Passive Pitch System

Abstract

The present invention provides a passive pitch system for a tidal turbine having a rated power output, the passive pitch system comprising: (a) a rotor rotatably engaged with a mechanical power transmission system of the tidal turbine, the rotor having a rotor hub rotatable about a rotor axis, the rotor hub having blade receiving portions for engagement with the blades; and (b) blades having (i) a first end and a second end opposite the first end, a blade inner portion extending from the first end toward the second end, and a blade outer portion extending from the second end toward the first end. a blade comprising: (i) an elongated body having a blade outer portion, the elongated body defining a leading edge extending between first and second ends and a trailing edge extending between the first and second ends opposite the leading edge; (ii) a blade root portion located at the first end of the blade, the blade root portion configured to fixedly engage the elongated blade body and to rotatably engage a blade receiving portion of the rotor about a pitch axis; and (c) a lever having: (i) a first engagement portion for fixedly engaging the blade root portion; and (ii) a resilient biasing means. a rotor comprising: (a) a lever having a body having (i) a second engagement portion opposing the first engagement portion for engagement with the rotor hub; and (ii) a third engagement portion for engagement with the stop member; d) a stop member fixedly engaging the rotor hub; and e) elastic biasing means having a first end engaging with the second engagement portion of the lever and a second end opposite the first end, the second end being coupled at a position away from the stop member and having a predetermined preload selected according to the rated power output of a tidal turbine with which the rotor hub is engaged in use; wherein, in use, when a hydrodynamic moment acting on the blade about the pitch axis is less than a moment applied to the blade about the pitch axis by the elastic biasing means, the elastic biasing means biases the lever into engagement with the stop member; and when the hydrodynamic moment acting on the blade about the pitch axis exceeds the moment applied to the blade about the pitch axis by the elastic biasing means, the blade is rotatable about the pitch axis and moves the lever away from the stop member.

Description

[Technical Field]

[0001] The present invention relates to a passive pitch system for controlling the pitch angle of rotor blades for tidal turbines. [Background technology]

[0002] Conventional tidal turbines have an actuator-driven "pitch system" that controls the pitch angle of the blades while generating power. The purpose of such a pitch system is to keep the loads on the turbine within acceptable levels.

[0003] The amount of lift or drag generated by a turbine blade depends on the flow velocity and the angle at which the flow approaches the blade (i.e., "angle of attack (AoA)"), so a pitch system can be used to control the load on the blade by controlling the blade angle. Because the loads passing through the turbine are biased towards the loads on the rotor, the effectiveness of the pitch system can have a significant impact on the cost of a tidal turbine.

[0004] Most commonly, the pitch system is used to control the turbine's power output to a predetermined level, but it can also be used to control other loads, such as rotor thrust or blade bending moment.

[0005] Most pitch systems are "actively" controlled. Sensors measure the loads that need to be controlled, and the measurements are fed into a control system, which uses an algorithm to calculate the pitch angle required to bring the loads to the desired level. A controller then commands actuators (e.g., motors, hydraulic cylinders) to drive the blades to the desired pitch angle. Active pitch systems are operable to control the blade pitch angle, thereby controlling the turbine's power output. Such active systems are very good at controlling average loads in response to low-frequency variations in mean flow velocity. They are also good at controlling loads due to local flow transients from waves and large-scale turbulence, which are relatively slow in onset and can be detected and responded to by the control system.

[0006] However, they are very poor at dealing with short-term local flow transients that have a huge effect on turbine loads: these effects are not easily detected in advance and occur so quickly that practically sized systems lack the power to pitch the blades fast enough to significantly reduce the load fluctuations.

[0007] Therefore, there is a need for an improved system for controlling turbine blade pitch, particularly in response to short-term local flow transients. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to address the problems of the prior art. [Means for solving the problem]

[0009] Aspects of the invention are set out in the accompanying claims.

[0010] A first aspect of the present invention provides a passive pitch system for a tidal turbine having a rated power output, the passive pitch system comprising: a) a rotor, a rotor in rotatable engagement with a mechanical power transmission system of the tidal turbine, the rotor having a rotor hub rotatable about a rotor axis, the rotor hub having blade receiving portions for engagement with the blades; b) a blade, (i) an elongated body having a first end and a second end opposite the first end, an inner blade portion extending from the first end toward the second end, and an outer blade portion extending from the second end toward the first end, the elongated body defining a leading edge extending between the first and second ends and a trailing edge opposite the leading edge and extending between the first and second ends; (ii) a blade including a blade root portion located at a first end of the blade, the blade root portion fixedly engaging the elongated blade body and configured to rotatably engage a rotor blade receiving portion about a pitch axis; c) a lever, (i) a first engagement portion for fixedly engaging the blade root portion; (ii) a second engaging portion facing the first engaging portion and adapted to engage with the elastic biasing means; (iii) a lever having a body having a third engagement portion for engaging with the stop member; d) a stop member fixedly engaging the rotor hub; e) a rotor comprising a resilient biasing means having a first end engaging the second engagement portion of the lever and a second end opposite to the first end, the second end being coupled at a position remote from the stop member, the resilient biasing means having a predetermined preload and stiffness selected according to the rated power of a tidal turbine with which the rotor hub engages in use; In use, when the hydrodynamic moment acting on the blade about the pitch axis is less than the moment applied to the blade about the pitch axis by the resilient biasing means, the resilient biasing means biases the lever into engagement with the stop member, and when the hydrodynamic moment acting on 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 allowed to rotate about the pitch axis and move the lever away from the stop member.

[0011] It should be understood that the inner portion of the blade extending from the first end toward the second end is located near the rotor hub, and the outer portion of the blade extending from the second end toward the first end is located away from the rotor hub.

[0012] In one embodiment, the second end of the resilient biasing means fixedly engages the rotor hub.

[0013] The passive pitch system of the present invention allows for control of pitch angle by rotating a rigid blade about its pitch axis during operation and balancing the hydrodynamic loads acting on the blade and the moments applied by the resilient biasing means.

[0014] The blade profile is preferably offset relative to the pitch axis along the length of the blade, and the resilient biasing means has a predetermined preload selected dependent on the power rating of the tidal turbine with which the rotor hub will engage in use.

[0015] In yet another embodiment, in use the blade inner portion is offset towards the trailing edge relative to the pitch axis, and optionally also away from the pitch axis P in the direction of current flow.

[0016] The offset of the inner blade portions relative to the pitch axis improves the response speed of the system while helping to balance the hydrodynamic pitching moment on the blade with the moment applied by the elastic biasing means in the rotor hub as the turbine moves through its rated operating flow speed range, i.e., the flow speed range over which the turbine normally produces power.

[0017] Additionally, or instead of offsetting the inner blade portion relative to the pitch axis, the outer blade portion may be offset in use relative to the pitch axis toward the trailing edge, and optionally away from the pitch axis P in the direction of the tidal current, thereby adjusting the sensitivity of the hydrodynamic pitching moment to current velocity at a given pitch angle.

[0018] Displacing the outer blade portions relative to the pitch axis improves response time to short-term transient disturbances in the current. Thus, the offset of the outer blade portions, combined with the offset of the inner blade portions relative to the pitch axis, provides good power control over the full operating range of current speeds as well as improved response time to short-term transient disturbances in the current.

[0019] In a preferred embodiment, the blade root portion is offset relative to the pitch axis greater than the blade tip portion is offset relative to the pitch axis.

[0020] Conventionally, the position of the pitch axis relative to the blade's hydrodynamic profile is set to minimize the hydrodynamic moment about the pitch axis. However, the hydrodynamic profile offset described above is intended to provide a larger hydrodynamic moment about the pitch axis to effectively operate the passive pitch system.

[0021] In yet another embodiment, the passive pitch system further comprises a tidal turbine controller operable to adjust the rotational speed of the rotor hub in response to a hydrodynamic moment acting on the rotor about the rotor axis, the rotational speed of the rotor hub, and a predetermined preload on the elastic biasing means, so that at a pitch angle corresponding to the rated power output of the tidal turbine, the hydrodynamic moment acting on the blades about the pitch axis is equal to the moment about the pitch axis applied to the blades by the elastic biasing.

[0022] In yet another embodiment, the passive pitch system further includes a preload adjustment member fixedly engaged with the second end of the resilient biasing means, the preload adjustment member being movable relative to the rotor hub and rotatable therewith, such that the preload adjustment member can be moved relative to the rotor lever to change the distance between the second engagement portion of the lever and the second end of the resilient biasing means, thereby adjusting the preload of the resilient biasing means.

[0023] Preferably, moving the preload adjustment member away from the lever to increase the distance between the lever and the second end of the elastic biasing means decreases the preload of the elastic biasing means, and moving the preload adjustment member towards the lever to decrease the distance between the lever and the second end of the elastic biasing means increases the preload of the elastic biasing means.

[0024] The passive pitch system may further include a wheel gear positioned in fixed engagement with the blade root portion, the wheel gear operable to control rotation of the blade about the pitch axis independently of a hydrodynamic moment about the pitch axis acting on the blade.

[0025] In one embodiment, the passive pitch system further includes an adjusting plate fixedly engaged with the stop member and the second end of the resilient biasing means, the adjusting plate operable to control rotation of the blade about the pitch axis independent of a hydrodynamic moment acting on the blade about the pitch axis. [Brief explanation of the drawings]

[0026] [Figure 1A] 1 shows a rotor for a tidal turbine. [Figure 1B] 1 shows a rotor for a tidal turbine. [Figure 1C] FIG. 1 is a perspective view of a blade for a tidal turbine. [Figure 1D] FIG. 1D is a view from one end of the blade of FIG. 1C. [Figure 1E] FIG. 1D is a view from the other end of the blade of FIG. 1C. [Figure 2A] 1 shows the blade and lever configuration of a first embodiment of a rotor of a passive pitch system according to the present invention. [Figure 2B] 1 shows the blade and lever configuration of a first embodiment of a rotor of a passive pitch system according to the present invention. [Figure 2C] 2A and 2B disposed within a rotor hub, and the rotor hub engaged with a plurality of the blade and lever arrangements shown in FIGS. 2A and 2B. [Figure 2D] 2C and 2E with the rotor hub omitted for clarity. [Figure 2E] 2C shows a rotor hub engaged with the multiple blade and lever arrangement shown in FIGS. 2A and 2B. [Figure 2F] 2C and 2E with the rotor hub omitted for clarity. [Figure 3A] Snapshots of a range of blade equilibrium positions are shown as the blade moves toward or approaching the desired pitch angle with increasing flow velocity. [Figure 3B] Snapshots of a range of blade equilibrium positions are shown as the blade moves toward or approaching the desired pitch angle with increasing flow velocity. [Figure 3C] Snapshots of a range of blade equilibrium positions are shown as the blade moves toward or approaching the desired pitch angle with increasing flow velocity. [Figure 4] 10 shows a graph illustrating the change in pitch angle β with increasing flow velocity. [Figure 5] 1 is a graph showing the relationship between flow velocity, pitch angle, and turbine output. [Figure 6] A graph showing typical total hydrodynamic moments at blade 16 for a 3 MW, 24 m rotor rotating at 14.3 rpm, observed at different pitch angles β, is shown. [Figure 7] 1 shows a conventional blade for an active pitch system. [Figure 8A] 1 shows a modified blade. [Figure 8B] 1 shows a modified blade. [Figure 9A] 10 is a graph showing the total aerodynamic moment versus pitch angle when the inner blade portion is offset relative to the pitch axis. [Figure 9B] 1 is a graph showing the hydrodynamic moment about the blade pitch axis at a rotor speed of 13 rpm for a range of flow velocities and pitch angles for a particular spring stiffness and preload. [Figure 9C] 1 is a graph showing the hydrodynamic moment about the blade pitch axis at a rotor speed of 14.6 rpm for a range of flow velocities and pitch angles for a particular spring stiffness and preload. [Figure 9D] 1 is a graph showing the hydrodynamic moment about the blade pitch axis at a rotor speed of 15.2 rpm for a range of flow velocities and pitch angles for a particular spring stiffness and preload. [Figure 9E] 1 is a graph showing that the rotor speed is set by the turbine controller to produce the required pitch angle β for rated power output for a particular spring preload and stiffness. [Figure 10] 1 shows a modified blade. [Figure 11]1 is a graph showing the hydrodynamic moment of the blade 16 at different pitch angles β for a 3 MW, 24 m diameter rotor rotating at 14.3 rpm, showing the spring moment at various preload settings. [Figure 12A] 1 illustrates one embodiment of an arrangement for achieving spring preload adjustment according to the present invention. [Figure 12B] 1 illustrates one embodiment of an arrangement for achieving spring preload adjustment according to the present invention. [Figure 12C] 1 illustrates one embodiment of an arrangement for achieving spring preload adjustment according to the present invention. [Figure 13A] 1 illustrates one embodiment of an arrangement for achieving blade feathering using a wheel gear that is fixedly engaged with the blade. [Figure 13B] 1 illustrates one embodiment of an arrangement for achieving blade feathering using a wheel gear that is fixedly engaged with the blade. [Figure 13C] 1 illustrates one embodiment of an arrangement for achieving blade feathering using a wheel gear that is fixedly engaged with the blade. [Figure 13D] 1 illustrates one embodiment of an arrangement for achieving blade feathering using a wheel gear that is fixedly engaged with the blade. [Figure 14A] 10 shows an embodiment of a further configuration for achieving blade feathering with an adjusting plate fixedly engaged with the lever and rotatable with the lever about pitch axis P, with the lever in a first position. [Figure 14B] 14B shows an embodiment of a further configuration for achieving blade feathering, with the lever rotated relative to FIG. 14A using an adjustment plate fixedly engaged with the lever and rotatable with the lever about pitch axis P. [Figure 14C] The configuration of FIG. 14D , with the rotor ham omitted for purposes of clarity, is shown with an adjusting plate fixedly engaged with the lever and rotatable with the lever about pitch axis P, illustrating one embodiment of a further configuration for achieving blade feathering. [Figure 14D] 14B illustrates one embodiment of a further configuration for achieving blade feathering, in which the rotor hub is engaged with the plurality of blades of FIG. 14A using an adjusting plate fixedly engaged with the lever and rotatable with the lever about pitch axis P. DETAILED DESCRIPTION OF THE INVENTION

[0027] 1A and 1B show a rotor 10 for a tidal turbine. The rotor 10 includes a rotor hub 12 having a plurality of blade engagement portions 14, each of which is engaged by a blade 16. The flow direction is indicated by arrow F. Hydrodynamic forces from water flowing over the blades 16 generate a moment about the rotor axis R, causing the rotor to rotate about the rotor axis R. This kinetic energy is converted to electrical energy by a generator engaged with the rotor 10.

[0028] Each blade 16 has an inner portion 18 i extending from the end of the blade 16 adjacent the rotor hub 12 and an outer portion 18 o extending from the opposite end of the blade 16 remote from the rotor hub 12 .

[0029] Each blade 16 has a blade root 20 at its inner portion 18i (see FIG. 1C). The blade root 20 of each blade 16 engages with a respective blade engagement portion 14, allowing the blade 16 to rotate relative to the rotor blade hub 12 about the pitch axis P.

[0030] The blades also have leading and trailing edges 11, 13 that extend longitudinally along the length of the blade. Each blade also has two faces: a pressure face 15 on one side of the blade 16 and a suction face 17 on the opposite side from the pressure face 15. The pressure face 15 and the suction face 17 meet at the leading and trailing edges 11, 13 (see FIG. 1E).

[0031] Relative rotation between each blade 16 and rotor hub 12 is typically achieved by providing a bearing between each rotor blade engagement portion 14 and its respective blade root 20 .

[0032] 1A and 1B also show the orientation of the rotor hub 12 and the leading edge 11 and trailing edge 13 of each blade 16 relative to the current flow direction F. In use, hydrodynamic forces from the water flow (indicated by arrows F) past the blades 16 generate moments about the rotor axis (indicated by arrow A in FIG. 1A ), causing the rotor hub 12 to rotate. These moments are reacted by a generator, which converts the mechanical energy produced in the rotor hub 12 into electrical energy.

[0033] It is important to control the amount of power and load on the turbine to avoid overloading the turbine's subsystems, including but not limited to the generator, gearbox, and electrical cables. This is achieved by rotating the blades 16 about a pitch axis P to control the orientation of the blades 16 relative to the flow direction F.

[0034] As shown in Figure 5, at flow speeds below the rated flow speed, the turbine sets its blades at a so-called pitch angle β=0 so that the blades can capture the maximum amount of energy from the flow.

[0035] Once the flow speed exceeds the rated flow speed, i.e., the flow speed at which the blade pitch angle β=0 and the turbine operates at rated power, the pitch angle of the blades 16 must be increased to limit the power produced to the turbine's rated power. The faster the flow speed, the greater the pitch angle required to limit the force to rated power.

[0036] In an active pitch control system, the orientation of the blades 16 relative to the rotor hub 12 is controlled by an actuation system that sets the blade position according to the flow speed and the turbine's rated power output. Such actuation systems involve the use of electric actuators and mechanisms such as hydraulic rams, levers, motors, sensors, and gearboxes, all of which are costly to install, complex to maintain, and provide multiple points of potential failure.

[0037] However, the flow velocity through the rotor area is not constant but varies with tidal cycles, wave dynamics, turbulence, water depth, and the influence of the installation structure on the operation of the turbine and its surrounding environment.

[0038] Traditionally, active pitch control systems respond to these fluctuations in a way that levels out the power generated by the turbine. In practice, however, active control systems are slow to respond to flow fluctuations, which can result in the turbine operating above rated power for short periods of time, which requires higher capacity and therefore increases manufacturing costs.

[0039] 2A and 2B show the rotor blade and lever configuration in a first embodiment of a passive pitch system according to the present invention.

[0040] 2C and 2D show the blade and lever arrangement of Figures 2A and 2B engaged with rotor hub 12. Blades 16 are held in place relative to rotor hub 12 by bearings fixed to blade engagement portions 14 of rotor hub 12, which allow blades 16 to rotate about pitch axis P to change the orientation of blades 16 relative to the flow direction. Rotor hub 12 is fixed to a shaft (not shown) that supports and drives a gearbox and generator to convert kinetic energy generated by rotor 10 into electrical energy.

[0041] 2E shows a rotor hub 12 with multiple blades 16 engaged with respective blade engagement portions 14 of the rotor hub 12, each defining an individual pitch axis P. A lever 22 is engaged with each blade 16, and movement of the blade about pitch axis P results in movement of the lever 22 relative to the rotor hub 12. FIG. 2F shows the positions of the blades 16 of FIG. 2E without the rotor hub to more clearly show the positions of the blades 16 and levers 22 within the rotor 10.

[0042] For purposes of clarity, the following description will refer to one of the blades 16 engaged with the rotor hub 12, but it should be understood that this applies to each blade 16.

[0043] The blade and lever arrangement includes a blade 16, a lever 22, and a resilient biasing means 30. In the illustrated embodiment, the lever 22 is attached to the blade root via a pintle shaft 32. In the illustrated embodiment, the resilient biasing means 30 is a coil spring. However, it should be understood that any other suitable resilient biasing means may be used in addition to or in place of the coil spring, including, but not limited to, a leaf spring, a torsion spring, a hydraulic ram with accumulator, or a pneumatic ram with accumulator.

[0044] Lever 22 has a first engagement portion 24 that fixedly engages blade root 20, a second engagement portion 26 opposite the first engagement portion that engages spring 30, and a third engagement portion 28 that engages stop member 34. Stop member 34 is fixedly engaged with rotor hub 12. The location of stop member 34 on rotor hub 12 and the location of fixed end 31 of spring 30 are selected so that spring 30 has a preloaded force therein to urge lever 22 against stop member 34.

[0045] Thus, as current F passes over the hydrodynamic control surfaces of blade 16, i.e., pressure surface 15 and suction surface 17, current F creates a pressure distribution that results in a net moment about pitch axis P. In addition, spring 30 exerts a moment about pitch axis P on blade 16.

[0046] When the flow velocity is zero, all of the force from the spring 30 is exerted by the stop member 34 and the blade 16 is at rest with a zero pitch angle, i.e., the blade orientation is completely determined by the preload of the spring 30 .

[0047] In an ideal system, when the velocity of flow F reaches rated speed, i.e., the flow velocity at which the turbine operates at rated power, the hydrodynamic moment acting on blade 16 is equal to and opposite to the moment caused by the preload of spring 30 (see FIG. 2B).

[0048] When the flow velocity exceeds the rated flow velocity, the hydrodynamic moment on blade 16 becomes greater than the preload of spring 30, which causes lever 22 and blade 16 to rotate about pitch axis P relative to rotor hub 12, thereby changing the orientation of blade 16. This rotation increases the moment exerted by spring 30, changing the hydrodynamic moment on blade 16 so that a new equilibrium position is achieved. This rotation is expressed as an angle relative to the orientation of blade 16 at zero pitch angle.

[0049] 3A, 3B, and 3C show snapshots of a range of equilibrium positions for the blades as they move toward or approach the desired pitch angle β as the flow rate increases. FIG. 3A shows the equilibrium position when the pitch angle β is zero, which occurs at flow velocities below the rated flow rate. That is, the hydrodynamic moment on the blades 16 is less than the moment created by the preload of the springs 30. FIG. 3B shows the equilibrium position when the pitch angle β is approximately 10 pitch angles from zero, which occurs when the flow rate exceeds the turbine's rated power output. FIG. 3C shows the equilibrium position as the flow rate increases further, further increasing the hydrodynamic moment on the blades 16 and further rotating the lever and blades about the pitch axis P, increasing the pitch angle β.

[0050] The graph in Figure 4 shows the variation of pitch angle β with increasing flow velocity, with the boxed region indicating when β is zero (i.e., Figure 3A), and the equilibrium positions in the snapshots in Figures 3B and 3C marked accordingly.

[0051] These observations were carried out using a turbine with a rated power of 3 MW and a rotor of 24 m.

[0052] FIG. 6 shows a graph of how the total hydrodynamic moment at the blade 16 varies with pitch angle β for a typical tidal turbine blade 16. Separate curves show how the hydrodynamic pitching moment varies with pitch angle β for different flow speeds. Markers plotted on each curve indicate the pitch angle β and hydrodynamic pitching moment when the turbine is producing rated power at a given flow speed. This figure shows that the elastic biasing means 30, whose load varies linearly with displacement, can provide a relatively good match to these markers, meaning that the passive pitch system of the present invention provides a relatively close match to the desired performance across a range of flow speeds. However, this match is not perfect; using the lever 22 and spring 30 configuration described above, it was observed that at low flow speeds, the hydrodynamic moment at the blade 16 is greater than the moment added by the spring 30. This results in the blade 16 beginning to pitch, i.e., rotate about the pitch axis P, at a speed below the turbine's rated power, resulting in some power loss at flow speeds close to the rated speed. This has the undesirable effect of reducing the amount of energy or power generated by the system.

[0053] Also note that as the flow velocity increases, there is a relatively small increase in the hydrodynamic pitching moment, as shown in Figure 6. This leads to a relatively small rotational acceleration of the blade 16 about the pitch axis P, which means that the blade 16 responds to changes in the flow relatively slowly. A faster response is more desirable because it can reduce fluctuations in turbine load and power output.

[0054] Note that Figure 6 is for a 3 MW, 24 m diameter rotor operating at 14.3 rpm, but the key characteristics are extensible and similar graphs can be obtained for turbines of different sizes and ratings.

[0055] One way to improve the system's response speed is to modify the blade's shape. By moving different portions of the blade 16, or even the entire blade 16, away from the pitch axis P, the system's response speed to changes in the flow can be increased. FIG. 7 shows a conventional blade 16 for an active pitch system. By comparison, FIGS. 8A and 8B show modified blades 16. FIG. 8A shows a modified blade shape in which the hydrodynamic shape along the inner portion 18i of the blade 16 is offset toward the trailing edge 13. Furthermore, FIG. 8B shows a modified blade shape in which the hydrodynamic shape along the inner portion 18i of the blade 16 is offset toward the trailing edge 13 and away from the pitch axis P in the flow direction F. A further modification of the blade 16 is shown in FIG. 10, in which the outer portion 18o of the blade 16 is offset toward the trailing edge.

[0056] These offsets result in an increase in the hydrodynamic moment on blade 16 as the flow velocity increases, and therefore blade 16 rotates more quickly about pitch axis P. This is shown in the graph of Figure 9A. Note how the difference in hydrodynamic pitching moment at an associated pitch angle β for a given flow velocity difference is significantly larger than in Figure 6.

[0057] Note that Figure 9A is for a 3 MW power, 24 m diameter rotor operating at 14.3 rpm, but the key characteristics are extensible and similar graphs can be obtained for turbines of different sizes and power ratings.

[0058] Displacing the inner portion 18i of the blade 16 provides a small improvement in speed of response while maintaining relatively accurate output control when using a constant stiffness spring 30. Displacing the outer portion 18o of the blade 16 provides a greater improvement in speed of response, at the expense of less accurate output control when using a constant stiffness spring 30.

[0059] Speed ​​Control To avoid loss of power generation caused by restriction by constant stiffness springs 30, rotor speed can be adjusted to vary the hydrodynamic moment applied to blades 16 about pitch axis P. Increasing rotor speed increases the moment, and decreasing rotor speed decreases the moment, over the turbine's normal operating range.

[0060] Figure 9A shows the hydrodynamic moment characteristics about the pitch axis P for a constant rotor speed of 14.3 rpm. The markers indicate the pitch angle β at which the turbine produces rated power at each flow rate. It is clear that at speeds above 3.4 m / s, the constant stiffness of the spring 30 accurately controls the turbine to rated power.

[0061] However, it is also clear that at zero pitch angle (β=0), the hydrodynamic moment at rated power is much greater than the moment due to spring 30. Therefore, the hydrodynamic moment applied to blade 16 about pitch axis P will exceed the spring moment at lower flow velocities, causing blade 16 to pitch away from zero pitch angle. This will result in the turbine operating at a lower power output and a corresponding reduction in power generation until the flow velocity reaches approximately 3.4 m / s.

[0062] However, reducing the rotor speed also reduces the hydrodynamic moment applied to the blades 16 about the pitch axis P.

[0063] As an example, Figures 9B, 9C, and 9D show the hydrodynamic moment of the blade about the pitch axis P for a range of flow velocities and pitch angles for a particular spring stiffness and preload. Figure 9B shows a rotor speed of 13 rpm, Figure 9C shows a rotor speed of 14.6 rpm, and Figure 9D shows a rotor speed of 15.2 rpm.

[0064] It should be noted that Figures 9B, 9C and 9D are for a 3 MW, 24 m diameter rotor, but the key characteristics are scalable and similar graphs can be obtained for turbines of different sizes and ratings.

[0065] As the flow velocity approaches approximately 3 m / s, the rotor speed reaches 13 rpm. As the flow velocity increases from 3 m / s, the rotor speed gradually increases until, at the pitch angle β that produces rated power, the hydrodynamic moment exerted on the blades 16 about the pitch axis P matches the moment exerted by the springs 30. This is 14.6 rpm at 3.2 m / s and 15.2 rpm above 3.4 m / s.

[0066] In this example, the rotor speed is set by the turbine controller to produce the pitch angle β required to obtain rated power output for a particular preload and stiffness of the spring 30, which is illustrated in FIG. 9E.

[0067] The turbine controller does not directly measure the flow velocity through it, but rather monitors the converted power and the rotor rotational speed and uses these data to calculate the rotor speed required so that the hydrodynamic moment applied to blades 16 about pitch axis P matches the moment applied by springs 30 at the pitch angle β required for rated power.

[0068] Spring preload adjustment A further means of avoiding the loss of power output due to the limitations of a constant stiffness spring is to control the preload of the spring 30. By gradually adjusting the preload of the spring 30 according to the flow velocity, the moment exerted by the spring 30 can be varied.

[0069] The adjustment of the preload of the spring 30 is effected by an actuation system controlled by the turbine controller and may be performed instead of or in addition to controlling the rotational speed of the rotor 10 as described above.

[0070] Figure 11 shows the spring moment at various preload settings. This preload will be set so that the hydrodynamic moment matches the spring moment at the pitch angle corresponding to the rated power. The preload setting is calculated based on the measured power level and blade pitch angle.

[0071] Note that Figure 11 is for a 3 MW, 24 m diameter rotor operating at 14.3 rpm, but the key characteristics are extensible and similar graphs can be obtained for turbines of different sizes and ratings.

[0072] One embodiment of an arrangement for achieving spring preload adjustment is shown in FIGS. 12A-C. FIG. 12A shows a single blade 16 with the lever 22 and spring 30 arrangement described above. A preload adjustment member, here a preload adjustment ring 36, is fixedly engaged with the end of the fixed end of spring 31 remote from the lever (see FIG. 12B). This preload adjustment ring 36 is movably engaged with rotor hub 12. Moving the preload adjustment ring 36 relative to the hub changes the relative angle between lever 22 and blade 16, thereby adjusting the distance between lever 22 and preload adjustment ring 36 and, consequently, changing the preload of spring 30. FIG. 12C shows the arrangement of FIG. 12B deployed with rotor hub 12.

[0073] Although the present embodiment uses a preload adjustment ring 36, it should be understood that the preload adjustment member need not be ring-shaped, and that other alternatives may be used, provided that any suitable component capable of performing a similar function is used.

[0074] It should be understood that any means for moving the preload adjustment member relative to the blade 16 may be used, as long as it changes the distance between the end of the spring 30 secured to the preload adjustment ring 36 and the lever 22.

[0075] Blade feathering Blade feathering involves rotating the blades 16 about the pitch axis P and orienting the blades at approximately β=90 degrees in order to reduce the rotor torque to zero. This is done to stop the rotation of the rotor 10, for example, in an emergency under excessive flow speeds or to bring the system to a shutdown.

[0076] An example of how this is accomplished with the passive pitch system of the present invention is shown in Figures 13A-13D using a wheel gear 38 fixedly engaged to blade 16. In the example shown in Figures 13A-13D, wheel gear 38 is fixedly engaged to pintle shaft 32 of blade 16. A worm 40 is located within lever 22 and a motor (not shown) is engaged to worm 40. Thus, during normal operation, the passive pitch system functions as described above.

[0077] However, when adjustment of the pitch angle β is required, for example when feathering the blades is desired, the motor can be activated to rotate the worm 40, thereby moving the wheel gear 38 relative to the lever 22, thereby changing the pitch angle β of the blades 16 without moving the lever 22.

[0078] Thus, a 90° rotation of the blade about the pitch axis P is effected, which allows the blade to be set in the feathering position without moving the lever 22 at the fixed end of the spring 30.

[0079] An alternative embodiment that allows for blade feathering is shown in Figures 14A-14D.

[0080] In this configuration, the adjusting plate 42 is pivotable about the pitch axis P and is fixedly engaged with the stop member and the second end of the resilient biasing means.

[0081] The angular position of the adjusting plate 42 is set by an actuation system (not shown).

[0082] During normal operation, the adjusting plate 42 is fixed and the system operates as previously described.

[0083] To move the blade to the feather position, the actuation system is controlled by the turbine controller and actuates the adjusting plate 42, the stop member 34, and the second end of the resilient biasing means to rotate, thereby rotating the lever 22 together with the blade 16 about the pitch axis P.

[0084] It should be understood that the actuator (eg, linear actuator) may be electric or hydraulic.

[0085] This configuration would need to be adapted to allow for spring preload adjustment. For example, the fixed end 31 of spring 30 would need to be mounted on a sliding system such as, but not limited to, a lead screw. An actuator would be required for each blade to control the position of the lead screw.

[0086] Advantages of a passive pitch system: Passive pitch systems reduce the ultimate and fatigue loads on the blades, rotor, and supporting structure. This significantly reduces the cost of the entire system's structural components, from the blades to the foundation. This load reduction is achieved through a passive pitch rotor design, which allows the blade pitch angle to change very quickly to accommodate changes in flow velocity, eliminating power and load fluctuations. Increased reliability as it does not rely on the correct operation of motors, drives, controllers, gearboxes, brakes, encoders and load sensors required for traditional active pitch systems. Only very simple mechanical components are required for operation (springs, end stops, i.e. stiffer springs). Passive pitch systems do not require load sensors or encoders, as traditional active pitch systems require.

Claims

1. 1. A passive pitch system for a tidal turbine having a rated power output, the passive pitch system comprising: a) a rotor, a rotor in rotatable engagement with a mechanical power transmission system of a tidal turbine, the rotor having a rotor hub rotatable about a rotor axis, the rotor hub having blade receiving portions for engagement with the blades; b) a blade, (i) an elongated body having a first end and a second end opposite the first end, an inner blade portion extending from the first end toward the second end, and an outer blade portion extending from the second end toward the first end, the elongated body defining a leading edge extending between the first and second ends and a trailing edge opposite the leading edge and extending between the first and second ends; (ii) a blade comprising: a blade root portion located at the first end of the blade, the blade root portion fixedly engaging the elongated blade body and configured to rotatably engage the rotor blade receiving portion about a pitch axis; c) a lever, (i) a first engagement portion for fixedly engaging the blade root portion; (ii) a second engaging portion facing the first engaging portion and adapted to engage with a resilient biasing means; (iii) a third engagement portion for engaging the stop member; d) a stop member fixedly engaging said rotor hub; e) a rotor comprising a resilient biasing means having a first end engaging the second engagement portion of the lever and a second end opposite the first end, the second end being coupled at a position remote from the stop member, the resilient biasing means having a predetermined preload selected according to the rated power of the tidal turbine with which the rotor hub is engaged in use; a resilient biasing means biasing the lever into engagement with the stop member when, in use, a hydrodynamic moment acting on the blade about the pitch axis is less than a moment applied to the blade about the pitch axis by the resilient biasing means, and the blade is rotatable about the pitch axis, causing the lever to move away from the stop member when the hydrodynamic moment acting on the blade about the pitch axis exceeds the moment applied to the blade about the pitch axis by the resilient biasing means.

2. 2. The passive pitch system of claim 1, wherein said second end of said resilient biasing means fixedly engages said rotor hub.

3. 3. The passive pitch system of claim 1, wherein the blade inner portion is offset relative to the pitch axis toward the trailing edge.

4. 4. The passive pitch system of claim 3, wherein the blade inner portion is offset relative to the pitch axis toward the trailing edge and away from the pitch axis P in the direction of current passing the blade in use.

5. 5. A passive pitch system according to claim 1, wherein the blade outer portion is offset relative to the pitch axis in a direction towards the trailing edge.

6. 6. The passive pitch system of claim 5, wherein the blade outer portion is offset away from the pitch axis P in the trailing edge direction relative to the pitch axis and in the direction of current past the blade in use.

7. 7. A passive pitch system according to any one of claims 1 to 6, further comprising a tidal turbine controller operable to adjust the rotational speed of the rotor hub according to the hydrodynamic moment acting on the rotor about the rotor axis, the rotational speed of the rotor hub, and the predetermined preload of the elastic biasing means, so that, at a pitch angle corresponding to a rated power output of the tidal turbine, the hydrodynamic moment acting on the blade about the pitch axis is equal to the moment added by the elastic biasing means to the blade about the pitch axis.

8. 8. The passive pitch system of claim 1, further comprising a preload adjustment member fixedly engaging the second end of the resilient biasing means, the preload adjustment member being movable relative to the rotor hub and rotatable therewith, wherein the preload of the resilient biasing means can be adjusted by moving the preload adjustment member relative to the lever to change the distance between the second engagement portion of the lever and the second end of the resilient biasing means.

9. 9. The passive pitch system of claim 8, wherein moving the preload adjustment member away from the lever to increase the distance between the lever and the second end of the resilient biasing means decreases the preload of the resilient biasing means, and moving the preload adjustment member toward the lever to decrease the distance between the lever and the second end of the resilient biasing means increases the preload of the resilient biasing means.

10. 10. The passive pitch system of claim 1, further comprising a wheel gear in a fixedly engaged position at a root of the blade, the wheel gear operable to control rotation of the blade about the pitch axis independently of the hydrodynamic moment acting on the blade about the pitch axis.

11. 11. A passive pitch system according to claim 1 and any one of claims 3 to 10 depending on claim 1, wherein the passive pitch system further comprises an adjusting plate fixedly engaged at a second end of the lever, a fixed stop, and the resilient biasing means, the adjusting plate being operable to control rotation of the blade about the pitch axis independently of the hydrodynamic moment acting on the blade about the pitch axis.

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