GB2486279A

GB2486279A - Wave energy converter with generation and damping modes

Info

Publication number: GB2486279A
Application number: GB1021031.8A
Authority: GB
Inventors: Stephen John Packard; Malcolm Van Den Burgh
Original assignee: Trident Energy Ltd
Current assignee: Trident Energy Ltd
Priority date: 2010-12-10
Filing date: 2010-12-10
Publication date: 2012-06-13
Anticipated expiration: 2030-12-10
Also published as: EP2649304A1; GB201021031D0; GB2486279B; WO2012076851A1

Abstract

A wave energy converter which can be operated in a generation mode for converting wave energy into electricity or a damping mode in which wave motion is reduced more than in generation mode and the amount of wave energy converted into electricity is less than in generation mode. The converter has at least one float 1 for immersion in waves 20 connected to an armature 2 of a generator and a controller 30. Preferably the damping mode actively moves the float relative to the generator stator by using the generator as a motor. The wave energy converter may be used on a floating marine platform, such as a floating wind turbine, where the damping mode is used to reduce the motion of the platform. Also described is a method of boarding a marine platform by using the wave energy converter in the damping mode to aid boarding.

Description

WAVE ENERGY CONVERTER

Description

The present invention relates to the conversion of sea wave energy to electricity and in particular to a wave energy converter.

Off-shore installations require maintenance and/or operations to be performed by humans. For this purpose humans must often land on off-shore installations, * sometimes at short notice. Landing on an off-shore installation or working on an off-shore installation can be made hazardous by the presence of waves. That is, wave motion can make access onto an off-shore installation or working on an off-shore installation hazardous. Additionally wave motion can destabilize an off-shore installation and potentially capsize the off-shore installation, particularly if the off-shore installation is a platform such as a floating platform.

The marine environment is becoming a more and more important source for renewal energy, in particular renewable electricity. One of the many difficulties of operating in the marine environment is the presence of waves. Waves can make access to off-shore installations difficult, dangerous or even impossible.

It would be advantageous to have a means for damping wave motion, both in terms of the amplitude of the wave and/or the motion of a platform which is moving due to wave action. This allows boarding of the platform in a safe way at any time, irrespective of the sea conditions, and work to be carried out on the platform irrespective of sea conditions and can reduce the chance of breakage or capsize of an off-shore installation, such as a platform, due to wave motion.

In a first aspect of the present invention there is provided a wave energy converter comprising: at least one float for immersion in waves and connected to an armature or a stator of a generator; a controller for operating the generator in a generation mode for converting wave energy into electricity or a damping mode in which wave motion is reduced more than in generation mode and the amount of wave energy converted into electricity is less than in generation mode.

In this way a marine energy converter which is designed for taking energy from the waves can be operated in a mode in which the marine energy converter reduces wave motion. in this way the amplitude of a wave passing under the wave energy converter can be reduced. This allows access to marine installations in the path of the wave downwind or downwave of the wave energy converter. The marine installation may be fixed to the sea bed or floating.

Additionally or alternatively the wave motion of a platform (e.g. movement of the platform due to wave action) may be reduced thereby allowing easier working on the platform and/or reducing chance of damage to components on the marine platform and/or reducing the chance to capsize.

in an embodiment, in damping mode, coils of the generator are short circuited, or the total effective length of coils in the generator is changed, or current through the coils is controlled or current is injected from an external source into the coils such that the float experiences a force from the generator different to the force experienced in generation mode.

These are ways in which the generator of the wave energy converter can change the character of waves thereby to change their amplitude, for example and/or their effect on the wave motion of a platform.

In an embodiment during damping mode or during initializing of damping mode the generator is driven as a motor.

In this way the float can be driven to a position or held at a position specifically for damping the wave motion and/or the float can be moved so as to apply a force to the wave specifically to reduce the wave motion. in an embodiment the generator is driven as a motor using energy converted into electricity from the waves by the wave energy converter. The energy may be stored in a storage unit such as a capacitor, a battery etc.. in an embodiment the generator is driven as a motor using electricity from a grid to which the generator is attached and to which, in generation mode, the generator provides electricity.

In one embodiment, a plurality of wave energy converters are provided. A control system can drive one or more of the plurality of wave energy converters in generation mode and/or one or more of the wave energy converters in damping mode.

In one embodiment power generated by a wave energy converter driven in generation mode is used to drive another wave energy converter in damping mode.

In one embodiment the wave energy converter generates electricity in damping mode. In that case the electricity generated may be used to operate the generator in damping mode, for example to drive the generator as a motor during specific times of damping mode and/or to short circuit coils of the armature, and/or to change the total effective length of coils in the armature and/or to change current through the coils.

The present invention also relates to a marine platform on which is mounted at least one of the above wave energy converters. In this way in damping mode the motion of the platform may be reduced and/or the amplitude of waves passing under the platform may be reduced.

Particularly if the marine platform is a floating marine platform, the wave energy converter can be used to damp the wave motion of the platform. That is, the motion of the marine platform due to the action of waves can be damped using the wave energy converter. For example, as a wave passes under the marine platform and the marine platform tends to fall at a first end and rise at the other second end, if the wave energy converter is mounted at the first end, in damping mode it can be driven so as to provide more buoyancy thereby to reduce the amount the first end of the platform falls. Conversely, if the first end of the platform were to be lifted up by the wave, in damping mode the wave energy converter could be driven to reduce the amount of buoyancy provided by the float thereby reducing the amount of upward motion at the first end of the platform. During passing of the wave under the generator, the position of the float can be changed (e.g. in a feedback or feedforward manner) to provide additional or less buoyancy, as required.

In one embodiment a plurality of wave energy converters is mounted to the marine platform.

In one embodiment at least one wind energy conversion apparatus is mounted on the marine platform. The wind energy conversion apparatus may be the main source of renewable energy generated by the marine platform. Energy from the wind energy conversion apparatus may be used to drive the wave energy converter in damping mode.

The invention may relate to a marine energy generation apparatus comprising a wind energy conversion apparatus and at least one of the above wave energy converters. The marine energy generation apparatus may be a wind farm comprising a plurality of wind energy conversion apparatus. The at least one wave energy converter may be present around at least part of the wind farm, for example may be positioned in a direction from which inclement weather is expected at the wave farm.

In normal operation the at least one wave energy converter may be driven to generate electricity. In inclement weather the wave energy conversion apparatus may be driven in damping mode thereby to damp the amplitude of waves and protect and/or allow access to one or more of the wind energy conversion apparatus of the wind farm.

The wind energy conversion apparatus may either be fixed to the sea bed in a static way or may be provided on a floating platform which is anchored to the sea bed.

The invention will now be described with reference to the accompanying drawings in which: -Figure 1 illustrates, in cross section, an example of a wave energy converter; -Figure 2 illustrates in perspective view a wind farm protected by a plurality of wave energy converters according to the present invention; and -Figure 3 illustrates a marine platform for a wind energy conversion apparatus and including wave energy converters according to the present invention.

Figure 1 illustrates, schematically in cross-section, a wave energy to electrical energy conversion apparatus 10. The apparatus comprises a float 1 which is connected to an armature 2 of a generator 3. The armature 2 is (co-axially) surrounded by a stator 4. The armature 2 is arranged for reciprocating movement in the stator 4. The armature 2 is configured as an elongate rod.

The stator 4 of the wave energy converter comprises a plurality of coils. The coils coaxially surround the armature 2. The armature 4 is held substantially stationery relative to the sea bed 30. As a wave 20 passes under the generator 3, the float I rises up with the wave thereby moving the armature 2 relative to the coils in the stator 4.

The armature 2 comprises a series of permanent magnets. Movement of the permanent magnets of the annature 2 within the coils of the stator 4 results in generation of electricity.

A controller 30 is provided for controlling the generator 3 as described below.

The apparatus of Figure 1 may be configured differently. For example, the stator may be in the form of an elongate rod with permanent magnets and fixed relative to the sea bed 30 and the armature comprising coils may be within the float 1.

In such an embodiment, as the float moves up and down in the waves, the coils are moved relative to the permanent magnets thereby generating electricity.

In one embodiment the generator 3 is a linear generator.

The wave energy converter 10 may be as described in EP-A1-l,l96,690, EP-A1-1,646,785, EP-Al-l,196,691 or WO 2006/075 147, the entire contents of all of which are hereby incorporated herein by reference thereto. The float I may be any type of float which provides enough buoyancy in order for the generator 3 to generate electricity on the passing of a wave 20 under the generator 3. The float 1 may be of the type described in WO 2007/042800, the entire contents of which is hereby incorporated herein by reference thereto.

A controller 30 is provided for controlling the generator 3. The controller 30 can operate the generator 3 in two modes. In a first mode (generation mode), the generator 3 is driven purely as a generator 3 and wave energy is converted into electricity. For example, the generator 3 is operated in such a way that as much energy as possible is extracted by the generator 3 from the waves. Operating parameters are changed to adjust, for example, the speed of the armature relative to the stator and the depth of the float relative to the wave surface.

The apparatus operates in generating mode is as follows. As a wave 20 arrives, the natural buoyancy of the float 1 causes the whole assembly comprising the float I and armature 2 to rise. This may be assisted by the pressure of the rising water acting against paddles or surfaces of the float I. Thus relative motion arises between the armature 2 and stator 4 of the (linear) generator 3 and alternating current is generated within the coils of the generator 3, the amplitude and frequency of which depend upon the vigour of the wave motion. The current is conducted to a shore station by a suitably armoured and flexible cable.

(Note, means, not shown, are present to prevent rotation of the wave energy converter and therefore unwanted tensioning of the cable.) Once the wave 20 has reached its zenith, and begins to fall, the weight of the float 1/armature 2 assembly causes the same also to fall. Power again is generated as the armature 2 traverses the stator I. If the upthrust experienced by the generator 3 is substantially the same as the weight of the float 1/armature 2 assembly, electricity is generated reasonably consistently both upon the rise and fall phases of the wave 20.

There is some natural phase lag between the ascending of the assembly relative to the waves, and its fall, due to the natural damping effect of the electromotive force generated. The load impedance presented to the generator 3, and the overall weight of the moving assembly, may be so selected as to optimise generation for any particular wave condition.

The apparatus of the invention thereby generates electrical energy.

The controller 30 can also operate the generator 3 in a damping mode in which wave motion is reduced more than in generation mode and the amount of wave energy converted into electricity is less than in generation mode, for example, no converstion into electricity may take place.

In damping mode the controller 30 changes operating parameters of the generator 3 to reduce wave motion. The wave motion reduced may be one of two types of wave motion as discussed in more detail with reference to Figures 2 and 3.

Wave motion may be the motion of the wave 20 (as explained with reference to Figure 2), for example its amplitude or may be the motion of a platform due to waves passing under the platform (as explained further with reference to Figure 3). To this end, various detection and measurement instruments may be provided with the wave energy converter 10. To achieve the best result knowledge of the wave period will enable the generator to be operated such that the float oscillates out of phase with the waves.

Detection and/or measurement means 17 located to detect and/or measure the rate of movement of the armature 2 of the generator 3, and also the extent of its movement. The detection means 17 may comprise Hall effect detectors located adjacent to the magnetic armature 2 of the generator 3, so providing information on its movement as the series of armature magnetic fields passes the detection means 17.

By using two detectors with a phase displacement of 90°, i.e. in 5 phase quadrature, information is made available in terms of direction, position and speed.

Alternatively, as an addition to or in place of use of the Hall effect components, the emf voltages generated by the stator 4 may be measured and assessed. In accordance with electrical engineering theory, the amplitude of these voltages provides information on the rate of movement, and the number of cycles generated in a particular movement of the armature 2 provides information on the overall distance travelled.

The signals so provided, by either or both of the Hall effects and the emf waveforms, are processed by the controller 30.

A further detector 21 (a wave sensor) is provided, for example on the same platform 40 as the generator 3. The detector 211s capable of detecting the level of the surface of the water under it. This provides information regarding the speed, amplitude and wave length of waves and allows the calculation of when the wave front of a wave can be expected under the generator 3. This information along with the information from the detection and measurement means 17 is passed to the controller 30 which can control operating parameters of the generator 3 accordingly.

The detector 21 may be upwave and/or downwave of the generator 3 (mounted to the same or (a) different platform(s) to the generator 3). A downwave detector 21 detects information about the waves as influenced by the generator 3. Such information can be used to control the generator 3 in damping mode, for example in a feedback manner. Upwave and/or downwave information (e.g. one or more selected from predicted time of arrival, wave period, wave speed, wave amplitude) may be used in a control algorithm to alter the behaviour of the generator 3 in damping mode by means of changing the resistance, the stroke and/or the speed of the armature.

The float will produce its own waves due to reflection. By adjusting the pattern of the reflected waves to interfere with the underlying wave cycle, a calming effect will be achieved. The effect on the waves will not simply be downwave but will be in an arc radiating from the generator. If a number of generators are therefore coordinated in an array, the combined effect will be better than a generator operating in an uncoordinated manner.

The controller can drive the generator in damping mode in a feedforward way, using a pre-determined algorithm which relates the characteristics of a wave front as sensed upwave of the generator to a particular operating characteristic needed of the generator to reduce wave amplitude downwave of the generator. For an array of generators, the controller may co-ordinate the control of each generator in a feedback or feedforward manner as described above in order to achieve a desired result (e.g. wave pattern) at a certain location.

Damping mode may be a passive mode in which the float I does not move relative to the stator 4 and the float 1 and armature 2 merely act as a barrier to the passage of a wave or in an active mode in which the float 1 and armature 2 move relative to the stator 4 (and sea bed 30).

In active mode the resistance of the float 1 and armature 2 to movement relative to the stator 4 can be increased or decreased relative to generation mode. This can be achieved by short circuiting one or more of the coils of the stator 4, or by varying the total effective length of coils in the stator 4 or by changing the current through the coils of the stator 4. Current may be injected from an external source into the coils. This latter technique enables larger resistance to movement of the float by waves to be generated than is possible by short circuiting the coils.

In one embodiment (e.g. in active mode or during initialisation/stopping of passive mode) current is passed through coils in the stator 4 thereby to drive the generator 3 as a motor. Energy may be provided from other generators operating in generation mode, from a storage unit such as a capacitor or battery or from a grid to which the generator is attached and to which, in generation mode, the generator optionally provides electricity.

The controller 30 controls the generator 3 in damping mode according to input parameters including the results from the detection and measurement means 17 and the wave sensor 21.

The controller 30 can automatically enter damping mode depending upon signals received from, for example, the detection and measurement means 17 or the wave sensor 21 or in response to a signal received from an operator.

In the embodiment of Figure 2 it is likely the controller 30 will be set up to receive a communication from an operator that wave damping is required (for example to allow technicians access to a wind energy conversion apparatus). In the embodiment of Figure 3 an operator may provide a signal for the wave energy converter 10 to enter damping mode. Alternatively, a detected motion of the platform 40 or signals from the detectors 17 may indicate that damping mode should be entered (for example because a certain parameter is above a certain threshold parameter). In either case, the controller 30 then enters damping mode and controls the operating parameters of the generator 3 to reduce wave motion.

In an embodiment it may be most effective if the generators act as motors and to ensure that the float or floats move out of phase with the waves.

It will be clear from the above that in damping mode wave motion is reduced more than in generation mode (in generation mode wave motion may be reduced because energy is taken out of the wave). In damping mode the amount of wave energy converted into electricity is less than in generation mode for a given wave.

That is, in generation mode the amount of energy converted into electricity is substantially optimised whereas this is not necessarily the case in damping mode. The efficiency of energy conversion in damping mode is lower than in generation mode.

in damping mode no energy may be generated by the wave energy converter 10.

Energy may be used by the wave energy converter 10 in damping mode, for example by driving the generator 3 as a motor.

Energy generated by the wave energy converter during generation mode or damping mode may be used in damping mode. For example, the energy may be used to drive the generator 3 as a motor or to change the operating parameters of the generator 3 (for example short circuiting coils of the stator 4, changing the total effective length of coils in the stator 4, varying the current through coils in the generator 3).

Varying the current of coils in the generator 3 may include driving a current through coils in the generator 3 to drive the generator 3 as a motor or may, for example, be attaching a resistance to the coils thereby to increase the resistance of the generator 3 to movement of the armature 2 relative to the stator 4. The controller 30 may control the operating parameters of the generator 3 depending on the position of the wave 20 relative to the generator 3 as measured by wave sensor 21. The controller 30 may vary the operating parameters of the generator 3 according to one or more of the shape of the wave, the velocity of the wave, the amplitude of the wave and the wavelength of the waves.

An energy storage unit 35 may be provided for storing energy generated by the wave energy converter 10 and for use in damping mode.

In one embodiment a plurality of wave energy converters 10 (e.g. in an array) are provided. The electricity generated by one wave energy converter 10 may be used by the controller 30 for controlling the generator 3 of another wave energy converter in damping mode.

The wave energy converter 10 of the present invention can be used to damp wave motion of or in proximity to off-shore installations. Off-shore installations include, but are not limited to, a marine platform of any type, in particular a marine platform for the mounting thereon of one or more wind energy conversion apparatus (such as illustrated in Figure 3). In another embodiment the wave energy converter apparatus is useful in a marine energy generation apparatus 100 which comprises a wind energy conversion apparatus 50 (such as a turbine), such as illustrated in Figures 2and3.

In one embodiment, the wind energy conversion apparatus 50 is secured in permanent position relative to the sea bed (Figure 2). In that embodiment the wave energy converter 10 may be used to reduce the amplitude of waves, as desired, to allow operator access to one or more wave energy conversion apparatus 50.

Maintenance of wind energy conversion apparatus 50 can only be carried out if wave amplitude is below a pre-defined maximum limit. It may be unusual for waves of above the maximum pre-defined limit to arrive from a particular direction.

If that is the case, it may only be necessary partly to surround the farm of wind energy conversion apparatus 50 on a side from which waves above the pre-determined maximum are likely. Alternatively a plurality of wave energy converters 10 may surround the wind farm.

If access to a particular wind energy conversion apparatus 50 is required, one or more of the wave energy converters 10 may be switched to damping mode thereby to damp the amplitude of waves in the region of the wind energy conversion apparatus 50 needing attention. In this way the wave energy converter 10 of the present invention can be used to ensure that access to wind energy conversion apparatus 50 is possible during most weather conditions, including weather conditions at which the wave amplitude would otherwise be above the maximum allowable amplitude.

During other times the wave energy converter(s) 10 may be driven in generation mode and contribute to the energy converted by the marine energy generation apparatus 100.

It will be appreciated that the wave energy converter 10 described above in relation to Figures 1 and 2 could be used to protect other types of off-shore structure and is not limited to protecting wind energy conversion apparatus 50.

Figure 3 shows a plurality of wave energy converters 10 attached to a marine platform 40. The marine platform 40 is a floating marine platform. The marine platform 40 is attached to the sea bed 30 via cables and anchors 60. The marine platform 50 is still effected by waves 20 and will have a wave motion as a result of those waves. Optionally the marine platform 50 may have mounted on it any type of marine installation including, but not limited to, a wind energy conversion apparatus 50.

The wave energy converters 10 may be installed both to convert wave energy into electricity (in generation mode) either to generate electricity to be transmitted to the shore or to generate electricity to be utilized on the marine platform 40. The wave energy converters 10 may be driven in damping mode in which case they are effective to damp the (wave induced) motion of the platform 40. In this embodiment, movement of the platform 40 may be detected (for example by an inclinometer or accelerometer 25 illustrated in Figure 1).

In Figure 3, the position of the wave results in a rotation of the platform 40 counter clockwise, as illustrated, This information is provided to the controller 30.

The controller 30, if in damping mode, then controls one or more of the wave energy converters 10 to provide more or less buoyancy than is normal. For example, if more buoyancy is required at a position (for example on the left hand side of the platform 40, as illustrated), the controller 30 ensures that the float I of a wave energy converter at that position is further from the stator 4 than would be the case in generation mode. Thus, the float I will be more immersed in the wave 20 than is normally the case and thereby provide additional buoyancy. At that point a larger than normal upward force on the platform 40 will be present thereby helping to damp the motion of the platform 40.

Conversely, if less buoyancy is required at the position of a wave energy converter 10 (e.g. on the right hand side of the platform 40, as illustrated), the controller 30 ensures that the float 1 is closer to the stator 4 than normal. As a result, the float 1 would be less immersed in the wave 20 than normal (or out of the water, as illustrated) and the wave energy converter 10 at that position will provide less buoyancy at its location. As a result, the motion of the platform 40 will be reduced, The controller 30 may control the distance between the float I and the stator 4 either by increasing or decreasing the resistance to motion between the armature 2 and stator 4 or by driving a current through coils in the armature 4 and driving the generator as a motor.

One or more of the wave energy converters 10 may be controlled in the generating mode whilst others are in the damping mode.

The controller 30 may automatically enter damping mode when sensors associated with the controller 30 indicate that certain conditions have been met (e.g. to stabilize the platform in inclement weather when the inclinometer or accelerometer indicate large movement of the platform). Additionally or alternatively the controller 30 may enter damping mode in response to a user signal. This could, for example, reduce the motion of the platform 40 during maintenance or at least during a time when an operator boards the platfonn 40.

In an embodiment the wave energy converters 10 mounted on the platform 40 can be operated in a similar manner to the wave energy converters as described in Figure 2. Thus, for example, in a wave farm one or more platforms 40 associated with one or more wind energy conversion apparatus 50 could surround other wind energy conversion apparatus 50 (either in fixed position relative to the sea bed or mounted on a floating marine platform) to reduce wave amplitude at the other wind energy conversion apparatus 50 thereby to protect and/or allow access to the wind energy conversion apparatus.

It will be apparent that the features of any above described embodiments can be combined with the features of any other described embodiment.

Claims

CLAIMS1. A wave energy converter comprising: at least one float for immersion in waves and connected to an armature of a generator; a controller for operating the generator in a generation mode for converting wave energy into electricity or a damping mode in which wave motion is reduced more than in generation mode and the amount of wave energy convened into electricity is less than in generation mode.
2. The wave energy converter of claim 1, wherein in damping mode coils of the generator are short circuited, or the total effective length of coils of the generator is changed, or current through the coils is controlled or current is injected from an external source into the coils such that the float experiences a force from the generator different to the force experienced in generation mode.
3. The wave energy converter of claim 2, wherein during damping mode or during initializing of damping mode the generator is driven as a motor.
4. The wave energy converter of any of claims 1-3, wherein the damping mode is an active mode in which the float moves relative to the stator of the generator.
5. The wave energy converter of any of the preceding claims, further comprising a wave sensor for providing information to the controller about a wave front either upwave or downwave or both of the wave energy converter.
6. The wave energy converter of claim 5, wherein in damping mode the controller controls the generator in a feedback or feedforward manner based on the information provided by the wave sensor.
7. A marine platform on which is mounted at least one wave energy converter of any of claims 1-6.
8. The marine platform of claim 7, wherein the marine platform is a floating marine platform.
9. The marine platform of claim 8, wherein the wave motion reduced in damping mode is motion of the platform.
10. The marine platform of claim 7, 8 or 9, on which is mounted at least one wind energy conversion apparatus.
11. A marine energy generation apparatus comprising a wind energy conversion apparatus and at least one wave energy converter of any of claims 1-6.
12. The marine energy generation apparatus of claim 11, wherein the wind energy conversion apparatus is the wind energy conversion apparatus of claim 10.
13. The wave energy converter of any of claims 1-6, the marine platform of any of claims 5-8 or the marine energy generation apparatus of claim 11 or 12, wherein the wave motion reduced in damping mode is the amplitude of waves.
14. A method of boarding a marine platform comprising instructing the controller of a wave energy converter of any of the preceding claims to enter damping mode and boarding the marine platform.
15. A wave energy converter, marine platform or marine energy generation apparatus substantially as hereinbefore described with reference to and/or as illustrated in the accompanying drawings.