Wave energy converter

Abstract

The present invention relates to a wave energy converter apparatus for harvesting electrical energy from sea waves, the apparatus comprising at least one volume-variable chamber comprising at least one deformable electroactive material wherein as sea waves consecutively pass over the apparatus, the chamber defines an oscillatory volume causing the deformable electroactive material to deform and enable harvesting of electricity, the apparatus further comprising a fixed-volume vessel for containing at least part of the working fluid, where said vessel is in fluid communication with said volume-variable chamber to allow movement of the working fluid therebetween.

Description

WAVE ENERGY CONVERTERTECHNICAL FIELD

[0001] The present disclosure relates to a wave energy converter, particularly, but not exclusively, a wave energy converter situated on, within, or below a body of water, such as an ocean; and which utilises a deformable electroactive material to convert mechanical wave energy into electrical power.BACKGROUND

[0002] Traditional methods of wave energy conversion typically rely upon mechanical systems such as turbines or floats. These can be complex, expensive, and prone to maintenance issues. It also is known to use deformable electroactive material, e.g. in the form of electroactive polymers (EAPs) used as generators, to convert mechanical energy from sea wave motion into electrical power. EAPs offer an alternative approach to energy harvesting by directly converting the mechanical movement of waves into electrical energy through their inherent electroactive properties. Potential advantages include flexibility, scalability, and simplicity, which can lead to more efficient and reliable wave energy harvesting systems. For example, one such prior art device utilising EAPs to generate electrical power from wave energy is described in United States Patent Publication No. 2022 / 0403811 Al. This discloses a structure defined by a deformable envelope which is provided with “elastomer generators” which deform when subjected to ocean energy.

[0003] However, prior art wave energy converters utilising EAP generators cannot optimally convert energy from waves of differing sizes and strengths. This is because EAP materials typically have a minimum deformation threshold below which little or no electrical power will be generated by the EAP; and a maximum deformation threshold above which mechanical failure or irreversible deformation of the EAP material may occur. Operation of a wave energy converter wherein its EAP materials can safely deform only within a predetermined range which is fixed and restricted. This poses significant design challenges and limitations.

[0004] In known wave energy converters utilising several EAP generators, the amount of deformation cannot be controlled or tuned, and all of the EAP generators participate in power production all of the time, reducing their lifetime.

[0005] Moreover, known wave energy converters comprise a single chamber comprising EAP materials. These chambers are filled with fluid (preferably a gas). During passages of waves, the resulting pressure variations compress the fluid and thus stretch the EAP material. However, due to the low compressibility of most fluids, the smaller the volume of the chamber (and thus of fluid) the lower stretch ratio of the EAP material and thus the lower the efficiency of energy harvesting. Therefore, to provide efficient wave energy converters, the chamber has to have very large volume. But large chambers are complicated to manufacture, to transport and to install and limit the number of locations where they can be placed. Moreover, the size of the EPA generators increases with the size of the chamber. This is a problem because manufacturing large EAP generators is difficult, and large EAP generators are prone to problems such as electrical breakdown, which reduce their lifetime.

[0006] The inventor of the present invention has recognised that it would be desirable to address the aforementioned challenges, and others, by providing an innovative solution for the efficient, sustainable, and reliable harnessing of sea wave energy using electroactive materials during a wider range of sea conditions. The solution of this disclosure allows to control the amount of deformation of EAP generators resulting from pressure variations, so that they can operate in their recommended deformation range. This solution can also increase the lifetime of the EAP generators by rotating their usage. The solution may also allow a single device to harvest a wide range of pressure variations. This is important because in most desirable wave energy conversion locations, a typical large wave is usually 20 times more powerful than a typical small wave, which limits the interest of wave energy converters with a single mode of operation. The solution may also enhance resonance by adapting the natural frequency of the wave energy converter to the frequency of the waves. The solution may also reduce the volume required for systems of a given power, by compensating the low compressibility of fluids such as air by the use of low-pressure cavities, in which low compressibility of fluid is not an issue.SUMMARY

[0007] According to a first aspect of the present invention, there is provided a wave energy converter apparatus for harvesting electrical energy from sea waves, the apparatus comprising: at least one volume- variable chamber for containing a working fluid, and isolating the working fluid from ambient surroundings; said chamber comprising at least one deformable electroactive material; and wherein as sea waves consecutively pass over the apparatus, the chamber defines an oscillatory volume causing the deformable electroactive material to deform and enable harvesting of electricity.

[0008] It will be appreciated that volume-variable chambers are isolated from ambient seawater (i.e. the apparatus is a closed system), and therefore the deformable electroactive material is not exposed to abrasion, fouling, or entanglement with sea life / creatures etc. The internal pressure of the working fluid within the oscillatory volume alternates between a pressure which is above and below the external pressure of the water column applied from above the apparatus.

[0009] Preferably, the apparatus comprises at least one fixed- volume vessel for containing at least part of the working fluid, wherein said vessel is in fluid communication with said volume-variable chamber to allow movement of the working fluid therebetween in response to ambient pressure changes caused by the sea waves consecutively passing over the apparatus.

[0010] The fixed-volume vessel is unresponsive to ambient pressure changes caused by the sea waves consecutively passing over the apparatus. The fixed-volume vessel provides a total oscillatory volume defined by a sum of a volume of each of the at least one volume- variable chamber and the volume of each of the at least one fixed- volume vessel.

[0011] This is particularly advantageous, because the fixed- volume vessel provides an additional reservoir of working fluid which is freely shared with the volume-variable chamber(s). The overall volume of working fluid therefore undergoes acompression / expansion cycle according to the variation of ambient pressure. In other words, the apparatus comprises a large quantity of working fluid allowing to increase the compression / expansion ratio of said fluid and thus the deformation of the deformable electroactive material without increasing the size of the volume-variable chamber(s) and thus the size of the deformable electroactive material.

[0012] Optionally, the fixed- volume vessel comprises a volume controller configured to control the volume of the one fixed- volume vessel.

[0013] Indeed, this allows to modify the total oscillatory volume (fixed- volume vessel and volume-variable chambers) undergoing compression / expansion during the passage of the waves to adapt the apparatus to the resonant frequency of the waves. This thus allows to reach different resonant frequencies of the waves without need of large volumevariable chamber or several volume-variable chambers disposed at a specific distance according to the wavelength of the waves.

[0014] Optionally, a valve is associated with said volume- variable chamber to selectively permit and prevent passage of the working fluid into and out of the volume-variable chamber.

[0015] It will be appreciated that the ability to selectively isolate a volume-variable chamber (or groups of chambers) from others within the apparatus provides several advantages. Firstly, it allows a degree of control over the amount of deformable electroactive material that is being used to harvest electricity, providing control over the amount of deformation of said material; it provides a way to disable any defective chambers whilst leaving other chambers operational; and it permits the dielectric material within chambers to be “rested” (perhaps in a rotating fashion) to reduce their fatigue, increase their lifespan (perhaps by a factor of five), and preserve their effectiveness. In some examples, the working fluid (which may be a gas such as air) is wholly or substantially removed from the isolated chambers so that they collapse to reduce or minimise their volume.

[0016] Optionally, the volume-variable chamber comprises two opposed non-deformable wall members, and wherein said deformable electroactive material is coupled betweensaid opposed non-deformable wall members to enable it to deform and generate electricity in response to a distance change between said non-deformable wall members.

[0017] It will be appreciated that when the external pressure of the water column becomes greater than the pressure of the working fluid between the non-deformable walls of a chamber, they move closer to one another. The volume of the chamber is correspondingly reduced, and a corresponding volume of working fluid is expelled and transferred to another (remote) volume- variable chamber and / or to a (remote) fixed-volume vessel for containing at least part of the working fluid. Since the deformable electroactive material(s) is / are connected between at least two opposed walls, the relative movement of those walls causes the deformable electroactive material(s) to deform (contract), allowing the amount of electrical energy to increase, and to be harvested once a state of minimal deformation has been reached. Once a sea wave retreats, the pressure of the water column above the device decreases as compared to the pressure of the working fluid between the walls. This causes the two opposed walls to move apart from one another. This deforms (stretches) the electroactive material, increasing its capacitance, and allowing it to receive an electrical charge. Once maximal deformation (stretching) has been achieved, the electroactive material can then receive an amount of electrical energy that will increase when it relaxes again as the next waves arrives.

[0018] Optionally, the opposing ends of said deformable electroactive material are coupled between said non-deformable wall members in a laterally offset manner in a length direction of said volume- variable chamber.

[0019] It will be appreciated that the orientation of the webs of dielectric material relative to the planes of the non-deformable walls may be adapted as required. By arranging prestretched webs of dielectric material in an inclined or diagonal manner, then even if the separation of the two non-deformable wall members reduces to zero, the laterally offset points of connection between webs and the wall members maintain a predetermined minimum stretch ratio of dielectric material.

[0020] Optionally, the apparatus comprises a limit stop configured to restrict the maximum distance of separation of said two opposed non-deformable wall members.

[0021] Optionally, the limit stop is a non-extensible connecter.

[0022] Optionally, the two opposed non-deformable wall members are connected by at least one non-extensible connecter configured to restrict the maximum distance of separation of said two opposed non-deformable wall members.

[0023] Examples of non-extensible connecters include strings or ropes, or any other suitable lengths of substantially non-stretchable material capable of preventing the non- deformable wall members from moving too far away from one another, thus placing an upper limit on the volume of each chamber. It will be appreciated that the at least one non-extensible connecter allows the non-deformable walls to move towards one another when a non-deformable wall is directly or indirectly subjected to the mechanical force of a sea wave. Importantly, whilst the non-extensible connecter is incapable of stretching, it is capable of compression. This feature places a maximum limit on the degree to which the deformable electroactive material can stretch, thus safeguarding it against mechanical failure or irreversible expansion. Furthermore, the ability to control the overall internal volume of the apparatus by selectively disabling and collapsing one or more chambers, or one or more layers of chambers, has the effect of moving the uppermost non- deformable wall towards or away from the sea surface, and hence subjects it to an increase or decrease in wave excitation. In other words, by controlling the overall internal volume of the apparatus in the vertical direction, the apparatus can be actively “tuned” to generate more or less electricity in a safe manner dependent on prevalent wave sizes and sea conditions. NB: the terms uppermost and lowermost are not intended to be limiting, but are relevant to the illustrated examples. In practice, these wall surfaces may be orientated at any suitable angle.

[0024] Optionally, the non-deformable wall members are defined by reinforced inflatable members.

[0025] It will be appreciated that, in the context of the present invention, the term “non- deformable” is intended to encompass semi-solid or even hollow wall members which are inflatable with a pressurised gas to facilitate their conversion from a non-operativecondition (e.g. for storage, transportation, or when temporarily taken out of operation) to then become substantially non-deformable when in an operative condition.

[0026] Optionally, the apparatus comprises one or more pressure sensors for monitoring pressure inside said volume- variable chamber.

[0027] Optionally, the apparatus comprises one or more pressure sensors for monitoring ambient pressure surrounding said volume- variable chamber.

[0028] It will be appreciated that pressure and / or force sensors may be employed within the apparatus such that, for each wave, it can be determined when the deformable electroactive material needs to be charged (i.e. typically when the distance between the non-deformable walls reaches a local maximum); and when the deformable electroactive material needs to be discharged in order to harvest electricity (i.e. typically when the distance between the non-deformable walls reaches a local minimum).

[0029] Optionally, the deformable electroactive material comprises a dielectric elastomer.

[0030] Optionally, the dielectric elastomer forms part of a dielectric elastomer generator.

[0031] Optionally, the dielectric elastomer is connected to an electrical source and / or an electrical load and / or to electrical storage to enable it to be charged and / or discharged and / or to store electricity.

[0032] Optionally, the volume-variable chamber comprises a protuberance extending beyond the plane of one or more of the non-deformable walls and configured to impose a minimum distance of separation of said two opposed non-deformable wall members.

[0033] Optionally, the protuberance is height-adjustable to enable said minimum distance of separation to be altered.

[0034] Optionally, the protuberance is defined by a reinforced inflatable member to provide said height-adjustability.

[0035] It will be appreciated that the protuberance places a minimum limit on the degree to which the deformable electroactive material can compress, thus safeguarding it against mechanical failure or irreversible compression. The protuberance may comprise a stopper in the form of a solid boss or protrusion; or a hollow or partially hollow inflatable protrusion.

[0036] Optionally, the working fluid is air, and an air compressor is provided on the apparatus, or connected thereto, to pressurise said protuberance such that said protuberance acts as an air spring.

[0037] Optionally, the apparatus comprises one or more sensors for monitoring the distance of separation of said two opposed non-deformable wall members.

[0038] It will be appreciated that distance sensors may be employed within the apparatus such that, for each wave, it can be determined when the deformable electroactive material needs to be charged with a given voltage (i.e. typically when the deformable electroactive material is stretched and the distance between the non-deformable walls reaches a local maximum); and when energy needs to be harvested from the deformable electroactive material (i.e. typically when deformable electroactive material is relaxed and the distance between the non-deformable walls reaches a local minimum).

[0039] Optionally, an inflatable cavity is attached to the uppermost of said two opposed non-deformable wall members for controlling the degree of wave excitation imparted to the uppermost non-deformable wall member.

[0040] Optionally, the inflatable cavity is isolated from the working volume within the volume- variable chamber.

[0041] Optionally, the wave energy converter comprises a low-pressure cavity disposed outside the at least one volume-variable chamber, the low-pressure cavity being hermetically connected to one of the two opposed non-deformable wall members of at least one said volume-variable chamber, the low-pressure cavity comprising a cavity fluid, the low-pressure cavity being fluidically connected to a pressure controller configured to control a difference of pressure between the working fluid and the cavityfluid so as to modify a distance between the two opposed non-deformable wall members and deform to the deformable electroactive material. Preferably, the low-pressure cavity is adjacent to one of the two opposed non-deformable wall members.

[0042] Controlling the difference of pressure between the working fluid and the fluid in the low-pressure cavity advantageously allows to set the deformable energy harvesting material in an initial deformation, therefore adapting the range of stretching ratio allowed for said deformable energy harvesting material. This therefore allows to configure the apparatus so that it is in the highest efficiency range of energy conversion.

[0043] According to a first aspect of the present invention, there is provided a wave energy converter apparatus for harvesting electrical energy from sea waves, the apparatus comprising: at least one volume- variable chamber for containing a working fluid, and isolating the working fluid from ambient surroundings, said volume- variable chamber comprising at least one deformable electroactive material, said volume- variable chamber comprising two opposed non-deformable wall members, said deformable electroactive material being coupled between said opposed non-deformable wall members; and a low-pressure cavity disposed outside the at least one volume-variable chamber, the low-pressure cavity being hermetically connected to one of the two opposed non- deformable wall members of at least one said volume-variable chamber, the low-pressure cavity comprising a cavity fluid, the low-pressure cavity being fluidically connected to a pressure controller configured to control a difference of pressure between the working fluid and the cavity fluid so as to modify a distance between the two opposed non- deformable wall members and deform to the deformable electroactive material; wherein as sea waves consecutively pass over the apparatus, the chamber defines an oscillatory volume causing the opposed non-deformable wall members to deform the deformable electroactive material enabling harvesting of electricity.

[0044] According to another aspect of the present invention, there is provided a wave energy converter apparatus for harvesting electrical energy from sea waves, the apparatus comprising:at least two volume-variable chambers for containing a working fluid, and isolating the working fluid from ambient surroundings, the wave energy converter comprising a fluid conduit connecting two or more of said volume-variable chambers and permitting movement of said working fluid therebetween; each volume-variable chamber comprising at least one deformable electroactive material; wherein as sea waves consecutively pass over the apparatus, each volume- variable chamber defines an oscillatory volume causing the deformable electroactive material to deform and enable harvesting of electricity; and wherein adjoining spaced apart volume-variable chambers are separated by a distance corresponding to between one quarter and one half of a typical or average wavelength of the sea waves such that said working fluid moves into and out of each volume- variable chamber via the fluid conduit in response to ambient pressure changes caused by sea waves traversing above each volume- variable chamber.

[0045] According to another aspect of the present invention, there is provided a wave energy converter apparatus for harvesting electrical energy from sea waves, the apparatus comprising: at least one volume- variable chamber for containing a working fluid, and isolating the working fluid from ambient surroundings, said chamber comprising at least one deformable electroactive material; and a low-pressure cavity disposed outside the at least one volume-variable chamber, the low-pressure cavity being hermetically connected to one of the two opposed non- deformable wall members of at least one said volume-variable chamber, the low-pressure cavity comprising a cavity fluid, the low-pressure cavity being fluidically connected to a pressure controller configured to control a difference of pressure between the working fluid and the cavity fluid so as to modify a distance between the two opposed non- deformable wall members and deform to the deformable electroactive material up to a predetermined initial deformation; and wherein as sea waves consecutively pass over the apparatus, the chamber defines an oscillatory volume causing the deformable electroactive material to deform and enable harvesting of electricity.

[0046] Optionally, the working fluid is air, and wherein an air compressor is provided on the apparatus, or connected thereto, to pressurise said volume-variable chamber.

[0047] Optionally, the apparatus comprises a fluid conduit connecting two or more of said volume-variable chambers and permitting movement of said working fluid therebetween.

[0048] Optionally, adjoining spaced apart volume-variable chambers are separated by a distance corresponding to between one quarter and one half of a typical or average wavelength of sea waves experienced in the location where the apparatus is located such that said working fluid moves into and out of each volume- variable chamber in response to ambient pressure changes caused by sea waves traversing above each volume- variable chamber.

[0049] Optionally, the plurality of volume-variable chambers is provided in multiple layers, one on top of another.

[0050] In some examples, the volume-variable chambers are arranged in a stacked array of layers thus allowing for a more efficient means of the scaling the apparatus to capture and convert larger amounts of energy from incident sea waves. In such an arrangement, the outermost layers (i.e. at the top and bottom layers of a stack) are directly exposed to pressure variations from the water column, whilst inner layers experience those pressure variations indirectly. The non-extensible connecters within each layer may be arranged in a laterally offset manner relative those of adjacent layers. Each layer within a stacked array may comprise chambers which contain a different mass of electroactive material and / or have different maximum volumes from the chamber in other layers, e.g. the uppermost layer may contain the smallest amount of electroactive material and / or have the largest maximum volume with the amount of electroactive material contained in the chambers reducing progressively in each new layer beneath. Using the top layers to capture pressure variations caused by small waves can be advantageous because those layers are closer to the surface, where the amount of available wave excitation is greater than at greater depths. Also, when the waves are large, reducing fatigue becomes a concern and collapsing the top layers can reduce the exposure of the device.

[0051] Optionally, an uppermost wall member of an uppermost layer of said multiple layers is non-deformable; and wherein a lowermost wall member of a lowermost layer of said multiple layers is non-deformable; and wherein said deformable electroactive material is coupled between the opposed wall members within each layer to enable it to deform and generate electricity in response to a distance change between said uppermost and lowermost non-deformable wall members.

[0052] Optionally, some or all of the intervening wall members are deformable.

[0053] Optionally, a valve is associated with each volume-variable chamber to selectively permit and prevent passage of the working fluid into and out of each layer, thus enabling control of which layer(s) participate in the generation of electricity. This advantageously allows to control which volume-variable chambers participate in the generation of electricity.

[0054] Optionally, the apparatus comprises a fluid conduit connecting two or more layered groups of volume- variable chambers and permitting movement of said working fluid therebetween.

[0055] The fluid conduit may have non-deformable walls. The non-deformable walls may be inflatable walls which become substantially non-deformable once they are inflated. In this embodiment, the non-deformable walls may comprise reinforced inflatable members.

[0056] Optionally, adjoining spaced apart layered groups of volume-variable chambers are separated by a distance corresponding to between one quarter and one half of a typical or average wavelength of sea waves experienced in the location where the apparatus is located such that said working fluid moves into and out of each layered group of volumevariable chambers in response to ambient pressure changes caused by sea waves traversing above each layered group of volume- variable chambers.

[0057] It will be appreciated that a half-wavelength is the optimal distance of separation between adjacent stacks of volume-variable chambers. For example, when a sea wave imparts a mechanical force upon a non-deformable wall of the first group, the workingfluid therein is expelled from its chambers, it moves through the connecting fluid conduit, and is transferred into the chambers of the adjacent second group whose walls are not (yet) subjected to the mechanical force of the wave. As the wave passes the first stack, and impacts against a non-deformable wall of the second stack, the working fluid therein reverts, via the connecting fluid conduit, and is transferred back into the chambers of the first group. The back-and-forth movement of the working fluid between the respective chambers of the adjacent groups causes the deformable electroactive material within each chamber to alternately stretch and compress (within its defined limits) and produce electricity.

[0058] Optionally, the apparatus comprises at least one fixed- volume vessel for containing at least part of the working fluid, where said vessel is in fluid communication with said multiple layers of volume- variable chambers to allow movement of the working fluid therebetween in response to ambient pressure changes caused by sea waves passing over the apparatus.

[0059] Optionally, the apparatus is connected to the sea bed or a structure via mooring lines.

[0060] Optionally, the apparatus is partially embedded within the sea bed.

[0061] Optionally, the apparatus is connected to a pre-existing marine structure.

[0062] According to another aspect of the present invention, there is provided a method of harvesting electrical energy from sea waves at a sub-sea location, the method comprising: providing an apparatus comprising one or more volume- variable chambers at said sub-sea location for containing a working fluid, and isolating the working fluid from ambient surroundings; providing at least one deformable electroactive material within each volumevariable chamber; setting an internal pressure of the working fluid within each volume- variable chamber to a pressure level substantially equal to an average external pressure applied to each volume- variable chamber by water column at the depth of said sub-sea location; andexposing each volume-variable chamber to the external force of consecutive sea waves to oscillate the volume within each volume- variable chamber, thereby causing each deformable electroactive material to deform and generate electricity.

[0063] It will be appreciated that the step of setting an internal pressure of the working fluid can additionally, or alternatively, include setting an internal volume of the working fluid. In essence, the amount of working fluid (e.g. air) added to each volume-variable chamber is calculated such that the deformation amplitude of the deformable electroactive material is around half way between the preferred minimal deformation amplitude and the preferred maximal deformation amplitude (e.g. its stretch ratio is in the middle of the desired stretching range) when the pressure of the water column above the apparatus reaches its average value at the depth location of the apparatus. In some examples, a combination of an air compressor and suitable valves may be used to inflate and deflate each volume-variable chamber to control the working fluid pressure / volume therein. An air intake may be situated on-shore, or fixed to an off-shore structure above the water line, or arranged to float above the water line.

[0064] According to another aspect of the present invention, there is provided a method of harvesting electrical energy from sea waves at a sub-sea location, the method comprising: providing an apparatus comprising one or more volume-variable chambers at said sub-sea location for containing a working fluid, and isolating the working fluid from ambient surroundings; providing at least one deformable electroactive material within each volume- variable chamber; providing a mass of working fluid within the apparatus so that a corresponding volume of working fluid fills the at least one of the volumevariable chambers to deform the deformable electroactive material up to a predetermined initial deformation to the deformable electroactive material; and exposing at least one of the volume-variable chambers to pressure variations due to consecutive sea waves to modify the volume within eachvolume- variable chamber, thereby causing each deformable electroactive material to deform and generate electricity.

[0065] Optionally, the predetermined initial deformation of the deformable electroactive material is comprised in the middle of a harvesting stretching range of the deformable electroactive material.

[0066] Optionally, the method comprises the step of continuously or periodically adjusting the internal pressure of the working fluid to maintain the degree of deformation of said deformable electroactive material between predetermined minimum and maximum levels.

[0067] Optionally, the method comprises the steps of connecting said deformable electroactive material to an electrical load and / or to electrical storage; charging said deformable electroactive material when said external pressure reaches a local minimum; and discharging said deformable electroactive material to said electrical load or electrical storage when said external pressure reaches a local maximum.

[0068] Optionally, the apparatus comprises a low-pressure cavity and the volumevariable chamber comprises two opposed non-deformable wall members, the deformable electroactive material being coupled between said opposed non-deformable wall members, the low-pressure cavity being disposed outside the at least one volume-variable chamber, the low-pressure cavity being hermetically connected to one of the two opposed non-deformable wall members of at least one said volume-variable chamber, the low- pressure cavity comprising a cavity fluid, the low-pressure cavity being fluidically connected to a pressure controller, the method comprising a step of controlling, by the pressure controller, a pressure inside the low-pressure cavity so that a difference of pressure between the working fluid and the cavity fluid modifies a distance between the two opposed non-deformable wall members and deforms the deformable electroactive material.

[0069] Optionally, the apparatus comprises at least one fixed-volume vessel for containing at least part of the working fluid, said fixed- volume vessel being in fluid communication with said volume-variable chamber, the apparatus further comprising avolume controller, the method comprising a step of controlling, by the volume controller, the volume of the one fixed-volume vessel to adapt a compression cycle of the working fluid to periodic movements of the waves.

[0070] Optionally, the volume controller comprises a volume separator hermetically separating the volume of the fixed- volume fluid chamber in a working volume comprising part of the working fluid and a buffer volume, the method comprising a step of controlling, by the volume controller, a pressure of a fluid comprised in the buffer volume so that the pressure of the fluid comprised in the buffer volume is lower than the pressure of the working fluid.

[0071] Optionally, the at least one fixed- volume vessel is inflatable, the method further comprising inflating the at least one fixed- volume vessel to achieve a predetermined level of rigidity leading the at least one fixed- volume vessel to be unresponsive to ambient pressure changes caused by sea waves consecutively passing over the apparatus.

[0072] Optionally, the method comprises further comprises: providing a plurality of volume- variable chambers in multiple layers; and activating all of the plurality of layers, or a subset thereof, dependent on the prevailing sea state, such that a range of deformation of said deformable electroactive material is maintained within an optimal deformation range,

[0073] Optionally, the method further comprises before activating all of the plurality of layers or a subset thereof: selecting at least one of the volume-variable chambers, said selection depending on an amplitude of pressure variations resulting from the movement of consecutive sea waves and a mass of electroactive material that can be deformed by said pressure variations while maximising the deformation ratio of said material, computing a mass of working fluid to be provided within the apparatus so that a deformation of the deformable electroactive material of the selected volume-variable chambers reaches a predetermined value, preferably avalue in the middle of a harvesting stretching range of said deformable electroactive material, and providing the computed mass of working fluid within the apparatus.

[0074] Optionally, the method further comprises rotating the activation of layers of volume-variable chambers in order to increase the longevity of the deformable electroactive material therein.

[0075] Optionally, the method further comprises rotating the activation of volumevariable chambers or of layers of volume-variable chambers so that the number of harvesting cycles can be evenly spread between the volume-variable chambers.

[0076] By way of example, if four layers of volume-variable chambers are available then, depending on the wave size, one, two, three, or all four of those layers may be activated for use at the same time, to distribute load. If the size of the waves does not require all layers to be active, instead of always using the same layers, their usage can be rotated so as to increase the longevity of its associated deformable electroactive material.

[0077] Optionally, the working fluid is air, and the method further comprises temporarily deflating the volume-variable chambers to reduce the overall volume of the apparatus for the purpose(s) of storage and / or transportation and / or to protect the apparatus from the impact of storms.

[0078] It will be appreciated that the ability to selectively place volume- variable chambers into a non-operative condition provides several advantages. Firstly, it allows a degree of control over the electricity generated by the apparatus; it provides a way to disable defective chambers whilst leaving other chambers operational; and it permits chambers to be “rested” (perhaps in a rotating fashion) to increase their lifespan and preserve their effectiveness.

[0079] Further features and advantages of the present disclosure will become apparent from the claims and the following description.BRIEF DESCRIPTION OF DRAWINGS

[0080] Embodiments of the present disclosure will now be described by way of example only, with reference to the following diagrams, in which:

[0081] Fig. 1 is schematic cross-sectional view through a volume-variable chamber forming part of a wave energy converter apparatus;

[0082] Fig. 2 is schematic cross-sectional view through a modified volume-variable chamber forming part of a wave energy converter apparatus;

[0083] Fig. 3 is schematic cross-sectional plan view of the volume-variable chamber of Fig- 1;

[0084] Fig. 4 is schematic cross-sectional view through a volume-variable chamber forming part of a wave energy converter apparatus, including additional features for controlling its volume;

[0085] Fig. 5 is schematic cross-sectional view through a wave energy converter apparatus, comprising four stacked layers of volume-variable chambers each connected via a fluid conduit;

[0086] Fig. 6 is schematic cross-sectional view of the apparatus of Fig. 5 wherein two of the layers are in a collapsed state;

[0087] Fig. 7 is schematic cross-sectional view through two spaced apart stacks of volume-variable chambers which are joined via fluid conduit permits movement of a working fluid between the two;

[0088] Fig. 8 is schematic cross-sectional view of the apparatus of Fig. 7 showing one stack in a minimised (compressed) state, and the other stack in a maximised (expanded) state;

[0089] Fig. 9 is schematic cross-sectional view through a wave energy converter apparatus comprising four stacked layers of volume- variable chambers capable of fluidcommunication with a first lower fixed-volume vessel, and provided with a first upper fixed-volume vessel; and

[0090] Fig. 10 is schematic cross-sectional view of a further alternative example of the apparatus partially embedded into a sea bed, showing its expanded volume state in dashed lines;

[0091] Fig. 11 is schematic cross-sectional view through a wave energy converter apparatus, comprising four stacked layers of volume-variable chambers each connected via a fluid conduit. The uppermost layers of chambers have lower mass of dielectric elastomer compared to the lowermost layers of chambers;

[0092] Fig. 12 is schematic cross-sectional view through the wave energy converter apparatus of Fig. 9 wherein the vessel comprises a volume controller comprising an inflatable element allowing to vary the available volume of the vessel;

[0093] Fig. 13 is schematic cross-sectional view through the wave energy converter apparatus of Fig. 9 wherein the vessel comprises a volume controller comprising a volume separator and three elastic members to vary the available volume of the vessel by sliding the volume separator;

[0094] Fig. 14 is schematic cross-sectional view through a wave energy converter apparatus comprising a low-pressure cavity wherein one of the two opposed non- deformable wall members slides;

[0095] Fig. 15 is schematic cross-sectional view through a wave energy converter apparatus, comprising two stacked layers of volume- variable chambers and comprising five low-pressure cavities located between the two stacked layers.DETAILED DESCRIPTION

[0096] Fig. 1 shows a volume-variable chamber 10 for containing a working fluid, such as air. As will be described subsequently, a wave energy converter apparatus may comprise a plurality of volume-variable chambers in fluid connection with one another. The wave energy converter apparatus is an isolated system when submerged in water, i.e.the working fluid moving between the internal volumes of the chambers is isolated from ambient sea water.

[0097] The volume-variable chamber 10 further comprises at least one deformable electroactive material 18. The at least one deformable electroactive material 18 are stretched by the deformation of the volume- variable chamber 10 when a wave passes over the volume- variable chamber 10. The deformable electro active material 18 has a minimum deformation threshold below which little or no electrical power will be generated; and a maximum deformation threshold above which mechanical failure or irreversible deformation of the deformable electroactive material 18 may occur. To maximize the energy harvesting from the waves, it is important that the deformation of each deformable electroactive material 18 leads to a stretch (deformation) that remains in a harvesting stretching range (also named optimal deformation range). The harvesting stretching range may be delimited by the minimum deformation threshold and maximum deformation threshold. The harvesting stretching range may be comprised within a larger range delimited by the minimum deformation threshold and maximum deformation threshold. The wave energy converter apparatus of this disclosure is configured to set and maintain the deformable electroactive material 18 in the harvesting stretching range.

[0098] The deformable energy harvesting material 18 may each be in the form of an electroactive polymer, such as a dielectric elastomer also named dielectric material in this disclosure. It can be organised in energy harvesting units named EAP generators. The deformable energy harvesting material 18 may take the form of dielectric elastomer generators, ionic dielectric elastomer generators, magnetostrictive generators, and / or a combination of flexible generators. Dielectric elastomer generators are embeddable energy converters whose power take off schemes are based upon a dynamically stretched capacitor.

[0099] The volume-variable chamber 10 may comprise two opposed non-deformable wall members (12, 14). The variation of the distance between the non-deformable wall members (12, 14) resulting from pressure variations due to the passage of waves deforms the deformable energy harvesting material 18 fixed to the non-deformable wall members (12, 14). The volume-variable chamber 10 may further comprise a peripheral non-extensible wall member 16. The peripheral non-extensible wall member 16 may be flexible. It has to be noted that, in this disclosure, the expression “non-deformable wall members” refers to elements which do not deform because of the passage of the waves above the apparatus. However, the shape of the non-deformable wall members (12, 14) may be adapted to provide the optimal efficiency to the apparatus according to the characteristic of the waves.

[0100] The volume- variable chamber 10 may further comprise limit stops. The limit stops may take the form of non-extensible connecters 17 but other elements configured to restrict the maximum distance of separation of said two opposed non-deformable wall members can be used. Lengths of deformable energy harvesting material 18 are connected between the two non-deformable wall members (12, 14), between each non-extensible connecter 17. The limit stops allows that the deformation of the deformable energy harvesting material 18 does not exceed the upper limit of the harvesting stretching range.

[0101] The dielectric elastomer may be provided as multiple deformable electrodes and deformable webs of dielectric material arranged alternately in a layered form. Importantly, when a volume- variable chamber is ‘active’ (i.e. when air is moving in / out as described subsequently and the dielectric elastomer are being cyclically charged and discharged in order to harvest electricity) their stretch ratio must not fall below a minimum threshold (of around 110%- 120% for most kinds of silicone rubber) of their relaxed length even when the two non-deformable wall members 12, 14 move towards one another.

[0102] In order to maintain a minimum stretch ratio and to maintain the deformation of the deformable energy harvesting material 18 in the harvesting stretching ratio range during operation there are several possible solutions.

[0103] The first solution (see Fig. 2) is to incline the dielectric elastomer 18 layers diagonally (e.g. in a zig-zag manner) within the internal volume of the volume- variable chamber 10. By doing so, even if the separation of the two non-deformable wall members 12, 14 were to reduce to zero, the laterally offset points of connection between the dielectric elastomer 18 and the wall members 12, 14 in the lengthwise direction of thevolume- variable chamber 10 are arranged to maintain the predetermined minimum stretch ratio of the dielectric elastomer 18.

[0104] The second solution is to provide one or more protuberances or stoppers 15 attached to one or both of the internally facing surfaces of the two non-deformable wall members 12, 14 of the volume-variable chamber 10. As shown in Fig. 4, the protuberances or stoppers 15 define a reduced vertical distance dmbetween their opposing surfaces. A minimum separation equal to the height of the protuberances or stoppers is defined between the two non-deformable wall members 12, 14. The layered dielectric elastomer 18 extends perpendicularly relative to the two non-deformable wall members 12, 14 and the protuberances or stoppers 15 prevent over slackening of the dielectric elastomer 18 layers.

[0105] In order to set the deformable electroactive material 18 in the harvesting stretching range, a predetermined initial deformation of the deformable electroactive material 18 may be achieved.

[0106] For example, also shown in the example of Fig. 4 are additional features for controlling the volume of the volume-variable chamber 10. These include a pump 20 for introducing compressed (pressurized) working fluid (such as air) into the volume- variable chamber 10 via a compressed (pressurized) working fluid conduit 22. The volume of working fluid may be injected from any part of the apparatus in fluidic connection with the volume-variable chamber 10. A conduit 24 is provided to allow working fluid to escape from the volume-variable chamber 10 and / or to allow working fluid to be introduced into the volume-variable chamber 10 from a separate volume- variable chamber (or another source). The passage of working fluid through conduit 24 is controlled via a valve 26. Varying the volume of the working fluid allows the deformation of the dielectric elastomer 18 to reach a chosen value, which is typically close to the middle of the optimal the harvesting stretching range. Controlling the volume of working fluid may be performed before exposing each volume-variable chamber 10 to the external force of consecutive sea waves. Controlling the volume of working fluid may also be performed when the number of volume- variable chambers 10 participating in the power generation is changed, or when switching from volume-variable chambers 10 that stretcha lower mass of dielectric elastomer 18 to volume-variable chambers that stretch a higher mass of dielectric elastomer (and vice-versa). Controlling the volume of working fluid may also be performed according the type and number of chambers selected to participate in the energy harvesting process. By varying the volume of the working fluid, the internal pressure of the working fluid may be adjusted to maintain the degree of deformation of said deformable electroactive material between predetermined minimum and maximum levels to preferably maintain the deformation of the dielectric elastomer 18 within the harvesting stretching range. The adjustment of the internal pressure may be continuous or periodic. Controlling the volume of working fluid may be performed by injecting or removing a mass of working fluid. The mass of working fluid subjected to the pressure within the apparatus will take a given volume depending on said pressure.

[0107] Another embodiment to generate the predetermined initial deformation of the deformable electroactive material 18 is illustrated in Fig. 14. In this embodiment, the apparatus comprises a low-pressure cavity 19 disposed outside the at least one volumevariable chamber(s) 10. The low-pressure cavity 19 is hermetically connected to one of the two opposed non-deformable wall members (12, 14) of at least one the volumevariable chambers 10. The low-pressure cavity 19 comprises a cavity fluid and is fluidically connected to a pressure controller configured to control a pressure of the cavity fluid inside the low-pressure cavity 19. For example, the pressure controller controls the pressure of the cavity fluid inside the low-pressure cavity 19 so that the pressure of the cavity fluid is lower than the pressure of the working fluid inside the volume-variable chamber(s) 10. Indeed, by controlling the difference of pressure between the volumevariable chamber(s) 10 and the low-pressure cavity 19, the connected non-deformable wall member (12, 14) moves relatively to the opposed non-deformable wall member (14, 12) because of the force applied by the highest pressure (in the volume- variable chamber 10 or the low-pressure cavity 19). For example, if the pressure in the low- pressure cavity 19 is lower than in the volume-variable chamber 10, the connected non- deformable wall member (12, 14) is pushed in the direction of the low-pressure cavity 19 by the pressure inside the volume-variable chamber 10 thereby increasing the distance between the opposed non-deformable wall members (12, 14). The modification of the distance between the non-deformable wall member (12, 14) thereby deforms thedeformable electroactive material 18. The deformable energy harvesting material 18 can thus be set in an initial deformation so as to be within the harvesting stretching range. Preferably, the initial deformation corresponds to the middle of the harvesting stretching range. The middle of the harvesting stretching range is a sub-range centred on the mean of the harvesting stretching range. For example, the middle of the harvesting stretching range covers 10% of the harvesting stretching range leading to an initial deformation comprised in the mean of the harvesting stretching range ±5%. In another example, the middle of the harvesting stretching range covers 20% of the harvesting stretching range. This is advantageous because the presence of low-pressure cavities adjacent to the non- deformable wall members lower the total volume requirement: when the pressure inside the variable-volume chambers increase, the opposed non-deformable walls can move away from one another without requiring the working fluid to be expelled from the volume- variable chamber (until the pressure inside the low-pressure cavity 19 increases to a certain level).

[0108] In another embodiment, the wave energy converter comprises at least one volumevariable chamber(s) 10 and the low-pressure cavity 19. In this specific embodiment, the wave energy converter does not comprise any fixed-volume vessel.

[0109] For example, in Fig. 14, the low-pressure cavity 19 is connected to the bottom non-deformable wall member 14 which is configured to slide in the low-pressure cavity 19. However, in other examples the low-pressure cavity 19 is connected to the top non-deformable wall member 12 or the low-pressure cavity 19 respectively disposed above and below the volume-variable chamber 10 is connected to each of the bottom non- deformable wall members (12, 14). In other embodiments, only part of the non- deformable wall members (12, 14) is configured to move relatively to the opposed non- deformable wall members (14, 12) as represented in Fig. 15, the part of the non- deformable wall members (12, 14) being attracted by the low-pressure cavity 19. Preferably, the low-pressure cavity 19 is disposed so that at least one deformable energy harvesting material 18 is fixed to the part of the non-deformable wall member (12, 14) connected to the low-pressure cavity 19. In the embodiment wherein the volume-variable chambers 10 are disposed in layers as described hereafter and shown in Fig. 15, the low-pressure cavity 19 may be disposed between two layers so that the top non-deformable wall member 12 of the bottom layer and the bottom non-deformable wall member 14 of the top layer are both connected to the low-pressure cavity 19. In this last embodiment, the low-pressure cavity 19 may be an intermediate layer disposed between two layers of volume-variable chambers 10. The low-pressure cavity 19 may also comprise several cavities as for example in Fig. 15 comprising five cavities. This advantageously allows to individually deform the deformable energy harvesting material 18.

[0110] The apparatus may further comprise a fixed-volume fluid chamber 70. It has to be noted that, in this disclosure, the expression “fixed-volume fluid chamber” refers to a volume which does not vary as a result of pressure variations due to the passage of waves above the apparatus. However, the fixed- volume fluid chamber may be adapted to provide the optimal efficiency to the apparatus according to the characteristic of the waves. The fixed- volume fluid chamber is also named fixed- volume fluid vessel, fixed- volume vessel or fixed- volume chamber in the following description.

[0111] The fixed- volume fluid chamber 70 acts as a fluid reservoir freely connected to at least one of the volume-variable chambers 10 to provide a compression ratio of the working fluid which is equivalent to those of a total volume of the fluid equal to a sum of the volume of the fixed- volume fluid chamber 70 and the connected volume-variable chamber(s) 10. In other words, the overall fluid contained in the volume-variable chamber(s) 10 and the fixed-volume fluid 70 oscillates when a wave is passing over the apparatus. Therefore, this allows to take advantage of the small sized volume- variable chamber(s) 10 in terms of convenience of manufacturing and transporting and in terms of lifetime of the dielectric material. In addition, this also allows to take advantage of large volume of working fluid which to efficiently use the largest range of contraction of the dielectric material 18 which is thus not limited by the compressibility of a small volume of fluid.

[0112] It has to be noted that the fixed- volume fluid chamber 70 is not used to for introducing a pressurized fluid into the volume- variable chamber 10. Indeed, no pump is used and the pressure of the working fluid is the same in the fixed-volume fluid chamber 70 than in the volume-variable chamber(s) 10 since the they freely share thesame working fluid. The fixed- volume fluid chamber 70 is only used to increase the total available volume of working fluid.

[0113] In Fig. 12 and Fig. 13, the fixed-volume vessel 70 comprises a volume controller configured to control the volume of the one fixed- volume vessel 70. This allows to adapt the compression cycle of the working fluid to the resonance frequency of the waves. The apparatus of this embodiment is thus configured to harvest an optimum energy of the waves by increasing the stretching range of the deformable energy harvesting material 18. This is particularly advantageous when a plurality of volume-variable chambers 10 is connected to the fixed- volume vessel 70. Indeed, without the fixed- volume vessel 70, an adaptation of the volume of each volume- variable chamber 10 is needed.

[0114] In the embodiment represented in Fig. 12, the volume controller comprises an inflatable element for containing a volume controlling fluid 75 isolated from ambient surroundings and a pressure controller configured to inflate or deflate the inflatable element by adding or removing the controlling fluid 75 from the inflatable element. For example, by inflating the inflatable element (by adding controlling fluid 75), the volume of the inflatable element increases. Consequently, the volume 72 of working fluid comprised in the fixed- volume fluid chamber 70 and thus the total volume of the working fluid decreases proportionally.

[0115] In the embodiment represented in Fig. 13, the volume controller comprises a volume separator 76 and three elastic members 74 to vary the volume of the working fluid in the fixed- volume fluid chamber 70 by sliding the volume separator 76. The elastic members 74 may be springs, pistons, air springs, or other elements providing a resistance. The volume separator 76 hermetically separates the volume of the fixed- volume fluid chamber in a working volume 72 comprising the working fluid and a buffer volume 71. The position of the volume separator 76 can vary thereby varying the ratio between the working volume 72 and the buffer volume 71. Preferably, the buffer volume 71 comprises a fluid having a pressure lower than the working fluid to reduce the forces acting against the movement of the volume separator 76 when the pressure of the working fluid increases. Therefore, given the stiffness of the elastic members 74 and the stiffness of the EAP generators in the selected volume- variable chambers 10, the resonance of thesystem can be maximised for a given sea state. The volume controller may be further configured to control the pressure of the fluid in the buffer volume 71. This embodiment does not limit the number of elastic members 74. Indeed, the number of elastic members 74 may be chosen according to the overall geometry of the apparatus. This embodiment allows to automatically adjust the volume of the fixed- volume vessel according to the pressure of the working fluid so that the compression cycle of the working fluid brings the apparatus in the resonant frequency of the waves.

[0116] As shown in the example of Fig. 5, volume-variable chambers 10 may be arranged in multiple layers stacked one on top of the other. In the illustrated example, four such layers are shown. It will be appreciated that the various layers need not be identical in structure. For example, it may be desirable to vary the number and / or length and / or spacing of the non-extensible connecters 17 and / or the mass and / or lengths and / or orientations of deformable energy harvesting material 18 extending between their opposed wall members 12, 14. For example, as illustrated in Fig. 11, the mass of the deformable energy harvesting material 18 increases towards the bottom layers. This is advantageous because the stretching ratio of the deformable energy harvesting material 18 depends on two factors: the stretching force proportional to the area of the opposed wall members (12, 14) and to the variation of pressure due to sea waves and the mass of the deformable energy harvesting material 18. The total mass that can be deformed depends on the active surface (the surface of the moving walls accessible to the working fluid) at a given moment and the amplitude of the pressure variations at the same time. The force required to deform the generators typically depends on their mass and Young’s modulus. Therefore, in an apparatus as illustrated in Fig.°l l, for the same area of the opposed wall members (12, 14), the harvesting stretching ratio of the deformable energy harvesting material 18 will be different for the different layers (higher for the uppermost layers). Therefore, for a wide range of waves thus generating a wide range of forces, the apparatus of Fig. 11 has at least one layer in the most efficient configuration leading to the most efficient stretch ratio of the deformable energy harvesting material 18. Indeed, when a large wave is passing above the apparatus, the variation of pressure is large and thus the force applied is optimal for the deformable energy harvesting material 18 of large mass (those of the bottom layers) whereas for small waves generating smallpressure variation, the deformable energy harvesting material 18 of low mass (those of the top layers) will be stretched optimally (i.e., withing the harvesting stretching range).

[0117] When using several volume- variable chambers 10, it is advantageous to determine previously the number of volume-variable chambers 10 which will be used and / or which volume-variable chambers 10 will be used. This choice may depend on the pressure variations resulting from the waves passing over the apparatus. The number of volumevariable chambers 10 that are used may preferably be adapted in real time according to the sea state. Then, an initial deformation of the deformable energy harvesting materials 18 may be generated to set them in the harvesting stretching range, for example, by controlling the volume of working fluid in the selected volume- variable chambers 10, by controlling the pressure in buffer volume 71, or by controlling the pressure in the low- pressure cavity 19, or by using any combination of these three methods.

[0118] The intermediate non-deformable wall members 13 located between the uppermost and lowermost non-deformable wall members (12, 14) each act as both an upper and lower non-deformable wall members in respect of the volumes located below and above them respectively. Such diversity in terms of the specific structure of each volume-variable chamber 10 may ensure that the overall apparatus can more efficiently generate electricity over a wider range of sea states.

[0119] A common fluid conduit 28 is in fluid communication with each layer via a number of openings which may be controllable via valves 26 to selectively open and close each opening. In the illustrated example, the vertical part 16 of the wall of the common fluid conduit 28 is non-extensible. In some embodiments, the common fluid conduit 28 leads to other volume-variable chambers (not shown), typically located at another other location (chosen so that there is most of the time a pressure difference due to waves). In other embodiments, the common fluid conduit 28 leads to a fixed- volume fluid chamber (not shown) in which case the volume of the fixed-volume fluid chamber (which may be, or may include, the volume of the fluid conduit 28) is between two and ten times more than the cumulative internal (maximal) volume of the volume- variable chambers 10.

[0120] In some embodiments, the wall members 16 of each volume-variable chamber 10 are resiliently compressible and are filled with a gas and arranged to be inflatable / defeatable to control their rigidity / compressibility.

[0121] In the example illustrated in Fig. 6, two of the four layers of volume-variable chambers 10 within a wave converter apparatus have been caused to collapse to reduce or minimise their internal volumes. This can be achieved by closing valves 26 (not visible) and opening a non-return valve (not shown) that only lets the working fluid out of the volume-variable chambers 10. Therefore, the valves 26 allows to control which volumevariable chambers 10 participate in the generation of electricity. This allows to adjust the type and number of volume- variable chambers 10 connected to the fixed-volume fluid chamber 70 according to the amplitude of the pressure variations caused by the waves, so as to maximize the mass of deformable electroactive material 18 that can be deformed using these pressure variations. This allows to maximize the mass of deformable electroactive material 18 that works at a deformation level close to the desired maximum deformation.

[0122] Once collapsed, three of the intermediate non-deformable wall members 13c lie in closer proximity to one another, thus reducing the overall height of the wave converter apparatus as measured between the uppermost and lowermost non-deformable wall members 12, 14.

[0123] Figs. 7 and 8 show a submerged wave converter apparatus comprising two groups 30, 40 of layered volume-variable chambers 10 connected together via a rigid fluid conduit 28. In one example, each group 30, 40 may measure approximately 5 metres in diameter, with larger scale models being up to around 8 metres in diameter. The fluid conduit may have a diameter of approximately 1 metre.

[0124] The apparatus is connected to the sea bed via tether lines 50 and maintain the overall apparatus floating beneath the water’s surface. For reasons which will be explained subsequently, each group 30, 40 is separated by a distance equivalent to between one quarter and one half of a typical or average or median wavelength of waves prevalent at the location where the apparatus is situated. For example, based on arepresentative wave within an energetic sea state at a reference location, and dependent on wave frequency and distance from the shore, each group 30, 40 may be separated by a distance of between 10 and 50 metres.

[0125] In another embodiment, the wave energy converter comprises a plurality of volume- variable chambers 10 separated by a distance equivalent to between one quarter and one half of a typical or average or median wavelength of waves prevalent at the location where the apparatus is situated. In this specific embodiment, the wave energy converter does not comprise any fixed-volume vessel nor low-pressure cavity.

[0126] As is evident in Fig. 7, the two groups 30, 40 of layered volume-variable chambers 10 have substantially the same internal volume, i.e. the height of each group 30, 40 (as measured between the uppermost and lowermost non-deformable wall members 12, 14) is substantially equal. The two groups 30, 40 can be considered to each be in an intermediate state whereby the stretch ratio of the dielectric elastomers 18 within each volume- variable chamber 10 has a value in the middle of the desired stretch ratio range.

[0127] By contrast, in Fig. 8 the two groups 30, 40 of layered volume- variable chambers 10 have different internal volumes, i.e. the height of each group 30, 40 (as measured between the uppermost and lowermost non-deformable wall members 12, 14) is unequal, for the reasons described subsequently.

[0128] An alternative example of the submerged wave converter apparatus of the present invention is shown in Fig. 9. It also comprises a layered stack of volume-variable chambers 10. However, the structure of the wave converter apparatus of Fig. 9 differs from those previously described in a number of respects. A first fluid vessel 60 of a fixed volume is provided immediately above the uppermost non-deformable wall member 12; and a second (larger) fluid vessel 70 of a fixed volume is provided immediately below the lowermost non-deformable wall member 14.

[0129] A peripheral fluid conduit 28 is in fluid communication both with each layer of volume-variable chambers 10, and with each of the fluid vessels 60, 70. Movement of fluid between each volume- variable chambers 10 and / or to / from each of the fluid vessels60, 70 is controlled via valves 26. Tether lines 50 connect the second fluid vessel 70 to the sea bed and maintain the overall apparatus floating beneath the water’s surface 55.

[0130] A yet further alternative example of the present invention is shown in Fig. 10 comprising a layered stack similar to that previously described, yet different in a number of respects. Firstly, the layered stack of volume- variable chambers 10 is located on, or partially embedded in, soft sediments on the sea bed, 100.

[0131] A first fluid vessel 80 of a fixed volume surrounds the stacked layers of volumevariable chambers 10. In the illustrated example, the fluid vessel 80 is toroidal in shape, and rigidified using compressed air. A non-deformable base 82 (which may also be rigidified using compressed air) connects to, or is contiguous with, the lowermost non- deformable wall member 14 of the stacked layers of volume-variable chambers 10. A wall connects an upper portion of the layered stack to an upper portion of the first fluid vessel 80. A second annular fluid vessel 84 is located in the space between the first fluid vessel 80 and the layered stack of volume- variable chambers 10, and bounded by the resiliently wall 16a and the non-deformable base 82.

[0132] A third (larger) fluid vessel 86 of a fixed volume is located beneath the stacked layers of volume-variable chambers 10. In the example of Fig. 10, the third fluid vessel 86 is embedded within a body of sand in the sea bed 100. A number of openings 22 (which may be selectively closable via valves 26) are formed within the non-deformable base 90 to link the second and third vessels 84, 86 and permit the passage of a working fluid between the two.

[0133] Referring back in particular to the example of Fig. 8, in use, the force W of a wave (travelling from left to right) firstly applies a downward force W against the uppermost non-deformable wall member 12 of the first group 30 of layered volume- variable chambers 10 and, in doing so, reduces its overall internal volume. When subjected to the maximum force of an incident wave, the first group 30 can be considered to be in a minimised state whereby electricity can be harvested from the dielectric elastomers 18 and transferred to an electrical load (not shown) or an electrical storage (not shown).

[0134] As the volume of the first group 30 of layered volume-variable chambers 10 reduces, a working fluid is transferred to the second group 40 of layered volume- variable chambers 10 via the fluid conduit 28 as indicated by the arrow. The introduction of the working fluid into the second group 40 of layered volume- variable chambers 10, coupled with a pressure reduction above the uppermost non-deformable wall member 12 of the second group 40 (caused by a retreating wave force W), concurrently causes an increase of the overall internal volume of the second group 40. The second group 40 can be considered to be in a maximised state whereby the dielectric elastomers 18 within each volume- variable chamber 10 undergo an oppositely directed deformation.

[0135] The working fluid oscillates between the first and second groups 30, 40 as waves pass over them; and the layers of dielectric elastomers 18 within each group 30, 40 are concurrently oppositely deformed during each variation of the internal volumes of the chambers 10 thus allowing the dielectric elastomers 18 to receive electrical energy each time they reach a state of maximum deformation, and provide an increased amount of electrical energy each time they reach a state of minimal deformation. When appropriately scaled, it is envisaged that the wave converter apparatus may yield a maximum power output of between approximately 500kW and 1 MW.

[0136] Dependent on the material of the dielectric elastomers 18, small deformations may produce significant energy loss whilst large deformations may result in premature material fatigue. Therefore, it may be beneficial to selectively activate or deactivate one or more of the stacked layers with a view to maintaining the deformation range of the dielectric elastomers 18 within a predetermined optimal range. Pressure sensors (not) shown may be provided for actively monitoring pressure inside one or more of the volume-variable chambers 10 and / or for monitoring external ambient pressure. Alternatively, or additionally, sensors may be provided for actively monitoring the distance of separation of opposing wall members 12, 13, 14. The output(s) from the sensor(s) may be used to calculate the effects of a given sea state and to initiate any adjustments necessary to the volume- variable chambers 10 to maintain the dielectric elastomers 18 within their predetermined optimal operating ranges.

[0137] The other illustrated examples of the apparatus operate on a similar basis. For example, in the apparatus of Figs. 5 and 6, the working fluid oscillates back and forth between the stacked layers of volume- variable chambers 10 and the fluid conduit 28 and any connected fluid vessel (not shown). In doing so, the layers of dielectric elastomers 18 are deformed during each variation of the internal volumes of the chambers 10.

[0138] In the arrangement of Fig. 9, the stacked layers of volume-variable chambers 10 is periodically deformed as waves apply a force W to the exterior surface of the uppermost fluid vessel 60. The increased proximity of fluid vessel 60 with the water’s surface 55 increases the amount of wave excitation that can be captured. In this arrangement, fluid chambers 60 and 70 have a fixed volume. Pressure variations due to passing waves cause the working fluid to move between the variable- volume chambers 10 and fixed- volume chamber(s) 60 and / or 70. When the pressure of the water column increases, the working fluid moves out of the variable- volume chambers 10, allowing the dielectric elastomers 18 to relax. When the pressure of the water column decreases, to working fluid moves back to the variable-volume chambers 10, stretching the dielectric elastomers 18.

[0139] Finally, in the arrangement of Fig. 10, the stacked layers of volume-variable chambers 10 is periodically deformed as waves apply a force W to the exterior surface of the uppermost non-deformable wall 12 which is connected to a resiliently compressible wall member 16. During each deformation, working fluid is expelled from the stacked layers of volume- variable chambers 10 via the fluid permeable walls 16 into the second fluid vessel 84, and into the larger third fluid vessel 86 via openings controlled by valves 26. Once a wave passes, the direction of the wave force reverses, fluid returns from the larger third fluid chamber 86 into the stacked layers of volume-variable chambers 10 to reverse their deformation. The layers of dielectric elastomers 18 are deformed during each variation of the internal volumes of the chambers 10.

[0140] Although particular embodiments of the disclosure have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims.

[0141] For example, in any of the aforementioned examples, the orientation of the dielectric elastomer 18 connecting opposing non-deformable wall members 12, 13, 14 may be varied, i.e. it need not extend perpendicularly between the opposing non- deformable wall members.

[0142] Furthermore, the ability to control the internal variable volume of the apparatus by selectively disabling and collapsing one or more one or more layers of chambers 10, has the effect of moving the uppermost non-deformable wall 12 (i.e. the wall which primarily receives the force W of a wave) towards or away from the sea surface, and hence subjects it to an increase or decrease in wave excitation. In other words, by actively altering the overall internal volume of the deformable parts of the apparatus in the vertical direction, the vertical stroke of the apparatus can be actively “tuned” to optimise its resonance and generate more or less power in a safe manner dependent on prevalent wave sizes and sea conditions.

[0143] Similarly, (either as an alternative or additional feature) the ability to actively alter the internal volume(s) of one or more of the fluid vessels 28, 60, 70, 80, 84, 86 means the apparatus can be actively “tuned” to optimise its resonance and generate more or less power in a safe manner dependent on prevalent wave sizes and sea conditions.

[0144] The limit stops such as the non-extensible connecters 17 serve to prevent the opposed wall members 12, 13, 14 from separating beyond a distance determined by the maximum length of the connecters. An alternative, or additional, means of preventing such separation would be to provide a physical limit stop feature externally of the volumevariable chambers 10. This may take the form of a rigid envelope partially surrounding the wave converter apparatus, whilst still allowing the force W of a wave to be transferred to the uppermost non-deformable wall member 12.

[0145] In an embodiment, at least part of the elements of the apparatus are inflatable. This is advantageous for the transport and storage of the apparatus since the apparatus may be deflated outside the sea. This is also advantageous when the apparatus is disposed in the sea. Indeed, the apparatus may be deflated during a storm to avoid any damage to the apparatus. For example, the fluid vessels 60 and 70 and / or the non-deformable wallmembers 12 and 14 are inflatable. The apparatus may thus be inflated before use to achieve the desired level of rigidity during harvesting operations. For example, the fixed- volume chamber(s) 60 and / or 70 may be inflated to achieve a level of rigidity leading to a minimal reaction of the fixed-volume chamber(s) to external pressure variations. The apparatus may be deflated in the event of storm of for the transport and storage by removing the fluid from the inflatable elements.

[0146] Unless the context requires otherwise, the features of the various examples of the invention as described above are combinable and / or interchangeable.

[0147] In summary, features of the present invention afford a number of advantages in the field of wave energy converters, including: (i) the ability to adapt to varying wave powers whilst maintaining dielectric materials close to their optimal operating range; (ii) the ability to rotate usage of the dielectric materials to evenly spread the number of harvesting cycles to avoid fatigue, thus increasing the longevity of the deformable electroactive material 18; (iii) the fact that the dielectric materials are protected from the harsh effects of the marine environment by being housed and isolated within the volumevariable chambers; (iv) the ability to protect the wave energy converters from the effects of storms by locating them on a sea bed and / or partially sinking them within the sea bed; (v) the ability to reduce the costs of transportation and installation by providing all or some of the parts of the wave energy converter apparatus as selectively inflatable components; (vi) built-in redundancy by arranging the dielectric materials within large arrays or stacks of independent volume- variable chambers, e.g. such that if one or a small number of chambers (or a certain amount of electroactive material within the chambers) become non-operational, this does not significantly affect the total power output of the overall apparatus; and (vii) flexibility in terms of the deployment of the apparatus, e.g. located and anchored offshore, or nearshore, or attached to an existing structure.

Claims

CLAIMS1. A wave energy converter apparatus for harvesting electrical energy from sea waves, the apparatus comprising: at least one volume-variable chamber (10) for containing a working fluid, and isolating the working fluid from ambient surroundings, said volume- variable chamber (10) comprising at least one deformable electroactive material (18); and at least one fixed- volume vessel (70) for containing at least part of the working fluid; wherein as sea waves consecutively pass over the apparatus, the volume-variable chamber (10) defines an oscillatory volume causing the deformable electroactive material (18) to deform and enable harvesting of electricity, a volume of the at least one fixed-volume vessel (70) being unresponsive to ambient pressure changes caused by the sea waves consecutively passing over the apparatus; and wherein said fixed- volume vessel (70) is in fluid communication with said volumevariable chamber (10) to allow movement of the working fluid therebetween generated by the oscillatory volume of the volume-variable chamber (10) in response to ambient pressure changes caused by the sea waves consecutively passing over the apparatus, a sum of a volume of each of the at least one volumevariable chamber (10) and the volume of each of the at least one fixed- volume vessel (70) defining a total oscillatory volume.

2. A wave energy converter according to claim 1, wherein a valve (26) is associated with at least one of said volume-variable chamber (10) to selectively permit and prevent passage of the working fluid into and out of the volume-variable chamber (10).

3. A wave energy converter according to claim 1 or 2, wherein said volume- variable chamber (10) comprises two opposed non-deformable wall members (12, 14), and wherein said deformable electroactive material (18) is coupled between said opposed non-deformable wall members (12, 14) to enable it to deform and generate electricity in response to a distance change between said non-deformable wall members (12, 14).

4. A wave energy converter according to claim 3, wherein opposing ends of said deformable electroactive material (18) are coupled between said non-deformable wall members (12, 14) in a laterally offset manner in a length direction of said volume- variable chamber (10).

5. A wave energy converter according to any of claims 3 to 4 comprising at least one limit stop configured to restrict the maximum distance of separation of said two opposed non-deformable wall members (12, 14).

6. A wave energy converter according to claim 5, wherein the limit stop is a non- extensible connecter (17), said two opposed non-deformable wall members (12, 14) being connected by the at least one non-extensible connecter (17).

7. A wave energy converter according to any of claims 3 to 6, wherein said non- deformable wall members (12, 14) are defined by reinforced inflatable members.

8. A wave energy converter according to any of claims 1 to 7, comprising one or more pressure sensors for monitoring pressure inside said volume-variable chamber (10).

9. A wave energy converter according to any of claims 1 to 8, comprising one or more pressure sensors for monitoring ambient pressure surrounding said volume-variable chamber (10).

10. A wave energy converter according to any of claims 1 to 9, wherein the fixed- volume vessel (70) comprises a volume controller configured to control the volume of the one fixed- volume vessel (70).

11. A wave energy converter according to any of claims 3 to 10, wherein said volumevariable chamber (10) comprises a protuberance (15) extending beyond the plane of one or more of the non-deformable wall members (12, 14) and configured to impose a minimum distance of separation of said two opposed non-deformable wall members (12, 14).

12. A wave energy converter according to claim 11, wherein said protuberance (15) is height-adjustable to enable said minimum distance of separation to be altered.

13. A wave energy converter according to any of claims 11 to 12, wherein the working fluid is air, and wherein an air compressor is provided on the apparatus, or connected thereto, to pressurise said protuberance (15) such that said protuberance (15) acts as an air spring.

14. A wave energy converter according to any of claims 3 to 13, wherein an inflatable cavity is attached to an uppermost of said two opposed non-deformable wall members (12, 14) for controlling a degree of wave excitation imparted to the uppermost non-deformable wall member (12).

15. A wave energy converter according to any of claims 3 to 14, comprising a low- pressure cavity (19) disposed outside the at least one volume- variable chamber (10), the low-pressure cavity (19) being hermetically connected to one of the two opposed non-deformable wall members (12, 14) of at least one said volumevariable chamber (10), the low-pressure cavity (19) comprising a cavity fluid, the low-pressure cavity (19) being fluidically connected to a pressure controller configured to control a difference of pressure between the working fluid and the cavity fluid so as to modify a distance between the two opposed non-deformable wall members (12, 14) and deform to the deformable electroactive material (18).

16. A wave energy converter according to any preceding claim, wherein the working fluid is air, and wherein an air compressor is provided on the apparatus, or connected thereto, to pressurise said volume-variable chamber (10).

17. A wave energy converter according to any preceding claim, wherein the apparatus comprises a fluid conduit connecting two or more of said volume-variable chambers (10) and permitting movement of said working fluid therebetween.

18. A wave energy converter according to claim 17, wherein adjoining spaced apart volume- variable chambers (10) are separated by a distance corresponding to between one quarter and one half of a typical or average wavelength of sea waves experienced in the location where the apparatus is located such that said working fluid moves into and out of each volume- variable chamber (10) in response toambient pressure changes caused by sea waves traversing above each volumevariable chamber (10).

19. A wave energy converter according to any preceding claim, wherein said plurality of volume- variable chambers (10) is provided in multiple layers.

20. A wave energy converter according to claim 19, wherein an uppermost wall member of an uppermost layer of said multiple layers is non-deformable; and wherein a lowermost wall member of a lowermost layer of said multiple layers is non-deformable; and wherein said deformable electroactive material (18) is coupled between the opposed wall members within each layer to enable it to deform and generate electricity in response to a distance change between said uppermost and lowermost non-deformable wall members (12, 14).

21. A wave energy converter according to claim 20, wherein a valve (26) is associated with each layer to selectively permit and prevent passage of the working fluid into and out of each layer, thus enabling control of which layer(s) participate in the generation of electricity.

22. A wave energy converter according to any of claims 20 to 21 , wherein the apparatus comprises a fluid conduit connecting two or more layered groups of volumevariable chambers (10) and permitting movement of said working fluid therebetween.

23. A wave energy converter according to claim 22, wherein adjoining spaced apart layered groups of volume-variable chambers (10) are separated by a distance corresponding to between one quarter and one half of a typical or average wavelength of sea waves experienced in the location where the apparatus is located such that said working fluid moves into and out of each layered group of volumevariable chambers (10) in response to ambient pressure changes caused by sea waves traversing above each layered group of volume-variable chambers (10).

24. A method of harvesting electrical energy from sea waves at a sub-sea location, the method comprising:providing an apparatus comprising one or more volume-variable chambers (10) at said sub-sea location for containing a working fluid, and isolating the working fluid from ambient surroundings; providing at least one deformable electroactive material (18) within each volume- variable chamber (10); providing a mass of working fluid within the apparatus so that a corresponding volume of working fluid fills the at least one of the volumevariable chambers (10) to deform the deformable electroactive material (18) up to a predetermined initial deformation to the deformable electroactive material (18); and exposing at least one of the volume- variable chambers (10) to pressure variations due to consecutive sea waves to modify the volume within each volume-variable chamber (10), thereby causing each deformable electroactive material (18) to deform and generate electricity.

25. A method of harvesting electrical energy from sea waves according to claim 24 wherein the predetermined initial deformation of the deformable electroactive material (18) is comprised in the middle of a harvesting stretching range of the deformable electroactive material (18).

26. A method of harvesting electrical energy from sea waves according to claim 24 or 25, comprising the step of continuously or periodically adjusting the internal pressure of the working fluid to maintain the degree of deformation of said deformable electroactive material (18) in the harvesting stretching range.

27. A method of harvesting electrical energy from sea waves according to any of claims 24 to 26, comprising the steps of connecting said deformable electroactive material (18) to an electrical load and / or to electrical storage; charging said deformable electroactive material (18) when said external pressure reaches a local minimum; and discharging said deformable electroactive material (18) to said electrical load or electrical storage when said external pressure reaches a local maximum.

28. A method of harvesting electrical energy from sea waves according to any of claims 24 to 27, wherein the apparatus comprises a low-pressure cavity (19) and the volume-variable chamber (10) comprises two opposed non-deformable wall members (12, 14), the deformable electroactive material (18) being coupled between said opposed non-deformable wall members (12, 14), the low-pressure cavity (19) being disposed outside the at least one volume-variable chamber (10), the low-pressure cavity (19) being hermetically connected to one of the two opposed non-deformable wall members (12, 14) of at least one said volumevariable chamber (10), the low-pressure cavity (19) comprising a cavity fluid, the low-pressure cavity (19) being fluidically connected to a pressure controller, the method comprising a step of controlling, by the pressure controller, a pressure inside the low-pressure cavity (19) so that a difference of pressure between the working fluid and the cavity fluid modifies a distance between the two opposed non-deformable wall members (12, 14) and deforms the deformable electroactive material (18).

29. A method of harvesting electrical energy from sea waves according to any of claims 24 to 28 wherein the apparatus comprises at least one fixed- volume vessel (70) for containing at least part of the working fluid, said fixed-volume vessel (70) being in fluid communication with said volume-variable chamber (10), the apparatus further comprising a volume controller, the method comprising a step of controlling, by the volume controller, the volume of the one fixed-volume vessel (70) to adapt a compression cycle of the working fluid to periodic movements of the waves.

30. A method of harvesting electrical energy from sea waves according to claim 29 wherein the volume controller comprises a volume separator (76) hermetically separating the volume of the fixed- volume fluid chamber in a working volume (72) comprising part of the working fluid and a buffer volume (71), the method comprising a step of controlling, by the volume controller, a pressure of a fluid comprised in the buffer volume (71) so that the pressure of the fluid comprised in the buffer volume (71) is lower than the pressure of the working fluid.

31. A method of harvesting electrical energy from sea waves according to any of claims 29 to 30, wherein the at least one fixed- volume vessel (70) is inflatable, the method further comprising inflating the at least one fixed-volume vessel (70) to achieve a predetermined level of rigidity leading the at least one fixed-volume vessel (70) to be unresponsive to ambient pressure changes caused by sea waves consecutively passing over the apparatus.

32. A method of harvesting electrical energy from sea waves according to any of claims 24 to 31, further comprising: providing a plurality of volume-variable chambers (10) in multiple layers; and activating all of the plurality of layers, or a subset thereof, dependent on the prevailing sea state, such that a range of deformation of said deformable electroactive material (18) is maintained within a harvesting stretching range.

33. A method of harvesting electrical energy from sea waves according to claim 32 further comprising, before activating all of the plurality of layers or a subset thereof, selecting at least one of the volume-variable chambers (10), said selection depending on an amplitude of pressure variations resulting from the movement of consecutive sea waves and a mass of electroactive material that can be deformed by said pressure variations while maximising the deformation ratio of said material, computing a mass of working fluid to be provided within the apparatus so that a deformation of the deformable electro active material (18) of the selected volume- variable chambers (10) reaches a predetermined value, preferably a value in the middle of a harvesting stretching range of said deformable electroactive material (18), and providing the computed mass of working fluid within the apparatus.

34. A method of harvesting electrical energy from sea waves according to any of claims24 to 33, further comprising:Rotating the activation of volume-variable chambers (10) or of layers of volume- variable chambers (10) so that the number of harvesting cycles can be evenly spread between the volume- variable chambers (10).

35. A method of harvesting electrical energy from sea waves according to any of claims 24 to 34, wherein the working fluid is air, and further comprising temporarily deflating the volume- variable chambers (10) to reduce the overall volume of the apparatus for the purpose(s) of storage and / or transportation and / or to protect the apparatus from the impact of storms.

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