GB2131887A - Wave energy conversion unit - Google Patents

Abstract

The unit comprises a conversion device attached to a frame movably and reacting to the waves so as to utilize the kinetic energy of the waves by converting the said energy to a high- pressure fluid for driving a tubine operating as the central machinery of the wave-energy power plant system. The conversion device comprises a supporting carriage 2 movable horizontally relative to the frame 11, 16, a pontoon construction 1 connected to the supporting carriage and moving along with the wave movement, locking means 9 for locking the pontoon construction 1 in the highest and lowest positions reached by it along with the wave in the vertical direction and for releasing it again after the wave crest/ trough has moved forwards so that the pontoon construction is then almost completely above/below the water surface, and single-acting or double- acting high-pressure fluid pumps 12 connected to the pontoon construction, for converting the energy of the free movement of the pontoon construction to hydraulic pressure. <IMAGE>

Description

SPECIFICATION Wave energy conversion unit The present invention is concerned with a wave energy conversion unit, which comprises a frame and a conversion device movably attached to the frame for reacting to waves so as to utilize the kinetic energy of the waves by transferring the said energy to a high-pressure fluid driving a turbine, for example a Pelton turbine, operating as the central machinery of the wave-energy power plant system.

After solar and wind energy, wave energy is the greatest pure, renewable energy form, whose potential within ice-free sea areas has been estimated at about 50 TW. Methods aimed at utilization of wave power have been suggested eversincethe 19th century, and in particular in recent years great attention has been paid to research into wave energy.

Among the solutions that have been suggested should be mentioned the cell converter of Prof. Rolf Törnqvist, the swinging wing of Salter, and many suggestions based on bending movement. So far, however, owing to constructional problems and/or low efficiency, among these solutions, there has not emerged a solution that differs from the others clearly advantageously and that has thereby gained extensive use.

The energy of a sea wave is, in principle, distributed uniformly over its entire wavelength, whereat the power Pin, for example waves of wavelength L = 180m, height H = 4.5m, and period T = 10 s, is 250 kW per metre of the wave front (Pim = 0.92 x Vt H2).

Thus, the energy content of one cycle of the example wave is 2,500 kWs, i.e. 13.88 kWs/m2.

This is, however, only a value indicating the average energy content of a cycle. As a matter of fact, at each metre of length of a wave, during a cycle, two accumulations of energy occur: at the crest ofthe wave, the movement of water in the vertical direction is zero, so that, underneath a surface square metre placed at the wave crest, there is a water column of which the top portion H/2 = 2.25m is placed above the average surface level. Its static energy is 2,250 kg x 2.25/2 m = 2,531 kgm, which is, thus, as a whole concentrated in the portion ofthewater column placed above the surface level. As the velocity of movement ofthe portion above the surface level in the horizontal direction is, on the average, 1.1 m/s, corresponding to an energy quantity of 139 kgm, its overall energy content is consequently 2,670 kgm, i.e.

26.18 kWs/m2. At the trough ofthe wave, the situation is the same.

If the portion placed above the surface level were allowed to fall freely to the surface level, it would attain a velocity of 4.7 m/s in 0.75 seconds, the friction and the resistance accompanying displacement not being taken into account. In practice, a falling velocity of 1.29 m/s is reached in 2.22 seconds, whereatT/4 = 2.5s;the lowervalue results from the inclination of the path of movement of the particles of water, of a shape of a relatively circular ellipse, angled by about 30 in the direction ofthe wind. Underthese circumstances, it has been left with only about 1/13 of its static energy, the overwhelming majority of the said energy being consumed for accelerating the underlying water masses both upwards and downwards, when the wave height becomes lower, also laterally.

Thus, the sequence of events is as follows: the wave crest is generated by the joint effect ofthe kinetic energies ofthe large water masses placed underneath as well as in front of and behind the wave crest. Awave trough is generated correspondingly when the force of gravity makes the wave crest collapse, the water masses in the wave crest, owing to the moments of inertia of the underlying layers, largely spreading forwards and rearwards and, owing to the horizontal component of movement obtained bythem in this way, generating a wide depression in the water surface. The momentary negative pressure caused by the depression and the resulting masses ofwater seeking their way into the said depression from ahead and from the rear, when meeting, generate a new crest of wave, etc.Moreover, owing to the pressure ofthe wind, the wave crest additionally has a rather strong horizontal component of movement in the direction of the wind, and the wave trough has a compensating current in the opposite direction.

Sincethe static energy ofthe wave is so strongly concentrated in its surface layer, a remarkable quantity of static energy could be recovered by maintaining a certain volumetric proportion of the wave crest in its position until the rest of the wave crest has moved forwards. In this case this static energy would not be available to the surface level only, but down to the following wave trough.

In accordance with the present invention there is provided an energy conversion unit for a wave-energy power plant, comprising a frame and a conversion device movably attached to the frame for reacting to waves, wherein the conversion device comprises a supporting carriage arranged for movement horizontally relative to the frame, a pontoon construction connected to the supporting carriage for movement along with the wave movement, locking means for locking the pontoon construction in the highest and lowest positions reached by it along with the wave in the vertical direction and for releasing it again afterthe wave crest/trough has moved forwards so that the pontoon construction is then almost completely abovelbelowthe watersurface, and single-acting or double-acting high-pressure fluid pumps connected to the pontoon construction, for converting the energy of the free movement ofthe pontoon construction to hydraulic pressure.

The carrying part of the pontoon construction preferably comprises several vertical cylindrical containers shaped slightly hydrodynamically at both their ends. The mass of the pontoon construction is preferably approximately one half or slightly less than one half of the mass ofthe quantity of water displaced by its containers when fully submerged.

In use of embodiments in accordance with the invention,the appropriateiy dimensioned and shaped pontoon, having risen onto the wave crest/sunkto the wave trough, is stopped at its extreme position until it may, afterthewave crest/trough has moved sufficiently forwards, freely drop/rise almost with its full weight/lifting force.

As can be seen there is provided a wave energy conversion unit that operates in accordance with the above, that has a very robust construction and high efficiency, and that is reliable in operation.

An embodiment ofthe invention will now be described byway of example and with reference to the attached drawings, in which: Figure lisa schematical side view of an exemplifying embodiment of a wave energy conversion unit in accordance with the invention, Figure 2 shows the unit of Fig. 1 as viewed from the front, and Figure 3 is a top view of the unit of Fig. 1 with the supporting carriage removed.

Figures 1 to 3 illustrate a more detailed construction of an embodiment of the wave energy conversion unit, illustrated somewhat schematically. The part of the unit actually reacting to the waves is the pontoon construction 1, which consists of longitudinal and transverse beams 6 and 7 placed crosswise relative to each other and ratherfar apart, of hydrodynamically shaped containers 3 attached to the beams by intermediate arms 4, and of control and support bars 5. The support bars 5 connect the pontoon construction to the supporting carriage 2 placed above the pontoon construction. The supporting carriage 2 is placed between pairs of columns 11 a, 11 b, 11 c, and so on supported or anchored in some other way.

Betweenthe columns 11a,11 b and 11c, bridges 16 have been attached, inside which closed grooves 27 have been formed, in which hard-rubber wheels 18 journalled to the supporting carriage 2 move, the number of the said wheels being for example four pairs. In this way, the supporting carriage may move in the horizontal direction relative to the columns 11 a, 11 band 1 1c. This horizontal movement is limited and utilized by two single-acting high-pressure fluid pumps 19 placed between the middle column 11 band the supporting carriage.

The guide and support bars 5 connected to the pontoon construction pass through the supporting carriage 2 and through guide sleeves 8 placed on the top face ofthe said carriage. The top ends of the guide sleeves 8 are provided with, locking means 9 for example magnetic locking means for locking the pontoon construction atthe desired level. The movement ofthe pontoon construction is limited and utilized by high-pressure fluid pumps 12, always provided there is one pump per one pair of support bars, as well as by expansions 10 formed at the ends of the bars 5.The pontoon construction is also provided with intermediate flanges 20, which prevent horizontal movement of water in the pontoon construction, whereas the positioning of the cylinders 3 permits vertical movement ofwater. Besides by the guide and support bars 5, the movement of the pontoon construction upwards and downwards is also guided by rubber wheels 15 attached to the sides of the pontoon construction and supported against the columns 1 1a, 11 band 1 c, the wheels being provided with spring suspension if necessary.

Thefunction of the supporting carriage 2 isto guide the movementofthe pontoon construction 1 in the vertical direction and to keep it in position in its extreme positions, while at the same time permitting resilient movement of the entire equipment in the direction of the horizontal component ofthe waves.

Thus, the said carriage is alternatingly burdened by the weight ofthe entire pontoon construction and by its entire lifting force.

In order to permit the performance of the different stages ofthe operation ofthewave energy conversion unit, the opposite ends of at least one cylinder3 are provided with detectors 13 and 14, which give an impulse fortemporary elimination ofthe counterpressure of the high-pressure fluid pumps 12 in orderto give the pontoon construction a residual acceleration as the containers 3 reach contact with the water surface. The locking pulse is given to the locking means by a detector 17 reacting tothe stopping of the rising and lowering movementofthe pontoon construction, the said detector 17 being connected, e.g., to some locking means 9.The releasing pulse to the locking means 9 is given by a detector 21, connected, e.g., to the last pair of wheels 18 in the direction of the wave movement, which detector 21 acts on the basis ofthe axle load or axle deflection when the strain reaches the preset value.

One such wave energy conversion unit of a waveenergy power plant operates independently from any neighbouring modules as follows: When the pontoon construction 1 reaches its top level about 24" after the peak of the wave crest, the sensor 17 gives a locking impulsetothelocking means 9. The pontoon construction 1 remains in this position until the carrying force ofthe water, with the approach of the wave trough, has been reduced to substantially 6 to 10% of its maximum value. Thereat, the increasing axle load ofthe rear support wheels 18 activates the sensor 21, which gives an impulse to the locking means 9 and releases the pontoon construction.The release impulse may be given in advance, for owing to the low residual acceleration,the movement of the pontoon construction is initially quite slow, and the water surface, which is at this stage moving down rapidly, leaves it instantly completely free. An advance larger than that mentioned above is, however, not possible, because the counter-pressure in the high-pressure fluid pumps 12 would, in such a case, tend to raisethe pontoon construction. The required residual acceleration of about 0.051 g nowgivesthe pontoon construction a final velocity of 1.53 mls when it strikes against the wave trough. Owing to its cellular structure, it continues its movement while the quantity of water displaced by it raises the watersurface in excess ofthe rise in accordance with the wave. In the example, the area free for water in the pontoon construction is equal to the area ofthe containers, so that the excessive rise in the water surface is equal to the depth of penetration ofthe pontoon into the water. An increase in the free area would make the pontoon construction unreasonably longer, in which case its peripheral portions would meet the water surface substantiallyearlierthanthe middle portion would.

Ifthe pontoon construction 1 is allowed to go on working after contact with the water surface, it stops after it has progressed 0.4 m, in which case the starting pointforthe next rising step remains quite unfavourable. On the contrary, if the counter-pressure caused by the high-pressurefluid pumps 12 is eliminated by means of an impulse given bythe sensors 13 and 14 on contact taking place with the watersurface and the pontoon construction is allowed to continue its movementfreely, it is not stopped and locked until it has moved towards a depth of 3.36m from the point of contact with watersurface. With this procedure, the overall efficiency ofthe equipment is improved by almost20 per cent.

The removal ofthe utilized gross energy attenuates the waves so thatthe height of the out-going wave Hp = V-ri x H.ln the case ofthewaves ofthe example, Hp = 1 - 0A02 x 4.5 m = 3.48 m. Calculations concerning the efficiency of the equipment must be carried outin accordance with thislower height, i.e.

the working height When the height ofthe pontoon construction is 2.5 m andthe height oftheworking wave the said 3.48 m, the top level ofthe pontoon construction is now atth e level - 134-1.36+2.5 = - 0.6m.Thislevel-the permitted advance of about 0.1 mis reached by the rising wave crest about 24 before the surface level, at which timethe sensor 21 gives a releasing impulse to the locking means 9 and the rising stage begins.The wave's own rising movementthereat gives the pontoon construction an extra initial velocity, and as the hydrodynamicshape of its containers reduces the friction with an increasing velocity, the lifting force of the pontoon need hardly be much higherthan the weight of the pontoon forthe samefinal velocity to be reached.Afterthe pontoon construction has made contactwith the water surface,the sensor 14, by means of its impulse, eliminates the counter-pressure in the high-pressurefluid pumps 9, andthe pontoon construction continues its rising and reachesthetop height 1.74+1.36 = 3.1 m (top face), being locked in this position. The height of its lowerface above the average level ofthe water surface is thereat 0.6 m.

When the rearface ofthe out-going wave crest reaches this level plus the permitted advance, the sensor21 gives a releasing impulsetothe locking mechanisms 9 and a new period starts.

Owing to its in itself large volume as well asto the considerable masses of water bound to it, the pontoon construction 1 has excellent possibilities of also utilizing the horizontal components of movement of the wave crest and thewavetrough. This is achieved by braking the movement ofthe supporting carriage by means of single-acting high-pressurefluid pumps 19.

Owing to its low acceleration, atthe beginning of its rising stage,when the rate of rising ofthewave surface is alreadycloseto its maximum value, the pontoon construction remains remarkably deep be lowthe surface. In orderthatthe support structures of the pontoon do notslow down its rising movement at this stage, the containers ofthe pontoon must be provided with tubularfixing arms 4 whose length is 1/3to 112 ofthewave height H, depending on the period T ofthe chosen predominant average waves.

Correspondingly, at the beginning ofthe lowering stage,the bottom of the pontoon construction 1 remains remarkably high above the water surface.

This is why the intermediate flanges 20,which are important in view of the utilization ofthe horizontally effective kinetic energy, have been lenghtened so that they extend by about one height of a container3 belowth e bottom ofthe pontoon construction. By means of a slight bending oftheir bottom portions in the direction ofthe out-going wave, the overall output has been improvedadditionallytosome extent.

Forlong ocean waves, T = 10 to 15 seconds, in an anchored version of e wave-energy conversion unit the prolonged weight/lifting force ofthe pontoon construction gives the entire power-plant system a considerable acceleration. Thus, atthe end of the lowering stage ofthe pontoon construction,the entire plantcontinues its movement downwards, and this movement does not end until it has been overruled by the lifting force ofthe pontoon construction approximately at the average surface level during the next rising stage.This movement, which may amount to several metres, is not detrimental as it increases the length ofthe useful movement towards the rising and the lowering stage, butwhen uncontrolled, it may cause resonance distrubances when losing its timing relative to the wave movement This is why it is advisable to multiplythe mass ofthe power plant construction by therein binding underlying water masses by means ofsubstantially horizontal sheet planes,which are not shown, sothatthe mass in this way obtained is 10 to 20 times as large as the overall mass ofthe pontoon construction.

Thus, in the example case, the poweryielding bythe vertical movementofthe pontoon construction is2 x 200,000 x (0.6+1.74) x 0.949 = 88,000 kgm/10s/10m of wave front, i.e. 87.1 kWsA m of wave front, the efficiency 0.402 as calculated from the vertical compo nent ofthe wave energy.

The share ofthe energy component effective horizontally in the overall energy varies from almost zero, in a ]ong low roller, to almost one third, in short, steep storm waves. In the example waves, it is ofthe order of 12 to 15 %, as calculated on the basis of 13.5 %, 34 kWs. The pontoon construction with its long intermediate flanges binds a large water volume in itself and therefore reaches at least the same efficiency both from the horizontal and from the vertical energy component and therefore, calculated on the basis of 0.402,yields 13.7kWs/1 m of wave front.Thus, the gross output generated bytheequipmentwould be 100.8kWs/1 m of wavefront If, in the support construction, a second pontoon construction isinstalled afterthefirst one, and is dimensioned for a lower wave heightand is conse- quently lower, it isfurther possible to utilize 40% of the residual wave, i.e. 59.7 kWs, wherebythe overall output ofthe equipment is increased to 160.5 kWs/1 m of wave front, the efficiency being 0.642. The height of the out-going residual wave is thereby reduced to 2.69 metres.

The equipment herein described may be applied either mainly as a small-scale stationary or anchored wave-energy power plant as well as, for example, to oil drilling rigs if its horizontal pushing force is compensated.

The combination of pontoon construction and supporting carriage described above can be replaced bya pontoon construction which is connected bya long, vertically moving carrying arm to the frame of the power plant by the intermediate of an articulated jointthat is resilient in the horizontal direction and that can be locked.

Claims (9)

1. An energy conversion unitfora wave-energy power plant comprising a frame and a conversion device movably attached to the frame for reacting to waves, wherein the conversion device comprises a supporting carriage arranged formovementhorizon- tally relative to the frame a pontoon construction connected to the supporting carriageformovement along with thewave movement locking meansfor locking the pontoon construction in the highestand lowest positions reached by it along with the wave in thevertical direction and for releasing itagain afterthe wave crest/trough has moved forwards so that the pontoon construction is then almost completely above/belowthe watersurface, and single-acting or double-acting high pressurefluid pumpsconnectedto the pontoon construction, for converting the energy of the free movementofthe pontoon construction to hydraulic pressure.

2. Energy conversion unit as claimed in claim 1, wherein the pontoon construction has a carrying part which comprises several vertical cylindrical containers shaped slightly hydrodynamically at both ends.

3. Energy conversion unit as claimed in claim 2, wherein detectors are connected to at least one ofthe containersforgiving an impulse, on reaching contact with the watersurface, fortemporary elimination of the counter-pressure ofthe high-pressure fluid pumps in orderto give the pontoon construction a residual acceleration.

4. Energy conversion unit as claimed in claim 2 or 3, wherein the mass ofthe pontoon construction is approximately one halforslightly less than one half of the mass of the quantity of water displaced byits containers.

5. Energy conversion unit as claimed in claim 2,3 or4wherein the containers are located in the pontoon constructionapartfrom each other,sothatthey permit movement of water in the vertical direction, whereas movementofwater in the horizontal direction is prevented by intermediate flanges.

6. Energy conversion unitasclaimed in claim 15, wherein the intermediate flanges extend about one height of a container belowthe bottom ofthe pontoon construction and havetheirbottom portions bent slightly in the direction ofthe out-going wave.

7. Energy conversion unit as claimed in claim 2, wherein the containers are provided with tubular arms whose length is 113 to 1/2 ofthe height ofthe predominant average waves.

8. Energy conversion unit as claimed in claim 1, wherein for limiting the horizontal movement ofthe supporting carriage relative to the frame, singleacting pressure fluid pumps are fitted between the supporting carriage and the frame, the said pumps being attached at one end to the supporting carriage and attheotherendtotheframe.

9. Energy conversion unitforawave-energy power plant substantially as hereinbefore described with reference to the accompanying drawings.