WO1993002287A1

WO1993002287A1 - A system for the exploitation of sea wave energy attached to an enchored floating breakwater

Info

Publication number: WO1993002287A1
Application number: PCT/GR1992/000012
Authority: WO
Inventors: Constantine Goudas; Evangelos Mylonas; Vasilios Kouskoulas; Alexandros Papadopoulos
Original assignee: Constantine Goudas; Evangelos Mylonas; Vasilios Kouskoulas; Alexandros Papadopoulos
Priority date: 1991-07-25
Filing date: 1992-07-17
Publication date: 1993-02-04
Also published as: GR1000799B; AU2313292A

Abstract

Sea waves, guided by the lower surface of the Shell (13) of the hydro-air-compressor, direct their critical Moment Wedge (Greek abbreviation SKO) inside the compression chamber of this Shell and thus operate as hydro-pneumatic compression pistons for the air trapped therein. During the ascent phase of the wave motion, the compressed air opens a Non-Return Outlet Valve (10) and proceeds through an air duct to the Air Tank (24), while during the descent phase of the waves, a second Non-Return Inlet Valve (8) opens to let new air from the atmosphere enter the compression chamber. The cycle is repeated with the frequency of the sea waves. The compressed air can produce electric power as it is directed to a system that consists of an air action turbine, a system of rotation regulating gears and an electric generator.

Description

A SYSTEM FOR THE EXPLOITATION OF SEA WAVE ENERGY ATTACHED TO AN ANCHORED FLOATING BREAKWATER

The international energy crisis lead during recent years to the search and utilization of new alternative sources of energy and new methods for their exploitation, with special emphasis on those sources and methods that combine such characteristics as ecological eligibility and financial profitability.

The theoretical study, the experimental application and the exploitation of such methods is developing fast enough to support the forecast that their use in the future will cover a substantial portion of the overall energy needs.

Specifically, for Greece, a country with limited conventional natural energy resources but of particularly abundant resources in renewable forms of energy, the exploitation of these alternative forms of energy and especially the energy of the sea waves, is indeed necessary and practically possible.

The energy of the sea waves is doubtlessly the most "dense" form of perpetual energy. The current research effort aims at the theoretical study and practical design of simple systems that will make possible the exploitation of this form of energy in the most economical way.

The invention presented in this document is a contribution to this cause and consists of a particularly simple and low cost structure entitled System Exploiting Wave Energy (in Greek Systema

Ek etallefseos Kymatikis Energeias, abbreviation SEKE) of sea waves with electric power as the end product.

The SEKE is a structural part of an original Anchored Floating Breakwater (in Greek Statheros Plotos Kymatothrafs is, abbreviation SPK), repeated, without gaps, along the side of the Breakwater exposed to the sea wave action. The Floating Breakwater is designed and constructed for the protection of a harbor or marina, or for the construction of a floating road link between shores, etc. It is stabilized using cables connecting it to concrete anchors resting at the sea bed. The above floating structures, beside their normal function (as piers, jetties, road links, etc) with the systems SEKE attached to them along the side of the wave action will also operate as electric power plants. This invention consists of the design of SEKE, of SPK and of combination of SEKE with SPK.

The Anchored Pontoons (in Greek Stathera Plota, abbreviation

SP) used instead of the classical breakwater are known and widely used off-shore structures. There are many companies in the world designing and constructing floating breakwaters, piers, jetties, docks, marinas and even airports.

The known classical SPs, however, are common breakwaters of mechanical reaction and reflection of the sea waves. Their operation implies absorption by the structure of part of the water momentum, oscillation of the structure, stresses on the cables and the anchors, i.e. reduced useful life span, and hence need for increased strength requirements.

On the contrary, the SPKs with the SEKE systems presented in this invention, absorb most of the energy of the impacting waves and turn, it into compressed air (subsequently converted into electric power) and in this way they are subject to lower stresses, less fatigue and hence longer useful life span. This combination results in a breakwater that in effect is dumping the sea water action rather than acting as an impact receiving body. Moreover, the interior empty spaces of the SPK of the present invention are designed so that they can be used as the air tank of the SEKE system, thus securing cost-free storage space for the large quantities of compressed air.

It is obvious that the system SEKE, which is designed by the authors of this paper, can operate either by itself, or as part of a larger off-shore structure which we already named Anchored Floating Breakwater (SPK).

The design also includes the proper combination of the SPK with the system SEKE, since in this way we secure technological optimization and economic viability. In addition, this combination aims at technological and financial support of both systems (i.e. SPK and SEKE), so that the patent rights applied are to cover the design of the system SEKE and the combined system SPK-SEKE.

At this point we should note that the corresponding breakwaters with foundations in the sea bed are less suitable for combination with SEKE systems, or at best present bigger technical difficulties and, most important, they require increased expenditure.

It is also necessary to emphasize the advantages of the floating breakwaters, over the conventional, not only with regard to the cost (it is estimated that the cost of the conventional ones is between three and nine times more expensive), but also with regard to environmental protection, to protection of the biosphere and the protection of the archaeological treasures often resting on the beds of the Greek seas.

The SEKE system that utilizes the renewable and "dense" sea wave energy without any pollution, adds to the advantages mentioned earlier its capability to pay its total construction cost and more through the power it will produce. The systems developed up to now for exploitation of the sea wave energy consisted of complicated articulated arrangements with hinges or pistons, usually of high cost and low reliability, the latter due to the even higher complexity and occasional violent behavior of the sea waves. Unlike them, the SEKE system with its simple structure, the absence of moving parts and its high efficiency, gives answers to the known problems and opens the way to the utilization of the sea wave energy in a way economically and environmentally acceptable.

In the following paragraphs we present briefly and in a simplified way the basic theory of the sea waves and then the parts constituting the systems SEKE and SPK.

A sea wave whose "crest" is moving toward any direction parallel to the free sea level is defined as "running wave". The waves in the open seas are running waves. The propagation speed of running waves depends on the their wave-length (L) as well as the depth of the sea (in case this is not big enough). In shallow seas the waves move with the same velocity regardless of their wavelength. The characteristics and process of displacement of the running waves is shown in Figures 1 to 4. Another type of wave is the "standing wave" which does not exhibit any directional displacement but any surface point moves in successive phases from the peak to the free sea level and from there to the valley (see Figure 5). Points that remain still on the free water level at all times are called "nodes" and they are half a wave length (L/2) apart.

Standing waves are created when running waves hit a vertical wall and are reflected backwards. In fact, a standing wave is produced by the interference at the same phase of two waves of the same length and period and of opposite directions. The motion of the sea particles, under the periodically changing outer surface of the standing wave, is shown in Figure 6. The running wave is not produced by mass transfer of water particles but by transfer of their kinetic energy to the adjacent ones. In this way the sea particles perform circular motions with approximately constant orbital speed, as shown in Figures 2 and 3.

In open seas, where the large depth to the sea bed permits the development of waves of large wavelength and of relatively small height, the motion of the water particles is theoretically circular. In reality, however, a small excess in velocity during the ascent to the wave crest phase produces a relatively small displacement of the sea particles along the direction of the blowing wind (see Figure 4). This roughly circular motion, which creates the impression of displaced and running waves, is performed on circles of radii decreasing exponentially with increasing depth. At a depth equal to L/4 the radius of the circular motion becomes equal to zero as is shown in Figure 1. Furthermore, in a shallow sea, of depth less than L/4, the circular motion near the surface turns, with increasing depth, into elliptical of increasing eccentricity and with the major axis parallel to the sea bed. The eccentricity becomes equal to one and the motion linear and oscillatory on the sea bed. The momentum of the particles performing these motions is a decreasing function of the depth. In this basically laminar motion of the sea particles, where all perform circular or elliptical motion, transfer of momentum occurs smoothly from each particle to the adjacent ones. The quantity M, where

and u = particle velocity, δ = density, V = volume and dV = ele¬ ment of volume, when computed for t = constant, gives the total momentum of the fluid contained inside the volume V. For fixed V, both in size and geometrical form, placed at depth H below the sea surface, (this implies that V = V(H)), we can search for the critical value H of H, such that

The critical depths H computed for any continuous sea area define a surface-locus of the "centers of critical moments". This surface is a horizontal plane at fixed depth for seas of large depth.

The characteristic ratio H/(L/4) increases for decreasing depth. This means that the Center of Critical Moment at a given sea location (it is the point at depth H below the water surface) approaches the sea bed as the depth of the sea decreases, or as we approach the seashore (see Figure 8). As is mentioned below, by taking advantage of these properties, we can collect a large part of the kinetic and potential energy (the latter due to the height of the water above the mean free surface).

As mentioned earlier, when a running wave falls upon a vertical wall it is reflected back and, for collisions at the proper phase, standing waves are produced. These waves retain only a portion of the original energy since the rest is lost in various ways (internal and external friction).

If the sea bed in front of the above vertical wall is given a suitable form similar to the surface of the locus of the Centers of Critical Moment, then the motion of the running wave during its ascending phase follows the sea bed surface carrying with it most of its kinetic energy. This energy, at the end of the phase of ascent becomes potential, i.e. "water level".

The height to which the wave rises before its surface

"breaks", depends on the energy that its water mass includes and is proportional to the square of wave height (A). The motion of the running waves (successive points along their paths) is shown in

Figures 9 and 10.

As is expected, once the kinetic energy of the wave turns into potential (water level), its velocity vanishes and the phase of descent begins. After the completion of this phase, the phenomenon repeats itself with a period equal to the period of the wave motion.

The motion of the Centers of Critical Moment follows the path of the running waves, while the paths of the sea particles contained inside the volume V(H ) form a group motion which we call "Critical Momentum Wedge" (in Greek "Sfena Krisimis Ormis, abbreviation SKO). As shown in Figure 11, the formation SKO comes gradually closer to the sea bed, whereas its direction between point A (initial) and point E changes by 90°. The curves A through E are the locus of the centers of the circular or elliptical motions of the sea particles at various positions during the motion of the running wave.

It transpires from the hitherto presentation that it is possible , through properly shaped sea floor or through an artificial immersed surface, to direct the Critical Momentum Wedge inside a special device that will receive the energy of the sea particles participating in the motion. Such a suitable device will be described immediately below.

Assume that a truncated cone made of material resistant to sea corrosion (e.g. polyurethene, concrete, etc) is placed at a fixed semi-immersed position with respect to the mean free sea surface, as shown in Figure 12. The large base of the cone is open so that the undulating water can move in its interior. The small base of the cone, resting above sea level and at a height that the waves can never reach, is sealed with a lid bearing two non-return air-valves, one inlet and one outlet. Such are the lids of the common alternating air-compressors.

It is now obvious that the sea water during its alternating motion will essentially operate inside the cone like a piston of variable active surface depending on the geometry of the cone and the water level. Hence, in the phase of ascent the air trapped in the interior of the cone will be compressed up to the time that the outlet valve opens. The compressed air is directed though a suitable pipe to an air tank such as the interior of the Anchored Floating Breakwater (SPK). Subsequently, during the phase of the wave descent, the air pressure inside the cone drops below the free air pressure and hence the inlet valve opens to let air from the atmosphere fill the gap created. The cycle will be repeated by the phases of the next wave. The following are useful remarks concerning the operation of the above original hydro-air-compressor:

- The cycle of operation of this compressor is not different from the cycle of the common alternating air compressor.

- The air inside the compression chamber absorbs a maximum of the wave energy when the sea floor, or the artificial floor, is shaped in such a way as to guide the Critical Moment Wedge inside the compression chamber. The dimensions of the opening should be calculated in a way that the compression chamber will receive a broad range of waves of various characteristics. Each application should be optimized by adjusting the geometrical dimensions of the system SEKE to the data of the site of application, most important of which is the length of the prevailing waves.

- The geometrical shape of the cone defines also the specific characteristics of the operation cycle. It is obvious that the tilt of the cone has an essential effect upon the ratio V/P of the air under compression as well as upon the ultimate pressure to be achieved. This means that there is a broad range of possibilities in the design of the system.

The calculation of the efficiency of the SEKE system is based upon the estimate of the amount of sea wave energy that can be "drawn" in the form of compressed air. This energy, per unit of mouth surface of the SEKE system, is proportional to the square of the wave height, which, in turn, is known to depend upon the air velocity, the wave attenuation and the depth of the sea floor. In addition, the produced energy depends upon the direction of the waves and the range of directions from which, at different times, the hydro-air-compressor is activated.

The average power of the Aegian waves is approximately between 12 and 30KW per m² of sea surface per m of wave front. For a typical SEKE-SPK installation exposed to the waves of an off-shore site in the Aegian the expected average energy to be produced is estimated as follows:

Assuming an efficiency coefficient of 0.60 for the hydro-air- compressor, the pressures expected to develop on the basis of the range of activity known to prevail in this sea are estimated to vary between 1.5 and 3.5Kg/cm².

The efficiency coefficient of the power production unit (air turbine - gear box - power generator) is assumed to be equal to 0.15. Under these assumptions the energy to be produced is:

(12 to 30KW/m²)X0.6X0.15 = 1.1 to 2.7KW/m of SEKE.

Hence, a floating breakwater 1000m long equipped with the SEKE system along all its length of the side exposed to the wave action as shown in the Drawings No 1 and No 2, will produce:

1.1 to 2.7(KW/m)X1000mX8000(h/year) = 8.8 to 21.6 MKWh/year.

The SEKE arrangement is shown in DRAWINGS No 1 and No 2. Drawing No 1 gives a perspective view of the SEKE-SPK system and Drawing No 2 the basic section of the system. The various parts of the system, as presented in Drawing No 2, are indicated by numbers and their description is as follows: (1) Sea level. (2) Platform Surface (Anchored Floating Breakwater). Cast reinforced concrete, ordinary or prestressed,able to carry continuous, constant, isolated, or moving loads, depending upon the power production plans or other operations to be performed. (3) Vehicle in scale. Example of moving load, suitable for deducing the order of physical size of the SEKE-SPK system. (4) Protection barrier. Barrier for the protection of people and the plant. Construction material: concrete or steel. (5) Walls covering the air-valves. Concrete walls for the protection of the air-valve openings. (6) Details of the air pipes and air-valves. Magnified section for clarification. (7) Wave deflection wall. Concrete wall for the deflection of the small part of the waves that will not enter the hydro-air- compressor. (8) Inlet air-valve. Non-return inlet air-valve made of materials resisting corrosion, e.g. polyester or coated steel. (9) Inlet air hatch. Opening on wall 5 for the air inlet. (10) Outlet air-valve. Non-return valve made of the same material as item 8. 11. Inlet air pipe. Pipe allowing air to enter the compression chamber. (12) Outlet air pipe. Pipe for the transfer of the compressed air to the air tank. (13) Shell of the hydro-air-com¬ pressor.Made of sea corrosion free material (polyester, fiberglass, coated sheet-steel, concrete, etc.). Its concave surfaces are described by exponential functions, as mentioned earlier. At regular intervals there are dividing walls of the same material thus securing many independent compression chambers, or SEKE systems in sequence. At the top there are, under cover, pairs of non-return, one inlet and one outlet, air-valves. The opening of the shell is placed at the depth, below water surface, that corresponds to the mean of the wavelength range produced in the area of operation of the SEKE system, so that the compression chamber to be acted upon by the Critical Momentum Wedge of the majority of waves. (14) Surface of the impacting wave. 15. Critical Moαentun Wedge (Greek abbreviation SKO). (16) Paths of water molecules within SKO. (17) Dividing interior walls of the pontoon.

Walls dividing the hollow space (used as air tank) of the floating platform, made of ordinary or prestressed concrete and cast together with the entire structure. (18) Openings for equalizing air-pressure within the air tank. These are openings permitting air to move to all segments of the air tank. (19) Opening for transfer¬ ring ballast. These are openings at the bottom of the dividing interior wall permitting ballast (sea water) to move from segment to segment and facilitating the uniform removal of ballast by the pump (item 20). (20) Ballast control pump. Water pump for the control of water ballast and pumping out excess water entering the air tank through the inlet air-valve in drops. This pump should be of the type used for pumping polluted waters and made of metals resisting sea corrosion. Its motor will be placed on top of the floating platform. (21) Side walls. Walls, made of ordinary or prestressed concrete, supporting the shell of the hydro-air- compressor. (22) Horizontal support walls. Same as item 21 for horizontal support. (23) Walls deflecting-regulating residuals of water αααentiα. They are walls of cast concrete, ordinary or prestressed, deflecting backwards the part of the waves that does not enter the compression chamber. (24) Air tank. It is the airtight hollow part of the floating platform. Its walls are made of ordinary or prestressed concrete and are designed to operate at air pressures up to 6 atmospheres and test pressures of 10 atmospheres. Its segments communicate through air viaducts placed at the top of the dividing concrete walls so that uniform pressure prevails though out the unified air tank. Depending on the size of the air tank, considerable quantities of energy can be stored so that variations in the wave activity to be averaged, particularly in the cases of small islands with sensitive power network. Compressed air is transferred from the air tank to the appropriate power generating system (gas turbine - rotation regulating system - power generator). (25) Anchors. The system for anchoring the floating platform does not appear in this or the previous Drawing since the pertinent technology is well known. It consists of braided cables made from coated steel wires able to stand high tension and sea corrosion. They connect the platform to heavy anchors, made of concrete and sunk to the sea floor at appropriate locations about the platform. Tension control devices placed at their connection points with the platform take care of the sea level changes caused by tides.

LIST OF FIGURES AND DRAWINGS

1. list of figures.

Figure 1. Running wave. The numbers on this Figure indicate: 1. Mean free surface of the sea.

2. Water surface.

3. Wave crest.

4. Wave propagation velocity.

5. Wave points on the mean free surface. 6. Wave valley.

7. Wave length.

Figure 2. Motion of the molecules of water of a running wave. Figure 3. Velocity field of a running wave. Number 1 indicates the direction vector of wave propagation. Figure 4. Gradual displacement of the sea molecules. Figure 5. Standing waves. The numbers on this Figure indicate:

1. Mean free surface.

2. Wave crest.

3. Wave valley. 4. Water surface.

5. Wave node.

6. Wave node.

Figure 6. Velocity field of the sea molecules in the case of standing waves. Figure 7. Sea molecule paths at various depths. Figure 8. Motion of the critical moment. Figure 9. Motion of running waves of the same wave length but of different heights.

Figure 10. Motion of running waves of the same wave length but of different heights.

Figure 11. Displacements of the centers of critical moment in a wave modified by the sea floor. Symbol I indicates the direction of the running wave. Figure 12. Design of a unit of the System Exploiting Wave Energy (SEKE). The numbers on this Figure indicate: 1. Sea floor properly shaped.

2. Critical Momentum Wedge.

3. Mean free surface of the sea.

4. Hydro-Air-Compressor.

5 5. Hydro-air-compressor top.

6. Inlet-outlet non-return air valves.

7. Air tank.

8. Compressed air.

9. Air turbine. 0 10. Constant pressure air.

11. Generator.

12. Power transport.

2. list of drawings. 5

Drawing No 1. Presentation of a floating platform with hydro-air- compressors attached to it.

Drawing No 2. Section and constituent parts of the floating plat¬ form with hydro-air-compressors attached to it. The _Q numbers placed on this Drawing indicate:

1. Mean water level.

2. Platform Surface (Anchored Floating Breakwater).

3. Truck, indicating scale. .4. Protection barrier. 5. Walls covering the air-valves.

6. Details of the air pipes and air-valves.

7. Wave deflection wall.

8. Inlet air-valve.

9. Inlet air hatch. ₀ 10. Outlet air-valve.

11. Inlet air pipe.

12. Outlet air pipe.

13. Shell of the hydro-air-compressor.

14. Surface of the Impacting wave. ₅ 15. Critical Momentum Wedge (Greek abbreviation SKO). 16. Paths of water molecules within SKO.

17. Dividing interior walls of the pontoon.

18. Openings for equalizing air-pressure within the air tank.

19. Opening for transferring ballast. 20. Ballast control pump.

21. Side walls.

22. Horizontal support walls.

23. Walls deflecting-regulating residuals of the water momentum. 24. Air tank.

25. Anchors.

Claims

C L A I M S

1. Any Fixed Floating Breakwater (Greek abbreviation SPK) or Floating Platform with the System SEKE for the exploitation of Wave Energy embodied or attached to it for any purpose such as stabilization, absorption of wave momentum, production and/or utilization of compressed air for any purpose. All arrangements and its parts, by themselves or in combination, as described and analysed and given the following brief titles: (1) Platform Surface, (2) Protection barrier, (3) Walls covering the air- valves, (4) Details of the air pipes and air-valves, (5) Wave deflection wall, (6) Inlet air-valve, (7) Inlet air hatch, (8) Outlet air-valve, (9) Inlet air pipe, (10) Outlet air pipe, (11) Shell of the hydro-air-compressor, (12) Critical Momentum Wedge (Greek abbreviation SKO), (13) Paths of water molecules lSwithin SKO, (14) Dividing interior walls of the pontoon, (15) Openings for equalizing air-pressure within the air tank, (16) Opening for transferring ballast, (17) Ballast control pump, (18) Side walls, (19) Horizontal support walls, (20) Walls deflecting-regulating residuals of the water momentum, (21) Air tank.

2. Any system consisting of repetitions of each of the SEKE or the SPK arrangement or their combination that operate as floating platforms, docks, breakwaters, marinas, floating links of shores, or floating roads.

3. Any system utilizing the arrangement SPK and/or SEKE but with its air tank or air tanks located apart but receiving compressed air from this arrangement and exploiting the energy of this compressed air in any way.

4. Any SEKE system of the above specifications and under demands I, II and III, but mounted on a pre-existing dock or breakwater, founded on the sea floor or operating under any other principle, or operating at a sea shore with proper shape of the adjacent sea floor.