GB2031527A - A device for extracting hydrodynamic energy from waves - Google Patents

Abstract

A buoyant platform (1) is moored to the sea bed by mooring lines (2). Mounted on the platform are a number of piston-and-cylinders (4), the pistons being connected to the mooring lines. As the platform is moved by a wave in one direction, the piston-and-cylinders pump hydraulic fluid to a high pressure reservoir (8) and as the platform is moved by a wave in the other direction, hydraulic fluid flows from a low pressure reservoir (6) to the piston-and cylinders. A sea condition monitor (15) measure parameters of the sea and feeds a signal to an intelligent controller (12) which thereby controls the pressure in the high pressure and low pressure reservoirs. An hydraulic pump (9), driving an electrical generator (10) is connected to be driven by flow of hydraulic fluid from the high pressure reservoir to the low pressure reservoir. <IMAGE>

Description

SPECIFICATION A device for extracting hydrodynamic energy from waves This invention relates to devices for extracting hydrodynamic energy from waves.

According to the present invention there is provided a device for extracting hydrodynamic energy from waves comprising: a buoyant platform; mooring means for mooring the platform at sea; pump means mounted on the platform and connected to the mooring means; a first reservoir for containing hydraulic fluid at a relatively high pressure; a second reservoir for containing hydraulic fluit at a relatively low pressure; hydraulic flow control means for causing the pump means to pump hydraulic fluid to the first reservoir when the platform is moved by a wave in one direction and for causing hydraulic fluid to flow from the second reservoir to the pump means when the platform is moved by a wave in the opposite direction; measuring means for measuring parameters of the sea adjacent the platform; a control device connected to control the pressure in the first reservoir and/or second reservoir in dependence upon the parameters of the sea determined by the measuring means; and hydraulic motor means connected to be driven by flow of hydraulic fluid from the first reservoir to the second reservoir.

Preferably the platform has a length and width of at least 30 metres. The platform may have a vertical mode natural frequency due to buoyancy of the water of 0.5 Hz.

The motor means preferably is mechanically connected to drive an electrical generator.

In the preferred embodiment the pump means comprises a plurality of piston-and-cylinders, the pistons of which are connected to the mooring means. The device may include hydrostatic bearing means acting between cylinder and the respective piston.

The device may include a polymeric coating on the interior of each cylinder and on the respective piston. The polymeric coating may include a matrix of a corrosion-resistant metal.

The device may include a flexible linkage between each piston and a piston shaft connected to the mooring means.

In one embodiment the mooring means includes an anchor plate having means to reduce or present horizontal movement thereof on the sea bed. Thus the mooring means may comprise a plurality of symmetrically arranged mooring lines, the lines being closer together on the anchor plate than on the platform.

The device may include piles for locating the anchor plate on the sea bed.

Additionally or alternatively there may be a concentration of mass around the periphery of the anchor plate.

The device may include an upstanding wall around the anchor plate for containing ballast placed on the anchor plate.

The platform may be hollow in which case it may be filled with a buoyant material.

The control device may be a minicomputer or microprocessor.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a plan view of a device according to the present invention for extracting hydrodynamic energy from waves; Figure 2 is an elevational view of the device of Figure 1; Figure 3 is a schematic diagram of a hydraulic energy conversion system of the device of Figure 1, Figure 4 shows graphically wave energy conversion efficiency against length of a platform of the device of Figure 1; Figure 5 is a section of an hydraulic piston-andcylinder of the device of Figure 1; Figure 6 is a plan view of a mooring system of the device of Figure 1; and Figure 7 is an elevational view of the mooring system of Figure 6.

A device according to the present invention for extracting hydrodynamic energy from waves comprises a floating platform 1 of buoyant construction having a rigid top surface. A plurality of anchor or mooring lines 2 pass over respective guide pulleys 3 on the platform 1 and each is connected to the end of a shaft of a respective hydraulic piston located in an hydraulic cylinder 4 which is securely fastened to the platform 1.

When employed in continental waters in a reasonably vigorous wave climate typical overall dimensions of the platform 1 might be 30 metres by 1.5 metres high and have a vertical mode natural frequency due to buoyance of the water of 0.5 Hz.

The edges of the platform 1 preferably are rounded to reduce wave reflection. Numerous variations to the specific shape illustrated in Figure 1 are possible.

However, shapes such as regular polygons having more than three sides and truncated vertices, or a circular shape are preferred when the direction of wave propagation is variable, so as to reduce tendanciestoward rotation of the platform about its vertical axis of symmetry, due to asymmetrical wave action. The closer the approximation to a circular shape, the lesser will be the tendency to rotate. The platform 1 can be of a hollow, relatively thin, steel plate construction with internal stiffening members and reinforcing structures (not shown) as necessary to provide sufficient support for the various elements to be mounted on the platforms and to provide adequate flexural strength for the platform 1 itself. To protect the platform from corrosion, the exposed outer surfaces are preferably covered with a non-corroding coating of, for example, polymeric material. The interior of the platform may be filled with a buoyant floating material to prevent flooding in the case of accidental rupture of the outer surface.

A closed cell polymeric foam material is quite suitable for this purpose. By generating the foam within the platform 1 so as to fill tightly all the free space, the flexural strength of the platform 1 can be significantly enhanced.

An alternative to mounting the various elements of the present invention on the top surface of the platform 1 is shown in Figure 1 is to mount therm within the platform 1 and provide the platform with suitable ports and closures to allow proper operation, and to facilitate maintenance.

The mooring lines 2 may be steel chains and/or cables or any other material having comparable flexibility, tensile strength and extensional modulus.

Particularly suitable for the mooring lines 2 is cabling of polymer bonded oriented alloy steel filaments since this has superior wear and corrosion resistance.

While Figures 1 and 2 show the hydraulic cylinders 4 in a horizontal attitude, a vertical attitude is also posible, and eliminates the need for the guide pulleys 3. However, a gimballed support system may be necessary if the cylinders 4 are in the vertical attitude to reduce or present large bending moments on the shafts of the pistons during operation.

Flexible fluid line connections to the hydraulic cylinders 4 are then necessary. In the case of vertically mounted hydraulic cylinders 4, extra long shafts of the pistons can be used in place of the mooring lines 2. While Figures 1 to 3 illustrate a device according to the present invention having four hydraulic cylinders, any number of cylinders could be used. Each piston-and-hydraulic cylinder is employed as a piston pump but could be replaced by a rotary pump and windlass arrangement.

As shown in Figure 3 each cylinder 4 is in communication with an hydraulic check valve 5 which permits hydraulic fluid from an air cushion low pressure reservoir 6 to flow into the respective hydraulic cylinder 4. An hydraulic check valve 7 connected to each cylinder 4 permits hydraulic fluid to flow from the respective hydraulic cylinder to an air cushion high pressure reservoir 8. An hydraulic motor or turbine 9 is hydraulically connected between the two reservoirs 6, 8 and mechanically coupled to an electrical generator 10.

In order to keep down the size and number of hydraulic cylinders needed, a relatively high fluid pressure is desirable. For example, the high pressure fluid reservoir 8 may operate around 70 kg/cm2 and a generator 10 may be 1 megawatt electric generator, 6 hydraulic cylinders 4 each having a cylinder bore of the order of 0.5 m. being required. A device according to the present invention with an output greater than 1 megawatt capacity is not expected to be functionally or economically attractice, and operating fluid pressures greater than 105 kg/cm2 are not anticipated.

While Figure 3 shows one high pressure reservoir 8 and one low pressure reservoir 6, which are shared by four hydraulic cylinders 4, any number of such reservoirs can be employed, connected either in series or in parallel.

An intelligent pressure controller 12 regulates the pressure of the air cushion in the reservoirs 6,8 depending upon information received from a sea condition monitor 15. The controller 12 obtains air from a high pressure source 14 and releases excess air through a vent 13.

A suitable alternative method for maintaining optimum pressures in the high pressure reservoir 8 and the low pressure reservoir 6 is to adjust the quantity of hydraulic fluid in each instead of the quantity of air. This alternative is particularly attractice in the case where the hydraulic fluid is sea water and significant leakage flows exist, since a source of high pressure makeup fluid is easiiy provided by means of a small auxiliary pump arranged to take water directly from the sea. However, filtering of the sea water is necessary to avoid fouling.

The sea condition monitor 15 can be any variety of continuous transmitting water level gauge such as bottom pressure sensing transducers, surface penetrating capacitance probes, wave riders, etc. While each device according to the present invention can have its own sea condition monitor 15, one such monitor could be shared between several devices which are all located in the same general sea location.

The intelligent pressure controller 12 receives a continuous signal from the sea condition monitor 15, the signal being proportional to the instantaneous water level in the vicinity of the monitor. This signal can be processed to determine such information as mean water depth, mean wave height, mean wave period, mean wave length as well as wave height, period and length statistical spectra. A control algorithm can then use such wave condition information to calculate the optimum hydraulic fluid pressure needed in the two reservoirs 6, 8 for maximum power conversion efficiency. The calculated optimum pressure then becomes the set point for a conventional closed loop pressure control system. To handle such a control algorithm, a small minicomputer or microprocessor is preferred.

While the controller 12 could be used to make rapid adjustments to the operating pressures of the hydraulic fluid, it is considered more practical only to employ it as a means of adjusting for the relatively slow changes that may occur in the sea condition overtime periods of the order of 1 to 2 hours. There are a number of suitable means for transmitting the signal from the sea condition monitor 15 to the controller 12, for example, direct electrical cabling and AM or FM radio wave transmission.

The valves 5, 7, the reservoirs 6, 8, the motor 9, the generator 10, the controller 12, the high pressure source 14 and the sea condition monitor 15 can be located on the platform 1 and can be dedicated to a single device according to the present invention or can be remotely located to service any number of devices.

Referring to Figure 5, there is illustrated a preferred form of hydraulic piston-and-cylinder having an extremely long maintenance free life. An hydraulic cylinder shaft 16 is mechanically coupled to a piston 17 by means of a flexible link 18. The flexible link 18 can be used because, in operation, it will always be in tension. The flexible link may be a mechanical universal joint, a wire rope or metal chain. A flexible link of rubbery material is les desirable since rubbery material tends to introduce a greater degree of unnecessary axial elasticity. If a wire rope cable is employed as the flexible link, it can be provided with a smooth polymeric coating and extended to take place of the shaft 16.

A pressurised working fluid space 19 is located at the end of the hydraulic cylinder 4 through which the shaft 16 passes. The working fluid in this case is well-filtered sea water, since uncollected leakage flows will be non-polluting to the sea environment.

However, any other incompressible fluid could be used if adequate means for collecting leakage flows is provided. A small annular space 20 is provided between the piston 17 and the internal surfaces of the hydraulic cylinder 4. A small annular space 21 is provided between the shaft 16 and the cylinder 4.

Fluid distribution channels 24 in the piston 17 allows a small leakage flow of working fluid, which is radially directed into the spaces 20, 21. By providing a series of three or more channels 24, which are symmetrically arranged about the longitudinal axis of the piston 17, and allowing radial discharge of fluid from at least two well-separated points along each channel, a hydrostatic bearing is formed within the spaces 20, 21 to maintain mutual concentricity and axial alighment of the piston 17 in the cylinder4 without direct physical contact.

Athin protective coating 23 of polymeric material is applied to the surfaces of the piston 17, the inner wall of the cylinder 4 and a shaft guide 24. The main purpose of the coating is to protect these surfaces from corrosion by the working fluid. The coating can be applied to the shaft 16 as well or any other surfaces that need such protection. Since the various components to which the coating is applied may undergo a significant amount of flexing during normal operation, a structurally compliant coating material is desirable. Various known polymeric substances are excellent materials for such an application, TEFLON (Trade Mark) in paricular probably being the best due to its extremely inert characteristics and exceptional ani-fouling properties.

Since thin coatings of polymeric material can be mechanically fragile and lacking in dimensional stability, it may be desirable physically to combine the polymeric material with relatively corrosionresistant metal. There are commercial processes for producing such polymeric/metal composite coatings. An example is a TEFLON/nickel alloy composite called NEDOX (Trade Mark). Such a coating is an excellent alternative to a pure polymeric material.

However, the specific polymer and metal used depends upon the choice of working fluid. When sea water is the working fluid, a combination of TEFLON and tin is preferred. Corrosion rates for tin in sea water are of the order of only 1.25 x 10-4 cm per year. Another possibility is TEFLON and a special nickel/chromium alloy containing small quantities of molybdenum and niobium which is used in underwater bearing applications in the off-shore oil industry. In fact, this alloy itself can be used as the coating 23.

A port 25 is located in the hydraulic cylinder 4 near the shaft 16 to permit flow of working fluit into and out of the hydraulic cylinder as the piston 17 travels back and forth along the longitudinal axis. A port 26 is located near the opposite end of the hydraulic cylinder 4 to allow the discharge of leakage fluid from the annular space 20. If desirable, the leakage flow from the annular space 22 and the port 26 can be collected, and the collected leakage flows combined and returned to the low pressure fluid reservoir 6, by means of a small auxiliary pump. The hydraulic cylinder 4 is closed by a plate 27.

The device according to the present invention and described above requires a mooring system not only to maintain a desired physical location at sea, but also to allow proper functioning of the device. A number of anchoring means can be considered, such as conventional ship anchors, massive deadweight anchors, piles driven into the sea bed, augers screwed into the sea bed etc. Most of these methods work best when the mooring forces have a relatively small vertical component. However, a device according to the present invention requires a large vertical mooring force for its proper functioning. Therefore, a modified reaction plate system is preferred as shown in Figures 6 and 7.

A flat horizontal reaction plate 28, rests on the sea bed 29 and has a top surface geometry which is similar to, but slightly larger than the bottom surface geometry of the platform 1. With such an arrangement, the same hydrodynamic action that produces lifting forces on the bottom surface of the floating platform 1 also produce downward acting reaction forces of a comparable magnitude which result in a natural means of holding the reaction plate 28 in place. The reaction plate 28 is made slightly larger than the floating platform 1 to allow tor some degree of horizontal offset as well as for other edge effects on performance. The reaction plate 28 is preferably constructed from non-buoyant materials such as steel or steel-reinforced concrete, so as to realise a degree of supplementary securing force due to the effective weight of the plate itself. In fact, the weight of the plate can be chosen so as to compensate entirely for any inefficiencies in the reaction plate system.

A plurality of cleats 30 are securely attached to the bottom surface of the reaction plate 28 and pressed into the sea bed indicated by reference numeral 29 by the weight of the reaction plate 28. The purpose of the cleats 30 is to discourage sliding of the reaction plate over the sea bed 29 due to unbalanced horizontal force components which act on the reaction plate 28 during operation of the device. The cleats 30 may have a number of shapes and they are preferably constructed from non-corroding metal, such as stainless steel, or MONEL (Trade Mark), but can also be constructed from carbon steel or other materials, and provided with a corrosion-resistant coating.

The action of waves passing over the reaction plate 28, produces a downward acting force fields which travels over the top surface of the reaction plate 28. At the same time, a corresponding force field of comparable magnitude and phase passes under the bottom surface of the floating platform 1.

However, the reaction forces in the mooring lines 2 are acting only at fixed locations on the surface of the reaction plate 28. Consequently moments are produced as a wave passes, which tend to first rotate the reaction plate 28 about one edge and then about the opposite endge. The magnitude of this effect can be reduced by arranging the mooring lines 2 so that under quiescent conditions, they are mutually up wards divergent in a symmetrical pattern as illustrated in Figures 6 and 7. Another means of reducing the tendency toward rotation of the reaction plate 28 about its edge is to distribute the excess mass of the reaction plate 28 such that it is concentrated at the edges. The concentrated weight can be in the form of a vertical peripheral wall which then provides a means of additional position stability because of the inertia of the water contained within the wall. Also, the contained water is easily replaceable with sand or other high density debris as an inexpensive means of increasing the weight of the reaction plate.

A plurality of piles 31 or similar devices having good horizontal stability are located about the perimeter of the reaction plate 28 in a symmetrical pattern. A plurality of elastic connectors 32 physically join the piles 31 to the reaction plate 28. The purpose of the elastic connectors 32 is to provide a repositioning effect in the event that the reaction plate 28 is significantly displaced, as may occur under violent sea conditions. The elastic connectors 32 can be simple coil springs as illustrated in Figures 6 and 7, or any of several alternative means, such as elastomeric rope, pneumatic springs etc. While a reaction plate 28 resting on the sea bed 29 is preferred, it can also be employed at any intermediate elevation between the sea bed 29 and the floating platform 1.

The operation of the device illustrated in the drawings will now be described. Having located the device in a desirable spot in the sea and made appropriate power transmission connections from the electrical generator 10 to an on shore or other energy using, distributing, storage or handling system, the device will proceed to extract and convert hydrodynamic energy from waves in the following manner. As the crest of a wave approaches the device, the water surface indicated by reference numeral 11 increases its elevation relative to the platform 1 causing an increase in its upward buoyant force. The increased buoyant force is matched by equivalent increases in tension in the mooring line 2.

This increase in tension in turn causes movement of the pistons in their respective hydraulic cylinders 4 and forces hydraulic fluid through the check valves 7 into the high presure reservoir 8.

As the crest of the wave passes, and the platform begins to settle into the trough of the wave, the slack in the mooring lines is accommodated by the flow of hydraulic fluid from the low pressure reservoir 6, through the check valves 5 and into the hydraulic cylinders 4, causing retraction of the pistons into the respective hydraulic cylinders and tightening of the mooring lines.

Thus energy is extracted and converte during a half cycle of each wave as the platform rides from trough to crest. The other half cycle is used for resetting the device for the next following half cycle.

The hydraulic fluid power represented by the press uredifferential and quantity of fluid in the two reservoirs 6, 8 is converted to electrical power by allowing fluid to flow through the hydraulic motor 9 which in turn drives the electrical generator 10.

The sea condition monitor 15 transmits information such as wave hight, wave length, wave period and water depth to the intelligent controller 12, which regulates the pressure of the air cushion in the reservoirs 6, so as to achieve optimum power conversion efficiency regardless of varying sea conditions.

Energy conversion efficiency is enhanced if the platform has a length and width similar to the length of the waves, and a mass sufficiently low to produce a natural frequencyforvertically directed mechanical oscillation due to the buoyancy of the water which is high compared to the wave propagation frequency.

While the foregoing efficiency conditions are not necessarily apparent from existing literature, they are well supported by tests that have been conducted by the present inventor. Figure 4 shows graphically the result of such tests where various platforms of high natural frequency were employed to show the effect of platform length on power conversion efficiency. Other tests have been conducted which show that the optimum hydraulic fluid pressure in the high pressure reservoir 8 for maximum energy conversion efficiency depends upon wave height, wave length and water depth. The device can accommodate changes in water level when sufficiently long hydraulic cylinders 4 are employed.

Claims (19)

1. A device for extracting hydrodynamic energy from waves comprising: a buoyant platform; mooring means for mooring the platform at sea; pump means mounted on the platform and connected to the mooring means; a first reservoir for containing hydraulic fluid at a relatively high pressure; a second reservoir for containing hydraulic fluid at a relatively low pressure; hydraulic flow control means for causing the pump means to pump hydraulic fluid to the first reservoir when the platform is moved by a wave in one direction and for causing hydraulic fluid to flow from the second reservoir to the pump means when the platform is moved by a wave in the opposite direction; measuring means for measuring parameters of the sea adjacent the platform; a control device connnected to control the pressure in the first reservoir and/or second reservoir in dependence upon the parameters of the sea determined by the measuring means; and hydraulic motor means connected to be driven by flow of hydraulic fluid from the first reservoir to the second reservoir.

2. A device as claimed in claim 1 in which the platform has a length and width of at least 30 metres.

3. A device as claimed in claim 1 or 2 in which the platform has a vertical mode natural frequency due to buoyancy of the water of 0.5 Hz.

4. A device as claimed in any preceding claim in which the motor means is mechanically connected to drive an electrical generator.

5. A device as claimed in any preceding claim in which the pump means comprises a plurality of piston-and-cylinders, the pistons of which are connected to the mooring means.

6. A device as claimed in claim 5 including hydrostatic bearing means acting between cylinder and the respective piston.

7. A device as claimed in claim 5 or 6 including a polymeric coating on the interior of each cylinder and on the respective piston.

8. A device as claimed in claim 7 in which the polymeric coating includes a matrix of a corrosionresistant metal.

9. A device as claimed in any of claims 5 to 8 including a flexible linkage between each piston and a piston shaft connected to the mooring means.

10. A device as claimed in any preceding claim in which the mooring means includes an anchor plate having means to reduce or prevent horizontal movement thereof on the sea bed.

11. A device as claimed in claim 10 in which the mooring means comprises a plurality of symmetrically arranged mooring lines, the lines being closer together on the anchor plate than on the platform.

12. A device as claimed in claim 10 or 11 including piles for locating the anchor plate on the sea bed.

13. A device as claimed in any of claims 10 to 12 in which there is a concentration of mass around the periphery of the anchor plate.

14. A device as claimed in any of claims 10 to 13 including an upstanding wall around the anchor plate for containing ballast placed on the anchor plate.

15. A device as claimed in any preceding claim in which the platform is hollow.

16. A device as claimed in claim 15 in which the platform is filled with a buoyant material.

17. A device as claimed in any preceding claim in which the control device is a minicomputer or microprocessor.

18. A device for extracting hydrodynamic energy from waves substantially as herein described with reference to and as shown in the accompanying drawings.

19. A device for efficiently extracting hydrodynamic energy from ocean waves, comprising a floating platform structure, an anchoring means which is connected to the piston shafts of an array of hydraulic cylinders within the platform structure which act through a network of check valves as piston pumps for transferring hydraulic fluid from an air cushioned low pressure reservoirto an air cushioned high pressure reservoir, a sea condition minitor and intelligent controller which regulates the air cushion pressure in the reservoirtanks so that optimum energy conversion efficiency is achieved for any given sea condition, and a hydraulic motor or turbine mechanically coupled to an electric generator and hydraulically connected between the two resevoir tanks which produces electricity as fluid is allowed to flow through it from the high pressure reservoir to the low pressure reservoir.