WO2017164803A1

WO2017164803A1 - Power take-off, wave energy converter comprising such power take-off and method for controlling such power take-off

Info

Publication number: WO2017164803A1
Application number: PCT/SE2017/050277
Authority: WO
Inventors: Mikael Sidenmark
Original assignee: Ocean Harvesting Technologies Ab
Priority date: 2016-03-22
Filing date: 2017-03-22
Publication date: 2017-09-28
Also published as: EP3433486A1; EP3433486A4

Abstract

A power take-off device for use in a wave energy converter comprises a buoy (1) and a means (2, 3) for converting linear motion from the movements of the buoy (1) into rotary motion. By providing at least one flywheel arrangement comprising a flywheel (6) and a transmission means (5; 9) with a stiff connection adapted to engage and disengage the flywheel to and from the means (2, 3) for converting linear motion from the movements of the buoy (1) in relation to the frequency of the wave, in different sea states and wave sizes, to increase power capture by matching the natural frequency with the waves to make the buoy resonate, and also to limit end stop forces by de-tuning the natural frequency in large waves and thereby reduce the amplitude of the buoy motion. A method of controlling a power take-off device is also provided.

Description

POWER TAKE-OFF, WAVE ENERGY CONVERTER COMPRISING SUCH POWER TAKE-OFF AND METHOD FOR CONTROLLING SUCH POWER TAKE-OFF

Technical field

[0001 ] The present invention relates generally to wave energy conversion and more particularly to a power take-off device with improved control function to increase power capture and reduce end stop forces.

Background art

[0002] A wave energy converter has greatly increased power capture at the point where the natural frequency of the buoy correlates with the wave frequency, i.e. where the buoy becomes resonant with the waves. However, the buoy has to have a very large mass to get a natural frequency that matches with the wave frequency in the average sea state. A wave energy converter with a large mass will also not be resonant in higher or lower wave frequencies, i.e. the natural frequency is quite narrow banded in relation to the frequency range in ocean waves.

[0003] It has been proposed different techniques to use phase control in order to keep a light weight buoy resonant within a wider range of sea states and wave frequencies.

[0004] Reactive control is well known for being the optimal control in terms of power capture but too costly to be used in reality. The force applied to the point absorber by the power take-off is the sum of two different components, one corresponding to a damping component of the buoy motion and the other resembling the effect of a mass variation on the point absorber (reactive component). The ideal case (optimum control) would be to use the reactive component in the control to exactly cancel the effect of the difference between the effects of the hydrostatic stiffness and of the mass of the point absorber. The result would be to have a light weight point absorber oscillate in phase with the excitation force from the waves. The component of the PTO force that resembles the effect of mass variation is however in the order of a magnitude larger than the damping component and contains a reactive part which means that power flow is reversed in the power take-off to push the buoy before the end strokes. In a direct drive system, the generators has to be greatly oversized to apply reactive control and there are great losses associated with reversing the flow of energy and taking power from the grid to apply reactive force to the buoy.

[0005] Instead of applying the reactive component through the power take-off, a similar force that cancel the effect of the difference between the effects of the hydrostatic stiffness and of the mass of the point absorber can be applied through a negative spring component. This allows the power take-off to be sized only for the damping component while the phase control is done completely through the negative spring arrangement. A negative spring can be implemented with pivoting gas cylinder springs. Pivoting cylinders applies a force which is proportional to the position of the buoy, i.e. the angle of the gas cylinders. One drawback with this method is that the position of the gas cylinders has to be adjusted for changing sea levels due to tides etc., the cylinders should be horizontal close to the sea level. Another drawback is that a negative spring is broad banded. This is positive for the power capture but makes it difficult to rapidly de-tune the spring in order to limit the stroke of the rack for individual waves, which may result in high end stop forces that drives the cost of the wave energy converter.

Summary of invention

[0006] An object of the present invention is to provide a power take-off for wave energy converters wherein the drawbacks of prior art are mitigated and wherein the efficiency and control functions are improved.

[0007] According a first aspect of the invention there is provided a power takeoff device comprising a buoy and a means for converting linear motion from the movements of the buoy into rotary motion, characterized by at least one flywheel arrangement comprising a flywheel and a transmission means adapted to engage and disengage the flywheel to and from the means for converting linear motion from the movements of the buoy into rotary motion, wherein the transmission means comprises any of the following: a clutch and a continuous variable transmission. By providing a flywheel with a stiff connection, such as a clutch or a continuous variable transmission, which enables controlled coupling and decoupling of the flywheel, the effective mass of the buoy can be changed in order to match the natural frequency of the buoy with the waves, thereby improving the power capture of the wave energy converter by putting the buoy in a resonant state with the waves. With a continuous variable transmission, which enables stepless control of the gear ratio to the flywheel and thus it^'s speed in relation to the rack velocity and thereby also the effective mass can be controlled by means of changing the gear ratio.

[0008] In a preferred embodiment, the means for converting linear motion from the movements of the buoy into rotary motion comprises a rack and a pinion. This provides an arrangement adapted to have the flywheels engaged with the rack in both directions of the stroke.

[0009] In a preferred embodiment, a plurality of flywheel arrangements, preferably two, are provided. This increases the possibility to control the natural frequency of the buoy and also distributes the forces applied from the set of flywheels on multiple rotary means, to reduce the weight and cost of the

arrangement. If the flywheels are of different sizes, the number of available inertias is increased.

[0010] In a preferred embodiment, at least one flywheel arrangement comprises two flywheels, each flywheel being connected to a common shaft by means of a respective transmission means. This provides for a compact arrangement with the possibility to control the natural frequency of the power take-off device.

[001 1 ] In a preferred embodiment, at least one flywheel arrangement comprises one flywheel connected to a shaft by means of a transmission means, the flywheel arrangement further comprising a fixed displacement hydraulic pump connected to the shaft.

[0012] In a preferred embodiment, a plurality of flywheel arrangements is provided, each comprising a fixed displacement hydraulic pump, the power takeoff device further comprising hydraulic control valves adapted to independently disengage and engage the fixed displacement hydraulic pumps. Thereby, the damping and power extraction can be controlled by means of engaging / disengaging pumps in the arrangement, while the natural frequency of the buoy can be controlled by engaging / disengaging flywheels in the arrangement.

[0013] In a preferred embodiment, at least one flywheel arrangement comprises a planetary gearbox adapted to increase the speed of the flywheel. This reduces the weight required for each flywheel for a given inertia force.

[0014] In a preferred embodiment, at least one flywheel arrangement comprises one flywheel connected to a shaft by means of a transmission means, the flywheel arrangement further comprising a variable displacement pump connected to the shaft. This enables tuning of the damping force.

[0015] In a preferred embodiment, a control system is provided for carrying out the method as set out below.

[0016] According to a second aspect of the invention there is provided a wave energy converter comprising a power take-off device according to the invention connected, by means of a hydraulic connection, to a hydraulic motor, and an electrical generator operably connected to the hydraulic motor for the generation of electrical energy.

[0017] In a preferred embodiment, the hydraulic motor and an electrical generator are provided in a central unit. It is preferred that a plurality of power take-off devices are provided, each located in a buoy connected to the central unit by means of a respective hydraulic connection. In one embodiment, the hydraulic motor and the electrical generator are provided in each buoy.

[0018] In a preferred embodiment, at least one hydraulic accumulator is connected to the hydraulic motor, to provide power smoothing to the WEC, preferably arranged to have a high and narrow pressure range to provide a high torque from each engaged pump independently of the energy stored in the accumulator. [0019] According to a third aspect of the invention method of controlling a power take-off device as described above, comprising the following steps: a) detecting the nature of an incoming waves, preferably the frequency of incoming waves, b) determining a number of flywheels to be engaged in the next stroke in the wave, and, c) connecting and/or disconnecting flywheels to match the number of flywheels to be engaged.

[0020] In a preferred embodiment, the step b) comprises determining in which wave period the buoy will be resonant.

[0021 ] In a preferred embodiment, step c) comprises connecting and/or disconnecting flywheels to satisfy the following equation: s 771

oom_m + ωτη_ν = 0

ύύ wherein ω is the wave frequency, m_m is the effective mass of the system, m_r is the added mass, predominantly mass from the water, and S_m is the hydrostatic stiffness.

[0022] In a preferred embodiment, the step c) is performed in an end stroke where the speed of the flywheel is zero.

[0023] In a preferred embodiment, the step b) is performed to optimize the number of flywheels to be engaged only depending on the sea state.

Brief description of drawings

[0024] The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 a is side view of a power take-off with flywheels connected to a rack and pinion drive.

Figure 1 b is a top view of the same configuration as seen in figure 1 a, where a back to back configuration of the rack is shown. Figure 2a is a side view of a power take-off with multiple modules connected with the same gear rack, comprising flywheel, clutch, pinion and fixed displacement pump.

Figure 2b is a top view of the same configuration as seen in figure 2a.

Figure 3a is a side view of a power take-off similar to figure 2a, with a planetary gearbox added between the pinion and the clutch to each flywheel.

Figure 3b is a top view of the same configuration as seen in figure 3a.

Figure 4a is a side view of a power take-off with multiple modules connected with the same gear rack, comprising flywheel and CVT in combination with a variable displacement pump.

Figure 4b is a top view of the same configuration as seen in figure 4a.

Figure 5 shows an embodiment of the power take-off in a point absorbing wave energy converter with single point mooring, which exports power in the form of high pressure fluid to a central unit where electricity is generated.

Figure 6 shows a power take-off combining flywheel phase control with hydraulic pumps, accumulators, motor and generator for generation of electricity on board the buoy.

Description of embodiments

[0025] In the following, a detailed description of a power take-off and a wave energy converter according to the invention.

[0026] Figure 1 a shows a power take-off device for a wave energy converter in the form of a buoy 1 comprising a flywheel 6 with a shaft attached to a means for converting linear motion from the movements of the buoy 1 into rotary motion. In the shown embodiment, this means comprises a pinion 3, which is connected to a gear rack 2. A stiff connection in the form of a clutch 5 is located on the shaft of the flywheel to the pinion 3 and enables the flywheel 6 to be engaged and disengaged to and from the rack and pinion 2, 3. The total inertia connected to the rack 2 from the flywheel 6 affects the natural frequency of the buoy 1 and thus in which wave period the buoy will be resonant. This is governed by the following equation:

P ^ra,max - _{r 9} . , -„_η ι/2 _m ^l J

R_r + \β_γ + (o)m_m + >m_r -S-m/ω) ^Δ j wherein

Pa.max is the maximum power from the buoy,

F_e is the external force,

R_r is resistance or damping of the buoy, ω is the wave frequency, m_m is the effective mass of the system, rrir is the added mass, predominantly mass from the water, and

Sm is the hydrostatic stiffness.

[0027] This equation gives where maximum power can be captured from the waves. It should be noted that the effective mass of the system is the weight or mass of the buoy plus any inertia force added from the rotation of the flywheels.

[0028] The system is in resonance when the following equation is satisfied: ωητ_πι + ωιη_ν— = 0 (2)

[0029] The system should be designed so that equation (2) is satisfied with no flywheels engaged at around 5 second wave period. In larger wave periods, different combinations of flywheels are engaged to keep equation (2) satisfied until about 12 second wave periods. This strategy will keep the system in resonance to capture maximum power across a frequency range that will cover most of the wave frequencies in a wave spectrum, although the frequency range will depend on the selected site and need to be selected accordingly.

[0030] Figure 1 b shows a top view of same power take-off device of Figure 1 a with a flywheel 6, clutch 5, rack 2 and a pinion 3.

[0031 ] Any number of take-off devices can be used to provide sufficient detail in the phase control as well as distribute the force from each take-off device to the gear rack.

[0032] Typically the clutch will be operated in every end stroke where the speed of the flywheel is zero and a control system (not shown in the figures) will determine the optimal number of flywheels to be engaged in the next stroke in the wave, which can be to adjust the natural frequency to match with the incoming wave to keep the buoy in resonance or to offset the natural frequency to limit the stroke length. A more simple control strategy is to optimize the number of flywheels to be engaged only depending on the sea state.

[0033] Figure 2a shows a power take-off device for a wave energy converter designed according to the same basic principles as the one described above with reference to Figures 1 a and 1 b but comprising two sets of flywheels 6 attached on either side of the pinion 3, which is connected to the gear rack 2 in order to convert linear motion from the movements of the buoy 1 into rotary motion. Two clutches 5 are located on a shaft common for the two flywheels 6 in a set of flywheels, one to each pinion 3 and they enable each flywheel 6 to be independently engaged and disengaged to and from the rack and pinion 2, 3. The total inertia connected to the rack 2 from the flywheels 6 affects the natural frequency of the buoy 1 and thus in which wave period the buoy will be resonant. Using multiple flywheels adds tuning capabilities to enable the buoy to be resonant in a wider frequency range, as well as making it possible to de-tune in strong sea states for survivability.

[0034] Figure 2b shows a top view of the power take-off device of Figure 2a with the two sets of flywheels 6 and pinions 3, each set comprising two flywheels 6. A total of 4 flywheels are thus included in this configuration, providing 5 different inertias to tune the natural frequency. The flywheels can be of the same size or different sizes to increase the number of available inertias. For example, 16 inertias can be used if the size of the flywheels are selected according to 1x, 2x, 4x and 8x. Where typically 1x + 8x will be connected to one pinion and 2x + 4x to the other pinion in order to distribute the load more evenly between the pinions.

[0035] Any number of power take-off devices can be used to provide sufficient detail in the phase control as well as distribute the force from each module across the optimal number of pinions. Distributing the force on multiple pinions reduces the size of the rack and allows smaller radius on the pinions and thus also higher speed of the flywheels, but the device becomes more complicated with more parts.

[0036] Figures 3a and 3b show a power take-off system for wave energy converters designed according to the same basic principles as the one described above with reference to Figures 2a and 2b but with discrete control of both phase and damping force by replacing one of the flywheels 6 in each set of flywheels with a fixed displacement hydraulic pump 7 in each set of flywheels. A power take-off device can comprise one or multiple power take-off devices connected to a single or back to back gear rack 2. The phase is controlled by the flywheels 6 and the damping / power extraction is controlled by the pumps 7. The pumps are

independently disengaged and engaged through hydraulic control valves 10, see the embodiment of Figure 6. The pumps 7 are connected to a common hydraulic circuit 15 and 16 when engaged and the arrangement thereby provides a total feed force to the rack depending on the total number of engaged pumps and the total displacement in relation to the pressures on high and low pressure circuits.

[0037] Figures 4a and 4b show a similar configuration as Figures 3a and 3b with the only difference that planetary gearboxes 4 are added to increase the speed of each flywheel 6 and thereby reduce the weight required for each flywheel 6 for a given inertia force to the rack 2.

[0038] Figure 5a and 5b show a similar power take-off module as Figures 3a and 3b but with stiff connections in the form of continuous variable transmissions (CVT) 9 instead of mechanical clutches 5 that can be used to change the gear ratio to the flywheels 6 and thereby the inertia force applied to the rack 2 in order to tune the natural frequency. Variable displacement pumps 8 can be used to tune the damping force in more detail compared to using fixed displacement pumps.

[0039] A CVT can be implemented with hydraulic pump-motor with variable displacement in one or both units, or the mechanical equivalent. Alternatively a mechanical fixed step gearbox, such as one commonly used in automobiles, can be used.

[0040] Figure 6 shows an embodiment of a power take-off device similar to the one shown in Figure 2b in connection with a wave energy converter 20. This figure also includes control valves 10 that are used to rectify the flow from the bidirectional pumps 7, i.e. the valve position is changed depending on the rotational direction of the pump to always connect the output flow to a high pressure circuit 15 and the input flow to a low pressure circuit 16. Passive rectification with a Graetz bridge can also be used. The rack 2 is connected to a mooring rope 17 which is attached to the sea bed 18 in the other, lower end. Thus, the power takeoff device moves with the buoy while the rack is held steady in the projection of the mooring rope. Such system is normally uni-directional, i.e. captures power only in the up stroke, although there are solutions for this type of WEC system that enables bi-directional power capture, and there are also other types of WEC^'s that provide bi-directional power capture.

[0041 ] Figure 6 further shows a system where the high and low pressure circuits are connected to a hydraulic collection system with central unit for generation of electricity, comprising hydraulic motor, gravity storage and generator.

[0042] Figure 7 shows an arrangement similar to the one of Figure 5, but with hydraulic accumulators 13, 14, a hydraulic motor 12 and a generator 1 1 on board the wave energy converter for on board electricity generation. The hydraulic accumulators are preferably sized for a high and narrow pressure range to provide a more or less fixed interval for the damping force that can be applied to the rack through the pumps. [0043] Preferred embodiments of a power take-off device and a wave energy converter have been described. It will be realized that this can be varied without departing from the inventive idea as defined in the appended claims. For example, in the shown embodiments the flywheel is connected to the means for converting linear motion from the movements of the buoy into rotary motion by means of a clutch or a continuous variable transmission, but it will be realized that other means of connection are possible, as long as they provide a stiff connection to the flywheel.

Claims

1 . A power take-off device comprising a buoy (1 ) and a means (2, 3) for converting linear motion from the movements of the buoy (1 ) into rotary motion, c h a ra c t e r i z e d b y at least one flywheel arrangement comprising a flywheel (6) and a transmission means (5; 9) adapted to engage and disengage the flywheel to and from the means (2, 3) for converting linear motion from the movements of the buoy (1 ) into rotary motion, wherein the transmission means comprises any of the following: a clutch (5) and a continuous variable transmission (9).

2. The power take-off device according to claim 1 , wherein the means (2, 3) for converting linear motion from the movements of the buoy (1 ) into rotary motion comprises a rack (2) and a pinion (3).

3. The power take-off device according to claim 1 or 2, comprising a plurality of flywheel arrangements.

4. The power take-off device according to claim 3, comprising two flywheel arrangements.

5. The power take-off device according to claim 3 or 4, wherein flywheels are of different sizes.

6. The power take-off device according to any one of claims 1 -5, wherein at least one flywheel arrangement comprises two flywheels (6), each flywheel being connected to a common shaft by means of a respective transmission means (5).

7. The power take-off device according to any one of claims 1 -6, wherein at least one flywheel arrangement comprises one flywheel (6) connected to a shaft by means of a transmission means (5), the flywheel arrangement further comprising a fixed displacement hydraulic pump (7) connected to the shaft.

8. The power take-off device according to claim 7, comprising a plurality of flywheel arrangements, each comprising a fixed displacement hydraulic pump (7), the power take-off device further comprising hydraulic control valves (10) adapted to independently disengage and engage the fixed displacement hydraulic pumps (7).

9. The power take-off device according to any one of claims 1 -8, wherein at least one flywheel arrangement comprises a planetary gearbox (4) adapted to increase the speed of the flywheel (6).

10. The power take-off device according to any one of claims 1 -9, wherein at least one flywheel arrangement comprises one flywheel (6) connected to a shaft by means of a transmission means (5), the flywheel arrangement further comprising a variable displacement pump (8) connected to the shaft.

1 1 . The power take-off device according to any one of claims 1 -10, comprising a control system for carrying out the method according to claim 17.

12. A wave energy converter comprising a power take-off device according to any one of claims 1 -10 connected, by means of a hydraulic connection (15, 16), to a hydraulic motor (12), and an electrical generator (1 1 ) operably connected to the hydraulic motor (12) for the generation of electrical energy.

13. The wave energy converter according to claim 12, wherein the hydraulic motor (12) and an electrical generator (1 1 ) are provided in a central unit (20).

14. The wave energy converter according to claim 13, comprising a plurality of power take-off devices, each located in a buoy (1 ) connected to the central unit (20) by means of a respective hydraulic connection.

15. The wave energy converter according to claim 12, wherein the hydraulic motor (12) and the electrical generator (1 1 ) are provided in the buoy (1 ).

16. The wave energy converter according to claim 15, comprising at least one hydraulic accumulator (13, 14) connected to the hydraulic motor (12).

17. A method of controlling a power take-off device according to any one of claims 1 - 1 1 , comprising the following steps: a) detecting the nature of incoming waves, preferably the frequency of incoming waves, b) determining a number of flywheels to be engaged in the next stroke in the wave, and, c) connecting and/or disconnecting flywheels to match the number of flywheels to be engaged.

18. The method according to claim 17, wherein the step b) comprises determining in which wave period the buoy will be resonant.

19. The method according to claim 17 or 18, wherein step c) comprises connecting and/or disconnecting flywheels to satisfy the following equation: ωπί_ΎΏ + ωτη_ν—— = 0 wherein ω is the wave frequency, mm is the effective mass of the system, rrir is the added mass, predominantly mass from the water, and

Sm is the hydrostatic stiffness.

20. The method according to any one of claims 17-19, wherein the step c) is performed in an end stroke where the speed of the flywheel is zero.

21 . The method according to any one of claims 17-20, wherein the step b) is performed to optimize the number of flywheels to be engaged only depending on the sea state.