GB2409898A

GB2409898A - A rocking motion energy converter

Info

Publication number: GB2409898A
Application number: GB0402875A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-01-10
Filing date: 2004-02-10
Publication date: 2005-07-13
Also published as: GB0402875D0

Abstract

A wave energy converter buoy 10 has a plurality of gyroscopic energy conversion modules 20 arranged symmetrically around a centre of buoyancy 12 which also serves as the connecting point of a cable 14 which tethers the device to the sea bed. The energy conversion module comprises a circular housing 22 with an internal ring track or groove 21, a moveable frame 30 supported resiliently by flexible seals 31 revolving about an axis 32, and a spinning rotor 40 supported within the revolving frame to rotate about another axis 42. An electric generator 44 is driven by the spinning rotor. The module also comprises an electric motor 46 for driving the rotor at times when it is not gaining rotational energy from the tilting motion of the buoy.

Description

ROCKING MOTION ENERGY CONVERTER

Field of the invention

The present invention relates to a device and control system for converting rocking motion energy to electricity.

In particular it relates to a device and control system for converting ocean or sea wave energy to electricity.

0 Background of the invention

Most developments of wave power have concentrated on the conversion of the energy in the wave into a form which can be converted to electrical energy by conventional high speed rotary generators. Examples such as the Salter Duck, McCabe Wave Pump, OPD Pelamis_, OPT Powerbuoy_ eta employ hydraulic power systems to convert the low speed reciprocating motion of the waves (typically 0.5 to 5 m/s) into high rotational speed suitable for the generator running at an air-gap velocity of typically 50 m/s or higher. This mechanical interface required for speed conversion is the most complicated and costly part of a WEC (Wave Energy Converter). In addition, hydraulic systems contribute to energy losses and require regular maintenance which is difficult on the high sea.

Summary of the invention

According to the present invention, there is provided an energy conversion control system installed on a rocking platform for extracting energy from the periodic tilting - motlon of the platform, the system comprising a circular housing with an internal ring track or groove, a moveable frame supported resiliently within the housing to revolve about an axis substantially concentric with the said ring track, a spinning rotor supported within the said revolving frame to rotate about another axis which is perpendicular to - 2 the said revolving axis of the frame and which lies in the same plane as the said ring track in the housing such that the spindle of the spinning rotor engages loosely at both ends within the said ring track while revolving with the said revolving frame around the ring track, an electric generator driven by the said spinning rotor for converting the rotational energy of the rotor to electricity, and a controller for controlling the rotation speed of the spinning rotor in response to the periodic tilting motion of lo the rocking platform such that the rotor gains rotational energy by synchronized transfer of angular momentum from the periodic tilting motion of the axis of the resiliently supported revolving frame which is the precession axis of the spinning rotor, achieved by actively matching the precession frequency of the rotor with the imposed tilting frequency of the platform thereby causing the spindle of the said spinning rotor to tilt by gyroscopic action into rolling contact with the side walls of the said ring track in the correct phase relative to the tilting frequency for it to drive the rotor while revolving in precession motion around the ring track.

The rocking platform may be a floating vessel set into periodic rocking motion by ocean or sea waves for the system to extract energy from the waves. It may be a submerged platform set into periodic rocking motion by ocean or river currents for the system to extract energy from the currents.

Indeed, it may be any platform set into periodic rocking motion by any energy means for the system to extract energy from that means.

In the invention, as the rocking platform is set into periodic pitching and rolling motion, the energy conversion control system operates such that the rocking energy is converted to rotational energy transferred synchronously to the spinning rotor of a gyroscopic device. This converts, in one step, low frequency oscillation to high speed rotation suitable for power generation using a generator of small size and high efficiency driven at high speed by the spinning rotor.

In the context of ocean wave energy, compared with other known WEC's, the invention does not rely on intermediate mechanical interface for speed conversion, therefore has low cost and low energy loss, and can be completely sealed and maintenance free. The invention is 0 applicable to many known wave energy absorbers such as the Salter Duck, McCabe Wave Pump, OPG Pelamis_ etc. which are linked buoys or floating vessels designed to rock, tilt, or wiggle with the frequency of the waves, and could all use the energy conversion control system of the present invention mounted on-board in substitution of their previous hydraulic interfaces.

The energy conversion control system of the present invention further comprises an electric motor for driving the rotor at times when the rotor is not gaining rotational energy from the tilting motion of the rocking platform. For compactness, this may be integrated with the said electric generator into a combined generator/motor unit.

Preferably, the axis of the revolving frame which is the precession axis of the spinning rotor is resiliently supported substantially vertically relative to the rocking platform for the axis to be tilted in response to both pitching and rolling motion of the platform.

In the invention, the spinning speed of the rotor is variable by varying the electrical loading of the said generator or the said motor in order to vary its precession speed about the revolving frame axis such that the precession frequency about the frame axis is the same as the tilting frequency exerted on the frame axis, which is the condition for the rotor to gain rotational energy by - 4 synchronized transfer of angular momentum in response to the forcing frequency of the rocking platform.

The invention makes use of a known principle of a gyroscopic device described in Patent US3726146 and marketed under the trade name Powerball_. This is a body-building ball for hand and wrist exercise composed of gyro ball mounted on ring track. In use, under swinging of the hand, the rotation speed of the gyro ball can be increased from lo hundreds of RPM to thousands of RPM, resulting in a great force exerted on the hand. When applied correctly, the swinging frequency of the hand (which is variable and adjusted adaptively by feel to the precession by the user) would be synchronized with the precession frequency, gaining speed in step with one another.

In the present invention using a parallel principle, because the tilting frequency of the precession axis is imposed externally dictated by the excitation frequency of the rocking platform, an active control system is necessary to vary the precession frequency adaptively in response to the excitation frequency to achieve synchronization.

Furthermore, the phasing of the precession frequency relative to the excitation frequency must be controlled such that rotational energy is transferred smoothly and efficiently to the rotor.

The invention differs from other known gyro-based WECs such as those described in Patents GB2058938 and US4300871, where the method of energy conversion relies on capturing the reactive movements of the precession frame by connecting the frame to a reciprocating piston pump operated at low speed, and the pumped fluid is used to power a hydraulic motor to drive a rotary generator at high speed. As explained earlier, such mechanical interface for speed conversion is the most complicated and costly part of a WEC.

Also the hydraulic system contributes to energy losses and I -5 - require regular maintenance which is difficult on the high sea. All these problems are mitigated by the present invention.

In the present invention, a sensor is provided for measuring the frequency and phase of precession of the revolving frame. Another sensor is provided for measuring the frequency and phase of pitching and rolling of the rocking platform. The latter sensor may be a small lo directional gyroscope or a set of accelerometers mounted on the rocking platform.

Using data from the sensors and advanced control methods, the controller regulates the generator or the motor in order to control the rotor speed and the associated precession speed such that the measured precession frequency matches the measured frequency of pitching and rolling of the platform, and the measured phasing of the precession relative to the pitching and rolling is optimum for smooth energy transfer to the rotor.

Finally, the controller varies the electrical load on the generator according to the energy available from the rotor while controlling the frequency and phasing of the precession motion of the rotor such that there is sufficient residual rotational energy in the rotor to sustain the rotor speed for an indefinite period against bearing and windage losses. To reduce such losses, the system may be housed within a sealed chamber which is evacuated of air.

The invention has the advantage that energy conversion would continue as long as periodic rocking motion continues driven at any frequency. The system is self-contained and self-regulated, automatically locking on to the excitation frequency of rocking motion in any operating environment, without requiring attention from the user. - 6 -

Brief description of the drawing

The invention will now be described further by way of example with reference to the accompanying drawings in which Figure 1 shows a schematic cross-sectional view of a WEC (Wave Energy Converter) having an energy conversion module controlled by a controller shown in Figure 2, and Figure 2 shows a schematic view of an energy lo conversion control system with a controller controlling an energy conversion module of Figure 1.

Detailed description of the preferred embodiment

: 15 Figure 1 shows a WEC buoy 10 having a plurality of : energy conversion modules 20 for extracting energy from ocean or sea waves, arranged symmetrically around the centre of buoyancy 12 of the buoy 10 so that the buoy may be tethered by a cable 14 directly at the centre of buoyancy 12 without interference with the modules 20. This arrangement allows the buoy 10 to pitch and roll freely with maximum amplitude on the waves and with minimum lateral movement of the cable 14. The cable 14 is connected to a submerged sink 16 which in turn is tethered by another cable 18 to the sea bed.

Figure 1 shows the energy conversion module 20 comprising a circular housing 22 with an internal ring track or groove 21, a moveable frame 30 supported resiliently by flexible seals 31 within the housing 22 to revolve about an axis 32 substantially concentric with the ring track 21, a spinning rotor 40 supported within the revolving frame 30 to rotate about another axis 42 which is perpendicular to the revolving axis 32 of the frame 30 and which lies in the same plane as the ring track 21 in the housing 22 such that the spindle 41 of the spinning rotor 40 engages loosely at both ends within the ring track 21 while revolving with the - 7 revolving frame 30 around the ring track 21, and an electric generator 44 driven by the spinning rotor 40 for converting the rotational energy of the rotor 40 to electricity. A controller 38 shown in Figure 2 controls the energy conversion module 20 in Figure 1 where the rotation speed of the spinning rotor 40 is regulated in response to the periodic tilting motion of the buoy 10 such that the rotor gains rotational energy by synchronized transfer of angular momentum from the periodic tilting motion against lo the flexible seals 31 of the axis 32 of the resiliently supported revolving frame 30 which is the precession axis of the spinning rotor 40, achieved by actively matching the precession frequency of the rotor 40 with the imposed tilting frequency of the buoy 10 thereby causing the spindle 41 of the spinning rotor 40 to tilt by gyroscopic action into rolling contact with the side walls of the ring track 21 in the correct phase relative to the tilting frequency for it to drive the rotor 40 while revolving in precession motion around the ring track 21.

It would be clear from the above that the ratio of rotor speed versus precession speed is fixed when the spindle 41 is forced into rolling contact with the ring track 21, and is determined by the diameter ratio of the ring track 21 and the spindle 41. Thus, the energy conversion module 20 controlled by the controller 38 serves as a speed conversion device from oscillating motion directly to rotating motion, and at a very high step-up ratio and high efficiency.

The energy conversion module 20 further comprises an electric motor 46 for driving the rotor 40 at times when the rotor is not gaining rotational energy from the tilting motion of the buoy 10. For compactness, this may be integrated with the electric generator 44 into a combined generator/motor unit, which could be one or both of the units 44 and 46. - 8 -

In Figure 1, the axis 32 of the revolving frame 30 which is the precession axis of the spinning rotor 40 is resiliently supported substantially vertically relative to the buoy 10 for the axis to be tilted in response to both pitching and rolling of the buoy 10. Thus, the axis 32 will wobble in elliptical or circular motion energised by the waves in step with the wobbling of the buoy 10.

In Figure 1, the spinning speed of the rotor 40 is lo variable by varying the electrical loading of the generator 44 or the motor 46 in order to vary its precession speed about the revolving frame axis 32 such that the precession frequency about the frame axis 32 is the same as the tilting frequency exerted on the frame axis 32, which is the condition for the rotor 40 to gain rotational energy by synchronized transfer of angular momentum in response to the forcing frequency of the waves.

Figure 2 shows the energy conversion control system comprising an energy conversion module 20 and a controller 38. A sensor 34 measures the frequency and phase of precession of the revolving frame 30. Another sensor 36 measures the frequency and phase of tilting of the axis 32 corresponding to the tilting of the buoy 10 at its centre of buoyancy 12. The latter sensor 36 may be a small directional gyroscope or a set of accelerometers mounted on the buoy 10, or a set of articulated feeler arms extending from the buoy 10 for sensing wave movements at a distance from the buoy for anticipatory control.

Using data from the sensors 34, 36 and advanced control methods, the controller 38 regulates the generator 44 or the motor 46 in order to control the rotor speed and the associated precession speed such that the precession frequency measured by the sensor 34 matches the excitation frequency of pitching and rolling measured by the sensor 36, and the phasing of the precession relative to the pitching - 9 - and rolling is optimum for smooth energy transfer to the i rotor 40. The optimum phasing may be determined by maximizing the rate of change in precession speed measured ! by the sensor 34, or by maximising the electrical load produced by the generator 44 measured by a load sensor 48.

The method is applicable to both steady waves with regular frequency and unsteady waves with changing frequencies which could be filtered. The controller 38 may also have internal memory for storing historical data of pitching and rolling lo of the waves for predictive control.

A viscous or friction damper 50 for influencing the frequency and phasing of the precession motion may also be provided between the revolving frame 30 and the housing 22.

The damper 50 may be a variable damper regulated by the controller 38 in conjunction with regulation of the generator 44 and the motor 46.

Finally, the controller 38 would vary the electrical load on the generator 44 according to the energy available from the rotor 40 while controlling the frequency and phasing of the precession motion of the rotor 40, such that there is sufficient residual rotational energy in the rotor to sustain the rotor speed for an indefinite period against bearing and windage losses. To reduce such losses, the housing 22 may be sealed and evacuated of air.

Typical ocean waves has frequencies between 1 Hz and 0.1 Hz. The inertia mass and speed range of the rotor 40 should therefore be designed for a range of precession frequencies between 1 Hz and 0.1 Hz. Also the inertia mass and speed of the rotor 40 could influence the effective amplitude of pitching and rolling of the buoy 10, which must be taken into account in determining the rated power output.

The motor 46 is required to drive the rotor 40 up to a minimum speed for the gyro effect to take hold, after which - 10 the rotor speed could be self-sustaining by regulating the generator 44 alone even in small waves. If the sea is calm, I the rotor may slow down or stop, then it must be restarted using the motor 46 when there are sufficient waves.

The generator 44 feeds electricity into an external power grid 24 connected to the energy conversion control system 20 via the controller 38 and sliding contacts (not shown) between the revolving frame 30 and the housing 22.

The same grid 24 may supply electric power to the motor 46 when required. Alternatively, the generator 44 may be used to charge an on-board battery (not shown) which then supplies the motor 46 when required.

The invention enables direct conversion of wave energy to electricity using a generator of small size and high efficiency driven at high speed by the spinning rotor.

Compared with other known WECs, the invention does not rely on intermediate mechanical interface for speed conversion, therefore has low cost and low energy loss, and is completely sealed and maintenance free. Moreover, the invention is applicable to many known wave energy absorbers such as the Salter Duck, McCabe Wave Pump, OPG Pelamis_ etc. which are linked buoys designed to rock, tilt, or wiggle with the frequency of the waves, and could all use the energy conversion control system of Figure 2 mounted on- board in substitution of their previous hydraulic interfaces.

The invention is also applicable to a known type of marine current energy converter such as the Stingray_ reciprocating wing concept. In this concept, a hydrofoil wing is triggered to move up and down in a tidal current by changing the angle of attack of the wing alternately at a variable frequency depending on the speed of the current.

By mounting the energy conversion control system of Figure 2 to a rocking platform tilting with the reciprocating wing - 11 arm, and controlling the precession frequency of the spinning rotor so that it matches the reciprocating frequency of the wing arm in the correct phasing, the hydrodynamic energy may be converted to rotational energy transferred smoothly to the spinning rotor, thus generating electricity directly without relying on hydraulic interface for speed conversion used in the original concept.

In general, the control problem posed by the present lo invention is easier to tackle on a submerged platform than on a floating vessel because the periodic tilting motion of a submerged platform in a flowing current is more regular and predictable than the wobbling motion of a floating vessel on top of the waves. The problem will be most severe within or close to the wave generation area by the wind, i.e. storm waves which exhibit a very irregular pattern.

These will progressively become more regular and smooth as they travel out of the storm area, retaining most of the energy in the form of deep water waves or swells.

Consequently, the invention is best applied to off-shore WEC's deployed over deep waters.

Finally, the energy conversion control system of Figure 2 is applicable in a wide variety of platforms set into periodic rocking motion by any energy means for the system to extract energy from that means. Depending on application, the energy conversion module 20 may have a wide range of sizes from mega-watts size to mini-watt size.

The invention has the advantage that energy conversion would continue as long as periodic rocking motion continues driven at any frequency. The system is self-contained and self-regulated, automatically locking on to the excitation frequency of rocking motion in any operating environment, without requiring attention from the user. - 12

Claims

1. An energy conversion control system installed on a rocking platform for extracting energy from the periodic tilting motion of the platform, the system comprising a circular housing with an internal ring track or groove, a moveable frame supported resiliently within the housing to revolve about an axis substantially concentric with the said ring track, a spinning rotor supported within the said lo revolving frame to rotate about another axis which is perpendicular to the said revolving axis of the frame and which lies in the same plane as the said ring track in the housing such that the spindle of the spinning rotor engages loosely at both ends within the said ring track while revolving with the said revolving frame around the ring track, an electric generator driven by the said spinning rotor for converting the rotational energy of the rotor to electricity, and a controller for controlling the rotation speed of the spinning rotor in response to the periodic tilting motion of the rocking platform such that the rotor gains rotational energy by synchronized transfer of angular momentum from the periodic tilting motion of the axis of the resiliently supported revolving frame which is the precession axis of the spinning rotor, achieved by actively matching the precession frequency of the rotor with the imposed tilting frequency of the platform thereby causing the spindle of the said spinning rotor to tilt by gyroscopic action into rolling contact with the side walls of the said ring track in the correct phase relative to the tilting frequency for it to drive the rotor while revolving in precession motion around the ring track.

2. An energy conversion control system as claimed in claim 1, wherein the system further comprises an electric motor for driving the rotor at times when the rotor is not gaining rotational energy from the tilting motion of the rocking platform. - 13

3. An energy conversion control system as claimed in claims 1 and 2, wherein the said electric generator and motor are integrated into a combined generator/motor unit.

4. An energy conversion control system as claimed in claim 1, wherein the axis of the revolving frame is resiliently supported substantially vertically relative to the rocking platform for the axis to be tilted in response to both pitching and rolling motion of the rocking platform.

5. An energy conversion control system as claimed in any preceding claim, wherein the spinning speed of the rotor is variable by varying the electrical loading of the said generator or the said motor in order to vary its precession speed about the revolving frame axis such that the precession frequency about the frame axis is the same as the tilting frequency exerted on the frame axis, which is the condition for the rotor to gain rotational energy by synchronized transfer of angular momentum in response to the forcing frequency of the rocking platform.

6. An energy conversion control system as claimed in any preceding claim, wherein a sensor is provided for measuring the frequency and phase of precession of the revolving frame.

7. An energy conversion control system as claimed in any preceding claim, wherein a sensor is provided for measuring the frequency and phase of pitching and rolling of the rocking platform.

8. An energy conversion control system as claimed in claims 6 and 7, wherein the controller regulates the said generator or the said motor in order to control the rotor speed and the associated precession speed such that the measured precession frequency matches the measured frequency of pitching and rolling. - 14

9. An energy conversion control system as claimed in claim 8, wherein the controller further controls the measured phasing of the precession relative to the pitching and rolling such that rotational energy is transferred smoothly and efficiently to the rotor.

10. An energy conversion control system as claimed in claims 8 and 9, wherein the controller varies the electrical load on the said generator according to the energy available lo from the rotor while controlling the frequency and phasing of the precession motion of the rotor such that there is sufficient residual rotational energy in the rotor to sustain the rotor speed against bearing and windage losses.

11. An energy conversion control system as claimed in any one of claims 1 to 10, installed on a floating vessel set into periodic rocking motion by ocean or sea waves for the system to extract energy from the waves.

12. An energy conversion control system as claimed in any one of claims 1 to 10, installed on a submerged platform set into periodic rocking motion by ocean or river currents for the system to extract energy from the currents.

13. An energy conversion control system as claimed in any one of claims 1 to 10, installed on any platform set into periodic rocking motion by any energy means for the system to extract energy from that means.

14. An energy conversion control system as claimed in any preceding claim, designed to be self-contained and self regulated, automatically locking on to the excitation frequency of rocking motion in any operating environment.