator to move ring 60 such that each adjustment element engagement portion 50 moves relative to each respective blade receiving portion 14 to rotate each respective lever 22 and change the pitch of each respective blade 16 to bring the blades to feather. Although the embodiment shown uses a ring 60 to control the change of pitch angle for all three blades together, it is to be appreciated that any suitable alternative to ring 60 may be used provided it engages with the adjustment element engagement portion 50 of each respective blade 16 and is movable such that each adjustment element engagement portion 50 moves relative to each respective blade receiving portion 14 to rotate each respective lever 22 and change the pitch of the blades 16 to bring the blades to feather. This allows concurrent and equal adjustment of the pitch angle of all blades using a single actuator. Figures 5A to 5D illustrate the movement of the blade i.e. adjustment of the pitch angle of the blade, in dependence upon movement of ring 60 in the direction of the rotor axis R. When the ring 60 is moved from the position shown in figures 5A and 5B in the direction of the rotor axis R to the position shown in figures 5C and 5D when the nut 41 is in the stop position, the movement of the rod causes the lever to rotate, thereby rotating the blades and increasing the pitch angle to circa 90 ° to reduce the rotor torque to near zero, depending on the rotor speed i.e. bringing the blades to feather. However, it is to be appreciated that the pitch angle of the blades 16 may be selected as desired by selective degree of movement of the ring 60 in the direction of the rotor axis R, and need not be restricted to movement of the blades to feather. Thus, the pitch angle of the blades can be adjusted independently of the operation of the passive pitch system and independently of any adjustment to the preload of spring 30 via nut 41. Actuator driven adjustment of spring preload It is to be appreciated that the embodiments of figures 3A to 4D could be modified by providing a second actuator (not shown) to drive, under command from the controller, the movement of nut 41 along the screw thread of rod 31 to change the stop position. This would then enable both actuators, the first to move axis X and the second to move nut 41 to change the distance between each end of the spring without changing the pitch angle of the blade. This results in a change in preload of the spring. A further embodiment is shown in figures 9A and 9B in which an abutment element 100 is provided adjacent to the first end portion 38 of spring 30. Gear 110 is provided adjacent to abutment element 100. Gear 110 is driven by an actuator (not shown) and is rotatable about rod 31. Abutment element 100 has threads which engage with gear 110, such that rotation of gear 110 about rod 31 slides abutment element 100 axially along rod 31. Thus, the spring preload can be adjusted by retaining the adjustment element engagement portion 50 in position relative to rod 31, and moving the abutment element 100 in a direction towards or away from the second end 40 of spring 30. Such an arrangement allows the adjustment of the preload of spring 30 without changing the position of axis X and nut 41. A further embodiment of the present invention is shown in figures 6A to 6D, in which the passive pitch system is operable to adjust the preload at the spring during use without adjusting the pitch angle of the blades. To achieve this, the adjustment engagement portion comprises two elements: a sliding shaft 80 which abuts the second end 40 of spring 30 and a gear 82 located adjacent to rod 31 between shaft 80 and nut 41. Please note that, although the embodiment shown in figures 6A to 6D shows gear 82 located between shaft 80 and nut 41, it is to be appreciated that other arrangements are possible and gear 82 does not have to be located between shaft 80 and nut 41. An actuator is provided in engagement with gear 82 and is operable, under command from the turbine controller, to drive gear 82 to move the shaft 80 along rod 31, thereby adjusting the distance between the second end 40 of spring 30 and the second engagement portion 26 of lever 22, and adjusting the preload of spring 30 without changing the pitch angle. This arrangement, allowing actuator operated adjustment of the spring preload without changing the pitch angle, can be provided for each respective blade and passive pitch mechanism where the pitch angle adjustment is driven by separate actuators for each blade and passive pitch mechanism . Alternatively, in a further embodiment as shown in figures 7A to 7D, a ring 60 as previously described is provided having a plurality of adjustment element engagement portions 50, each adjustment element engagement portion 50 being in engagement with the second end 40 of the respective spring 30 of each respective blade and passive pitch mechanism. In such an arrangement, ring 60 would operate as previously described such that actuator-driven movement of the X axis of each adjustment element engagement portion 50 away from the respective pitch axis on command by the turbine controller would cause the pitch angle of that respective blade to increase when the nut 40 is at the stop position, thereby allowing the blade pitch angle to be adjusted on command by the turbine controller without altering the load at the spring. In addition, the load at the spring would be adjustable during use of the tidal turbine without altering the pitch angle as described above. Such an arrangement would allow concurrent and equal adjustment of all blade pitch angles using one actuator and coordinated adjustment of the preload at each respective spring 30 using a separate actuator for each spring 30. Finally, additional improvements to the embodiment of the present invention include incorporating a resilient member such as a spring or the like between the stop member i.e. nut, and the adjustable engagement member, to reduce any impact load during use and / or incorporating damping systems to run in parallel with the springs to smooth out any impact events.

Claims

1. A passive pitch system for a tidal turbine, the passive pitch system comprising a rotor having:a) a rotor hub for rotatable engagement with a mechanical power transmission system of a tidal turbine and rotatable about a rotor axis, the rotor hub having a blade receiving portion for engagement with a blade;b) a blade comprising:(i) an elongate blade body having a first end and a second end opposing the first end, wherein the elongate body defines a leading edge extending between the first and second ends and a trailing edge extending between the first and second ends and opposing the leading edge;(ii) a blade root portion located at the first end of the blade in fixed engagement with the elongate blade body and configured for rotatable engagement with the rotor blade receiving portion about a pitch axis;c) a passive pitch mechanism comprising:(i) a moment application element comprising a body having:a. a first engagement portion for engagement with the blade root portion; andb. a second engagement portion opposing the first engagement portion;(ii) an adjustable portion comprising:a. a force application element having a rigid elongate body having a first end portion in engagement with the second engagement portion of the moment application element and a second end portion opposing the first end portion;b. a resilient biasing means with a first end in engagement with the first end portion of the force application element and a second end opposing the first end, the resilient biasing means extending from the first end portion of the force application element towards the second end portion of the force application element;c. a stop member mounted on the force application element and located distal to the first end portion of the force application element; andd) an adjustment element comprising an adjustment element engagement portion defining an axis X, the adjustment element engagement portion being located distal to the moment application element, wherein the resilient biasing means has a predetermined preload in dependence upon the distance between the adjustable element engagement portion and the first end of the resilient biasing means,wherein when the hydrodynamic moment acting at the blade about the pitch axis exceeds the moment applied to the blade about the pitch axis by the resilient biasing means, the blade is rotatable about the pitch axis and the force application element moves relative to the adjustment element engagement portion to move the stop member from a first configuration where it abuts the adjustment element engagement portion to a second configuration where the stop member is moved away from the adjustable element engagement portion.

2. A passive pitch system as claimed in claim 1, further comprising:an adjustment element actuator in engagement with the adjustment element; anda controller in communication with the adjustment element actuator and operable to drive the adjustment element actuator to move the adjustment element relative to the rotor hub to move the moment application element relative to the rotor hub blade receiving portion and adjust the pitch angle of the blade.

3. A passive pitch system as claimed in claim 2, further comprising a stop member actuator in engagement with the stop member, wherein the controller is operable to drive the stop element actuator to move the stop member along the force application element and adjust the distance between the stop member and the first end portion of the force application element, andwherein the controller is operable to drive both the adjustment element actuator and the stop member actuator to adjust the preload within the resilient biasing means without changing the pitch angle of the blade when the position of the second engagement portion of the moment application element remains unchanged.

4. A passive pitch system as claimed in any preceding claim, further comprising an abutment element located adjacent the first end portion of the resilient biasing means and movable relative to the force application element, and an abutment element actuator in engagement with the abutment element and operable to move the abutment element relative to the force application element to adjust the preload of the resilient biasing means.

5. A passive pitch system as claimed in claim 1 or claim 2, wherein the adjustable portion further comprises an abutment member in moveable engagement with the force application element and abutting the second end of the resilient biasing means, wherein the passive pitch system is operable, under command from the controller, to move the abutment member along the force application element to move the second end of the resilient biasing means relative to axis X to adjust the preload within the resilient biasing means.

6. A passive pitch system as claimed in claim 1, wherein the passive pitch system comprises a plurality of blades, each blade having a respective passive pitch mechanism and adjustment element.

7. A passive pitch system as claimed in claim 6, wherein each passive pitch mechanism further comprises:an adjustment element actuator in engagement with a respective adjustment element; andwherein the passive pitch system further comprises a controller in communication with each respective adjustment element actuator and operable to drive each respective adjustment element actuator to move the respective adjustment element relative to the rotor hub and move the respective moment application element relative to the respective rotor hub blade receiving portion and adjust the pitch angle of each respective blade.

8. A passive pitch system as claimed in claim 7, wherein each passive pitch mechanism further comprises a stop member actuator in engagement with the respective stop member of that passive pitch mechanism, wherein the controller is operable to drive each stop element actuator to move the respective stop member along the respective force application element and adjust the distance between the stop member and the first end portion of the respective force application element to adjust the preload within the respective resilient biasing means, andwherein the controller is operable to drive both the adjustment element actuators and the stop member actuators to adjust the load within each resilient biasing means without changing the pitch angle of each respective blade when the position of the second engagement portion of the respective moment application element of that passive pitch mechanism remains unchanged.

9. A passive pitch system as claimed in claim 6 or claim 7, wherein the adjustable portion of each passive pitch mechanism further comprises an abutment element located adjacent the first end portion of the resilient biasing means and movable relative to the force application element, and an abutment element actuator in engagement with the abutment element and operable to move the abutment element relative to the forceapplication element to adjust the preload of the resilient biasing means10.A passive pitch system as claimed in claim 6 or claim 7, wherein the adjustable portion of each passive pitch mechanism further comprises a. an abutment member in moveable engagement with the respective force application element and abutting the second end of the respective resilient biasing means, andb. an actuator operable, under command from the controller, to move the abutment member along the respective force application element to move the second end of the respective resilient biasing means relative to axis X to adjust the preload within the resilient biasing means.11.A passive pitch system as claimed in claim 1, wherein the passive pitch system comprises a plurality of blades, each blade having a respective passive pitch mechanism, the passive pitch system having a single adjustment element, wherein the adjustment element comprises a plurality of adjustment element engagement portions, each adjustment engagement element portion being in engagement with the force application element of a respective passive pitch mechanism, and abutting the second end of the respective resilient biasing means of that passive pitch mechanism, the passive pitch system further comprising: a. An adjustment element actuator in engagement with the adjustment element; andb. a controller in communication with the adjustment element actuator and operable to drive the actuator to move the adjustment element such that each respective adjustment element engagement portion moves relative to the respective rotor hub blade receiving portion and adjusts the pitch angle of each respective blade by the same amount.

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