WO2018226152A1

WO2018226152A1 - Power take off device comprising a variable transmission for use in a wave energy converter

Info

Publication number: WO2018226152A1
Application number: PCT/SE2018/050599
Authority: WO
Inventors: Mikael Sidenmark
Original assignee: Ocean Harvesting Technologies Ab
Priority date: 2017-06-09
Filing date: 2018-06-09
Publication date: 2018-12-13

Abstract

A power take-off device for use in a wave energy converter comprises a buoy, a drive unit (10) comprising a rotary input shaft (12), and a variable transmission means (20) including an energy storage device (22) connected to the rotary input shaft, and a means for converting linear motion from the movements of the buoy (1) into rotary motion, connected to the rotary input shaft (12) of the drive unit (10). The variable transmission means (20) is adapted to control the amplitude of power stored and retrieved to/from the energy storage device (22) and the amplitude and direction of the force applied to the buoy from the drive unit (10).

Description

POWER TAKE OFF DEVICE COMPRISING A VARIABLE

TRANSMISSION FOR USE IN A WAVE ENERGY CONVERTER

Technical field

[0001 ] The present invention relates generally to wave energy conversion and more particularly to a power take-off device (PTO) with features to provide reactive force control in a more economical way compared to prior art.

Background art

[0002] Different types of wave energy converters (WEC) have been proposed, in which a power take-off is used for applying a force to the buoy to capture power from the waves and to control the phase of the buoy to be resonant with the waves.

[0003] Different control techniques for the PTO force have been proposed. A so called reactive control strategy is known to provide the optimal PTO force in order to extract close to the maximum power from the waves, but is not easy to implement in a real system due to high cost and high losses in the typical power take-off system. With the reactive control strategy, the PTO force applied to a point absorbing wave energy converter can be described as the sum of two different components, one damping component and one reactive component, resembling the effect of a combined mass and hydrostatic stiffness variation on the point absorber that controls the resonance frequency of the buoy. The ideal case (optimum control) would be to use the reactive component in the control to exactly match the natural frequency of the system with the frequency of the incoming wave, or said in another way, to eliminate the phase difference between buoy velocity and wave excitation force.

[0004] One challenge with reactive control is that power flows back and forth through the PTO in every wave cycles, causing in the order of 100 million high and short power pulses in both directions through the PTO in a 20-25-year design life. The amplitude of each power pulse varies but can easily exceed 10 times the average output power. Prior art has not shown solutions that can provide such reactive control in a cost-effective way. Summary of invention

[0005] An object of the present invention is to provide a power take-off for wave energy converters (WEC) which can provide reactive control in a cost effective way by using the same principle as a kinetic energy recovery system (KERS) used in vehicles to recover brake energy, wherein the amplitude of power stored and retrieved to/from an energy storage device and the amplitude and direction of the force applied to the buoy from the power take-off can be controlled with full flexibility to provide advanced force control to increase power output and control the buoy motion. A second objective of the present invention is to connect an active power transfer device to the KERS in order to output constant and/or time shifted power by means of energy stored in the energy storage device.

[0006] According to a first aspect of the invention a power take-off device for use in a wave energy converter is provided comprising a buoy, a drive unit comprising a rotary input shaft, and a variable transmission means including an energy storage device connected to the rotary input shaft, a means for converting linear motion from the movements of the buoy into rotary motion, connected to the rotary input shaft of the drive unit, which is characterized in that the variable transmission means is adapted to control the amplitude of power stored and retrieved to/from the energy storage device and the amplitude and direction of the force applied to the buoy from the drive unit.

[0007] In a preferred embodiment, the buoy is provided with a buoy hull to be put into motion by motions of water in which the buoy is placed.

[0008] In a preferred embodiment, the drive unit is arranged inside the buoy.

[0009] In a preferred embodiment, the rotary input shaft of the drive unit is connected to a pinion.

[0010] In a preferred embodiment, the drive unit 10) comprises an active power transfer device in the form of at least one of a hydraulic pump and an electric generator. [001 1 ] In a preferred embodiment, the amplitude of power stored and retrieved to/from the energy storage device and the amplitude and direction of the force applied to the buoy from the drive unit are controlled by changing the conversion ratio, preferably a gear ratio, in the variable transmission means.

[0012] In a preferred embodiment, the variable transmission means comprises a first electric generator/motor with variable speed drive interconnected with an energy storage device in the form of an electric battery, and a power output cable.

[0013] In a preferred embodiment, the energy storage device is a flywheel.

[0014] In a preferred embodiment, an active power transfer device is provided comprising any of a hydraulic pump and an electric motor connected to the flywheel.

[0015] In a preferred embodiment, active power transfer device is provided after the variable transmission means, as seen from the input shaft of the drive unit, wherein the active power transfer device during operation is adapted to output an essentially constant sea state tuned power, while the conversion ratio of the variable transmission means is used for controlling the total force applied by the power take-off device on the buoy.

[0016] In a preferred embodiment, the drive unit comprises a variable

transmission means in the form of an infinitely variable transmission connected to the input shaft of the drive unit, wherein the infinitely variable transmission is adapted to provide a positive gear ratio when the input shaft rotates forward and to apply a negative gear ratio applied when the input shaft 12) rotates backwards, whereby a uni-directional rotation of the flywheel is provided.

[0017] In a preferred embodiment, the infinitely variable transmission comprises a mechanical variator, preferably a toroidal variator coupled to an epicyclic gear stage, in which the gear ratio between input and output shaft of the infinitely variable transmission is adapted to be controlled infinitely between forward and reverse ratios by changing the angle of rollers between two discs and thereby the contact radius on each disc. [0018] In a preferred embodiment, the infinitely variable transmission comprises a hydrostatic variator with at least one first hydraulic pump/motor connected to the input shaft and at least one second hydraulic pump/motor connected to the output shaft, wherein all first and second pumps/motors are interconnected by means of a hydraulic circuit, and at least one of the first and second hydraulic pumps/motors has variable displacement, preferably variable reversible displacement.

[0019] In a preferred embodiment, the infinitely variable transmission comprises an electric variator with at least one first electric generator/motor connected to the input shaft and at least one second electric generator/motor connected with the output shaft, wherein all first and second electric generators/motors are connected with variable speed drives interconnected with a common electric circuit, whereby the conversion ratio between an input shaft and an output shaft of the electric variator is infinitely controllable from forward to reverse gear ratios by changing the amplitude and direction of the torque applied on the input shaft by the first generator/motor while the second generator/motor is controlled to provide charge and discharge power corresponding to the control force.

[0020] In a preferred embodiment, a plurality of first electric generators/motors are provided, each connected to a separate input shaft.

[0021 ] In a preferred embodiment, a plurality of second electric

generators/motors are provided, each connected to a separate output shaft .

[0022] In a preferred embodiment, a continuously variable transmission is provided after the infinitely variable transmission as seen from the input shaft (12) of the drive unit.

[0023] In a preferred embodiment, the active power transfer device is located before the variable transmission means, as seen from the input shaft, wherein the active power transfer device applies, during operation, a damping force of variable amplitude to extract power from the waves which is converted into hydraulic or electric power, and the variable transmission and the flywheel apply, during operation, a reactive force with variable amplitude to control the phase of the buoy. [0024] In a preferred embodiment, the variable transmission means, during operation, is adapted to apply a pre-tensioning force on a mooring rope indirectly connected to the input shaft, in addition to the reactive and damping forces, wherein the total force applied to the mooring rope by the power take-off device is uni-directional.

[0025] In a preferred embodiment, the means for converting linear motion into rotary motion of the wave energy converter comprises an internal gear, wherein a plurality of pinions meshing with the internal gear is connected to a respective input shaft 12) of a drive unit.

[0026] In a preferred embodiment, the means for converting linear motion from the movements of the buoy into rotary motion is further connected to a fixed point of reference for the linear motion.

[0027] According to a second aspect of the invention, a method of controlling a power take-off device according to the invention is provided, comprising the following steps: a) detecting the nature of incoming waves, preferably the frequency and amplitude of incoming waves, b) determining the optimal PTO force to capture the highest amount of energy from the incoming waves, c) applying the determined PTO force with a limited maximum amplitude.

[0028] In a preferred embodiment, the step a) is performed by using weather forecasting to determine the current sea state, from which a sinusoidal wave is determined with fixed amplitude and frequency.

[0029] In a preferred embodiment, the step a) is performed by a vision system to detect, trace and calculate a trajectory for all waves moving towards the WEC, whereby the movement of the water in the location of the WEC can be calculated by super imposing the incoming wave trajectories.

[0030] In a preferred embodiment, the step b) comprises determining the total PTO force by satisfying the following equations: the absorbed power from the power take-off given by:

where m₆ - buoy mass i_rx - added mass at infinity frequency ω - wave frequency

X - PTO reactive

R - PTO damping

i?„ - radiation damping s - hydrostatic stiffness

F„ - excitation force wherein the power attains maximum when:

A' =— <»<m_fe 4- « —

[0031 ] The method according to claim 22-23 and 25 wherein the step b) comprises determining the PTO damping component by a polynomial function, preferably multiplying the buoy velocity with the cube of a constant, where the constant is preferably tuned to the sea state.

[0032] The method according to any one of claims 22-26, wherein the step c) comprises shifting the phase and reducing the amplitude of the PTO force relative to the optimality condition in order to reduce recirculating power flows and losses to the point where the power capture starts to reduce faster than the losses, thereby maximizing the energy output. [0033] In a preferred embodiment, the step c) comprises shifting the phase of the PTO force relative to the optimality condition to also limit the height and velocity of the buoy motion.

[0034] In a preferred embodiment, the step c) comprises shifting the phase of the PTO force to also limit the energy capture.

[0035] In a preferred embodiment, the step c) is performed by applying a continuous damping force component according to Ropt with the active power transfer device, and by applying a continuous reactive force component according to Xopt by means of the IVT and flywheel.

[0036] In a preferred embodiment, the step c) is performed by applying a constant average damping force component according to Ropt for each stroke of the buoy with the active power transfer device, and by applying a continuous reactive force component according to Xopt by means of the IVT and flywheel.

[0037] In a preferred embodiment, the step c) is performed by applying a constant average damping force component according to Ropt for the sea state with the active power transfer device, and by applying a compensating damping force component to provide a total continuous damping force component according to Ropt and a continuous reactive force component according to Xopt by means of the IVT and flywheel.

[0038] In a preferred embodiment, the step c) is performed by continuously controlling the damping force component applied from the active power transfer device relative to the speed in order to transfer a constant sea state tuned power, and by applying a total PTO force to satisfy Ropt and Xopt by means of the IVT and flywheel.

[0039] In a preferred embodiment, the step c) is performed by a power take-off comprising multiple drive units where the number of active power transfer devices engaged with the system us used to control the total damping force provided to transfer power from the wave energy converter. [0040] In a preferred embodiment, a model predictive control is used to determine the power take-off force according to the optimality condition while taking losses into account and with constraints, preferably for the maximum force, stroke length and velocity, to allow in the power take-off.

Brief description of drawings

[0041 ] The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 a is a schematic view of a drive unit comprising a KERS and a

pump/generator;

Fig. 1 b shows a drive unit similar to that of Fig. 1 a with a disc brake and clutch added to the input shaft to the KERS;

Fig. 1 c shows the drive unit similar to that of Fig. 1 a with a shuttling gear rectifier device added before the KERS;

Fig. 2a shows a KERS comprising an IVT and a flywheel energy storage device;

Fig. 2b shows a KERS similar to that of Fig. 2a with an epicyclic gear added between the KERS and the flywheel storage device;

Fig. 2c shows a KERS similar to that of Fig. 2a with a CVT added between the KERS and the flywheel storage device;

Fig. 3 shows the schematics of a CVT based on a single cavity double roller toroidal variator;

Fig. 4a shows the schematics of an IVT system with an epicyclic gear added to the CVT shown in Fig. 3;

Fig. 4b shows a schematic view of an IVT system based on a twin cavity double roller toroidal variator; Fig. 4c shows a schematic view of an IVT system based on a twin cavity single roller toroidal variator;

Fig. 4d shows a schematic view of a hydrostatic IVT system.

Fig. 4e shows a schematic view of an electric IVT system.

Fig. 4f shows a schematic view of an electric IVT system with multiple input shafts and a single output shaft.

Fig. 4g shows a schematic view of an electric IVT system with a single input shaft and multiple output shafts.

Fig. 4h shows a schematic view of an electric IVT system with a multiple input shaft and multiple output shafts.

Fig. 5a shows the schematic view of a flywheel energy storage device;

Fig. 5b shows a flywheel energy storage device similar to that of Fig. 5a with an epicyclic gear added between the input shaft and the flywheel;

Fig. 5c shows a flywheel energy storage device similar to that of Fig. 5b with a primary flywheel added to the ring gear of the epicyclic gear and a clutch between the carrier shaft and ring shaft;

Fig. 5d shows a flywheel energy storage device with integrated electric motor.

Fig. 6a shows a drive unit comprising a shuttling gear according to Fig. 1 c, an IVT according to Fig. 4b and a epicyclic gear before the flywheel and pump/generator, with all components comprised in a common vacuum case;

Fig. 6b shows a drive unit comprising an electric KERS with an electric flywheel battery;

Fig. 6c shows a complete drive unit similar to the one of Fig. 6b but with an energy storage device comprising first and second counter rotating flywheels; Fig. 6d shows a drive unit comprising an electric KERS with an electric general battery;

Fig. 7a shows a side view of a power take-off device comprising a drive unit connected to a rack and pinion actuator;

Fig. 7b shows a side view of a power take-off device comprising a pair of drive units with pinions connected to a double-sided rack;

Fig. 7c shows three drive modules according to Fig. 6, mounted to three double sided racks in a triangular framed structure;

Figs. 7d and 7e show alternative configurations for a rack assembly;

Fig. 8a shows a side view of a power take-off device comprising a drive unit connected to a winch drum and rope/wire actuator;

Fig. 8b shows a side view of a power take-off according to Fig. 8a with the drive unit connected to the winch drum by means of an external gear;

Fig. 8c and 8d shows different side views of a power take-off according to Fig. 8a with multiple drive units connected to the winch drum by means of an internal gear;

Figs. 8e and 8f show a wave energy converter with an involute gear winch system comprising multiple drive units;

Fig. 9 shows a wave energy converter with a power take-off and with the rack leg extending to a universal joint at the seabed, and with elastic mooring ropes connected between the hull and the seabed;

Fig. 10 shows a similar WEC as Fig. 9 but with a shorter rack leg connected to the seabed through a mooring rope and without the elastic mooring ropes; and

Fig. 1 1 shows a Wave Energy Converter with an on-board hydraulic system. Description of embodiments

[0042] In the following, a detailed description of a power take-off device (PTO) for use in a Wave Energy Converter (WEC) according to the invention, comprising a kinetic energy recovery system to control the PTO force in combination with an active power transfer device to output constant and/or time shifted power, will be described in detail.

[0043] In this document, the term "Kinetic Energy Recovery System" or "KERS" refers to an arrangement, which provides a fully flexible PTO force in amplitude and direction by storing / retrieving power corresponding to this force to/from an energy storage device, preferably by means of an IVT system connected to a flywheel. The term "Continuous Variable Transmission" or "CVT" refers to a transmission with variable gear ratio with a typical range from reduction ratio 6: 1 to step up ratio 1 :6. The term "Infinite Variable Transmission" or "IVT" refers to a transmission with an infinitely variable gear ratio from full forward to full reverse and with torque transfer available across the full speed range for both input and output shafts independently from each other.

[0044] The optimal PTO force to capture maximum power from the waves can be derived by describing the dynamics of an unconstrained heaving wave energy converter as a simple mass-spring-damper system with excitation force input from the waves and control force input from the power take-off: »* 4- m_x)∑ -f- R_r ^■h S∑ = F_a— F^ where m_& - buoy mass

- added mass at infinity frequency R_r - radiation damping S - hydrostatic stiffness F_s - excitation force

F_?to - PTO force ζ, ζ, ζ - buoy heave displacement, velocity and acceleration respectively

[0045] The most general PTO force comprises of a damping and reactive part, where the damping force is in phase with the buoy velocity, i.e. the damping force always has the same direction and/or sign with the buoy velocity, and reactive force is 90 degrees out of phase with the buoy velocity, i.e. the force and velocity have the same direction during 2 quarters of a wave period and have opposite direction during the other 2 quarters. Therefore, the damping part of the PTO force constitutes the active power transfer from the power take-off, corresponding to the useful power recovered from the waves. The reactive force constitutes the reactive power, which goes back and forth between buoy and power take-off and is used for controlling the resonance frequency of the buoy. The general PTO force is represented as

= *έ +jx

[0046] Where the first term R is the damping force with and the second term X is the reactive force.

[0047] The absorbed power from the power take-off is given by

[0048] The power attains maximum when

[0049] The first equation is the phase or resonance condition which depend on the reactive PTO force component and the second equation is the amplitude condition which depend on the damping PTO force component. It should be noted that the optimal reactive force depends on the mass, hydrostatic stiffness and wave frequency and the optimal damping force depends only on the radiation damping when the phase condition is satisfied.

R = R_T

[0050] It should be realized that the resulting PTO force that satisfies the optimal phase and amplitude conditions is very large and causes high reciprocating power flows. It is not economical to build a PTO that can provide this force and power and, unless the PTO is extremely efficient, the losses will also be large in proportion to the active power transfer. The PTO force as well as stroke length and velocity can however be limited by having a PTO and control system that is capable of providing control of the PTO force to only partly satisfy the optimal phase and amplitude conditions.

[0051 ] The power take-off device and control method according to the invention, aims at using KERS technology to provide a fully flexible PTO force in both direction and amplitude, which can include all force components comprising reactive and active forces as well as a pre-tension force that is used to maintain a uni-directional force from the power take-off, enabling reactive control to be applied with a tether moored point absorbing WEC. By shifting the total PTO force slightly away from the optimal phase condition, the output power increases until the control force is shifted to the point where the power capture starts to reduce faster than the losses. The shift of the PTO force is also used for limiting the stroke length and velocity of the actuation system in the WEC as well as the maximum tether force applied by the PTO, i.e. the output power is optimized within selected constraints, preferably for stroke, velocity and force, to make the WEC more economical to build. These constraints limits power capture above the design sea state where the WEC produces its rated power. A model predictive controller is used to calculate the optimal PTO force taking into account the losses of the power take-off and the above-mentioned constraints.

[0052] A flywheel based KERS according to the invention uses gear ratio control of an IVT system connected to a flywheel to control the direction and amplitude of the force applied to the transmission, and thereby the storage and retrieval of energy to / from the flywheel, which will be further explained by way of example.

[0053] The change rate of the gear ratio in the IVT, relative to the velocity of the buoy, determines how fast a flywheel is accelerated or decelerated and thereby the amplitude and direction of the force applied to either pull or push the buoy in a given direction of motion. The KERS unit applies a pushing force on the buoy by reducing the gear ratio so that the speed of the flywheel reduces, whereby stored energy is dissipated from the flywheel. The KERS unit applies a pulling force on the buoy by increasing the gear ratio so that the speed of the flywheel increases, whereby energy is stored in the flywheel.

[0054] Using an IVT in the KERS unit enables the alternating direction of the rotary input shaft of the drive unit, which is proportional to the motion of the buoy, to be converted into a uni-directional speed for the flywheel which can be kept within a high speed interval. The rectification of the reciprocating rotary input motion is accomplished by changing between reverse and forward gear ratio of the IVT in the turning points of the buoy motion, i.e. a positive gear ratio is used in a first direction of the buoy movement and a negative gear ratio is used in the second direction of the buoy movement. The flywheel is kept spinning also when the pinion is not rotating by using the geared neutral state of the IVT. It is in this way possible to use a very high speed of the flywheel to store larger quantities of energy with a smaller flywheel at the same time as a PTO force can be provided instantly independently from the input speed and direction of rotation to the drive unit. The energy storage capacity can be selected to store energy over

consecutive waves. A typical KERS used in a 500 kW WEC uses a 500-1000 kg flywheel which is kept within the range of 10 000 - 20 000 rpm. This is sufficient to smooth captured power to a constant level in irregular waves. The storage capacity can be increased further to offer time shifting capabilities, which can increase revenues from sold electricity by adjusting the production of electricity to short term changes in demand for electricity. Frequency regulation is a typical service that can be provided.

[0055] A flywheel energy storage device is capable of cost effectively providing the high cycle life and high power rating required for reactive control. The storage capacity can also be increased as desired to provide the above-mentioned grid services without reducing the overall efficiency of the system.

[0056] An IVT system can be implemented with mechanical, hydrostatic or electric variators as will be shown by wave of example.

[0057] With a mechanical IVT system, preferably implemented with a toroidal variator coupled to an epicyclic gear stage, the gear ratio between input and output shaft of the infinitely variable transmission is adapted to be controlled infinitely between forward and reverse ratios by changing the angle of rollers between two discs and thereby the contact radius on each disc.

[0058] With a hydrostatic IVT system, the variator comprises a first hydraulic pump/motor and a second hydraulic pump/motor interconnected by means of a hydraulic circuit, wherein at least one of the first and second hydraulic pump/motor has variable displacement, preferably variable reversible displacement.

[0059] With an electric IVT system, the variator comprises a first electric generator/motor and a second electric generator/motor with variable speed drives interconnected with a common electric circuit, whereby the conversion ratio between an input shaft and an output shaft of the electric variator is infinitely controllable from forward to reverse gear ratios by changing the amplitude and direction of the torque applied on the input shaft by the first generator/motor while the second generator/motor is controlled to provide charge and discharge power corresponding to the control force. [0060] An active power transfer device in the form of a hydraulic pump or electric generator is connected to the flywheel and used to provide a constant output power from the WEC. In case of an electric generator, the torque is controlled to balance against the varying speed of the flywheel to provide a constant output. In case of a hydraulic pump, the displacement is changed for the same purpose.

[0061 ] A KERS can also be implemented with an electric battery, in which case the drive unit comprises an electric generator/motor connected to the input shaft which is interconnected with the electric battery, preferably lithium Ion or super capacitors, in which case the PTO force is controlled with torque control of the electric generator / motor in the same wave as in the case with an electric IVT system.

[0062] In of KERS with an electric battery, the active power transfer device can be incorporated in the power electronics interconnecting the electric

generator/motor, energy storage device and power output cable. This can be used also in a flywheel based KERS with an electric IVT, in which case the flywheel storage unit can be seen to include the electric motor and is equivalent with any other type of electric battery. This way of implementing the active power transfer device reduces the energy exchange with the flywheel since only excess power above the output is stored in the flywheel, as apposed to the case with the using an active power transfer device in the form of a separate electric generator connected to the flywheel.

[0063] The reactive part of the PTO force alternates between pulling and pushing on the buoy, resulting in an even exchange of energy between the power take-off and the buoy, which is provided by means of the flywheel energy storage in the KERS unit. The PTO force is bi-directional relative to the seabed. To use reactive control in a point absorber WEC with a single point tether mooring, which is more economical compared to a WEC with a float moving against a fixed structure that can counteract the PTO force in both directions, the PTO control force can be shifted by using a constant or variable pre-tension force with an amplitude that exceeds the control force, which enables reactive control to be applied while maintaining a uni-directional force in the PTO applied in the direction that pull the buoy down towards the mooring point. This way tension is always maintained in the tether mooring. The hydrodynamic interaction between buoy and the wave remains unchanged and only depend on the control force.

[0064] Different control strategies can be used to calculate the optimal PTO force for the highest power output from irregular waves. The parameters in the above-mentioned equations to calculate the optimal PTO force can be sea state tuned or calculated for each individual wave. In a sea state tuned controller, the reactive force is controlled with a combination of spring constant affecting the hydrostatic stiffness of the system, i.e. responds to the position of the buoy, and with an inertia constant effecting the effective mass of the system, i.e. responding to the acceleration of the buoy. The damping force component can be calculated with a fixed constant depending on the buoy velocity, preferably the cube of the velocity. A constant damping force can also be used. A sea state tuned controller has the advantage that it does not require any information about the incoming wave, the PTO responds only to the buoy motion. Constraints on the maximum force can be used and the control force can be shifted to reduce the reactive part of the control force and thereby the reciprocating power flows and losses to maximize output power.

[0065] A model predictive control strategy can be used to calculate the parameters for optimal PTO force individually for each wave, which increases the active power transfer from the system and also improves the control of the height and velocity of the buoy motion even with a limited PTO force, by finding the optimal phase shift of the control force to satisfy the desired amplitude and velocity of the buoy motion for every individual wave. The pre-tension offset can be tuned by the sea state or calculated on a wave-by-wave basis to minimize losses. The power take-off according to the invention can also benefit from other control strategies, e.g. based on neural networks, machine learning and Artificial

Intelligence (Al). [0066] Advanced control algorithms with capabilities to optimize the control force for each individual wave requires information about the incoming wave in advance of time. This is provided by using a combination of a vision system, preferably of the type Light Imaging Detection and Ranging (LIDAR), and wave prediction algorithms. LIDAR uses laser beams to build a real time 3D image of the

surroundings, which is a technology widely used in autonomous vehicles. Applied in wave energy, algorithms are used to analyze the 3D image feed to detect, trace and calculate trajectories for all waves moving towards the WEC. This information is then used to calculate the water movements at the location of the WEC by super imposing all wave trajectories.

[0067] Fig. 1 a shows a general view of a drive unit 10 comprising an input shaft 12, a KERS unit 20 and an active power transfer device 30 in the form of a pump/generator, as a general component that can be used in different WEC systems together with different actuation devices as will be described in further detail.

[0068] Fig. 1 b shows a similar drive unit according to Fig. 1 a with the addition of a brake 14 and a clutch 16, where the brake 14 is used to lock the position of a linear actuator when there is no force applied from the drive unit, and where the clutch 16 is used to disconnect the KERS unit from the linear actuator. The disconnection by the clutch can be triggered when the torque exceeds a limit and this can be done through passive mechanical mechanisms for torque overload protection or by means of the control system using a torque sensor as input.

[0069] Fig. 1 c shows a similar drive unit according to Fig. 1 a with a shuttling gear rectifier mechanism 18 added between the input shaft 12 and the KERS unit 20. The shuttling gear 18 comprises two epicyclic gear stages 18a, 18b with disk brakes 18c mounted to each ring gear. Both sun gears are connected to the KERS unit 20 by means of a common sun shaft 18d. Both planet carriers are connected to a common input shaft 12. The first gear stage 18a comprises single planets and the second gear stage 18b comprises double planets. When only the disc brake 18c to the first stage is locked, the first epicyclic gear stage is active, applying a gear ratio between the input and output shaft of the shuttling gear and maintaining the same direction of rotation on both shafts. When only the disc brake 18c to the second stage is locked, the second epicyclic gear stage is active, applying a gear ratio between the input and output shaft of the shuttling gear and reversing the direction of rotation from the input shaft to the output shaft.

[0070] The purpose of using the shuttling mechanism is to improve the efficiency by avoiding using the IVT of the KERS unit 20 in reverse gear. The reverse gear of an IVT causes high recirculating power flows with high associated losses of energy, as is further explained with the description of Fig. 4b.

[0071 ] Fig. 2a shows a KERS unit comprising an IVT 21 and an energy storage device in the form of a flywheel 22. There are many different types of KERS units available, but an essential part of all KERS types is to use a variable transmission to control the amplitude and direction of the torque applied from the flywheel in the KERS unit to an input shaft 23 of the KERS unit. It is preferred to use a type of KERS unit with an infinitely variable transmission (IVT), which can also be of different types, exemplified but not limited to the designs shown in Figs. 4a, 4b, 4c, 4d and 4e. The flywheel 22 is connected to an output shaft 24 of the KERS unit 20. Also, the flywheel energy storage device 22 can be of different types

exemplified but not limited to the designs shown in Figs. 5a, 5b and 5c.

[0072] Fig. 2b shows a KERS unit 20 similar to the one shown in Fig. 2a but with the addition of an epicyclic gear stage 25 between the IVT 21 and the flywheel 22. The purpose of this epicyclic gear stage 25 is to increase the speed of the flywheel 22, which increases the energy storage capacity relative to the size of the flywheel.

[0073] Fig. 2c shows a KERS unit 20 similar to the one of Fig. 2a but with the addition of a continuously variable transmission (CVT) 26 between the IVT 21 and the flywheel 22. This means that continuously variable transmission is provided after the IVT as seen from the input shaft 23. The purpose of using a CVT instead of an epicyclic gear stage is to allow the IVT 21 to operate with a narrow ratio spread, which improves the efficiency of the IVT system. This configuration is furthermore favorable to use in combination with any of the rectifier mechanisms shown in Figs. 1 c, where the IVT is used in only forward direction, to increase the useful speed range of the flywheel and increase the maximum allowed buoy velocity. Using the IVT 21 with only forward ratio allows infinite gear ratio only in the direction from the input shaft to the flywheel, meaning that the input shaft can stop while a high speed of the flywheel is maintained. However, the maximum allowed speed of the input shaft 23 depends on the current speed of the flywheel 22 and the total maximum allowed gear ratio, which is limited by the ratio spread of the CVT 26 in this direction.

[0074] Fig. 3 shows a schematic of a CVT type known as Double roller Full Toroidal Variator (DFTV). This type of CVT is a traction drive that transfer torque from an input disc 26a to an output disc 26b by clamping the discs together with a high force by means of a clamping device 26c. The input disc 26a is connected to an input shaft 26d and journaled around an output shaft 26e with a trust bearing 26f between the end of the output shaft 26e and the input disc 26a. The output disc 26b is preferably mounted around the output shaft 26e with a spline to allow axial movement of the output disc 26b. A hydraulic clamping device 26c with a thrust bearing 26f is journaled around the output shaft 26e and arranged to apply a force on the output disc 26b to push it towards the input disc 26a. The gear ratio is controlled by changing the angle of rollers, which sets the contact radius between the rollers and the input and output discs 26a, 26b. The gear ratio applied from the input to the output shaft 26d, 26e is given by the contact radius on the input disc divided by the contact radius on the output disc. The ratio shown in Fig. 3 is 1 : 1. The roller angle may be controlled by a step motor turning a worm gear journaled around the output shaft 26b, which is connected to ratio gears of each double roller pair, not shown in the figure. Also, hydraulic cylinders can be used to adjust the angular position of each roller to provide the desired gear ratio.

[0075] Fig. 4a shows an IVT 21 based on the CVT type shown in Fig. 3. A planetary gear stage 21 a is added before the input disc of the CVT 26 with the ring gear connected to an input shaft 21 b of the IVT, the carrier gear connected to the input disc of the CVT 26 and the sun gear connected to the output disc of the CVT 26. The IVT 21 is in geared neutral when the CVT 26 applies a ratio from input to output disc that matches with the ratio between the planet carrier and sun gear in the epicyclic gear stage 21 a, as shown in Fig. 4a. When the CVT ratio is higher than this value, the output shafts to the IVT 21 will rotate with the same direction with higher speed than the input shaft. When the CVT ratio is lower, the input and output shafts 21 b, 21 c will rotate in opposite directions. This way the gear ratio applied by the CVT 26 effectively controls the relation between the input and output shafts 21 b, 21 c of the IVT 21 from maximum reverse to maximum forward speed, crossing a neutral state when the flywheel can rotate even when the output shaft is not rotating at all. The neutral state makes it possible to keep the flywheel spinning through the turning points of the buoy.

[0076] Fig. 4b shows an IVT 21 similar to the one shown in Fig. 4a but with twin cavities and a secondary epicyclic CVT reduction stage 26g. The twin cavity design doubles the torque capacity by using twice as many rollers. The efficiency is also improved since the output discs 26b are clamped between the two input discs 26a, 26a' rotating with the same speed. There is no need for any thrust bearing between the discs, only for the clamping device 26c. The secondary epicyclic CVT reduction stage is used for shifting the roller angle where the geared neutral is located, to increase the portion of the discs that is used for the forward ratio of the IVT. The geared neutral can this way be set to occur when the rollers are in parallel with the output shaft, which is preferred when no external rectifier mechanism is used, i.e. to have equal positive and negative ratios available in the IVT system. When using external rectifier mechanisms according to Fig. 1 c, the angle for geared neutral can be shifted even further to give even more room for the forward ratio, since the reverse ratio is not used. Using an external rectifier mechanism reduces the recirculating power from using the IVT in reverse gear, which is relative to the difference in speed between input and output shaft. It is therefore more efficient to operate the IVT in only forward direction where the input and output shafts rotates with the same direction and the relative speed difference is lower. [0077] Fig. 4c shows an IVT type similar to Fig. 4b with the equivalent functionality, but with single rollers and a parallel CVT reduction stage 26g.

[0078] Fig. 4d shows a hydrostatic IVT 121 where ports of a first hydraulic pump/motor 121 c and a second hydraulic pump/motor 121 d are interconnected by means of a hydraulic system 121f, with the first pump/motor connected to the input shaft 121 b and the other pump/motor connected to the output shaft 121 c of the IVT 121 . At least one of the pump/motors has variable displacement, which enables the gear ratio of the IVT 121 to be changed by adjusting the displacement. The direction can be changed by using a pump/motor with a variable tilt plate driving the cylinders inside the pump/motor. The angle of the tilt plate is used to control the displacement and the variable tilt plate can also be tilted in the opposite direction whereby the direction of rotation in the output shaft is reversed. Also pump/motors with digitally controlled valves, so called displacement motors, can reverse the rotational direction of the output shaft by means of valve control.

[0079] It should be realized that a hydrostatic IVT system can comprise multiple input shafts with one first pump/motor connected with each input shaft, and also multiple output shafts with one second pump/motor connected with each output shaft, with similar embodiments as shown for an electric IVT system in fig. 4f-h. Such arrangement can e.g. be used in case of using multiple linear actuators in the power take-off and/or multiple energy storage devices, in which case all pumps/motors are interconnected through the same hydraulic circuit and operating with the same working pressure enabling an even load sharing between all first pumps/motors and all second pumps/motors when using the same displacement on all first and/or all second pumps/motors.

[0080] Fig. 4e shows an electric IVT 221 where a first electric generator/motor 221 d and a second electric generator/motor 221 e with variable speed drives are connected to a common electric circuit 221 f. The first generator/motor 221 d is connected to the input shaft 221 b of the IVT 221 and the second generator/motor 221 e is connected to the output shaft 221 c of the IVT 221 . When a flywheel in the drive unit in which the IVT is provided is charged, the first generator/motor 221 d is driven like a generator and the second generator/motor 221 e like a motor, transferring power from the first generator to the second motor. The torque of the first generator 221 d is controlled for the force control, and the resulting motor torque depends on the speed of the second motor 221 e and the power transferred from the first generator 221 d. When the flywheel is discharged, the second generator/motor 221 e is operated as a generator and the first generator/motor 221 d is operated as a motor, and the discharge is controlled with torque on the second motor.

[0081 ] Fig. 4f shows an electric IVT according to Fig. 4e with multiple input shafts 221 b with one first electric generator/motor 221 d connected to each input shaft. Multiple output shafts 221 c with one second electric generator/motor 221 e connected to each output shaft 221 ccan be used as shown in Fig. 4g and 4h. This enables multiple linear actuators to be used in the power take-off to share the PTO force and/or multiple energy storage devices to share the power and storage capacity on smaller units, where all first and second electric generators/motors are connected to variable frequency drives interconnected with power cables on a common bus system. Load balancing between the actuators and energy storage devices can be done through generator/motor torque control.

[0082] It will be realized that the hydraulic and electric IVT described above can provide the equivalent function as a mechanical IVT, but with the advantage of a more flexible design and simpler solutions for load sharing when using multiple actuators and/or multiple energy storage devices.

[0083] Fig. 5a shows an energy storage device 22 comprising a flywheel 22a enclosed in a vacuum case 22b, with an input shaft 22c and an output shaft 22d.

[0084] Fig. 5b shows an energy storage device similar to the one of Fig. 5a but with the addition of a clutch 22e that enables the flywheel 22a to be disconnected from the IVT unit, which can be used to preserve energy when a WEC goes into standby mode. [0085] Fig. 5c shows an alternative energy storage device 22 comprising a primary flywheel 22a and a secondary flywheel 22a' and an epicyclic gear stage 22f, with the input shaft 22c coupled to the carrier, the ring gear coupled to the primary flywheel 22a through the clutch 22e and the sun gear coupled to the secondary flywheel 22a'. The purpose of this arrangement is to return the carrier speed close to zero after each force cycle of the tethering force, in order of reducing the re-circulation of power in the IVT when the tethering force is high by limiting the gear ratio to be as close as possible to 1 : 1 , where the IVT of the preferred type with a double roller toroidal variator is most efficient. This improves the overall efficiency of the power take-off to deliver the required force trajectory for the total PTO force, at the same time as the generator is controlled to output a constant sea state tuned power.

[0086] Input power from the PTO is split between both the primary and the secondary flywheel 22a, 22a', while only the secondary flywheel 22a' outputs active power to drive the generator. The brake to the primary flywheel 22a is used to direct enough power to the secondary flywheel 22a' to prevent the primary flywheel 22a from gradually increasing after each force cycle. When the buoy starts to rise, the input power from the PTO to the carrier of the flywheel energy storage device 22 is lower than the output power from the sun, the ring gear and primary flywheel 22a will rotate backwards, feeding power from the primary flywheel 22a to the secondary flywheel 22a' to maintain a constant output power to the generator, while the carrier torque is maintained by the PTO. When the input power has increased to reach the same level as the output power from the sun to the secondary flywheel 22a' and generator, the ring gear will have zero speed at which point it is locked by the brake for a timed duration. When the input power increases further, all power is directed to the secondary flywheel 22a' until the ring gear and primary flywheel 22a are released, after which most of the excess power from tethering and reactive forces is directed to the ring gear. This is due to the higher torque on the ring gear compared to the sun gear in a planetary gear train. The locking mechanism is timed in such a way that the amount of energy directed to the secondary flywheel 22a' corresponds to the total active energy transfer from the system during a complete PTO force cycle. When the buoy descends in the wave, the primary flywheel will discharge and return just enough energy to the PTO to return the carrier speed to zero when the cycle completes.

[0087] Fig. 6a shows a complete drive unit 10 comprising shuttling gear 18 as shown in Fig. 1 c, an IVT 21 as shown in Fig. 4b, an epicyclic step up gear stage 21 a, an energy storage device 22 as shown in Fig. 5a and an active power transfer device in the form of a pump/generator 30. All components are enclosed in a common vacuum chamber 17. Therefore, there are no high-speed shafts penetrating the vacuum chamber.

[0088] Fig. 6b shows a complete drive unit with an electric IVT 221 and a flywheel energy storage device 22, in which case the active power transfer device is integrated in the interconnection between the electric generators/motors and the output cable. Output power is this way extracted directly from the electric circuit by means of power electronics. Active power is extracted independently from the direction of power through the IVT, alternately provided from a part of the captured power and by discharging the flywheel.

[0089] Fig. 6c shows a complete drive unit 10 similar to the one of Fig. 6b, but with an energy storage device comprising first and second counter rotating flywheels 22a, 22a', each driven by a respective electric generator/motor 121 e, 121 g. The electric generators/motors are interconnected by means of a power cable 30' and the entire assembly is preferably enclosed in a vacuum case. This assembly cancels angular momentum and precession due to gyroscopic effects, and the flywheel energy storage device thereby do not affect the buoy motion.

[0090] Fig. 6d shows a similar embodiment as Fig. 4e, where the flywheel energy storage device including the second generator/motor is replaced with an energy storage device in the form of an electrical battery 22'. The PTO force is controlled in the same way by controlling the torque of the electric

generator/motor, which exchanges power to/from the electric battery through variable frequency drives. The electric generator/motor 221 d, the electric battery storage 22' and output cables are interconnect through an active power transfer device 30' in the form of power electronics and electric cables, preferably a DC bus.

[0091 ] Fig. 7a shows a power take-off with a drive unit according to any of Figs. 1 a-1 c and 6a-c connected to a means for converting linear motion into rotary motion 310 comprising a rack 312 and pinion 314 actuator.

[0092] Fig. 7b shows a similar embodiment as Fig. 7a with two drive units 10 connected to a double-sided rack 312'. Chords 316 are mounted on the side of the rack 312' to make the rack stiffer to enable long unsupported length, forming a drive module 310.

[0093] Fig. 7c shows a complete power take-off assembly 300 where three drive modules 50 according to Fig. 7b is placed around a triangular framed structure 320 with three integrated double-sided racks 312'. Such framed structure is very stiff and suitable to provide long stroke length of a rack and pinion actuator.

[0094] Figs. 7d and 7e show two alternative geometries 320', 320" of a rack leg with four integrated double-sided racks.

[0095] In order to apply the damping and reactive force components through the full wave cycle, so called bi-directional power take-off, the buoy must provide enough force to drive the PTO in both directions of the wave motion and a counteracting force to the buoy motion must also be provided.

[0096] Fig. 8a shows a power take-off comprising a drive unit according to any of Fig. 1 a-1 c and 6a-c connected to a winch drum actuator comprising a winch drum 332 and a wire 334.

[0097] Fig. 8b shows a similar embodiment as Fig. 8a but with the drive unit 330 connected to an external gear 336 on the side of the winch drum 332 meshing with a pinion 338 connected to the input shaft 12 of a drive unit 10. This arrangement increases the input velocity and reduced the input torque to the drive unit.

[0098] Figs. 8c and 8d show a similar embodiment as in Fig. 7b but with multiple drive units 10 connected to an internal gear 336' on the side of the winch drum 332, sometimes called a slew drive. In other words, an internal gear is provided, wherein several pinions 338' are connected to a respective input shaft 12 of a drive unit 10. The load on each gear mesh is shared between all drive units, enabling the pinion diameters to be reduced, to increase the gearing before each drive unit.

[0099] Figs. 8e and 8f show different views of an embodiment of a wave energy converter with a winch drum 332 with involute gear and multiple drive units 10 according to Figs. 8c, 8d. The KERS provides both reactive PTO force and pretension while the pumps/generators provide the damping PTO force in the same way as will be described with reference to Fig. 10. In this case there is no rod that needs to be encapsulated above and below the buoy hull. A sheave 336 below the winch drum 332 is used to position the winding correctly on the drum surface while guiding the wire through the movements of the waves. The diameter of the winch drum should be large to hold a wire 334 or fiber rope with large diameter, required to manage the high pulling forces applied. A large diameter also reduces the bending angle and bending fatigue which increases the life time of the wire. The hull of the buoy 1 on the water surface 9 can e.g. be 8 meters in diameter and the drum diameter is preferably 6 meters in diameter in this case, giving 19 meters length of the wire 4c for each revolution on the drum, which is sufficient stroke length for the buoy.

[00100] The wire 334 can be wound in one single furrow in a right angle to the drum, which terminates at the end of the furrow. When all wire is unwound a soft end stop is provided which pulls the buoy below the surface in the large 100 year waves, without the need to use active force control from the KERS units.

[00101 ] Fig. 9 shows an embodiment of a wave energy converter (WEC) with a buoy 1 housing a power take-off device according to Fig. 7a, in which the rack 312 according to Fig. 7a is of sufficient length to rest on the seabed 9 in the bottom. This WEC device uses external spring like pre-tension force through flexible mooring ropes 5 attached between the sea bed and the buoy hull, which reduces the mass required for bi-directional power take-off, thus improving the economy of the WEC. The mass of the buoy is selected so that the water line will be at the center line of the buoy in the state of equilibrium with the pre-tension ropes attached.

[00102] In this embodiment, the KERS in each drive unit 10 provides the reactive part of the PTO force, the pumps/generators provide the damping part of the PTO force either by using variable displacement or variable torque in each unit or by engaging disengaging pumps/generators in a discrete control scheme, which allows each engaged unit to operate at an ideal torque or displacement which improves the efficiency.

[00103] Fig. 10 shows a similar embodiment as Fig. 9 but without the flexible ropes to provide external pre-tension spring force. A pre-tension force is instead provided by the KERS in each drive unit 10 in the power take-off assembly together with the reactive force component. Providing pre-tension through the KERS units makes it possible to hold a more ideal constant pre-tension force which does not degrade the power capture performance like a spring like pretension force. A shorter rack rod can be used in this case with a mooring wire between the bottom end of the rod and the sea bed.

[00104] A pre-tension force can only be provided when the system is operational and there is energy stored in the KERS units. The mass of the buoy is selected in such a way that the water level is at the center of the buoy hull at the point of equilibrium, when the pre-tension force is applied. When the WEC is not in operation, the buoy will take a higher position relative the sea level. A buoy that is not operational is in this way easier to access for maintenance.

[00105] Using a pre-tension force from the KERS unit also makes it possible to increase the pre-tension force temporarily in order to submerge the buoy below the water line in large waves, which effectively limits the maximum height of the buoy and the maximum required stroke length of the rack.

[00106] Fig. 1 1 shows an embodiment of a wave energy converter with a power take-off comprising drive units 10 according to Figs. 1 a-c and 6a-c and a double sided rack 312', as shown in Fig. 7b, where control valves 412 are added to the pumps in order to shift ports and to disconnect the pump from the hydraulic circuit. High and low pressure hydraulic accumulators 414 are connected to the hydraulic circuit and also a hydraulic motor 416 that drives a generator 418 for on board electric power generation.

[00107] In this embodiment, the KERS units may provide only the reactive force component while the pumps provide the pre-tension force when engaged to the circuit, and with the control valves set in a direction that applies a force to pull the buoy down towards the mooring point. The hydraulic accumulators 414 are charged when the buoy 1 is lifted by the wave, and discharged when the buoy 1 descends. The hydraulic motor 416 connected to the circuit taps power from the system and is controlled by the generator torque connected with the motor. The energy extracted through the hydraulic motor 416 and generator 418 results in a gradual pressure drop through the pre-tension cycle, i.e. the force applied by the pumps gradually decreases in a way that applies a higher average force in the rise of the wave compared to the decent of the wave. The difference between the average force applied by the pumps during the rise and the average force during the decent corresponds to the damping force applied by the PTO through a complete wave cycle. The speed of the generator is controlled to output a constant power corresponding to the average sea state tuned energy extracted from the waves. The KERS units are used to add wave-by-wave tuning of the damping force on top of the sea state tuned force applied by the pumps, i.e. the damping force is reduced in small waves and increased in large waves to optimize power capture. An energy balance to the KERS units exist across a time frame of approx. 1 -2 minute, thus the extracted power through the hydraulic motor and generator controls the trend of the energy storage in the KERS units.

[00108] The pre-tension force is released by disconnecting the pumps from the hydraulic circuit by means of the control valves, whereby the pumps no longer apply a force to the PTO. This simplifies installation and retrieval of the WEC, which can be done without the pre-tension force applied to the mooring rope. [00109] A similar embodiment without the hydraulic motor and generator on board the WEC can be connected to a hydraulic collection system with the hydraulic motor and generator located in a central unit in an array of multiple wave energy converters, as shown in prior art. In this case the pre-tension function as well as the reactive PTO force is provided by the KERS units, while the damping PTO force is provided by the pumps. The hydraulic accumulators are in this case used to reduce the pressure variations from inertia and pressure drops that varies due to the variation in the flow rate from the pumps.

[001 10] Different embodiments of power take-off device and a wave energy converter according to the invention have been described. It will be realized that these can be varied within the scope of the appended claims.

Claims

1 . A power take-off device for use in a wave energy converter comprising

- a buoy (1 ),

- a drive unit (10) comprising a rotary input shaft (12), and a variable transmission means (20) including an energy storage device (22) connected to the rotary input shaft,

- a means (312; 312'; 314; 332, 334; 336', 338') for converting linear motion from the movements of the buoy (1 ) into rotary motion, connected to the rotary input shaft (12) of the drive unit (10), c h a ra c t e r i z e d i n t h a t the variable transmission means (20) is adapted to control the amplitude of power stored and retrieved to/from the energy storage device (22) and the amplitude and direction of the force applied to the buoy (1 ) from the drive unit (10).

General features for support of priority:

2. The power take-off according to claim 1 , wherein the buoy (1 ) is provided with a buoy hull to be put into motion by motions of water in which the buoy is placed.

3. The power take-off according to claim 1 or 2, wherein the drive unit (10) is arranged inside the buoy (1 ).

4. The power take-off according to any one of claims 1 -3, wherein the rotary input shaft (12) of the drive unit (10) is connected to a pinion.

5. The power take-off according to any one of claims 1 -4, wherein the drive unit (10) comprises an active power transfer device in the form of at least one of a hydraulic pump and an electric generator (30).

6. The power take-off according to any one of claims 1 -5, wherein the amplitude of power stored and retrieved to/from the energy storage device (28) and the amplitude and direction of the force applied to the buoy (1 ) from the drive unit (10) are controlled by changing the conversion ratio, preferably a gear ratio, in the variable transmission means.

7. The power take-off device according to any one of claims 1 -6, wherein the variable transmission means (20) comprises a first electric generator/motor (221 d) with variable speed drive interconnected with an energy storage device in the form of an electric battery (22'), and a power output cable (30').

8. The power take-off device according to any one of claims 1 -6, wherein the energy storage device is a flywheel (22a, 22a').

9. The power take-off device according to claim 8, comprising an active power transfer device (30) comprising any of a hydraulic pump and an electric motor connected to the flywheel.

10. The power take-off device according to claim 9, wherein the active power transfer device (30) is provided after the variable transmission means (20), as seen from the input shaft (12) of the drive unit (10), wherein the active power transfer device (30) during operation is adapted to output an essentially constant sea state tuned power, while the conversion ratio of the variable transmission means (20) is used for controlling the total force applied by the power take-off device on the buoy (1 ).

1 1 . The power take-off device according to any one of claims 8-10, wherein the drive unit (10) comprises a variable transmission means (20) in the form of an infinitely variable transmission (21 ; 121 ; 221 ) connected to the input shaft (12) of the drive unit, wherein the infinitely variable transmission (21 ; 121 ; 221 ) is adapted to provide a positive gear ratio when the input shaft (12) rotates forward and to apply a negative gear ratio applied when the input shaft (12) rotates backwards, whereby a uni-directional rotation of the flywheel (22) is provided.

12. The power take-off device according to claim 1 1 , wherein the infinitely variable transmission (21 ) comprises a mechanical variator, preferably a toroidal variator (26a-f) coupled to an epicyclic gear stage (21 a), in which the gear ratio between input and output shaft (26d, 26e) of the infinitely variable transmission (21 ) is adapted to be controlled infinitely between forward and reverse ratios by changing the angle of rollers between two discs (26a, 26b) and thereby the contact radius on each disc. (Figs. 3, 4a)

13. The power take-off device according to claim 1 1 , wherein the infinitely variable transmission (121 ) comprises a hydrostatic variator with at least one first hydraulic pump/motor (121 d) connected to the input shaft and at least one second hydraulic pump/motor (121 e) connected to the output shaft, wherein all first and second pumps/motors are interconnected by means of a hydraulic circuit, and at least one of the first and second hydraulic pumps/motors (121 d, 121 e) has variable displacement, preferably variable reversible displacement. (Fig. 4d)

14. The power take-off device according to claim 1 1 , wherein the infinitely variable transmission (221 ) comprises an electric variator with at least one first electric generator/motor (221 d) connected to the input shaft and at least one second electric generator/motor (221 e) connected with the output shaft, wherein all first and second electric generators/motors are connected with variable speed drives interconnected with a common electric circuit (221 f), whereby the

conversion ratio between an input shaft (221 b) and an output shaft (221 c) of the electric variator is infinitely controllable from forward to reverse gear ratios by changing the amplitude and direction of the torque applied on the input shaft (221 b) by the first generator/motor (221 d) while the second generator/motor (221 e) is controlled to provide charge and discharge power corresponding to the control force. (Fig. 4e).

15. The power take-off device according to any one of claims 7-14, comprising a plurality of first electric generators/motors (221 d), each connected to a separate input shaft (221 b).

16. The power take-off device according to any one of claims 8-15, comprising a plurality of second electric generators/motors (221 d), each connected to a separate output shaft (221 c).

17. The power take-off device according any one of claim 1 1 -16, comprising a continuously variable transmission (26) provided after the infinitely variable transmission (21 ) as seen from the input shaft (12) of the drive unit (10).

18. The power take-off device according to any one of claims 9-17, wherein the active power transfer device is located before the variable transmission means, as seen from the input shaft, wherein the active power transfer device applies, during operation, a damping force of variable amplitude to extract power from the waves which is converted into hydraulic or electric power, and the variable transmission and the flywheel apply, during operation, a reactive force with variable amplitude to control the phase of the buoy.

19. The power take-off device according to any one of claims 1 -18, wherein the variable transmission means (21 ), during operation, is adapted to apply a pre- tensioning force on a mooring rope indirectly connected to the input shaft, in addition to the reactive and damping forces, wherein the total force applied to the mooring rope by the power take-off device is uni-directional.

20. The power take-off device according to any one of claims 1 -19, wherein the means for converting linear motion into rotary motion of the wave energy converter comprises an internal gear (336'), wherein a plurality of pinions (338') meshing with the internal gear (336') is connected to a respective input shaft (12) of a drive unit. (Figs. 8c, 8d).

21 . The power take-off device according to any one of claims 1 -20, wherein the means (312; 312'; 314; 332, 334; 336', 338') for converting linear motion from the movements of the buoy (1 ) into rotary motion is further connected to a fixed point of reference for the linear motion.

22. A method of controlling a power take-off device according to any one of claims 1 -21 , comprising the following steps: a) detecting the nature of incoming waves, preferably the frequency and amplitude of incoming waves, b) determining the optimal PTO force to capture the highest amount of energy from the incoming waves, c) applying the determined PTO force with a limited maximum amplitude.

23. The method according to claim 22, wherein the step a) is performed by using weather forecasting to determine the current sea state, from which a sinusoidal wave is determined with fixed amplitude and frequency.

24. The method according to claim 22, wherein the step a) is performed by a vision system to detect, trace and calculate a trajectory for all waves moving towards the WEC, whereby the movement of the water in the location of the WEC can be calculated by super imposing the incoming wave trajectories.

25. The method according to claim 22, wherein the step b) comprises determining the total PTO force by satisfying the following equations: the absorbed power from the power take-off given by:

where m_b - buoy mass

-m_∞ - added mass at infinity frequency ω - wave frequency

X - PTO reactive

R - PTO damping

i?„ - radiation damping s - hydrostatic stiffness F_s - excitation force wherein the power attains maximum when:

26. The method according to claim 22-23 and 25 wherein the step b) comprises determining the PTO damping component by a polynomial function, preferably multiplying the buoy velocity with the cube of a constant, where the constant is preferably tuned to the sea state.

27. The method according to any one of claims 22-26, wherein the step c) comprises shifting the phase and reducing the amplitude of the PTO force relative to the optimality condition in order to reduce recirculating power flows and losses to the point where the power capture starts to reduce faster than the losses, thereby maximizing the energy output.

28. The method according to any one of claims 22-27, wherein the step c) comprises shifting the phase of the PTO force relative to the optimality condition to also limit the height and velocity of the buoy motion.

29. The method according to any one of claims 22-28, wherein the step c) comprises shifting the phase of the PTO force to also limit the energy capture.

30. The method according to any one of claims 22-27, wherein the step c) is performed by applying a continuous damping force component according to R_opt with the active power transfer device, and by applying a continuous reactive force component according to X_opt by means of the IVT and flywheel.

31 . The method according to any one of claims 22-27, wherein the step c) is performed by applying a constant average damping force component according to Ropt for each stroke of the buoy with the active power transfer device, and by applying a continuous reactive force component according to X_opt by means of the IVT and flywheel.

32. The method according to any one of claims 22-27, wherein the step c) is performed by applying a constant average damping force component according to Ropt for the sea state with the active power transfer device, and by applying a compensating damping force component to provide a total continuous damping force component according to R_opt and a continuous reactive force component according to X_opt by means of the IVT and flywheel.

33. The method according to any one of claims 22-27, wherein the step c) is performed by continuously controlling the damping force component applied from the active power transfer device relative to the speed in order to transfer a constant sea state tuned power, and by applying a total PTO force to satisfy R_opt and Xopt by means of the IVT and flywheel.

34. The method according to any one of claims 22-33, wherein the step c) is performed by a power take-off comprising multiple drive units where the number of active power transfer devices engaged with the system us used to control the total damping force provided to transfer power from the wave energy converter.

35. The method according to any one of claims 22-34, wherein a model predictive control is used to determine the power take-off force according to the optimality condition while taking losses into account and with constraints, preferably for the maximum force, stroke length and velocity, to allow in the power take-off.