WO1998032967A1

WO1998032967A1 - Wave energy converter

Info

Publication number: WO1998032967A1
Application number: PCT/GB1998/000283
Authority: WO
Inventors: Alan Arthur Wells
Original assignee: Applied Research & Technology Limited
Priority date: 1997-01-29
Filing date: 1998-01-29
Publication date: 1998-07-30
Also published as: AU5872998A; GB9701818D0

Abstract

A buoyant body (1) for a wave energy converter is generally ring-shaped. It is adapted to float horizontally. It can generate electricity and/or pumped water or other forms of energy.

Description

WAVE ENERGY CONVERTER

The present invention relates to a structure for harnessing waves, offshore or in a large body of inland water, for the purpose of transforming wave energy into electrical or another readily utilisable form of energy.

Since the 1970's there has been increasing concern about the earth's finite fossil fuel resource and about environmental pollution resulting from combustion of such fuels. This has led to considerable efforts to develop new and cost-effective means of obtaining energy from renewable sources. In particular, there has been much interest in how to derive usable energy from solar, geothermal, wind and wave sources. The latter two offer the prospect of generating electricity by directly converting the kinetic energy of wind or waves into electrical energy.

Theoretically, waves constitute an enormous reservoir of energy to be exploited. However, the random nature and the hostility of the marine environment pose significant problems to the practical realisation of a cost-effective wave-to-electricity energy converter.

Although deep water offers the largest incident power flows, it has been realised that the mass, rigidity and complex engineering necessary to enable fixed structures to extract power and to survive in that environment makes them uneconomic.

The applicants have previously filed International patent application No. PCT/GB 92/00711, published as WO 92/18704. This describes a number of different configurations of shallow- water wave collectors/generators, including some of toroidal design. These generators are fixed structures designed to rest on the sea bed.

It has previously been proposed to generate electrical power from a floating disc buoy having a horizontal or inclined channel beneath, so that movement of water in the channel as the buoy "rocks" in the waves can be harnessed to generate electricity.

Whilst the aforementioned and other earlier proposals represent significant advances in the development of a practical wave-to-electricity energy converter, in practice it is quite difficult to design a shape of wave collector structure which is efficient in converting wave energy to electrical or other energy and vet is able to withstand the strongest waves which are encountered perhaps once onlv in fiftv \ears

A number of earlier proposals for wave energy converters employ a semi-submersible (1 e floatable) vessel which adopts the shape of a vertical torus (1 e with a horizontal axis of symmetry) Such devices are known from GB-Λ-2 056 574, FR-A-2 435 387 and FR-A-2 436 070

Another buoyant device is disclosed in FR-A-2 375 463 In this apparatus, a tank of liquid is disposed on a pair of floats As the apparatus rocks in the waves, the liquid in the tank sloshes back and forth and this sloshing motion is converted into electrical energy

In WO 94/15096, there is described a device in which a plurality of interconnected units are attached to a respective float and as they bob up and down on the waves, fluid is passed from one unit to another, depending on their relative heights The movement of the fluid drives a turbine for the generation of electricity

Two mutually similar devices are respectively disclosed in WO 97/41349 and WO 97/41350 (both of which were published after the priority date of the present invention) In these, energy conversion is effected by movement of a working piston within a so- called acceleration tube which extends vertically under water, below a buoyant body (float)

None of the aforementioned devices is capable of harnessing each of the heave, pitch and surge of the waves For example, those described WO 97/41349 and WO 97 41350 are responsive substantially only to heave In none of these devices is liquid within the buoyant body (float) itself utilised for energy conversion A new form of structure has now been devised capable of fulfilling both strength and efficiency criteria This embodies a number of inventive concepts

Thus, a first aspect of the present invention prov ides a wave energy convener comprising a generally ring-shaped buoyant body and conversion means for converting wave energy incident upon the buoyant body when floating, into another form of energy, the buoyant body adapted to float substantially horizontally

The wave energy converter according to the present invention is capable of harnessing heave, pitch and surge motion whilst being sufficiently robust to withstand the testing conditions of the marine environment

The buoyant body, when in the floating position may be of any convenient shape but is preferably such that in this position, its height is less than its maximum width or diameter In any event, it may be of any shape in plan view (1 e from above), so long as it is generally ring shaped This means that it may be generally toroidal, 1 e in the form of a continuous circular tube (see Fig 7A), this being a preferred form However in plan view, such a tube could be in an endless polygonal, e g hexagonal (see Fig 7B), square (see Fig C), or triangular (see 7D) form Such a ring-shaped endless tube could even be ellipsoidal (see Fig 7E) Such ring-shaped forms are preferably endless but a short break could be provided in the ring (see Fig 7F)

In order to maintain buoyancy, it is generally preferred that the buoyant bodv should be at least partially hollow, although as explained further hereinbelow, many embodiments function by virtue of the buoyant bodv being at least partially filled with liquid However, forms which do not rely on use of liquid within the buoyant body could be made of a foamed or solid material of a density sufficiently low to enable them to float

In cross-section, the tube forming the ring shape can be of any convenient form e g circular semi-circular, elliptical, or polygonal The cross section need not necessarily be of uniform shape or area, around the ring, although such uniformity is preferred Unlike the toroidal devices described in GB-A-2 056 574, FR-A-2 435 387 and FR- - 2 436 070, the generally ring-shaped buoyant body is adapted to float horizontally, i e if floating on a substantially flat body of water, the buoyant body floats so that the plane of the ring is substantially level with, or parallel to, the surface of the water, whereas with the aforementioned known devices, the plane of the torus would be substantially at right angles to the water surface

Moreover, if the buoyant body is generally toroidal or has another shape having an axis of symmetry, it will adopt a position such that is the absence of waves or swell, the axis of symmetry is substantially vertical The aforementioned known vertical torus devices would adopt a position with their axis of symmetry parallel to the surface of the water

The '"eye^" of the generally ring-shaped buoyant body (i e the central "hole^") can be enclosed to form a central chamber Alternatively, a housing can be located inside or extending through the eye The conversion means and/or other ancillary equipment can be located within such central chamber our housing

The buoyant structure will normally float by being at least partially hollow but in manv embodiments, the hollow part will be partially filled with water or another liquid but floats when in use, so that air above the internal water or other liquid provides the necessary buoyancy If the buoyant body is partially filled with water this can be used to drive the conversion means by one of several arrangements which will be described in more detail hereinbelow Many embodiments do not provide communication between internal water and the water in which it floats but exceptions are possible and some are described in further detail hereinbelow

If the buoyant body is generally toroidal, this means it is generally in the shape of a (generally hollow) torus A torus is an annular or doughnut-shaped structure The geometric definition of a torus is that formed by rotating a circle of radius (a) about an axis in the plane of the circle at a distance (b) from the centre of the circle A torus i;> sometimes described as an "anchor ring" However, any generally toroidal structure does not have to be a perfect torus In fact, it is particularly preferred that the structure is formed as an isotorus An isotorus is a toroidal shell of uniform thickness having a modified circular cross-section so that it has constant axial membrane stress throughout A common example of an isotorus is a pneumatic tyre having a small πm diameter An approximate example is equivalent to a torus but being the volume swept out by a circle cut-off by a diameter or by a chord parallel to the axis of rotation A full description of an isotorus is given in WO 92/18704

A central structure for housing power generating and/or other equipment can be located inside the eye of the generally toroidal structure, preferably in a central chamber as described hereinbefore

A second aspect of the present invention provides a floatable structure for a wave energy converter, the structure being generally toroidal In combination with conversion means for converting wave energy to another form of energy, this provides a wave energy converter which optionally, may embody any of the aforementioned optional or preferred features of the wave energy converter according to the first aspect of the present invention The following apparatus features are preferred features of any wave energy converter according to the first aspect of the present invention or any wave energy converter embodying a floatable structure according to the second aspect of the present invention, where buoyant body ' and floatable structure ^' can be taken to be synonymous

When in use, the buoyant body forms part of a wave energy converter which also comprises means for converting wave energy to another form of energy One class of such converter converts the wave energy to electrical energy Another form converts the wave energy to mechanical" energv in the form of highly pressurised pumped water A device of the latter kind mav also comprise a subsidiary electrical generator for internal or subsidiary power purposes The pumped water can be used for purposes such as oil extraction or pumping or itself may be directed to another location, e g onshore for remote generation of electrical power Preferably, the type of energy converter primarily intended for generation of electrical energy comprises a buoyant bodv which is sub-divided to define a plurality of chambers, at least in an upper region thereof, e g by means of baffles which extend to below the internal water level Such baffles are preferably sealed to the inside surface of the structure However, they do not totally sub-divide the water into chambers, 1 e internal water flow beneath or through a lower part of the baffles is permitted Ducting means can then be provided for interconnecting the plurality of chambers to permit flow of air therebetween Thus, at least one turbine may be located in the ducting means for driving an alternator or generator The ducting means may comprise a plurality of air ducts a respective turbine being located in at least some of these air ducts

Preferably, the turbine is a self-rectifying turbine This means, a rotor which turns in the same direction, regardless of which direction the air is incident thereupon More preferably, it is a zero incidence self-rectifying turbine

One form of self-rectifying turbine is the so-called Darius rotor Essentially the Darius rotor comprises a plurality of aerofoil-shaped blades, preferably of symmetric cross-section, extending longitudinally between an upper and a lower support so as to form a pseudo-spherical cage

Another kind of self-rectifying turbine is described in UK Patent Specification GB 1 595 700

In the case of a wave energy converter intended to produce pumped water instead of electrical generation, the converter is provided with pumping means Such pumping means will normally consist of a valve arrangement for restricting direction of flow to be in the direction intended Pumping can be effected through suitable ducts which is preferably in the form of one or more hose pumps A hose pump is a flexible tube which contracts in diameter when axially stretched It is therefore capable of pumping if repeatedly stretched and released, provided at least one directional flow valve is arranged in the flow path more preferably an inlet valve (external or internal to the hose pump) at or near one end and an outlet (delivery) valve at or near the other end again internal or external to the hose pump rjer se, provided that it is also within the flow path In a preferred embodiment, the hose pumps are stretched from the periphery of the buoyant body to the sea bed. either directly in shallow water, or by intermediate connection to the mooring in deep water so that motion of the structure due to incident waves causes the hose pumps to be alternately stretched and relaxed Hoses which transmit hydraulic flows and possess the hose pump property are characteristically reinforced with appropriately handed double helical wire windings

In the case of the converter primarily designed to generate electrical energy, the buoyant body will have water or another liquid sealed inside, which water or liquid does not communicate with the body of water in which the structure is floating However, one form of water pumping device has the buoyant body partially filled with water which enters from the body of water in which the device is floating Valve means permitting water only to enter via an inlet and only to exit via an outlet means that the 'sloshing" of water inside the buoyant body, due to the incidence of waves, causes pumping Part of the valve means enables water only to flow out of the body through the outlet and to deliver it to a point of use by means of ducting, piping or a hose or other flexible delivery means Outlet valving may be provided anywhere along the length of such hose or ducting or may be located internally in the body of the wave energy converter

However, one or more hose pumps having a respective inlet attached to the outside of the buoyant bodv and in which water does not pass between the internal water and the body of water in which the structure floats, may also be employed for the water pumping function

A device primarily intended for producing pumped water may nevertheless have an auxiliary internal electrical generator for providing some subsidiary electrical power, for example to power the systems of the wave energy converter device Instead of the internal chamber/pneumatic turbine configuration referred to above for a converter primarily intended for electrical generation, such a subsidiary electrical power unit may be powered by water pressure, driven bv a hose pump which is re-entrant to the dev ice wherein the auxiliary generator is housed

The pumped water can be delivered to the bed of the sea or other body of water in which the device is floating, to provide forced extraction of oil deposits or to dri e sub-sea oil pumps, or for similar applications However, if the device is floating near the shore, the pressurised water could be pumped ashore to drive an electrical power generator on land

In passing, it should be noted that the form of device primarily intended for electrical generation, as described above, preferably uses changes in internal air pressure driving one or more relevant turbines However, the internal electrical power generator could be water-driven, driven by changes in internal water pressure as water sloshes within the buoyant body However, the slow and faltering changes in water pressure make this generally less preferred It is generally preferred to utilise changes in air pressure above the moving internal body of water, preferably above the surface of water in each of the respective chambers It should be noted that use of internal "sloshing" to pump water need not necessarily be performed with a generally toroidal structure

Thus, a third aspect of the present invention provides an apparatus for pumping water, the apparatus comprising a floatable structure (preferably to be partially filled when in use, with water or another liquid) and water pumping means being mechanically operable by movement of the structure when floating Preferably however, the floatable structure comprises a buoyant body of the kind which is one integer of the wave energy converter according to the first aspect of the present invention However, it could be any other known form of floating structure in which sloshing" of internal water occurs

Devices according to the first, second or third aspect of the present invention will normally be anchored by tethers to the bed of the body of water with anchor means but freely floating, at the site of operation

A preferred form of device according to the present invention comprises an enclosed floating, hollow, thick-walled toroidal structure, having a similarly enclosed hollow infill machinery space at its eye, and an articulated mooring connection below its centre with chain or cable flexible mooring to the seabed Passing wave undulations cause the buoyant body to incur pitch, heave and surge motions, most of which effects may be converted to electric or seawater hydraulic power for local use, either at the seabed or ashore For both deep and shallow water power extraction, the generally buoyant bodv may optionally be partially filled with an annular volume of water, limited bv overall floating stability considerations, which oscillates from side to side under the action of pitch, heave and surge of the external wave motions A first general form of embodiment designed for production of electricity, comprises at least two diametral equi-spaced internal dams provided within the torus, each of which encloses the remnant air space above the mean level of internal water, without substantially impeding water flow, so that air is displaced from side to side by oscillation of the water The displaced air is directed through at least 4 equi-spaced pneumatic fan- turbines at the eye, arranged with approximate radial axes, and these are conveniently provided with zero incidence aerofoil blades, so that they spin unidirectionally at high speed and extract power pneumatically in a self-rectifying manner Such turbines have been well-known since 1974, and are characteristically described as Wells turbines They may be directly connected to electric generators, or seawater hydraulic pumps, and benefit from total enclosure within the main pressure shell This configuration is capable of extracting wavepower from any azimuth direction and may also be used in shallower water with an appropriate mooring harness

A second general form of embodiment is designed to deliver pressurised seawater at the seabed (after anti-bacterial dosing), for water injection at sub-sea oil wells or for boosting flow of oil products in pipelines This generalised embodiment also provides for some local conversion to hvdrauhc or electric power

In an embodiment for shallow water power extraction near-shore, with a narrow arc of wave incidence directions, the eve of the torus is also enclosed The interior of the structure may be partially filed with water when in use, as for the pneumatic turbine application, but no internal dams need be provided, nor the pneumatic turbines Flexible mooring is similarly attached at the base, and long linear flexible hose pumps are extended from the base to seabed fastenings in the directions off-shore and ashore Each hose pump is closed at the outer end, with both the inlet and outlet valves at the inner end, and coupling to a delivery hose Flexible rubber hose pumps with pressure containing double helical wire windings have long been known and extension of hose pump lengths reduces internal volumes for pumping, and relaxation induces infill It has been estimated by the inventor that ideally, for a deep water generally ring-like device, power is required as an electric drive for downhole pumping operations. Whereas the electric power is required to be steady, the hydraulic surplus could reflect seasonal differences in wave flux. These requirements justify the direct use of hose pumps at high pumping efficiency, since the proportion drawn off for electric power would be applied to one Pelton generator of about lMw capacity, inside the eye of the structure, also operating at high efficiency compared with a group of 4 pneumatic electric generators.

The hose pump arrangement may retain the sloshing internal water volume with bactericide dosing, also providing internal corrosion protection. Internal dams can be eliminated,, so that no power is diverted pneumatically, but the sloshing water may provide tuned stiffness in surge, which is wholly mobilised to generate pitching motions of the device. The pitching motions are externally damped by the hose pumps so that the mathematical modelling of the device is unchanged from the pneumatic counterpart, and the sloshing amplitude is unchanged.

Pelton turbines can be directly connected to an alternator and, in turn through a hydraulic torque converter to a diesel generator, connected by oil filling the torque converter at infrequent times of low wave power. The main component of alternating electric power would pass to the downhole pumps, with a smaller component frequency charged or rectified to supply ancillary power for control, dosing etc., using battery storage if required.

A small number of hose pump lengths are preferred, since this both minimises horizontal force components on the tension mooring at the bunched hose pump ends, and reduces the total number of valves in terms of infrequent maintenance. Independence of wave azimuth direction would be retained. Advantages of the configuration are ( 1) improved efficiency (>80%) of power conversion, both hydraulic and electric. (2) elimination of internal dams at the device, (3) elimination of 4-8 fan generators of large diameter, with large structural openings in the isotorus shell, and efficiencies of conversion of about 50-60%, leading to laree air frictional heat release. and corresponding temperature rises with closed circuit air flows and (4) fewer more compact plant items for installation and removal through the top shell of the device These advantages offset the need for diver assembly of the hose pumps

When anchored in shallow water, the system is also appropriate for direct pumping of seawater ashore to a Pelton generator Thus, with further manifolding at the seabed, hydraulic power can additionally or alternatively be passed ashore from a group of collectors Hydraulic power transfer has the merit of greater efficiency than a pneumatic-electric counterpart, and the hose pump system permits the power buov to be almost passive with regard to conversion machinery requirements on-board, and facilitates maintenance

It has also been shown in wavetank model studies that arrays of such devices offer significant coastal protection from wave erosion It may also be noted that clean seawater at high hydrostatic pressure possesses sufficient intrinsic energy for practical application of the vacuum vapour compression-condensation desalination process, without use of extra heat

Both general forms of embodiment of wavepower collector have a negligible visual profile, and similarly bypass a larger fraction of extreme wave force than a comparable fixed collector Extreme wave impulsive forces are also more flexibly resisted by partial mooring restraint, so that limited recoil is permitted

The generally toroidal shape is significant in terms of its high external pressure strength when overtopped by extreme waves in storms, a feature not exhibited bv equivalent flat ellipsoids of revolution with vertical axes, although the latter have also been shown to function well as converters of wavepower The generic form of these devices may be considered to be point or disc absorbers, which have certain limitations to be described

The reference power absorbed by a disc absorber is limited by its diameter in relation to the input from an impinging wave crest of length equal to one ore more diameters It has been shown bv Evans and Srokosz that a disc absorber responding only to heave has a limiting capture width, l_ma given by 1/k, where k = co^" g g is the gravity constant and o is the angular frequency of incident waves Thus as co zzz> 0 \_mάK zz> x and at anv particular tuned frequency ω₀, the absorption length is g ω₀ ^" However, when the tuned non-dimensional wave-number, k_f,a, (= ω₀ ^" a g, where a is the device radius), is less then 0 5 the corresponding amplitude of heave motion required to extract the maximum power available will be in excess of twice the wave amplitude and this factor provides the practical limitation on device design

If the disc absorber mobilises surge and/or pitch motions in addition to heave in its power extraction mode the limiting capture width l_ma is increased to 3 k The absorption length under these conditions is thus 3g/ω The non-dimensional capture factor l/2a for a disc absorber of diameter 2a at anv frequency ω is thus given by 3/2ka (= 3g/2ω^"a) It should be noted that the maximum possible value of l/2a is not necessarily at the tuned non-dimensional wave-number koa Although l/2a touches the curve \_maκ 2a at k^a higher values of l/2a can occur, the only restriction is that the curves for l/2a all lie below the curve for l_ma-/2a and touch that curve at k₀a As with pure heave mode extraction there is a practical limitation on the minimum tuned non- dimensional wave-number, l^a of approximately koa=0 6, below which the amplitudes of motion for the disc absorber in the surge and heave modes would exceed a factor of two on incident wave amplitudes, as illustrated by Srokosz

For typical ocean waves centred on a period of 10 seconds, o = 0 6283 and l_ma„ = 74 6m if a practical limit of k^a = 0 6 is adopted it may be shown that the typical device radius, a is given as 15m so that the potential advantage of a disc absorber mav be appreciated, in terms of compact structural dimensions

The stiffnesses of the device which engenders resonance due to wave action is traceable to flotation, influencing both heave and pitch, and the equivalent internal oscillatory surge stiffness of the annular water, mooring stiffness is also included The objective from dynamic analysis is then approximately to match the resonant periods through each of these influences with the incident wave period Power output is determined through the damping constant at the internal water due to the pneumatic turbines or the hose pumps It has been discovered from calculations such as described with verification from models in wave testing tanks, that a sufficient coincidence of natural periods in pitch, heave and surge depends critically upon the relative internal and external water levels The relationship is predictable, sustainable but complex in that a simple geometric expression cannot readilv be derived at the present time Nevertheless, a device such as described can be designed to possess a capture factor significantly greater than unity, even as an annual average in random waves conforming with an observed spectral description This may be contrasted with the previously known floating buoy with a central vertical or inclined oscillating water column, displacing air through a self-rectifying pneumatic turbine, or moored through hose pumps where only heave power is mobilised, and the capture factor is in practice significantly less than unity

The requirements both for stability and coupled resonances of the preferred toroidal form of device suggest useful secondary shape modifications, as shown Thus, the simplest is to limit the diameter of the internal annular water volume through interposing a coaxial conical diaphragm, with an enclosed air buoyancy space beyond it An alternative is to substitute a so-called isotorus cross-section (as mentioned above) which is both deeper and has a cylindrical eye, so that the internal water volume is placed lower The added advantage of the latter is that the pressure shell shape may be proven to sustain equal biaxial membrane stresses throughout, from external pressure difference, it therefore forms a stronger, more economical structure

The physical effect which permits pitch, heave and surge wave oscillations to be converted into surge-pitch oscillations of the annular internal water and of the device itself, may be more clearly demonstrated by observing the calculated wave particle motions near the πm of the device Where a plane incident wave has circular orbits in deep water, these orbits are modified by the circular shape of the device, and may be better described in terms of locally convergent waves for which the hydrodynamic treatment may be traced to Cauchy and Poisson early in the 19th century The resulting calculated inclined elliptic orbits of wave particles at the front and rear of the device which can be plotted, indicate that surge and pitch are relatively close in phase and in quadrature with heave, so that the expected motion of the device is pitch about a centre near the rear

Application of these criteria of engineering mechanics and hydrodynamics permits the design of a wavepower conversion device which is robust in terms of resistance to extreme waves, yet economic to construct and place in deep or shallow water, efficient with regard to power conversion, and therefore may be claimed to be productive of electric or hydraulic power at a cost competitive with most fossil fuel systems

It is also possible for the device to be provided with a further rotor intended to be driven by wind power, preferably positioned at or near the top of the structure

The present invention will now be explained in more detail by reference to the following description of a preferred embodiment and with reference to the accompanying drawings in which -

Figure 1 shows an axial cross-section through a first embodiment of wave energy converter according to the present invention, intended for generation of electrical power,

Figure 2 shows a radial cross-section through the first embodiment of wave energy converter shown in Figure 1,

Figure 3 shows a perspective view of the first embodiment of a wave energy converter according to the present invention, when tethered for operation,

Figure 4 shows an axial cross-section through a second embodiment of a wave energy converter according to the present invention ,

Figure 5 shows the anchoring and hose pump distribution arrangement of the second embodiment of wave energy converter shown in Fig 4,

Figure 6 shows an axial cross-section through a third embodiment of a wave energy converter according to the present invention, and

Figures 7A-7F shown plan views of different shapes of generally ring-shaped buoyant bodies which may be used in wave energy converters according to the present invention

As shown in Figure 1, a first embodiment of a wave energy converter according to the present invention comprises a hollow isotoroidal buoyant structure 1, an upper part 3 of which is normally visible above the sea level 5 in which the structure is floating A lower part 7 of the structure is mostly beneath the surface of the sea The structure is partially filled with water 9 up to a level 1 1 approximately occupying the lower quarter of the toroidal structure The remaining upper portion 13 constituting about three quarters of the structure, contains air which provides for the necessary buovancy As seen better in Figure 2, the toroidal structure 1 is divided internally into four chambers, 15, 17, 19, 21 by four vertically extending baffle plates 23, 25, 27, 29 (not seen in Figure 1 ). These baffle plates extend partially below the surface 1 1 of the internal water so that it is only the air above the water 9 which is completely subdivided into chambers.

The body of air in each of the respective chambers communicates with the other chambers by way of respective air ducts 31, 33, 35, 37 and a respective self-rectifying turbine 39, 41, 43, 45 is located in each of these turbines. The air ducts and turbines communicate with each other via a central structure 47 in the "eye" 49 of the buoyant toroidal structure. This central structure 47 is sub-divided into various rooms or compartments for housing control and ancillary equipment.

Each of the self-rectifying turbines drives a respective generator of electricity

The rocking motion induced by waves causes the internal water 9 to "slosh" from one side to the other, between the chambers 15, 17, 19, 21. The "air column" above the water in each chamber 15, 17, 19, 21 thus changes in volume and pressure, causing air to flow between the upper parts of the chamber, through the respective ducts, 31, 33, 35, 37. Since the turbines are self-rectifying turbines, they produce substantially constant motion to drive the generators.

The device floats and is anchored to the seabed by means of chain anchors 51, 53. A power cable 55 conveys the generated power to the point of use.

As shown in Figure 3, when anchored in place, the total wave energy converter device may be provided with a helicopter platform 57 on the upper portion thereof and a wind-powered subsidiary electrical generator unit 59, also extending from the upper surface thereof.

The electrical power from the cable 55 may be distributed via a distribution network 61 on the seabed, to power production wells 63 for oil extraction.

A second embodiment of wave energy converter according to the present invention may be seen in Figures 4 and 5. Where integers are substantially the same as those of the first embodiment, the same reference numerals are used. This second embodiment is analogous to the first embodiment shown in Figures 1-3. except that the isotoroidal buoyant body 71 shown in this embodiment is not sub-divided into compartments There is no air ducting corresponding to the ducts 31, 33, 35, 37 in the first embodiment The internal water 9 enters through an inlet 73 in the lower region 75 of a central structure 77 in the 'eye" 79 in the centre of the buoyant structure The water entering through inlet 73 enters the toroidal structure via a valve 81 and then water ducts 83, 85 Since the upper part of the toroidal structure is sealed, buoyancv is maintained The valve prevents water from exiting the structure

The "rocking" of the structure due to wave motion causes water to enter via the inlet 73 and valve 81 only, and only to exit via hose pipes 87, 89 which communicate with the internal water, one respective end 91, 93 of each hose pipe being located in the lowermost regions 95, 97 of the toroidal structure Valve means (not shown) ensure that water can only exit and not enter through these hose pumps This pressurised water can be transmitted to the sea bed to provide forced oil extraction However as seen better in Figure 5, a proportion of it can be redirected upwardly through a distribution valve 99 and then, up a central hose 101 to drive an internal generator 103 in the central structure 77 of the device, powered by a Pelton turbine, discharging to the interior or exterior

A third embodiment of wave energy converter according to the present invention is shown in Figure 6

In this third embodiment, a hollow toroidal buoyant body 1 1 1 surrounds a central cylindrical spar 113, an upper portion 1 15 of which extends above the buoyant body 1 1 1 The central spar floats free inside the buoyant body 1 1 1 and hoses 1 12, 1 14 as shown are part of the water circulating system and not rigid links A lower portion 1 17 of the central spar 1 13 extends below the buoyant body 1 1 1 and below the surface 5 of the sea in which the apparatus is floating The wave energy converter is anchored to the sea bed bv means of anchor chains 1 19 attached to the lowermost part 121 of the central spar 1 13

Lower hose pumps 123, 125 communicate with ducting (not shown) inside the lower portion 1 17 of the central spar 1 13, via respective non-return valves 127, 129 similarly upper hose pumps 13 1 133 communicate with ducting (not shown) inside the upper portion 1 15 of the central spar 1 13 The hose pumps take water \ la inlets 135 137 form water 1 1 which partially fills the buoyant body 1 1 1

The hose pumps operate according to the same principles as those of the second embodiment and drive a Pelton turbine and associated electrical generator (not shown) housed inside the central spar 113

A sump (not shown) for the circulating water is also housed inside the lower part 1 17 of the central spar 1 15 Although as described, the circulating water is contained within a closed loop, in an alternative of this third embodiment, the circulating water can be inlet from and discharged into the surrounding sea Similarly, the water can be inlet to the hose pumps directly from the sea without first being stored inside the buo ant body 1 1 1, which then is not partially filled with water

In the light of this disclosure, modifications of the described embodiments, as well as other embodiments, all within the scope of the present invention as defined by the appended claims, will now become apparent to persons skilled in this art

Claims

1 A wave energy converter comprising a generally ring-shaped buoyant bodv and conversion means for converting wave energy incident upon the buovant bodv when floating, into another form of energy, the buoyant body being adapted to float substantially horizontally

2 A wave energy converter according to claim 1, wherein the conversion means is arranged to be driven by liquid within the buoyant body

3 A wave energy converter according to either preceding claim wherein the buoyant body is generally toroidal

4 A wave energy converter according to any preceding claim, wherein the eye of the generally ring-shaped buoyant body is enclosed to form a central chamber

5 A wave energy converter according any of claims 1-3, wherein a housing is located inside or extends through the eve of the generally ring-shaped buoyant body

6 A wave energy converter according to claim 4 or claim 5, wherein at least part of the conversion means is housed within the central chamber or housing respectively

7 A wave energy converter according to any preceding claim, wherein the conversion means is functional to produce electrical energy

8 A wave energy converter according to claim 7, wherein the buoyant bodv is internally subdivided to define a plurality of chambers at least in an upper region thereof, ducting means being provided for interconnecting the plurality of chambers to permit flow of air therebetween

9 A wave energy converter according to claim 8 wherein at least one turbine is located in the ducting means

10 A wave energy converter according to anv of claims 1 -6 wherein the conversion means is functional to pump water

1 1 A wave energy converter according to claim 10, wherein the conversion means comprises at least one hose pump

12 A wave energy converter according to claim 1 1, wherein at least two hose pumps are provided, one of which is connected to an internal hydraulic turbine electrical generator

13 A wave energy converter according to claim 1 1 or claim 12, further comprising a water inlet to permit entry of water to said buoyant body and at least one water outlet respectively communicating with at least one hose pump

14 A wave energy converter according to any preceding claim, further comprising anchor means for anchoring the converter while allowing it to float

15 A wave energy converter according to claim 14, when anchored bv the anchor means

16 A wave energy converter according to claim 14 or 15, wherein the structure is partially filled with water or another liquid

17 A wave energy converter according to anv of claims 14 to 16, when dependent upon anv of claims 8- 10, wherein a hose pump connects the converter with the bed below the bodv of water for pumping water into said bed

18 A method of extracting oil from the ground beneath a bed of a body of water, the method comprising pumping water into said bed using a wave energy converter according to claim 17

19 An apparatus for pumping water, the apparatus comprising a buoyant body (preferably to be partially filled when in use with water or another liquid) and water pumping means being mechanically operable by movement of the structure when floating

20 .An apparatus according to claim 19, wherein the buoyant body is generally toroidal