WO2010007607A1

WO2010007607A1 - A wave energy converter

Info

Publication number: WO2010007607A1
Application number: PCT/IE2009/000049
Authority: WO
Inventors: Patrick Joseph Duffy; Jocelyn Raymond Fitzsimons
Original assignee: Jospa Limited
Priority date: 2008-07-17
Filing date: 2009-07-16
Publication date: 2010-01-21
Also published as: EP2307705A1; US20110116942A1

Abstract

A wave energy harnessing converter (1) has a tube (3) which floats on the sea. A water inlet (2) delivers water to the tube, and an air inlet (2) delivers air to the tube (3). The tube has sufficient buoyancy and flexibility to float on the water and conform to the shape of waves when the tube extends substantially in the direction of travel of the waves, causing water in the tube to be conveyed from the inlet and to be pressurised and the air to be compressed in a series of moving air pockets. The tube is reinforced to minimise energy losses through distortion or elongation. A converter output section (10) for receiving water and compressed air from the tube (3) for providing energy.

Description

"A Wave Energy Converter"

INTRODUCTION

Field of the Invention

The invention relates to a wave energy converter.

Prior Art Discussion

The oceans contain vast amounts of concentrated energy in the form of waves but harnessing this energy economically is very difficult.

A typical approach to wave energy harvesting is to provide an oscillating water column which pumps air through a turbine.

WO2008/036141 (Catlin) describes an ocean power harvester having a network of interconnected semi-submerged devices with air compressors.

The invention is directed towards providing an improved converter and method for harvesting wave energy, which is more efficient, and/or more robust, and/or simpler.

SUMMARY

According to the invention, there is provided a wave energy converter comprising: at least one tube to float on the sea or other water body, a water inlet for delivering water to the tube, an air inlet for delivering air to the tube, wherein the tube has sufficient buoyancy and flexibility to float on the water and conform to the shape of waves when the tube extends substantially in the direction of travel of the waves, causing water in the tube to be conveyed from the inlet and to be pressurised and the air to be compressed; a system output section for receiving water and compressed air from the tube for providing energy. In one embodiment, the tube has a curved cross-sectional shape.

In one embodiment, said tube has a diameter is in the range of 100mm to 2m.

In one embodiment, the diameter is in the range of 500mm to 1500mm.

hi one embodiment, the length of at least one tube is in the range of 100m and 1000m.

hi one embodiment, the length is in the range of 200m to 600m.

hi one embodiment, the tube has a longitudinal stiffener.

Li another embodiment, the stiffener extends along the neutral plane of the tube.

Li one embodiment, there is a pair of stiffeners, one on each opposed side of the tube.

In one embodiment, the converter comprises a plurality of juxtaposed and interconnected tubes forming a tube assembly.

Li one embodiment, the tube assembly comprises a skirt along at least one side of the assembly to reduce air ingress under the tubes.

Li one embodiment, the converter further comprises a tensioning mechanism for varying overall length of the tube in the horizontal plane.

In one embodiment, the tensioning mechanism comprises tensioning ropes extending between the ends of the tube, and a control mechanism to adjust the length of the ropes.

In one embodiment, the converter further comprises water outtake means for removing water from a location to define a plurality of tube stages, in which pressure of stages increases with distance from the inlet end. In one embodiment, the converter further comprises a manifold between the stages for routing of air and water between different tubes.

In one embodiment, there are progressively fewer tubes as pressure increases.

In one embodiment, the tubes of the successive stages are arranged in parallel.

In one embodiment, the higher pressure stages are biased towards being located centrally.

In one embodiment, the water and the air inlets are combined in a combined inlet comprising a mouth to receive water and air and buoyancy means to position the combined inlet to receive air and water.

In one embodiment, the mouth is arranged to receive water from crests of waves.

In one embodiment, the mouth comprises a tapered or curved guide for guiding water into the mouth inlet.

In another embodiment, the guide extends downwardly below the mouth inlet.

In one embodiment, the mouth comprises a plate located to cut the top of a wave to take advantage of the momentum of the forward-rotating portion of the water in the top of the wave.

In one embodiment, the water inlet comprises means for being partly submerged.

In one embodiment, the water inlet is in the form of a substantially vertical riser, and comprises a pumping means to pump water upwardly through the riser.

In another embodiment, the pumping means comprises a feedback link from the outlet section arranged to deliver compressed air to the riser to provide an air lift pump, said link providing at least part of the air inlet.

In one embodiment, the feedback link includes an air storage tank, and the storage tank is adapted to release air into the riser. In one embodiment, the water inlet comprises an oscillating water column.

In one embodiment, the air inlet comprises a one-way valve at an upper end of the oscillating water column.

In a further embodiment, the air inlet comprises a bellows.

In one embodiment, the inlet comprises a buoy for supporting the bellows.

Ih one embodiment, the air inlet comprises a floating air trap having an inlet valve and an outlet for pulsed air driven by rising waves.

In one embodiment, the air inlet and the water inlet are arranged to deliver air and water into the tube at a volume ratio of substantially 60:40 +/-6%

In one embodiment, the output section comprises a flow restrictor to build pressure of air and water in each tube.

In one embodiment, the flow restrictor is an electricity generator such as a turbine.

In one embodiment, the output section comprises an air/water separator.

hi one embodiment, the output section comprises at least one turbine. There may be an air turbine and a separate water turbine.

In one embodiment, the output section is adapted to feed water to a reservoir.

In one embodiment, the output section is adapted to feed compressed air to an external entity.

DETAILED DESCRIPTION OF THE INVENTION

Brief Description of the Drawings. The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 is a cross-sectional diagram showing a wave energy converter of the invention;

Fig. 2 is a pair of perspective views showing a combined water and air inlet for the system;

Figs. 3 (a) and 3(b) are views showing construction of the tube;

Figs. 4(a) to 4(f) are diagrams illustrating the manner in which wave energy is harnessed by the system;

Fig. 5 is a diagram showing a system having serial stages of low pressure, mid-range pressure, and high pressure, and Fig. 6 is a diagram showing a parallel arrangement;

Fig. 7 shows the principle of use of a "lilo" arrangement of juxtaposed tubes to form a tube assembly;

Fig. 8 is a diagram illustrating a system in which there are submerged air and water inlets;

Fig. 9 is a diagram showing a water inlet with an oscillating water column arrangement;

Fig. 10 is a set of diagrams showing how a system maybe tensioned for optimum control; and

Figs. 11 and 12 are diagrams showing alternative outlet sections.

Description of the Embodiments

First Embodiment Figs. 1-3

Referring to Fig. 1 a wave energy converter 1 comprises a combined air and water inlet 2 at a leading end of a flexible compressor tube 3 of approximately 250m in length, and terminating in a power output section 10. In this embodiment, the output section 10 has an air turbine 11 using compressed air exiting the tube 3 for electricity generation, and a water turbine 12 for electricity generation from water exiting the tube under pressure. The system 1 is anchored to the sea bed by an anchor 15, however in other embodiments it may be anchored to a structure such as a wind turbine column.

The inlet 2 provides a sequence of water and air plugs. The air plugs are akin to air locks, except that they move along the tube. Wave action on the tube 3 causes pressurisation of the water and compression of the air. The output section 10 comprises a level sensor and a separator which maintain water level to control flow through the turbines.

Referring to Fig. 2 the inlet 2 comprises a water guide 20 comprising a pair of curved vertical plates arranged to focus water near the crests of waves into an inlet formed by a bottom plate 21 and side and top walls 22 and 23. The bottom plate 21 skims water from the crests of waves. A wave moves along horizontally but the water moves up and down and within a wave body the water is circulating. Hence at the crest of a wave the water is moving substantially horizontally in the direction of the wave, at the top of its circular motion. The inlet 2 is arranged to receive this water and, in-between crests, air. Thus the water enters the inlet 2 with a certain kinetic energy to propel it into the inlet 2 and the tube 3.

The inlet 2 combines three actions: tapered channel, wave cutting, and pitching to achieve sufficient momentum to feed the tube 3. First the concentrator plates 20 concentrate a wide wave front into a narrow width with an opening width of approximately 3 times the tube diameter. The wave crests become higher and the wave troughs become lower. Secondly, the level of the horizontal cutting plate 21 can be say 1 to 3 m above the mean water level, due to buoyancy, not shown. This might be higher than the tops of the surrounding sea waves, but because the curved guides 20 have amplified the waves and we only want to take the top 20% or so, the plate 21 can be set high up. Thus the inlet 2 is sloped downwardly from its front, giving extra acceleration at tube entry. Also, the top portion of the wave contains the maximum forward motion, and thus it 'shoots' into the rigid inlet 2 (at 5 to lOm/sec) and on into the flexible tube 3. The reason for a length of rigid tube before entry to the flexible working tube is that if it was flexible it could collapse after a water slug and not open up again until the next slug. Therefore insufficient air would be drawn in. An alternative to a rigid tube inlet would be a flexible tube with a coiled spring embedded in the rubber to hold it open so that it will not collapse between water slugs. The inlet 2 and other inlets of the invention are arranged to provide an intake airrwater ratio of approximately 60:40 by volume (+/-6%). It has been found that this is particularly effective due to progressive compression of the air pockets as they are conveyed along the tube towards the outlet.

Referring to Figs. 3 (a) and 3(b) the tube 3 has woven fibres 31 of material such as polyester or steel to provide excellent robustness and flexibility. Importantly, there is a stiffening rib 30 along the tube at opposite sides, at the tube's neutral bending plane. In some embodiments the system comprises multiple juxtaposed interconnected tubes arranged as shown in Fig. 3(b). The arrows in these diagrams show clockwise and anti clockwise windings going down into the page, to illustrate that there are two separate sets of windings.

To maintain the diameter of the tube it is wrapped with embedded spiral reinforcements, as shown in Fig. 3 (a), with half spiralling clockwise and the rest counter clockwise. This arrangement is known in principle for hoses such as industrial hoses. The spiralling reinforcements may be made of any suitable filament type such as polyester.

If one employs longitudinal reinforcements evenly around the diameter of the tube under certain conditions the longitudinal reinforcements may tend to straighten, possibly leading to kinking. In mechanical and structural engineering, the concept of a "neutral bending plane" is well known. The material neither stretches nor compresses at the neutral bending plan'. As shown in Fig. 3(a) by placing two longitudinal reinforcements on opposite sides of the diameter of the tube, they will together define a neutral bending plane. The longitudinal reinforcements remain with a constant length while the tube is bent downwards. The material, or matrix, stretches more as the distance from this neutral plane increases. The longitudinal reinforcements may be made of twisted steel cable, Vectran™ or any suitable high modulus material.

It will thus be appreciated that the construction of the tube is such that diameter increasing under pressure and longitudinal stretching is avoided, while permitting vertical compliance with the sea waves.

Energy Conversion Mechanism As shown in Figs. 4(a) to 4(f) air is compressed in the flexible tubes, which float while following the surface of the waves. The tubes are filled with sequential volumes of air and water in a pattern which matches the waves. Air tends to rise to the parts of the tubes floating at the crests of the waves while water tends to gather at the hollows between the waves. Both air and water move within the tubes at a speed which matches the movement of the waves. If no restriction is placed on the outflow from the tubes, they would simply act as low pressure pumps conveying the water and air in the direction of the waves. If, however, restrictions such as electricity- generating turbines are placed at the outlets of the rubes the water segments move backwards somewhat and opposite to the general flow. Small pressure heads are thus created at each wave, each one like a small air lock moving towards the outlet. The cumulative addition of all of these pressure heads generates a substantial pressure at the outlet if the length of the tube is sufficiently long. The air segments compress in volume as they move towards the outlet, where the pressure is highest.

Air compression as described above is (desirably) isothermal, due to the cooling effect of the water and slow compression rate. Air volume is halved when compressed to one atmosphere (1 Bar, gauge). Thus, inside the tube 3 the volume of the air portion reduces as it moves downstream. When 1 Bar is reached the volume is halved, thus taking up an increasingly large proportion of the tube volume with water which is not adding to the head, and therefore not being used to any advantage. Figs. 4(a) to 4(c) illustrate the progress of one water/air segment along a tube. Initially, the air volume is large and only part of the water is contributing to the working part of the compressor. As shown in Fig. 4(a) the head hi is not perfect but satisfies a necessary condition to allow for progress down the tube. Moving onwards to the next stage (Fig. 4(b)) there are optimal conditions where all the water is contributing to the head h50 and there is no water which is not contributing to the head and thus the work of compression. But next we move on to hlOO and see the head has reduced again, there is backwards spill of water (but not air) and energy is being lost in turbulence. This is a sub-optimal situation in so far as it dissipates energy, but is necessary to achieve high pressures. In irregular sea waves there will be a mixture of these three conditions happening along the tube at any given moment, with the condition at Fig. 4(a) dominating at the beginning and at Fig. 4(c) dominating towards the outlet. To continue incrementally increasing pressure the system can remove much of the water so that the air/water returns to where it was at hi, but now at the higher pressure. Since the air volume has reduced by compression and water is removed, we need fewer tubes to continue the compression in a second stage, or alternatively a slower overall velocity which extracts more energy from slower moving waves. This excess pressurised water can be collected from all the tubes and usefully used. It is under useful pressure, equivalent to a head of about 10m and therefore it can be used to compress atmospheric air to say 9m head, and this can then be fed back upstream and into the tubes where air at this pressure may enter. Alternatively, the water could be used in a turbine to generate power. The number of working tubes is preferably reduced when this water is removed, as we have a reduced air volume coupled with water removal. While the first bar of pressure above atmospheric leads to a reduction in air volume to half, a further pressure rise to 3 bar is required for the air volume to again be halved. Thus, as pressures rises the need to remove excess water reduces rapidly. Water take out therefore is predominately closer to the intake/low pressure end of the system.

Figs. 4(d) to 4(f) illustrate the same length of tube 3 at time intervals of a few seconds apart. At the left of Fig. 4(d) (above the formula) is the moving slug of water 'W'. In Fig. 4(e) the same slug W has moved to the right and in Fig. 4(f) it has moved further to the right. As the oncoming wave lifts the tube 3 the water flows to the right in front of the oncoming wave as if it were surfing the front of the wave. The air is trapped between successive slugs of water so that it can not move backwards in the direction of the oncoming waves. This trapped air in a tube is normally referred to an air lock. In this case the air locks are also being moved forward within the tube. Multiple air locks have the potential to cause a large pressure difference between the inlet and outlet of a tube. As the waves move the air locks towards the outlet the pressure builds up and up and is equivalent to the sum of the small pressure heads either side of each moving air lock. So, the pressure at the outlet is equal to the sum of all the air lock differentials within the tube.

It is preferred that the water has a speed at the inlet which matches that of the waves so that the water slugs 'surf along in the tube in a manner analogous to a human surfer on a wave. As the water has useful kinetic energy as it exits the tube a diffuser in the output section can be used to convert this kinetic energy into extra pressure as it enters the collection tank.

As the waves move the tube 3 in a manner to follow the general shapes of the waves the water inside the tube is pressurised as the air between the slugs is compressed. As sea waves have a spectrum of speeds we try to match the speed of maximum energy. For example on the inlet 2 there may be a chamber in the buoyancy for adjustment of the level in the water. Where the inlet comprises an air lift arrangement as described below, air flow rate, depth of the riser tube, and bubble size for example could be controlled.

The converter 'tunes' to the wave spectrum by choosing the input speed. The tube has a relatively wide capture bandwidth. Inside the tube the velocity remains fairly constant, only reducing slightly as the air portion compresses. Energy is pressure x volume, and as the volume flow rate remains approximately constant the pressure rises continually as it travels along the tube.

The waves cause the tubes to move in a wave motion, transferring energy to the air and water in the tube. This energy takes the form of compression of the air and a rise in water pressure.

Power is energy/sec = pressure x flowrate. The tube 3 has two flows out, water and air. For the water with a cross sectional area of say Im² and velocity of 5m/sec, and coming out 50% of the time, there is a flow of Im² x 5m/sec x Vi = 2.5m³/sec. If exit pressure is 1 bar (or 10m H₂O) = 100 000N/m2. Then the water power exiting the tube is 2.5 m3/sec x 100 000N/m2= 250KNm/sec= 250KW. (lNm/sec=lwatt). The air portion is also 50% of the volume at exit and the same pressure x flowrate. Air could normally be expected to exceed 25OkW for the same flow rate and pressure.

Arrangement of Multiple Tubes

Referring to Figs. 5 and 6 the manner in which a system of the invention with multiple juxtaposed tubes may be arranged in low, medium, and high pressure sections is illustrated. In Fig. 5 a wave energy converter 50 has first, second, and third compression stages 51, 52, and 53. As the pressure increases along the length of the tubes some back spill of water to the upstream air segment becomes inevitable (Fig. 4(c)), wasting some energy in turbulence, but also beneficially helping to reduce the air volume and increase the air pressure in that upstream segment. This dissipation of energy by back-spilling eventually leads to a substantial fall off in energy gathered per unit length of tube. Thus, for improved efficiency, a second stage compression is preferable whereby some water is removed for enhanced step-by-step progress towards higher pressures. Two and three stage compression is sometimes used in standard air compressors; with inter-cooling between stages, in order to better approximate isothermal compression. In this case however, there is isothermal compression and the need for more than one stage of compression is to re-establish the optimum airwater ratio and thus optimise gathered energy per unit length and to reduce back-spill energy losses. As shown in Fig. 5 the stages may be sequential, whereas as shown in Fig. 6 they may be parallel, with the high pressure stage preferably located in the centre. There may be interleaving of high, medium and low pressure tubes, overall having a higher concentration of high pressure towards the centre.

Multi stage compression, aimed at achieving higher pressures, benefits from the lack of air underneath the tube assembly. This means that the atmospheric pressure presses the tubes against the water surface ("suction effect"). The outer tubes of the assembly may be assigned first stage compression duties of, say, up to one Bar, while some inner tubes, towards the centre, may be carrying out second or third stage compression duties of several Bar. They are therefore suffer, but where the suction effect is at its most dependable and best able to counteract this extra stiffness.

As described above with reference to Fig. 6 second and third stage compression stages may be in parallel and within the same tube assembly as the first low pressure stage and not in series, or downstream as illustrated in Fig. 5. This will mean returning pressurised air and some water back upstream to near the air intakes. This may be carried back upstream in straight pipes at a much higher velocity than the velocity in the working tubes, and would therefore be of much smaller diameter; this diameter being only sufficient to avoid excessive friction losses. In one arrangement, this pressurised air and water travels back upstream through tensioned span limiters, consisting of hollow pipes under tension. The system can be arranged so that one span limiter would carry pressurised water only, while anotiier carries air only. As these span limiters are very long the buoyancy of the air pipes may be useful as a structural support for the water pipes.

An advantage in combining in a parallel arrangement first stage compression and higher pressure stages in the one tube assembly is to make best use of the "suction effect". The higher pressure tubes should be located to dominate towards the centre, to reduce edge lifting and air ingress at the outer edges, as shown in Fig. 6.

Referring to Fig. 7, tubes may be arranged juxtaposed in a lilo-like assembly 100 of tubes 101. There are preferably side skirts 103 to minimise unwanted air ingress from the sides. In one embodiment, the arrangement is 250m long by 25m wide, with each tube having a diameter of about Im. Fig. 7 also shows one of many diverter films 102 for diverting water off the assembly 100. This avoids wasting energy by flowing water on top of the assembly 100.

Distinct individual tubes as described will work, but with huge areas to cover, and the need to withstand storm conditions, it may be advantageous in some conditions to join the tubes side-by- side. This way, the encircling reinforcements also secure the longitudinal reinforcements and join the tubes into the "IUo" arrangement, as illustrated in Fig. 3(b). Also, as described in more detail below, the energy extracted per meter per tube for this arrangement is higher than for individual tubes.

The reduction in energy extraction resulting from the tendency of the tubes to straighten and cut through the waves is substantially lessened with a wide lilo-like arrangement. Because the crests can not break up through the impervious lilo-like layer above and also, since air can not easily find its way underneath there is a greater compliance of the wide lilo arrangement to the wave shape than the single tube.. Air attempting to get underneath the lilo, where its edges meet the wave hollows, are faced with a moving labyrinth seal. Also, suction is created in the wave hollows, if the tube assembly attempts to rise away from the wave hollows. This is referred to as the "suction effect". Thus a wide tube assembly is forced to comply with the wave pattern much more effectively than a single tube.

To enhance the "suction effect" the sealing skirts 103, on each side of the assembly are incorporated to block unwanted air ingress, as shown in Fig. 7. One or more skirts may incorporate non return flap valves on the downstream end to evacuate any unwanted air ingress.

Alternatively or in addition a higher water:air ratio in the outermost tubes would also help to keep these down on the water in the wave hollows, thus helping to prevent air ingress, and preventing it getting to the stiffer high pressure tubes towards the centre.

Embodiment with an air-lift inlet CFig. 8)

Referring to Fig. 8 a system 200 has a tube 201 and a submerged combined air and water inlet 202 having a riser 203 having a length of 10m. The depth can be chosen to a level to set a desired pressure of the entire system. It is calculated that for each additional 10m riser depth the tube air pressure rises by 1 bar. There is a compressed air storage reservoir 204 for controlled delivery of compressed air to the riser 203. The reservoir 204 is fed by a feedback air link 205 from the outlet section 210. The feedback air link may in some embodiments have an in-line air turbine for extraction of some of the air energy if the pressure available is greater than necessary to supply the riser 203.

Injection of compressed air into the bottom of the riser 203 provides an "air-lift" pump analogous to those sometimes used in the mining industry. Advantageously, the system has compressed air available in the outlet section 210. Fine air bubbles are introduced into the bottom of the riser. As the bubbles rise the combined density of air and water in the vertical column is substantially lower than the outside water density. Being lighter than the water it rises. The bubbles rise and combine to form air slugs in the tube 201, leaving the water to form the water slugs. The water already has velocity so this supplies the necessary momentum as it enters the tube. This is a closed air circuit (with some top up to make up for dissolved air, and to control the overall pressure in the system, as well as the air/water ratio). The water on the other hand is an open circuit.

There is considerable kinetic energy in the riser, helping to maintain flow during a short lull in the waves. A major advantage is that the inlet of the tube is not exposed to storm damage! An approaching wave would only 'see' the tubes rising up in a curve from below. The air lift pump would be mainly located below the most powerful wave action. The curved tube would present a small resistance to large oncoming waves which would tend to pass over.

The reservoir 204 can be filled with high pressure air during energetic wave conditions from tube outlets, or it could be topped up with air from a compressor powered by a wind turbine or other type of wave energy converter.

There may in some embodiments be a means to vary the depth of the air inlet to avoid stalling the output from the tubes. There may be more than one air in-feed in the riser, to select the level (or pressure) of air input.

Embodiment with Oscillating Water Column f"OWC") Inlet. (Fig. 9)

Referring to Fig. 9 a system 300 comprises an oscillating water column 301 with an air valve 302 above the column, and a tube 303. It is known to provide an oscillating water column having a Wells air turbine to extract the energy, hi the OWC 301 there is water overtopping and air pumping via the non return valve 302. Both of these feed air and water to the tube 303. In more detail, there is overtopping on the upswing of the oscillating water, followed by air suction on the downswing. On the upsurge the displaced air is blown into the tube followed by overtopping water which drives the entrained air into the tube 303 before it. Then as the oscillating water falls back the air is again drawn in through the non return air inlet valve. As the water swings up again the cycle repeats itself.

Referring to Fig. 10, a system 400 has a tensioning mechanism 401 which winds lengths of stiffening cable 403 to set the length of the tube assembly in the horizontal plane.

Tubes tend to straighten when pressurised and overcoming this tendency is advantageous. Otherwise, the tubes may not follow the wave shape sufficiently well, and may tend to ride on top and through the waves, thus gathering little energy. For most efficient harnessing of wave energy the tubes must follow the wave pattern reasonably closely.

The reinforced tubes as described would, if closed at both ends and pressurised, flex up and down easily. If anchored at one end and filled with sea water to give an overall density less than but close to the density of sea water, it would follow the waves up and down closely. However, if the outlet end of this water-filled tube was forcefully pulled, the up and down waveform would straighten out, cutting into the tops of the waves and hanging free of the wave hollows. The sinusoidal amplitude would lessen. The potential 'heads' obtainable within this shape tube would be less than if the tube more accurately followed the waveform. The efficient use of tube material would be lessened, as if the seas were calmer than in reality. The Watts per meter would suffer.

As the system is not handling a tube filled with stationary sea water because it is continually compressing air along the length, the force, (pressure x area) or tension grows as the internal pressure grows. This has an effect similar to catching the end of the water-filled tube and pulling against the anchor so that the tube will tend to cut into the crests of the waves and hang above the wave hollows.

The tendency of the tubes to straighten and cut through the waves is largely solved by the longitudinal reinforcements along the neutral bending plane, and enhanced by the "suction effect" but to achieve yet higher pressures, an additional and different type of structure is advantageously employed. The tensioning mechanism (Fig. 10) prevents the exit ends of the tubes moving away from the entry ends. The span (overall length of the tube assembly as viewed in plan) is adjustable, depending on weather and wave conditions. In a very calm sea the wave height will be so small that the span will almost become equal to the overall length of the tube. Ih high wave conditions however, these tension members are shortened to permit a high amplitude-pattern matching the waves. A shorter span is needed for higher waves, but the span may be set at longer than optimal length to avoid the situation where the water in the tube is moving so quickly in relation to the tube diameter that friction and turbulence become excessive leading to a loss of power. A control system, detecting upstream wave conditions, and a motorised arrangement to permit adjustment of the span solves this problem. Thus, compliance with the shape of the waves is enhanced for all wave conditions and the energy intake per unit length is optimised. These long span control tensioners may be coated steel cables as, they may serve a dual purpose and be hollow straight pipes for carrying pressurised air and/or water.

Together, longitudinal reinforcements, the "suction effect", span limitation, and a pliable matrix material improve waveform compliance and high pressure air compression.

Alternative Output Sections

Referring to Fig. 11, in a system 700 the exhaust water is fed to a high reservoir to provide a head for pumped storage power generation.

Referring to Fig. 12, compressed air may be pumped to a low water depth, causing it to rise as bubbles and causing what is known as up-welling. Up- welling is proposed as a way of raising the nutrient deep waters to the surface where photosynthesis can take place. This up-welling technique is proposed as a way of causing great masses of algae growth to be collected at a great distance from the up-welling and used as biomass or fertiliser.

Alternative Embodiments hi most of the embodiments described both fresh air and fresh sea water enter and leave the tubes. However, there are some situations where it may be worthwhile to return the water in a closed loop for reuse, in such a way that the system is an open system for air but a closed system for water. This almost eliminates the problem of having to filter the water for seaweed before it enters the pipes. Filter screen cleaning is also virtually eliminated. It also avoids the potential problem of energy losses due to air dissolving in the water, and so, not being available for use as compressed air. When the oxygen content of the compressed air is important for some applications, for oxygen production or some combustion systems, a closed water system would reduce oxygen dissolving in water and being wasted.

Regarding tube diameter, smaller diameter tubes have lower anchorage requirements but also lower throughput in all but calm seas. The material cost per unit power is higher. The choice of diameter is primarily a compromise between wave conditions and tube friction losses. .It also depends on whether one wants a reliable power supply at a relatively low level most of the time or maximum energy over time. An island, with no connection to a mainland power line and with limited or no storage might require power for the maximum number of hours per annum as opposed to the maximum overall energy.

A major advantage of the invention is that it lends itself to continuous process manufacture. There is no welding, chopping, plating, screwing, pivots, seals or like fabrication. This is very amenable for large scale, low cost, continuous manufacture. Ideally, the tube assembly would exit from the production facility directly on to water, eliminating the need for very wide conveyors. A production facility could also be based on a ship that could travel to the designated location and produce in situ, the ship being also used as operational headquarters, for ancillary production and assembly.

The converter tube or tubes are very tough and are flexible enough to yield under storm conditions as it is reinforced extensively but not rigid. The fact that air and water are both combined in the tubes means that potentially destructive oscillations are damped out. There is also scope to change the storm-resistant properties if bad storms are forecast, such as: fill tubes mainly with air and change span control, and/or fill with water mainly and adjust span control, and/or inject air below the tube assembly to break the "suction effect".

Lower the inlet feed to allow larger waves to pass over

It will also be appreciated that the arrangement of the system lends itself to a low maintenance requirement. Absence of metal parts means there is little corrosion. Also, as the tubes lie so close to the water surface that they should not be visible when just a few kilometres off shore. The ancillary equipment, feed and power, are smaller than the tube assembly and can also be designed to be low profile.

For fish conservation the need for marine reservations is well accepted. An energy farm employing a system of the invention and a marine reservation could advantageously co-exist. Some of the oxygen in the tubes will dissolve in the water given the pressure, time and movement involved. When this water is exhausted from a turbine this enriched oxygen water would be available to marine life.

It will be appreciated that the invention overcomes the main potential difficulties such as storm damage, expense, maintenance in a hostile environment, visual impact, and high strain anchorage, and wide bandwidth.

It is also envisaged that the outlet end of the tubes may be anchored on land or an island or structure such as an oil rig.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims

1. A wave energy converter comprising: at least one tube to float on the sea or other water body, a water inlet for delivering water to the tube, an air inlet for delivering air to the tube, wherein the tube has sufficient buoyancy and flexibility to float on the water and conform to the shape of waves when the tube extends substantially in the direction of travel of the waves, causing water in the tube to be conveyed from the inlet and to be pressurised and the air to be compressed; a system output section for receiving water and compressed air from the tube for providing energy.

2. A wave energy converter as claimed in claim 1, wherein the tube has a curved cross- sectional shape.

3. A wave energy converter as claimed in claim 1, wherein said tube has a diameter is in the range of 100mm to 2m.

4. A wave energy converter as claimed in claim 3, wherein the diameter is in the range of 500mm to 1500mm.

5. A wave energy converter as claimed in any preceding claim 1 or 2, wherein the length of at least one tube is in the range of 100m and 1000m.

6. A wave energy converter as claimed in claim 5, wherein the length is in the range of 200m to 600m.

7. A wave energy converter as claimed in any preceding claim, wherein the tube has a longitudinal stiffener.

8. A wave energy converter as claimed in claim 7, wherein the stiffener extends along the neutral plane of the tube.

9. A wave energy converter as claimed in either of claims 7 or 8, wherein there is a pair of stiffeners, one on each opposed side of the tube.

10. A wave energy converter as claimed in any preceding claim, comprising a plurality of juxtaposed and interconnected tubes forming a tube assembly.

11. A wave energy converter as claimed in claim 10, wherein the tube assembly comprises a skirt along at least one side of the assembly to reduce air ingress under the tubes.

12. A wave energy converter as claimed in any preceding claim, further comprising a tensioning mechanism for varying overall length of the tube in the horizontal plane.

13. A wave energy converter as claimed in claim 12, wherein the tensioning mechanism comprises tensioning ropes extending between the ends of the tube, and a control mechanism to adjust the length of the ropes.

14. A wave energy converter as claimed in any preceding claim, further comprising water outtake means for removing water from a location to define a plurality of tube stages, in which pressure of stages increases with distance from the inlet end.

15. A wave energy converter as claimed in claim 14, further comprising a manifold between the stages for routing of air and water between different tubes.

16. A wave energy converter as claimed in claims 14 or 15, wherein there are progressively fewer tubes as pressure increases.

17. A wave energy converter as claimed in any of claims 14 to 16, wherein the tubes of the successive stages are arranged in parallel.

18. A wave energy converter as claimed in claim 17, wherein the higher pressure stages are biased towards being located centrally.

19. A wave energy converter as claimed in any preceding claim, wherein the water and the air inlets are combined in a combined inlet comprising a mouth to receive water and air and buoyancy means to position the combined inlet to receive air and water.

20. A wave energy converter as claimed in claim 19, wherein the mouth is arranged to receive water from crests of waves.

21. A wave energy converter as claimed in claim 20, wherein the mouth comprises a tapered or curved guide for guiding water into the mouth inlet.

22. A wave energy converter as claimed in claim 21, wherein the guide extends downwardly below the mouth inlet.

23. A wave energy converter as claimed in claim 22, wherein the mouth comprises a plate located to cut the top of a wave to take advantage of the momentum of the forward- rotating portion of the water in the top of the wave.

24. A wave energy converter as claimed in any preceding claim, wherein the water inlet comprises means for being partly submerged.

25. A wave energy converter as claimed in claim 24, wherein the water inlet is in the form of a substantially vertical riser, and comprises a pumping means to pump water upwardly through the riser.

26. A wave energy converter as claimed in claim 25, wherein the pumping means comprises a feedback link from the outlet section arranged to deliver compressed air to the riser to provide an air lift pump, said link providing at least part of the air inlet.

27. A wave energy converter as claimed in claim 26, wherein the feedback link includes an air storage tank, and the storage tank is adapted to release air into the riser.

28. A wave energy converter as claimed in claim 24, wherein the water inlet comprises an oscillating water column.

29. A wave energy converter as claimed in claim 28, wherein the air inlet comprises a oneway valve at an upper end of the oscillating water column.

30. A wave energy converter as claimed in any preceding claim, wherein the air inlet comprises a bellows.

31. A wave energy converter as claimed in claim 30, wherein the inlet comprises a buoy for supporting the bellows.

32. A wave energy converter as claimed in any preceding claim, wherein the air inlet comprises a floating air trap having an inlet valve and an outlet for pulsed air driven by rising waves.

33. A wave energy converter as claimed in any preceding claim, wherein the air inlet and the water inlet are arranged to deliver air and water into the tube at a volume ratio of substantially 60:40 +1-6%

34. A wave energy converter as claimed in any preceding claim, wherein the output section comprises a flow restrictor to build pressure of air and water in each tube.

35. A wave energy converter as claimed in claim 34, wherein the flow restrictor is an electricity generator such as a turbine.

36. A wave energy converter as claimed in any preceding claim, wherein the output section comprises an air/water separator.

37. A wave energy converter as claimed in any preceding claim, wherein the output section comprises at least one turbine.

38. A wave energy converter as claimed in claim 37, wherein there is an air turbine and a separate water turbine.

39. A wave energy converter as claimed in any preceding claim, wherein the output section is adapted to feed water to a reservoir.

0. A wave energy converter as claimed in any preceding claim, wherein the output section is adapted to feed compressed air to an external entity.