Improvements in and relating to fluid flow-driven generators

Abstract

A fluid flow-driven generator 1 for generating energy from a flow of liquid or gas comprises a fluid-driven device 3, 4 with at least one movable foil 41 that has an adjustable angle of attack. A sen

Description

Field of the Disclosure

[0001] The present disclosure provides improvements in and relating to fluid flow-drive generators. More particularly, but not exclusively, the present disclosure relates to hydropower generators of the kind comprising a flapping mechanism that includes a hydrofoil arranged to move back and forth on a mechanical linkage in a flow of water, the mechanical linkage being connected to an electric generator for converting the reciprocal motion of the hydrofoil into electrical energy. Aspects of the present disclosure are also applicable to other fluid flow-driven generators such for example as water and wind turbines.

[0002] Background

[0003] The conversion of kinetic energy in flowing air or water into electrical energy by rotary turbines or by oscillating wings or sails is well known. Hydropower extracts power from flowing water. The water flow could be due to tides or river currents, or could be the flow past a vessel in motion such, for example, as a sailing or motor yacht.

[0004] All hydropower systems generate power by slowing the flow. There is a physical limit (the Betz limit) to the maximal power that can be generated by slowing the flow, because if the flow is stopped completely, no power can be generated. The power available from the flow is:

[0005] Pfl0W=±rSV\

[0006] where Pfiow is the power available from the flow; r is fluid density; S' is the area swept by the turbines, wings or sails; and Vis the flow velocity.

[0007] In a horizontal axis hydropower turbine system such, for example, as a turbine, the flow acts on turbine blades to generate a hydrodynamic force, which rotates the turbine and drives a generator. The magnitude of the hydrodynamic force depends on the twist of the blade, its size, and the angle of attack between the blade and the flow. Meanwhile, a flapping hydropower system uses a fin in the flow to generate hydrodynamic forces and requires a mechanism to adjust the angle of attack of the fin to the flow to generate a flapping motion and thus the forces required to drive the generator. At any instant the force on the fin depends on the flow velocity, the size of the fin and the angle of attack of the fin relative to the flow. Some of the interest in flapping hydropower systems stems from the fact that for the same span and amplitude, a turbine sweeps a circle with ?r / 4 smaller area than the square swept by a flapping foil, and therefore the available power is lower.

[0008] Real hydropower systems are neither optimal nor 100% efficient. Conventionally, inefficiencies are captured in a power-coefficient:

[0009] P = Cp|rW3.

[0010] The power that can be extracted by slowing the flow thus varies with a power coefficient, Cp. which is a maximum at the Betz limit of 16 / 27 or 0.59. When the flow passing through the hydropower system is slowed to about one-third of its initial velocity, the power coefficient is maximised. At the time of writing, wind turbines typically slow the flow by only about 0.5-0.7 at rated power, giving Cp= 0.4-0.5.

[0011] Prior approaches to flapping hydropower prescribe the kinematics and geometry of the flapping stroke using actuators or mechanical systems to set the angle-of-attack of the fin and the shape of the stroke. These mechanisms require the angle of attack of fin to be specified and controlled throughout the stroke. Many different methods have been developed to control fin angle of attack.

[0012] McKinney, W., &DeLaurier, J. (1981). The Wingmill: An oscillating-wing windmill. Journal of Energy, 5(2), 109-115 describe a a horizontally mounted wing whose plunging motion is transformed into a rotary shaft motion. The wing is pivoted to pitch at its half-chord location by means of a fitting which, itself, is rigidly attached to a vertical support shaft. Also fixed to the support shaft is an outer sleeve of a push-pull cable whose end pivots on a wing-fixed lever to control the wing's pitch. The up-and-down motion of the support shaft is transformed, through a Scotch-yoke mechanism, into a rotary motion of a horizontal shaft. This shaft, in turn, operates a crank at its far end which actuates the previously mentioned pitch-control cable. Hence the wing's pitching and plunging motions are articulated together at a given frequency and phase angle. The phase angle is controllable while the wingmill is running.

[0013] US 7989973 B2 discloses power generation apparatus including a wing-shaped blade having opposite sides, opposite ends and leading and trailing edges extending between those ends. A lift differential producing device in the blade produces a lift differential at the opposite sides of the blade and that device is switched so that one blade side or the other produces the greater lift. A blade shaft extends along an axis in the blade that is in close parallel relation to the leading edge of the blade and that shaft is fixed to move with the blade. Supports support the blade shaft so that the blade can be positioned in a fluid stream with the leading edge facing upstream and swing about the axis between first and second extreme positions on opposite sides of a neutral position, the blade shaft oscillating with the blade. A coordinating device coordinates the switching of the lift differential producing device with the swinging of the blade so that the switching occurs at the extreme positions of the blade.

[0014] US 2009 / 0121490 Al discloses a method for converting kinetic energy of wind or water flows into electric energy in which wings or sails are mounted on swing arms or guide rails in such a way that the air or water flow induces an oscillatory wing or sail motion with a phase angle between the wing's or sail's pitch and plunge motion of about ninety degrees. Stroke reversal of the oscillatory motion is initiated by an aerodynamic / hydrodynamic mechanism such that the air or water flow induces a pitching moment on the wing or sail which rotates the wing or sail and thereby reverses the lift acting on the wing or sail. Two switching rods are mounted in such a way that a spike attached to a wing leading edge starts to touch a right or left switching rod at the end of right and left strokes, respectively. This forces the wing to rotate about the switching rod because an aerodynamic or hydrodynamic pitching moment is generated which changes the pitch angle from positive to negative on a right end of the stroke and from negative to positive on a left end. Other aerodynamic or hydrodynamic methods to generate the aerodynamic or hydrodynamic forces and moments necessary to initiate the stroke reversals include, for example, control surfaces mounted on the wing.

[0015] Platzer, M. F., Ashraf, M. A., Young, J., &Lai, J. C. S. (2009, January). Development of a new oscillating-wing wind and hydropower generator. In 47 th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition (AIAA 2009-1211). Orlando, FL: American Institute of Aeronautics and Astronautics disclose a mechanism consisting of a hydrofoil having tips, that is able to move in a coupled pitch and plunge motion. Attached to each tip of the hydrofoil are aluminum blocks that contain mounting points for bearings that fit into slots in rails. These bearings can be adjusted to allow the pivot position to vary. The aluminum blocks provide a place to connect the hydrofoil to the rest of the mechanism. Thin push rods attach the hydrofoil via ball-joints to a swing arm of the mechanism. An aluminum plate mounted on the opposite side of the swing arm pivot counterbalances the swing arm and hydrofoil. Through a push rod and an intermediate swing arm, the rocking swing arm rotates a phasing gear which through another gear rotates the main shaft. The main shaft has a flywheel to smooth out the cycle at the extremes of the plunge amplitude. On one end of the shaft is a pitch arm which through a linkage drives a bell-crank back and forth. The bell-crank is located on the end of the swing arm above the hydrofoil. A thin push rod connects the trailing edge of the hydrofoil with the bell-crank. When the hydrofoil plunges up and down, the swing arm moves with it, turning the main shaft and the pitch arm. The pitch arm moves the bell-crank back and forth, which in turn pitches the hydrofoil. A flywheel is used to conserve energy so that the hydrofoil does not stop at the deadpoints. The hydrofoil is capable of pitch amplitudes of 65° and plunge amplitudes of 125 mm. During an upstroke, the hydrofoil is at a positive angle of attack, thus lift and motion are in the same direction. During a downstroke, the hydrofoil is at a negative angle of attack and therefore the (negative) lift is again in the direction of the motion. Thus work is done by the water flow on the hydrofoil over most of the cycle. Tests of this first hydropower generator in a water tunnel at flow speeds up to 3 ft / sec (0.9144 m / s) showed satisfactory performance.

[0016] Lessons learned from these tests were incorporated in the design of a second power generator that employs two hydrofoils in a tandem arrangement. The hydrofoils oscillate with a ninety degree phase difference, such that a null spot of one coincides with a power stroke of the other, thereby making the generator self-starting regardless of initial hydrofoil positions. The rails for the plunge motion were replaced by pitching motion about pivot points, thus eliminating any friction problems. Also, the flywheel used on the first generator could be dispensed with because of the mutual reinforcement between the two hydrofoils. Otherwise, control of the pitch angle of the hydrofoils was accomplished in a manner similar to the method used on the first generator. Water tunnel tests of this second generator showed that the tandem arrangement indeed leads to a much smoother running machine.

[0017] Kinsey, T., Dumas, G., Lalande, G., Ruel, J., Mehut, A., Viarouge, P., Lemay, J., &Jean, Y. (2011). Prototype testing of a hydrokinetic turbine based on oscillating hydrofoils. Renewable Energy, 36(6). 1710-1718 describe a hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational). Testing of an experimental 2 kW prototype including two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration is reported. Pitching motion of each hydrofoil is coupled to their cyclic heaving motion through four-link mechanisms, which effectively yield a one-degree-of-freedom system driving a rotating shaft. The rotating shaft is connected to a speed-controlled electric generator. The turbine was mounted on a custom-made pontoon boat and dragged on a lake. The heaving and pitching amplitudes of the hydrofoils were kept constant for all runs at values of 1 chord and 75° respectively. Measured power extraction efficiency reached 40% once overall losses in the mechanical system were taken into account.

[0018] WO 2012 / 040834 Al discloses a system and method for converting kinetic energy from a fluid flow into mechanical energy, the method comprising the steps of: a) providing a turbine including first and second hydrofoils, each of the hydrofoils being able to move linearly in a heaving motion, and being able to oscillate about a spanwise axis in a pitching motion, said heaving and pitching motions being quasi-sinusoidal, b) coupling the heaving motions of the first and second hydrofoils to the pitching motions of the second and first hydrofoils respectively, with the pitch-heave motion phase being substantially equal to the inter-hydrofoil phase, the heaving motion of one of the hydrofoils thereby driving the pitching motion of the other hydrofoil; and c) transforming the heaving motions of the hydrofoils into a rotational movement of a rotatable shaft, with linear-to-rotary transmission means.

[0019] US 9562434 B2 discloses an oscillating foil turbine that has a foil having a first fluid dynamic surface for producing lift in a fluid flow, a support for the foil, and a second fluid dynamic surface, wherein the support allows for cyclic motion of the first and second surfaces with respect to each other. A driven member is provided to tap energy from a flow throughout each cycle. Throughout at least part of the cyclic translation, the fluid dynamic surfaces are oriented sufficiently parallel, and separated by a distance that is sufficiently small, to achieve a substantial wing-in-ground effect.

[0020] The first commercial prototype of an oscillating hydrofoil hydrokinetic turbine was the 150 kW “Stingray” prototype from Engineering Business Ltd, which was tested in 2003 in Scotland. The “Stingray” design employed a single hydrofoil comprising a wing-like hydroplane, which was attached to a supporting frame by a moveable arm. The supporting frame was seabed mounted, which was expensive and caused substantial damage to the marine habitat. As tidal currents passed over the hydroplane, lift and drag forces caused the hydroplane to lift. Hydraulically powered cylinders were used to alter the hydroplane angle, allowing pitch and plunge motion to be controlled directly, such that the apparent angle of attack, relative to the oncoming current, was maintained at its optimum angle. As the current lifted the hydroplane, the arm was caused to lift, actuating hydraulic cylinders at the arm / frame junction. High-pressure oil developed by the cylinders turned a hydraulic motor which, in turn, drove an electric generator. When the hydroplane, and arm, reached their upper limit, the hydroplane angle was reversed such that the arm was driven down, and the cycle was repeated. In the tests, the “Stingray” system reached a maximal production of 85 kW for a modest power extraction efficiency of 11.5%, but the hydraulic mechanisms required a complex control system which, in practice, was found to require substantial maintenance, leading to significant down time in operations; the hydraulically powered cylinders for altering the hydroplane angle consumed significant amounts of energy; and the system was tagged as non-economically viable following poor results.

[0021] In each of the above-mentioned mechanisms, the geometric angle of the wing, sail, blade, hydrofoil or the like is controlled and defined relative to the mechanism, and the motion of the same is defined by the kinematics of the drive mechanism. The mechanisms either impose a mechanically prescribed phase relationship between pitch and plunge (Kinsey et al.), impose a fixed phase relationship between pitch and plunge (McKinney &DeLaurier), trigger a pitch change at fixed plunge end-stops, or require a complex (and therefore fragile) mechanism to determine the phase relationship between pitch and plunge (Stingray). This means that these systems require input forces to drive the mechanism. In some cases, such forces are provided by static elements. However, none of the designs includes any facility to accommodate changes in flow direction, such as might be caused by turbulence or wave-induced motion.

[0022] The flow environment in the natural world is complex, involving long-term changes in flow velocity owing to seasonal or weather-related changes, short-term stochastic variations owing to waves, and periodic repeatable changes in flow velocity between slack-water at the peaks of high and low tide, where the stream velocity may be zero, and flood tide where the tidal stream velocity reaches a peak. There may also be substantial velocity gradients with depth or distance from a shoreline because friction slows the flow adjacent to a solid surface. The above-referenced oscillatory hydropower systems lack any mechanical system that can cope with such temporal or spatial variations in flow velocity and conditions.

[0023] The conventional approach for hydropower systems is to design them with fins or blades of the right size for a rated flow rate, which is often the maximum flow rate they will be used in. There is then a kick-in velocity that must be attained before the hydropower system can start to generate power. In tidal hydropower systems, this typically leads to a 0.4 capacity factor, meaning that the system only generates power for 40% of the tidal flow cycle since velocities are below the kick-in velocity around slack water at high and at low tide. For river and yacht hydropower systems, this typically means that the fins or turbine blades of the system are sized to generate power over a limited range of flow velocities, targeting the expected mean flow rate or yacht speed, and the fins or turbine blades are then typically too small to generate significant power in low speed flows, and are too large when flow speed is very high, generating excessive load: as noted above, the power Pfiow in a flow varies with the flow velocity V cubed. Hydropower systems designed for low-speed flows are thus quickly overloaded at higher flow rates. For yachts, at least, large hydropower systems that can generate power at low speeds are therefore typically removed from the water when the flow is too fast, to avoid generating excess power and drag. None of the known systems is capable of responding to wind-loads or other loads.

[0024] More recent designs have focussed on exploitation of a phenomenon known as aeroelastic flutter, i.e. passive oscillations of foils mounted on suitably tuned damped-elastic mounts. Such systems use high-lift, unsteady fluid dynamics to expand the operating flow speed range, but they require flapping foils.

[0025] Duarte, L., Dellinger, N., Dellinger, G., Ghenaim, A., &Terfous, A. (2019). Experimental investigation of the dynamic behaviour of a fully passive flapping foil hydrokinetic turbine. Journal of Fluids and Structures, 88. 1-12 for example disclose a flapping foil prototype comprising a foil mounted on a pitching shaft by means of a sliding box. The pitching shaft is supported by tapered roller bearings inside a shaft casing, which is then mounted on a heaving rail through a linear bearing carriage. Extension springs are used for heaving stiffness. Heaving translational motion is converted through a rigid linkage into alternative rotational motion of a transmission belt system, which is linked to an electric servo motor. The pitching motion is linked to an electric servo motor through a transmission belt system. The study focuses on investigating the dynamic behaviours of the foil: neither heaving nor pitching motions are forced or constrained.

[0026] In passive hydropower systems, an element of damping is provided by a generator system. Passive hydropower systems have the advantage that they require no control systems once a resonant condition is achieved. Damping could be used to control resonance, but control in relation to varying fluid flow velocities is inevitably an issue. Inherent coupling between vortex shedding and structural aeroelasticity allows such systems to operate in a high-lift, unsteady fluid dynamics range, but it is unclear whether the forces they generate can be moderated to allow operation across a range of velocities, while retaining resonance.

[0027] The present disclosure seeks to alleviate one or more of the above-mentioned problems.

[0028] Inter alia, the present disclosure seeks to provide an improved fluid flow-driven generator such for example as a hydropower generator that can automatically respond to changes in prevailing conditions without the need for a complex, active control system.

[0029] Another object of the present disclosure is to provide an improved fluid flow-driven generator such for example as a hydropower generator that can operate in a wide range of flow velocities. In particular, the present disclosure seeks to provide a fluid flow-driven generator that can operate with a relatively high power coefficient at low flow velocities and at a lower power coefficient at higher flow velocities, thereby to moderate the drag on the fluid flow-driven generator.

[0030] Another object of the present disclosure is to provide a fluid flow-driven generator such for example as a hydropower generator that has an improved capacity factor as compared to prior fluid flow-driven generators.

[0031] Alternatively or additionally, the present disclosure seeks to provide a floating fluid flow-driven generator such for example as a floating hydropower generator that does not need to be permanently fixed to the ground underwater, thus obviating the need for expensive and environmentally undesirable foundations.

[0032] Yet another object of the present disclosure is to provide an improved hydropower generator of the kind that comprises a fluid-driven flapping mechanism that operates substantially or entirely passively without a consuming large amounts of energy.

[0033] It will be understood that not all implementations of the present disclosure will meet all of the above-mentioned objects or aims: different implementations of the disclosure will achieve one or more of them, as is clear from the following. Summary of the Disclosure

[0034] In accordance with a first aspect of the present disclosure there is therefore provided a fluid flow-driven generator for generating energy from a flow of liquid or gas, which fluid flow-driven generator comprises a fluid-driven device comprising a mounting for fixing the fluid-driven device to a supporting structure that is positioned in or adjacent a fluid flow and at least one movable fluid foil having an adjustable angle of attack that is configured to be driven by the fluid flow; an electric generator; a power take-off mechanism being operably connected between the fluid-driven device and the electric generator, such that motion of the fluid-driven device drives the electric generator; and a governor being configured and arranged for controlling the angle of attack of the fluid foil. The governor comprises a governor mechanism for adjusting the angle of attack of the fluid foil, a sensor for sensing a magnitude of a dynamic environmental or structural factor that is indicative of a dynamic load on the fluid-driven device, the supporting structure, or a positioning system for the supporting structure when the fluid-driven device is in use, and a governor actuator for operating the governor mechanism to adjust the angle of attack of the fluid foil according to the sensed magnitude of the dynamic environmental or structural factor, thereby to moderate the drag on the fluid foil in the fluid flow.

[0035] By “fluid foil” herein is meant a structure configured to generate lift in a flowing gas or liquid. A fluid foil for use in the air includes is commonly called an aerofoil or airfoil and includes structures such as wings and turbine blades. A fluid foil for use in water is commonly called a hydrofoil, which typically has a wing-like shape. As is well known by those skilled in the art, a fluid foil - such as an aerofoil or hydrofoil - functions by interacting with the surrounding fluid to produce lift owing to differences in pressure across its surfaces.

[0036] Typically, the governor may operate to reduce the angle of attack of the fluid foil at low flow velocities and to increase it in high flow velocities. It will be understood that by adjusting the angle of attack of the fluid foil according to the sensed magnitude of the dynamic environmental or structural factor, the fluid flow-driven generator can be operated in a wide range of flow velocities. At low flow velocities, the coefficient of power (Cp) may be optimised to the Betz limit, or as close to that value as is possible, given the fluid dynamic efficiency (lift to drag ratio) of the fluid flow-driven generator. Meanwhile, at higher flow speeds, the angle of attack of the fluid foil may be reduced, thereby reducing the coefficient of power, but allowing the fluid flow-driven generator still to be operated without unduly straining the fluid-driven device, the supporting structure, or a positioning system for the supporting structure. In very high flow velocities, the angle of attack may be reduced to zero to minimise the drag on the fluid foil.

[0037] The fluid flow-driven generator of the present disclosure may therefore have an advantageous capacity factor as compared with prior fluid flow-driven generators, because it may be operated at high flow velocities without straining or risking damage to the fluid-driven device, the supporting structure, or a positioning system for the supporting structure by reducing the angle of attack of the fluid foil as aforesaid, and it may also be operated efficiently at lower flow velocities with a high angle of attack of the fluid foil, because there is no need to fix the angle of attack for a high rated flow rate (e.g. the maximum fluid flow rate in which the fluid flow-driven generator will be used) to avoid overloading the fluid-driven device, the supporting structure, or a positioning system for the supporting structure in stronger flows.

[0038] In some implementations, the angle of attack of the fluid foil may be reduced progressively as the dynamic environmental or structural factor sensed by the sensor increases, thereby to optimise the ratio of the coefficient of power (Cp) to the drag on the fluid flow-driven generator.

[0039] In some implementations, the governor actuator may be configured to reduce progressively the angle of attack of the fluid foil according to the sensed magnitude of the dynamic environmental or structural factor above a first lower threshold, thereby to moderate the drag on the fluid flow-driven generator when the sensed magnitude of the dynamic environmental or structural factor exceeds the first lower threshold. The angle of attack may be maintained in a maximal setting when the sensed magnitude is below the first lower threshold.

[0040] In some implementations, the governor actuator may be configured to make the angle of attack of the fluid foil minimal (for example substantially zero) when the sensed magnitude exceeds a second upper threshold. Thus, when the magnitude of the dynamic environmental or structural factor is such as to make continued operation of the fluid flow-driven generator unpractical, for example owing to the magnitude of the fluid flow or other structural or environmental loads on the fluid flow-driven generator, its supporting structure or a positioning system for the fluid flow-driven generator, the angle of attack of the fluid foil may be made as small as possible to reduce the drag thereon as much as possible until the conditions improve.

[0041] The fluid may typically be water, the fluid flow-driven generator being a hydropower generator such for example as a tidal turbine or tidal stream generator, but in some implementations, the fluid may be air, the fluid flow-driven generator being a wind turbine such, for example, as a horizontal-axis wind turbine (HAWT), a vertical-axis wind turbine (VAWT) or a floating offshore wind turbine.

[0042] In some implementations of the present disclosure, the fluid foil may comprise a turbine blade of the kind that is adapted for use in a wind turbine or tidal turbine. The governor mechanism may be configured for reducing the angle of attack of a turbine blade for example by rotating the blade along its long axis around a hinge. Those skilled in the art will be aware of the opposite use case (i.e., where power is put in to generate thrust) of propellers where the blades of variable pitch propellers can be adjusted by rotation about the blade’s long axis to change the overall angle of attack of the blade and force produced at any given motor RPM. It will be understood that in the case of a wind or tidal turbine, the fluid-driven device may typically comprise multiple turbine blades that may be operated in concert.

[0043] In some preferred implementations of the present disclosure, the fluid flow-driven generator may comprise a fluid-driven flapping mechanism that is configured such that in use the movable fluid foil is driven by the fluid flow to undergo a reciprocal pitch-plunge motion. By “pitch-plunge” motion herein is meant a complex motion that combines rotation (pitching) and translation (plunging) of the fluid foil relative to the direction of the fluid flow. In such an arrangement, the governor may be configured to control the angle of attack of the fluid foil according to the sensed magnitude of the dynamic environmental or structural factor, thereby to modulate the wavelength of the pitch-plunge motion motion of the flapping mechanism in the fluid flow.

[0044] The fluid-driven flapping mechanism preferably comprises a mechanical linkage that is adapted to be fixed to the supporting structure at one end and carries the fluid foil at another end. The fluid foil can pitch relative to the mechanical linkage, and the mechanical linkage constrains the fluid foil to reciprocal plunging motion across the direction of the fluid flow. In some implementations, mechanical linkage may comprise a support shaft, guide rails or the like, which may constrain the fluid foil to a substantially linear path; or a swing arm, which may constain the fluid foil to an arcuate path. The fluid foil may be configured and arranged to be driven the fluid flow to cause the mechanical linkage to undergo reciprocal motion relative to the supporting structure, which is used to drive the electric generator.

[0045] In such an arrangement, in accordance with the present disclosure, the drag on the fluid foil in the fluid flow may be moderated by adjusting the amplitude of the reciprocal pitch-plunge motion as well as, or instead of, adjusting the angle of attack of the fluid foil.

[0046] Thus, in accordance with a second aspect of the present disclosure there is provided a fluid flow-driven generator for generating energy from a flow of liquid or gas, which fluid flow-driven generator comprises a fluid-driven device comprising a mounting for fixing the fluid-driven device to a supporting structure that is positioned in or adjacent a fluid flow, and a fluid flow flapping mechanism that comprises at least one movable fluid foil and is configured to be driven by the fluid flow to undergo a reciprocal pitch-plunge motion; an electric generator; a power take-off mechanism being operably connected between the fluid-driven flapping mechanism and the electric generator such that motion of the fluid-driven flapping mechanism drives the electric generator; and a governor for controlling the amplitude of the reciprocal pitch-plunge motion of the fluid-driven flapping mechanism; the governor comprising a governor mechanism for adjusting the amplitude of the reciprocal pitch-plunge motion, a sensor for sensing a magnitude of a dynamic environmental or structural factor that is indicative of a dynamic load on the fluid-driven device, the supporting structure, or a positioning system such, for example, as a mooring or anchor arrangement for the supporting structure, when the fluid-driven device is in use, and a governor actuator for operating the governor mechanism to reduce the amplitude of the reciprocal pitch plunge motion according to the sensed magnitude of the dynamic environmental or structural factor, thereby to moderate the drag on the fluid foil in the fluid flow.

[0047] Typically, the governor may operate to reduce the amplitude of the reciprocal pitch-plunge motion of the fluid flow flapping mechanism at low flow velocities and to increase it in high flow velocities. In an analogous manner to the first aspect of the present disclosure, the governor actuator of the fluid flow-driven generator of the second aspect of the present disclosure may be configured to reduce progressively the amplitude of the reciprocal pitch-plunge motion of the fluid-driven flapping mechanism according to the sensed magnitude of the dynamic environmental or structural factor above a first lower threshold, thereby to moderate the drag on the fluid flow-driven generator when the sensed magnitude of the dynamic environmental or structural factor exceeds the first lower threshold. The amplitude of the reciprocal pitch-plunge motion may be maintained at a maximal setting when the sensed magnitude is below the first lower threshold. In very high flow velocities, the amplitude of the reciprocal pitch-plunge motion may be reduced to zero to minimise the drag on the fluid foil.

[0048] In some implementations, the governor actuator may therefore be configured to make the amplitude of the reciprocal pitch-plunge motion of the fluid-driven flapping mechanism minimal (for example substantially zero) when the sensed magnitude exceeds a second upper threshold. Thus, when the magnitude of the dynamic environmental or structural factor is such as to make continued operation of the fluid flow-driven generator unpractical, the amplitude of the reciprocal pitch-plunge motion may be made as small as possible to reduce the drag thereon as much as possible until the conditions improve.

[0049] A particular advantage of the present disclosure is that when the sensor is adapted and arranged to sense the magnitude of a structural factor indicative of a dynamic load on the supporting structure or a positioning system for the supporting structure such, for example, as ground tackle or a mooring, rather than the fluid flow velocity, adjustment of the angle of attack of the fluid foil and / or amplitude of the reciprocal pitch-plunge motion (in the case of a fluid flow-driven generator comprising a fluid-driven flapping mechanism) may be based on changing structural loads on the supporting structure or positioning system that arise, not just from increased flow velocities, but also from other dynamic forces acting thereon such for example as storm loads from windage, waves and spray on exposed parts of the supporting structure.

[0050] In some implementations, the fluid-flow driven generator may be mounted to a supporting structure that is fixed to or otherwise supported immovably by the ground. Where the fluid-flow driven generator is a hydropower generator or an offshore wind turbine, the supporting structure may be disposed wholly or partially in or adjacent a body of water, such for example as a bridge, pier, dock, jetty, tower, pile, plinth, barrage, or other permanent or semi-permanent structure.

[0051] In such implementations, the sensor may be adapted and arranged to sense the velocity of the fluid flow. However, the sensor is preferably adapted and arranged to sense the strain in the fluid-driven device, or the supporting structure. For example, the sensor may be adapted and arranged to sense the strain in the mounting between the fluid-driven device and the supporting structure.

[0052] In preferred implementations, the fluid-driven generator may be mounted to a floating supporting structure such, for example, as vessel that is adapted to move through the water or to a platform that is fixed or maintained in position by a suitable positioning system such for example as a mooring, anchor arrangement or other form of ground tackle. In this way, substantial damage to the natural environment at the sea-, river or estury bed may be avoided, not to mention the expense of foundations fixed into the ground under water. In some implementations, the vessel may comprise a yacht, boat, ship, barge, or submersible. The platform may comprise such a vessel, or a barge, pontoon, or other floating platform that is anchored or moored to the ground beneath or adjacent the body of water.

[0053] In such implementations, the sensor may be adapted and arranged to sense the velocity of the fluid flow, or the strain in the fluid-driven device or the supporting structure. However, the sensor is preferably adapted and arranged to sense the strain in the positioning system for the supporting structure.

[0054] Generally, where the fluid flow-driven generator is a hydropower generator, the supporting structure may be capable of motion through the water or positioned in or next to a moving body of water, such that there is a flow of water relative to the supporting structure for driving the fluid-driven device.

[0055] Those skilled in the art will be aware of numerous different ways to sense the magnitude of the dynamic environmental or structural factor that is indicative of the dynamic load on the fluid-driven device. In some implementations, it is unnecessary for the sensor actually to measure the magnitude of the dynamic environmental or structural factor, but only to control operation of the governor mechanism to reduce the angle of attack of the fluid foil, or amplitude of the reciprocal pitch-plunge motion (in the case of a fluid flow-driven generator comprising a fluid-driven flapping mechanism) in accordance with the magnitude of the dynamic environmental or structural factor. Thus, in some implementations, the sensor may comprise a mechanical sensor. The sensor may be integrated with the governor actuator. For example, where the supporting structure comprises a floating vessel or platform, the sensor may comprise a snubber that is incorporated into the positioning system, e.g. an anchor cable or mooring line, and arranged to transmit variable loads in the positioning system to the governor mechanism for operating the same. The snubber may for example be arranged to pull against a suitable tension member such for example as an extension spring, the extension of the spring being used to control the governor mechanism.

[0056] In some implementations, the sensor may comprise an electro-mechanical sensor that is adapted to measure the dynamic condition and to generate an output signal that represents the magnitude of the dynamic environmental or structural factor. The governor actuator may comprise a controller that is configured to receive the output signal and to operate the governor mechanism according to the magnitude of the dynamic environmental or structural factor. For example, in some implementations, the controller may be configured to operate a servo drive that is arranged to operate the governor mechanism.

[0057] As mentioned above, in some preferred implementations of the present disclosure, the fluid flow-driven generator may comprise a fluid-driven flapping mechanism comprising a mechanical linkage and a fluid foil that undergoes reciprocal pitch-plunge motion in a fluid flow. At the end of each stroke, the pitch of the fluid foil must be inverted to reverse the direction of lift generated by the fluid foil. In some implementations of the present disclosure, this periodic inversion of the fluid foil may be powered. However, in some preferred implementations of the present disclosure, operation of the the fluid-driven flapping mechanism may be substantially passive, thus reducing the power consumption of the fluid flow-driven generator.

[0058] In accordance with a third aspect of the present disclosure, there is provided a fluid-driven flapping mechanism for a hydropower generator comprising a mounting for fixing the flapping mechanism to a supporting structure that is positioned in or adjacent a body of flowing water, a fin having a hydrofoil and at least one elevon, a mechanical linkage that is attached at one end to the mounting and at another end to the fin and is configured and arranged to hold the fin in the water when attached to the supporting structure and to permit and constrain movement of the fin relative to the mounting to a reciprocal plunging motion across the direction of the flow, and an elevon angle control mechanism. The fin is pivoted to the mechanical linkage at a fulcrum, such that it can pitch freely about the fulcrum in two opposite directions relative to the direction of the flow. The hydrofoil has two opposite surfaces that are each shaped to generate lift in the flow when the fin is pitched in either direction. The at least one elevon is pivoted to the hydrofoil for movement between two angled states, in each of which the elevon is angled towards a respective opposite surface of the hydrofoil. The elevon angle control mechanism is configured to retain the at least one elevon in different respective ones of the angular states during opposite strokes of the flapping mechanism, and to pivot the elevon to the other angled state when the mechanical linkage is moved by the fin in either direction relative to the mounting past an actuation point.

[0059] When the at least one elevon is held in one of its angled states by the elevon angle control mechanism in a flow of water, the elevon generates a turning moment on the fin, thereby causing the fin to pitch in one direction about the fulcrum until it reaches an equilibrium position in which the resulting lift produced by the hydrofoil in one direction generates an equal and opposite turning moment about the fulcrum. This lift causes the fin to plunge on the mechanical linkage in the one direction relative to the mounting, thereby driving motion of the mechanical linkage in the one direction relative to the mounting until it moves beyond the actuation point, at which the elevon angle control mechanism pivots the elevon to its other angled state, thereby inverting the pitch of the fin in the flow. The resulting lift in an opposite direction from the one direction causes the fin to plunge in the opposite direction relative to the mounting; whereby the mechanical linkage is caused to undergo reciprocal motion relative to the mounting.

[0060] In some implementations, as mentioned above, the fin may be constrained by the mechanical linkage for substantially linear plunging motion across the direction of the fluid flow. In principle, the mechanical linkage may comprise any suitable arrangement of supports, support shafts, guide rails or the like that affords such substantially linear plunging motion of the fin. However, in preferred implementations of the present disclosure, the mechanical linkage may comprise a swing arm and a hinge assembly for hinging the swing arm to the mounting at a proximal of the mechanical linkage, with the fin being fulcummed to a distal end of the swing arm. The plunging motion of the fin may therefore be constrained by the swing arm to an arcuate path.

[0061] By “elevon” herein is meant a flap on a leading or trailing edge of the hydrofoil that can be used to generate a pitching moment on the hydrofoil about the fulcrum. Preferably the at least one elevon is pivoted to the trailing edge of the hydrofoil.

[0062] By an “angled state” of the at least one elevon herein is meant that the elevon is pivoted with respect to the hydrofoil such that a centreline of the elevon subtends an acute angle with a centreline of the hydrofoil. The term is not meant to imply any specific angle, only that the elevon is angled with respect to the hydrofoil. In one angled state, the elevon may be pivoted in one direction towards one of the opposite surfaces of the hydroil, while in the other angled state, the elevon may be pivoted in an opposite direction towards the other opposite surface of the hydroil. In some implementations, the fluid-driven flapping mechanism may be configured to be fixed to the supporting structure such that the plunging motion of the fin is in a substantially vertical plane. In such case, the hydrofoil may be arranged to pivot on the fulcrum about a substantially horizontal axis, and the at least one elevon may likewise be pivoted to the hydrofoil on a substantially horizontal axis such that the elevon may be angled “up” or “down”, as would be well understood by those skilled in the art. In other implementations, the flapping mechanism may be configured to be fixed to the supporting structure such that the plunging motion of the fin is in a non-vertical plane, e.g. a horizontal plane, in which case the at least one elevon may be angled to one side or another of the hydrofoil.

[0063] By the “neutral state” herein is meant that the at least one elevon is aligned with the hydrofoil such that the centreline of the elevon is aligned with a centreline of the hydrofoil. When the elevon is in the neutral state, it generates no pitching moment on the fin.

[0064] In principle, any suitable mechanism may be used in accordance with the present disclosure to drive the at least one elevon from one angled state to the other when the mechanical linkage moves in either direction beyond the actuation point. Preferably, however, the mechanical linkage is drivably connected to the elevon angle control mechanism, such that the plunging motion of the mechanical linkage drives the elevon angle control mechanism automatically to invert the angle of the elevon as the mechanical linkage moves past the actuation point during each stroke of the flapping mechanism. In this way, the operation of the flapping mechanism may be entirely or almost entirely passive.

[0065] In some implementations, the elevon angle control mechanism may be configured and arranged to pull the at least one elevon in the one direction relative to the hydrofoil according to the position of the mechanical linkage against the action of an elevon biasing device, such for example as an elevon spring, that is arranged to bias the elevon in the opposite direction.

[0066] In some implementations the actuation point may be any point along the path of movement of the mechanical linkage where the elevon angle control mechanism is triggered to invert the angle of the at least one elevon from one angled state to the other. It will be understood that the position of the mechanical linkage at the actuation point may be different for opposite strokes of the mechanical linkage, e.g. upstrokes and downstrokes, since the actuation point effectively defines the end point of each stroke. The degree of motion of the mechanical linkage during each stroke before it reaches the actuation point determines the amplitude of its plunging motion, as disclosed below in more detail.

[0067] Preferably, the location of the actuation point is defined by the elevon angle control mechanism, which is arranged to be operated by the mechanical linkage. Thus, the existence and position of the actuation point may be an intrinsic design feature of the elevon angle control mechanism. Preferably, the mechanical linkage is arranged to drive the elevon angle control mechanism through a bidirectional lost motion mechanism such that the position of the mechanical linkage at the actuation point is different for opposite strokes of the mechanical linkage as mentioned above.

[0068] Preferably the elevon angle control mechanism incorporates a bistable toggle mechanism that is connected between the at least one elevon and the mounting and is arranged to be driven by the mechanical linkage. Preferably the actuation point is defined by the bistable toggle mechanism.

[0069] Preferably, the bistable toggle mechanism is operably connected to the at least one elevon and switches automatically between first and second stable configurations when it is moved past the actuation point, thereby to drive the elevon from one of its angled states to the other angled state. The bistable toggle mechanism is stable in its first and second stable configurations and unstable at an intermediate point that defines the actuation point.

[0070] Suitably, the bistable toggle mechanism may comprise an input toggle member that is fixed relative to the mounting, an output toggle member, and a toggle biasing device, such for example as a toggle tension spring, that acts between the input and output toggle members, the output toggle member being movable with respect to the input toggle member through an overcentre position corresponding to the actuation point, such that the toggle biasing device biases the output toggle member bistably into first and second positions corresponding respectively to the first and second stable configurations of the bistable toggle mechanism. The output toggle member may be operably connected to the at least one elevon, for example via a suitable control cable or other mechanical transmission. The output toggle member may be arranged to pull the elevon in the one direction relative to the hydrofoil when it moves from the second position to the first position, against the action of the elevon biasing device, which is arranged to bias the elevon in the opposite direction.

[0071] Thus, when the bistable toggle mechanism is disposed in its first stable configuration, the at least one elevon may be retained in the one angled state by the toggle biasing device, against the action of the elevon biasing device. When the bistable toggle mechanism is disposed in its second stable configuration, the elevon may be retained in the other angled state by the toggle biasing device and the elevon biasing device.

[0072] The output toggle member is preferably arranged to be driven by the mechanical linkage. Suitably, at least the output toggle member may comprise an output toggle arm that is arranged for rotation between the first and second positions. The input toggle member may be fixed relative to the mounting. Such an arrangement may be especially preferred where the mechanical linkage comprises a swing arm, so that pivoting of the swing arm about the hinge assembly to the mounting at the proximal end may conveniently be arranged to cause rotation of the output toggle arm relative to the input toggle member.

[0073] Preferably, the mechanical linkage is arranged to drive the output toggle member through the bidirectional lost motion mechanism, the arrangement being such that when the output toggle member passes through the actuation point in either direction at the end of one stroke, its movement to the first or second position is “lost” relative to the mechanical linkage. This necessitates opposite movement of the mechanical linkage on the next stroke to return the output toggle member to the actuation point to end the next stroke. It will be appreciated therefore that the amount of lost motion that is permitted by the lost motion mechanism, and thus the corresponding amount of movement by the mechanical linkage that is required to drive the output toggle member back through an equal and opposite distance to the actuation point, determines the amplitude of each stroke.

[0074] In some implementations, the bistable toggle mechanism may, for example, comprise a flange with a slot that is connected to the mechanical linkage for movement therewith. In preferred implementations in which the mechanical linkage comprises a swing arm, the flange may be arranged for rotation with the swing arm, and the slot may be arcuate. The output toggle member may comprise an output toggle limit pin that is received in the slot. In each of the first and second stable configurations of the bistable toggle mechanism, the limit pin may be held against the flange at a respective end of the slot by the toggle biasing device. Motion of the mechanical linkage causes corresponding movement of the flange to drive the output toggle member from one of its first and second positions until it reaches the actuation point with respect to the input toggle member, at which point the output toggle member is disposed over-centre with respect to the input toggle member, allowing the output toggle limit pin to be pulled to the other end of the slot by the toggle biasing device, the output toggle member thus being caused to move to the other of the second and first positions, thereby inverting the angled state of the at least one elevon.

[0075] Preferably, in accordance with with the present disclosure, the fluid-driven flapping mechanism may further comprise a governor mechanism for controlling the degree of movement of the mechanical linkage relative to the mounting in each stroke before the actuation point is reached, thereby to control the amplitude of each stroke. In this way, the governor mechanism may be configured to control the amplitude of the plunging motion of the flapping mechanism.

[0076] Suitably, the governor mechanism may be configured and arranged for altering the positions, and thus the distance or angular distance between, the first and second positions of the output toggle member to either side of the actuation point. Preferably, the governor mechanism may be configured and arranged for adjusting the degree of lost motion of the output toggle member that is permitted by the lost motion mechanism. Preferably, the governor mechanism may be configured and arranged for adjusting the degree of lost motion of the output toggle member that is permitted by the lost motion mechanism after the output toggle member has past the actuation point in each stroke and moves independently of the mechanical linkage into a respective one of its first and second positions. In this way, the governor mechanism may be operated to moderate the drag on the fluid-driven flapping mechanism in the flow of water by progressively reducing the degree of movement of the mechanical linkage relative to the mounting that is afforded by the lost motion mechanism on each stroke before it reaches the actuation point defined by the elevon angle control mechanism.

[0077] Preferably, the elevon angle control mechanism may be configured to set the maximum angle of the at least one elevon relative to the hydrofoil in each angled state. In some implementations, the output toggle member of the bistable toggle mechanism may be operably connected to the elevon in such a manner as to set the maximum angle of the elevon in each angled state. In some implementations, the output toggle member may be operably connected to the elevon such that movement of the output toggle member from its second position to its first position causes the at least one elevon to moved in the one direction against the action of the elevon biasing device. Movement of the output toggle member from its first position to its second position may allow the at least one elevon to be moved in the opposite direction by the elevon biasing device.

[0078] Preferably, the amount of lost motion of the output toggle member permitted by the lost motion mechanism after it has passed the actuation point in either direction may determine the positions of the first and second positions of the output toggle member and thus the maximum angle of the at least one elevon relative to the hydrofoil in each angled state. Accordingly, operation of the governor mechanism to adjust the degree of lost movement of the output toggle member after it passes the actuation point in either direction may limit the degree to which the elevon may be angled away from the neutral state in either angled state and, in consequence of that, the maximum pitch of the fin in the equilibrium position during each stroke. In this way, the governor mechanism may be used to control the maximum angle of attack of the fin during each stroke.

[0079] In some implementations, the governor mechanism may comprise a pair of opposing toggle limit jaws juxtaposed the slot, which may be operated to control the length of a portion of the slot along which the output toggle limit pin may move. In this way, the opposing toggle limit jaws may be used effectively to vary the length of the slot, and thus the spacing between the first and second positions of the output toggle member, for controlling the amplitude of each stroke of the reciprocal plunging motion of the mechanical linkage and the maximum angle of attack of the fin during each stroke.

[0080] In some embodiments, the elevon angle control mechanism may be configured to progressively alter the angle of the at least one elevon relative to the hydrofoil in each angled state, according to the position of the mechanical linkage. Preferably, the output toggle member of the bistable stable toggle mechanism may be operably connected to the elevon such that movement of the output toggle arm before it reaches the actuation point causes or allows a certain degree of corresponding movement of the elevon in either angled state before the output toggle member reaches the actuation point and the elevon is moved to its other angled state. Thus, progressive movement of the output toggle member from its second position to its first position, which may be driven by the mechanical linkage for example, may cause corresponding movement of the elevon in the one direction, while progressive movement of the output toggle member from the first position to the second position may allow corresponding motion of the elevon in the opposite direction under the influence of the elevon biasing device. During each stroke therefore, plunging motion of the mechanical linkage may thus drive the elevon angle control mechanism to decrease slightly the angle of the elevon relative to the hydrofoil in the respective angled state before the actuation point is reached and a change of angled states is triggered. In this way, the angle of the at least one elevon in each angled state may be progressively trimmed to maintain a stable angle of attack of the fin as the mechanical linkage is moved from the start of each stroke to the actuation point. This arrangement may be especially useful where the mechanical linkage comprises a swing arm with the fin pivoted on the other end thereof, such that the fin tends naturally to pitch as the swing arm rotates about the hinge assembly, so that the angle of the at least one elevon can be adjusted to counteract the changing angle of the swing arm.

[0081] In some implementations, the fluid-driven flapping mechanism may further comprise a sensor for sensing the magnitude of a dynamic environmental or structural factor that is indicative of the load on the flapping mechanism, or the strain in the supporting structure or a positioning system for the supporting structure, when the flapping mechanism is in use; and a governor actuator for operating the governor mechanism according to the sensed magnitude of the dynamic environmental or structural factor.

[0082] As disclosed above, the dynamic environmental or structural factor may be the velocity of the flow of water past the flapping mechanism, the strain in the flapping mechanism, the supporting structure or the mounting between the flapping mechanism and the supporting structure, or the load in an anchor cable or mooring line for the supporting structure, where the supporting structure is a floating vessel or platform.

[0083] Preferably, the governor actuator may be configured to operate the governor mechanism to control the amplitude of the plunging motion of the mechanical linkage according to the sensed magnitude of the dynamic environmental or structural factor.

[0084] Preferably, the governor actuator may be configured to operate the governor mechanism to control the maximum angle of attack of the fin according to the sensed magnitude of the dynamic environmental or structural factor.

[0085] More preferably, the governor actuator may be configured to reduce progressively the maximum angle of attack of the fin as the magnitude of a dynamic environmental or structural factor increases on the flapping mechanism, structural support or a positioning system for the structural support, in connection with the first aspect of the present disclosure.

[0086] More preferably, the governor actuator may be configured to reduce progressively the amplitude of the plunging motion of the mechanical linkage as the magnitude of a dynamic environmental or structural factor increases on the flapping mechanism, structural support or a positioning system for the structural support, in connection with the second aspect of the present disclosure.

[0087] According to a fourth aspect of the present disclosure there is provided a hydropower generator comprising the flapping mechanism in accordance with the third aspect of the disclosure, an electric generator and a power take-off mechanism that is operably connected between the mechanical linkage and the electric generator such that reciprocal motion of the mechanical linkage drives the electric generator.

[0088] In some implementations, the fluid-driven device of the fluid flow-driven generator according to the first aspect of the disclosure may incorporate the flapping mechanism of the third aspect.

[0089] In some implementations, the fluid flow-driven generator according to the second aspect of the disclosure may comprise the fluid-driven flapping flapping mechanism of the third aspect.

[0090] It will be appreciated that features described in relation to one aspect of the present disclosure may be incorporated into other aspects of the present disclosure. For example, the fluid flow-driven generators of the first and second aspects of the disclosure may incorporate any of the features described with reference to the fluid-driven flapping mechanism of the third aspect of the disclosure, and vice versa. Brief Description of the Drawings.

[0091] Following is a detailed description by way of example only with reference to the accompanying drawings of example implementations of the present disclosure:

[0092] In the drawings:

[0093] FIG. 1 is a system diagram of a yacht at anchor which is fitted with a hydropower generator in accordance with a first example implementation the present disclosure.

[0094] FIG. 2 is a side view of the hydropower generator of FIG. 1, showing a swing arm with a hinge assembly at one end for attachment to a supporting structure, such for example as the yacht of FIG. 1, and a fin at the other end for driving reciprocal plunging motion of the swing arm in a flow of water, and an electric generator, which is coupled to the swing arm through a power take-off mechanism comprising a connecting rod and a crank shaft;

[0095] FIG. 3 is a side elevation of the fin, swing arm and hinge assembly of the hydropower generator of FIG. 2.

[0096] FIG. 4 is a plan view of the fin, which comprises a bipartite hydrofoil having two foil portions, each with a respective elevon on a trailing edge thereof, the swing arm and the hinge assembly of FIG. 3.

[0097] FIG. 5 is an enlarged rear view of part of the the fin, swing arm and hinge assembly of FIGS. 3 and 4.

[0098] FIG. 6 is an enlarged sectional side view along the line VI-VI of FIG. 5, including a control cable for an elevon on one of the foil portions.

[0099] FIG. 7 is a perspective view from above, behind and to one side of the hinge assembly of FIGS. 2 and 3, including a bistable toggle mechanism for automatically reversing the direction of flapping of the fin at the end of each stroke, and a governor mechanism including a governor control ring, toggle limit jaws, a servo drive and drive pinion for controlling the pitch and plunge amplitudes of the fin in use.

[0100] FIG. 8 is a plan view of the hinge assembly of FIG. 7.

[0101] FIG. 9 is a cross-section through the hinge assembly of FIGS. 7 and 8 on the line IX-IX of FIG. 8, showing details of the bistable toggle mechanism and governor mechanism.

[0102] FIG. 10 is a perspective view of the bistable toggle mechanism shown in FIGS. 7-9, with the governor mechanism removed.

[0103] FIG. 11 is an exploded view of the part of the bistable toggle mechanism of FIG. 10.

[0104] FIG. 12 is an enlarged side view, from the front, of the governor mechanism shown in FIGS. 7-9.

[0105] FIG. 13 is cross-section through the governor mechanism shown in FIGS. 7-9 and 12, on the line XIII-XIII of FIG. 8.

[0106] FIGS. 14A-D are side views from the front of the bistable toggle mechanism of FIGS. 10 and 11, showing input and output toggle arms and an annular flange formed with an arcuate slot: FIGS. 14A-C show the bistable toggle mechanism with the output toggle arm in a first stable configuration relative to the arcuate slot in the annular flange, with the input toggle arm being rotated through successively greater angles relative to the output toggle arm as the swing arm pivots on the hinge assembly, until it is overcentre with respect to the output toggle arm at an actuation point; FIG. 14D shows the output toggle arm in a second stable configuration relative to the annular flange, having flipped from the first stable configuration.

[0107] FIGS. 15A-C are side views from the front of the governor mechanism shown in FIGS. 7-9 and 12-13, showing opposing toggle limit jaws in closed, intermediate and fully open positions respectively.

[0108] FIGS. 16A-H are side views of the fin, swing arm and hinge assembly of FIGS. 3 and 4 shown in successive positions during a stroke of the swing arm: in FIG. 16A the swing arm reaches the bottom of a downstroke with the hydrofoil of the fin oriented leading edge down, and the elevons angled down; in FIG. 16B, the elevons are angled up following operation of the bistable toggle mechanism at the bottom of the downstroke; in FIG. 16C the change in angled state of the elevons causes the hydrofoil to pitch to a leading edge up position; in FIG. 16D, the swing arm begins to move upwards, owing to upward lift generated by the hydrofoil; in FIG. 16E the swing arm reaches the top of an upstroke; and in FIG. 16F, the elevons are angled down again following operation of the bistable toggle mechanism at the top of the upstroke.

[0109] FIG. 17 is a cascading flowchart of factors that affect the angle of attack of the fin of the first example hydropower generator of FIGS. 1-16F during each stroke.

[0110] FIG. 18 is a flow diagram illustrating operation of the hydropower generator of FIGS. 1 -16F.

[0111] FIG. 19A is a diagram showing a waveform of relatively short wavelength that is swept by the fin of the hydropower generator of FIGS. 1 -16F around slack water when deployed in a tidal flow.

[0112] FIG. 19B is is a diagram showing a waveform of relatively long wavelength that is swept by the fin of the hydropower generator of FIGS. 1-16F around peak tidal flow velocity when deployed in a tidal flow.

[0113] FIG. 20 is a graph showing how the angle of attack of the fin (dashed line) and power produced (dotted line) by the hydropower generator of FIGS. 1 -16 when installed on a yacht as shown in FIG. 1, or to a similar floating support structure varies over the course of a complete tidal phase. Also shown are the variation in tidal flow velocity (chain-dotted line) and the anchor load (solid line).

[0114] FIG. 21 is an input-output model diagram showing functional relationships between components of the hydropower generator ofFIGS. 1-16F. Detailed Description

[0115] The present disclosure relates to a fluid driven generator of the kind comprising a fluid flow-driven mechanism that can be mounted to a supporting structure and is adapted to be driven by a relative flow of fluid past the supporting structure, over at least one reciprocally movable fluid foil. The present disclosure relates especially to a hydropower generator having at least one hydrofoil that is adapted to be driven by water passing over the hydrofoil.

[0116] In preferred implementations of the disclosure, the fluid flow-driven mechanism comprises a fluid-driven flapping mechanism of the kind comprising a mechanical linkage that is attached at one end to the supporting structure and has the hydrofoil fulcrummed to another end thereof. The mechanical linkage preferably comprises a swing arm. The supporting structure may be adapted to float on or in the water, or be fixedly secured to the ground or another immovable object in or adjacent a body of flowing water, with the hydrofoil submerged in the water. The flapping mechanism is configured and arranged to flap continuously by reciprocal motion of the hydrofoil when the water moves over it, either by movement of the supporting structure through the water, or of the water past the supporting structure, or both, thereby driving movement of the mechanical linkage which can be used to drive an electric generator.

[0117] In some implementations, the hydropower generator advantageously requires no additional power input to drive or control the reciprocal motion of the hydrofoil.

[0118] In some implementations, the power coefficient Cp of the hydropower generator may advantageously be adjusted automatically according to the relative speed of the flow of water past the supporting structure, the strain on the supporting structure, and / or the load on its fixings such for example as an anchor system or mooring, thereby to optimise the performance of the hydropower generator according to the prevailing conditions. Adjusting the power coefficient Cp of the hydropower generator in accordance with the strain on the supporting structure and / or the load on its fixings is an especially advantageous feature of some implementations of the present disclosure.

[0119] The hydropower generator may be mounted to a floating structure such, for example, as vessel that is adapted to move through the water, or to a moored platform. In some implementations, the vessel may comprise a yacht, boat, ship, barge, or submersible. The moored platform may comprise a vessel of the kind just mentioned, or a barge, pontoon, or other floating platform that is anchored to the ground beneath or adjacent the body of water. Alternatively, the hydropower generator may be mounted to a supporting structure that is fixed to or otherwise supported immovably by the ground beneath or adjacent the body of water such as a bridge, pier, dock, jetty, tower, pile, plinth, barrage, or other permanent or semi-permanent structure. More generally, the supporting structure should either be capable of motion through the water or positioned in or next to a moving body of water, such that there is a flow of water relative to the supporting structure for driving the flapping mechanism as disclosed herein.

[0120] FIG. 1 of the accompanying drawings shows a hydropower generator (1) in accordance with a first example implementation the present disclosure, which is fixedly secured to the transom of a yacht (2) as a supporting structure. As mentioned above, in different implementations, the hydropower generator of the present disclosure may be fixed to different kinds of supporting structure. It will be understood that the hydropower generator may be implemented in different sizes and / or capacities, depending on the size and strength of the supporting structure. In FIG. 1, the hydropower generator (1) of the present example implementation is suitably dimensioned to be supported by the yacht (2) when it is afloat, but a hydropower generator for mounting to a barge or other larger structure may be significantly bigger and heavier, and have a greater output capacity.

[0121] As described in more detail below, the hydropower generator (1) shown in FIG. 1 comprises a fluid-driven flapping mechanism comprising a swing arm (3), which is pivotably attached to the yacht (2) at a proximal end of the swing arm (3) and extends rearwardly relative to the yacht (2), and a fin (4) that is fiilcrummed to a distal end of the swing arm (3), away from the yacht (2), which fin (4) is arranged to be submerged in the water (10), and an electric generator (12), which is mounted on the yacht and arranged to be driven by the swing arm (3) through a suitable power take-off mechanism as described below. A larger supporting structure may support a hydropower generator comprising two or more swing arms, which may be arranged to drive one or more electric generators, or multiple hydropower generators, each comprising one or more swing arms, fins and an electric generator.

[0122] In FIG. 1, the yacht (2) is shown floating on a body of water (10) and anchored to the ground (11) under the water, for example a sea or river bed, by a suitable positioning system such for example as an anchor (5) and anchor cable (6). For the sake of the following description, the body of water (10) is flowing past the yacht (2) in the direction of the arrow (F). In the case of a river or reservoir, the flow (F) will typically always be in the same direction. However, in the case of a tidal waterway such, for example, as an estuary or the open sea, the body of water (10) may flow in opposite directions cyclically. Those skilled in the art will appreciate that where the direction of flow (F) reverses periodically, the yacht (2) will swing around on its anchor cable (6) such that the yacht (2) is always positioned downstream of the anchor (5) relative to the instant direction of flow (F), such that the proximal end of the swing arm (3) is disposed forwards of the distal end, relative to the direction of flow (F).

[0123] As described below, the fin (4) comprises a hydrofoil (41) that is configured and arranged to pitch on the distal end of the swing arm (3), thereby to generate lift in the flow of water. The swing arm (3) serves as a mechanical linkage between the yacht (2) and the fin (4), to allow the fin (4) to move tranverse the direction of the flow (F) of water (10) by dint of the generated lift, and to constrain its movement to a particular path relative to the yacht (2). In the present example implementation, the fin (4) is mounted on the distal end of the swing arm (3) for flapping motion along an arcuate path, as indicated in FIG. 1 by the double-headed arrow (P). The swing arm is mounted to the yacht (2) and configured for motion in a substantially vertical plane, but in other implementations, the swing arm may be mounted for movement in a horizontal (or any other) plane. In other implementations, instead of a swing arm, the mechanical linkage may comprise a guide frame, shaft or the like for constraining motion of the fin (4) to a substantially linear path, which may similarly be oriented vertically or horizontally in the water.

[0124] The swing arm (3) is also connected, via the power take-off mechanism comprising for example a connecting rod (31) and crank shaft (32) as shown in FIG. 2, to the electric generator (12) for generating power from the transverse motion of the fin (4) through the water. The swing arm (3) thus serves to drive the electric generator (12). In implementations in which the hydropower generator comprises multiple swing arms, or the supporting structure carries multiple hydropower generators, the swing arms may be arranged to drive a single or multiple electric generators. The electrical energy generated by the electric generator may suitably be stored by a storage battery, which may also be mounted to the supporting structure, or elsewhere: for example, on the land.

[0125] As described in more detail below, the direction and amount of lift generated by the fin (4) is controlled by elevons (42) on the hydrofoil (41), and the swing arm (3) is attached to the yacht (2) by a hinge assembly (35) that incorporates an elevon angle control mechanism for holding the elevons (42) at a particular angle relative to the hydrofoil (41) and automatically inverting the angle of the elevons (42) between two opposite angled states when the swing arm (3) is pivoted in either direction beyond an actuation point at a particular angle relative to the yacht (2) as the supporting structure, thereby to cause the fin (4) to pitch about the distal end of the swing arm (3) and reverse the direction of the lift generated by the hydrofoil (41), and thereby to reverse the direction of transverse motion of the fin (4) through the water. Advantageously, as described below, the elevon angle control mechanism is driven entirely by the motion of the swing arm (3) and does not require the input of any additional power, which would decrease the efficiency of the hydropower generator. However, it will be understood that in other implementations, the elevons (42) may be powered. This would not represent a serious disadvantage, because actively moving the elevons (42) between their angled states would require significantly less energy than actively driving the pitching motion of the whole fin (4).

[0126] The reciprocal motion of the fin (4) is controlled by a governor (21) comprising a governor mechanism (25) operated by a governor actuator which, in the present example implementation, comprises a servo drive (22). In turn, the servo drive (22) is controlled by a controller (23), suitably a microcontroller, which receives an input signal from a load sensor (24) that is arranged to measure the load in the anchor cable (6). The load sensor (24) may, for example, comprise a strain gauge. The controller (23) may be configured to receive a digital or analogue input signal from the load sensor (24) and to output a control signal to the servo drive (22) to operate the governor mechanism (25). The controller (23) may be configured to control operation of the governor (21) according to the load in the anchor cable (6), as represented by the input signal, as disclosed herein.

[0127] Using the load in the anchor cable (6) for controlling operation of the governor (21) is especially advantageous for the reasons outlined herein, but in other implementations, the sensor (24) may be adapted and arranged to sense any other relevant dynamic environmental or structural factor that is indicative of the load on the flapping mechanism, or the strain in the supporting structure or in a positioning system for the supporting structure, when the flapping mechanism is in use. Thus, as described below, in the present example implementation, the governor (21) may be configured and arranged to adjust the pitch of the fin (4) and / or the amplitude of the reciprocal motion of the fin (4) according to the load in the anchor cable (6), thereby to modulate the power coefficient Cp of the hydropower generator (1) according to the detected anchor load. However, in other implementations, the sensor (24) may be adapted and arranged for sensing the velocity of the water flow (F) past the fin (4), the strain in the supporting structure or a connection between the flapping mechanism and the supporting structure, or the strain or load in a positioning system for a floating supporting structure such, for example, as a mooring line, anchor cable or other ground tackle.

[0128] In other implementations, the motion of the swing arm (3) or other mechanical linkage may be controlled by a wholly mechanical or electromechanical governor (21). In the case of a purely mechanical governor, the sensor may be adapted to relay the sensed load or strain for actuating the governor mechanism (25), while an electromechanical sensor may be adapted to measure the load or strain and generate an output signal that is processed by the controller (23) in a similar manner to the present example implementation.

[0129] As shown in FIGS. 2-5, the swing arm (3) of the flapping mechanism of the hydropower generator (1) of the present example implementation comprises an elongate shaft (36), which is attached at a first proximal end (37) to the hinge assembly (35) that is fixedly secured to the transom of the yacht (2), as described in greater detail below, and at a second distal end (38) to the fin (4). The shaft (36) may have any suitable configuration, but in the present example implementation is tubular. A wide range of suitable materials for making the shaft (36) and the fin (4) are available to those skilled in the art, including aluminium, carbon fibre and glass fibre, which are generally lightweight, strong materials which retain a degree of flexibility to absorb impacts and other transient stresses on the flapping mechanism.

[0130] The fin (4) is pivotably attached to the second distal end (38) of the shaft (36) at a fulcrum (43). The position of the fulcrum (43) between the end of the shaft (36) and the fin (4) is important to the correct operation of the flapping mechanism, as described below. In the present example implementation, the fin (4) is pivoted to the distal end (38) of the shaft (36) on a substantially horizontal axis when the hydropower generator (1) is secured to a supporting structure, such as the yacht (2), such that the fin (4) can pitch in opposite directions, up and down, relative to the shaft (36).

[0131] As best seen in FIG. 4, the hydrofoil (41) has a bipartite structure, comprising two foil portions (41 A, 4 IB) that are mirror images of each other and are disposed to each side of the distal end (38) of the shaft (36). The two foil portions (41 A, 4 IB) form the hydrofoil (41) which is unitary and moves together as a single part, about the fulcrum (43) to the shaft (36). Each foil portion (41A; 41B) is symmetrical about its centreline (Cf), having equally and oppositely cambered upper and lower surfaces (47, 48), as shown in FIG. 6, and has a generally trapezoidal planform, which is swept relative to the shaft (36), as best seen in FIG. 4, to afford satisfactory hydrodynamic properties, as will be well understood by those skilled in the art. In other implementations, the precise shape and configuration of each foil portion (41 A, 4IB), including its span, chord-length, upper and lower surface profile, planform and sweep may be varied as desired. However, it is desirable that the hydrofoil (41) should be configured and pivoted to the distal end (38) of the shaft (36) such that its centre of lift (CL) is located behind the fulcrum (43) relative to the direction of flow of water (F), as shown in FIG. 6. In a flow of water the fin (4) will therefore pivot naturally about the fulcrum (43) to a neutral hydrodynamic centre position, with the centerline (Cf) of each foil portion (41 A, 4 IB) being oriented substantially parallel to the direction of the flow (F), generating little or no lift, only drag. As indicated in the cascading flow chart of FIG. 17, the neutral centre position (201) of the hydrofoil (41) is determined by the planform and sweep (202) of each foil portion (41 A, 4 IB) and the net pitch moment (203) (if any) of the hydrofoil (41).

[0132] As shown best in FIGS. 4-6, each foil portion (41A, 41B) has an elevon (42) that is pivoted to a trailing edge (46) of the respective foil portion (41 A, 4 IB). In the present example implementation, each elevon (42) is laterally coterminous with atip (45) of the respective foil portion (41A, 41B), but in other implementations it may be spaced inwardly of the tip (4), i.e. elsewhere along the span of the foil portion (41 A, 4 IB), if desired. Each elevon (42) is pivoted to its respective foil portion (41 A, 4 IB) about a respective axis that is substantially parallel to the span of the respective foil portion (41 A, 4 IB) and can pivot up and down relative to the foil portion (41 A, 4IB) between two angled states through an angle of about 40°, being ±20° each side of the centreline (Cf), although it will be appreciated that this range may be varied in the other implementations. Generally, the elevon (42) subtends an acute angle with the respective foil portion (41A, 4IB) in each angled state.

[0133] As best seen in FIG. 6, a double-ended lever (49), which forms part of the elevon angle control mechanism, is fixedly secured to each elevon (42), or to its respective pivot, for rotation with the elevon (42). Upper and lower ends (49A, 49B) respectively of the double-ended lever (49) protrude above and below upper and lower surfaces (47, 48) of the foil portion (41 A, 4 IB) respectively. The upper end (49A) of the lever (49) is attached to a respective distal end (52) of a dual-output control cable (50) such, for example, as a split Bowden cable that extends along the length of the shaft (36) of the swing arm (3) to the hinge assembly (35), as described in more detail below, while the lower end (49B) is attached to one end of an elevon spring (55), the other end of which is attached to the lower surface (48) of the respective foil portion (41A, 41B). It will be appreciated that the control cable (50) may be used selectively to pull the elevon (42) in one direction into an “elevon up” angled state relative to the chordline, as shown in FIGS. 3 and 6, while the elevon spring (55) serves to bias the elevon (42) in an opposite direction into an “elevon down” angled state relative to the centreline (Cf) of the foil portion (41A, 41B) (e.g. as shown in FIG. 16A which is discussed below).

[0134] The terms “up” and “down” as used herein in relation to the elevons (42) have their conventional meanings that describe a respective angled state of the elevons (42), i.e. when the elevons (42) are angled “down”, each elevon (42) extends downwards rearwardly away from the trailing edge (46) of the respective foil portion (41A, 41B) towards the lower surface of the hydrofoil portion (41A, 41B), such that the elevon (42) increases drag on the lower surface (48) of the foil portion (41 A, 4 IB) when the fin (4) is fitted to the yacht (2) or other supporting structure. When the elevons (42) are angled “up”, each elevon (42) extends upwards rearwardly away from the trailing edge (46) of the respective foil portion (41A, 41B) towards the lower surface of the hydrofoil portion (41A, 41B), such that the elevon (42) increases drag on the upper surface (47) of the foil portion (41A, 41B).

[0135] When the elevon (42) is disposed in a neutral state, with the centreline (Ce) of the elevon (42) level with the centreline (Cf) of the foil portion (41A, 4 IB), there is no turning moment on the fin (4) resulting from the flow of water (10) passing over the foil (41). However, when the elevon is rotated to an “up” or “down” angled state away from the neutral state by pulling or releasing the control cable (50), the flow of water over the elevon (42) produces a turning moment on the hydrofoil (41), indicated at (205) in the flowchart of FIG. 17, which causes the fin (4) to pitch up or down about the fulcrum (43) to the distal end (38) of the shaft (36), according to whether the elevon is angled up or down respectively. The fin (4) then rotates upwards or downwards respectively about its fulcrum (43) to the distal end (38) of the shaft (36) until it reaches a stable equilibrium position in which the foil portion (41 A, 4IB) has an angle of attack (208) at which the turning moment (205) generated by the elevon (42) is counterbalanced (204) by an equal and opposite pitching moment generated by the foil portion (41 A, 4IB) that depends on the position (206) of the fulcrum (43) and the velocity (207) of the water flow (F) over the foil portion (41 A, 41B). It will be understood that the forces and moments acting on the two foil portions (41A, 41B) are substantially the same, since the elevons (42) are operated together with the dual-output control cable (50). Further, it will be understood that the greater the angle of the elevons (42) in one or other of their angled states away from the neutral state, the greater the pitch of the hydrofoil (41) that is needed to counterbalance the forces and moments acting thereon, the actual pitch of the hydrofoil (41) also being dependent upon the speed of the water flow (F).

[0136] When the fin (4) pitches up or down relative to the flow of water (10), the hydrofoil (41) generates lift (L) as shown in FIG. 3 and plunges upwards or downwards respectively on the end of the swing arm (3), transverse the direction of flow (F), as indicated at (209) in FIG. 17, the swing arm (3) pivoting around its proximal end (37) to permit the movement of the fin (4). The effective flow direction of water arriving at the fin (4) is then not just the direction of flow (F) of the water (10), but is the sum of that flow and the fin’s (4) motion, the balance of moments and forces causing the fin (4) to rotate about its fulcrum (43) to the distal end (38) of the shaft (36), so that it maintains the same angle of attack relative to the resultant flow, with no further adjustment of the elevons (42) required. It will be understood that the motion of the fin (4) is fully passive: once the elevons’ (42) position is set, the fin (4) adjusts to a given angle of attack relative to the resultant flow it experiences throughout its motion, thereby automatically accommodating changes in the speed and direction of the water flow (F). The lift generated by the fin (4) is thus automatically adjusted according to the speed of the water.

[0137] As best seen in FIG. 7, the hinge assembly (35) at the proximal end (37) of the shaft (36) comprises a generally U-shaped hinge outer (61) having two opposing outer arms (62’, 62”) on opposite sides of a central portion (63) that is fixedly secured to the shaft (36) by a suitable fastening such for example as a socket head cap screw (64), as shown in FIGS. 8 and 9. In some implementations, the hinge outer (61) may be integral with the shaft (36). The hinge outer (61) defines a recess between the outer arms (62’, 62”), which is dimensioned to receive two opposing inner arms (72’, 72”) of a hinge bracket (71), the two inner arms (72’, 72”) being connected by a central portion (73) that is fixedly attached to an inline hinge flange (75) by means of socket head cap screws, one of which is shown at (74), or the like, which prevent rotation of the hinge bracket (71) relative to the hinge flange (75). In some implementations, the hinge bracket (71) may be integral with the hinge flange (75). The hinge flange (75) is fastened to the transom of the yacht (2) (not shown in FIG. 7) and thus serves as a mounting for fastening the hydropower generator (1) to the supporting structure.

[0138] As best shown in FIG. 8, each outer arm (62’, 62”) is disposed closely adjacent to a respective one of the inner arms (72’, 72”) to form a stable hinge. One of the outer arms (62’) is pivoted to its corresponding inner arm (72’) by a stub axle (81) that extends through axially-aligned bores (76’, 66’) formed in the inner and outer arms (72’, 62’), is fixedly sercured to the inner arm (72’) by a first grub screw (82’) and is journalled to the outer arm (62’) by a ball bearing assembly (83’).

[0139] As best shown in FIG. 9, the other outer arm (62”) is pivoted to its corresponding inner arm (72”) by means of a generally cylindrical, hollow trunnion (90) having an inner portion (91) that is fastened within a bore (66”) formed in the outer arm (62”) and is supported rotatably on a central portion (102) of an elongate hollow input toggle shaft (100) that extends through the hollow trunnion (90), an inner portion (101) of which input toggle shaft (100) is fixedly secured by a second grub screw (82”) in an axially-aligned bore (76”) formed in the inner arm (72”). Suitably the inner portions (91, 101) of the trunnion (90) and input toggle shaft (100) may be slightly rebated as shown in FIG. 10 to facilitate a tight fit in the bore (66”) in the outer arm (62”) and the bore (76”) in the inner arm (72”) respectively. Rotation of the trunnion (90) relative to the input toggle shaft (100) is facilitated by by two axially spaced ball bearing assemblies (83”, 83’”).

[0140] The bores (66’, 66”, 76’, 76”) formed in the two outer and inner arms (62’, 62”, 72’, 72”) are mutually aligned to define the pivot axis of the swing arm (3) relative to the yacht (2) as the supporting structure in the present example implementation.

[0141] As shown in FIG. 9, an outer portion (92) of the trunnion (90) extends beyond the bore (66”) formed in the corresponding outer arm (62”) and is formed at its outer extremity with an annular flange (94) which is formed with an arcuate slot (95), forming part of a lost motion mechanism whose purpose is described below. An outer portion (103) of the input toggle shaft (100) protrudes outwardly beyond the annular flange (94).

[0142] The input toggle shaft (100) rotatably accommodates a coaxial, elongate output toggle shaft (110) having an inner portion (111) that extends inwardly beyond the inner portion (101) of the input toggle shaft (100) and terminates approximately midway between the two inner arms (72’, 72”), and an outer portion (112) that extends outwardly beyond the outer portion (103) of the input toggle shaft (100). The inner and outer portions (111, 112) of the output toggle shaft (110) may be slightly rebated as shown in FIG. 11, and carry respective ball bearing assemblies (113’, 113”) for journalling the output toggle shaft (110) within the input toggle shaft (100).

[0143] As shown in FIG. 11, the outer portion (103) of the input toggle shaft (100) is attached to a teardrop shaped input toggle arm (105) which extends generally downwardly from the input toggle shaft (100) when the hydropower generator (1) is installed on the supporting structure, substantially orthogonally to the axis of the input toggle shaft (100), and tapers to an outer end (106) that carries an outwardly extending input toggle spring pin (107).

[0144] The outer portion (112) of the output toggle shaft (110) is attached to a tear-drop shaped output toggle arm (115) which extends generally upwardly from the output toggle shaft (110), substantially orthogonally to the axis thereof, and tapers to an outer end (116) that carries an outwardly extending output toggle spring pin (117) and an inwardly extending output toggle limit pin (118). As shown in FIG. 9, although the output toggle shaft (110) extends outwardly beyond the outer portion (103) of the input toggle shaft (100), the input toggle spring pin (107) is somewhat longer that the output toggle spring pin (117), so the ends of the input and output toggle spring pins (107, 117) lie in substantially the same plane orthogonal to the axis through the input and output toggle shafts (100, 110). The ends of the input and output toggle spring pins (107, 117) are interconnected by a toggle tension spring (120) as shown in FIG. 10, which acts to pull the ends of the input and output toggle spring pins (107, 117) together by rotating the output toggle shaft (110) within the input toggle shaft (100), the latter being fastened to the inner arms (72’, 72”) of the hinge bracket (71) such that it cannot rotate. While in the present example implementation, the input toggle arm (105) extends generally downwardly from the input toggle shaft (100) and the output toggle arm (115) extends generally upwardly from the output toggle shaft (110), these orientations may be varied as desired, provided the input and output toggle arms (105, 115) generally extend away from each other on opposite sides of the axis of the input and output toggle shafts (100, 110).

[0145] As shown in FIG. 10, the output toggle limit pin (118) is received in the arcuate slot (95) in annular flange (94) of the trunnion (90) which thus provides for lost motion between the shaft (36), which is fixed to the annular flange (94) through the hinge outer (61) and trunnion (90), and the output toggle shaft (110). Rotation of the output toggle shaft (110) relative to the trunnion (90) is limited by the length of the arcuate slot (95).

[0146] The inner portion (111) of the output toggle shaft (110) is fastened to a radially extending horn (130) that is attached to a proximal end (51) of the control cable (50) and extends away from the axis of the output toggle shaft (110) in an opposite direction from the output toggle arm (115), i.e. downwardly in the present example implementation as best shown in FIG. 9, when the hydropower generator (1) is fixed to the supporting structure. It will be understood therefore that rotation of output toggle shaft (110) clockwise as shown in FIG. 7 causes the horn (130) to pull the control cable (50) forwards, thereby pulling the elevons (42) “trailing edge up”, while rotation of output toggle shaft (110) anticlockwise as shown in FIG. 7 causes the horn (130) to push the control cable (50) rearwards, thereby allowing the elevons (42) to pitch “trailing edge down” under the influence of the elevon springs (55). It will be understood that the greater the degree of rotation of the output toggle shaft (110), the greater the pitch of the elevons (42) and the greater the resulting pitch of the hydrofoil (41) that is required to balance the forces and moments thereon, as described above. In this way, the elevon angle control mechanism is configured to be driven by motion of the swing arm (3), through the lost motion mechanism formed by the slotted annular flange (94) of the trunnion (90) and the output toggle arm (115) and toggle limit pin (118).

[0147] Ignoring the operation of the governor (21) for the time being, which is described in more detail below, the hinge outer (61), hinge bracket (71), trunnion (90), annular flange (94) and arcuate slot (95), input and output toggle shafts (100, 110), input and output toggle arms (105, 115) and toggle tension spring (120) form a bistable toggle mechanism (80), which operates as described below with reference to FIGS. 14A-D. The bistable toggle mechanism (80) works automatically to reverse the direction of movement of the fin (4) across the flow of water when the swing arm (3) is pivoted in each direction to a particular angle relative to the yacht (2) as the supporting structure.

[0148] It will be understood that the bistable toggle mechanism (80) of the present example implementation is given by way of example only and those skilled in the art will be able to substitute a range of alternative mechanisms for achieving the same result.

[0149] In FIG. 14A, the bistable toggle mechanism (80) is shown in a first stable configuration, in which the output toggle limit pin (118) is held stably against the annular flange (94) in a first position at a first end (96) of the arcuate slot (95): the toggle tension spring (120) biases the output toggle arm (115) clockwise as shown in FIG. 14A towards the input toggle arm (105), the position of the input toggle arm (105) being fixed relative to the yacht (2) by dint of being fastened to the hinge bracket (71). Meanwhile, the annular flange (94) is constrained to rotate with the swing arm (3) by dint of the trunnion (90) being fastened to the hinge outer (61) that is attached to the proximal end (37) of the shaft (36). While not shown in FIGS. 14A-D, in the first stable configuration, the swing arm (3) is positioned at the start of an upstroke, with the elevons (42) disposed “trailing edge up” in the one angled state, as shown for example in FIG. 16B, which is described in more detail below. Upwards movement of the swing arm (3) from this position by dint of the lift generated by the fin (4) as described above, causes the trunnion (90) to rotate forwards relative to the yacht (2) when viewed from above, i.e. counterclockwise as shown in FIG. 14A. Rotation of the trunnion (90) with its annular flange (94) thus causes corresponding counterclockwise rotation of the output toggle arm (115), thereby extending the toggle tension spring (120), as shown in FIG. 14B. (In FIGS. 14A-D, the annular flange (94) is depicted as being stationary for convenience, but in reality the trunnion (90) that rotates relative to the yacht (2), pushing against the toggle limit pin (118) of the output toggle arm (115), while the input toggle shaft (100) and input toggle arm (105) remain fixed relative to the yacht (2)).

[0150] Upon continued anticlockwise rotation of the trunnion (90), the annular flange (94) continues to drive the output toggle arm (115) until it reaches an unstable actuation point as shown in FIG. 14C, where it is arranged diametically opposite the input toggle arm (105) and the toggle tension spring (120) is maximally extended. Continued movement of the trunnion (90) in the same direction then pushes the output toggle arm (115) over-centre relative to the input toggle arm (105), allowing the toggle tension spring (120) to snap the output toggle arm (115) towards the input toggle arm (105) on an opposite side of the flange (94) from its starting position, thereby driving the toggle limit pin (118) along the arcuate slot (95) with lost motion with respect to the trunnion (90) to a second end (97) of the arcuate slot (95), as shown in FIG. 14D, the bistable toggle mechanism (80) now being arranged in a second stable configuration, with the swing arm (3) disposed at the end of its upstroke, as shown in FIG. 16E, and the output toggle arm (115) being disposed in a second stable position.

[0151] As described above, the output toggle arm (115) is attached to the output toggle shaft (110) whose inner portion is fastened to the horn (130) that is connected to the proximal end (51) of the control cable (50). Accordingly, progressive upwards movement of the swing arm (3) and concomitant anticlockwise rotation of the trunnion (90) as illustrated in FIGS. 14A-C prior to the actuation point causes a corresponding certain degree of rearwards motion of the horn, which depends below the output toggle shaft (110), thereby pushing the control cable (50) towards the fin (4) and allowing the trailing edges of the elevons (42) to rotate downwards somewhat (while still remaining trailing edge up overall) relative to their respective foil portions (41 A, 4 IB) under the force of the elevon springs (55), thereby to adjust the equilibrium of the fin (4) on the distal end (38) of the shaft (36) such that it maintains a substantially constant angle of attack relative to the direction of flow (F), for a given flow velocity.

[0152] Once the swing arm (3) reaches the actuation point defined by the input and output toggle arms (105, 115) and the toggle tension spring (120) as described above, the output toggle arm (115) rotates over-centre of the input toggle arm (105), the resulting lost motion of the output toggle arm (115) along the arcuate slot (95) in the annular flange (94) to the second end (97) causing sufficient rotation of the horn (130) on the inner portion (111) of the output toggle shaft (110) to allow the elevon springs (55) to rotate the elevons (42) to the other angled state, i.e. trailing edge down relative to foil portions (41 A, 4 IB), as shown in FIG. 16F. As mentioned above, this causes a change in the balance of the forces and moments acting on the fin (4), causing the hydrofoil (41) to pitch down relative to the flow of water (F) as shown in FIG. 16A, thereby reversing the plunge direction of the fin (4) across the flow. The angle of the elevons (42) in each of the “up” and “down” angled states is determined by the extent of rotation of the output toggle shaft (110) and thus by the length of the arcuate slot (95) in the annular flange (94). The lost motion mechanism formed by the input and output toggle arms (105, 115) and toggle tension spring (120) thus defines the extent to which the output toggle arm (115) must rotate in either direction with respect to the input toggle arm (105) before it reaches the actuation point, where the bistable toggle mechanism (80) switches between the first and second stable configurations. In the present example implementation, the length of the arcuate slot (95) thus also defines the amplitude of the plunging motion of the swing arm (3).

[0153] As already mentioned, the motion of the fin (4) on the swing arm (3) as the mechanical linkage is controlled by the governor mechanism (25), which comprises a pair of planar arcuate limit jaws (141A, 14 IB) that are rotatably attached to an outer face (98) of the annular flange (94), as best seen in FIG. 12, and serve to adjust the effective length of the arcuate slot (95) as described below. As shown in FIG. 13, each limit jaw (141A, 14 IB) comprises an arcuate central portion (142’, 142”) that terminates in a generally circular end portion (143’, 143”) at one end and a substantially flat end surface (144’, 144”) at the other end. As shown in FIGS. 12, 13 and 15A-C, the central portion (142’, 142”) of each limit jaw (141A, 14 IB) is shaped to extend around the outer portions (103, 112) of the input and output toggle shafts (100, 110) that protrude from the outer surface (98) for the annular flange (94), so as not to impinge thereon. Intermediate its circular end portion (143”) and flat end surface (144”), an exterior surface (146”) of one of the limit jaws (14 IB), which is designated an active jaw, is formed with a first cam surface (147”). The other limit jaw (141A) is passive, as described below.

[0154] The circular end portions (143’, 143”) are mounted to the annular flange (94) at spaced locations, generally opposite the arcuate slot (95) by means of short pan head screws (150’, 150”) that extend through the limit jaws (141A, 14 IB) and are screwed into the annular flange (94) to form a pivot joint therebetween. A washer and spacer (not shown) are disposed around each pan head screw (150’, 150”) to facilitate movement of the limit jaws (141A, 14 IB) with respect to the outer surface (98) of the annular flange (94), the washer being interposed between an inner face of the respective limit jaw (141A, 14 IB) and the outer face (98) of the annular flange (94), and the spacer being disposed around the shaft of the pan head screw (150’, 150”) and extending through the corresponding washer.

[0155] The circular end portions (143’, 143”) of the limit jaws (141A, 14 IB) are disposed adjacent one another and are formed with interengaging gear segments (145’, 145”) that serve to link rotation of the two limit jaws (141A, 14 IB) about their respective pivot joints, whereby rotation of the active limit jaw (14 IB) about its respective pivot joint causes equal and opposite rotation of the passive limit jaw (141A) about its respective pivot joint.

[0156] The limit jaws (141A, 14 IB) are shaped and dimensioned such that the end surfaces (144’, 144”) thereof are juxtaposed the acruate slot (95) formed in the annular flange (94), with the radius of each end surface (144’, 144”) from its respective pivot joint being substantially equal to the radius of the arcuate slot (95) so that as each limit jaw (141A, 14 IB) is pivoted about its respective pivot joint, its end surface (144’, 144”) it substantially follows the path of the arcuate slot (95). The two end surfaces (144’, 144”) oppose one another along the line of the arcuate slot (95) and, as best seen in FIG. 9, the toggle limit pin (118) extends between the opposing end surfaces (144’, 144”) into the arcuate slot (95). It will be appreciated therefore that the end surfaces (144’, 144”) thus serve to control the effective length of the arcuate slot (95) by limiting the extent of travel of the toggle limit pin (118) along the arcuate slot (95).

[0157] The limit jaws (141A, 14 IB) are thus movable between a fully closed position, as shown in FIG. 15A and a fully open position, as shown in FIG. 15C, in which the input and output toggle shafts (100, 110) and input and output toggle arms (105, 115) are omitted for the sake of clarity; an intermediate position is shown in FIG. 15B. In the fully closed position of FIG. 15A, the toggle limit pin (118) is held between the opposing end surfaces (144’, 144”) of the limit jaws (141A, 14 IB) at a point approximately half-way along the arcuate slot (95) between its first and second ends (96, 97) and is unable to move. In this position, the output toggle shaft (115) is positioned such that the control cable (50) holds the elevons (42) on the hydrofoil (41) in the neutral state, level with the centre-lines of the respective foil portions (41A, 41B). In this orientation, as described above, the elevons (42) produce substantially no moment on the fin (4), which therefore remains substantially flat relative to the direction of the flow of water, producing no lift. In the fully open position of FIG. 15C, the opposing end surfaces (144’, 144”) of the limit jaws (141A, 14 IB) are disposed clear of the ends (96, 97) of the arcuate slot, so the toggle limit pin (118) is free to travel along the full length of the arcuate slot (95), thereby to allow periodic inversion of the angle of the elevons (42) relative to the hydrofoil (41) and a maximal rotational movement of the output toggle shaft (110), thereby to move the elevons (42) to their maximum pitch positions in each angled state to either side of the neutral state, i.e. ± about 20° in the present example implementation. When the limit jaws (141 A, 14 IB) are fully open, the output toggle shaft (110) can rotate through an angle that corresponds to the maximum angular displacement of the elevons (42) relative to the foil portions (41 A, 4IB) as mentioned above. In intermediate positions between the fully closed and fully open positions, for example the position shown in FIG. 15B, the limit jaws (141A, 14 IB) serve to limit the extent of travel of the toggle limit pin (118) along the arcuate slot (95) to each side of the actuation point and the degree of rotation of the output toggle shaft (110), and therefore also the maximum angle of the elevons (42) relative to the hydrofoil (41) in each angled state.

[0158] The governor mechanism (25) further comprises a governor control ring (160) that comprises a first relatively narrow inner portion (161) that is journalled to the outer portion (92) of the trunnion (90), where it protrudes outwardly of the other outer arm (62”) of the hinge outer (61), such that the governor control ring (160) can rotate freely with respect to the trunnion (90), an intermediate annular web portion (163), and second relatively wide outer ring portion (162) that is arranged circumjacent the annular flange (94) of the trunnion (90) and comprises interior and exterior cylindrical surfaces (164, 165). As best seen in FIG. 13, the interior surface (164) is formed with a second cam portion (167) that is configured and arranged to engage and cooperate with the first cam portion (147”) on the active limit jaw (14 IB), such that rotation of the governor control ring (160) relative to the trunnion (90) causes rotation of the active limit jaw (14 IB) and, therefore, through the interengagement of the interengaging gear segments (145’, 145”) as described above, equal and opposite rotation of the passive limit jaw (141A), thereby to move the opposing end surfaces (144’, 144”) of the limit jaws (141A, 14 IB) towards or away from other, depending on the direction of rotation of the control ring (160).

[0159] The exterior surface (165) of the outer ring portion (162) is formed with a gear segment (166) that operably engages a pinion (172) that is fastened on an output shaft (171) of the servo drive (22), which is fixedly mounted in a rebate (68) formed in the other outer arm (62”) of the hinge outer (61), such that the servo drive (22) moves with the hinge outer (61) and, in consequence of the interengagement of the gear segment (166) and pinion (172), the governor control ring (160) normally rotates with the trunnion (90) when the swing arm (3) rocks about the hinge assembly (35) at its proximal end (37).

[0160] As described above, the servo drive (22) is arranged to receive the control signal from the controller (23) in response to the measured load in the anchor cable (6). In particular, in accordance with the present disclosure, the controller (23) is configured to transmit a control signal to the servo drive (22) to actuate the governor mechanism (25) by rotating the governor control ring (160) relative to the trunnion (90) and, more specifically, the annular flange (94), to adjust the positions of the limit jaws (141A, 14 IB) as described above - and thus the angle of the elevons (42) on the hydrofoil (41) -according to the load in the anchor cable (6) or, in other implementations, the strain in the supporting structure or speed of the flow of water (F).

[0161] Thus, as described in more detail below, when the load in the anchor cable (6) is below a certain first lower threshold, TL, (which may be determined according to the size and capacity of the hydropower generator (1) and the strength of the anchor cable (6) or other supporting structure), the controller (23) transmits a control signal to the servo drive (22) to rotate the governor control ring (160) relative to the trunnion (90) until the opposing limit jaws (141A, 14 IB) are in their widest apart configuration, as shown in FIG. 15A. When the load in the anchor cable (6) rises above the first lower threshold, TL, the controller (23) signals to the servo drive (22) to rotate the governor control ring (160) relative to the trunnion (90) to close the limit jaws (141A, 14 IB) to an intermediate position, for example as shown in FIG. 15B, thereby attenuating the drag on the fin (4) as described in more detail below and modulating the load in the anchor cable (6). When the load on the anchor cable (6) exceeds a second upper threshold, the controller (23) signals to the servo drive (22) to rotate further the governor control ring (160) relative to the trunnion (90), to close the limit jaws (141A, 14 IB) fully, as shown in FIG. 15A, thereby minimising the drag on the hydrofoil (41) by holding the elevons (42) in their neutral state as described above.

[0162] FIGS. 16A-F illustrate operation of the hydropower generator (1). FIG. 18 is a flow diagram that illustrates the sequence of steps involved in a complete cycle (comprising a downstroke and an upstroke) of the flapping mechanism.

[0163] In FIG. 16A, the swing arm (3) is disposed at the bottom of a downstroke. The load sensor (24) continuous monitors the load in the anchor cable (6) as indicated by step (309) in FIG. 18. In a low speed water flow (F), for example around slack water in a tidal environment, the load sensor (24) measures a relatively low load in the anchor cable (6), below the first lower threshold, TL, which causes the controller (23) to control the servo drive (22) to rotate the governor control ring (160) as indicated by step (310) in FIG. 18 so that the limit jaws (141 A, 14 IB) are arranged in the fully open position of FIG. 15C, such that the toggle limit pin (118) is able to traverse the entire length of the arcuate slot (95). Just before the swing arm (3) reaches the bottom of the downstroke, the toggle limit pin (118) is located at the second end (97) of the arcuate slot (95), and the trunnion (90) is rotated almost fully rearwards when viewed from above (i.e. clockwise as shown in FIGS. 14A-D) relative to the input toggle shaft (100), such that the the output toggle arm (115) is disposed at the actuation point, substantially in line with the input toggle arm (105), with the toggle tension spring (120) being maximally extended. The radially extending horn (130) on the inner portion (111) of the output toggle shaft (110) is disposed in a rearward position, such that the control cable (50) is pushed rearwards, towards the distal end (38) of the shaft (36), to allow the elevons (42) to be angled down under the influence of of the elevon springs (55). The elevons (42) are thus angled down with respect to the hydrofoil (41), causing the fin (4) to pitch leading edge (44) down (clockwise as shown in FIG. 16A) until the downward moment on fin (4) about its fulcrum (43) to the distal end (38) of the shaft (36) is counterbalanced by an equal and opposite leading edge (44) upward moment at its centre of lift (CL), rearward of the fulcrum (43), that results from the (downward) lift (L) generated by the hydrofoil (41). In this position, the actual pitch of the fin (4) is determined by the size and geometry of the hydrofoil (41) and elevons (42), including the location of the centre of lift (CL) of the hydrofoil (41), the location of the fulcrum (43) between the fin (4) and the elongate shaft (36) and the angle of the elevons (42), which is determined by the configuration and location of the horn (130) and the configuration of the bistable toggle mechanism (80).

[0164] When the swing arm (3) reaches the bottom of the downstroke, the output toggle arm (115) is rotated over-centre relative to the input toggle arm (105) past the actuation point, thereby allowing the toggle tension spring (120) to pull the output toggle arm (105) rearwardly along the entire length of the arcuate slot (95) (i.e. clockwise as shown in FIGS. 14A-D) to its first stable configuration, as shown in FIG. 14A, in which the toggle limit pin (118) is engaged with the annular flange (94) at the first end (96) of the arcuate slot (95) and is held stably in that position by the toggle tension spring (120), as indicated by step (301) in FIG. 18. This motion of the output toggle arm (105) causes rearwards rotation of the output toggle shaft (110) when viewed from above and thus forwards movement of the radially extending horn, which depends below the output toggle shaft (110). The movement of the horn (130) pulls the control cable (50) forwards, against the restoring force of the elevon springs (55), thereby causing the elevons (42) to angle up relative to the centrelines (Cp) of the foil portions (41 A, 4 IB), as shown in FIG. 16B and indicated by step (302) in FIG. 18. Since the limit jaws (141A, 14 IB) are arranged in their fully open position, the output toggle shaft (110) is able to rotate relative to the trunnion (90) to its maximum extent, thereby allowing the elevons (42) to be pulled to their maximum angled up state, i.e. about +20° in the present example implementation.

[0165] In this configuration, the elevons (42) produce a leading edge (44) upward pitching moment on the hydrofoil (41) in the flow of water (F), causing the fin (4) to pivot leading edge (44) upwards about its fulcrum (43) to the distal end (38) of the elongate shaft (36), as shown in FIG. 16C and indicated at step (303), until it is balanced by a leading edge down pitching moment produced by (upwards) lift (L) generated by the hydrofoil (41) that effectively acts at the centre of lift (CL) about the fulcrum (43). In this orientation, the hydrofoil (4) is generates lift, causing the fin (4) to move upwards as shown, across the direction of the water flow. As the fin (4) moves upwards, the swing arm (3) rotates clockwise as shown in FIG. 16D and indicated by step (304) in FIG. 18, causing the trunnion (90) to rotate forwards when viewed from above. This rotation of the trunnion (90) causes the annular flange (94) to push against the toggle limit pin (118), which remains held stably at the first end (96) of the arcuate slot (95), causing the ouput toggle shaft (110) to rotate anticlockwise, as shown in FIGS. 14A and B. The output toggle shaft (110) thus rotates forwardly when viewed from above, causing the horn (130) to move rearwards. In so doing, the control cable (50) is pushed rearwards, allowing the angle of the elevons (42) to be progressively decreased relative to the hydrofoil (41) by the elevon springs (55), thereby decreasing the counter-balancing pitch of the fin (4) relative to the direction of flow (F), to maintain a substantially constant pitch of the hydrofoil (41) relative to the swing arm (3).

[0166] Forwards rotation of the trunnion (90) as the swing arm (3) is raised through the water causes forwards movement of the output toggle arm (115), as shown in FIG. 14B, until, just before the swing arm (3) reaches the top of an upstroke, as shown in FIG. 16E, the output toggle arm (115) is once again disposed at the actuation point, in line with respect to the input toggle arm (105) (which remains fixed at all times), as shown in FIG. 14C. It will be appreciated that because the toggle limit pin (118) is free to travel along the entire length of the arcuate slot (95), as described above, the trunnion (90) must rotate through a maximum angular displacement before it reaches the actuation point, corresponding to a maximum angular motion of the swing arm (3) during the upstroke.

[0167] At the top of the upstroke, the output toggle arm (115) is moved over-centre of the input toggle arm (105), allowing the toggle tension spring (120) to pull the output toggle arm (115) forwards along the entire length of the arcuate slot (95) until the toggle limit pin (118) engages the second end (97) thereof, as shown in FIG. 14D and indicated by step (305) in FIG. 18. This forwards movement of the output toggle arm (115) causes forwards rotation of the output toggle shaft (110) and thus rearward movement of the horn (130), thereby allowing the elevon springs (55) to invert the orientation of the elevons (42) to their maximum angled down state (i.e. -20° in the present example implementation), as shown in FIG. 16F and indicated by step (306) in FIG. 18, whereupon the elevons (42) produce a leading edge (44) downwards moment on the hydrofoil, causing the fin (4) to pitch downwardly about its fulcrum (43) on the distal end (38) of the elongate shaft (36) as indicated by step (307) in FIG. 18 until the upwards moment produced by the hydrofoil (41) about the fulcrum (43) counterbalances the downwards moment produced by the elevons (42). The lift on the hydrofoil (41) then causes the fin (4) to plunge downwards across the direction of the water flow (F) through its maximum angular displacement, until it reaches the bottom of another downstroke as shown in FIG. 16A and indicated by step (308) in FIG. 18.

[0168] As long as the water continues to flow past the fin (4), the hydrofoil (41), elevons (42), swing arm (3) and bistable toggle mechanism (80) in the hinge assembly (35) cause automatic inversion of the angle of the elevons (42) at the end of each up- and downstroke, thereby automatically reversing the direction of plunging of the fin (4) and producing an entirely passive reciprocal flapping motion, which is used to drive the generator (12) through the connecting rod (31) and crank shaft (32). The displacement of the fin (4) relative to the flow of water (F), i.e. a waveform swept by the fin (4), is illustrated schematically in FIG. 19A, in which the numbers for the ordinate and abscissa are arbitrary.

[0169] The kinematics of the motion of the flapping mechanism are thus driven by the loads generated by the fin (4) but are not prescribed. Instead, the balance of forces and moments generated by the fin (4) are set by the elevons (42) to a pre-selected value, which balances the moments around the fulcrum (43) between the fin (4) and the distal end (38) of the swing arm (3), so that the fin (4) naturally and passively adopts an angle of attack relative to the flow (F) where the angle of attack represents a stable equilibrium position defined by the balance of pitching moments around fulcrum (43) between the fin (4) and the swing arm (3). The result is that the flapping mechanism moves up and down of its own accord (auto flapping) with no external control inputs required. At every stage during stroke reversal, the stability of the fin (4) and the elevon-induced pitching moment about the fin’s (4) fulcrum (43) keep the fin (4) balanced passively at an angle of attack that can be controlled by the angle of the elevons (42). Further, as soon as the fin (4) moves, the local flow direction changes, but the fin (4) automatically adjusts under its own inherent stability to maintain the same angle of attack relative to the new local flow velocity.

[0170] FIG. 20 illustrates how the load in the anchor cable (6), represented by the solid line (401), the angle of attack of the hydrofoil (41), represented by the dashed line (402), and the power produced by the hydropower generator (1), indicated by the dotted line (403), varies with the velocity of the water flow (F), represented by the chain-dotted line (404), over a complete flood or ebb tide in such a tidal environment between high and low tides.

[0171] At low flow velocities, e.g. at and around high and low tides, when the load measured in the anchor cable (6) is below the first lower threshold TL, the governor (21) is arranged to allow the toggle limit pin (118) to travel along the full length of the arcuate slot (95) and to allow the elevons (42) to be raised and lowered to their maximum extent, as described above in detail. In this state, the hydrofoil (41) adopts a maximal angle of attack to balance the pitching moment owing to the elevons (42), which produces the maximal available lift for the flow velocity, and the reciprocal plunging motion of the swing arm (3) has a maximum amplitude. The power output of the hydropower generator (1) thus changes relatively steeply with the flow velocity, as shown at (406) and (407) in FIG. 20. In this position, the power coefficient, Cp, of the hydropower generator (1) may be advantageously optimised to the Betz limit of about 16 / 27 or 0.59, or as close to that value as is possible, given the hydrodynamic efficiency (lift to drag ratio) of the hydropower generator.

[0172] When the tension in the anchor cable (6) is sensed as indicated at step (309) of FIG. 18 to increase above the first lower threshold, TL, for example when the water flow speed is close to peak tidal current, or as a result of windage, waves, or other forces acting on the yacht (2) or other supporting structure, the controller (23) transmits a control signal to the servo drive (22) to rotate the governor control ring (160) relative to the trunnion (90) so as to move the toggle limit jaws (141A, 14 IB) closer together, as indicated at step (310) in FIG. 18, thereby to limit the extent of travel of the toggle limit pin (118) along the arcuate slot (95). The degree to which the limit jaws (141A, 14 IB) are closed depends on the load measured in the anchor cable (6) by the load sensor (24): the greater the measured load above the first lower threshold, TL, the closer the limit jaws (141A, 14 IB) are moved together and thus the shorter the effective length of the slot (95). This can be achieved by programming the controller (23) in a manner familiar to those skilled in the art, and causes adjustment of the above-described lost motion mechanism formed by the input and output toggle arms (105, 115) and the trunnion (90) to require the output toggle arm (115) to be rotated by the toggle tension spring (120) through a shorter arc relative to input toggle arm (105) before it reaches the actuation point, where the ouput toggle arm (115) moves over-centre relative to the input toggle arm (105) at the end of each up- and downstroke of the fin (4), than it does when the limit jaws (141A, 14 IB) are fully opened. This has two effects on the motion of the swing arm (3) and fin (4) in the present example implementation as illustrated schematically in FIG. 21.

[0173] First, the operation of the governor (21) in response to a greater load (501) in the anchor cable (6) results in the swing arm (3) rocking through a smaller angle on each up- and downstroke before it reaches the actuation point to trigger the bistable toggle mechanism (80) to invert the angle of the elevons (42) as described above, thereby reversing the direction of motion of the fin (4). This reduces the amplitude of the plunging motion (502) of the flapping mechanism in higher velocity flows when the drag on the yacht (2) is greater, or when the supporting structure is subjected to other increased loads.

[0174] Second, it limits the extent of rotation of the horn (130) on the inner portion (111) of the output toggle shaft (110), thereby reducing the angle (503) to which the elevons (42) are moved up or down relative to the foil portions (41A, 41B) at the start of each up- and downstroke, which in turn reduces the equilibrium pitch (504) of the fin (4) (which, as described above, is also determined by the geometry (505) of the hydrofoil (41), the location of its fulcrum (43) to the distal end (38) of the swing arm (3), and the water flow (F) speed), thereby increasing the wavelength and reducing the frequency (506) of the flapping motion of the fin (4) through the water flow (F), as shown in FIG. 19B, reducing the drag on the fin (4), and alleviating the load in the anchor cable (6).

[0175] In the present example implementation, the governor (21) thus serves to adjust automatically amplitude of the flapping motion of the fin (4) and the angle of the elevons (42) according to the load in the anchor cable (6) for controlling the angle of attack of the fin (4), thereby to modulate the drag exerted on the fin (4) while optimising the power output of the hydropower generator (1).

[0176] As shown in FIG. 20, therefore, when the load (401) in the anchor cable (6) increases above the first lower threshold, TL, the angle of the elevons (42) is automatically reduced correspondingly by the governor (21) as shown by region (408) of the dashed line (402), to reduce the angle of attack (402) of the hydrofoil (41) and thus the drag on the the flapping mechanism. The power output (403) of the hydropower generator (1) continues to increase with increased flow velocities (405), but less steeply than before, as indicated by region (409) of the dotted line (403). Meanwhile, the load in the anchor cable (6) is substantially capped at the first lower threshold, TL, as indicated by the substantially flat region (410) of the solid line (401). In this way, the hydropower generator of the present disclosure is adapted to operate with a variable flow induction factor by extracting power at a rate that maximises the ratio of the power coefficient, Cp, to the load in the anchor cable (6) or other supporting structure.

[0177] The power output (403) continues to increase until the flow velocity (405) reaches a maximum around the flood tide as indicated at (405) when the angle of attack (402) of the fin (4) is minimal. The power output (403) of the hydropower generator (1) peaks at this point and then progressively decreases with the slowing water speed until the next period of slack water.

[0178] If the load in the anchor cable (6) exceeds the second upper threshold (not shown in FIG. 20), the controller (23) signals to the servo drive (22) to rotate the governor control ring (160) relative to the trunnion (90) to close the toggle limit jaws (141A, 14 IB) completely, thereby holding the elevons (42) in their neutral state, such that there is substantially no pitching moment on the hydrofoil (41) which, in turn, maintains a neutral, no-lift, orientation relative to the swing arm (3), minimising the drag on the fin (4).

[0179] Using anchor load on a floating hydropower generator (1) to govern the angle of attack of the hydrofoil (41) and / or amplitude of the motion of the flapping mechanism and thus moderate the drag produced by the hydropower generator (1) is advantageous because the anchor load depends not just on the flow rate of the current, but also on wind loads (and any other loads) on the floating supporting structure. In a storm wind, loads may be dominant, and wave loads may be large, and modulating the angle of attack according to the load in the anchor cable (6) is particularly appropriate, since that is what is needed to be moderated in order for the hydropower generator to ride out a storm.

[0180] In some implementations of the present disclosure, the flapping mechanism may be configured such that when the elevons (42) are disposed in their maximum angled up and down states relative to the hydrofoil (41) at low loads in the anchor cable (6), low measured strain in the supporting structure or low flow velocities, the fin (4) may pitch to such a degree that the hydrofoil (41) operates in a beyond-stall angle of attack regime that is characterised by a high-lift unsteady separated vortex flow across the hydrofoil (41). The flapping mechanism may be configured (e.g. by appropriate selection of foil portion (41 A, 4 IB) sweep and of stroke amplitude) such that leading edge vortices are created at the start of each up- and downstroke, and the stroke may be timed to match the duration of the vortex formation, growth, and shedding process. Stroke reversal may be triggered by the bistable toggle mechanism (80) as the leading edge vortex is shed. Travel of the leading edge vortex towards the trailing edge (46) as it is shed may aids in rotating the fin (4) ready for the start of the next stroke.

[0181] When the load in the anchor cable (6), strain measured in the supporting structure or flow velocity increases, the maximum pitch angle of the fin (4) may be reduced by controlling the angle of the elevons (42) as described herein, so that the hydropower generator (1) transitions from separated to attached-flow fluid dynamics, which deliver lower lift but a much higher lift to drag ratio. This allows anchor cable or supporting structure loads to be maintained within designed limits as the anchor load, supporting structure strain or flow velocity increases, while still extracting useful power. As described above, the angle of the elevons (42) may be continually reduced until the centreline, Ce, of each elevon (42) is aligned with the centreline, Cf, of its respective foil portion (41 A, 4 IB) in the neutral state, in which state the fin (4) no longer generates lift, and only suffers drag owing to surface friction. This configuration, with the hydrofoil (41) and the elevons (42) feathered to align with the flow (F), affords minimum drag for storm-survival, or when the supporting structure for the generator (1) has to be moved.

[0182] As described above, while the present example implementation measures the load in the anchor cable (6) to regulate automatically the angle of attack of the fin (4) and amplitude of it oscillatory motion through the water, in other implementations, the angle of attack of the fin (4) and / or plunge amplitude may be adjusted according to strain measured in the supporting structure or the flow velocity of the water itself. Utilising the load in the anchor cable (6) may be especially germane for a hydropower generator (1) according to the present disclosure that is mounted to a moored floating platform such for example as the yacht (2) of the present example implementation or a barge or the like, but measuring strain in the supporting structure may also be applicable for fixed supporting structures such as towers, pylons and the like. Both arrangements allow the drag on the fin (4) to be modulated according to the total loads or strain on the supporting structure, not just those that result from the drag on the fin (4), but using a direct measurement of flow speed may also be useful for hydropower generators according to the present disclosure that are mounted to a freely floating supporting structure, such for example as a sailing or motor vessel, or ship, underway.

[0183] Whilst the present disclosure has been described and illustrated with reference to particular example implementations, it will be appreciated by those skilled in the art that the present disclosure lends itself to many different variations not specifically illustrated herein. In particular, the abovedescribed example implementation comprises a specific bistable toggle mechanism (80) and a specific governor (21). However, it is envisaged that any bistable toggle mechanism may be employed that automatically inverts the orientation of the elevons (42) at the end of each stroke to cause automatic reversal of the direction of motion of the fin (4). Any governor may be employed that actively or passively adjusts the maximum angle of the elevons (42) and / or the amplitude of each stroke according to the load measured in the anchor cable (6), the strain measured in the supporting structure or the measured flow velocity of the water over the fin (4). The governor of the above-described example implementation utilises a controller (23) and servo drive (22) to drive the governor control ring (22), but in other implementations, a completely passive governor may be used that, for example, transmits force from the anchor cable mechanically to the governor for setting the angle of the elevons (42) according to the size of the transmitted force. In this connection, a force in a snubber or similar device connected in the anchor cable (6) may be used to drive a mechanical governor arrangement.

[0184] Although aspects of the present disclosure have been described with reference to particular example implementations, it is to be understood that these example implementations are merely illustrative of the principles and applications of the disclosure. It is therefore to be understood that numerous modifications may be made to the example implementations and that other arrangements may be devised without departing from the scope of the present disclosure as defined by the appended claims.

[0185] It will be appreciated by those skilled in the art that features of the example implementations may be combined in other implementations that fall within the scope of the present disclosure.

[0186] While various details have been set forth in the foregoing description, it will be appreciated that the various aspects of the hydropower generator may be practised without these specific details. Those skilled in the art will recognise that the herein described components, devices, objects, and the discussion accompanying them are used as examples for the sake of conceptual clarity and that various configuration modifications are contemplated. Consequently, as used herein, the specific exemplars set forth and the accompanying discussion are intended to be representative of their more general classes. In general, use of any specific exemplar is intended to be representative of its class, and the non-inclusion of specific components, devices, and objects should not be taken limiting.

[0187] Further, while several forms have been illustrated and described, it is not the intention of the applicant to restrict or limit the scope of the appended claims to such detail. Numerous modifications, variations, changes, substitutions, combinations, and equivalents to those forms may be implemented and will occur to those skilled in the art without departing from the scope of the present disclosure. Moreover, the structure of each element associated with the described forms can be alternatively described as a means for providing the function performed by the element. Also, where materials are disclosed for certain components, other materials may be used. It is therefore to be understood that the foregoing description and the appended claims are intended to cover all such modifications, combinations, and variations as falling within the scope of the disclosed forms. The appended claims are intended to cover all such modifications, variations, changes, substitutions, modifications, and equivalents.

[0188] Whilst in the foregoing description, integers or elements are mentioned which have known obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as advantageous, convenient or the like are optional, and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some implementations of the disclosure, may not be desirable and may therefore be absent in other implementations.

[0189] The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases.

[0190] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified.

[0191] The terms “approximately” and “about” may be used to mean within ±20% of a target value in some implementations of the present disclosure, within ±10% of a target value in some implementations, within ±5% of a target value in some implementations, and yet within ±2% of a target value in some implementations. The terms “approximately” and “about” may include the target value.

[0192] In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. The transitional phrases “consisting of’ and “consisting essentially of’ shall be closed or semi-closed transitional phrases, respectively.

[0193] Where a range or list of values is provided, each intervening value between the upper and lower limits of that range or list of values is individually contemplated and is encompassed within the disclosure as if each value were specifically enumerated herein. In addition, smaller ranges between and including the upper and lower limits of a given range are contemplated and encompassed within the disclosure. The listing of exemplary values or ranges is not a disclaimer of other values or ranges between and including the upper and lower limits of a given range.

[0194] The use of headings and sections in the application is not meant to limit the disclosure; each section can apply to any aspect, implementation, or feature of the disclosure. Only those claims which use the words “means for” are intended to be interpreted under 35 USC 112, sixth paragraph, and even then, only in the United States of America. Absent a recital of “means for” in the claims, such claims should not be construed under 35 USC 112. Outside the United States, the words “means for” are intended to have their natural meaning. Uimitations from the specification are not intended to be read into any claims unless such limitations are expressly included in the claims.

Claims

1. A fluid flow-driven generator (1) for generating energy from a flow of liquid or gas, which fluid flow-driven generator (1) comprises a fluid-driven device (3, 4) comprising a mounting (75) for fixing the fluid-driven device (3, 4) to a supporting structure (2) that is positioned in or adjacent a fluid flow (F) and at least one movable fluid foil (41) having an adjustable angle of attack that is configured to be driven by the fluid flow (F); an electric generator (12); a power take-off mechanism (31, 32) that is operably connected between the fluid-driven device (3, 4) and the electric generator (12) such that motion of the fluid-driven device (3, 4) drives the electric generator (12); and a governor (21) being configured and arranged for controlling the angle of attack of the fluid foil (41); the governor (21) comprising a governor mechanism (25) for adjusting the angle of attack of the fluid foil (41), a sensor (24) for sensing a magnitude of a dynamic environmental or structural factor that is indicative of a dynamic load on the fluid-driven device (3, 4), the supporting structure (2), or a positioning system (5, 6) for the supporting structure (2) when the fluid-driven device (3, 4) is in use, and a governor actuator (22, 23) for operating the governor mechanism (25) to reduce the angle of attack of the fluid foil (41) according to the sensed magnitude of the dynamic environmental or structural factor, thereby to moderate the drag on the fluid foil (41) in the fluid flow.

2. The fluid flow-driven generator (1) of claim 1, the governor actuator (22, 23) being configured to reduce progressively the angle of attack of the fluid foil (41) according to the sensed magnitude of the the dynamic environmental or structural factor between a first lower threshold and a second upper threshold; the angle of attack being maximal when the sensed magnitude is below the first lower threshold (TL) and being minimal when the sensed magnitude is above the second upper.

3. The fluid flow-driven generator (1) of claim 1 or claim 2, the fluid-driven device (3, 4) comprising a fluid-driven flapping mechanism that is configured to be driven by a fluid flow (F) to undergo a reciprocal pitch-plunge motion, thereby to drive the power take-off mechanism (31, 32); the governor (21) being configured to control the angle of attack of the fluid foil (41) according to the sensed magnitude of the dynamic environmental or structural factor, thereby to modulate the wavelength of the pitch-plunging motion of the flapping mechanism in the fluid flow.

4. A fluid flow-driven generator (1) for generating energy from a flow of liquid or gas, which fluid flow-driven generator comprises a fluid-driven device comprising a mounting (75) for fixing the fluid-driven device to a supporting structure (2) that is positioned in or adjacent a fluid flow, and fluid flow flapping mechanism (3, 4) that is configured to be driven by the fluid flow (F) to undergo a reciprocal pitch-plunge motion; an electric generator (12); a power take-off mechanism (31, 32) that is operably connected between the fluid-driven flapping mechanism (3, 4) and the electric generator (12) such that motion of the fluid-driven flapping mechanism (3, 4) drives the electric generator (12); and a governor (21) for controlling the amplitude of the reciprocal pitch-plunge motion of the fluid-drivenflapping mechanism (3, 4); the governor (21) comprising a governor mechanism (25) for adjusting the amplitude of the reciprocal pitch-plunge motion, a sensor (24) for sensing a magnitude of a dynamic environmental or structural factor that is indicative of a dynamic load on the fluid-driven device, the supporting structure (2), or a positioning system (5, 6) for the supporting structure (2), when the fluid-driven device is in use, and a governor actuator (22, 23) for operating the governor mechanism (25) to reduce the amplitude of the reciprocal pitch-plunge motion according to the sensed magnitude of the dynamic environmental or structural factor, thereby to moderate the drag on the fluid foil (41) in the fluid flow (F).

5. The fluid flow-driven generator (1) of claim 4, the governor actuator (22, 23) being configured to reduce progressively the amplitude of the reciprocal pitch-plunge motion according to the sensed magnitude of the dynamic condition between a first lower threshold and a second upper threshold; the amplitude of the reciprocal pitch-plunge motion being maximal when the sensed magnitude is below the first lower threshold (TL) and being minimal when the sensed magnitude is above the second upper threshold.

6. A fluid-driven flapping mechanism for a hydropower generator (1) comprising a mounting (75) for fixing the flapping mechanism to a supporting structure (2) that is positioned in or adjacent a body of flowing water (10), a fin (4) having a hydrofoil (41) and at least one elevon (42), a mechanical linkage (3, 35, 43) that is attached at one end (37) to the mounting (75) and at another end (38) to the fin (4) and is configured and arranged to hold the fin (4) in the water when attached to the supporting structure (2) and to permit and constrain movement of the fin (4) relative to the mounting (75) to a reciprocal plunging motion across the direction of the flow (F), and an elevon angle control mechanism (21, 50, 55, 80); the fin (4) being pivoted to the mechanical linkage (3, 35, 43) at a fulcrum (43) such that it can pitch freely about the fulcrum (43) in two opposite directions relative to the direction of the flow (F), the hydrofoil (41) having two opposite surfaces that are each shaped to generate lift (L) in the flow (F) when the fin (4) is pitched in either direction, and the at least one elevon (42) being pivoted to the hydrofoil (41) for movement between two angled states, in each of which angled states the elevon (42) is angled towards a respective opposite surface of the hydrofoil (41); the elevon angle control mechanism (50, 55, 80) being configured to retain the at least one elevon (42) in different respective ones of the angular states during opposite strokes of the flapping mechanism, and to pivot the elevon (42) to the other angled state when the mechanical linkage (3, 35, 43) is moved by the fin (4) in either direction relative to the mounting (75) past an actuation point; the arrangement being such that when the at least one elevon (42) is held in one of its angled states by the elevon angle control mechanism (50, 55, 80) in a flow of water (F), the elevon (42) generates a turning moment on the fin (4), causing the fin (4) to pitch in one direction about the fulcrum (43) until it reaches an equilibrium position in which the resulting lift (L) produced by the hydrofoil (41) generates an equal and opposite turning moment about the fulcrum (43), the lift (L) causing the fin (4) to plunge on the mechanical linkage (3, 35, 43) in one direction relative tothe mounting (75), thereby driving motion of the mechanical linkage (3, 35, 43) in the one direction relative to the mounting (75) until it moves beyond the actuation point, whereupon the elevon angle control mechanism (50, 55, 80) pivots the elevon (42) to its other angled state, thereby inverting the pitch of the fin (4) in the flow (F), the resulting lift (L) causing the fin (4) to plunge in the opposite direction relative to the mounting (75); whereby the mechanical linkage (3, 35, 43) is caused to undergo reciprocal motion relative to the mounting (75).

7. The fluid-driven flapping mechanism of claim 6, the mechanical linkage (3, 35, 43) being operably connected to the elevon angle control mechanism (50, 55, 80), thereby to operate the elevon angle control mechanism (50, 55, 80) automatically to invert the angle of the at least one elevon (42) when the mechanical linkage (3, 35, 43) moves past the actuation point in either direction, during each stroke of the flapping mechanism.

8. The fluid-driven flapping mechanism of claim 7, the elevon angle control mechanism (21, 50, 55, 80) incorporating a bistable toggle mechanism (80) that is operably connected between the at least one elevon (42) and the mounting (75) and is arranged to be operated by the mechanical linkage (3, 35, 43); the bistable toggle mechanism (80) defining the actuation point and being configured to switch automatically between first and second stable configurations when it is moved in either direction past the actuation point by the mechanical linkage (3, 35, 43), thereby to pivot the elevon (42) between its angled states.

9. The fluid-driven flapping mechanism of claim 8, the bistable toggle mechanism (80) comprising an input toggle member (105) that is fixed relative to the mounting (75), an output toggle member (115) that is drivably connected to the at least one elevon (42), and a toggle biasing device (120) that acts between the input and output toggle members (105, 115) to bias the output toggle member (115) bistably into first and second positions relative to the input toggle member (105), corresponding respectively to the first and second stable configurations of the bistable toggle mechanism (80); the output toggle member (115) being movable with respect to the input toggle member (105) by the mechanical linkage (3, 35, 43) through an overcentre position that defines the actuation point.

10. The fluid-driven flapping mechanism of claim 9, the output toggle member (115) being arranged to pivot the at least one elevon (42) in one direction relative to the hydrofoil (41) against an elevon biasing device (55) that is arranged to bias the elevon (42) to pivot in an opposite direction.

11. The fluid-driven flapping mechanism of claim 9 or claim 10, the mechanical linkage (3, 35, 43) being arranged to drive the output toggle member (115) through a bidirectional lost motion mechanism (94, 95, 118); the arrangement being such that when the output toggle member (115) passes through the actuation point in either direction, its movement to the first or second position is lost relative to the mechanical linkage (3, 35, 43), thereby determining the amplitude of each stroke.

12. The fluid-driven flapping mechanism of any of claims 6-11, further comprising a governor mechanism (25) for controlling the degree of movement of the mechanical linkage (3, 35, 43) relative to the mounting (75) during each stroke before the actuation point is reached, thereby to control the amplitude of each stroke.

13. The fluid-driven flapping mechanism of any of claims 6-11, further comprising a governor mechanism (25) for controlling the maximum angle of the at least one elevon (42) in each angled state during each stroke before the actuation point is reached, thereby to control the angle of attack of the fin (4).

14. The fluid-driven flapping mechanism of claim 11, further comprising a governor mechanism (25) configured and arranged for varying the separation of the first and second positions of the output toggle member, thereby to control the degree of movement of the mechanical linkage (3, 35, 43) relative to the mounting (75) during each stroke prior to the actuation point, thereby to control the amplitude of each stroke.

15. The fluid-driven flapping mechanism of claim 14, the governor mechanism (25) being configured to adjust the degree of lost motion of the output toggle member (115) relative to the mechanical linkage (3, 35, 43) that is permitted by the birectional lost motion mechanism, thereby to adjust the separation of the first and second positions.

16. The fluid-driven flapping mechanism of any of claims 6-15, the elevon angle control mechanism (50, 55, 80) being configured to set the maximum angle of the at least one elevon (42) relative to the hydrofoil (41) in each angled state, thereby to determine the maximum angle of attack of the fin (4) during each stroke.

17. The fluid-driven flapping mechanism of claim 15, the output toggle member (115) being operably connected to the at least one elevon (42) such the maximum angle of the elevon (42) relative to the hydrofoil (41) in each angled state is determined by the location of the respective one of the first and second positions of the output toggle member (115); whereby operation of the governor mechanism (25) to adjust the degree of lost motion of the output toggle member (115) allowed by the bidirectional lost motion mechanism after it has passed the actuation point in either direction correpondingly varies the maximum angle of the elevon (42) relative to the hydrofoil (41) in each angled state, thereby controlling the maximum angle of attack of the fin (4) during each stroke.

18. The fluid-driven flapping mechanism of claim 11, claim 14, claim 15, or claim 17, the bidirectional lost motion mechanism (94, 95, 118) comprising a flange (94) that is drivably connected to the mechanical linkage (3, 35, 43), and an output toggle limit pin (118) that is attached to the output toggle member (115); the flange (94) having a slot (95) therein, and the output toggle limit pin (118) being arranged to move in the slot (95) to afford lost motion of the output toggle limit pin (118) relativeto the mechanical linkage (3, 35, 43); the arrangement being such that the length of the slot (95) determines the first and second positions of the output toggle member (115).

19. The fluid-driven flapping mechanism of claim 18, the governor mechanism (25) comprising two opposing toggle limit jaws (141A, 14 IB) juxtaposed the slot (95), which are operable to control the length of a portion of the slot (95) along which the toggle limit pin (118) can move.

20. The fluid-driven flapping mechanism of any of claims 9-11, 14-15 or 17-19 , the output toggle member (115) being operably connected to the at least one elevon (42) such that movement of the output toggle arm (115) before it reaches the actuation point causes or allows corresponding movement of the elevon (42) in each angled state, such that plunging motion of the mechanical linkage (3, 35, 43) during each stroke causes or allows the elevon angle control mechanism (50, 55, 80) progressively to decrease the angle of the elevon (42) relative to the hydrofoil, thereby to maintain a stable angle of attack of the fin (4).

21. The fluid-driven flapping mechanism of any of claims 6-20, the mechanical linkage (3, 35, 43) comprising a swing arm (3) having a hinge assembly (35) at one end (37) for attachment to the mounting (35) and the fin (4) pivoted to the other end (38) thereof.

22. The fluid-driven flapping mechanism of any of claims 12-15 or 17-19, further comprising a sensor (24) for sensing the magnitude of a dynamic environmental or structural factor that is indicative of the load on the flapping mechanism, or the strain in the supporting structure (2) or a positioning system (5, 6) for the supporting structure, when the flapping mechanism is in use; and a governor actuator (22, 23) for operating the governor mechanism (25) according to the sensed magnitude of the dynamic environmental or structural factor.

23. The fluid-driven flapping mechanism of claim 22, the sensor (24) being adapted and arranged to sense a load in a positioning system (5, 6) for the supporting structure (2), where the supporting structure is a floating vessel or platform.

24. The fluid-driven flapping mechanism of claim 22 or claim 23, the governor actuator (22, 23) being configured to operate the governor mechanism (25) to control the amplitude of the plunging motion of the mechanical linkage (3, 35, 43) according to the sensed magnitude of the dynamic environmental or structural factor.

25. The fluid-driven flapping mechanism of claim 22, claim 23 or claim 24, the governor actuator (22, 23) being configured to operate the governor mechanism (25) to control the maximum angle of attack of the fin (4) according to the sensed magnitude of the dynamic environmental or structural factor.

26. A hydropower generator (1) comprising the fluid-driven flapping mechanism of any of claims 6-25, an electric generator (12) and a power take-off mechanism (31, 32) that is operably connectedbetween the mechanical linkage (3, 35, 43) and the electric generator (12) such that reciprocal motion of the mechanical linkage (3, 35, 43) drives the electric generator (12).

27. The fluid flow-driven generator (1) of any of claims 1-3, the fluid-driven device (3, 4) comprising the fluid-driven flapping mechanism of any of claims 6-25.5 28. The fluid flow-driven generator (1) of claim 4 or claim 5, the fluid-driven flapping mechanism (3,4) comprising the fluid-driven flapping mechanism of any of claims 6-25.

29. The fluid flow-driven generator (1) of any of claims 1-5, 27 or 28, the sensor (24) being adapted for sensing the magnitude of the strain in the supporting structure (2), or the load in a positioning system (5, 6) for the supporting structure (2).10 30. The fluid flow-driven generator (1) of any of claims 1-5, or 27-29, the sensor (24) comprising amechanical sensor.

31. The fluid flow-driven generator (1) of any of claims 1-5 or 27-30, the sensor (24) comprising an electro-mechanical sensor that is adapted to measure the dynamic environmental or structural factor and to output a signal representing the magnitude of the dynamic environmental or structural factor; the15 governor actuator (22, 23) comprising a controller (23) that is configured to receive the signal and to operate the governor mechanism (25) according to the magnitude of the dynamic environmental or structural factor.A

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