Field of the invention

The present invention relates to a wave power hydraulic system and method. Background to the invention The extraction of energy from wave power presents technical difficulties due in particular to the oscillating nature of the waves and to the significant variations in prevailing wave conditions over time.

There are numerous examples of wave power capture systems. Such systems include mechanical members that are moved by operation of the waves, and power conversion systems that convert the resulting mechanical energy into electrical energy.

A previous patent application, WO2006/100436, filed in the name of the present applicant, disclosed a wave power capture system comprising a wave energy conversion device for use in relatively shallow water, which addressed some of the problems associated with previously known wave power capture systems.

The wave energy conversion device of WO2006/100436 comprises a mechanical member in the form of a flap biased to the vertical in use and formed and arranged to oscillate backwards and forwards about the vertical in response to wave motion acting on faces of the flap. The flap is coupled to a hydraulic circuit via a positive displacement pump such that oscillation of the flap causes the flow of fluid through the hydraulic circuit, which drives operation of a variable displacement hydraulic motor.

In order to maximize the extraction of power using wave energy conversion devices it has been recognised that tuning of properties of the devices to the wave conditions can be important, so that such properties are optimised for the amplitude and frequency of waves that are experienced by the devices.

Generally speaking larger waves require higher resistive or damping torque to be provided by a power capture system to the wave energy conversion device than smaller waves to extract the most energy. In real seas devices experience a spectrum of wave periods and amplitudes, and in order to obtain the maximum energy abstraction the oscillator torque would in theory have to be continuously tuned to the current wave, which would require advance knowledge of the size of the wave and a power takeoff system which could adjust immediately to the size of the wave. In practice, perfect continuous tuning and immediate adjustment is not possible but various practical arrangements have been suggested to provide tuning of the properties of wave energy conversion devices to wave conditions.

Some known systems include a piston and cylinder arrangement to drive pressurised fluid to an electrical generator system via an arrangement of check valves. For example, WO2006/100436 describes such an arrangement. The use of check valve arrangements can provide for a high pressure flow of fluid downstream of the check valve arrangement regardless of the direction of movement of the mechanical member driven by action of the waves. However, there can still be significant pressure variations and variations in torque, due to the oscillating nature of wave motion, the wide spectrum of wave energies and the variation in sea conditions over time. Furthermore, the presence of a piston rod on one side of the piston means that the piston area used to pump fluid is different for one direction of movement than for the other direction of movement, which provides an asymmetry in the damping torque for different directions of movement.

Fluid accumulators can be used to smooth pressure variations downstream of a check valve arrangement of a wave power system. It has also been suggested in WO 2010/084305 to use a variable opening valve downstream of a check valve arrangement in a water-based hydraulic power extraction system to provide continuous control of the rate of flow of fluid to a turbine apparatus. In WO 201 1/010102 it has been suggested to use accumulators, or mechanical springs, to assist the movement of the mechanical member when it is stationary at the extremes of its range of movement. Those measures can provide additional pressure and torque smoothing or tuning in the context of a hydraulic system including an arrangement of check valves downstream of a piston and cylinder arrangement.

In wave power systems that use water hydraulic systems, the wave energy conversion device comprising a mechanical member can be located offshore, and the electrical power generator can be located on-shore, with high pressure water being driven to the electrical power generator via the water hydraulic system by action of the wave energy conversion device.

It has been suggested to use an oil hydraulic power take-off system located offshore close to the wave energy conversion device. Such an oil hydraulic power take-off system may allow for rapid control of the system pressure (for example using a swash plate motor and controller that can provide real-time control of operating parameters) which in turn affects the resistive torque at the mechanical member that moves in response to wave motion. The pressure could be adjusted continuously in dependence, for example, on an oil flow rate through the system. However, amongst other things, the use of oil hydraulic systems in the under-sea environment can result in high heat loads being generated in high sea states.

As an alternative to the use of check valve arrangements and accumulators, it has been suggested to use digital hydraulics to control the output of fluid from the cylinder chambers of a piston cylinder arrangement directly. However, in such a system it is necessary to provide real time, active control of flow from the cylinder chambers using digital hydraulics many times during each wave period in order to provide a reasonably constant flow of fluid to an electrical generator during each wave period. The switching of chamber outputs many times during each wave cycle can also result in the accumulation of significant compression losses due to the switching of fluid outputs from each cylinder chamber from high pressure to low pressure. Summary of the invention

In a first independent aspect of the invention there is provided a wave power hydraulic system comprising at least one piston unit, wherein:- each piston unit comprises a respective piston moveable within a respective cylinder, each piston unit comprises a first cylinder chamber on one side of the piston and a second cylinder chamber on the other side of the piston, the hydraulic system is connectable to a mechanical member to drive movement of the or each piston in response to wave motion, thereby to drive fluid from each chamber along a respective fluid flow path, each fluid flow path being connectable, for example via a non-return valve arrangement, to a power takeoff system, and the system further comprises control means for varying the configuration of the hydraulic system to vary the flow path from at least one of the chambers. That may vary the volume of fluid provided via the check valve arrangement to the power takeoff system in response to a given piston movement.

By thus varying the configuration of the hydraulic system the power transferred to the hydraulic system by motion of the flap can be controlled, and fluid flow and pressure to the power takeoff system can be controlled.

The term check valve is intended to encompass the terms non-return valve and one-way valve.

The varying of the configuration of the hydraulic system may comprise varying the fluid flow path to vary a damping torque provided to the mechanical member. Thus, the power transferred to the hydraulic system by movement of the flap may be varied. That may enable control of the level of heat generated in the hydraulic system and/or power takeoff system.

The at least one piston unit may comprise a plurality of piston units, and the cross-sectional area of the cylinder chambers for a first one of the piston units may be different to the cross-sectional area of the cylinder chambers for a second one of the piston units.

The use of a cylinder chambers having different cross-sectional areas can provide for selection of a larger number of possible active piston areas and/or torques. Configurations in which substantially equal active piston areas for different directions of piston movement are obtained may be provided.

For the or each cylinder, the piston may comprise a piston rod such that the area of the piston in the first chamber is different to the area of the piston in the second chamber.

The at least one piston unit may comprise a plurality of piston units, and the sum of the cross-sectional areas of at least two of the pistons on one side of the pistons may be substantially equal to the cross-sectional area of one of the pistons, or the sum of the cross-sectional areas of two or more of the pistons, on the other side of the pistons.

The cross-sectional areas of two of the piston rods may be substantially equal to the cross-sectional area of the piston in one of the chambers, optionally the chamber of the cylinder having the smallest cross-sectional area.

The term substantially equal as used herein in connection with a first and second value (for example a first and second torque, area, pressure, flow rate or volume) may refer to the first value being within 30%, optionally 20%, optionally 10%, further optionally 5% or 2% of the second value.

The control means may be operable to select a configuration in which the fluid flow to the power takeoff system from the cylinder chambers in response to a given movement of the mechanical member in a first direction is substantially the same as the fluid flow to the power takeoff system from the cylinder chambers in response to substantially the same given movement of the mechanical member in a second direction.

The first direction of the movement of the mechanical member may be a direction of movement that causes movements of each of the pistons in a forward direction and the second direction of movement of the mechanical member may be a direction of movement that causes movement of each of the pistons in a backward direction. The control means may be operable to select a configuration in which the fluid flow to the power takeoff system from the cylinder chambers in response to a given movement of the piston or pistons in a forward direction is substantially the same as the fluid flow to the power takeoff system from the cylinder chambers in response to substantially the same given movement of the piston or pistons in a backward direction.

Operation of the hydraulic system and power takeoff system may provide a torque opposing movement of the mechanical member, and the control means may be operable to select a configuration in which the torque is substantially the same for a first direction of movement of the mechanical member as for a second direction of movement of the mechanical member.

The control means may comprise, for the or each piston unit, means for activating or deactivating at least one of the chambers, wherein when a chamber is activated movement of the piston drives fluid to the power takeoff system from that chamber and when a chamber is deactivated movement of the piston drives substantially no fluid to the power takeoff system from that chamber.

The control means may be operable to activate the first chamber of a cylinder unit independently of whether the second chamber of that cylinder unit is activated or deactivated.

The control means may comprise means for selectively connecting one of the chambers to another of the chambers.

The control means may comprise, for each piston unit, means for varying the configuration to provide a level of fluid flow to the power takeoff system from the first chamber in response to a given piston movement in a first direction that is independent of the level of fluid flow to the power takeoff system from the second chamber in response to the same piston movement in a second, opposite direction.

The control means may be operable to vary the active area of the piston or pistons.

The control means may be operable to select the active area of the piston or pistons for a first direction of movement of the piston or pistons independently of the active area of the piston or pistons for a second direction of movement of the piston or pistons.

The control means may be operable to divert flow from each of one or more of the chambers to a respective alternative fluid flow path.

The control means may comprise at least one valve and/or means for overriding operation of at least one check valve of the check valve arrangement. Overriding operation of the at least one check valve may comprise maintaining the at least one check valve in an open state.

For at least one of the chambers, the alternative fluid flow path may comprise a low pressure fluid flow path, the low pressure fluid flow path being at lower pressure than the path to the power takeoff system for that chamber via the arrangement of check valves.

The power takeoff system may comprise a low pressure return path and the control means is operable to connect at least one of the chambers to the low pressure return path.

Operation of the at least one valve may bypass the flow path to the power takeoff system via the arrangement of check valves.

The control means may be configured to maintain the configuration of the hydraulic system for an interval between each change of the configuration of the hydraulic system, the interval being set to be greater than a wave period or a plurality of wave periods. Thus, wave period compression losses may be reduced.

The control means may be configured to vary the configuration of the hydraulic system when the pistons are substantially stationary. That can reduce wear on valves. The control means may be configured to vary the flow path for a chamber when the chamber is in a low pressure state. That may reduce compression losses.

The control means may be configured to provide a plurality of modes of operation, and to select the mode of operation in dependence on sea state and/or in dependence on at least one operating parameter. A first of the modes of operation may comprise a mode in which the configuration of the hydraulic system is selected to provide a torque that is substantially the same for a first direction of movement of the mechanical member as for a second direction of movement of the mechanical member, and a second of the modes of operation may comprise a mode in which the configuration of the hydraulic system is selected to provide a torque that is different for the first direction of movement of the mechanical member and for the second direction of movement of the mechanical member.

The system may further comprise at least one accumulator between the check valve arrangement and the power takeoff system.

The power takeoff system may comprise at least one electrical generator and/or at least one swash plate motor.

The hydraulic system may be mounted in a unit with the power takeoff system.

The working fluid of the hydraulic system may comprise oil. The mechanical member may comprise a flap biased to the vertical and arranged to oscillate about the vertical in response to wave motion.

In a further, independent aspect of the invention there is provided a method of controlling operation of a wave power hydraulic system that comprises at least one piston unit, wherein each piston unit comprises a respective piston moveable within a respective cylinder, each piston unit comprises a first cylinder chamber on one side of the piston and a second cylinder chamber on the other side of the piston, the hydraulic system is connectable to a mechanical member to drive movement of the or each piston in response to wave motion, thereby to drive fluid from each chamber along a respective fluid flow path, each fluid flow path being connectable, optionally via a non-return valve arrangement, to a power takeoff system, and the method comprises varying the flow path from at least one of the chambers, which may vary the volume of fluid provided via the check valve arrangement to the power takeoff system in response to a given piston movement.

The method may comprise selecting a configuration in which the fluid flow to the power takeoff system from the cylinder chambers in response to a given movement of the mechanical member in a first direction is substantially the same as the fluid flow to the power takeoff system from the cylinder chambers in response to substantially the same given movement of the mechanical member in a second direction.

Operation of the hydraulic system and power takeoff system may provide a torque opposing movement of the mechanical member, and the method may comprise selecting a configuration in which the torque is substantially the same for a first direction of movement of the mechanical member as for a second direction of movement of the mechanical member.

The method may comprise activating or deactivating at least one of the chambers, wherein when a chamber is activated movement of the piston drives fluid to the power takeoff system from that chamber and when a chamber is deactivated movement of the piston drives substantially no fluid to the power takeoff system from that chamber.

The method may comprise selectively connecting one of the chambers to another of the chambers.

The method may comprise varying the active area of the piston or pistons.

The method may comprise diverting flow from each of at least one of the chambers to a respective alternative fluid flow path. The method may comprise operating at least one valve and/or overriding operation of at least one check valve of the check valve arrangement in order to vary the configuration of the hydraulic system.

The alternative fluid flow path may comprise a low pressure fluid flow path, the low pressure fluid flow path being at lower pressure than the path to the power takeoff system for that chamber via the arrangement of check valves.

The power takeoff system may comprise a low pressure return path and the method may comprise connecting at least one of the chambers to the low pressure return path. The method may comprise overiding a check valve that connects a chamber to the power takeoff system via a high pressure path.

The method may comprise maintaining the configuration of the hydraulic system for an interval between each change of the configuration of the hydraulic system. The interval may be set to be greater than a wave period or a plurality of wave periods.

The method may comprise selecting a configuration of the hydraulic system in dependence on sea state and/or in dependence on at least one operating parameter.

The method may comprise selecting a mode of operation in dependence on sea state and/or in dependence on at least one operating parameter. A first of the modes of operation may be a mode in which the configuration of the hydraulic system is selected to provide a torque that is substantialiy the same for a first direction of movement of the mechanical member as for a second direction of movement of the mechanical member. A second of the modes of operation may be a mode in which the configuration of the hydraulic system is selected to provide a torque that is different for the first direction of movement of the mechanical member and for the second direction of movement of the mechanical member.

There may also be provided a wave power hydraulic system and/or a method substantially as described herein with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. For example, system features may be applied as method features and vice versa.

Detailed description of embodiments

Embodiments of the invention are now described, by way of non-!imiting example, and are illustrated in the following figures, in which :-

Figure 1 is a schematic illustration of a wave power conversion system according to an embodiment; Figure 2 is a further illustration of the embodiment of Figure 1 ;

Figure 3 is a further schematic illustration of the embodiment of Figure 1 , showing some parts in more detail;

Figure 4 shows a table that includes data representing various configurations of the embodiment of Figure 1 ;

Figure 5 is a bar chart showing active areas for forward and backward directions of movement for different configurations of the embodiment of Figure 1 ; and

Figure 6 is a diagram showing schematically the faces of the piston for both chambers of each cylinder and indicating which areas of the piston faces are active, for various configurations of the embodiment of Figure 1.

A power conversion system for conversion of the oscillating motion of a wave energy conversion device to electricity is illustrated in Figure 1.

The system includes a wave energy conversion device 2, coupled by a first piston rod 4a to a first piston 6a that reciprocates in a cylinder 8a, and by a second piston rod 4b to a second piston 6b that reciprocates in a cylinder 8b. The first piston rod 4a, first piston 6a and first cylinder 8a form a first piston unit, and the second piston rod 4b, second piston 6b and second cylinder 8b form a second piston unit.

The piston 6a divides cylinder 8a into a first cylinder chamber 40a on one side of the piston 6a and a second cylinder chamber 42a on the other side of the piston 6a. Similarly, the piston 6b divides cylinder 8b into a first cylinder chamber 40b on one side of the piston 6b and a second cylinder chamber 42b on the other side of the piston 6b. The cross-sectional area of the cylinder chambers 40a, 42a is different to the cross-sectional area of the cylinder chambers 40b, 42b in the embodiment of Figure 1 . The cross-sectional area of the rod 4a is also different to the cross- sectional area of the rod 4b in the embodiment of Figure 1 .

The wave energy conversion device 2 comprises a base anchored to the bed of the sea or other body of water and a mechanical member in the form of an upstanding flap 5, of generally rectangular form, mounted for rotation about a pivot axis to the base 2. An example of a suitable wave energy conversion device 2 is described, for example, in WO 2006/100436.

The cylinders 8a, 8b form part of a hydraulic circuit 10 including conduits 17, 24 to which they are connected by inlet/outlet ports 12a, 12b at one end of the cylinders 8a, 8b, inlet/outlet ports 14a, 14b at the opposite end of the cylinders 8a, 8b, and arrangements of non-return valves 15a, 15b. Each of the arrangements 15a and 15b of non-return valves comprises four check valves 16a, 18a, 20a, 22a and 16b, 18b, 20b, 22b, which are arranged to ensure that hydraulic fluid is driven under pressure along conduit 24, and returned to the cylinders via conduit 17, for both directions of movement of the flap 5.

The conduit 24 leads to a power takeoff system in the form of an arrangement of swash plate motors 36 that are linked via a common drive train to one or more electrical generators 38. The return conduit 17 returns hydraulic fluid from the swash plate motor arrangement 36 to the cylinders 8a, 8b via the check valve arrangements 15a, 15b.

An accumulator 30 is connected to the fluid conduit 24 between the non- return valves 20a, 22a, 20b, 22b and the swash plate motors 36. A further accumulator 36 is also connected to the fluid conduit 24 and may be connected or disconnected to the fluid conduit 24 using a control valve, for example in dependence on sea state or other operating conditions. Another accumulator 34 is connected to the fluid conduit 17 between the non-return valves 16a, 18a, 16b, 18b and an output of the swash plate motors 36.

The accumulators 30, 34 each comprise a pressure cylinder containing air. For each of the accumulators, the mass of air in the accumulator, its precharge pressure and the volume of the accumulator are known.

The working fluid of the hydraulic system of Figure 1 is oil, although any suitable alternative working fluid can be used in alternative embodiments.

In the embodiment of Figure 1 , all of the components of the hydraulic system, the swash plate motors and the generator 38, are provided in a single hydraulic unit 50 mounted to the base of the wave energy conversion device 2. The hydraulic unit 50 is shown in cutaway in Figure 2. In Figure 2, two generators 38 are shown but any suitable number of generators can be used.

The hydraulic system further comprises a controller 60 that is shown in Figure 3. The controller of Figure 3 comprises a processor that is programmed to receive sensor signals from various sensors (not shown) that sense operating parameters of the system, and to provide control signals that control operation of various valves. The controller 60 is also configured to control operation of the swash p!ate motors 36 and the generators 38.

Figure 3 shows certain parts of the hydraulic system of Figure 1 in more detail. The accumulators 30, 32, 34, swash plate motors 36, and generators 38 are not shown in Figure 3 for clarity. Figure 3 presents a view from the other side of the flap to that shown in Figure 1 , so the positions of the larger and smaller cylinders 8a, 8b appear to be reversed. Both Figures 1 and 3 are schematic views intended to show the connections between components and are not to scale. The system includes a control valve 62 that can be operated, under control of the controller 60, to connect the chamber 42a of one of the cylinders 8a to the chamber 40b of the other of the cylinders.

The system also includes control lines 64a, 66a, 64b, 66b each of which can be used by the controller 60 to send override signals to a respective one of the check valves 16a, 20a, 18b, 20b. The override signals act to maintain the check valve open regardless of the pressure difference across the check valve. In this case, the override of each of the check valves 16a, 20a, 20b connects the output of a respective one of the cylinder chambers 40a, 42a, 42b to the low pressure return conduit 17 regardless of the directions of movement of the flap 5 and regardless of the pressure difference between the selected chamber(s) and the return conduit 17. The override of the check valve 18b connects the output of the cylinder chamber 40b, to the high pressure conduit 24 regardless of the directions of movement of the flap 5 and regardless of the pressure difference between the selected chamber(s) and the return conduit 1 In a variant of the described embodiment, the check valves are not overridable and instead bypass valves are provided that can be operated selectively by the controller 60 to connect a selected one or more of the cylinder chambers 40a, 42a, 40b, 42b to the return conduit 17 or high pressure conduit 24 regardless of the direction of movement of the flap 5 and regardless of the pressure differences between the selected chamber(s) and the conduit 24 or return conduit 17.

The effect of overriding the check valves, or operating the control valve 62, will be considered in more detail below. Firstly, operation of the system with the control valve 62 closed, and the check valves operating normally without being overriden, is considered.

In operation the flap 5 is placed to face the general direction of wave motion, and the wave motion causes the flap 5 to oscillate about the pivot axis, which in turn drives the pistons 6a, 6b back and forth in the cylinders 8a, 8b.

On each forwards stroke of the pistons 6a, 6b, low pressure fluid from inlet pipe 7 is drawn into chambers 40a, 40b of the cylinders 8a, 8b through ports 14a, 14b via non-return valves 16a, 16b, and high pressure fluid is pumped out of chambers 42a, 42b of the cylinders 8a, 8b through ports 12a, 12b and via respective fluid flow paths to non-return valves 22a, 22b and then into the fluid conduit 24. On each backwards stroke of the pistons 6a, 6b low pressure fluid from inlet conduit 17 is drawn into chambers 42a, 42b of the cylinders 8a, 8b through ports 12a, 12b via non-return valves 18a, 18b and high pressure fluid is pumped out of the cylinders 8a, 8b through ports 14a, 14b and via respective fluid flow paths to non-return valves 20a, 20b and then into the fluid conduit 24. Thus, regardless of whether the direction of movement of the flap 5 is in a forwards or backwards direction, fluid is driven along the conduit 24 to the swash plate motors 36.

The flow of high pressure fluid along conduit 24 drives operation of the swash plate motors 36, which in turn drive operation of the generator 38. As high pressure fluid flows along conduit 24, air in the accumulator 30 is compressed to store some of the pressure produced by the pumping action of the pistons 6a, 6b. Although fluid will flow along conduit 24 to the swash plate motors 36 regardless of the direction of movement of the flap 5 due to the check valves, in the absence of additional smoothing measures the pressure and flow rate in the conduit 24 will vary during each cycle. The accumulator 30 assists in smoothing such pressure and flow rate variations. The accumulator 34 has a similar effect in relation to fluid flowing along the return conduit 17.

For a range of sea states, the hydraulic accumulators 30, 34 can store peak power obtained from the waves leading to a reduced size of the swash plate motors and smoothed electric power output of the generator 38. The damping torque can also be controlled by the controller 60 by controlling the displacement of the swash plate motors. By use of the accumulators 30, 34 and by control of the swash plate motors variations in the speed of operation of the generator 38 can be smoothed.

It can be seen in Figure 1 that the piston rods 4a, 4b are on the same side of the pistons 6a, 6b relative to the flap 5. The space taken by the piston rods 4a, 4b on the surface of the pistons 6a, 6b means that the piston area used to pump fluid to the swash plate motors 36 via the conduit 24 for one direction of movement of the flap (in this case the forward direction of movement) is greater than for the other direction of movement of the flap (in this case the rearward direction of movement), when the check valves are operating normally and the further valve 62 is closed. Thus, in principle, the pressure in the fluid conduit 24, and the damping torque provided to the flap 5, can be significantly different for one direction of movement of flap than for the other direction of movement of the flap 5. Such variations can in principle lead to inefficiencies in power generation, as the electrical generators 38 may operate most efficiently when operating at constant speed. In practice, for low and medium sea states, the accumulators 30, 34 can smooth pressure and flow rate variations. Active control of the swash plate motors 36 by the controller 60 can also smooth variations in the speed of operation of the swash plate motors and thus the speed of rotation of the generators 38.

In higher sea states, the capacity of the accumulators 30, 34 is often not sufficient to absorb the pressure and flow rate variations that may be experienced, and the power generated may exceed the rating of the generators potentially leading to inefficiency or even malfunction. Furthermore, in such higher sea states, large quantities of heat may be generated in the hydraulic system, which must be dispersed.

It is a feature of the embodiment of Figures 1 to 3 that the controller 60 is operable to override one or more of the check valves and/or to operate the further valve 62, thereby to vary the configuration of the hydraulic system so as to vary the flow path from at least one of the cylinder chambers. That in turn can vary the volume of fluid provided via the check valves to the swash plate motors 36 in response to a given movement of the pistons 6a, 6b.

For example, if the controller 60 overrides operation of check valve 20a then the cylinder chamber 42a will be open to the low pressure return conduit 17 regardless of the pressure in the chamber 42a and regardless of the movement of the piston 6a. As the pressure in the chamber 42a will thus always be relatively low in comparison to the pressure in the conduit 24, the check valve 22a will remain closed throughout the wave cycles whilst the check valve 24a is overriden and substantially no fluid will be pumped from chamber 42a to the power takeoff system, and thus chamber 42a can be considered to be deactivated. Thus, overriding of check valve 20a deactivates chamber 42a and alters the active piston area of the system for one of the directions of movement of the flap member 5 and the pistons 6a, 6b. Overriding of check valve 20a provides an alternative fluid flow path from chamber 42a, directly to low pressure return conduit 17. The alternative fluid flow path in this case can be considered to be a low pressure fluid flow path that is at lower pressure than the path to the power takeoff system via the arrangement of check valves. The alternative fluid flow path may be a path that returns fluid to the cylinder chambers in various embodiments. In alternative embodiments the alternative fluid flow path may be to a reservoir or other storage device, for example via a selectively operable control valve.

Each of the chambers 40a, 40b, 42a, 42b, or any combination of those chambers, can be activated or deactivated by the controller 60 independently, thus selectively varying the active piston area and damping torque of the system.

It is a feature of the embodiment of Figures 1 to 3, that the cylinders 8a, 8b have different cross-sectional areas. Thus, selective activation or deactivation of particular cylinder chambers can provide a range of different active piston areas and damping torques.

Furthermore, operation of the valve 62 can connect the chamber 42a to the chamber 40b. Due to the different cylinder diameters and piston areas for those chamber 42a, 40b, operation of the valve 62 causes a reduction in the volume of fluid that is passed to the load via fluid conduit 24 in response to piston movement but does not cause a complete deactivation of the cylinder chamber 40b.

In a two cylinder system of unequal cylinder chamber areas, in which any cylinder chamber can be deactivated or connected directly to any other cylinder chamber without going through the check valve arrangement there are 2 ^G4 different configurations of the hydraulic system that can be achieved. The system of Figures 1 to 3 provides fewer possible configurations as only one pair of chambers 40b, 42a can be connected together to provide fluid flow therebetween without passing through the check valve arrangement.

In embodiments such as that of Figures 1 to 3 it is possible to obtain the same damping torque for both directions of movement of the flap 5 by deactivation of only one chamber, if the rods and cylinders are chosen to have appropriate cross- sectional areas. For example, if the cylinder chamber cross-sectional area of the smaller of the cylinders (for example cylinder 8b in Figure 1 ) is substantially equal to the sum of the cross-sectional areas of the pistons on the side of the pistons on which the rod is located, then substantially equal active areas (and substantially equal torques, and transfer of substantially equal volumes of fluid in response to a given piston movement) can be obtained for both directions of movement of the pistons by deactivating the chamber of the larger cylinders on the side on which the rod is not located (for example 42a in Figure 1 ).

By providing substantially equal active areas for both directions of movement of the flap, the variation in pressure and flow rate for different directions of movement of the flap can be reduced, ensuring that the generator or generators operate more efficiently. That can be particularly important at higher sea states, when the power that can be generated by movement of the flap approaches or exceeds the power rating of the generator or generators, and/or when the capacity of the accumulators to smooth pressure variations may be exceeded. In such states, it has been found to be more important to reduce differences in flow rate produced by forward and backward movement of the flap, thereby to increase efficiency of operation of the generator or generators, than to maximise the power captured by the hydraulic system. Indeed, by reducing the power captured by the hydraulic system in such high sea states, by varying the flow paths from the chambers, the amount of heat generated can be reduced and kept within acceptable limits. The need to operate pressure relief valves when system pressures become high in high sea states may also be avoided, as the system pressures can be kept relatively low by suitable deactivation of chambers. By providing for t e alteration of flow paths from the different chambers, and for example the activation, deactivation or connection of individual chambers, a necessity to increase the capacity of the generators and/or the accumulators in order to cope with high sea states can be avoided.

The controller 60 can be programmed to alter the configuration of the hydraulic system, for example by activating, deactivating or connecting individual chambers, automatically in dependence upon measured sea states or in dependence upon measured operating parameters (for example pressures and/or flow rates in one or both of the conduits 17, 24, speed or power output of the generator(s) 38, pressure and/or fill level of one or more of the accumulators). Sensors are provided to measure the various operating parameters in the embodiment of Figures 1 to 3, and to provide measurement data to the controller 60.

In one configuration, the controller 60 is programmed with at least two modes of operation and automatically selects the mode of operation in dependence on sea state or measured operating parameters. In a first one of the modes of operation, the configuration of the hydraulic system is selected by the controller 60 by activation, deactivation or connection of individual chambers to provide a torque that is substantially the same for the forward direction of movement of the flap as for the backward direction of movement of the flap. In a second one of the modes of operation, the configuration of the hydraulic system is selected by the controller 60, by activation, deactivation or connection of individual chambers or otherwise altering flow paths, to provide a torque that is different for the forward direction of movement of the flap than for the backward direction of movement of the flap. In the second configuration, the hydraulic system may be configured to provide maximum active area of the pistons and thus maximum power transfer from the flap to the generator(s). The first configuration may be suitable for higher sea states and the second configurations may be suitable for lower sea states.

The controller 60 can also be configured to change configuration of the hydraulic system, for example by activation, deactivation or connection of individual chambers or otherwise altering flow paths from the chambers, only after a minimum interval since the last change of configuration. The minimum interval in some embodiments is set to be greater than an expected wave period, or greater than a selected multiple of expected wave periods. For example, in some configurations the minimum interval is greater than or equal to 10s, 30s, 5 minutes or 10 minutes. By limiting the frequency with which the configuration of the hydraulic system is changed, compression losses can be reduced. The use of the check valves and accumulators can ensure that variations in pressure and flow rate to the swash plate motors can be smoothed, and efficiency of operation of the generators maintained, despite the configuration of the hydraulic system being altered less than once per wave cycle.

In some modes of operation the controller 60 is configured to vary the configuration of the hydraulic system when the pistons are substantially stationary. The controller 60 can also be configured to vary the flow path for a chamber only at a point in the operation cycle when the chamber is in a low pressure state.

In the embodiment of Figures 1 to 3, one of the cylinders 8a has a 490mm piston diameter and 290 mm rod diameter, and the other cylinder 8b has a 370mm piston diameter and a 250 mm rod diameter. The maximum damping torque provided by the system is defined as being 12 MNm. Figure 4 is a table that shows the total active piston area obtained for different directions of movement of the flap 5 (referred to as A_retract and A_extend in the table) for different configurations of the hydraulic system obtained by activation, deactivation or connection of different cylinder chambers. In the table the different chambers are referred to as Piston 1 (the chamber of the first cylinder in which no rod is present), Rod 1 (the chamber of the first cylinder in which a rod is present), Piston 2 (the chamber of the second cylinder in which no rod is present) and Rod 2 (the chamber of the first cylinder in which a rod is present). If there is a 1 in the table for a particular chamber that indicates that the check valve for that chamber has been overriden and is maintained in an open state. There is also a column labelled "Connection Piston 1 and Rod 2" which indicates with a 1 whether valve 62 has been activated to connect the chambers 40b and 42a. Chamber 42a can also be connected to the high pressure conduit 24 by overriding of check valve 18b.

It can be seen that for certain configurations the active piston area is shown as being negative (see lines 4, 15, 20 and 31 of the table. In those configurations, the check valve (18b) from Rod2 (40b) to high pressure is actively opened, and thus high pressure oil can flow back from the accumulators to the chamber Rod2 (40b). Thus, depending on the particular configuration, the hydraulic and power take off systems may not dampen the flap but may instead actively move it.

For instance, when chamber Rod2 (40b) is connected to Pistonl (40a) by valve 62 and at the same time the check valve (18b) from Rod2 (40b) to high pressure is actively opened, and high pressure oil can flow back from the accumulators to both chambers. Since the piston area of chamber Pistonl (40a) is bigger than that of chamber Rod2 (40b) the hydraulic and power take off systems will not dampen the flap but actively move it.

Figure 5 is a bar chart showing active areas for forward and backward directions of movement of the flap 5 for different ones of the configurations, labelled 0 to 6. The configurations 0 to 6 illustrated in Figure 5 are also marked in the last column of the table of Figure 4.

Figure 6 is a diagram showing schematically the faces of the piston for both chambers of each cylinder and indicating for each face whether the area of the face is active (ie pumping fluid to the load) or inactive for the extend and retract (forward and backward directions) for each of configurations 1 to 6. Red indicates an active area, blue indicates an inactive area.

Embodiments can provide any desired hydraulic configurations, and providing any desired active piston areas or damping torques. The active piston areas and damping torques for different directions of movement of the mechanical member may be selected independently by suitable variation of flow paths from the chambers, for example by suitable activation, deactivation or connection particular chambers. The desired configuration for a particular system can be selected to suit the particular property to the system, for example in order to provide desired power takeoff characteristics, and can be

Although it may be advantageous in particular systems to provide only two cylinder units and to provide equal active piston areas by deactivation of only a single cylinder chamber, as that may provide a simple system and reduced compression losses, any desired number of piston units can be provided.

It has been found in practice that in a multiple cylinder flap-type wave power system it can be simpler to provide all pistons on the same side of the flap. However, embodiments are not limited to having all cylinders on the same side of the flap. Cylinders can be provided on different sides of the flap.

The power takeoff system is not limited to comprising one or more swash plate motors and one or more generators. For example, any device or combination of devices that can be used to generate electricity from fluid flow or pressure can be used as a power takeoff system. Any suitable hydraulic motor, for example any suitable variable displacement hydraulic motor, can be used instead of the swash plate motor.

The check valves can be any suitable type of check valve, for example any suitable type of one-way valve or non-return valve.

Embodiments are not limited to flap-type wave energy conversion devices, and the mechanical member does not have to be a flap. Any suitable wave energy conversion device in which movement of one or more mechanical members causes movement of a piston in a cylinder can be used. It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.