"WAVE ENERGY SYSTEM" Field of the Invention

The present invention relates to a system for generating energy from wave motion. Background to the Invention

Many systems have been proposed for utilising wave motion from the oceans to generate energy. One such type of system utilises a buoy located at the surface or submerged near the surface of the water which is connected by a tether to a device located below the buoy. The reciprocating motion of the buoy and tether with the waves is then used as a source of energy for generating electricity or for other applications.

Such system may present difficulties in optimising the arrangement for different conditions. The systems will generally perform at their optimum when the wave frequency approaches the natural frequency of the device. Also, variations in tide alter the system dynamics and therefore present a challenge for optimisation.

The present invention relates to a system for converting wave motion into usable energy aimed at providing both reliability and flexibility in optimising the system for various conditions. References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a wave energy system comprising: a buoy coupled to a tether, the tether being anchored in such a manner as to provide resistance to upward movement of the buoy;

a first drive wheel onto which is wound a portion of the tether wherein, in use, the first drive wheel is driven in a first direction during upwards movement of the buoy as the tether unwinds and in a second opposite direction during downwards movement of the buoy; a drive shaft connected to the first drive wheel; and, energy conversion means coupled to the drive shaft for converting rotational motion of the drive shaft to a form of energy for storage.

In one embodiment the first drive wheel, the drive shaft, and the energy conversion means are all mounted inside the buoy. Preferably said energy conversion means comprises a hydraulic pump. Preferably the hydraulic pump comprises one or more hydraulic cylinders coupled to a crank shaft which is coupled to the drive shaft, wherein rotation of the drive shaft drives the crank shaft which in turn actuates the hydraulic cylinder(s) in a reciprocal motion. The hydraulic cylinder(s) ports are valved such that the reciprocal motion of the cylinder piston(s) causes hydraulic fluid to be pumped in one direction from a source to a motor (on the pressure side).

Preferably the system further comprises a mechanical rectifier wherein, in use, the drive shaft is driven to rotate in a first direction when the first drive wheel is rotating in the first direction and the drive shaft is decoupled from the first drive wheel and free to continue rotating in the first direction when the first drive wheel rotates in the second direction.

Preferably the first drive wheel is coupled to the energy conversion means by a gearing system such that the energy conversion means is driven at a higher rotational speed than the drive shaft.

According to another aspect of the present invention there is provided a wave energy system comprising: a buoy coupled to a tether;

a first drive wheel onto which is wound a portion of the tether such that the first drive wheel rotates in a first direction when movement of the buoy is such that the tether unwinds from the first drive wheel and in a second direction when movement of the buoy is such that the tether winds onto the first drive wheel; and a drive shaft connected to the first drive wheel; wherein the drive shaft is driven to rotate in a first direction when the first wheel is rotating in the first direction and the drive shaft is decoupled from the first drive wheel and free to continue rotating in the first direction when the first wheel rotates the second direction.

Preferably, the drive shaft is coupled to an energy conversion means to convert rotational motion of the drive shaft to a form of energy for storage and the first drive wheel is coupled to the energy conversion means by a gearing system such that the energy conversion means is driven at a higher rotational speed than the drive shaft. In one embodiment, the energy conversion means comprises a hydraulic pump which has one or more hydraulic cylinders coupled to a crank shaft such that the rotation of the drive shaft drives the crank shaft which in turn actuates the hydraulic cylinder(s) in a reciprocal motion. The hydraulic cylinder(s) ports are valved such that the reciprocal motion of the cylinder piston(s) causes the hydraulic working fluid to be pumped in one direction from a source (or suction) to a motor (on the pressure side).

In one embodiment the first drive wheel, the drive shaft, and the energy conversion means are all mounted inside the buoy. A control system is also preferably provided which allows the number of hydraulic cylinders actively pumping to be varied according to changing environmental conditions such as wave height and wave period. The control system may also actively control extension of the first tether to optimise motion of the buoy.

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In a preferred embodiment, a support platform is provided to be fixed on the seabed, the first drive wheel and the drive shaft being secured onto the support platform.

Preferably, the buoy comprises a primary buoy connected to a first tether and a recovery buoy is provided connected to a second tether wound onto a recovery wheel, wherein the recovery buoy is connected to the first drive wheel such that the recovery buoy applies a reverse torque to the first drive wheel so that the first tether is wound back onto the first drive wheel during downwards movement of the buoy. This arrangement ensures that tension is maintained on the first tether and prevents 'slack line' in the first tether.

Preferably a sealed housing is provided on the support platform, the drive shaft passing from the first drive wheel to the inside of the housing through a sealed aperture in the housing.

Preferably, the hydraulic pump driven by the drive shaft delivers pressurised fluid for storage in an accumulator module. The accumulator module preferably includes one or more pressure tanks for storing pressurised fluid.

In one embodiment, a generator module is provided comprising a hydraulic motor and an electrical generator, the generator module receiving pressurised fluid from the accumulator module such that the pressurised fluid provides motive power to the hydraulic motor which in turn drives the electrical generator for creating electrical energy.

Preferably the support platform includes a deflector sheave mounted such that the first tether extends upwardly from the deflector sheave to the primary buoy and horizontally from the deflector sheave to the first drive wheel. Preferably, the deflector sheave is hinged to rotate along an axis from the horizontal to allow the sheave to align with the direction of the tether and therefore minimise fleeting angles to the sheave.

According to a further aspect of the present invention there is provided a wave energy device comprising: a drive arrangement including a drive adapted to have a tether wound there around with the tether extending from the drive to a riser; an energy recovery mechanism for recovering energy; and

a connection facility for coupling the energy recovery mechanism to the drive arrangement, the connection facility for allowing the energy recovery mechanism to recover energy as the riser moves upwards and pulls the tether to unwind the tether and turn the drive. Preferably the connection facility includes a decoupling mechanism for decoupling the drive arrangement from the energy recovery mechanism to readily allow the drive to be rotated to wind the tether back around the drive upon downward movement of the riser.

Preferably the wave energy device includes a winding facility for biasing the drive to wind the tether back around the drive upon downward movement of the riser. Preferably the winding facility includes a riser adapted to exert a force on the drive biasing the drive to wind the tether back around the drive.

Preferably the energy recovery mechanism includes a plurality of linear hydraulic cylinders. The drive arrangement may include a drive shaft. Each of the linear hydraulic cylinders is preferably directly connected to the drive shaft to pump a fluid to recover energy.

The drive shaft may be provided in the form of a crank shaft with each linear cylinder being matched by a corresponding linear cylinder to offset forces on the crank shaft. A first set of the pistons preferably extends from a first side wall of the energy recovery mechanism towards the crank shaft. A second set of the pistons preferably extends from a second opposite side wall of the energy recovery mechanism towards the crank shaft.

Preferably the wave energy device includes a control facility adapted to selectively activate or isolate one or more of the linear cylinders. The control facility is preferably adapted to tune the energy recovery mechanism to the movement of the riser and thereby increase the recovery of energy.

According to a still further aspect of the present invention there is provided a method of recovering wave energy comprising: winding a tether around a drive with the tether extending from the drive to a riser; and by coupling the

drive to an energy recovery mechanism recovering energy as the riser moves upwards to pull and unwind the tether from the drive.

Preferably the method includes providing a plurality of linear hydraulic cylinders connected to a crank shaft and recovering energy comprises turning the crankshaft so that the linear hydraulic cylinders pump a working fluid. Preferably one upward stroke of the riser results in more than one rotation of the crankshaft.

Preferably the method includes decoupling the drive from the energy recovery mechanism to allow winding of the tether around the drive. The method preferably further includes connecting a winding riser to the drive to bias the drive to wind the tether about the drive on downward movement of the riser.

Other aspects and preferred features will be apparent from the drawings and detailed description of the preferred embodiments provided below.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word

"preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of several specific embodiments of the wave energy system, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a side view of a preferred embodiment of a wave energy system according to the present invention;

Figure 2 is a perspective view of the wave energy system of Figure 1 ;

Figure 3 is a perspective view of a generator of the wave energy system of Figure 1 ;

Figure 4 is a perspective view of an accumulator of the wave energy system of Figure 1 ;

Figure 5 is a first view of an array of support platforms;

Figure 6 is a second view of an array of support platforms including an accumulator and generator;

Figure 7 is a side view of an alternative embodiment of a support platform of the wave energy system according to the invention;

Figure 8 is a side view of a wave energy system including the support platform of Figure 7; Figure 9 is an illustration of a further preferred embodiment of a wave energy device according to the present invention;

Figure 10 is a front view of the wave energy device shown in Figure 9; Figure 11 is a top view of the wave energy device shown in Figure 9; Figure 12 is a side view of the wave energy device shown in Figure 9; Figure 13 is a top perspective view of one embodiment of an energy recovery mechanism used in the wave energy device of Figure 9;

Figure 14 is a plan view of the energy recovery mechanism shown in Figure 13;

Figure 15 is a side view of the energy recovery mechanism shown in Figure 13;

Figure 16 is an end view of the energy recovery mechanism shown in Figure 13;

Figure 17(a) is a side view of a further embodiment of an energy recovery mechanism; Figure 17(b) is a plan view of the energy recovery mechanism shown in Figure 17(a); and,

Figure 18 illustrates a still further preferred embodiment of a wave

energy device according to the present invention. Detailed Description of Preferred Embodiments

A preferred embodiment of the wave energy system 10 in accordance with the invention, as illustrated in Figures 1 to 8, comprises a support platform 12, (or manifold according to subsea engineering terminology) which is anchored below the ocean surface. The support platform 12 anchors a primary buoy 14 connected to the support platform 12 by a first tether 16.

The primary buoy 14 is connected to a first end of the first tether 16 and positioned such that wave motion of the ocean causes movement of the buoy 14. The second end of the first tether 16 is wound onto a first drive wheel 18 comprising a first sheave mounted on the support platform 12. Movement of the primary buoy 14 generally away from the first drive wheel 18 rotates the first drive wheel 18 in a first direction as the first tether 16 unwinds from the first drive wheel 18. Also provided is a recovery buoy 20 to apply a reverse torque to the first drive wheel 18 biasing the first drive wheel 18 to rotate in a second direction. The recovery buoy 20 is connected to a first end of a second tether 22. The second end of the second tether 22 is wound around a recovery wheel 25 comprising a recovery sheave.

The recovery wheel 25 is connected to the first drive wheel 18 and the second tether 22 is wound in an opposite direction to the first tether 16. The recovery buoy 20 thereby provides resistance against the primary buoy 14 such that when the primary buoy 14 is not subject to sufficient force by wave motion to move the primary buoy 14 away from the first drive wheel 18, the first drive wheel 18 rotates in the second direction and the first tether 16 is wound back onto the first drive wheel 18.

Generally, as the primary buoy is lifted up by the wave motion, the first drive wheel 18 rotates in the first direction and as the primary buoy 14 falls, the first drive wheel 18 rotates in a second direction. It will be appreciated that motion of the primary buoy 14 is unlikely to be only vertical and the primary buoy 14 will also undergo movement in a horizontal direction with the wave motion.

The support platform 12 is preferably provided with a sealed housing 24. The first drive wheel 18 is connected to a drive shaft 26 which passes into the housing 24 through an aperture. The aperture includes a seal such that the housing 24 is separated from the ocean environment. The housing 24 includes an energy conversion means to convert the rotational energy of the drive shaft 26 into a form that can be stored for use.

The drive shaft 26 is coupled to the first drive wheel 18 by a mechanical rectifier such that when the first drive wheel 18 is rotating in a first direction, the drive shaft 26 is driven also in the first direction and when the first drive wheel 18 is rotating in a second direction, the drive shaft 26 is decoupled from the first drive wheel 18 such that the drive shaft 26 continues to rotate freely in the first direction. The drive shaft 26 thereby continuously rotates in the same direction.

The energy conversion means comprises a hydraulic pump 28 within the housing 24. The rotational movement of the drive shaft 26 drives the hydraulic pump 28 to deliver pressurised fluid to an accumulator module 30. The use of an accumulator 30 allows the ability to smooth out the random nature of wave energy and to provide a means to improve the efficiency of operation of the electrical energy generator that will operate at a higher efficiency when subject to more constant energy input.

In this embodiment, the connection between the drive shaft 26 and the hydraulic pump 28 is provided with a gearing system. The gearing system converts the rotation of the drive shaft 26 in the first direction to rotation of the hydraulic pump 28 at a higher rotational speed. The drive shaft 26 may be provided with a second drive wheel 27, within the housing 24, connected to a pump drive wheel on the hydraulic pump 28 by a chain 29 to provide the gearing. The relatively slow rotational motion of the first drive wheel 18, caused by the reciprocal motion of the primary buoy 14, is thereby converted to the higher speed, and more continuous rotation of the hydraulic pump 28. The gearing system may comprise a simple chain gearing arrangement as shown or other suitable gear sets could be used.

An alternative embodiment is shown in Figures 7 and 8 in which the energy conversion means comprises a piston 46 having a cylinder 48 with an end mounted to a point on the periphery of the second drive wheel 27 such that rotation of the drive shaft 26 causes reciprocal motion of the piston 50. The support platform 12 is also preferably provided with a deflector sheave 32. The deflector sheave 32 is mounted such that the first tether extends upwardly from the deflector sheave 32 to the primary buoy 14 and horizontally from the deflector sheave 32 to the first drive wheel 18. The deflector sheave 32 may be hinged to rotate along an axis from the horizontal to provide suitable fleeting of the first tether 16 between the deflector sheave 32 and the primary buoy 14 as well as between the deflector sheave 32 and the first drive wheel 18. Alternatively, the first tether 16 may extend directly upwards from the first drive wheel 18 to the primary buoy 14.

The accumulator module 30 may comprise one or more pressure tanks 34 connected to the hydraulic pump 28 by pressure lines 36. The pressure tanks 34 store the pressurised hydraulic fluid from the hydraulic pump 28 for later use.

The wave energy system 10 may include one or more generator modules 38 connected to the accumulator module 30 by pressure lines 37. Each generator module 38 is provided with a sealed housing 42. The pressure lines 37 provide pressurised hydraulic fluid from the pressure tanks 34 which is used to drive a hydraulic motor 39 within the sealed housing 42. The hydraulic motor 39 in turn drives an electric generator 40 provided within the sealed housing 42 of the generator module 38. The generator 40 creates electrical energy for delivery to an onshore location by power lines 44 extending from the housing 42 through sealed openings.

It is expected that the wave energy system 10 may comprise a plurality of support platforms 12 each having associated primary buoys 14, located in an array on the seabed. An example of such an array is shown in Figures 7 and 8. As shown in Figure 8, a plurality of support platforms 12 provide a supply of pressurised hydraulic fluid to an accumulator module 30 and a generator module 38. The use of separate modules for generating energy, storing

energy and converting to electrical energy allows easier transport, installation and maintenance of the system 10.

The hydraulic lines 36 and 37 are provided with connectors at each end which are received in sockets provided in the housing 24 of the support platform 12 and the accumulator modules 30 and generator modules 38. Any of the support platforms 12, accumulator modules 30 or generator modules 28 can therefore be disconnected for maintenance. The length of the hydraulic lines

36 and 37 may be longer than the water depth at which the module is located to allow the modules to be brought to the surface for maintenance or replacement with disconnection.

A control system is preferably also provided to vary the characteristics of the system such as the gearing arrangement or to actively control the extension of the tether 16 and thereby the buoy motion. By varying the gearing arrangement and thereby the resistance provided against the primary buoy 14, it is possible to tune the resonant characteristics of the wave energy system 10 to optimise for varying conditions. The gearing may be varied by having multiple pump drive wheels on the hydraulic pump 28 whereby the chain 29 shifts between these pump drive wheels. This optimisation can be performed on a wave by wave basis or also on a seasonal basis. The control provided by the control system may be automatic in that changes are made based on sensed changes in the ocean conditions.

The active control on the tether extension 16 may be to latch or restrict the tether 16 from extending during portions of the cycle of motion of the primary buoy 14 to optimise the buoy 14 motion. Also, the control system may drive the hydraulic pump in reverse to rotate the first drive wheel 18 to wind or unwind the first tether 16 and thereby adjust the relative positions of the primary buoy 14 and the support platform 12. Such control can be used to account for tidal variations or to submerge floating buoys in severe weather conditions. Along with pressure lines 36 and 37, it is expected that power lines and control lines (not shown) may be provided between the support platforms 12,

accumulator modules 30 and generator modules 38 in order to transfer power and control signals between the modules. The power and control lines may be connectable to the modules in via suitable sockets in the same manner as the pressure lines. It will be appreciated that energy generated may be used for means other than generation of electrical energy. For example, the pressure created may be used to pressure seawater which can then be converted into potable water by standard reverse osmosis or other conversion methods. The pressurised seawater may be stored in an accumulator. The use of an accumulator provides the ability to smooth out the random nature of ocean wave energy and allows seawater to be supplied to the reverse osmosis module at a more constant pressure. The reverse osmosis module can therefore operate more efficiently.

In Figures 9 to 16 there is shown another preferred embodiment of a wave energy device 50 according to the present invention. The wave energy device 50 advantageously accommodates the upward movement of a riser in a manner that allows a relatively variable stroke length. Hence the wave energy device 50 is considered to be able to be operated in environments having variable wave heights and to recover a relatively high proportion of the available energy in comparison to other point absorber wave energy systems such as the 'Archimedes Waveswing' and the CETO system.

As shown in Figure 9 the wave energy device 50 includes a drive arrangement 52 and an energy recovery mechanism 54. The drive arrangement 52 includes a drive 56 in the form of a drive wheel 58. The drive wheel 58 is adapted to have a tether 60 wound there around.

In this embodiment the tether 60 is wound around the drive wheel 58 a number of times and extends from the drive wheel 58 to a riser 62. The riser 62 is provided as a self righting flotation cylinder 64 of the form shown in Figure 9. Other suitable buoy hull shapes may be employed. The drive arrangement 52 includes a connection facility 66 for coupling the energy recovery mechanism 54 to the drive arrangement 52. Advantageously

the connection facility 66 includes a decoupling mechanism 68 for decoupling the drive arrangement 52 from the energy recovery mechanism 54. With this embodiment the decoupling mechanism 68 is activated on the descent of the riser when the tension in the tether 60 drops below a predetermined amount. In this arrangement, the decoupling mechanism 66 comprises a rotational rectifier which directly couples a drive shaft to a crank shaft when the drive shaft is rotating in one direction (corresponding to the direction of turn induced by the primary buoy heaving upwards), and allows the drive shaft to rotate without turning the crankshaft when the drive shaft is rotating in the opposite direction (corresponding to the primary buoy moving downwards). Rotational rectifiers are known, for example ratchet type sailing winches allow rotation to wind a cable in one direction with movement in the opposite direction being decoupled from the cable.

The wave energy device 50 includes a rewinding facility 70 that biases the drive arrangement 52 to wind the tether 60 back around the drive wheel 58 upon downward movement of the riser 62 when the drive arrangement 52 is decoupled from the energy recovery mechanism 54.

The rewinding facility 70 includes a riser 72 connected to a tether 74 that is adapted to exert a winding force on the drive 56. Once the energy recovery mechanism 54 is decoupled from the drive arrangement 52, and the riser 64 moves downwardly, the reduction in tension in the tether 60 causes the riser 72 to take control and wind the tether 74 back around the drive wheel 56. A separate decoupling mechanism could be provided for the tether 74. In the arrangement shown in Figure 9 there is provided a "primary buoy" or "power buoy" 62 and a first tether, and a "recovery buoy" 72 and a "secondary tether". Both the buoys or risers comprise floats.

As would be apparent a limiting factor on the maximum wave height that can be accommodated by the wave energy device 50 is the length of the tether 60 wound around the drive wheel 58. However, unlike a lever which has a fixed stroke length, the length of the tether 60 wound on the drive wheel 58 can be configured to allow the wave energy device 50 to operate over a wide range of stroke lengths say in the order of 5 m, 10 m, 20 m or more depending upon

the predicted wave environment statistics for any given installation site. This is considered to be advantageous in comparison to current wave energy devices such as the 'Archimedes Waveswing' and the CETO system.

The energy recovery mechanism 54 advantageously includes a plurality of linear hydraulic piston arrangements in the form of linear hydraulic cylinders

78. Each of the linear cylinders 78 is directly connected to a drive shaft 80 of the drive arrangement 52. The linear cylinders 78 form part of a hydraulic circuit 82 that recovers energy by a pumping action provided by the hydraulic cylinders on rotation of the drive shaft 80. The drive shaft 80 is provided in the form of a crank shaft 84. Figures 10 to 12 provide further views of the wave energy device 50.

Figure 13 comprises a cutaway view of the energy recovery system 54. As shown in Figure 13 there are provided four (4) cylinders 78 each matched by a corresponding one of the cylinders 78 to compensate for forces on the crank shaft 84. For example, a piston 88 is matched by a corresponding piston 90 and a piston 92 is matched by a corresponding piston 94. The piston 88 and the piston 90 form a first set of the cylinders 78 that extends from a first side wall 96 of the energy recovery mechanism 54 towards the crank shaft 84. The piston 90 and the piston 94 form second set of the cylinders 78 that extends from a second opposite side wall 98 towards the crank shaft 84. The wave energy recovery mechanism 54 includes a plurality of crank supports 100 that extend from a wall 102 positioned between the side wall 96 and the side wall 98. A top view of the arrangement of the cylinders 78 is shown in Figure 14. Figure 15 provides a side view showing that the piston 88, piston 90, piston 92 and piston 94 form a parallelogram defined by extensions 104 of the crankshaft 84. The extensions 104 are arranged at 90 degrees.

With the present embodiment the number of cylinders 78 can be increased or decreased as required. For this purpose there is provided a control facility adapted to activate or isolate one or more pumping cylinders 78 from pumping circuit 82 by energising or de-energising a hydraulic isolation valve. This is considered to be advantageous because the hydraulic circuit can be

designed and used to pump fluid into an accumulator with a nearly constant charge pressure. The resistance torque applied to the crank shaft may then be varied by varying the number of cylinders hydraulically connected to the pumping circuit at any one time. This in turn will vary the tension in the tether connecting the point absorber to the anchor point.

The tension in the tether is a component of the 'stiffness' of the point absorber buoy as a hydrodynamically heaving device. By varying the stiffness of a heaving buoy, the natural heaving period can be altered. In this embodiment the natural heaving period of the point absorber system can be varied. The wave energy device can accordingly be 'tuned' to the existing wave conditions to maximise the heaving motion and hence maximise the power absorbed from the waves for a given wave environment. This form of control is considered to advantageously increase the power absorbed from the wave environment as the wave environment changes over time. This is considered to be an improvement over the conventional linear hydraulic cylinder as a power take off system.

Furthermore, the present embodiment does not require overly long hydraulic cylinders to accommodate relatively long stroke lengths. In arrangements such as in the 'Archimedes Waveswing' and the CETO system accommodating relatively long stroke lengths has not been possible. If relatively long hydraulic cylinders are used to accommodate relatively long stroke lengths and maximise the wave height in which the unit can operate the result would still, nonetheless, be expensive large cylinders that use only a fraction of the stroke for most of the time. The use of relatively long hydraulic cylinders may also eventually lead to uneven wear in the cylinder components along the length of the cylinder and, with a 'long' aspect ratio, to be susceptible to bending or buckling damage. The present embodiment is less susceptible to these problems.

The manner of operation of the wave energy device 50 will be briefly described. Initially the tether 60 is wound around the drive wheel 58. A passing wave increases the buoyancy force experienced by the riser 62. The riser 62 moves upwardly which increases the tension in the tether 60 which

unwinds from the drive wheel 58. The rotation of the drive wheel 58 in the unwinding direction causes the linear cylinders 78 to pump hydraulic fluid and recover energy.

Once the wave has passed, the riser 62 falls downwardly. This causes the tension in the tether 60 to reduce. The tension in the tether 74 caused by the riser 72 subsequently operates to rewind the tether 60 back onto the drive wheel 58. The decoupling mechanism 68 ensures that the drive arrangement

52 is decoupled from the energy recovery mechanism 54. When the next wave passes the process repeats with the decoupling mechanism deactivating, the drive wheel 58 turning in the unwinding direction and the energy recovering mechanism 54 recovering energy.

As would be appreciated the resulting pressure and flow rate of the hydraulic working fluid creates power that can be converted to electricity via a turbine or hydraulically driven electrical generator. The wave energy device 50 is considered to provide an advantageous wave energy pump (or wave engine). Whilst there are a number of technologies for converting wave energy into electrical power, that are based around a floating buoy attached to some mechanism for converting vertical motion into electrical power, the present embodiment advantageously allows a variable stroke length. The riser acts as a 'point absorber' absorbing energy from the ocean at a relative point in space as the riser typically has small dimensions compared to the wavelength to which it is exposed. The power take off mechanism may be anchored to the sea floor or partially anchored in vertical position by large plates that damp vertical motion. As shown in Figures 9 and 10 the wave energy system 50 includes an anchoring platform 75 having a first spacer 77 for the riser 62 and a second spacer 79 for the winding mechanism 70. The spacer 77 and the spacer 79 are provided with pulleys to separate the riser 62 and the riser 72 from the drive arrangement 52.

The wave energy device 50 is considered to advantageously combine the high efficiency of hydraulic cylinders at low and variable speed with a rotary mechanism that solves the problem of relatively limited stroke length. By using a flexible tether to connect the point absorber to the anchor point, and

turning this tether about a sheave connected to a crank shaft, the linear motion is converted to rotary motion. The crank shaft drives one or more hydraulic cylinders in oscillatory motion with a fixed stroke length.

Ideally a typical stroke length of relative motion between the point absorber and the anchor point will cause the drive wheel, which in this embodiment is sheaved, to rotate more than once so that at least one full crankshaft rotation is achieve for each cycle of relative motion. Because the crank shaft may rotate as many times as is required to achieve any given relative motion stroke, the maximum allowable relative motion stroke is not limited by the pump mechanism. This means that the wave energy device provides a pump that can accommodate any feasible predetermined maximum wave height. In addition, tidal variations affect only the mean extension between the point absorber and the anchor point, so large tidal variations do not require a modification to the power take off system. In another embodiment the rotational motion of the drive shaft is transmitted to reciprocating hydraulic cylinders via a series of drive wheels mounted along the drive shaft with chains or other belt type transmissions connecting the drive wheels to a series of power take off wheels to which the hydraulic cylinders are attached. This embodiment accordingly provides an alternative to the crank shaft 84. In the arrangement, one end of each cylinder is attached to a foundation, and the other end is attached to one of the series of power take off wheels at an appropriate radius from the turning axis so that the rotational motion of the power take off wheel is converted to reciprocating actuation of the hydraulic cylinder. An exemplary arrangement 110 of this basic form is shown in Figure 17. Other arrangements for converting rotational movement may be employed.

In each of the above described embodiments the support platform is located at or near the seabed. However, virtually the same general arrangement will operate equally well if the various components mounted on the support platform are housed within the buoy itself. Figure 18 illustrates a still further embodiment of the wave energy system 120 in which the first drive wheel, the drive shaft, and the energy conversion means are all mounted inside the

buoy. Those parts of the wave energy system 120 that are substantially the same as the similar parts of the previous embodiments will be identified using the same reference numerals and will not be described again in detail.

The wave energy system 120 comprises a buoy 122 coupled to a tether 60, the tether being anchored in such a manner as to provide resistance to upward movement of the buoy. One end of the tether 60 may be anchored to the seabed by a gravity base 124, as shown, or by a driven pile or a suction pile. Alternatively it may be anchored by a heave plate, which is not anchored to the seabed, but which still provides adequate resistance to upward movement of the buoy 122. The other end of the tether 60 passes through a tether guide 126 into the interior of the buoy 122. The tether guide 126 is designed to accommodate the free movement of the tether 60 as it unwinds and rewinds during heaving motion of the buoy 122 due to wave motion. The tether guide 126 may comprise a fixed deflector or alternatively it may comprise deflector sheaves or a plurality of roller devices to provide a suitable bending radius and fleeting of the tether 60 between the tether guide 126 and the drive wheel 58.

The wave energy system 120 comprises a first drive wheel 58 onto which is wound a portion of the tether 60 wherein, in use, the first drive wheel 58 is driven in a first direction during upwards movement of the buoy 122 as the tether 60 unwinds and in a second opposite direction during downwards movement of the buoy 122. A drive shaft 80 connected to the first drive wheel 58 is coupled to an energy conversion means for converting rotational motion of the drive shaft 80 to a suitable form of energy for storage. The energy conversion means or energy recovery mechanism 54 comprises a hydraulic pump having a plurality of linear hydraulic cylinders 78. Each of the linear cylinders 78 is directly connected to the drive shaft 80. The rotational movement of the drive shaft 80 drives the hydraulic pump to deliver pressurised fluid to a high pressure accumulator module 30. The accumulator module 30 comprises one or more pressure tanks 34 connected to the hydraulic pump by pressure lines 36. Pressure lines 37 provide pressurised hydraulic fluid from the pressure tanks 34 to drive a hydraulic motor 39. The

hydraulic motor 39 in turn drives an electric generator 40 which creates electrical energy for delivery to an onshore location by power lines 44.

In this embodiment, the return torque on the drive shaft 80 (used to rewind the tether on the buoy's downward movement) is provided either: (a) by a separate small hydraulic motor coupled to the drive shaft 80 and driven by the low pressure side of the hydraulic circuit to provide a rewinding torque to the first drive wheel 58 mounted in the buoy; or,

(b) by applying a back pressure to the hydraulic cylinders 78 during the buoys downward heaving motion such that they provide a rewinding torque to the first drive wheel 58 mounted in the buoy.

A low pressure accumulator module 126 is provided in the buoy 122 for this purpose.

In the each of the described embodiments sea water may be used as the hydraulic working fluid. The seawater may be used to drive a Pelton type turbine to rotate an electrical generator, or the high-pressure seawater can be passed through reverse osmosis filters to create potable water.

Alternatively to seawater, a mineral oil or water based hydraulic fluid may be used. This has the advantages that the hydraulic working fluid: (i) can be designed to be a good lubricant, which extends the life of the pump and reduces maintenance requirements; (ii) can contain anti-corrosive additives, which extends the operational life of the pump and reduces maintenance requirements; (iii) can contain additives to reduce the fluid's propensity to cavitate, which reduce a negative operational problem; (iv) can be formulated to contain no particulates that may cause abrasion; and (v) can be formulated to reduce or eliminate scaling or pipes and gel formation. The water based hydraulic working fluids can be designed so that they are benign to the ocean environment in the case of a release from the mechanism to the environment.

In the case where an installation requires a selection between electrical power generation and potable water production, it is preferred to provide

pressure and flow rate of the hydraulic working fluid which would then drive a hydraulic motor which would then in turn drive a specialist seawater pump to provide seawater to a reverse osmosis plant or other potable water production technology to produce potable water. The operational benefits of being able to use collected wave energy for water production in off peak electrical power demand times is preferably maintained in this way.

Having now described a number of embodiments it will be apparent that the preferred embodiments of the wave energy system provide a number of advantages including: (a) Recovering energy from variable riser stroke lengths where the system is configured to accommodate a predetermined maximum wave height say in the order of 10m.

(b) The provision of a variable stroke that can be used to accommodate tidal variations as well as dynamic wave elevation changes. (c) The provision of a linear cylinder pump that does not have to be sized in length to suit the wave environment in which it operates, forcing a matching of equipment to site conditions and a reduction in the opportunity for economies of scale.

(d) Systems that solve the problem of finite stroke length in conventional arrangements, such as in the 'Archimedes Waveswing' and the CETO system, which limits the maximum wave height in which the systems can operate.

(e) Systems that address the previous requisite use of relatively long hydraulic cylinders designed to accommodate relating long stroke lengths to maximise the wave height in which the systems operates.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. Therefore, it will be appreciated

that the scope of the invention is not limited to the specific embodiments described but is to be determined from the appended claims.