GB2108590A

GB2108590A - Liquid wave energy absorber

Info

Publication number: GB2108590A
Application number: GB08227594A
Authority: GB
Inventors: Nils Ambli; Knut Bonke; Oddbjorn Malmo
Original assignee: Kvaerner Brug AS
Current assignee: Kvaerner Brug AS
Priority date: 1981-10-02
Filing date: 1982-09-28
Publication date: 1983-05-18
Also published as: DE3235842A1; AU8880282A; NO153542C; JPH0128225B2; GB2108590B; SE450723B; AU550828B2; SE8205424L; FR2514048A1; FR2514048B1; NO153542B; SE8205424D0; NO813344L; JPS5872678A

Abstract

For liquid wave energy absorbers, it is proposed that confining surfaces be placed adjacent the absorber, optionally secured to the absorber, in such manner that the band width of the dependency of the energy absorption on the wave frequency obtains at least one new maximum at a frequency determined by the placement of the confining surface. The principle is explained in connection with a wave energy power plant which operates according to the oscillating water column principle and which is made as a concrete structure with a submersed opening 7. In front of the opening 7, wall projections 10 are arranged which act as confining surfaces. <IMAGE>

Description

SPECIFICATION Liquid wave energy absorber The invention relates to a liquid wave energy absorber of the type in which the efficiency of energy absorption is a function of the frequency of the incident liquid waves.

Typical examples of such wave energy absorbers are those which operate according to a resonant fixed-body (buoy) principle and energy absorbers which operate according to the oscillating water column principle.

The invention has particular utility in connection with a wave energy power plant, wherein the wave energy is not only absorbed but is utilized for producing useful power and typically electrical power.

In a wave energy absorber of the above-noted type, the amplitude of oscillation, at constant amplitude and varying frequency of the incoming waves, will vary according to a characteristic resonance curve. The band width of this mode of oscillation will be relatively narrow. There is clearly a need to obtain a greater band width. This may be obtained in accordance with the invention by utilizing the so-called "harbour resonance", thereby obtaining an additional mode of oscillation.

According to one aspect of the invention there is provided a liquid wave energy absorber of the type in which the efficiency of energy absorption is a function of the frequency of the incident liquid waves, in which the band width of the dependency of the energy absorption upon the incident wave frequency is increased by arranging waveconfining surfaces adjacent to the absorber in such wise that at least one new maximum of energy absorbed versus frequency is created by the placement of the confining surfaces.

In the case of harbour resonance, the position of the resonance curve on the frequency scale will be dependent upon the length of the confining surfaces. Thus, by suitably forming the confining surfaces and the wave energy absorber, one can determine the position of the two resonance curves so that they together give the system a favourable band width in the relevant frequency range.

The placement of the confining surfaces is calculated on the basis of known wave theory, preferably also taking into account the topography of the site where the absorber is located.

According to a further aspect of the invention there is provided a liquid wave energy absorber comprising a chamber with a submersed opening through which the liquid can flow in and out in response to incident liquid waves, thereby setting the liquid mass inside the chamber into oscillation, and a means which taps energy from the oscillating system by damping the oscillations of the liquid mass, which is characterized in that on each side of the opening, wall members acting as confining surfaces are located in such manner that the dependency of the energy absorption upon the wave frequency obtains at least one new maximum at a frequency determined by the placement of the confining surfaces.

The invention may be realized in many way. In the following discussion, it will be described in connection with a realizable wave energy power plant, of the type wherein a submerged concrete structure, secured to the sea bed, contains a chamber with a submersed opening through which liquid can flow in and out, whereby the mass of liquid inside the chamber, (e.g. sea water) is set in oscillation, and wherein the oscillations of the liquid mass act upon an air mass present at the top of the chamber, causing the air to flow through an air turbine which drives an electrical generator.

A wave energy power plant incorporating an embodiment of wave energy absorber in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the wave energy power plant in longitudinal section, Figure 2 is a vertical cross section through the plant of Figure 1, Figure 3 is a horizontal section, on an enlarged scale, through the plant along the line Ill-Ill in Figure 1, and Figure 4 shows a portion of the plant in which the turbine and generator are located.

The wave energy power plant illustrated in the drawings is constructed as a concrete structure having a base 1, upstanding side walls 2, 3, an upstanding rear wall 4, a roof 5 and a transverse wall 6 extending between the side walls 2, 3 and down from the roof 5, the transverse wall 6 terminating a distance above the base 1 to form an opening 7 into a chamber 8 defined by the side walls 2, 3, the rear wall 4, the roof 5 and the base 1.

The base 1, as may be seen from Figures 1 and 2, is hollow and is provided with ballast compartments 9. The transverse wall 6 is also hollow, as shown in Figures 1 and 3. Rooms to contain apparatus and facilities for personnel, etc., may suitably be provided in this wall. As shown parts of the side walls 2, 3 extend upwardly above the roof 5, forming a chamber 39 which is open at the top. This open chamber is intended for receving ballast.

Outside the transverse wall 6, i.e. on the inflow side of the opening 7, the side walls 2, 3 are extended by wall members 10, 11. As may be seen from Figure 3, these wall members 10, 11 as well as parts of the side walls 2, 3 are also hollow, for receiving ballast and to strengthen the structure.

At the top of the chamber 8, below the roof 5, walls 12, 13 extending downwardly from the roof, define a compartment in which an air turbine 1 5 and an associated electrical current generator 16, driven by the air turbine, are placed, the compartment opening freely outwardly via an opening 1 4. A grate 17 extending between the walls 12, 13, delimits the compartment at the bottom thereof. The grate 1 7 forms part of a protective device whose purpose is to protect the air turbine from penetration by water from the chamber 8. This protective device comprises in addition a float body 1 8 which is enclosed in a cage 19.

At the top of the chamber 8, downwardly open chamber sections 20 and 21 are formed between the respective side walls 2 and 3 and the respective walls 12, 13.

The wave energy power plant, being made as a concrete structure, can be cast in dry dock, the necessary machinery being subsequently installed and ballast added so that the completed structure floats stably and with the base 1 horizontal. The concrete structure may then be floated as a unit to the intended site and can be submerged into position without the use of cranes by filling the hollow compartments therein to the required extent with water Thereafter, most ballast may be added, inter alia in the said upwardly-open chamber 39 above the roof 5. Following connection of an electrical cable from the plant to a user location (e.g. adjacent land), the wave energy power plant will be ready for power production.

The power plant operates in the following manner: The chamber 8 constitutes an oscillation chamber communicating with the sea through the submersed opening 7 which desirably faces directly towards the prevaling direction of incidence of the waves. The incident waves will then set the water column in the chamber 8 into oscillation. At constant amplitude and varying frequency of the incident waves, the amplitude of oscillation will vary according to a resonance curve characteristic of the plant. The band width for this mode of oscillation, however, will be relatively narrow. To obtain a greater band width, the concrete structure is provided with the previously discussed wall projections 10 and 11 on either side of the opening 7. One thus obtains an additional mode of oscillation, a "harbour resonance", in front of the opening 7.

The position of the resonance curve on the frequency scale, in the case of the harbour resonance, will be dependent upon the length of the projecting walls 10, 11 which represent the confining surfaces discussed above, whereas the position of the resonance curve for the oscillation of the water column will be dependent upon the geometry of the oscillation chamber 8. The position of the two resonance curves can therefore be chosen independently of each other, but so that together they give the system an enhanced band width in the relevant frequency range.

The water column in the oscillation chamber 8 will operate as a piston, alternately pushing air out and sucking air in through the air turbine 1 5. The turbine is of a type, known per se, which rotates in the same direction irrespective of the throughflow direction of the drive medium (in this case air). It is thus not necessary to cause the air flow to be unidirectional by means of valves, which would be difficult to do with the large quantities of air in question here. The rotor on the air turbine 1 5 may suitably have eight blades, for example, consisting of symmetrical wing configurations with respect to the centre line in the plane of rotation. A set of fixed guide vanes 22 also serves as a support for the generator 1 6.The air turbine 1 5 produces the energy to drive the generator and at the same time provides a resistance to the air flow so that the oscillations in the air flow become damped to some extent. The turbine output is desirably chosen so that this damping is optimised, which results in the energy yield being maximised. In a practical embodiment, the rotor of the turbine 1 5 could for instance have an outside diameter of 8.4 metres and rotate at 428 revolutions per minute.

The turbine cannot easily be dimensioned to withstand the speed of revolution which will occur in an unloaded state ,(the racing speed) and to prevent racing speed arising, should the generator 1 6 become disconnected from the load it supplies (e.g. a power network), it is advisable to ensure that the generator 1 6 is automatically connected to an electrical braking resistance (not shown) in the event that the normal load connection is disrupted. The rate of revolution will then be reduced sufficiently to allow mechanical braking equipment on the turbine to take over, to stop and later hold the generator stationary. If everything else fails, the mechanical brake will be able to stop the generator from turning at full speed, any damage arising then being limited to the less expensive braking equipment.Details of the braking equipment are not shown, as this is considered to lie within the competence of the skilled person, and known components can be utilized.

In the illustrated example, the average maximum output over several wave periods will be 4 MW. A synchronous generator with a fixed number of revolutions per minute has been chosen; therefore, no equalization over the wave period will occur as a result of the fly-wheel effect from the rotating masses. The generator output will therefore vary between 0 and a maximum value of 8 MW twice per wave period. If this maximum power is exceeded, the generator 1 6 will be disconnected from the electrical load and braked to a stop.

If the turbine blades are immersed in water when rotating at full revolution, they could be broken off, resulting in very expensive damage to the apparatus. To prevent the water column from swinging up as high as the turbine on the occurrence of extremely high waves, the float body 1 8 is placed upstream of the turbine. This float body can consist of an elastic, inflatable rubber sleeve which will be lifted by the water column to lie against the grate 1 7, closing the openings in the grate and thus isolating the overlying compartment from the oscillation chamber 8. If the oscillating water column rises higher, it will compress the volumes of air enclosed in the chamber sections 20, to provide a soft damping effect and prevent the water column from striking the roof 5 of the concrete structure.

The wave energy power plant will normally be unmanned, but when starting it up for the first time and as required at other times, personnel may be accommodated on the structure. The power plant can be entered from a boat via a small quay 23 on the lee side, or from a helicopter. As mentioned previously, the hollow transverse wall 6 may be fitted out with rooms for apparatus and facilities for personnel. The compartment between the float valve, i.e. above the grate 17, and the generator 15, 1 6 may be reached via an air lock 24, and gangways 25 can be provided along the walls 12, 13 from the generator 15, 1 6 to the air lock 24.

The pressure oscillations in the upper air chamber will not usually be large enough to prevent personnel from using these gangways.

On the roof 5 of the power plant, a yoke 26 with two pulleys 27 and 28 is secured. When the generator 15, 1 6 is to be dismantled for maintenance, a winch 29 is first placed on the top of the yoke (see Figure 4), for instance by helicopter. A hoist cable is then led over the pulleys and passed through a hatch in the roof above the generator 1 6. The parts can then be let down into a boat 30, as shown in Figure 4. The generator is conveniently instailed/dismantled as a watertight unit.

The confining surfaces, in this case the extensions 10, 11 of the side walls, may be fixedly secured to the wave energy absorber, as shown in the illustrated example, but the wall members or confining surfaces may also be made as independent members. Optionally, they may extend upwardly as wall members from the base 1, thus providing access for the sea water to flow between the wall members and the faces of the wails 2 and 3 which define the opening 7.

The power plant may optionally have a plurality of adjacent oscillation chambers, but it is preferable to build them as shown and described above to facilitate construction and erection, for technical reasons which will be obvious to a person skilled in the art.

To obtain a satisfactory power production at one location, 50 units as shown and described might for instance be placed beside each other along a length of a coastline in question, for example spaced about 80 metres apart. To give one an impression of the dimensions it should be noted that the submersion depth shown on the drawings is about 30 metres.

The wave energy absorbers may be positioned arbitrarily in relation to each other, each selected with regard to the local prevailing direction of wave incidence. The absorbers may also be positioned in an equidistantly spaced row at right angles to the direction of incidence of the waves, or in an equidistantly spaced row parallel to the direction of incidence of the waves.

Claims

1. A liquid wave energy absorber of the type in which the efficiency of energy absorption is a function of the frequency of the incident liquid waves, in which the band width of the dependency of the energy absorption upon the incident wave frequency is increased by arranging waveconfining surfaces adjacent to the absorber in such wise that at least one new maximum of energy absorbed versus frequency is created by the placement of the confining surfaces.

2. A liquid wave energy absorber according to claim 1, in which the absorber operates according to the principle of an oscillating column of water.

3. A liquid wave energy absorber according to claim 1 , in which the absorber operates according to a resonant fixed body (buoy) principle.

4. A liquid wave energy absorber according to any one of the preceding claims, in which the placement of the confining surfaces is calculated having regard to the topography of the site where the absorber is located.

5. A liquid wave energy absorber comprising a chamber with a submersed opening through which the liquid can flow in and out in response to incident liquid waves, thereby setting the liquid mass inside the chamber into oscillation, and a means which taps energy from the oscillating system by damping the oscillations of the liquid mass, characterized in that on each side of the opening, wall members acting as confining surfaces are located in such manner that the dependency of the energy absorption upon the wave frequency obtains at least one new maximum at a frequency determined by the placement of the confining surfaces.

6. A liquid wave energy absorber according to claim 5, wherein the energy-tapping damping means comprises an air-filled compartment inside the chamber above the liquid mass, and an air turbine which is acted upon by the air flows resulting from the oscillation of the liquid mass inside the chamber, a float valve being provided to prevent the liquid mass from rising up to the air turbine.

7. A liquid wave energy absorber according to claim 6, in which the float valve comprises a horizontal grate inside the chamber above normai level of the liquid mass and a floating body which is lifted by the liquid mass into contact with the lower surface of the grate to close off the apertures in the latter when the liquid mass rises to a level where, apart from the float valve, it might contact the air turbine.

8. A liquid wave energy absorber according to claim 7, in which the floating body is an inflated resiiient body.

9. A liquid wave energy absorber according to any one of claims 6 to 8, in which a downwardly open chamber section is provided above and at one side of the float valve.

10. A wave energy power plant incorporating a wave energy absorber as claimed in any preceding claim.

11. A liquid wave energy power plant substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

12. Electrical power extracted from a power plant as claimed in claim 11.