WO2020055320A1

WO2020055320A1 - Power take-off device for use in a wave energy converter

Info

Publication number: WO2020055320A1
Application number: PCT/SE2019/050869
Authority: WO
Inventors: Mikael Sidenmark
Original assignee: Ocean Harvesting Technologies Ab
Priority date: 2018-09-14
Filing date: 2019-09-16
Publication date: 2020-03-19
Also published as: EP3850211A1; EP3850211A4

Abstract

A power take-off device (100) for use in a wave energy converter having a prime mover (404), the power take-off device having a PTO frame (110). A direct drive motor (14) mounted to the PTO frame (110) and electrically connected to an energy storage (16), a means (12) for converting linear motion from the movements of the prime mover (404) to rotary motion to the direct drive motor (14), and control means (206) adapted to control the direct drive motor (14) to apply a torque to the means for converting linear motion of the prime mover (404) into rotary motion to control the power take-off force by means of storing and retrieving energy in the energy storage (16) while outputting constant power to an output cable of the power take-off device. A wave energy converter and a wave energy converter system are also provided.

Description

POWER TAKE-OFF DEVICE 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 (PTO) device with features to provide PTO force control and energy smoothing. A wave energy converter and a wave energy converter system are also provided.

Background art

[0002] Different types of wave energy converters (WEC^'s) have been proposed, in which a power take-off is used for converting linear motion into rotary motion, and for applying a force to the buoy to capture power from the waves, controlling the motion of the buoy and smoothing the irregular input into a continuous output.

[0003] One challenge with a PTO force necessary to provide the above- mentioned features is that power flows back and forth through the PTO device in every wave cycle. To provide the necessary force control features in the PTO device, i.e., to control the phase of the buoy motion, to balance power capture with loads and losses in order to minimize the cost of energy, to provide a pre-tension force to keep tension in the tether mooring of a point absorbing WEC, and to output approximately 500 kW nearly constant output power, the system needs to manage approximately 5 MW peak power, > 30 kWh useful energy storage capacity and a cycle life in the order of 4 million full charge / discharge cycles and 100 million total charge / discharge cycles. To provide time shifting capabilities, the storage capacity needs to be extended to approx. 500 kWh.

[0004] Most conventional flywheel energy storage technologies are limited to approximately 500 kW maximum power and 100 000 full charge / discharge cycles, which is due to fatigue issues in the hub of barrel shaped flywheel connecting to an external generator / motor through a shaft, and wear issues caused by the rotor being journaled with mechanical bearings. Ultra-capacitors can provide approximately one million full charge / discharge cycles but become very bulky in size to provide the necessary lifetime and energy storage capacity. Summary of invention

[0005] An object of the present invention is to provide a power take-off device for wave energy converters (WEC^'s) with a more practical and durable design of a Kinetic Energy Recovery System (KERS), that will provide sufficient cycle life as well as power and energy ratings to smooth the irregular waves in to a constant / time shifted output power.

[0006] The invention is based on the realization that direct drive motors, preferably frameless torque motors, can be used with the linear actuators, to be electrically connected to a common electrical energy storage unit without using any mechanical gear system to form the KERS.

[0007] According to a first aspect of the invention, there is provided a power take-off device for use in a wave energy converter having a prime mover, the power take-off device having a PTO frame and being characterized by a direct drive motor mounted to the PTO frame and electrically connected to an energy storage, a means for converting linear motion from the movements of the prime mover to rotary motion to the direct drive motor, and control means adapted to control the direct drive motor to apply a torque to the means for converting linear motion of the prime mover into rotary motion to control the power take-off force by means of storing and retrieving energy in the energy storage while outputting constant power to an output cable of the power take-off device.

[0008] In a preferred embodiment, the means for converting linear motion to rotary motion comprises a non-rotating screw fixedly connected to the prime mover, and a rotating nut mounted to the PTO frame, preferably by means of thrust bearing, with the rotor of the direct drive motor mounted around the nut.

[0009] In a preferred embodiment, the screw is any of the following: ball screw, planetary roller screw, lead screw, hydrostatic screw, acme screw, and magnetic leadscrew.

[0010] In a preferred embodiment, comprising a pair of means for converting linear motion to rotary motion, wherein a first means comprises a left cut screw and a second means comprises a right cut ball screw, whereby the corresponding nuts are counter rotating relative to each other during operation.

[001 1 ] In a preferred embodiment, a plurality of pairs of means is provided for converting linear motion to rotary motion, preferably two or three pairs.

[0012] In a preferred embodiment, the direct drive motor comprises a rotor in fixed connection with the nut of the means to convert linear motion into rotary motion and a stator surrounding the rotor.

[0013] In a preferred embodiment, the energy storage comprises a flywheel energy storage comprising an electric generator/motor.

[0014] In a preferred embodiment, the flywheel energy storage comprises an electric generator/motor with a stator and a rotor which is integrated with a flywheel.

[0015] In a preferred embodiment, flux between the stator and the rotor (314) is any of the following: radial flux and axial flux.

[0016] In a preferred embodiment, the rotor is hub-less.

[0017] In a preferred embodiment, the flywheel comprises a magnetic levitation system levitating the rotor. The magnetic levitation system preferably comprises any of the following: active magnetic bearings and a combination of passive and active magnetic bearings. Preferably, the magnetic levitation system comprises a super conducting magnetic bearing.

[0018] In a preferred embodiment, the energy storage comprises two counter rotating flywheels, each comprising a rotor, whereby torque applied to the power take-off device from acceleration / deceleration of the rotors is cancelled.

[0019] In a preferred embodiment, a DC bus (18) is provided interconnecting the direct drive motor, the energy storage 16) and an output power cable, preferably by means of AC/DC-converters. [0020] In a preferred embodiment, a plurality of direct drive motors and a plurality of means for converting linear motion from the movements of the prime mover into rotary motion to the direct drive motors are provided.

[0021 ] In a preferred embodiment, power electronics is provided in the form of a ring surrounding a non-rotating screw of the means to convert linear motion into rotary motion.

[0022] In a preferred embodiment, a gas spring is adapted to provide a pre tensioning force. The gas spring preferably comprises a gas piston rod fixedly connected to the prime mover and a gas cylinder fixedly connected to the PTO frame.

[0023] In a preferred embodiment, the energy storage is provided remotely from the power take-off frame, preferably at a substation, to which multiple WEC units are connected. Alternatively, the energy storage is provided on the PTO frame.

[0024] According to a second aspect of the invention, a wave energy converter is provided comprising a power take-off device according to the invention, comprising a prime mover in the form of a buoy hull mechanically connected to the power take-off device, and a mooring device attached to a foundation on a seabed.

[0025] In a preferred embodiment, the mooring device comprises a universal joint attached to the foundation. Alternatively, the mooring device comprises a flexible pipe and a flange coupling on the seabed.

[0026] In a preferred embodiment, the power take-off device is connected to the mooring device by means of a screw, whereby the distance between the power take-off device and the mooring device can be adjusted.

[0027] In a preferred embodiment, the means for converting linear motion from the movements of the prime mover to rotary motion to the direct drive motor comprises vertical, non-rotating ball screws fixedly connected in their ends to a PTO top platform and a PTO bottom platform, respectively, wherein the PTO top and bottom platforms are fixedly connected to the prime mover.

[0028] The wave energy converter may alternatively comprise a prime mover in the form of a submerged hull is connected to a plurality of power take-off devices according to the invention, preferably three power take-off devices, each connected to a mooring device attached to a foundation on a seabed.

[0029] According to a third aspect of the invention, a wave energy system is provided comprising a plurality of wave energy converters according to the invention and a substation comprising an energy storage, preferably a flywheel energy storage, to which the wave energy converters are electrically connected.

Brief description of drawings

[0030] 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 KERS comprising a ball screw actuator with direct drive torque motor connected to an energy storage;

Fig. 1 b is a schematic view of a KERS according to Fig. 1 a in which power electronics and a disc brake are integrated with the drive torque motor.

Fig. 1 c is a schematic view of a KERS according to Fig. 1 a assisted by a passive constant spring force.

Fig. 2 shows a power take-off device with multiple ball screw actuators with direct drive torque motors connected to an energy storage on top of the PTO frame;

Fig. 3a shows a schematic view of a control unit, multiple ball screw actuators with direct drive motors and AC/DC converters, energy storage connected to AC/DC- converters, a grid DC/AC converter and a common DC bus system;

Fig. 3b shows a schematic view according to 3b with a transformer and without energy storage and grid DC/AC converter. Fig. 3c shows a schematic view of the substation with an energy storage device, and power cable connector interface to multiple WEC units.

Fig. 4a shows a top view of a ring flywheel energy storage device using a large diameter torque motor, which rotor is integrated with the flywheel rotor, and a magnetic bearing to journal the rotor;

Fig. 4b shows a sectional view of the ring flywheel energy storage shown in Fig. 4a;

Fig. 5a shows a top view of a large diameter ring flywheel energy storage device with an axial flux motor/generator design and a combination of passive and active magnetic bearings;

Fig. 5b shows a sectional view of the ring flywheel energy storage shown in Fig. 5a;

Fig. 5c shows an assembly of two counter rotating ring flywheel energy storage devices according to Figs. 5a and 5b;

Fig. 6a shows a power take-off assembly with a single ball screw and a ring flywheel energy storage device according to Figs. 5a and 5b;

Fig. 6b shows a power take-off assembly according to Fig. 2 with a ring flywheel energy storage device according to Figs. 5a and 5b;

Fig. 6c shows a power take-off assembly similar to that of Fig. 2, but with a ring flywheel energy storage device according to Fig. 5c and counter rotating ball nuts in the ball screw actuators;

Fig. 7a shows a wave energy converter with a power take-off system according to Fig. 6a and a mooring with a universal joint connection to the seabed;

Fig. 7b shows a wave energy converter with a power take-off system according to Fig. 6c and a mooring with a universal joint connection to the seabed; Fig. 7c shows a wave energy converter with a power take-off system according to Fig. 6c and a mooring with a flexible pipe and flange coupling to the seabed;

Fig. 7d shows a triple point submerged wave energy converter with three power take-off systems and moorings according to Fig. 7c;

Fig.7e shows a wave energy converter according to Fig. 7c with a power take-off system according to Fig. 1 c and a flexible hose used to contain pressurized gas for the tension gas spring.

Fig. 8a shows a spar buoy substation with one WEC unit connected to it.

Fig. 8b shows an array topology with WEC units located around a common substation as two rings.

Fig. 8c shows an array topology with WEC units located around a common substation as one ring.

Description of embodiments

[0031 ] In the following, a power take-off (PTO) device for use in Wave Energy Converters (WEC), comprising an improved design of an electrical kinetic energy recovery system (KERS) to better meet the requirements to provide PTO force control and constant / time shifted power output, will be described in detail.

[0032] 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.

[0033] The PTO force is controlled by means of torque control of direct drive torque motors, which can instantly provide any direction and amplitude of the torque within the design ratings as requested by the control system. An active power transfer device in the form of a grid inverter is controlled to output a constant power from the WEC, and the charge / discharge of the energy storage is controlled to store and retrieve energy as requested by the direct drive torque motors and the active power transfer device.

[0034] Fig. 1 a shows a principal sketch of a general electric KERS 10

comprising a ball screw actuator 12 comprising a non-rotating ball screw 12a and a rotating ball nut 12b, with the rotor 14a of a preferably frame-less direct drive torque motor 14 mounted around the ball nut 12b. The rotor 14a is adapted to rotate inside a fixed stator 14b. Windings of the torque motor 14 are electrically connected to an energy storage device 16 and to an output power cable 18. A ball screw actuator using a rotating nut and non-rotating screw can manage longer stroke at high velocities, compared to arrangements with rotating screws.

Furthermore, a frame-less torque motor provides 10 times higher power density and 20 times higher torque density to conventional framed generators / motors and is typically designed for lower speed that matches the speed of the ball nut and therefore does not require any gearbox transmission. Other types of power screws such as planetary roller screws, lead screws, hydrostatic screws and magnetic leadscrews can also be used.

[0035] Fig. 1 b shows a principal sketch similar to Fig. 1 a, but with power electronics 14d including capacitors, inverter and control unit, in the form of a ring with a hole in the middle where the ball nut will fit, i.e., the power electronics 14d surrounds the non-rotating ball screw 12a. A disc brake 14e is preferably integrated with the torque motor in frame 14c. The torque motor assembly 14 is mounted to the ball nut flange 12c. This type of assembly and KERS architecture is similar to the general KERS architecture with in-wheel motors and a common battery energy storage found in electric vehicles.

[0036] It should be realized that the disc brake 14e is an optional component used for adding an additional controllable braking force to slow and lock the heave actuation system without the use of motor torque.

[0037] Fig. 1 c shows a principal sketch of a general electric KERS 10 according to Fig. 1 a, assisted by a gas spring 20 to provide a pre-tensioning force that reduces the amplitude of the force from the ball screw 12a to half, to provide the same total force. A gas piston rod 22a is fixedly connected to a PTO top platform 130 of a power take off device, as described below with reference to Fig. 2, and a gas cylinder 24 is suspended below the PTO frame 1 10, to which the direct drive motor 14 is mounted. In this way the piston rod 22a is always in tensile load to prevent buckling forces. Both ends 24a, 24b of the gas cylinder 24 are reinforced. A gas port 24c to ambient pressure is located at the bottom above the lower reinforced part 24b, and when a piston 22b attached to the piston rod 22a passes the port 24c, the pressure and thereby the force rapidly increases and provides a bottom end stop buffer spring, which prevents the piston 22b from reaching the hard end stop of the cylinder 24. A hollow piston rod 22a is used with a piston rod gas port 22c to a gas chamber 24d of the cylinder, preferably located

approximately 0.5 meter above the piston 22b. When the piston rod gas port 22c moves outside the cylinder chamber 24d, the chamber pressure and force rapidly increase and provide a top end stop buffer zone. A second piston rod gas port 22d is located at the top of the piston rod, and connects to an external gas container 26, which has typically 5-6 times larger volume than the cylinder pressure chamber 24d. A cylinder 24e located above the main cylinder 24 blocks port 22c when the piston moves in the buffer zone.

[0038] Fig. 2 shows a power take-off device 100, wherein two vertical, non rotating ball screws 12a, 12a’ are fixedly connected in their ends to a PTO top platform 130 and a PTO bottom platform 140, respectively. These platforms are fixedly connected to a prime mover 404 of a WEC 400, such as a floating buoy hull. Direct drive motors 14, 14’ are connected to a respective rotating ball nut 12b, 12b’ according to Fig. 1 a connected by means of thrust bearings 15, 15’ to a PTO frame 1 10.

[0039] The power take-off device 100 also comprises a level actuation system 120 with a third ball screw actuator 122 with a direct drive motor 124 connected to a rotating ball screw 122a connected by means of a thrust bearing 125 to the PTO frame 1 10 and with a non-rotating ball nut 122b connected to the top of a mooring cylinder 126. The thrust bearings are oriented to hold tensile force from the top of the two first ball screw actuators, through the PTO frame 1 10, the third ball screw actuator and down to the mooring cylinder. This means that the PTO frame 110 is essentially at the same level with reference to a sea bed, to which the power take off device is moored.

[0040] The ball screws 12a, 12a’ in the first and second ball screw actuators 12,12’ are attached to a top ball screw plate 130 and a bottom ball screw plate 140 connected to the prime mover of a wave energy converter to be set in motion by the movements of the waves, such as a buoy hull (not shown). The third ball screw actuator 122, the ball nut of which is attached with the mooring cylinder to give a fixed point of reference to the movements of the prime mover, is arranged to adjust the height of the PTO frame 1 10 above the mooring cylinder 126 to submerge the prime mover.

[0041 ] It should be realized that although the thrust bearings shown in Fig. 2 are only oriented in one direction to handle only tensile load, another set of thrust bearings can be added to manage pulling force towards the bottom ball screw plate, which will be the case when using a PTO with pre-tension gas cylinders according to Fig. 1 c. In case a bi-directional force is applied by the ball nut and torque motor assembly, the ball screws are mounted in a way to the PTO top and bottom platforms to always maintain tensile load in the screws, by allowing the screw to move up relative to the top ball screw plate and down relative to the bottom ball screw plate, preferably by means of a spline connections and a stop block mounted outside the spline in each end of the ball screws. The stop blocks can be equipped with rubber pads to provide to reduce shock loads, e.g. when the torque motor brakes are locked.

[0042] It should also be realized that the energy storage can also be located remotely from the power take-off, such as at a substation, to which multiple WEC units are connected, whereby all WEC units have access to a common energy storage system. This will be further described below with reference to Figs. 8a-c .

In this case a transformer may be added in each WEC to increase the voltage level for the power cables between the WEC units and substation, in order to reduce the current and thereby cost of the cables. [0043] Fig. 3a shows a schedule of an electrical system 200 of a wave energy converter connecting five ball screw actuators: four heave actuators 12 and one level actuator 122, each with a direct drive AC torque motor, an energy storage and an output power cable to a common DC bus 18 by means of respective AC/DC-converters 16c. The AC/DC-converters 16c are active converters connected to a controller 206 adapted to control a PTO device in accordance to control schemes. The DC bus 18 is preferably connected to an energy storage 16 and an output power cable 208 by means of a respective AC/DC-converter 16a, 205a.

[0044] It should be realized that any even number heave actuators 12, preferably two, four or six, can be used to share the load in the heave system which is exposed to much more frequent load cycles compared to the level actuator 122. It should also be realized that the heave system can be composed of a combination of ball screw actuators and pre-tension cylinders according to Fig. 1 c, preferably 2 or 4 ball screw actuators and 2 pre-tension cylinders. It should furthermore be realized that although ball screw actuators are embodied in this document, also other types of linear actuators such as rack and pinion and winches can be used together with a KERS using direct drive motors for the actuators, and also other types of pre-tension springs such as rubber cords, steel spring coils or hydraulic oil cylinders can be used.

[0045] Fig. 3b shows a similar schedule of the electrical system of a wave energy converter as Fig. 3a, but without energy storage 16 and AC/DC converter 16a, and with a transformer 205b instead of DC/AC converter 205a. The

transformer increases the voltage level from the electrical system in the WEC to the export cable, preferably from 400-690V to 1 1 -33 kV, to allow higher power to be transferred over the cable without increasing the cable dimensions. The shown embodiment of the electrical system is connected to a substation by means of power cable 208 and control signal interface 206a.

[0046] Fig. 3c shows a schedule of the electrical system in a substation, with cables 208 from six WEC^'s connected by dry-mate cable connectors and switch gears 209a connected to a common DC bus system 202b, and an energy storage unit 16, preferably a flywheel, connected to the DC bus system by means of AC/DC converter 16a and transformer 16b. The transformer reducing the voltage level from the DC bus to the flywheel AC/DC converter to a suitable voltage level for the flywheel motor, preferably 690 V. Switch gear 209b connects the DC bus system to a DC/AC converter 205a and power grid cable 208e. Control unit 206b is connected to all switch gears and converters, and with control interface 206a, to each WEC.

[0047] It should be realized that any number of WEC units can be connected to the substation, preferably 20 WEC units, whereby each WEC unit can use energy from the common energy storage, to provide the same KERS functionality with reactive force control as with a system where the energy storage unit is located in each WEC unit. Since all WEC units are connected to a common DC system, the system will have an advantage of the shifted power output from the WEC unit due to the timing of the incident waves to reach the location of each WEC, resulting in reduced variation of the total power in the system before the central energy storage device 16, and also lower storage capacity to smooth the output to a constant power, typically the total peak power is reduced by 50% and the total storage requirement is also reduced by 50%.

[0048] Figs. 4a and 4b show a top and sectional view, respectively, of a ring flywheel energy storage device 300 comprising a large diameter radial flux torque motor 310 with an outer stator ring 312 and an inner rotor ring 314 integrated with a flywheel rotor. The rotor 314 is preferably provided with permanent magnets and the stator 312 is provided with windings so as to functioning as an electric motor/generator. The rotor 314 is levitated and centered in the stator ring 312 by means of active magnetic bearing elements 316 that are evenly spaced along the rotor ring. A vacuum case 318 may be used to reduce or eliminate windage losses and to seal the flywheel from the surrounding environment.

[0049] Figs. 5a and 5b show a top and sectional view, respectively, of a ring flywheel energy storage device 300 comprising a large diameter axial flux generator / motor 310 with the rotor ring 314 integrated with the flywheel rotor and vertically stacked between two stator rings 312, where also a passive magnetic bearing system is integrated with the rotor and stator rings to levitate the rotor and keep it in position when the flywheel unit moves around inside the wave energy converter. Using passive bearings reduces the energy used, but they function as an undamped spring and oscillations can easily build up. Active magnetic bearing elements 314a are located on the inside of the rotor 314 to stabilize the rotor and make it possible to maintain a homogenous airgap between rotor and stator.

[0050] Fig. 5c shows a ring flywheel energy storage device 300 comprising two counter rotating ring flywheels according to Figs. 5a and 5b. This arrangement cancels the torque applied to the WEC structure from the torque applied in the flywheel to accelerate the rotor when charging energy and decelerating the rotor when discharging, and the two counter rotating flywheel rotors also cancel each other’s gyroscopic forces, which is applied to the PTO frame in case of a single flywheel.

[0051 ] Fig. 6a shows a power take-off device 100 with a single ball screw actuator 12 using a non-rotation ball screw 12a to be actuated by a prime mover in a wave energy converter, preferably a buoy, and a rotating ball nut 12b connected to a direct drive motor 13 journaled with a thrust bearing 13a to a PTO frame 1 10 attached on top of the mooring cylinder 126. A ring flywheel 300 is arranged on top of the PTO frame 1 10 around the ball screw 12a.

[0052] Fig. 6b shows a power take-off device 100 according to Fig. 2 but with a flywheel energy storage 300 according to Figs. 5a and 5b, which is arranged with the ball screw actuators going through the center hole of the ring flywheel.

[0053] Fig. 6c shows a power take-off device 100 according to Fig. 6a but with an energy storage 300 according to Fig. 5c, and with the first ball screw 12a left cut and the second ball screw 12a’ right cut, whereby the rotating ball nuts 12b, 12b’ in the two ball screw actuators are counter rotating relative to the linear motion of the system, whereby the torque applied on the WEC structure by the torque motors 14, 14’ are cancelled in the same way as with the two counter rotating flywheels. This provision of counter rotating ball screw actuators can be applied in all embodiments.

[0054] Fig. 7a shows an embodiment of a point absorbing WEC 400 with a single point mooring, where a power take-off device 100 according to Fig. 6a is placed in the prime mover of the WEC in the form of a floating buoy hull 404 floating on a water surface 402, and where a stiff mooring cylinder 410 extends through a linear seal 420 in the hull 404 down to a universal joint connection 430 to a foundation 440 on the seabed 450.

[0055] Fig. 7b shows an embodiment of a WEC 400 according to Fig. 7a but with a power take-off 100 according to Fig. 6c.

[0056] Fig. 7c shows an embodiment of a WEC 400 according to Fig. 7b but with a mooring cylinder 410 attached to a flexible pipe 460 and flange 462 coupling to the foundation 440 on the seabed 450.

[0057] Fig. 7d shows an embodiment of a WEC with three power take-off devices 100 shown in Fig.7c attached to a common submerged prime mover 400, i.e., buoy hull, by means of a respective universal joint 406.

[0058] Fig. 7e shows an embodiment of a WEC according to Fig. 7c, with a power take-off device 100 according to Fig. 1 c and a gas container in the form of a flexible pipe 26 wound around at the wide part of the prime mover, i.e. the buoy hull 404. The flexible pipe 26 is connected to the top port on the piston rod 22a to the pre-tension gas cylinder 24, and a nozzle 26a is located close to the port. During installation the gas container is not pressurized, and pre-tension is therefore not applied. Once the mooring is attached, gas is filled through the nozzle 26a and the springs are pressurized, preferably until the hull is submerged to the mid-point of the wide part. The gas is also emptied through the nozzle to remove the pre-tension force before the WEC is removed from site for

maintenance or decommissioning.

[0059] It should be realized that the gas container can also be of different forms and locations. In one embodiment, not shown, the mooring cylinder can be used to contain the pressurized gas and connect to a gas port close to the top of the cylinder instead of to the piston rod.

[0060] Fig. 8a shows an embodiment of a wave energy converter 400 according to Fig. 7e connected to a substation 500, preferably a spar buoy 501 with heave plates 502 and weight 503 to maintain a steady position in the waves, by means of power cable 208 with buoyancy elements 208c to prevent the power cable from touching the sea floor. The power cable goes from the PTO frame inside the buoy, through the mooring cylinder and exits at its base 208b, where there is no vertical motion and minimal horizontal motion. The power cable is connected to the substation above the water surface with a dry-mate power cable connector 208d. The spar buoy 501 is moored to the seabed with slack moorings 505 and the export cable 208e goes down to the seabed and then further into the point of common coupling with the electrical grid. The substation 500 comprises electrical system according to Fig. 3c, with an energy storage unit 16d, preferably a flywheel energy storage, and power electronics and control system 16e.

[0061 ] It should be realized that different types of energy storage can be used in the substation, such as flywheel, gravity storage, batteries, super conductor and thermal energy storage.

[0062] Fig. 8b shows a top view of the topology for a cluster with 20 WEC units 400 in the form of two rings and with substation 500 in the middle, where the inner ring of WEC^'s has radius R1 , preferably 90 meters, and the outer ring has radius R2, preferably 135 meters. Each ring comprises 10 WEC^'s, with the distance X between each WEC in the inner ring is 56 meter and the distance Y between each WEC in the outer ring is 83 meters. R1 and R2 can be adjusted and optimized depending on the size and capacity of each WEC unit, and so that the radiation waves from each WEC unit will to a large extent match the incident wave frequency in a way that do not degrade the overall power capture in the array.

[0063] Fig. 8c shows a top view of the topology for a cluster with 20 WEC units 400 in a single ring and with substation 500 in the middle, preferably with radius R1 180 meter and distance X between each WEC being 56 meters. [0064] It should also be realized that several clusters, preferably 10 clusters, can be placed together to make a larger WEC array, in which case the grid AC/CD converter is only located in one of the substations, while the other substations export DC power, whereby all WEC^'s and energy storage units operate on a common DC power grid system.

[0065] 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. It should be realized that even if batteries, super/ultra-capacitors, other types of flywheels or any other form of electrical energy storage is not currently available to meet the

requirements outlined in this document, this can change in the future, in which case such energy storage can be used as an alternative to the ring flywheel.

[0066] It will also be appreciated that the ring flywheel can be used as an energy storage in other kinds of PTO devices.

Claims

1. A power take-off device (100) for use in a wave energy converter having a prime mover (404), the power take-off device having a PTO frame (1 10) and being c h a racte r i zed by

- a direct drive motor (14) mounted to the PTO frame (1 10) and electrically

connected to an energy storage (16),

- a means (12) for converting linear motion from the movements of the prime mover (404) to rotary motion to the direct drive motor (14), and

- control means (206) adapted to control the direct drive motor (14) to apply a torque to the means for converting linear motion of the prime mover (404) into rotary motion to control the power take-off force by means of storing and retrieving energy in the energy storage (16) while outputting constant power to an output cable of the power take-off device.

2. The power take-off device according to claim 1 , wherein the means (12) for converting linear motion to rotary motion comprises a non-rotating screw (12a) fixedly connected to the prime mover (404), and a rotating nut (12b) mounted to the PTO frame (1 10), preferably by means of thrust bearing (15), with the rotor of the direct drive motor (14) mounted around the nut.

3. The power take-off device according to claim 2, wherein the screw (12a) is any of the following: ball screw, planetary roller screw, lead screw, hydrostatic screw, acme screw, and magnetic leadscrew.

4. The power take-off device according to claim 2 or 3, comprising a pair of means (12) for converting linear motion to rotary motion, wherein a first means comprises a left cut screw (12a) and a second means comprises a right cut ball screw (12a’), whereby the corresponding nuts are counter rotating relative to each other during operation.

5. The power take-off device according to claim 4, comprising a plurality of pairs of means (12) for converting linear motion to rotary motion, preferably two or three pairs.

6. The power take-off device according to any one of claims 2-5, wherein the direct drive motor (14) comprises a rotor (14a) in fixed connection with the nut (12b) of the means (12) to convert linear motion into rotary motion and a stator (14b) surrounding the rotor.

7. The power take-off device according to claims 1 -6, wherein the energy storage (16) comprises a flywheel energy storage (300) comprising an electric generator/motor (310).

8. The power take-off device according to claim 7, wherein the flywheel energy storage (300) comprises an electric generator/motor (310) with a stator (312) and a rotor (314) which is integrated with a flywheel.

9. The power take-off device according to claim 8, wherein flux between the stator (312) and the rotor (314) is any of the following: radial flux and axial flux.

10. The power take-off device according to claim 8 or 9, wherein the rotor (314) is hub-less.

1 1. The power take-off device according to any one of claims 7-10, wherein the flywheel comprises a magnetic levitation system (316) levitating the rotor.

12. The power take-off device according to claim 1 1 , wherein the magnetic levitation system comprises any of the following: active magnetic bearings (316) and a combination of passive and active magnetic bearings.

13. The power take-off device according to claim 1 1 , wherein the magnetic levitation system comprises a super conducting magnetic bearing.

14. The power take-off device according to any one of claims 7-13, wherein the energy storage comprises two counter rotating flywheels, each comprising a rotor (314), whereby torque applied to the power take-off device from acceleration / deceleration of the rotors is cancelled.

15. The power take-off device according to any one of claims 1 -14, comprising a DC bus (18) interconnecting the direct drive motor (14), the energy storage (16) and an output power cable (208), preferably by means of AC/DC- converters (16c).

16. The power take-off device according to any one of claims 1 -15, comprising a plurality of direct drive motors (14) and a plurality of means (12) for converting linear motion from the movements of the prime mover into rotary motion to the direct drive motors.

17. The power take-off device according to any one of claims 1 -16, comprising power electronics (14d) in the form of a ring surrounding a non-rotating screw (12a) of the means (12) to convert linear motion into rotary motion.

18. The power take-off device according to any one of claims 1 -17, comprising a gas spring (20) adapted to provide a pre-tensioning force.

19. The power take-off device according to claim 18, wherein the gas spring (20) comprises a gas piston rod (22a) fixedly connected to the prime mover (404) and a gas cylinder (24) fixedly connected to the PTO frame (1 10) .

20. The power take-off device according to any one of claims 1 -19, wherein the energy storage (16b) is provided remotely from the power take-off frame (110), preferably at a substation (500), to which multiple WEC units (400) are connected.

21. The power take-off device according to any one of claims 1 -19, wherein the energy storage (16) is provided on the PTO frame (1 10).

22. A wave energy converter comprising a power take-off device according to any one of claims 1 -21 , comprising a prime mover in the form of a buoy hull (404) mechanically connected to the power take-off device (100), and a mooring device (410, 460) attached to a foundation (440) on a seabed.

23. The wave energy converter according to claim 22, wherein the mooring device comprises a universal joint (430) attached to the foundation.

24. The wave energy converter according to claim 22, wherein the mooring device comprises a flexible pipe (460) and a flange (462) coupling on the seabed.

25. The wave energy converter according to any one of claims 22-24, wherein the power take-off device (100) is connected to the mooring device by means of a screw (122), whereby the distance between the power take-off device and the mooring device (126) can be adjusted.

26. The wave energy converter according to any one of claims 22-25, wherein the means (12) for converting linear motion from the movements of the prime mover (404) to rotary motion to the direct drive motor (14) comprises vertical, non-rotating ball screws (12a, 12a’) fixedly connected in their ends to a PTO top platform (130) and a PTO bottom platform (140), respectively, wherein the PTO top and bottom platforms are fixedly connected to the prime mover (404).

27. A wave energy converter (400’) comprising a prime mover (404’) in the form of a submerged hull is connected to a plurality of power take-off devices

(100) according to any one of claims 1 -21 , preferably three power take-off devices, each connected to a mooring device attached to a foundation on a seabed.

28. A wave energy system comprising a plurality of wave energy converters (400) according to claims 22-27 and a substation (500) comprising an energy storage (16b), to which the wave energy converters (400) are electrically connected.

29. The wave energy system according to claim 28, wherein the energy storage (16b) is a flywheel energy storage.