EP2347120A2

EP2347120A2 - Device for a winch-operated wave-energy-absorbing buoy

Info

Publication number: EP2347120A2
Application number: EP09737187A
Authority: EP
Inventors: Ingvald Straume; Morten Sandnes; Arne Johannes Mo
Original assignee: Straumekraft AS
Current assignee: Straumekraft AS
Priority date: 2008-10-17
Filing date: 2009-10-12
Publication date: 2011-07-27
Also published as: NO20084372L; NO329152B1; WO2010044674A2; WO2010044674A3

Abstract

The invention relates to a wave-power plant where a floating buoy (23) arranges for energy absorption from the ocean waves. The buoy is anchored by a wire (14), which is reeled in on a self-tightening winch (12). When the wave motion(s) lifts the buoy, the winch cable drum is forced to rotate outwards. This rotating power motion is directed into a mechanical energy conversion subsystem and a hydraulic subsystem (20, 21) where the energy is converted by mechanic and hydraulic means and then stored in a hydraulic accumulator (8), from which a smooth flow of power may produced by a hydraulic power-take-off motor (10) coupled to an electric generator (28). The hydraulic means for converting the rotational energy also incorporate the mechanism that spools in the winch and keeps it tight, by using some of the stored energy in the accumulator to wind in the winch using a pump / motor as an agent for forcing the winch to rotate inwards, when the wave forces that previously forced it to rotate outwards fall off. The hydraulic subsystem may also provide overload protection of the plant by means of a pressure-limiting valve (6), limiting the maximum pressure of critical components. The mechanical energy conversion subsystem (20) may furthermore involve a slip clutch (16) between the winch cable drum and the hydraulic subsystem, which protects the power plant and the components in it against extreme loading during incidents of violent waves. The slip clutch determines the amount of energy per time unit that can be absorbed from a given wave, by slipping if too heavy torque or too high speed is attempted transmitted through it. Thereby, the wire is pulled out without offering increased resistance, and the buoy simply drifts with the wave until the wave has passed, with the result that the excess energy is left in the wave.

Description

DEVICE FOR A WINCH-OPERATED WAVE-ENERGY-ABSORBING BUOY

BACKGROUND OF THE INVENTION

The patent literature describes over 1000 devices for converting the motions of ocean waves into useful energy. Commercial exploitation of ocean wave energy has not yet been realized. Many of the design proposals will probably work, in the sense that they will be able to convert some of the wave energy into useful energy. Some of the concepts that have been tested in practice, have actually proven to work, in that sense. But the problem is that none of the proven concepts have been able to produce enough energy compared to what they cost to build and maintain. In other words: The specific energy cost (cost per produced kWh) is too high in these earlier concepts.

This patent application presents a design of a winch based wave energy absorbing buoy, where a self-tightening winch, mounted on or otherwise connected to the buoy, serves as anchoring system, and at the same time provide energy absorption. The system also comprises an overload protection strategy based on the simple principle of not letting more energy into the system than the system itself can handle. The self-tightening-mechanism of the winch is furthermore integrated in the energy-conversion and power-take-off system, which helps to lower the design costs of the wave power plant.

KNOWN TECHNOLOGY ON WHICH THE INVENTION IS BASED

The invention comprises the following elements and subsystems: a wave energy absorbing floating body or buoy, a winch, well known hydraulic components and standard means for converting hydraulic flow and pressure into electricity, among others. But the system assembly has certain characteristics, which the parts and subsystems do not have separately.

The key point is that when the various elements are put together in this particular arrangement, the parts and subsystems together make up the fundamental basics of a wave energy absorption and conversion system that has the ability to survive in extreme weather, and the potential to become profitable due to lower building costs.

Separately or put together differently, the elements and subsystems are not capable of solving the problem addressed by the invention described herein: to exploit energy from ocean waves with sufficiently low cost design of the plants without the plants and the components therein being destroyed by extreme waves. Winch-operated wave-power plants

There are several examples of wave-power systems based on wave energy absorbing floating buoys, where energy is transmitted mechanically, by means of a wire rolling on a drum. See, e.g. US 2005/0121915 and GR 990100030. However, these are missing the compact and integrated design to help keep building- and maintenance-costs sufficiently down. They also lack the overload-protection means which are necessary to allow the plants to survive the encounter with the most extreme waves in the worst stormy conditions without requiring to have such a robust design that they become unprofitable.

Slip clutch(es)

The fundamental principle of overload protection provided with the invention is about limiting power through-put by simply "letting go" and not absorbing more energy from the waves when maximum power input limit has been reached, so that the amount of energy conducted into the system never will become excessive. This fundamental principle has never elsewhere been described as part of a winch- anchored buoy-based ocean wave power absorption- and power conversion system's survivability strategy in extreme waves.

One of the overload protection means described in this document, to execute this principle in practice, involves a slip clutch 16 which is disengaged when the amount of energy per time unit transferred from the buoy 23 via the wire 14 and the winch 12 through the winch axle 18 has reached a certain upper limit.

The use of slip clutches in wave-power plants is mentioned in DE 2850293, WO 96/30646 and US 4228360. But these lack the necessary characteristics in order for a wave-power plant, without incurring unreasonably large design costs, to be capable of surviving the encounter with the at times extreme forces of the ocean waves in the event of storms and hurricanes.

DESCRIPTION OF WAVE-POWER TECHNOLOGY

The invention will now be described in more detail by means of examples of embodiments and with reference to the accompanying figures. In the figures, similar items have the same reference numbers.

Figure 1 shows one example embodiment of a system for energy conversion, energy storage and power-take-off according to the invention.

Figure 2 shows a cross section side view of one embodiment of the wave-power plant, where the winch 12 is placed inside the buoy 23. Figure 3 is a bottom view of the wave-power plant in the embodiment corresponding to figure 2.

Figure 4 shows a winch 12 with a slightly conical cable drum.

Figure 5 shows another example embodiment of the system for energy conversion, energy storage and power-take-off according to the invention.

Overview of the invention

The device according to the invention comprises a wave energy absorbing floating buoy with energy absorption- and conversion system connected to a generator, which may be placed inside the buoy. Figures 1, 2, 3, 5 and 6 illustrate the principle of the device according to the invention. A floating buoy 23 (fig. 2) acts as absorption element. This buoy is connected to a winch 12 with a winch wire 14. The buoy 23 and the winch 12 with the winch ,wire 14 are connected in such a manner that the winch is forced to rotate when the wave forces move the buoy 23 in the winch wire's longitudinal direction. The winch and the winch wire interconnect the buoy and a reference body below the waves of the ocean surface. This reference body may be a pelagic anchor plate, an anchor 19 at the seabed as shown in figure 2, an expansion bolt in the rock of the seabed, or a different anchoring device. In the embodiment shown in figure 2, the winch is in the buoy. But the winch may also, instead of being integrated in the buoy, be placed elsewhere, for example at the seabed or in a pelagic anchoring device. The energy absorbed from the waves when the winch is forced to rotate, is transmitted in the form of rotating motion from the winch axle 18 to a mechanical energy conversion subsystem 20, powering a hydraulic subsystem 21. In the hydraulic subsystem 21, the absorbed energy is converted into hydraulic potential energy in the form of p ^• V (pressure multiplied by volume) inside an accumulator 8, where a compressible fluid serves as energy storage medium. The accumulator powers a hydraulic motor 10, which in turn runs an electric generator 28. The accumulator also powers the winch-tightening mechanism which is integrated with the mechanical energy conversion subsystem and the hydraulic subsystem, ie. that the mechanical energy conversion subsystem and the hydraulic subsystem comprises functionality which inherently provides winch tightening. The fundamental functionality of this integration is that spooling- in the winch wire a certain length requires lesser volume of fluid to flow out of the accumulator, than the volume of fluid which is pumped into the accumulator when the winch wire is pulled out an equal length. Different approaches can be used to accomplish this functionality. The hydraulic subsystem and possibly parts of the mechanical energy conversion system may be assembled in such a manner that the displacement of the pump(s) is higher on outward rotations of the winch than on inward rotations. Throughout this document the term 'displacement' refers to pump volume capacity per revolution of the pump shaft. This may be accomplished by means of a variable displacement pump and a governing system which alters the displacement of the pump according to the direction of the hydraulic flow at any given moment, or by a system assembly like the one exemplified in figure 1, where instead of one pump, two pumps are connected to the mechanical energy conversion system; smaller pump 11 with a relatively lower displacement and a larger pump 3 with a relatively higher displacement. Both pumps may draw fluid from the same atmospheric pressure fluid reservoir 1. When the winch wire is pulled outwards (e.g. by rising wave motions lifting the buoy 23 up), both pumps pump fluid under high pressure into the hydraulic accumulator 8. The smaller pump 11 also acts in the opposite direction as a motor, powered by the accumulator, forcing the winch to wind in, when the forces that pull the wire out drop sufficiently. On inward rotations of the winch, the larger pump 3 is disengaged due to check valve 7, preventing fluid from flowing from the accumulator 8 through pump 3 back into the fluid reservoir 1, and a one way clutch 29 mounted on the shaft connecting the mechanical machinery 20 to the pump 3. Or a mechanical gear transfer system may be applied, that alters the gear ratio to provide a higher rotational speed of the pump(s) compared to the rotational speed of the winch axle 18, on outward rotations of the winch, than on spooling in. Such a mechanical gear transfer system may for example be an assembly of gears and clutches, like the one exemplified in figure 5, where the clutches arrange for the transfer of rotational motion to be channelled through a gear path with a higher gear ratio on outward rotations than the gear path through which the rotational motion is channelled on inward rotations. By employing a mechanical gear transfer system where the gear ratio is altered as the rotation is reversed, one fixed-displacement pump/motor 22 is sufficient. Now, this pump/motor acts as a pump on outward rotation of the winch, and as a motor on inward rotation of the winch.

The mechanical energy conversion subsystem

The mechanical energy conversion system 20 converts mechanically the rotational energy from the winch 12, and transfers that rotational energy to the hydraulic subsystem 21. The main purpose of the mechanical energy conversion system is to ensure that the mechanical energy transferred to the hydraulic subsystem has the qualities necessary to make the hydraulic subsystem function optimally with the highest possible efficiency. An additional purpose of the mechanical energy conversion system is to provide for winch tightening in co-operation with the hydraulic subsystem, as explained further down. In the embodiment according to figure 1, the mechanical energy conversion subsystem comprises the following parts: • a shock absorbing rubber link 13 in the extension of the winch axle 18

• a gear transmission system 15 in the extension of the shock absorbing rubber link, gearing up the rotational motion of the winch axle, by using the rotational motion from the shock absorbing rubber link as input, providing higher rotational speed, and consequently lower torque, at the output.

• a slip clutch 16 connected to the output of gear transmission system 15

• a gear transmission system 17, in the extension of slip clutch 16, gearing up the rotational motion further

• a one-way clutch (overrunning clutch) 29 mounted on one of two ends of the output shaft from gear transmission system 17

All the above listed elements in this mechanical energy conversion system, except the one-way clutch 29, are optional, and may be omitted, without taking away the fundamental functionality of the invention. They may, however, contribute to lowering the overall design costs of the system, by being present. Another example embodiment of the mechanical energy conversion subsystem 20 is shown in figure 5, where the gear transmission system 15' s output shaft 35, which is a gear-penetrating axle, has a second overrunning clutch 34 connected to one side, and where the slip clutch 16 is connected to the other side of the axle 35. The slip clutch 16 is further connected via an axle 25 to the first overrunning clutch 29, which is connected via an axle 27 to gear transmission systems 17 and 33, where the rotational motion is geared up and transferred — on outward rotations of the winch — to a gear-penetrating axle 26 which is also connected directly to axle 35 through the second overrunning clutch 34. Both gear transmission systems 17, 33 must be of the same type with regards to rotational direction of the output shaft in relation to the input shaft's rotational direction: Either none of them must cause direction reversal, or both have to cause direction reversal, in order to provide that the output axle 26 from gear transmission system 33 and the output axle 35 from gear transmission system 15 run in the same direction. The two overrunning clutches are coupled so they work in each other's reverse directions. On outward rotations of the winch, rotary power is transferred from the winch through the first overrunning clutch 29, whilst on inward rotations, rotary power is transferred to the winch through the second overrunning clutch 34. In other words: the first overrunning clutch 29 is engaged when the winch wire is pulled out, and disengaged on spooling in the winch, and vice versa for the second overrunning clutch 34. The axle 26 may be described as a fast-rotating axle, whilst the axle 35 may be described as a slowly- rotating axle, due to the fact that axle 26 rotates faster than axle 35 on outward rotations of the winch, because: on outward rotations, the first overrunning clutch 29 is engaged, directing rotational power through gear transmission systems 17 and 33, whilst the second overrunning clutch 34 is disengaged. On spooling in the winch, the two axles rotate at the same speed, because then they are interconnected directly by the second overrunning clutch 34, which now is engaged, whilst the first overrunning clutch 29 is disengaged. Gear transmission system 33 may be omitted, thereby improving the efficiency of the mechanical energy transmission- and conversion system in cases where the extra gear transmission system 33 is considered unnecessary, provided that the rotary direction of the output shaft 24 of gear transmission system 17 is not reversed. The transmission of rotational power from axle 24 to axle 26 could then be achieved by a belt drive or a chain drive. To reduce mechanical energy loss, such a drive may be omitted, and the axles 24 and 26 merged into one, provided that axles 35, 25 and 27 with the gear transmission system 17 and the corresponding slip clutch 16 and overrunning clutches are designed and arranged so that the axles 24 and 35 are aligned with their centre line along the same line. This could be accomplished by using an epicyclic gear system and by employing hollow axle-cylinders and having axles enclosed in each other.

The hydraulic subsystem

The hydraulic subsystem 21 comprises in the embodiment of figure 1 the parts listed below. The parts are coupled together by hydraulic pipes or hoses, arranged as shown in the hydraulic diagram of figure 1 :

• a larger hydraulic pump 3 connected to the output shaft of gear transmission system 17 via one-way clutch 29 in such a manner that the pump is activated when and only when the winch 12 rotates outwards, in other words: when the winch rotates in the direction that allows the winch wire 14 to roll out, and only when the winch rotates in that direction.

• a check valve 7, which permits fluid to flow from the pump 3 into the accumulator 8 or through the pressure limiting valve 6, but not in the opposite direction.

• an accumulator 8, containing a compressible fluid (e.g. nitrogen gas) which cannot escape the accumulator, so that that compressible fluid is forced to be compressed when fluid is pumped into the accumulator. • optionally: a pressure limiting valve 6, which under normal circumstances is closed, so that fluid pumped by pump 3 or pump / motor 11 cannot escape anywhere but through the hydraulic power-take-off motor 10, but which opens when the pressure in accumulator 8 increases to a certain level, then allowing fluid pumped by the pumps to bypass the hydraulic power-take-of motor, conducting the pumped fluid straight back into the fluid reservoir 1.

• a smaller hydraulic pump / motor 11 mounted directly on the output shaft of gear transmission system 17, acting as a pump when the winch 12 rotates outwards, otherwise acting as a motor.

• optionally: a flow control 31 between the accumulator 8 and the smaller pump / motor 11 that can be used to control the flow of fluid from accumulator 8 to pump / motor 11 when pump / motor 11 acts as a motor, and a check valve 32 allowing fluid pumped from pump / motor 11 to bypass flow control 31 when pump / motor 11 acts as a pump.

• a sequence valve 30 blocking fluid from flowing from the accumulator 8 and/or the pumps 3 and 11 into power-take-off motor 10 at pressure below a certain minimum threshold, but allowing it to flow at pressures above that threshold.

• optionally: a flow control 9 providing for a smooth and constant flow of fluid into the power take off motor, and also allowing the flow of fluid into the power-take-off motor to be regulated externally, manually or automated.

• a hydraulic power-take-off motor 10 which outputs steady, high speed rotational energy to a generator 28.

• optionally: a return line filter 5

• a breather 4

• optionally: a level/temperature gauge 2 • an atmospheric pressure fluid reservoir 1 supplying pump 3 and 11 with fluid when pumps are activated

In the embodiment shown in figure 5, the hydraulic subsystem contains fewer parts: there is only one pump 22, with a larger displacement, like pump 3 in the embodiment according to figure 1. The pump 22 is connected to the output shaft 26 of the mechanical energy conversion subsystem, according to figure 5. The pump 22 also serves as a motor, winding in and tightening the winch, like pump / motor 11 in the embodiment according to figure 1. But because the mechanical energy conversion system now contains a variable gear transmission mechanism with two reversely coupled overrunning clutches 29 and 34 and a step-up gear system 17, 33 functional only on outward rotations of the winch, the effect of the pump 22 acting as a motor when spooling in the winch wire, is as though it had a lower displacement. In this embodiment, there is no need for the check valve 7. However, a double-pipeline with a flow control 31 in one of the courses and a check valve 32 in the other course mounted in the same direction as check valve 7 in the embodiment according to figure 1, may optionally be included between the pump 22 and the accumulator 8 to control the activity of pump 22 when operating as a motor.

Power-take-off

The hydraulic subsystem powers a generator 28. One of the problems of exploiting wave energy is that the energy is unevenly distributed over short periods of time. In a particular wave, which typically has a wave period (wave cycle duration) of 4 - 15 seconds, a buoyancy body can only absorb energy in a part of the wave cycle. A generator needs a steady and invariable power input, in order to function optimally and to be able to generate electric power at required quality. A steady power input to the generator is provided by the accumulator 8 and optionally the flow control 9. The accumulator temporarily stores energy from the waves, so that it can be produced as a steady stream of fluid by the power-take-off motor 10. The accumulator inherently helps to equalize the differences. The larger size of the accumulator, the better equalization of the differences of power input. An unlimited large accumulator would, as such, be a perfect device for equalization. An unlimited large accumulator is, however impractical. Optionally, therefore, a flow control 9 can be employed to stabilize the speed of the power-take-off motor further. Also: Stabilizing the speed of the power-take-off motor regardless of the pressure in the accumulator, may be accomplished by using a variable displacement motor as the power-take-off motor 10, where the displacement is controlled and adjusted by mechanical, hydraulic or electronic feed-back-mechanisms based on measuring the speed of the generator 28, by means well known to engineers. Furthermore: By employing a variable displacement motor as the power-take-off motor 10, and by designing the generator 28 to allow the energy per time unit absorbed by the generator, and hence the torque produced by the generator on the axle connecting the power-take-off motor to the generator at any speed rate, to be altered — which can be done by means well known to power-engineers — the power-take-off motor and the generator together, will have great flexibility to drain out a wide range of power rates from the accumulator 8. This may make the system efficient in a wide range of different wave conditions.

The hydraulic winch tightening mechanism

Any winch-operated wave energy absorbing buoy-system will need a mechanism for tightening the winch. The winch in the invention described herein, is self- tightening. The arrangement of hydraulic and mechanical components provides for this. In the embodiment according to figure 1, it works like this: The accumulator 8 maintains a certain minimum pressure thanks to sequence valve 30. When the wave forces that pull the wire are sufficiently low, the pressure from the accumulator will power the smaller hydraulic pump / motor 11, now acting as a motor, causing the winch 12 to rotate inwards, tightening the wire. Due to the one-way clutch 29, the larger pump 3 will, unlike the smaller pump / motor 11, not rotate inwards with the winch. When the wave forces are strong enough to overcome the spool-in force from the pump / motor 11, the winch will rotate outwards, causing both the larger pump 3 and the smaller pump / motor 11 to rotate with the winch, pumping fluid from fluid reservoir 1 into the accumulator 8. The smaller pump / motor 11 preferably has a lower displacement than the larger pump 3. As a result, because the amount of fluid, and thus the amount of energy, directed into the accumulator when the winch rotates outwards a certain length, always is greater than the amount of fluid, and energy, directed the opposite way through the pump / motor 11 when spooling in the winch equally, the accumulator will always hold sufficient amount of pressure and energy to keep the winch tight at any time. The extra energy pumped into the accumulator on outward rotations of the winch, powers the power- take-off pump 10 and the generator 28. In the embodiment according to figure 5 the effect is the same, but brought about by means of the two counter-coupled overrunning clutches 29 and 34 and the extra gear ratio provided by gear transmission systems 17 and 33, making any extra pump / motor redundant.

Overload protection mechanism(s)

The basic principle for protecting the plant, and the parts and subsystems contained in it, against overload, is simple: When the amount of wave energy per time unit which encounters the buoy is excessive, the buoy simply does not absorb that energy. This is made possible by designing an energy conversion and absorption system which inherently limits the amount of energy per time unit that can be channelled into the system.

Different design approaches may be employed to accomplish this, as proposed in the following text. Regardless of the design, the basic concept is that the wave- power plant should be capable of withstanding the worst extreme waves because it does not try to resist the waves when the wave forces therein become too great, but instead gives way and allows most of the power in the extreme waves, the destructive energy peaks, to pass and remain in the sea.

Overload protection by hydraulic means

The proposed overload protection mechanism is engaged by a hydraulic pressure- limiting valve 6 in the hydraulic subsystem. When the wave forces on the heaving buoy causing the winch to rotate outwards, become excessive, the pressure limiting valve 6 will open, allowing the pump(s), 3 and 11, or 22, to discard the extra energy input which otherwise could damage the wave-power plant, by directing the flow of fluid from the pump(s) straight back into the reservoir 1, bypassing the power-take- off pump 10. The pipeline(s) or hose(s) leading from the pump(s), 3 and 11, or 22, through the pressure-limiting valve 6 to the reservoir 1, and the pressure-limiting valve itself, have to be sufficiently wide, to accomplish this.

Overload protecting slip clutch

The hydraulic overload protection arrangement described above, will set a maximum limit for how of high pressure the hydraulic system can be exposed to, provided that the check valve 7, the pressure limiting valve 6 and the pipes or hoses leading from the pumps 3 and 11 back into the reservoir 1 are dimensioned correctly. This arrangement will, however not limit the speed which the pumps 3 and 11 and the mechanical energy conversion subsystem 20 and the components in it, may be exposed to, caused by the wave motions.

In order to avoid excessive speeds in the system, a mechanical slip clutch 16 can be applied. This slip clutch is set to slip if the rotational speed or the torque transferred through it becomes to high. The slip clutch may be disengaged in events of extreme waves to protect the internal system from excessive speed, excessive forces and excessive energy input. Cone shaped winch cable drum

The winch tightening system should ensure that the winch wire 14 is tight at all times. However: during operation, special events may occur, which make the winch wire slow. This may for example happen when the buoy finds itself on top of a breaking freakwave followed by another freakwave. A slow wire is a problem. The wire is likely to wind in on the winch in a messy way in situations like that, and this may lead to greater wear and tear. To ensure that the wire is arranged properly again on the winch cable drum, after it has become disorganized by freak waves slowing the wire, a conical winch cable drum, as shown in figure 4, may be applied. By making the winch cable drum conical, and by attaching the winch wire to the slimmest end, the wire will slide towards the slimmest end, and arrange properly by itself. The gradient angle (α) on the cone can be equal to the friction angle between wire and drum when the wire is resting and not sliding on the drum (the static- friction coefficient). That is, tan a = μ, where μ is the coefficient of static friction between wire and drum, (α is the angle between the centre line of the winch axle and the peripheral edge line of the winch cable drum.) At this angle, the wire windings will remain close together without significant side load.

REFERENCE TERMS USED IN THE FIGURES:

1. fluid reservoir

2. level/temperature gauge

3. larger hydraulic pump 4. breather

5. return line filter

6. pressure limiting valve

7. check valve

8. accumulator 9. flow control

10. hydraulic power-take-off motor

11. smaller hydraulic pump / motor

12. winch

13. shock absorbing rubber link 14. winch wire

15. gear transmission system

16. slip clutch

17. gear transmission system

18. winch axle 19. anchor

20. mechanical energy conversion subsystem

21. hydraulic subsystem

22. hydraulic pump / motor

23. buoy 24. axle

25. axle

26. fast-rotating gear-penetrating axle

27. axle

28. generator 29. one way clutch (overrunning clutch) 30. sequence valve

31. flow control

32. check valve

33. gear transmission system

34. one way clutch (overrunning clutch)

35. slowly-rotating gear-penetrating axle

Claims

1 . A device for a winch-operated wave-power plant with a self-tightening winch (12) connecting a wave-energy-absorbing body (23) via a winch wire (14) to a reference point, where the winch cable drum is connected via a mechanical energy transmission- and conversion system (20) to a hydraulic subsystem (21) containing one or more pumps, c h a r a c t e r i s e d in that it comprises a hydraulic accumulator (8) where the hydraulic subsystem is arranged to pumping fluid into the hydraulic accumulator (8) as the winch wire is pulled out causing the winch cable drum to rotate outwards, thereby storing energy in the accumulator from which the energy may be conducted into a hydraulic power-take-off motor (10), the hydraulic subsystem and the mechanical energy transmission- and conversion system being arranged to spooling in the winch when the external spool-out force is low, by allowing the accumulator to direct some of its stored energy in the opposite direction using at least one pump, now as a motor, driving the winch cable drum inwards thereby tightening the winch wire.

2. A device according to claim 1, c h a r a c t e r i s e d in that outward rotations of the winch effected by external forces, cause a greater volume of fluid per revolution of the winch cable drum to flow into the accumulator than the volume of fluid per revolution of the winch cable drum that flows from the accumulator in the reverse direction on inward rotations, where "outward rotations" refers to rotations corresponding to the winch wire being wound out and "inward rotations" refers to rotations corresponding to the winch wire being wound in (and tightened).

3. A device according to claim 1, c h a r a c t e r i s e d in that the power-take- off motor (10) is connected to a generator (28) which generates electricity from the surplus of energy stored in the accumulator (8).

4. A device according to claim 1, 2 or 3 where the hydraulic subsystem comprises at least two pumps: a smaller pump (11) with a relatively lower displacement and a larger pump (3) with a relatively higher displacement, both drawing fluid from an atmospheric pressure fluid reservoir (1) pumping fluid under high pressure into the hydraulic accumulator (8) when an external spool-out force is applied to pull the winch wire (14) out, where the smaller pump can act both as a pump pumping fluid into the accumulator (8) when the winch wire is pulled out and in the opposite rotational direction as a motor to wind in the winch wire when the external spool- out force drops, and where the larger pump is activated only in one rotational direction, only when the winch wire is pulled out, due to a one way clutch (29) mounted on the shaft connecting the mechanical machinery (20) to the pump (3), c h a r a c t e r i s e d in that the winch tightening mechanism is integrated in the hydraulic subsystem along with the hydraulic power-take-off mechanism by means of using some of the accumulator's stored energy to power the smaller pump (11) which serves as a motor for tightening the winch.

5. A device according to claim 1, 2 or 3, c h a r a c t e r i s e d in that the winch tightening mechanism is integrated in the hydraulic subsystem along with the hydraulic power-take-off mechanism by means of having the hydraulic subsystem contain a variable displacement pump, that is: a pump where the amount of fluid pumped per revolution of the pump's shaft can be varied while the pump is running, where the pump's displacement is lowered on inward rotation of the winch cable drum and increased on outward rotation of the winch cable drum.

6. A device according to claim 1, 2 or 3, c h a r a c t e r i s e d in that the winch tightening mechanism is integrated in the hydraulic subsystem along with the hydraulic power-take-off mechanism by means of having the mechanical energy transmission- and conversion system (20) contain a variable gear transfer mechanism which allows the pump(s) to rotate at a higher speed compared to the rotational speed of the winch cable drum, on outward rotations of the winch cable drum, than on inward rotations of the winch cable drum.

7. A device according to claim 6 where the mechanical machinery (20) transferring energy in the form of rotational power from the winch (12) to the hydraulic subsystem (21), contains two overrunning clutches (29, 34) attached in opposite functional directions of each other to the same rotary output connected to the winch, coupled in such a way that rotational power from the winch to the hydraulic subsystem is transferred through the first clutch (29) only, on outward rotations of the winch, while the flow of power the other way, from the hydraulic subsystem to the winch, is transferred through the second clutch (34) only, on inward rotations of the winch, c h a r a c t e r i s e d in that the rotational power transmission mechanism from the winch via the first clutch (29) to the hydraulic subsystem contains one or more step-up gear transmission systems (17, 33) providing a higher rotary speed at the output on the hydraulic subsystem's side.

8. A device according to claim 4, characteri sed in that the hydraulic subsystem comprises a check valve (7) between the accumulator and the larger pump (3) blocking fluid from flowing from the accumulator back into the larger pump.

9. A device according to claim 1, 2 or 3, and according to claim 4, 6 or 7, characteri sed in that there is a sequence valve (30) in the passage from the accumulator (8) to the hydraulic power-take-off motor (10), where the sequence valve requires pressure above a certain level to open, ensuring a minimum pressure inside the accumulator.

10. A device according to claim 1, 2 or 3 and according to claim 4, 6 or 7, characteri sed in that there is one (or more) wide by-pass pipeline(s) or hose(s) with a wide pressure limiting valve (6) leading from the accumulator (8), or from the pump(s) connected to the accumulator, directly back into a low-pressure fluid reservoir (1) without going through the power-take-off-motor (10) thereby acting as an overload protection mechanism limiting the maximum pump pressure which the system may obtain and consequently limiting the maximum working torque of the parts in the mechanical machinery (20), shedding the wave-power- plant and the components in it from excessive stress forces, excessive energy absorption and excessive power-through-put.

11. A device according to claim 1, 2 or 3, and according to claim 4, 6 or 7, characteri sed in that the mechanical energy transmission- and conversion system (20) contains a slip clutch (16) which is disengaged on excessive rotational speed or if the torque that is attempted transferred through it at any time gets too high, thereby cutting or lessening the flow of mechanical power from the winch to the hydraulic subsystem, to protect the wave power plant and its components from damaging interaction from extreme waves.

12. A device according to claim 11 where the mechanical energy conversion subsystem (20) contains a gear transmission system (15) between the winch cable drum and the slip clutch (16) gearing up the rotational speed of the slip clutch.

13. A device according to claim 11, characteri sed in that the mechanical energy conversion subsystem (20) contains one or more gear transmission systems

(17, 33) between the slip clutch (16) and the hydraulic subsystem, gearing up the rotational speed so that the input shaft of the hydraulic subsystem may rotate faster than the slip clutch.

14. A device according to claim 1, 2, 3, 4, 6 or 7, characteri sed in that the winch cable drum is conical with the winch wire (14) attached to the slimmest end, to ensure that the winch wire will arrange properly again on the winch cable drum by itself, if it previously should happen to be disorganized, where the angle, α , between the centre line of the winch axle (18) and the conical winch cable drum's peripheral edge line has been chosen with the aim that tan α should be equal to the coefficient of static friction between wire (14) and drum, so that the wire windings will lay close together on the drum without significant side load.

15. A device according to claim 1, 2, 3, 4, 5, 6 or 7, characteri se d in that the mechanical energy conversion subsystem (20) contains a shock absorbing rubber link (13) between the winch cable drum and the rest of the mechanical energy conversion subsystem, protecting the components in the wave-power plant from extreme loads that could be generated by abrupt accelerations of the winch cable drum due to powerful jerks in the winch wire (14).