GB2460832A - Rocking buoy wave energy converter with submerged turbines - Google Patents

Abstract

A wave energy extraction device comprises a submersible sealed buoy 10 with a cylindrical wave energy catcher 12 that projects upwards from the buoy through the wave (surf) zone when the buoy is submerged. The wave energy catcher 12 is driven by wave impact to cause the buoy to rock and is symmetrically shaped so that it can be driven by waves from any direction. A plurality of water turbines 14 are mounted externally about the periphery of the buoy so that rocking of the buoy 10 causes water flow through the turbines which can extract energy from that movement. The turbines 14 may be separated by vanes 16 to control water flow. The turbines drive hydraulic pumps which supply pressurised fluid to an electricity generator within the buoy 10.

Description

Wave Energy Extraction Device

Technical Field of the Invention

The present invention relates to the extraction of energy from sea waves and, in particular, to the generation of electric power therefrom.

Background of the Invention

The generation of renewable energy from natural resources is the subject of increasingly intense study and development activity. Many prototype and full scale facilities have been built, employing wind, tidal and hydroelectric power. Wave energy has also been recognised as a promising potential source of renewable energy and several attempts have been made in the past decades to design and build wave energy extraction devices. Such devices harness wave motion to drive an electricity generator, the output of which may then be fed to shore via a suitable electric cable.

Such devices do not depend on tidal rise and fall but employ semi submersible slack anchored structures, whose heaving or rolling motion is converted to rotation of one or more turbines which in turn drive one or more generators.

One of the earliest such proposals, known as "Salter's Duck", is described in US patent 3928967 to S. Salter for an "Apparatus and method for extracting wave energy". This uses, as a basic component, an energy removing member (rotor), rotatably mounted on a submerged floating platform (stator). The member, shaped rather like an inverted comma, rotates like a bobbing duck as a result of impacting waves. This produces partial relative rotation between rotor and stator which causes water at high pressure to be pumped through internal manifolds to drive a power generator, not shown in any detail, which may be a turbine for generating electricity. Banks of rotors and stators may be linked together by pivot couplings to multiply the output proportionately. However, Salter's devices need to be oriented roughly orthogonally to the direction of the waves in order to work.

A later proposal to extract energy from waves is described in GB patent application 2056574A to Rinaldi for "Toroidal modular captors of the energy of sea waves". This shows a semi submersible toroidal vessel, designed to float upright with the plane of the torus vertical with respect to the surface of the sea. A minor part of the torus ring is above the surface and the major part is below. The toroidal vessel is anchored loosely so that it is free to move with wind and current up to the limits of an anchor chain. The swell of the waves causes the vessel to pitch. The pitching motion causes an internal plunger within a ballast chamber formed by the lower part of the toroid to move back and forth within the chamber and pump sea water under pressure into alternate pressure chambers. The pressurised sea water drives hydraulic turbines which in turn drive electricity generators.

Another approach to wave energy extraction is described in US patent 4221538 to A.A.Wells for a "Rotary Transducer. This shows a so-called "Wells turbine" in which rotary turbine blades are driven in the same direction by airflow in either axial direction. This is applied to a wave energy system in which a part-spherical buoy is open to the sea and supported on top of a submerged spherical ballast chamber. Rise and fall of the sea inside the buoy forces air out of the top of the buoy through a nozzle in which is located such an air driven unidirectional turbine. The system is neutrally buoyant and designed to float while anchored by a mooring chain. One variant mentioned is that multiple transducers (turbines) fed by parallel air streams may be employed.

A further patent to Wells EP 0037408B1 for "Wave Energy Apparatus" shows another buoy in the form of a quasi-spherical open topped flask which is designed to extract wave energy not only from heave but also from rocking movements. This has a single turbine in an upper nozzle fed by three circumferentially disposed ducts. At their lower ends, the ducts flare out around a trapped air space and open out into a shared water filled lower space. Heaving of the sea drives air in the same direction through all the ducts and pitching or rolling causes outflow of air through at least one duct and inflow through another, each flow driving the turbine in the same direction.

A yet further patent application to Wells WO 9832967 for a "Wave Energy Converter" employs, a ring shaped buoyant body, partly filled with and floating flat on the surface of the sea and surrounding a central turbine and generator core. There are four turbines at 90° positions, each driven by a duct from respective subdivided chambers which communicate via one way valves. "Sloshing" of water within the body of the ring under rolling conditions causes pressure changes in the chambers which drive air through the respective turbines. It is stated that "this configuration is capable of extracting energy from any azimuth direction....". Also of note, this patent application does mention the possibility of driving the turbines with water flow instead of air flow, though it states that "the slow and faltering changes in water pressure make this generally less preferred".

Finally, it is noted that deep water vessels known as FPSOs (Floating production, storage and offloading) which are employed in the offshore oil extraction industry also have to withstand severe weather conditions at sea. Stresses caused by movement under extreme weather conditions can cause a great deal of damage to the FPSO itself and to any subsea connections, as the FPSO strains against its moorings. A known solution to this problem is the use of internal turret systems about which the body of the vessel can rotate and which are connected to slack mooring chains. Cabling and piping can then be fed out through the turret without being damages by the motion of the vessel

Disclosure of the Invention

The abovementioned prior art does not provide an efficient device with turbines which can be driven by sea water and yet which is insensitive to wave direction.

Accordingly, the present invention provides a wave energy extraction device for generating electric power from sea waves comprising: a submersible sealed buoy; a wave energy catcher projecting upwardly from the buoy when the buoy is submerged, the catcher being driven by wave impact so as to cause the buoy to rock and being symmetrically shaped so as to react substantially identically to waves from any direction; a plurality of water turbines disposed externally about the periphery of the buoy so that water flows through at least one turbine to cause it to rotate whatever the axis of rocking; and electricity generation means within the buoy for producing electricity at an output thereof in response to rotation of the turbines.

Thus, the turbines are directly driven by their motion through the sea water to provide a low speed but high torque drive for the electricity generation means within the buoy. Placing them on the periphery of the buoy, rather than internally, increases the arc of travel and relative velocity of the water through the turbines. All the prior art devices rely on a more complex internal system of air or pressure displaced water to drive the turbines.

Preferably, the buoy is round, the turbines being disposed around a circumference of the buoy but the buoy could also take a regular polygonal shape. Either shape ensures an omnidirectional response of the turbines to the plane of rocking but the round shape is structurally the stronger. The optimum robust shape for the buoy to withstand adverse sea and weather conditions is a sphere. A sphere also has the advantage of being inclined to roll without too much water resistance when the catcher is driven by the waves.

Whatever the shape of the buoy, the turbines are preferably separated from each other by intervening vanes. This helps to direct the water flow through them, as the buoy rocks.

Preferably, the wave energy catcher is cylindrical in form, the axis of the cylinder passing through the centre of gravity of the buoy. This is again the most robust shape for the catcher and gives an even omnidirectional response to the motion of the waves. Other shapes such as fluted or polygonal, which would still have a substantially omnidirectional response, can be envisaged for the catcher.

The turbines are preferably double acting so as to extract energy from their movement through the water in both directions. One suitable double acting turbine is a Wells turbine, which rotates in a single direction whatever the direction of incident flow. However, alternating pairs of oppositely oriented single acting turbines could achieve the same effect, albeit at a lower packing density. Another alternative might be a double set of turbine blades on a single shaft which feather or free wheel in opposite directions. Using a variable pitch blade is another double acting variation to achieve maximum packing density and hence efficiency.

To prevent twisting of mooring cables with resultant strains and potential for damage, a swivel linkage about which the buoy is mounted for rotation is a desirable feature, the linkage being adapted for attachment to an external slack mooring system. A preferred form of swivel linkage is a turret internal to the buoy including bearings to permit relative rotation. The mass of the overall device and the anticipated sea-state would largely overcome any loss of efficiency caused by the drag of the turret and mooring. The motion concept is of a slow roll with high angular momentum and hence torque. Alternative swivel linkages such as ball and socket joints between sections of cable or at the attachment point of a cable to the buoy can also be used but are more suitable for lighter weather conditions.

Similar considerations apply to the connection of the electricity generating means to an external electric output cable attached to the external mooring system. This must permit relative rotation between the output and the cable, for example by means of a slip ring.

In the preferred device, each turbine has an output shaft coupled to a hydraulic pump to form a modular unit external to the buoy, the fluid output of the pump being ducted through the wall of the buoy to drive a hydraulic motor, the electricity generating means including an electric generator coupled to be driven by the hydraulic motor.

A preferred arrangement is for there to be one hydraulic motor and generator per modular turbine/pump unit, though it would also be possible that all the turbines and their respective pumps might feed a single hydraulic motor and generator via a manifold.

It is preferred that the output shaft extends from each end of the turbine and that there is a hydraulic pump on each end of the output shaft.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to preferred embodiments thereof as illustrated in the accompanying drawings, in which: Figure 1 is an isometric view of a wave energy extraction device according to the invention; Figure 2 is a side elevation, partly in section, showing how an anchor chain and electric output cable are connected to the device of Figure 1; Figure 3 is a schematic block diagram illustrating the main energy/power transmission sequences in a wave energy extraction device according to the invention; Figure 4 is a schematic cross section of a wave energy extraction device according to the invention attached to a mooring system; Figure 5 is a cross section through a turbine module suitable for use in the device of Figure 4; and Figure 6 is a view on line "A" of Figure 5.

Detailed description of the Invention

Figure 1 is an exterior view of one embodiment of a wave energy extraction device according to the invention, as it would appear when deployed at sea. The device is a relatively massive structure, comparable to a merchant ship in size. The main body of the device consists of a submerged ballasted floating sphere 10. The sphere is attached and controlled in the water via an anchor chain 11 which is fixed to the seabed, although it will be recognised that a single chain may not be adequate for deep water/offshore applications. Attached on top of the sphere is a cylinder 12. It is the cylinder section, protruding through the wave splash zone above the surface, which absorbs and transmits the wave forces/movements to the device. The single chain is envisaged for shallow/near shore applications.

Also attached around the equator of the sphere, on a horizontal plane, is a submerged ring 13. Within this ring are housed water turbines 14 located around a 360 degree circumference. Only the end cap and supportive cross struts are visible in the illustration of Figure 1 and the turbine blades can not be seen.

As shown in more detail in Figures 5 and 6, the optimal turbines are of "Wells" design, which rotate in the same direction, irrespective of the direction of fluid flow. The Wells turbine blades are flat and orthogonal to the axis of rotation. Three dimensional wave forces acting on the cylinder are transferred to the sphere, making it roll which in turn projects the attached turbines near vertically through the water in a reciprocating arc, causing seawater to flow through them. It should be noted that because "Wells" turbines respond to bi-axial flow, there is a double acting energy transfer/extraction for each direction of roll of the device.

As shown more clearly in Figure 2, the anchor chain 11 incorporates a watertight swivel joint 15 to relieve torsional strains. This is required to relieve twisting effects on the chain when the device rotates on its axis which could result in excessive torsional stresses in the chain. An electric cable 17, running alongside the chain 11, carries electricity, generated internally from the rotation of the turbines caused by the rolling motion of the device, out to the sea bed through a slip ring arrangement 18. The slip ring is housed within the swivel joint and similarly avoids torsional stresses in the cable. The cable is clipped to the anchor chain by cable clips 19 so that they do not twist around each other.

Referring again to Figure 1, as the waves impact on the exposed vertical cylinder the subsequent reactionary movement of the device results in a predominantly two dimensional rocking or rolling motion, although some heave is anticipated. The cylinder acts as an omnidirectional driver on which waves can impinge with similar effect from any incident source angle, that is, within 3600. The ballasting and shape of the device are such that it tends to right itself after being rocked by a wave and to overshoot in the opposite direction.

The device therefore is free to move and predominantly to roll in two of its six degrees of freedom (three translational and three rotational) although, as mentioned, it can also rise and fall, to some extent, and rotate on its vertical axis. The device is restricted to lateral movement within the given limits of the anchor chain tension/slack. The main motion contributing to power extraction is therefore the rolling of the device with minimum heave. This global rolling of the device translates into a reciprocating vertical arcing motion of the turbine ring 13 which results in each submerged turbine 14 being projected bodily through the water. Guide vanes 16 are located at either side of each turbine to channel the water flow more efficiently. The axial flow of water through the turbines, consequently cause the turbines to rotate.

As mentioned, the turbines are housed around the equator of the spherical section in the turbine ring 13, attached to the outside. Rocking of the device due to wave action, results in water passing through the submerged turbines in a double-acting manner. Note that in electricity generating operation, the turbines should always be under water, even after the most extreme roll, with the water level located above the top guide vanes.

In operation, it is anticipated that the cylinder will protrude through the splash zone, that is, the range of maximum to minimum wave height. In calm conditions, the water level would be half way up the cylinder. The total height of the cylinder is a trade off between overall device buoyancy (ballast) and extraction potential -the higher the cylinder, the more the mass driving the sphere. The cylinder height should be chosen such that the maximum height wave expected never reaches the top as, if waves go over the top of the cylinder, energy is lost. As well as the rolling motion, the device will heave in relation to swell. Because of the large mass of the device, it will find its own natural frequency in relation to applied wave frequency, much like any FPSO ship, and will heave up and down relative to the body of water in which it sits.

In this instance, more water will pass through the turbines and contribute to energy extraction.

When waves are significant, the roll of the device will become the predominant effect.

However, there would still be a heave component.

The movement of the device and the anchor tension will be linked but not over-restrictive such that the chains are in full tension all the time. This should not inhibit the roll in heavy sea. The wave energy extraction will be performed as waves impinge and the device finds its natural rolling motion/momentum. A combination of wave size and duration gradually causes the device to roll at its own natural reaction period.

No detail of the physical coupling of the turbines 14 to electricity generating equipment, internal to sphere 10 is shown in Figure 1. However, the principles of operation of a wave energy extraction device for electricity generation according to the invention, such as that of Figure 1, are illustrated schematically in Figure 3. Only the components directly in the power generation chain are illustrated. Further detail of the disposition of the generating equipment and its drive arrangements are shown in Figure 4.

Firstly, considering Figure 3, waves 20 impacting on the cylinder 12 of a wave energy extraction device, such as that of Figure 1, cause the spherical buoy 10 to roll which projects the turbines bodily in a vertical arc thus channelling sea water through the turbines, one of which is shown at 21. A positive displacement pump 22, located adjacent each turbine, is driven by an output shaft 23 of the turbine to pump a hydraulic fluid under high pressure through pipes or hoses 24 passing through the wall of the buoy. The pressurised fluid drives a hydraulic motor located within the sphere. The hydraulic motor's output speed is stepped up by gear box 26, the output shaft of which drives an electricity generator 27 to produce alternating current.

The AC so generated is connected by an electric output cable 28 to a transformer 29, also within the buoy, which transforms it to a voltage suitable for the transmission of power to the shore. The output cable 28 is connected via a slip ring arrangement, such as 18 in Figure 1, to a subsea cable 30 which leads to a subsea junction box 31. The junction box then supplies the power generated to the on shore electricity supply grid.

Multiple similar extraction devices may be located in arrays in any given selected sea area to concentrate the produced energy at one onshore location and cables from some of these devices are also shown terminating at the junction box 31. Such multiple unit arrays may also be positioned so as to reduce coastal erosion in problem areas. Existing, redundant, fixed platforms may also be suitable for acting as gathering stations for arrays.

A section through a wave energy extraction device operating in the manner of Figure 3 is shown in Figure 4. Identical numbers have been used to indicate identical parts.

Similarly to Figure 1, a spherical buoy 40 supports a cylindrical wave energy catcher or drive cylinder 41 which projects upwardly from the buoy when appropriately ballasted.

Ballasting is effected by means of conventional ballast tanks 43 which are filled with water which can be expelled by air as needed. A combination of ballasting and anchor lines 44, attached to anchor blocks 45 on the sea bed, keeps the drive cylinder 41 at the level of incident waves.

The power generation system conforms to that shown in Figure 3, that is, turbines 21 and pumps 22 are disposed around the equator of the sphere in turbine ring housing 46 and are separated by vanes 47.When the turbines rotate, they cause the pumps to pump hydraulic fluid under pressure through flexible feed and return hoses 24 to hydraulic motors 25 within the sphere 40. The hoses pass through watertight bulkhead penetrations 48 in the sides of the cylinder and of the sphere 40. For additional protection from the waves, especially at the cylinder level, the hoses can be run through rigid piping or through vanes 47. The hydraulic fluid then drives hydraulic motors 25 to drive generators 27 via gearboxes 26.The alternating current produced by the generator is transformed in transformer 29 and supplied to the electric output cable 28.

The output cable 28 is fed to a slip ring connector 49 which makes electrical connection with subsea cable 30, while allowing relative rotation between the cables as the buoy swivels round in the waves. The cable 30 is attached to one of the anchor chains by clips (not shown) and passes to one of the anchor blocks 45, from which it is led to the nearest junction box (not shown).

As previously discussed in connection with the device of Figure 1, it is important to prevent damage caused by torsional strains and twisting of the anchor cable or, as here, cables.

In the system of Figure 4, swivelling of the buoy is accommodated by mounting the sphere 40 on a central cylindrical turret 50 in the manner of an FPSO vessel. The buoy consisting of sphere 40 and cylinder 41 is free to rotate around the turret on roller bearings 51 while the anchor cables 44 are attached to the bottom of the turret by lugs 52. This arrangement has the advantage of allowing more than one anchor chain for safety.

One possible variant on this method of anchoring would be to have four anchor chains at approximately 900 angles to the buoy connect first to floats from each of which further chains would drop to anchor blocks on the sea bed. This mooring arrangement has been successfully employed in the positioning of heavy weather FPSO vessels.

Since the turret only rotates around a vertical axis, the drag of the anchor lines will tend to work against rotation of the device about horizontal axes. However, such an anchoring system is desirable for operation in very heavy seas at some distance from shore. The mass of the device and the characteristics of the deep ocean waves will still cause the device to roll and it will settle at a frequency determined by its mass and shape and the power and size of the waves.

A preferred way of mounting the turbines in modular fashion to assist with replacement is illustrated in Figures 5 and 6. In this arrangement, a Wells turbine comprising body 60 and blades 61 is mounted within an external casing 62. The turbine has a double ended output shaft 63 mounted in bearings 64. The bearing outer races are supported by cross struts 65. The two ends of the output shaft 63 are attached to respective pumps 66 via couplings 67. The pumps 66, shaft 63, bearings 64 and turbine body are located within an inner casing 68. This casing 68, -10 -together with end cones 69, while not watertight protects these components from damage caused by the repeated flow of water through the turbine. In fact, the external and inner casings 62 and 68 define a flow chamber through which the sea water flows bi-axially over the turbine blades 61, as indicated by the double ended arrows, as the buoy rolls. Ideally, the flow channel between inner and outer casings would not be parallel sided as shown but should taper towards the centre to increase the velocity of water across the turbine blades.

Inlet and outlet hoses 70 to the pumps are routed through one of the support struts.

These have plug-in/quick release watertight connections (not shown) to the hydraulic lines within the buoy to facilitate removal and replacement of the module.

An end view, along line A of Figure 5 is shown in Figure 6. This shows the end cone 69 and turbine blades 61, as well as the position of the cross struts 65, all within the internal casing 68.

If turbine modules have to be changed, in the event that a fault has developed, the whole energy extraction device would be de-ballasted allowing the turbine assembly of Figures 5 and 6 to be removed and a new unit installed and connected up. Alternatively, divers may be deployed to complete this task to avoid de-ballasting.

In summary, the wave energy devices described ensure robustness to enable energy extraction to be achieved in some of the harshest sea environments, where free unpolluted energy is at its most fruitful. To this end the devices described are of spherical construction with a vertical cylindrical driver attachment, as mentioned. The whole assembly is symmetrical to ensure a uniform response to source waves from any angle of incidence. Further internal and external stiffening/bracing than is shown may be used to strengthen and render the device fit-for-purpose when exposed to very heavy sea states. The devices move with the tide much like any fixed floating structure anchored by a modern mooring system and are free to move laterally due to current and tidal forces within a given restricted floating area limited by the anchor chain tension. As already noted, this has minimal effect on the local working roll of the device.

Claims (15)

1. CLAIMS1. A wave energy extraction device for generating electric power from sea waves comprising: a submersible sealed buoy; a wave energy catcher projecting upwardly from the buoy when the buoy is submerged, the catcher being driven by wave impact so as to cause the buoy to rock and being symmetrically shaped so as to react substantially identically to waves from any direction; a plurality of water turbines disposed externally about the periphery of the buoy so that water flows through at least one turbine to cause it to rotate whatever the axis of rocking; and electricity generation means within the buoy for producing electricity at an output thereof in response to rotation of the turbines.
2. 2. A device as claimed in claim 1 in which the buoy is round.
3. 3. A device as claimed in claim 2 in which the buoy is substantially spherical, the turbines being disposed around a circumference of the buoy.
4. 4. A device as claimed in any preceding claim in which the turbines are separated from each other by intervening vanes.
5. 5. A device as claimed in any preceding claim in which the wave energy catcher is cylindrical in form, the axis of the cylinder passing through the centre of gravity of the buoy.
6. 6. A device as claimed in any preceding claim in which the turbines are double acting.
7. 7. A device as claimed in claim 6 in which the turbines are Wells turbines.
8. 8. A device as claimed in any preceding claim further including a swivel linkage about which the buoy is mounted for rotation, the linkage being adapted for attachment to an external slack mooring system.
9. 9. A device as claimed in claim 8 in which the swivel linkage is a turret internal to the buoy including bearings to permit relative rotation.
10. 10. A device as claimed in either claim 8 or claim 9 further including electrical output connection means adapted to connect the output of the electricity generating means to an external electric output cable attached to the external mooring system while permitting relative rotation between the output and the cable.

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11. 11. A device as claimed in claim 10 in which the electrical output connection means includes a slip ring.
12. 12. A device as claimed in any preceding claim in which each turbine has an output shaft coupled to a hydraulic pump to form a modular unit external to the buoy, the fluid output of the pump being ducted through the wall of the buoy to drive a hydraulic motor, the electricity generating means including an electric generator coupled to be driven by the hydraulic motor.
13. 13. A device as claimed in claim 12 in which there is one hydraulic motor and generator per modular turbine/pump unit.
14. 14. A device as claimed in either claim 12 or claim 13 in which the output shaft extends from each end of the turbine and which includes a hydraulic pump on each end of the output shaft.
15. 15. A wave energy extraction device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.