WO2005095790A1

WO2005095790A1 - Wave energy conversion apparatus

Info

Publication number: WO2005095790A1
Application number: PCT/GB2005/001166
Authority: WO
Inventors: Neal A. Brown; Joseph E. LEARY
Original assignee: Hoffman Virginia L
Current assignee: Hoffman Virginia L
Priority date: 2004-03-27
Filing date: 2005-03-24
Publication date: 2005-10-13
Anticipated expiration: 2006-09-27
Also published as: GB0407014D0

Abstract

A wave energy conversion device is disclosed comprising a plurality of oscillating water columns. The oscillating water columns are of different sizes and each comprises a generally vertical portion (3) and a generally horizontal portion (5). The horizontal portion terminates in a mouth (6) facing the principal direction of incident waves. The individual oscillating water column are tuned by variation of their size to be responsive to different band widths within the frequency spectrum of incident waves over a given time period such as annually. The oscillating water columns are in particular constructed such that their natural resonant frequency is within that band width.

Description

Wave Energy Conversion Apparatus

Field of the Disclosure

The present invention relates generally to devices and systems intended to harvest the energy transported by ocean or sea waves and to convert it into usable electrical power or other products. The present invention relates specifically to ocean wave energy harvesting devices of the oscillating water column (OWC) type, particularly, but not exclusively, those in offshore locations where wave energy is not seriously attenuated by shallow water effects, and to the efficiency with which they harvest the wave energy available to them; their capacity, economy, durability and longevity. Secondarily, the present invention relates to the applicability to OWC wave energy systems of guiding principles known in unrelated fields of physical science, electrical engineering, and economics.

Prior Art

US 4,858,434 (Masuda)

An example of a wave energy device employing coupled motions is illustrated in Figure 7 showing the Taka-hiko Masuda "backward bent duct buoy" (BBDB). This has an L-shaped passage in an OWC in a moored, floating buoy. The area cross-section of the horizontal duct relative to that of the vertical, and the length of the former are not specified. Masuda achieves a measure of energy absorption bandwidth by including more than one mode of motion of the combined OWC and buoy dynamical systems. The horizontal duct opening faces downwind, explicitly so as to best take benefit from the heave (and pitch?) motions of the buoy. Masuda alleges, but does not demonstrate, that the orbital motion of the incident wave is so phased as r lυσtv; i vvw

to assist the outward flow of water from the leeward facing port of the OWC. While the wave's orbital velocity at the downstream port may be in phase with the velocity of the water in the horizontal channel, it is clear that the pressure under the associated wave crest opposes that motion. In fact, in the case cited by Masuda, it appears likely that the OWC is doing work on the ocean rather than absorbing energy from the ocean. The plot of Masuda's figure 8 (and also his figure 2) is non-informative on this vitally important point because it displays only the pressure generated in the OWC and does not inform us of the associated velocity or volume flow rate, if any, and phase. Masuda also shows in these two plots of figure (2) and figure (8) that a forward facing bent duct would significantly decrease the energy harvested by his ship-like buoy, which is moored at the bow. However, the data displayed in U. S. Patent No. 4,858,434 is insufficient to justify the conclusions drawn and is disputed by the present invention. To quote from McCormick⁽¹⁾: "Engineers have found that the available wave energy to the cavity is doubled if the cavity opening faces the oncoming wave. In this situation the wave pressure at the opening is not just the static pressure but the total wave pressure."

AERE Harwell

To quote further from McCormick¹: "A device designed to utilize the total pressure, called the oscillating water column, is illustrated in Plate 4.1." The Plate shows two floating, moored OWC units each having a number of adjacent horizontal rectangular openings underwater with "roof air vents, fewer in number. The caption reads: "An artist's conception of an oscillating water column wave energy conversion system. Courtesy of the Energy Technology Support Unit, AERE Harwell - The United Kingdom". (Without reference.) The patents on which the plates are based appear to be US4139984 & US4198821 , or their UK equivalents, and are noted here as

¹ Ref. 1. McCormick, Michael E., "Ocean Wave Energy Conversion", John Wiley & Sons, New York, 1981, ISBN 0-471-08543-X. prior art only because there is a superficial, external similarity of appearance. The present invention incorporates a nested set of a multiplicity of OWC units to provide adequate bandwidth with adequately small, real impedance to absorb wave energy in the important yearly average incidence.

GB2325964

GB2325964 describes a point collector OWC that attempts to harness wave energy by creating a resonant response to the incident waves. Being a point absorber, its available energy cross section (width) is limited to a fraction of the length of an incident wave. Its energy harvesting effectiveness is therefore biased toward long waves which, though energetic, occur less frequently. The energy availability of shorter, more frequently occurring waves is sacrificed, relatively. (Comparison of effective harvest cross section- to- physical cross section is irrelevant to a point absorber.)

The device is comprised of a bundle of vertical tubes of differing length, open at their bottoms. By opening downward, a significant fraction of the incident wave energy is lost through phase mismatch and turbulent edge losses. In order to be resonant at energetic wave frequencies, the tubes must be very long, hence deep. The device does not make use of tube cross-section variation (reduction) to achieve low frequency resonance with reduced tube length and mouth depth. Further, it does not aim its tube mouths in the direction of incident waves in order to maximize energy absorption, resulting in a loss of up to one-half of the available energy.

At depth, the hydraulic input impedance of a tube must be very low to approximately match that of an incident wave. In other words, the volume velocity of water flow into and out of the tube mouth must be very large despite a greatly depth-attenuated driving wave pressure. This implies, not only a resonance requirement, but one with a very high amplification factor, or "q". This implies, in turn, a very narrow bandwidth of response. To encompass the bandwidth of annually averaged available energy, numerous tubes must be employed, having differing lengths and resonant frequencies. As the damping is beneficially provided by the power generator (rather than by ever-present hydraulic friction losses), that power must be commensurately small for each tube in order to achieve high amplification. The performance is greatly compromised by frictional losses that replace useful power. The power capacity per unit investment suffers thereby.

Most such devices and systems as the above prove uneconomic and non- competitive because they are capable of capturing only a small portion of the annually available incident wave energy, while incurring disproportionately high initial, operating and maintenance costs. None are of utility scale. Existing systems and devices have often proven incapable of mechanically resisting storm conditions and of continuing to operate in such conditions. The present invention is as survivable as any offshore, moored, floating platform or structure and will continue to generate self-limiting, high power levels in storm conditions without being shut down. It does not depend on its motion in a sea-way to generate power. A perhaps larger first cost is amortizable over a very long useful lifetime, in analogy to hydro-power installations.

The present invention is meant to remedy the shortcomings revealed in the prior art while introducing some new concepts that enable economically competitive exploitation of the wave energy resource.

Summary of the Disclosure

According to a first aspect of the present invention there is provided a wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising a vertically inclined portion and a horizontally inclined portion extending from a lower part of, and in fluid communication with, the vertically inclined portion and defining a submerged mouth oriented in use towards a principal approach direction of incident waves at least one power take off device responsive to the movement of fluid in the respective oscillating water columns wherein the cross sectional area of each vertically inclined portion is greater than the cross sectional area of the respective horizontally inclined portion, and the oscillating water columns are selected to be of different respective sizes.

Preferably in this first aspect of the invention the mouths of the said horizontally inclined portions are arranged in a common plane.

The cross sectional areas of the respective horizontally and vertically aligned portions need not be constant. For example, one or other of the said portions may be tapered. In particular, the horizontally inclined portion may be tapered (i.e. of reducing cross section) towards the mouth. Preferably, however, the said portions are of regular cross section.

According to a second aspect of the invention there is provided a wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising a vertically inclined portion and a horizontally inclined portion extending from a lower part of, and in fluid communication with, the vertically inclined portion and defining a submerged mouth oriented in use towards a principal approach direction of incident waves at least one power take off device responsive to the movement of fluid in the respective oscillating water columns wherein the oscillating water columns are selected to be of different respective sizes, and the mouths of the said horizontally inclined portions are arranged in a common plane.

It is especially preferred in both the first and second aspects of the invention that the vertically inclined portion is substantially vertical. However, the vertically inclined portion may, in principle, adopt any angle with respect to the vertical of from 0° to less than 90°, preferably from 0° to 45°, more preferably from 0° to 30° and particularly 0⁶ to 20°. It is also especially preferred that the horizontally inclined portion is substantially horizontal. However, the horizontally inclined portion may, in principle, adopt any angle with respect to the horizontal of from 0° to less than 90°, preferably from 0° to 45°, more preferably from 0° to 30° and particularly 0° to 20°. The vertically inclined portion thus extends upwardly from its junction with the horizontally inclined portion. The horizontally inclined portion (if not in the most preferred substantially horizontal configuration), may extend upwardly or downwardly from its junction with the vertically inclined portion, but more preferably extends downwardly.

Preferably, in an embodiment of the second aspect of the invention the cross sectional area of each inclined portion is greater than the cross sectional area of the respective horizontal portion. In this embodiment, the cross sections of the respective horizontally and vertically aligned portions need not be constant. For example, one or other of the said portions may be tapered. In particular, the horizontally inclined portion may be tapered (i.e. of reducing cross section) towards the mouth. Preferably, however, the said portions are of regular cross section. According to a third aspect of the invention there is provided a wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising r luctj i vvυ an upper first end disposed in use above the water surface and a lower second end, said second end defining a submerged mouth oriented in use towards a principal approach direction of incident waves, and an arcuate portion between said first and second ends at least one power take off device responsive to the movement of fluid in the respective oscillating water columns, wherein the cross sectional area proximate said first end is greater than the cross sectional area proximate said second end, and the oscillating water columns are selected to be of different respective sizes.

Preferably in this aspect of the invention each oscillating water column is of tapered construction with the cross sectional area reducing from the first end to the second end.

Preferably the mouths of the oscillating water columns are arranged in a common plane.

According to a fourth aspect of the invention there is provided a wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising an upper first end disposed in use above the water surface and a lower second end, said second end defining a submerged mouth oriented in use towards a principal approach direction of incident waves, and an arcuate portion between said first and second ends at least one power take off device responsive to the movement of fluid in the respective oscillating water columns wherein the oscillating water columns are selected to be of different respective sizes, and the mouths of the said horizontally inclined portions are arranged in a common plane. Preferably in this fourth aspect of the invention the cross sectional area proximate said first end is greater than the cross sectional area proximate said second end.

In a particularly preferred embodiment the oscillating water columns are disposed in a nested arrangement in which adjacent oscillating water columns share common boundary walls.

Preferably the size of each oscillating water column is selected to provide respective oscillating water columns with a response band-width responsive to a frequency range selected from the frequency band-width of incident waves over a given time period. Typically, and preferably the frequency band-width of incident waves over a given time period represents an annualised average frequency band-width, that is, an annualised average of the range of frequencies experienced in a given location.

In one preferred form of the first and second aspects of the invention said size (of each oscillating water column) is selected by selecting the length of the said horizontally inclined portion and/or the height of the said vertically inclined portion.

In another preferred form of the first and second aspects of the invention invention, alternatively or additionally, said size is selected by selecting the ratio of the cross sectional areas of the respective horizontally and vertically inclined portions.

Preferably the response band-widths of at least two of said oscillating water columns are substantially contiguous. In especially preferred embodiments of the invention, the response band widths of all, or substantially all, of the oscillating water columns are contiguous and form a series spanning at least a major part of, and more preferably substantially all of, the frequency range of energetic incident waves. It is particularly preferred that each respective oscillating water column is constructed so that its natural resonant frequency lies within said respective frequency range selected from the frequency band-width of incident waves over a given time period. That is, the natural resonant frequency of a given oscillating water column of the invention is made to conform to the selected frequency band-width of incident waves for which the particular oscillating water column is intended to be responsive.

It is especially preferred that the, or each, power take off device provides respective oscillating water columns with a predetermined load such the input impedance of the oscillating water column is substantially matched to the characteristic impedance of incident waves having said frequency range selected from the frequency band-width of incident waves over a given time period. In this way, energy transfer from the incident waves is maximised.

Preferably, in embodiments of the first and second aspects of the invention, at least one said oscillating water column includes an array of turning vanes disposed proximate the junction of the horizontally and vertically inclined portions.

In an alternative embodiment of the first and second aspects of the invention the walls of the respective horizontally and vertically inclined portions are curved in the region of the junction of said portions, to provide a smooth transition between the respective portions. In this construction, the array of turning vanes may not be required.

Other preferred features, embodiments and variations of the apparatus of the invention include the following, which may be employed in the invention as defined above either alone or in combination with any one or more other features, embodiments and variations: Most preferably in each oscillating water column (OWC) the vertically inclined portion is partially water-filled and the horizontally inclined portion is fully water-filled and connected at the lower extremity of said vertically inclined portion; . .

As suggested above, the horizontally inclined portion of each OWC extends towards (and the mouth is open to the sea) in a direction facing that of the most likely incidence of energetic waves;

It is much preferred that each OWC is, by means of its geometry and submergence depth of the mouth, tuned in frequency of natural oscillation of water in the OWC to a frequency of wave among the range of energetic incident wave frequencies;

Preferably, each OWC, by means of its height-of-mouth opening, normalized to central depth of mouth opening, provides an energetic frequency bandwidth about its aforesaid resonant frequency;

As noted above, preferably each OWC is loaded by a power conversion (or power take off) device (such as a turbine) that provides a measure of damping to the water oscillation response at the aforesaid resonant frequency, which is consistent with the aforesaid normalized bandwidth;

Also as noted above, each such OWC, by dint of its appropriately damped resonance, presents a wave energy conversion device (WEC)-face area- averaged input impedance equal to the characteristic impedance of the waves having corresponding frequency;

Most preferably, each of a plurality of said OWCs is tuned to a resonant frequency different from the others. Of course the OWCs may be arranged in groups and within each respective group the OWCs are tuned to a resonant frequency different from the others. In the latter case, preferably each group spans the annuals average frequency band-width of incident energetic waves;

Thus, the said resonant frequencies preferably providing a sampling of the frequencies within the yearly-average power spectrum of energetic incident waves for the site location;

Preferably, the bounding frequencies of the frequency bands centred on the said resonances are the same as their neighbours, that is, the upper limit of the frequency band of a given OWC corresponds to the lower limit of the frequency band of an adjacent OWC;

Preferably, the aforesaid frequency bands, when taken together, form a continuous band that spans the economically harvestable portion of the yearly-average power spectrum of energetic incident waves for the site location;

Preferably in apparatus of the invention comprises a plurality of OWCs whose length in a direction perpendicular to that of expected incident energetic waves is not less than one-quarter of the length of that wave whose frequency is that of the lowest-frequency resonant, largest OWC; whose overall length in a direction perpendicular to that of expected incident energetic waves is some integral multiple of the aforesaid minimum length; and wherein each member of a plurality of OWCs is tuned to different wave frequencies, and has a length, in a direction perpendicular to that of expected incident energetic waves, that is not less than one-quarter of the length of that wave to which frequency it is tuned;

Preferably in the apparatus of the invention, each OWC length is bounded by transverse structural bulkheads that are both water- and air-tight; the said bounding bulkheads preferably extend transversely through all adjacent OWCs for structural purposes, but are not air-tight, unless coinciding with the bounding bulkhead of another OWC;

a plurality of structural transverse bulkheads, which may not be air-tight, are spaced lengthwise at an interval equal to the mouth-height of the largest, lowest-frequency OWC; such that the mouths of said largest, lowest frequency OWCs are nominally of rectangular (and especially nominally square) shape when viewed in elevation, waveward;

the bounding bulkheads of the plurality of OWCs are variously coincident with members of the aforesaid plurality of structural transverse bulkheads;

Preferably:

the aforementioned plurality of OWCs are made contiguous so that neighbouring units share both horizontal-longitudinal and vertical-longitudinal bounding walls as common between them, thereby forming a laterally- vertically nested array of OWCs;

the vertical OWC side walls are continuous in length and reach upward to a roof structure, both being air-tight;

the volume bounded by the OWC water surface and the roof, the left and right longitudinal-vertical sidewalls, and the air-tight bounding end bulkheads in each instance forms an independent air-filled plenum chamber;

Preferably in each OWC:

an air-flow rectifying cabinet resides in an airflow path between an atmospheric duct and an air plenum; an axial-flow air-turbine wheel pierces a bulkhead that divides said cabinet into a first half and a second half;

the first half cabinet is fitted with two doors or two sets of doors, each opening inwardly, one door or set of doors allowing, in an open position, communication with a duct which opens to atmosphere and the other door or set of doors allowing, in an open position, communication with the air plenum, which doors, or sets of doors, are nominally air-tight when closed and are actuated to open or close by small pressure gradients in the corresponding directions;

the second half cabinet is fitted with two doors or two sets of doors, each opening outwardly, one door or set of doors allowing, in an open position, communication with a duct which opens to atmosphere and the other door or set of doors allowing, in an open position, communication with the air plenum, which doors are nominally air-tight when closed and are actuated to open or close by small pressure gradients in the corresponding directions;

the air-turbine wheel penetrating the cabinet's dividing bulkhead is subject to air flow only in the direction from the first half cabinet-to-the second half cabinet and rotates in but one angular direction without regard to the rise or fall of water level below the attached air-plenum within the enclosing OWC and the subsequent direction of flow of the associated displaced air;

the air-turbine wheel is fitted with a shaft that reaches on one side or the other to a rotary electric generator situated outside of the air-flow rectifying cabinet;

the air-turbine optionally (alternatively) including two wheels, each piercing a separate bulkhead dividing the said air-flow rectifying cabinet into three sections: a first, a second and a centre section, wherein the first and second sections are connected together in common to the atmospheric duct and together to the air plenum;

an air- turbine-generator-cabinet unit resides in each OWC air plenum on a walkway at an elevation above the highest water level expected to.be intermittently reached in said OWC while encountering extreme sea-states;

each OWC unit between water- and air-tight bounding bulkheads is served by an air-turbine-generator unit;

the turbine-generator and air-flow rectifying cabinet are accessible to maintenance personnel, on foot, via connecting/surrounding walkways;

In further preferred features, preferably:

the apparatus of the invention is constructed of reinforced concrete, thereby requiring but minimal maintenance and inspection over a very long useful life;

the apparatus is supported upon (or integral with) a moored, floating, stable platform in deep water;

alternatively, the apparatus may be supported upon a grounded, gravity- base, fixed-legged or jack-up platform in shallow water;

the apparatus may be installed in a fixed shore emplacement;

In preferred forms the apparatus of the invention comprises a plurality of OWCs in which

"freeing ports" are located below the level of the turbine-generators, at a height above a statistically determined level expected to be exceeded only rarely in extreme sea states, which one-way freeing ports, by venting water back to the sea, are sized to prevent further rise of water level in the OWC

Brief Description of the Drawings

For a better understanding of the invention and to show how the same may be carried into effect reference will be made by way of example only to the following drawings in which:

Figure 1 shows a transverse plane, cross-sectional view of the nested OWC absorbers and associated air turbines and ducting, when looking in the longitudinal direction; Figure 2 shows a horizontal plane section plan view of OWC cell air spaces variously grouped into common plena serving air turbines.

Figure 3 illustrates schematically the rectification of the turbine airflow, by means of four sets of hinged louver-doors. Figure 4, 5 and 6 illustrate the development from a classic vertical OWC design, to a single area-changed "L" shape, and finally, to the nested "L" shapes.

Note: In the description and discussion of this device and system the terms longitudinal and transverse will be used to mean the following: The longitudinal direction will be parallel to the wave crests of normally incident waves. The transverse direction will be parallel to the direction that normally incident waves travel. It is recognized that waves can arrive at angles up to plus-or-minus 90 degrees to that normal, but do not change these definitions.

Description of Preferred Embodiments

The present invention makes use of an assemblage of absorbers of the oscillating water column (OWC) type to provide a frequency bandwidth-of- effectiveness, which purposely spans that of high annual energy for a specific oceanic locale. Each of the OWC absorbers is made resonant over a different sub-interval of the desired overall frequency bandwidth. The individual pass-bands are made contiguous, unless the available energy spectrum suggests otherwise.

The absorber is in basically a two-dimensional configuration extending in a direction perpendicular to the principal direction of incidence of energetic waves. It will be applied preferably in deep water where waves have not suffered attenuation, and will preferably be floating and moored, although fixed to the sea-bottom is possible using a "jacket" type structure. It will be sufficiently large that its supporting floating platform may fulfil other purposes, such as lifesaving or as an offshore fishery-port, which will allow dilution of its initial and operating costs. An example of such a large, stable platform may be found in Very Large Floating Offshore Structures (VLOFS) designs, of the type described in US Patent No. 5,375,550.

Each OWC of the nested set consists of a vertical column contiguous with a horizontal duct extending to windward and opening on a submerged common vertical face. The cross-sectional area of each such horizontal duct is generally selectably smaller th n that of its associated vertical column. The purpose of this feature is to allow the independent selection of the resonant natural frequency of oscillation of each OWC.

Most ocean wave energy absorbers fail economically because much of the energy incident upon them is either reflected from, or passes beyond them without being absorbed. This results from a neglect of the principle of impedance matching, wherein the transfer of energy between connected wave-guides is maximized when the input impedance of the receiver is equal, in magnitude and phase, to that of the supplier. Referring to Figure 4, it will be obvious to one skilled in the art that the characteristic impedance of ocean waves 31 , given by the product of water mass density and wave celerity, is considerably smaller than that at the input of an oscillating water column 32, unless the OWC is operating at or near resonance. Typically, the resonant frequency of an OWC is higher than that of energetic ocean waves. As that resonant frequency is inversely proportional to the square-root of the height 33 of the water column, it is clear that an OWC may be brought into resonance, and hence a condition of reduced input impedance, by lengthening, or increasing the depth 33 of its water column. Unfortunately this simple approach suffers from two practical defects; it is: a) expensive and b) subject to markedly reduced energy availability with depth.

In the present invention, the water column is indeed lengthened, but with reduced cross-sectional area 34, and is then turned into the horizontal 35, toward the direction 30 of wave approach. In this way, the effective mass of oscillating water is greatly increased, controllably reducing the resonant frequency of the OWC and pointing the mouth toward the waves to optimally match impedances. (When the wave elevation is at a maximum, the pressure and forward particle velocity are both also at a maximum; and the reverse is true.) At the same time, the depth of the OWC structure is reduced in comparison to that required of a straight vertical column of constant section, and places the mouth at a location near that of maximum energy availability. Example: for a 14 second resonant period, the depth of a straight, constant section column would be about 160 feet. (In any case, approximately 1/6^th of the length of the deep-water wave to which it is to be matched.) At this depth, the available wave energy is reduced to about 14% of that at the surface.

When impedances are matched, the incident wave is absorbed, thereby preventing reflection or transmission. In essence, the reflection or transmission is "swallowed". The oscillatory volume flux from the mouth of the OWC is, in two dimensions, just that required to generate, in still water, an opposing wave train of the same amplitude and frequency as that of the incident wave, but with reversed polarity. The impedance at resonance is resistive, that is, the required driving pressure is in phase with the water velocity 39 in the OWC. Beyond frictional losses, which are to be minimized, the resistive impedance is to be furnished by the oscillating air pressure load above the OWC water surface. That load is, in turn, provided by the power takeoff, which in this example, is shown as a turbine-generator (16) & (19) or by other means of extracting the energy which may be fluid, chemical, mechanical or electrical. Conventional turbines may be employed, or a

"Wells turbine" may be used which has the advantage of rotating in the same direction irrespective of the air flow direction, but subject to a possible loss of efficiency. The load must be adjusted to yield the correct impedance in the water in order to maximize energy absorption. If the load is too great, there will be pressure increase up to double, but little motion, hence little power developed. Alternatively, if the load is too slight, there will be much motion, but little pressure developed, hence, again, little power. These two cases relate to positive and negative reflections, respectively.

Those skilled in the art will recognize that several two-dimensional horizontal ducts extending from OWCs will be stackable in a contiguous manner, as shown in Figures 1 & 6, if each is tuned to a frequency in a geometric series, with frequency-proportional band-widths. The height of each duct will be proportional to the square root of the product of its mean depth with its normalized band-width. That band-width, normalized on center frequency, will be equal to the "loss factor" (the inverse of the peak resonant response factor) of the OWC oscillator with frictional and power loads. It will be further found that the progression of horizontal duct lengths and vertical OWC widths may be allocated so that the OWCs, 38 may be "stackable" horizontally in the same order as are the ducts in height. The result is that the L-shaped OWCs may be nested in a configuration requiring minimum water draft and large, but mostly continuous horizontal extent. Other bandwidth divisions and distributions are also possible with satisfactory geometric attributes, and may be desirable as dictated by the spectral distribution of annualized average incident energy. The lowest frequency OWC, and largest 36, lies at the bottom of the stack and farthest from the mouth plane, while the highest frequency OWC, and smallest 37, lies at the top of the stack and nearest the mouth plane. A quite I arge, but efficient, cellular structure results that is economical to be built in (preferably) reinforced concrete (or any other suitable medium capable of withstanding the demands placed on it by a hostile ocean environment). Such a structure will be extremely durable in the marine environment and require little or no maintenance whatever beyond periodic cleaning of marine growth.

Each nested OWC member is divided into cells in the longitudinal direction by transverse structural bulkheads 9. Where power takeoff is by air-turbine, each such cell will contain a complete air-turbine system. The maximum length of each of these cells is made somewhat greate r than one-quarter of the shortest trace wavelength expected to be experienced by the associated OWC at its resonant frequency. This relates to the expectation that the pertinent waves will not always approach from the perpendicular direction, so that their trace wavelength may vary as the inverse sine of their incidence angle, measured from the normal. A quarter wavelength assures that maximum air flow will be directed to the individual turbi ne; no "cancellation" will result within a cell. In the low frequency OWCs, where air-flow volumes are expected to be very large, cells shorter than the ab»ove may be selected in order to limit the size of the turbines. In any event, t e smaller OWCs may have numerous single-flow turbines while the larger ones will have fewer, large, double-flow turbines (two wheels on a single shaft). Triple-flow turbines are also a possibility. Enlarged rectifier plenum volumes may be enlisted as accumulators to reduce wave-frequency fluctuations in the output. The absorption bandwidth of the system at both ends, at the large, more expensive low-frequency end, and at the small, but numerous, high frequency end, must be the result of a careful an alysis of return on investment. The result will depend on the nature of the wave energy resource, the price of electricity and the costs of the asset and its operation.

The vertical excursion of the water surface in the columns will exceed the wave height by a factor equal to the inverse of t t e loss factor. In order to limit the height to which water may rise in the col umns, each cell is fitted with freeing ports situated at a height above the still waterline dictated by the height of a wave of resonant frequency which wil I occur with a selected small probability in extreme conditions. The freeing ports will open against pre-set spring load by combined air and water hydrostatic pressure. Referring to the air-turbine engines, these are located at a height safely above that of the freeing ports to avoid flooding them. Water discharged through freeing ports is allowed to cascade down the "roof of the facil ity toward the face of the absorber. Diverter structures above each freeing port will prevent the weight of such upper discharges from holding them closed. In the troughs of extreme waves the OWC duct mouths may be u ncovered. In that case, water will tend to drain from the ducts and columns and, in extreme seas, to vent in volumes of atmospheric air. The draining and refilling of the ducts need not threaten or damage the integrity of the structure or machinery. Indeed extra engulfed air will resist the subsequent rise of water level in the column until vented by the freeing ports. The freeing ports provide a self- limiting characteristic to the energy absorbing and power generating capacity of the system. The turbine-generators will not be overloaded, nor need they shut down.

Whereas the classical OWC (Figure 4) involves a simple vertical duct 32, the present invention, restricts the vertical dimension but compensates by adding a more-or-less horizontal extension 35 (Figure 5>), opening to windward, having a different, generally smaller, cross-sectional area 34. The natural period of oscillation for the "bent" OWC can be shown, in terms of the dimensions noted, to those skilled in the art, to be approximated by the relation:

T₀ *-2xJΫ l A_CX ^■ + 1 A Y

where Y is the nominal height 33 of the vertical section of the OWC, g is the acceleration of gravity, A_c is the cross-sectional area per longitudinal unit span of the ve rtical column 34a, A_w is the cross-sectional area per longitudinal unit span of the horizontal extension 34, and X is the nominal length of the horizontal extension 33a. The "longitudinal" direction is in the normal to the plane of the drawing.

Those skilled in the art will recognize that the product of those terms, exclusive of the right-most radical, comprise a mathematical expression for approximating the resonant period of a "classical" OWC having a water column height of dimension Y. The second radical has a value greater than unity. In fact, for approximately equal values of the dimensions X and Υ, the value of this radical may be made considerably greater than unity by incorporating a cross-sectional area A_w that is appreciably smaller than that represented by A_c.

For example, consider a height Y=32 feet. The classical OWC resonant period would be approximately 6 seconds. If now the length of the horizontal extension is taken equal to that of the vertical height, with equal cross- sectional areas, the resonant period would be increased by approximately 40%. If, further, the cross-section of the horizontal extension were made one-third that of the vertical column, the period would be increased by 100% rather than 40%. In this sequence of events, that period would be increased from 6 seconds to 8.5 seconds, and finally to 12 seconds, approximately. Those skilled in the art will recognize that there is much more energy available in ocean waves with periods about 12 seconds than in those with periods about 6 seconds. Further extensions of resonant period are available within practical dimensions.

Referring to Figures 1 & 2, the ocean wave energy harvester 1 is in the form of a power house mounted upon a large, stable floating platform 2. (A bottom-mounted gravity base structure would also serve as. a support, in shallow water depths.) Multiple (three shown) vertical columns 3 are flooded with water as indicated by a still water level line 4. Each of these vertical columns is extended horizontally at its lower end by a duct 5 reaching in an up-wave direction and terminating in a port 6 opening to the sea, said ports being generally aligned vertically in the plane of the face 7 of the absorber. The cross-sectional areas of the horizontal ducts 5 are generally smaller than those of their attached vertical columns 3. The combined L-shaped conduits are bounded, and separated one from another, by longitudinal, air- and water-tight structural walls 8. The L-shaped conduits are further divided into discrete cells by continuous transverse structural bulkheads 9. The individual cells comprise independent oscillating water columns. Turning vanes 10 traverse the cells, spanning between the transverse bulkheads in the transition area between the horizontal ducts 5 and the vertical columns 3. In the larger vertical columns, vertical anti-slosh plates 11 also span between the bulkheads.

Most of the transverse bulkheads 9 terminate at a height 12 that is above the statistical high water-level 13 of their particular column but below the overhead "roof 14. This provides that a number of longitudinally contiguous column air spaces are serially joined by a common plenum 15. In the uppermost regions of the plena there are located air-driven turbo-generators 16. The turbines run on air that is alternately discharged through and drawn in from atmospheric vent ducts 17. The turbines each sit in a cabinet- compartment 18 fitted with one-way doors 21 that provide rectification of the air-flow so that it always moves through the turbine 19 in the same direction, without regard to whether air is moving from plenum-to-atmosphere, as on the up-stroke of the OWC, or from atmosphere-to-plenum, as on the down- stroke.

Above the statistical high water-level 13, but well below the tu rbine generators 16, there are situated freeing ports 20 that insure the safe discharge, to the exterior of the absorber structure, of excess water that rises too far in oscillations in an extraordinarily high sea state. The freeing ports 20 are spring loaded so that they do not open under the influence of air pressure alone, but will when that pressure is augmented by a small head of water.

In the plan view, Figure 2, it is apparent that the gallery plena are subdivided into longitudinal cell cohorts, each combining an integral num ber of transverse cells. The transverse bulkheads that define these subdivisions extend upward to the overhead "roof to form airtight closures. It is obvious that the number of cells collected between bulkheads depends on the size, hence the resonant period , of the OWC set member. The fewest number, hence shortest cell cohort, is governed by the objective of maintaining phase coherence for the airflow within a turbine-served cohort of cells. Assuming wave incidence directions are possible anywhere within a 180 degree arc, the longest effective cohort of the smallest (shortest-period) OWC system would be equivalent to about one-quarter of the corresponding wave length. Considering that each cohort requires a dedicated turbine-ge nerator set, and that the probability of grazing-incidence waves is small, economy dictates cohorts somewhat longer than that. At the other extreme, the cell cohort length will be limited by the airflow capacity of economically practical turbines. As the airflow rates obtainable with large, long period OWCs are potentially immense, this calculation will be influenced by the availability of double-flow (even triple-flow) parallel turbine configurations.

Because the air pressure variations in the cell cohorts are comparable to those under the incident waves, maintenance personnel must be protected by tunnels enclosing their access walkways. Generators and other electrical equipment will be kept at constant pressure and will be accessible within these tunnel walkways. However, turbines and other flow-path parts must be outside the protection and will only be accessible during operation if their cells are vented to the atmosphere, essentially short-circuiting the air-path.

The turbines are located in compartments that are situated in, and controllably communicate with, the atmospheric air ducts, on the one hand, and the gallery air space of their cohort cells, on the other. From Figure 3 it will be obvious even to those not skilled in the art, that by means of four sets of hinged louvered doors, the flow oscillating in and out of a cell cohort can be rectified so that it moves through the turbine in only one direction, if unevenly. That unsteadiness of airflow velocity through the turbine can be accommodated while maintaining high efficiency by employing a reaction- type turbine. That turbine type, where most of the pressure drop takes place in the rotating blading vs. the stationary nozzle vanes, has a broad efficiency characteristic on the parameter of blade speed - to - air speed ratio. The turbines are located near the longitudinal centre of the cell cohorts that they serve, in order to minimize pressure losses in their airflow to- and from the OWC cells.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

1. A wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising a vertically inclined portion and a horizontally inclined portion extending from a lower part of, and in fluid communication with, the vertically inclined portion and defining a submerged mouth oriented in use towards a principal approach direction of incident waves at least one power take off device responsive to the movement of fluid in the respective oscillating water columns wherein the cross sectional area of each vertically inclined portion is greater than the cross sectional area of the respective horizontally inclined portion, and the oscillating water columns are selected to be of different respective sizes.

2. A wave energy conversion apparatus as claimed in claim 1 wherein the mouths of the said horizontally inclined portions are arranged in a common plane.

3. A wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising a vertically inclined portion and a horizontally inclined portion extending from a lower part of, and in fluid communication with, the vertically inclined portion and defining a submerged mouth oriented in use towards a principal approach direction of incident waves at least one power take off device responsive to the movement of fluid in the respective oscillating water columns wherein the oscillating water columns are selected to be of different respective sizes, and the mouths of the said horizontally inclined portions are arranged in a common plane.

4. A wave energy conversion apparatus as claimed in claim 2 or 3 wherein the cross sectional area of each vertically inclined portion is greater than the cross sectional area of the respective horizontally inclined portion.

5. A wave energy conversion apparatus as claimed in any of claims 1to 4 wherein said vertically inclined portion is substantially vertical.

6. A wave energy conversion apparatus as claimed in any of claims 1 to 5 wherein said horizontally inclined portion is substantially horizontal.

7. A wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising an upper first end disposed in use above the water surface and a lower second end, said second end defining a submerged mouth oriented in use towards a principal approach direction of incident waves, and an arcuate portion between said first and second ends at least one power take off device responsive to the movement of fluid in the respective oscillating water columns, wherein the cross sectional area proximate said first end is greater than the cross sectional area proximate said second end, and the oscillating water columns are selected to be of different respective sizes.

8. A wave energy conversion device as claimed in claim 7 wherein each oscillating water column is of tapered construction with the cross sectional area reducing from the first end to the second end.

. A wave energy conversion apparatus as claimed in claim 7 or 8 wherein the mouths of the oscillating water columns are arranged in a common plane.

10. A wave energy conversion apparatus comprising: a plurality of oscillating water columns, each water column comprising an upper first end disposed in use above the water surface and a lower second end, said second end defining a submerged mouth oriented in use towards a principal approach direction of incident waves, and an arcuate portion between said first and second ends at least one power take off device responsive to the movement of fluid in the respective oscillating water columns wherein the oscillating water columns are selected to be of different respective sizes, and the mouths of the said horizontally inclined portions are arranged in a common plane.

11.A wave energy conversion apparatus as claimed in claim 10 wherein the cross sectional area proximate said first end is greater than the cross sectional area proximate said second end.

12.A wave energy conversion apparatus as claimed in any preceding claim wherein the oscillating water columns are disposed in a nested arrangement in which adjacent oscillating water columns share common boundary walls.

13.A wave energy conversion apparatus as claimed in any preceding claim wherein the size of each oscillating water column is selected to provide respective oscillating water columns with a response band-width responsive to a frequency range selected from the frequency band-width of incident waves over a given time period.

14. A wave energy conversion device wherein the frequency band-width of incident waves over a given time period represents an annualised average frequency band-width.

15. A wave energy conversion apparatus as claimed in claim 13 or 14 when dependent on claim 1 or 3 wherein said size is selected by selecting the length of the said horizontally inclined portion and/or the height of the said vertically inclined portion.

16.A wave energy conversion apparatus as claimed in claim 13, 14 or 15 when dependent on claim 1 or 3 wherein said size is selected by selecting the ratio of the cross sectional areas of the respective horizontally and vertically inclined portions.

17. A wave energy conversion apparatus as claimed in any of claims 13 to 16 wherein the response band-widths of at least two of said oscillating water columns are substantially contiguous.

18. A wave energy conversion apparatus as claimed in any of claims 13 to 17 wherein each respective oscillating water column is constructed so that its natural resonant frequency lies within said respective frequency range selected from the frequency band-width of incident waves over a given time period.

19.A wave energy conversion device as claimed in any of claims 13 to 18 wherein the or each power take off device provides respective oscillating water columns with a predetermined load such the input impedance of the oscillating water column is substantially matched to the characteristic impedance of incident waves having said frequency range selected from the frequency band-width of incident waves over a given time period.

20.A wave energy conversion apparatus as claimed in claim 1 or 3 or any preceding claim dependent thereon wherein at least one said oscillating water column includes an array of turning vanes disposed proximate the junction of the horizontally and vertically inclined portions.

21. A wave energy conversion apparatus as claimed in any of claims 1 or 3 or any preceding claim dependent thereon wherein the walls of the respective horizontally and vertically inclined portions are curved in the region of the junction of said portions, to provide a smooth transition between the respective portions.