Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03B—MACHINES OR ENGINES FOR LIQUIDS
  • F03B13/00—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates
  • F03B13/12—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy
  • F03B13/14—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy
  • F03B13/16—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem"
  • F03B13/18—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore
  • F03B13/1845—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore and the wom slides relative to the rem
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03B—MACHINES OR ENGINES FOR LIQUIDS
  • F03B13/00—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates
  • F03B13/12—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy
  • F03B13/14—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy
  • F03B13/16—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem"
  • F03B13/20—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" wherein both members, i.e. wom and rem are movable relative to the sea bed or shore
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2260/00—Function
  • F05B2260/40—Transmission of power
  • F05B2260/403—Transmission of power through the shape of the drive components
  • F05B2260/4031—Transmission of power through the shape of the drive components as in toothed gearing
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2260/00—Function
  • F05B2260/40—Transmission of power
  • F05B2260/403—Transmission of power through the shape of the drive components
  • F05B2260/4031—Transmission of power through the shape of the drive components as in toothed gearing
  • F05B2260/40311—Transmission of power through the shape of the drive components as in toothed gearing of the epicyclic, planetary or differential type
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2260/00—Function
  • F05B2260/40—Transmission of power
  • F05B2260/406—Transmission of power through hydraulic systems
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/30—Energy from the sea, e.g. using wave energy or salinity gradient

Definitions

• the present invention relates generally to wave energy conversion and more particularly to a wave energy converter with improved power take-off.
• the so called reactive control strategy is known to provide the optimal PTO force to extract close to the maximum available energy from the waves but is not easy to implement in a real system due to high cost and high losses in the typical direct drive power take-off system. This is caused by the high amplitude of the total PTO force and a reactive part of the PTO force that alternate in direction relative the movement of the buoy, with significant round trip losses added when the reactive power flows back and forth through the generator of a typical direct drive power take-off system.
• An object of the present invention is to provide a cost-efficient wave energy converter with improved performance and reliability.
• a wave energy converter comprising a buoy and a means for converting linear motion into rotary motion connected to an input shaft of a power take-off device, which is
• the means for converting linear motion into rotary motion comprises a screw actuator.
• the buoy is adapted to move up and down with waves
• the power take-off device has a fixed vertical position
• the screw actuator comprises a non-rotating heave screw fixed to the buoy and a rotating heave screw nut journaled around the heave screw and arranged on a power take-off platform comprised in the power take-off device.
• a plurality of screw actuators is provided, each comprising a heave screw fixed to the buoy and a rotating heave screw nut journaled around the respective heave screw and located in the power take-off device.
• the buoy comprises a buoy hull, wherein the heave screw of each screw actuator is fixed to the buoy hull.
• the rotating heave screw nut of each screw actuator is arranged on the power take-off platform by means of thrust bearings.
• the screw actuator is any of the following: ball screw actuator, planetary roller screw actuator, lead screw actuator, hydrostatic screw actuator, acme screw actuator, and magnetic leadscrew actuator.
• a mooring arrangement is provided
• the mooring arrangement comprises a mooring pipe which in a first end is attached in the centre of a mooring yoke and in a second end to a sea bed, and a linear seal and bearing at the bottom of the buoy hull to prevent sea water from entering the buoy hull, and to provide a static route for a power cable, which preferably is arranged in a coil between the power takeoff platform and a mooring yoke in order to adjust the length when the height of the power take-off platform is adjusted.
• a level screw is connected to the power takeoff platform, preferably by means of a thrust bearing, and a clutch connecting the level screw to the power take-off device, preferably by means of a collection gear to a common drive unit, whereby the level screw is rotated by the power take-off device when the clutch is engaged in order to adjust the height above a seabed of the power take-off device.
• the screw actuator comprises a bevel gear with a first bevel pinion attached to an input shaft connected to the power take-off arrangement and a second bevel pinion attached to a ball nut mounted to a heave screw.
• the screw actuator comprises a second bevel gear comprising the first bevel pinion, wherein each bevel gear is mounted to the ball nut by means of a respective clutch.
• a collection gear is provided connecting a plurality of screw actuators to a common drive unit of the power take-off device.
• a gear ratio in the collection gear is adapted to provide the same travel speed for both heave screws and level screws, whereby the buoy will be kept in a steady position while the height of the power take-off platform above a seabed is adjusted by means of the power take-off device when level clutches are engaged.
• At least three screw actuators are provided connected to a first sun gear on a central output shaft and at least two level screws connected to a second sun gear axially displaced below the first sun gear, in order to allow different diameters of the sun and larger gear ratio from the heave screw gears to the heave sun pinion.
• a gas cylinder is provided connected to a gas bottle by means of pipes, wherein the gas cylinder is adapted to provide a pretension force between the buoy and the PTO platform.
• the power take-off device comprises: an input shaft connected to the screw actuator, a variable transmission means connected to the input shaft, the variable transmission means having an output shaft, an energy storage device comprising a flywheel, and an active power transfer device, wherein the variable transmission means is adapted to control the amplitude of power stored and retrieved to/from the flywheel energy storage device and the amplitude and direction of the force applied to the buoy from the power take-off device, by changing the gear ratio in the variable transmission means.
• the power take-off device comprises: an input shaft connected to the screw actuator, a variable transmission means comprising a hydrostatic ITV connected to the input shaft, the variable transmission means having an output shaft, an energy storage device comprising a flywheel, and an active power transfer device, wherein the variable transmission means is adapted to control the amplitude of power stored and retrieved to/from the flywheel energy storage device and the amplitude and direction of the force applied to the buoy from the power take-off device, by changing the ratio in the variable transmission means between the input shaft and the output shaft.
• the power take-off device comprises: an input shaft connected to the screw actuator, a variable transmission means in the form of an electric ITV connected to the input shaft, the variable transmission means having an output shaft, an energy storage device comprising a flywheel, and an active power transfer device, wherein the variable transmission means is adapted to control the amplitude of power stored and retrieved to/from the flywheel energy storage device and the amplitude and direction of the force applied to the buoy from the power take-off device, by changing the ratio in the variable transmission means between the input shaft and the output shaft.
• Fig. 1 a is a schematic view of a power take-off device comprising a kinetic energy recovery system and a generator;
• Fig. 1 b is a schematic view of a power take-off device comprising a kinetic energy recovery system, a CVT and a generator
• Fig. 1 c is a schematic view of a power take-off device comprising an IVT and a twin flywheel energy storage device with a brake to the primary flywheel and a generator to the secondary flywheel;
• Fig. 1 d is a schematic view of a power take-off device comprising an IVT and a twin flywheel energy storage device with one generator connected to each flywheel;
• Fig. 1 e is a schematic view of a power take-off device comprising an IVT and a twin flywheel energy storage device with a generator and a clutch to lock the primary flywheel to the input shaft;
• Fig. 1f is a schematic view of a power take-off device comprising a CVT and a twin flywheel energy storage device with a generator and a clutch to lock the primary flywheel to the input shaft;
• Fig. 2a shows a schematic view of a hydrostatic IVT system
• Fig. 2b shows a schematic view of an electric IVT system
• Fig. 2c shows a drive unit comprising an electric IVT and active power transfer device, and a flywheel;
• Fig. 2d shows a complete drive unit with two counter-rotating flywheels
• Fig. 4a is a schematic view of a wave energy converter with a ball screw actuator and a power take-off according to Fig. 1 a;
• Fig. 4b is a schematic view from a different angle of a wave energy converter according to Fig. 2a;
• Fig. 5 is a schematic view of a wave energy converter according to Fig. 2a with a pre-tension cylinder added;
• Fig. 6a and 6b are schematic views of the pre-tension gas spring cylinder system
• Fig. 7 a shows a side view of a power take-off with a drive unit connected to a ball screw actuator by means of a bevel gear
• Fig. 7b shows a power take-off similar to Fig. 9a with a rectifier mechanism comprising two bevel gears connected to the ball nut by means of clutches;
• Fig. 7c shows a side view of a power take-off with one drive unit vertically mounted and connected to two ball screw actuators through a collection gear;
• Fig. 7d shows a side view of a power take-off with one drive unit vertically mounted and connected to two ball screw actuators by means of a differential gear with a similar rectifier mechanism as shown in Fig. 9b;
• Fig. 7e shows a ball screw actuator with two ball nuts using a roller cage torque balancing mechanism
• Fig. 7f shows a power take-off using two ball screw actuators according to Fig. 9e, with a roller cage torque balancing mechanism between the two actuators;
• Fig. 8a shows a power take-off using two ball screw actuators connected to a common drive unit through a collection gear
• Figs. 8b and c show a top and a side view, respectively, of a collection gear system to connect for four ball screw actuators to a common drive unit;
• Fig. 8d shows a ball screw top plate with a hydrostatic torque balancing
• Fig. 9a shows a power take-off comprising a hydraulic cylinder actuator connected to a drive unit by means of hydraulic pump / motor;
• Fig. 10a is a schematic view of the collection gear in the screw actuator, joining two heave screws and two level screws to a common power take-off device;
• Fig. 10b is a schematic view of the collection gear in the screw actuator, joining three heave screws and three level screws to a common power take-off device;
• Fig. 1 1 a and 1 1 b are schematic views from different angles of a collection gear with separate sun pinions for three heave gears and three level gears;
• Fig. 1 1 c is a schematic view of a collection gear according to Fig. 1 1 a and 1 1 b only showing the heave mechanism;
• Fig. 1 1 d is a schematic view of a collection gear according to Fig. 1 1 a and 1 1 b only showing the level mechanism;
• Fig. 12a is a schematic view of a wave energy converter according to Fig. 2a in normal operational mode with moving hull and a fixed power take-off;
• Fig. 12b is a schematic view of a wave energy converter according to Fig. 2a in level adjustment mode with fixed hull and moving power take-off;
• Fig. 13a shows a wave energy converter (WEC) with the rack leg extending to a universal joint at the seabed, and with elastic mooring ropes connected between the hull and the seabed;
• WEC wave energy converter
• Fig. 13b shows a similar WEC as Fig. 13a but with a shorter rack leg connected to the seabed through a mooring rope and without the elastic mooring ropes;
• Fig. 14a shows a WEC with a similar power take-off comprising two drive units, which are fixed on a platform on top of a mooring cylinder attached to the seabed through a mooring rope;
• Fig. 14b shows a WEC with the drive unit fixed on a platform on top of a mooring cylinder attached to the seabed through a flexible pipe;
• Fig. 14c shows a similar WEC as Fig. 14b but with a level system added between the PTO platform and the mooring cylinder, and with the cylinder extending to a universal joint at the seabed.
• a power take-off device for use in a wave energy converter according to the invention, comprising a kinetic energy recovery system to control the PTO force and to smooth captured power into a constant output, and a ball screw actuator mechanism will be described in detail.
• PTO platform When reference is made to PTO platform, this expression should be construed to encompass all forms of structures enabling the provision of a PTO at a distance from a seabed, such as chassis, frames etc. Also, when it is stated that this PTO platform is provided on a fixed height or distance from the seabed, this refers to normal operation of the WEC. As described herein, it is possible to adjust the distance to the seabed in order to take account of tidal variations etc., but the PTO platform is still provided on a "fixed height".
• KERS Kinetic Energy Recovery System
• 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.
• IVT Infinite Variable Transmission
• IVT Infinite Variable Transmission
• PPG Planetary Gear Train
• 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:
• the damping component 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 the reactive component is 90 degrees out of phase with the buoy velocity, i.e. the reactive 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 component of the PTO force constitutes the active power transfer from the power take-off, and the reactive force constitutes the reactive power, which goes back and forth between buoy and power take-off.
• the general PTO force is represented as
• the first equation is the phase or resonance condition which depends on the reactive PTO force component and the second equation is the amplitude condition which depend on the damping PTO force component.
• the optimal reactive force depends on the effective mass (inertia), hydrostatic stiffness and wave frequency and the optimal damping force depends only on the radiation damping when the phase condition is satisfied.
• the wave energy converter (WEC) with a power take-off device aims at using a kinetic energy recovery system to control the total PTO force, and an active power transfer device in the form of a generator or a hydraulic or pneumatic pump for exporting a constant power from the WEC.
• optimal power capture is achieved when the natural frequency of the buoy correlates with the wave frequency.
• a buoy with a suitable diameter must have a very large mass to be naturally resonant in the dominant wave period and will also capture much less power in both longer and shorter wave periods, which is not ideal.
• a WEC should instead be designed to have a low mass and high natural resonance frequency and use phase control techniques to tune the resonance frequency of the buoy to match with the waves across the full range of occurring wave frequencies.
• a PTO capable of tuning the inertia, damping and spring coefficients of the PTO force can provide the optimal phase and amplitude conditions in all waves and thereby capture close to the theoretical maximum of available energy.
• a PTO only capable of providing optimal damping will capture only a fraction of that energy.
• a fixed point of reference is needed to counteract the PTO force. This can be provided by a fixed structure, or by adding a pre-tension force to the PTO force in a tethered point absorber. A pre-tension force is selected to always exceed the control forces, thereby maintaining a force in only one direction in the tethering. With pre-tension included in the PTO force, the combination of all components provides a total sinusoidal tethering force as shown in Fig. 8. The total PTO force acts against the buoy motion in the rise of the wave and with the buoy motion in the decent, resulting in an alternating power flow between wave and PTO, with the average power being the active power transfer from the system or the exported power.
• a Kinetic energy recovery system with flywheel storage is ideal to provide this force cycle which require high power rating, low energy rating and extreme cycle life.
• the power take-off device according to the invention can provide all three components of the PTO force in order to satisfy the optimal phase and amplitude condition, and also the pre-tension force, which will be shown by way of example.
• Fig. 1 a shows a power take-off device or drive unit, generally designated 10, with a mechanical IVT based kinetic energy recovery system 20 with an input shaft 12 and a generator 40.
• the IVT 22 comprises a CVT 24 with a single cavity double toroidal variator.
• the input disc of the CVT 24 is coupled to the sun gear of a planetary gear train (PGT) 26, the output disc to a ring gear of the PGT and the carrier gear of the PGT is coupled to a flywheel 30.
• PGT planetary gear train
• This arrangement provides geared neutral when the rollers are in an angle that provides the same gear ratio as the PGT 26, where the output disc of the CVT 24 rotate with the same speed in the opposite direction to the carrier.
• the total PTO force comprising reactive, active and pre-tension forces follows a sinusoidal wave form, which is provided by means of ratio control of the IVT 22.
• the gear ratio of the IVT 22 is increased with a controlled ratio to speed up and charge the flywheel while providing an amplitude that follows the required force trajectory in the opposite direction to the buoy motion.
• the IVT 22 is first set to reverse direction and then the gear ratio is reduced to slow the speed and discharge the flywheel by applying a controlled change rate to follow the force trajectory in the same direction of the buoy motion.
• a large flywheel 30, such as one with a weight of 500 - 1000 kg, can be used whose speed changes slowly to balance the variation of power extracted in irregular waves, with only small change in speed across individual waves. This speed may be around 20 000 rpm.
• the IVT mechanism allows the flywheel to keep spinning with high speed through the turning points of the buoy motion while the PTO force is maintained according to the force trajectory, at the same time as the generator can be controlled to output a constant sea state tuned power from the flywheel.
• Fig. 1 b shows a similar configuration of the power take-off according to Fig. 1 a with a CVT 24' added between the flywheel 30 and generator 40 to provide a constant speed and frequency of the generator, whereby the generator can be used without a frequency converter and output constant power on any level by means of torque control.
• Fig. 1 c shows an alternative configuration of the power take-off with an IVT 22 and a flywheel energy storage device comprising two flywheels 30, 30' and a planetary gear train 26', with the carrier coupled with the IVT 22, the ring gear coupled to a primary flywheel and the sun gear coupled to a secondary flywheel.
• a brake 38 is connected to the other side of the primary flywheel 30 and a generator 40 is connected to the other side of the secondary flywheel 30'.
• 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 22 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 torodial variator is most efficient. This will significantly improve 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.
• the input power from the PTO is split to both flywheels, while only the secondary flywheel outputs active power to drive the generator 40.
• the brake to the primary flywheel 30 is used to direct enough power to the secondary flywheel 30' to prevent the primary flywheel 30 to gradually increase after each force cycle.
• the buoy starts to rise the input power from the PTO to the carrier of the flywheel energy storage device is lower than the output power from the sun, the ring gear and primary flywheel will rotate backwards, feeding power from the primary flywheel 30 to the secondary flywheel 30' to maintain a constant output power to the generator 40, while the carrier torque is maintained by the PTO.
• the ring gear When the input power has increased to reach the same level as the output power from the sun to the secondary flywheel 30' and generator 40, the ring gear will have zero speed at which point it is locked by the brake for a timed duration.
• all power is directed to the secondary flywheel until the ring gear and primary flywheel 30 is 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 30', corresponds to the total active energy transfer from the system during a complete PTO force cycle.
• the primary flywheel 30 When the buoy descends in the wave, the primary flywheel 30 will discharge and return just enough energy to the PTO to return the carrier speed to zero when the cycle completes.
• FIG. 1 d shows a power take-off device according to Fig. 1 c with a primary generator 40 connected to the primary flywheel 30 instead of to the brake and with a secondary generator 40' connected to the secondary flywheel 30', whereby the torque on both generators is controlled to output the active power from each flywheel while maintain a constant power output from both generators together.
• FIG. 1 e shows a power take-off device according to Fig. 1 c with a clutch 36 between the carrier gear and ring gear instead of the brake in the twin flywheel energy storage device 30, 30', whereby the clutch 38 is closed for a short duration at the point where input power from the PTO becomes larger than the output power to the generator, which is at the moment when the carrier gear reaches the same speed as the ring gear at the same time as the PTO inputs power to the flywheel energy storage device, with the purpose to direct the equivalent of the full amount of active energy extracted from the waves to the secondary flywheel 30' and generator 40.
• Fig. 1f shows a power take-off device according to Fig. 1 e but with a CVT 24 instead of an IVT.
• This power take-off configuration is used in combination with external pre-tension gas springs shown in Fig. 4a, where the power take-off device is only arranged to provide the reactive and active forces through the CVT 24 and flywheel energy storage device 30, 30'.
• the sum of the reactive and active power is bi-directional and is only slightly out of phase with the buoy velocity.
• the flywheels are in this case charged through the first and third quarters of the cycle and are discharged through the second and fourth quarters.
• the carrier gear is in this case returned to zero also in the upper turning point of the buoy in the crest of the wave.
• the ratio spread of the CVT 24 can be limited through the full cycle, preferably within ratio 6: 1 to 1 :6.
• the carrier is furthermore rotating forward in the accent of the buoy motion and backwards in the decent of the buoy motion.
• the rectification of the reciprocating direction of the carrier gear input to the flywheel energy storage device to the sun gear is then provided by the ring gear.
• Fig. 2a shows a hydrostatic IVT 50 where the ports of two hydraulic pumps / motors 52, 52' are connected together, with one pump / motor 52 connected to an input shaft 54 and the other pump / motor 52' to an output shaft 56 of the hydrostatic IVT 50.
• At least one of the pump / motors 52, 52' have variable displacement, which enables the gear ratio of the hydrostatic IVT 50 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.
• pump / motors with digitally controlled valves so called displacement motors, can reverse the rotational direction of the output shaft by means of valve control.
• Fig. 2b shows an electric IVT 60 where two electric generators / motors 62, 62' with variable speed drives are connected to a common electric circuit 68.
• a first generator / motor 62 is connected to an input shaft 64 of the electric IVT 60 and a second generator / motor 62' is connected to an output shaft 66 of the electric IVT 60.
• the flywheel in the drive unit When the flywheel in the drive unit is charged, the first generator / motor 62 is driven like a generator and the second generator / motor 62' like a motor, transferring power from the first generator 62 to the second motor 62'.
• the torque of the first generator 62 is controlled for the force control, and the resulting motor torque depend on the speed of the second motor and the power transferred from the first generator.
• the second generator / motor 62' is operated as a generator and the first generator / motor 62 is operated as a motor, and the discharge is controlled with torque on the second motor.
• multiple inputs and outputs can be used.
• one generator / motor can be connected to each actuator and to a common electric circuit. Load balancing between the actuators can be done through generator / motor torque control.
• two counter rotating flywheels on the same axle can be used with one generator / motor connected to each flywheel, to cancel angular momentum and precession due to gyroscopic effects.
• Fig. 2c shows a complete drive unit 10 with an electric IVT 60 and flywheel 30, in which case the generator on the other side of the flywheel is not used.
• Active power is extracted directly from the electric circuit 68 by means of power electronics, i.e. a part of the input power to the electric IVT 60 is extracted as active power.
• 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.
• Fig. 2d shows a complete drive unit 10 similar to the one of Fig. 2c, but with an energy storage device comprising first and second counter rotating flywheels 30, 30', each driven by a respective electric generator/motor 62, 62'.
• the electric generators/motors are interconnected by means of a power cable 68 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.
• Figs. 3a and 3b show a front and side view, respectively, of a power take-off in a Wave Energy Converter (WEC) 100 according to the invention using a screw actuator to convert linear motion of the buoy into rotational input to the power take-off device.
• the WEC 100 comprises a buoy 1 10 with a buoy hull 1 10a.
• Two heave screws 120, 120' are fixed with the buoy hull structure 1 10a and a PTO 130 is located on a platform 130a with rotating heave screw nuts 132, 132', which are journaled around a respective of the heave screws 120, 120'.
• the heave screw nuts 132, 132' are journaled with thrust bearings from below the PTO platform 130a and coupled with gears to the sun gear of a collection gear 160, which will be explained in more detail below with reference to Figs. 6a-c.
• the sun gear is coupled to the input shaft of the power take-off device 130.
• the PTO platform 130a has a fixed vertical position while the buoy 1 10 with the buoy hull 1 10a moves up and down with the waves, as indicated by the three positions in Fig. 3c.
• the heave screws 120, 120' are preferably of the type ball screw or planetary roller screw, using high lead angle for high efficiency in both forward and backward driving.
• Two rotating level screws 134 are also attached to the PTO platform 130a through thrust bearings 134b from above the PTO platform, coupled to a clutch 134a and gears to the sun gear coupled with the power take-off.
• a mooring yoke 142 connected to a flexible mooring pipe 144 of a mooring arrangement 140 comprises one fixed nut for each level screw 134 and is moved in relation to the PTO platform when the screws are rotated.
• the level screws 134 are preferably of the type buttress thread screw with a low lead angle and self-locking to prevent back driving when the clutches are disengaged. It will be realized that a single level screw is sufficient for achieving the height adjustment.
• the level screw clutches 134a are normally open, whereby the screws are not driven by the power take-off device and locked from rotating by the self- locking mechanism. In this mode, the buoy hull heaves around the PTO platform as shown by Fig. 3c. When the level screw clutches 134a are closed, the level screws are rotated in the same direction as the heave nuts 132, moving the PTO platform down relative to the mooring pipe 144 at the same time as the buoy 1 10 is moving up relative to the PTO platform 130a.
• the PTO platform By selecting gear ratios between the heave and level gears in the collection gear to provide the same travel speed of both heave and level screws, the PTO platform is moved vertically while the buoy is kept steady when the level clutches are engaged. This mechanism is used for adjusting the length of the tethering for tidal variations and also to submerge the buoy in storm conditions, as shown be Fig. 3d.
• the arrangement with heave screws and level screws joining at a PTO platform also provides a relatively static route for a power cable 138, only subject to tidal adjustments and not heave motion, through the mooring yoke and a pipe which goes out of the bottom of the hull.
• part of the power cable is preferably configured as a coil 138a.
• the cable runs in the form of a cable coil.
• Linear bearings are used to resist the horizontal loading between the hull and the mooring pipe and seals are used to prevent sea water to enter the hull.
• the pipe can be of a flexible type to reduce the horizontal forces on the linear bearing.
• a linear bearing and pipe seal 46 is provided between the buoy hull 1 10a and the flexible mooring pipe 144.
• Fig. 4a shows a Wave Energy Converter similar to the one described above with reference to Figs. 3a and 3b with a pre-tension gas cylinder system 150 added to provide pre-tension force, whereby the power take-off device 130 mainly provides reactive and active force components.
• Gas cylinders 152 are fixed to the bottom of the buoy hull 1 10a and a rod connected to with the PTO platform 130a to push the buoy down relative to the PTO platform 130a.
• the pre-tension force should be close to constant, which requires a large external gas volume provided by gas bottles 154 connected to the gas cylinder 152 by means of gas pipes 156 relative to the cylinder volume, and a pre-charge gas pressure.
• the power take-off device 130 is used to balance the variable pressure in the cylinders due to the gas compression cycle to provide a constant pre-tension force on the buoy.
• Figures 4b and 4c show a front and side view, respectively, of only the pre-tension gas spring system 150, comprising two gas cylinders 152 and two gas bottles 154 connected to a common pneumatic system 156.
• the gas bottles 154 can alternatively be placed side by side with the gas cylinder to reduce the length of the gas pipes 156.
• Fig. 5a shows a power take-off with a drive unit 10 connected to a ball screw actuator 150 by means of a bevel gear 152 between the ball nut 132 and input shaft 12 of the drive unit 10.
• the bevel gear comprises a first bevel pinion 154 attached to the input shaft 12 and a second bevel pinion 156 attached to the ball nut 132.
• a ball screw actuator 150 using a rotating nut 132 and non-rotating screw 120 can manage longer stroke at high velocities, compared to arrangements with rotating screws.
• Other types of power screws such as planetary roller screws, lead screws, hydrostatic screws and magnetic leadscrews can also be used.
• Fig. 5b shows an embodiment similar to Fig. 5a, with two bevel gears 152, 152' are connected to the same bevel pinion 154, each bevel gear being mounted to the ball nut by means of a respective clutch 156, 156'.
• a respective clutch 156, 156' During the up stroke of the buoy, only the upper clutch 156' is engaged. During the down stroke of the buoy, only the lower clutch 156 is engaged. This way the rotational input to the drive unit is rectified to be in only one direction.
• both clutches 156, 156' engaged the actuator 150 is locked.
• both clutches 156, 156' open the pinion 154 connected to the drive unit 10 is decoupled from the ball screw actuator.
• Fig. 5c shows an embodiment with two ball screw actuators as shown in Fig. 5b connected to a common drive unit 10 through a collection gear 160. Such embodiment can be extended with further ball screws connected to a common drive unit.
• Fig. 5d shows similar embodiment as Fig. 5c but with only one bevel gear 152 on each ball screw nut 130 and with the bevel gears 152 of two ball screw actuators 150 connected to a collection gear in the form of a differential gear 134 to provide even load sharing between both ball screws.
• the rectifier mechanism is instead implemented in the differential gear 134 by using two bevel ring gears connected to the differential cage by means of clutches 134a, 134a'. The torque is reduced by the gear ratio of first bevel gears 134 on the ball nuts before the clutches.
• Fig. 5e shows a similar embodiment according to Fig. 5a, but with a double ball nut assembly to increase the load capacity of the ball screw actuator, using a stiff torque balancing mechanism in the form of a roller cage 153.
• the pins of the roller cage are mounted with spherical bearings 154 in the flanges 156a of the two ball nuts; an upper ball nut 156 and a lower ball nut 156'.
• a bevel gear 152 is placed in the center of the roller cage, with spherical bearings to hold the pins at the pivot point.
• Both ball nuts are mounted to a fixed structure with thrust bearings 158, in the figure only shown to take load in one direction.
• Another set of thrust bearings is added to take loads in both direction in embodiment with bi-directional force.
• the vertical alignment of both ball nuts and the bevel gear are fixed with thrust bearings.
• the angular position of the two ball nuts are in this way allowed to be slightly displaced from each other without affecting the angular position of the bevel gear. This way the torque is evenly distributed between the two ball nuts.
• multiple starts of the ball screw are used to increase the load capacity of a ball screw actuator.
• Fig. 5f shows a similar embodiment as Fig. 5d using the double ball nut assembly 150 shown in Fig. 5e, and the same type of roller cage 159 to balance the torque load between two ball screw assemblies instead of using a differential gear.
• a roller cage can be more suitable than a differential gear since the displacement of the angular position between the shafts are relatively constant.
• Fig. 6a shows a similar embodiment as Fig. 5c without the rectifier mechanism and with two ball screw actuators 150 connected to a common drive unit 10 through a collection gear 160 using spur or helical gears.
• Figs. 6b and 6c show different views of the embodiment according to Fig. 6a with collection of four ball screw actuators 150 connected to a common drive unit 10.
• a planet gear for each ball nut is connected to a common sun shaft 164 in two levels.
• An upper level with two planet gears 162 connects to an upper sun gear 164 and a lower level with two planet gears connects to a lower level sun gear 168.
• This arrangement enables larger gear ratio to be used between planet gears and sun gear when using more than two ball screws in the actuator system. It is possible to extend each level with further planet gears to increase the number of ball screws.
• Fig. 6d shows the top ball screw plate 170 used together with the ball screw actuation system shown in Fig. 6b, using a hydrostatic load balancing mechanism comprising a lever arm 172 connected to each ball screw and a hydraulic cylinder 174 connecting the tip of the lever arm 172 with the top ball screw plate at a joint 174a. All four hydraulic cylinders are connected to the same hydraulic circuit 176, this way each screw is rotated in relation to each other until the torque applied on all screws are equal.
• a hydraulic accumulator can be added to the circuit to introduce some elasticity to absorb shock loads.
• rotating hydraulic pumps/motors preferably of the type radial piston motor can be used to provide the equivalent load balancing mechanism.
• a hole 178 is provided in the top ball screw plate 172 for a drive unit.
• double ball nut assembly shown in Fig. 5e can be used also together with the embodiments shown in Figs. 6a-d, in which case the bevel gears in the ball nut assemblies are replaced with planet spur or helical gears.
• Figure 7a shows a collection gear 160 with two heave gears 160a, two level gears 160b and a sun gear 160c, where the diameter is selected depending on the lead angle and diameter of the screws to provide equal travel speeds of the heave and level screws when the level clutch is engaged.
• the sun gear 160c is smaller than the heave gears 160a, providing a step-up gearing from the screw to the sun gear to increase the input speed to the power take-off device.
• Fig. 7b shows an alternative collection gear with three heave gears 160a and three level gears 160b, with diameters also selected to provide equal travel speeds of the heave and level screws when the level clutch is engaged.
• Using three heave and level screws makes the screw assembly more stiff and resistant to side loads from all directions, also the tethering force is shared between three screws instead of two, which reduces the diameter of each screw and thereby increases the rotational speed for a given lead angle and travel velocity.
• Figures 8a-d show an alternative collection gear 160 with the heave gears 162a connected to a first sun gear 160c and the level gears connected to a second sun gear 160h.
• This arrangement makes it possible to use higher gear ratio from the heave gears to the sun gear.
• the heave nut and heave nut gears 162a are rotated by the heave screws 120 and attached with the heave sun collection pinion.
• the heave nut is journaled to the PTO platform 130a with a thrust bearing 160f to transfer force in only one direction.
• the level screw gear 160b is connected to the rotating screw 160g through the level clutch 160e and is journaled in the PTO platform with thrust bearings in the other direction.
• Fig. 9a shows a wave energy converter 100 with a ball screw actuator 150 according Fig. 5a, with two drive units 10 attached to a steel cylinder connected to a mooring rope 148, and with a non-rotating ball screw 120 attached to the buoy hull 1 10a.
• the mooring arrangement 140 comprises a linear bearing and seal 146, a mooring rope 148 attached to a seabed foundation 180 below the water surface 182, and a mooring cylinder 149.
• Fig. 9b shows a wave energy converter similar to Fig. 9a but with a ball screw actuator 150 according to Fig. 6a and a mooring pipe 148' instead of the mooring rope, which gives a protected and static route for a power cable inside the steel cylinder 149 and mooring pipe 148'.
• Fig. 9c shows a wave energy converter similar to Fig. 9b without the flexible pipe section of the mooring system. Instead the mooring cylinder 149 extends to a flexible joint 184 at the seabed.
• the PTO platform 130a is attached to the top of the mooring cylinder 149 through a lead screw mechanism, enabling the height of the PTO platform above the seabed to be adjusted.
• a motor 192 is arranged to rotate a screw 194 mounted with a thrust bearing 194a to the PTO platform 130a, a ball nut 196 is mounted at the entrance to the mooring pipe.
• Linear guides 147 block the mooring pipe and hull from rotating, thus the rotation of the lead screw or planetary roller screw result in a change of the vertical position of the PTO platform relative to the sea bed. This is used to adjust the PTO position for tidal variations and to submerge the buoy in case of extreme weather to protect it from damage. The buoy is also submerged when the waves are too small to extract energy from. This way potential energy is stored that can be used to spin the flywheels up to the operating range when the wave energy converter resumes normal operation.
• the power cable is wound on a real to manage the variation in height between the top of the mooring cylinder and the PTO platform when operating the level system.
• Using a separate level system enables heave system to be designed for a shorter stroke, to save cost and although long ball screws exist, they are not common for the required force levels and can be expensive.
• the ball screws used for the heave action is furthermore exposed to more than a thousand times more cycles, it is therefore more cost effective to use as short length of the heave system as possible and manage level adjustment with a separate system.
• the linear guide system shown in Fig. 9c is not entirely correct.
• two guide rails are arranged from the top ball screw plate all the way down to the bottom of the buoy hull, un-supported between the top and bottom plates for the ball screws, and then supported against the hull cylinder below the bottom ball screw plate.
• Guide wheels are mounted on the PTO platform and on top of the mooring cylinder.
• the guide system has the purpose of unloading the ball screws from radial loads and to cancel torsional forces from the ball screw actuation system to prevent the buoy from spinning.

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• Combustion & Propulsion (AREA)
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• General Engineering & Computer Science (AREA)
• Other Liquid Machine Or Engine Such As Wave Power Use (AREA)

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• 2018
  • 2018-09-10 EP EP18854672.5A patent/EP3728830A4/de active Pending
  • 2018-09-10 WO PCT/SE2018/050906 patent/WO2019050466A1/en unknown

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