A wave energy conversion system

Field of the Invention The present invention relates to a wave energy conversion system. In particular the present invention relates to a wave energy conversion system which includes a mechanical energy converter which has a generating mode for providing power to an electrical grid and a motoring mode for extracting power from the electrical grid.

Background

Wave energy conversion systems are known in the art. Examples of such systems include those described in patents EP1439306, EP1295031 and EP1036274 of which the present applicant is the proprietor. Such systems are usefully deployed in a maritime environment and generate useful power from wave motion.

Such wave energy conversion systems employ a wave energy absorber, a hydraulic/pneumatic circuit and a power generator. Wave energy is absorbed by the wave energy absorber which pumps oil through the hydraulic/pneumatic circuit. The pump action causes a rotary element of the power generator to rotate thereby translating rotational energy into electrical energy. The hydraulic/pneumatic circuit is coupled between the wave energy absorber and the power generator and acts as an intermediary control means. The hydraulic/pneumatic circuit is complex. The absorbed mechanical energy is first converted to hydraulic/pneumatic energy, which is then converted to electrical energy which results in a relatively inefficient conversion process as the conversion from mechanical energy to electrical is not direct. The operating parameters of the hydraulic/pneumatic circuit may be changed in response to changes in the prevailing ocean conditions. However, the hydraulic/pneumatic circuit is unable to react rapidly enough to these changes due to the inherent characteristics of such circuits.

There is therefore a need for a wave energy conversion system which is adaptable to varying wave regimes and more efficient in converting mechanical forces to useful power.

Summary

These and other problems are addressed by a wave energy conversion system which includes a mechanical energy converter which has a generating mode for providing power to an electrical grid and a motoring mode for extracting power from the electrical grid.

Accordingly, a first embodiment of the invention provides a wave energy conversion system as detailed in claim 1. The invention also provides a power take off system as detailed in claim 36. Additionally, the invention relates to an off-shore distributed electrical network as detailed in claim 37. The invention also relates to a wave energy electrical generation network as detailed in claim 38. Advantageous embodiments are provided in the dependent claims.

These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the teaching of the invention.

Brief Description Of The Drawings

The present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic view of a wave energy conversion system in accordance with the present invention. Figure 2 is a diagrammatic view of a detail of the mechanical energy converter of Figure 1.

Figure 3 is an inductance profile and force generation profile for the mechanical energy converter of Figure 1 , in both motoring and generating modes.

Figure 4 is a schematic circuit diagram of a control circuit.

Figure 5 is a diagrammatic illustration of the circuit of Figure 4 with transistors Ti and T ₂ switched on.

Figure 6 is a diagrammatic illustration of the circuit of Figure 4 with transistors Ti and T ₂ switched off.

Figure 7 is a diagrammatic view of another wave energy conversion system which is also in accordance with the present invention.

Figure 8 is a diagrammatic view of a distributed electrical network which includes a plurality of the systems of Figure 1.

Detailed Description Of The Drawings

The invention will now be described with reference to an exemplary system which is provided to assist in an understanding of the teaching of the invention.

Referring to Figure 1 there is illustrated an exemplary wave energy conversion system 100 for harnessing wave energy. The system 100 comprises a power take-off device provided by a mechanical energy converter 102. In the exemplary embodiment the mechanical energy converter is a linear switch reluctance (LSR) generator configured to convert mechanical energy into electrical energy. The generator 102 is driven by a wave energy absorber 105. Before describing specifics of the generator 102 aspects of the wave energy absorber 105 will first be described. It will be understood that wave energy absorbers are known in the art, an example of which is shown in European Patent no. 1 ,295,031 of which the present applicant is the proprietor and replicated in Figure 1 of the instant application. This exemplary wave energy absorber 105 comprises at least two devices (floats) 110, 111. While it is not intended to limit the teaching of the present invention to such a specific type of wave energy absorber, this specific absorber is described to assist in an understanding of how the parameters of a mechanical to electrical energy converter may be varied in response to changes in the prevailing wave conditions using power from an electrical grid such as an on-shore grid.

In the exemplary wave energy absorber, each of the two devices comprises a surface float and/or at least one submerged wave driven body 115 below the surface of the body of liquid. The outer surface float 111 is annular and surrounds the inner surface 110. Linkages 139 are provided between the at least two devices 110, 111. By configuring each of the two devices 110, 111 to oscillate at different frequencies relative to one another in response to passing waves, relative movement between the at least two devices 110, 111 may be used to generate an energy transfer which may be harnessed by the linkages 139 between the at least two devices 110, 111. The linkages are coupled to the generator 102 which harnesses the mechanical energy generated by the wave energy absorber 105 and converts the mechanical energy into electrical energy.

In a preferred arrangement which may be usefully employed within the context of the present teaching, the generator 102 is an LSR generator and is directly coupled to the wave energy absorber 105. The generator 102 comprises a translating member 120 of electrical steel which is moveable axially and intermediate to a pair of spaced apart stator members 125 also of electrical steel. The translating member 120 includes first and second sets of teeth 132 of rectangular cross section on its respective opposite sides which define translator poles. Each stator member 125 includes teeth 138 on one side thereof of rectangular cross section which define stator poles. The respective sides of the translating member 120 are associated with the corresponding stator members 125 such that the translator poles 132 and the stator poles 138 define opposing pole arrangements. The translating member 120 is operably coupled to the wave driven body 115 via linkages and is axially moveable along rails (not shown) such that the translating member 120 reciprocates in tandem with the oscillating wave driven body 115. The opposing pole arrangements are dimensioned such that air gaps 140 exist between the translator poles 132 and the stator poles 138. The translating member 120 is coupled to the inner surface float 110, and each stator member 125 is coupled to the annular outer surface float 111.

Copper coils 141 as illustrated in Figure 4 are wound around the stator poles 138. The sequential energisation of these poles creates a magnetic field and a steady aligning force between opposing stator poles 138 and translator poles 132. The translating member 120 moves against the steady aligning force thereby converting mechanical energy into electrical energy. The aligning force may be considered to be an operating characteristic of the generator 102. A person skilled in the art will appreciate that, in motoring operation, a forward electromagnetic force (forward EMF) is produced when electric current flowing in a coil 141 coincides with rising coil inductance. In generating operation, a backward electromagnetic force (back EMF) is produced when the coil 141 current coincides with falling coil inductance.

Referring now to Figure 2 which shows a section 143 of a 6/4 linear switched reluctance generator 102. It will be appreciated by those skilled in the art that the generator 102 may comprise any number of such sections 143 or any specific configuration of individual sections. In this exemplary arrangement, every six stator poles 138 are opposite four translator poles 132. The translating member 120 reciprocates under the influence of the oscillating wave driven body 115 of the wave energy absorber 105. Stator poles S1 , S4 and S7 are shown energised, driving magnetic flux in a closed loop through S1 , T1 , T3, S4 and back to S1. A similar closed flux loop is set up between S4, T3, T5 and S7. The magnetic flux in the air gaps 140 exerts a strong force pulling both the translating member 120 and the stator members 125 closer together horizontally, and also exerts a force vertically aligning the energised poles. The translating member 120 is urged against this force by the wave driven body 115 of the wave energy absorber 105 thereby converting the mechanical energy absorbed by the translating member 120 from the linkages of the wave energy absorber into electrical energy.

As the translator poles 132 moves further out of alignment, the alignment force on the energized poles T1 , T3 and T5 weakens, and reaches a minimum when T3 is mid-way between S4 and S5. At this point the next translator pole T2 has moved into alignment with S3 and T4 has aligned with S6, so S1 , S4 and S7 are de-energised, and S3 and S6 are energised. This drives flux across the air gap 140 through T2 and T4, exerting a backward alignment force as the translating member 120 moves past.

The mechanical power, P _m , absorbed by the translating member 120 from the wave energy absorber 105 is given approximately by:

ft ..,. = F -- X v _{(1 )}

Where: v is the velocity of the translating member 120, and Fe is the electromagnetic force exerted on it by the magnetic field.

Under generator operation the direction of F _e is opposite to the direction of motion of the translating member 120, so P < 0.

The electromagnetic force F _e is a function both of the displacement of the translating member 120 and the coil currents i _a , ib and i _c . Since the phase currents are turned on in turn, the total electromagnetic force can be found by considering a single phase 'a' only, with the assumption that phases 'b' and 'c' behave identically.

p _ i _a ² dL _as (xJ)

- = 2 _{ax (2)} Where:

Las is the inductance of phase 'a', and i _a is the phase 'a' current.

Referring now to Figure 3, an inductance profile for one phase of the generator 102 is shown. The inductance is a maximum when poles 132, 138 are in full alignment and a minimum when poles 132, 138 are completely out of alignment. A six-pole generator 102 has three independently-driven phases each phase consists of a number of pairs of coils connected in series. The 6/4 generator 102 may therefore be considered as a three-phase machine. Under normal operation only one phase of the machine 102 is switched on at a time, energising all the coils 127 wrapped around the stator poles 138 in alignment with the translator poles 132, which, in this exemplary embodiment, is every third stator pole 132. The profiles of Figure 3 assume an ideally square current waveform; in practice the maximum rate of rise and fall of the phase currents is finite, and depends on the phase inductances and resistances.

It will be appreciated that wave energy varies significantly depending on the conditions in the ocean. In periods of large swells, the wave energy absorber 105 generates a large amount of kinetic (mechanical) energy which drives the translating member 120 at a high speed so a large amount of electricity is generated. In periods of relatively small swells, the kinetic (mechanical) energy generated by the wave energy absorber 105 is significantly less than periods of large swells resulting in less kinetic energy and as a consequence the translating member 120 is driven at a slower speed resulting in less electricity being generated. As will be discussed in greater detail below, to address this variance in the output of the electricity generated as a result of varying conditions, a control means provided by the control unit 147 regulates the electromagnetic force F _e and in turn the damping force of the translating member 120 by selectively energising the coils 141 with power from an on-shore electrical grid 169 (shown in Figure 4) so that the velocity of the translating member 120 remains substantially constant irrespective of the ocean conditions. In this manner the operating characteristic of the generator 102 is dynamically varied to the prevailing ocean conditions (wave regime). If the generator 102 was not dynamically varied/tuned, the damping resistance of the translating member 120 would have to be set to cope with a wave regime which provides maximum wave energy. The generator 102 is a mechanical device with mechanical operating limitations, for example, the speed of the translating member 120 has to be within certain limits. If the speed of the translating member 120 exceeds these limits there is a significant risk that the generator 102 would malfunction due to its inability to cope with the excess kinetic energy provided by the wave absorber 105. The control unit 147 ensures that the speed of the translating member 120 operates within its design limits by varying the electromagnetic force F _e which in turn varies the damping resistance of the translating member 120. For example, in period of large swells it is desirable that the aligning force between the poles 132, 138 is large enough to provide a large damping resistance to the wave driven body 115 or otherwise the translating member 120 may operate outside its speed limits resulting in the generator 102 malfunctioning. In period of small swells it is desirable that the aligning force between the poles 132, 138 is relatively small to provide significantly less damping resistance to the wave driven body 115 than in period of large swells so that the translating member 120 moves at a speed to maximise the amount of electricity being generated. If the electromagnetic force was not varied, in periods of small swells very little electricity would be generated. Referring now to Figure 4, an exemplary circuit diagram of a single phase of the switch reluctance generator 102 is shown. It will be appreciated that the coils 141 previously described as being wound around the respective stator poles 138 are coupled to a corresponding circuit as illustrated in Figure 4. A power line 160 operably coupled to a mains electricity supply grid 169 relays AC power to a grid converter 167. It will be understood that the mains electricity supply grid 169 is remotely located to the operation of the wave energy absorber, i.e. it may be considered an on-shore arrangement whereas the wave energy absorber is an off-shore arrangement. The characteristics of the power line 160 are selected to be appropriate to effect a transfer of power from the grid 169 to the generator 102. These characteristics will depend on the distances between the generator 102 and the mains power supply and the voltage at which the power is provided.

The control unit 147 is in communication with the sensor 150 for reading the sensed velocity of the wave driven body 115. The control unit 147 is also electrically coupled to the bases of transistors T1 and T2 for switching on and off the transistors thereby dynamically varying the operating characteristic of the generator 102 in response to changes in velocity of the wave driven body 115. In this exemplary circuit which may be usefully employed, the coil 141 is coupled intermediate the pair of bipolar power transistors T1 and T2. A first free wheel diode D1 is coupled to a common node shared by the transistor T2 and the coil 141. A second free wheel diode D2 is coupled to a common node shared by T1 and the coil 141. The control unit 147 is operable for turning on/off transistors T1 and T2 so that the circuit can operate as a motor or a generator thus the circuit has two modes of operation. When transistors T1 and T2 are switched on the coil 141 is energised thereby drawing power from the grid 169 via the AC-DC inverter 168 of the grid converter 167. When the coil 141 is being energised in this fashion the circuit operates as a motor.

Figure 5 diagrammatically illustrates power being drawn from the grid 169 when the transistors T1 and T2 are switched on. In contrast, when the transistors T1 and T2 are switched off the circuit operates as a generator providing power from the coil 141 to the grid 169 via the DC-AC converter 170 of the grid converter 167. Figure 6 diagrammatically illustrates power being provided from the coil 141 to the grid 169 when the transistors T1 and T2 are switched off.

In operation, the generator 102 is operably coupled to the wave energy absorber 105. The submerged wave driven body 115 of the wave energy absorber 105 is forced to oscillate by wave energy, which in turn provides a driving force which drives the translating member 120 to reciprocate. The movement of the translating member 120 converts mechanical energy absorbed form the wave energy absorber 105 into electrical energy. The sensor 150 senses the velocity of the wave driven body 115 which is then read by the control unit 147.The control unit 147, in response to the velocity of the wave driven body 115, appropriately modulates the phase currents of the energy converter 102 for controlling the electromagnetic force F _e and in turn the damping resistance of the translating member 120. It will therefore be appreciated that the control unit 147 is co-operable with the sensor 150 for selectively controlling power from the electrical grid to the energy converter 102 thereby energising the converter 102 for varying its phase currents in response to the velocity of the wave driven body 115.

When the control unit 147 switches on transistors Ti and T ₂ a current circulates in a phase of the generator 102, increasing in magnitude. When the current rises above a threshold value, transistors Ti and T ₂ are switched off as illustrated in Figure 6. The energy stored in the winding of coil 141 keeps the current flowing in the same direction, decreasing quickly in magnitude below the threshold level. The diodes Di and D ₂ provide a path for the coil current to continue to flow, quickly decaying after Ti and T ₂ turn off. It should therefore be apparent that the power transistors Ti and T ₂ switch on and off many times during the excitation of a single phase. Modulating the phase current for changing damping resistance of the translating member 120 is achieved by increasing or decreasing the switching times of the power transistors Ti and T ₂ . It will be appreciated by those skilled in the art that other switching converters and switching strategies may be used as an alternative to the arrangement to that of Figure 4. The circuit arrangement of Figure 4 is given by way of example only and it is not intended to limit the invention to this arrangement.

Referring now to Figure 7 there is illustrated another embodiment of a wave energy conversion system 200 which is also in accordance with the present teaching. The system 200 of Figure 7 is substantially similar to the system 100 of Figure 1 , and like components are indicated by similar reference numerals. The main difference is that the system 200 includes two additional side generators 202 operably coupled to a central generator 204. The two side generators 202 and the central generator 204 are provided in a modular arrangement for facilitating modular assembly. It is envisaged that the modular arrangement may include any desired number of generators and configuration. The two side generators 202 and the central generator 204 operate substantially similar to the generator 102 of Figure 1. The dimensions (geometries) of the side generators 202 are less than that of the central generator 204 which facilitates fine tuning to the prevailing wave regime. The control means may activate or deactivate one or more generators of the modular arrangement when desired. The control means may also be configured so that only some of the coils 141 in a particular generator are energised while the other coils 141 of that generator 102 are not energised such an arrangement facilitate finely adjusting the parameters of the generator 102 to suit the prevailing wave regime. When desired the control means may selectively activate a combination of coils in a combination of generators. For example, four coils in one generator may be energised while six coils in the neighbouring generator may be energised, the two generators may have differing geometries.

Referring now to Figure 8 there is illustrated an off-shore distributed electrical network 300 which includes a plurality of wave energy conversion systems 100 and is also in accordance with the present teaching. The network 300 comprises a central hub 305 operably coupled to the off-shore grid 169 and the wave conversion systems 100 which facilitates duplexing of power between the grid 169 and the generators 102 as well as between the respective generators 102. The central hub 305 includes a control unit for controlling power to the generators 102 and may operate in a similar fashion to the control unit 147 of Figure 4. The grid converter 167 is provided on-shore and the central hub 304 is provided off-shore. It is envisaged that the hub 305 may also be provided onshore. The generators 102 may be energised from power from the grid 169 or from at least one of the other generators 102 via the central hub 305. The electrical grid 169 is AC compatible and the generators 102 are DC compatible. The grid converter 167 provides a bridge between AC to DC, and vice versa. The arrangement of the central hub 305 allows for the combining of DC power generated by the individual generators 102 which is then converted to AC by the grid converter 167. It will therefore be appreciated that it is not necessary to provide complex DC-AC or AC-DC conversion circuitry for each generator 102 as they share a common grid converter 167.

The advantages of a system provided in accordance with the teaching of the invention are many. In particular, the wave energy conversion system of the present application eliminates the need to use hydraulic/pneumatic circuits, thereby increasing efficiency as the mechanical energy is directly converted to electrical energy. Furthermore, the systems 100 and 200 can absorb and convert mechanical power in an irregular way and unsynchronised fashion which is particularly suitable for wave power generation. This converted mechanical energy may be coupled to an AC supply. The translating member 120 is robustly constructed from laminated electrical steel, with no copper coils, and is therefore suitable for the harsh environmental conditions which are commonplace of wave energy conversion systems. The coils 141 on the stator member 125 are easy to configure, as they are wound tightly around the stator poles 138. It will therefore be appreciated that complicated distributed coil arrangements, of the type found in synchronous and induction generators, are not required. Furthermore, the generator 102 does not require permanent magnets in its construction. It is undesirable to include permanent magnets in a wave energy conversion system as they must be sized to accommodate the largest energy flux anticipated from the wave climate where the wave absorber is located, resulting in that the generator 102 is overrated for most of the time. Also permananet magnets are unsuitable as they are relatively expensive, suffer from demagnetization over time and have to be replaced periodically, and are prone to oxidation in hostile environments. By avoiding the use of permanent magnets these problems are avoided.

Both generating and motoring action are possible in both directions using unipolar current, simply by adjusting the switching sequence in the phase coils. Since mutual coupling is absent, each phase is electrically independent of the others. A short-circuit fault in one phase therefore has practically no effect on the operation of the other phases. This is in direct contrast to a permanent magnet synchronous machine, where a failure of one phase puts the machine out of action.

It will be understood that what has been described herein is an exemplary embodiment of a wave energy conversion system. Such a system comprises a mechanical energy converter for converting mechanical energy absorbed from a wave energy absorber to electrical energy. A sensing means is provided for sensing a parameter of the wave energy absorber. The mechanical energy converter is coupled to a power line which may provide power to the mechanical energy converter from an electrical grid connection. A control means is co- operable with the sensing means for selectively controlling the power provided from the electrical grid to the mechanical energy converter thereby selectively energising the mechanical energy converter in response to the sensed parameter of the wave energy absorber and allowing for a varying of the performance of the mechanical energy converter to the ambient wave conditions. While the present invention has been described with reference to exemplary arrangements it will be understood that it is not intended to limit the teaching of the present invention to such arrangements as modifications can be made without departing from the spirit and scope of the present invention.

The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers , steps, components or groups thereof.