WAVE ENERGY CONVERTER
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.
1. Field of the Invention
The invention relates to a wave energy converter.
2. 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 inter-connected 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.
SUMMARYAccording to the invention, there is provided a wave energy converter comprising:
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- 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 100 mm to 2 m.
In one embodiment, the diameter is in the range of 500 mm to 1500 mm.
In one embodiment, the length of at least one tube is in the range of 100 m and 1000 m.
In one embodiment, the length is in the range of 200 m to 600 m.
In one embodiment, the tube has a longitudinal stiffener.
In another embodiment, the stiffener extends along the neutral plane of the tube.
In 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.
In one embodiment, the tube assembly comprises a skirt along at least one side of the assembly to reduce air ingress under the tubes.
In 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.
In 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 60:40-1-1+/−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.
In 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.
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:—
Referring to
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
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 10 m/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 air:water 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
To maintain the diameter of the tube it is wrapped with embedded spiral reinforcements, as shown in
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
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 MechanismAs shown in
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.
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×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×flowrate. The tube 3 has two flows out, water and air. For the water with a cross sectional area of say 1 m2 and velocity of 5 m/sec, and coming out 50% of the time, there is a flow of 1 m2×5 m/sec×½=2.5 m3/sec. If exit pressure is 1 bar (or 10 m H2O)=100 000 N/m2. Then the water power exiting the tube is 2.5 m3/sec×100 000 N/m2=250 KNm/sec=250 KW. (1 Nm/sec=1 watt). The air portion is also 50% of the volume at exit and the same pressure×flowrate. Air could normally be expected to exceed 250 kW for the same flow rate and pressure.
Arrangement of Multiple TubesReferring to
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 stiffer, but where the suction effect is at its most dependable and best able to counteract this extra stiffness.
As described above with reference to
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
Referring to
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 “lilo” arrangement, as illustrated in
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
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 (FIG. 8)Referring to
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 (“OWC”) Inlet. (FIG. 9)Referring to
Referring to
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×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 (
Together, longitudinal reinforcements, the “suction effect”, span limitation, and a pliable matrix material improve waveform compliance and high pressure air compression.
Alternative Output SectionsReferring to
Referring to
In 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
1-40. (canceled)
41. 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.
42. The wave energy converter as claimed in claim 41, wherein said tube has a diameter is in the range of 100 mm to 2 m.
43. The wave energy converter as claimed in claim 41, wherein the length of at least one tube is in the range of 100 m and 1000 m.
44. The wave energy converter as claimed in claim 41, wherein at least one tube has a longitudinal stiffener.
45. The wave energy converter as claimed in claim 41, wherein at least one tube has a longitudinal stiffener which extends along a neutral plane of the tube.
46. The wave energy converter as claimed in claim 41, wherein at least one tube has a longitudinal stiffener on each opposed side of the tube.
47. The wave energy converter as claimed in claim 41, wherein there is a plurality of juxtaposed and interconnected tubes forming a tube assembly.
48. The wave energy converter as claimed in claim 41, wherein there is a plurality of juxtaposed and interconnected tubes forming a tube assembly; and wherein the tube assembly comprises a skirt along sides of the assembly to reduce air ingress under the tubes.
49. The wave energy converter as claimed in claim 41, further comprising a tensioning mechanism for varying overall length of the tube in the horizontal plane.
50. The wave energy converter as claimed in claim 41, further comprising a tensioning mechanism for varying overall length of the tube in the horizontal plane; and 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.
51. The wave energy converter as claimed in claim 41, 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.
52. The wave energy converter as claimed in claim 41, 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; and further comprising a manifold between the stages for routing of air and water between different tubes.
53. The wave energy converter as claimed in claim 41, 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; and wherein there are progressively fewer tubes as pressure increases.
54. The wave energy converter as claimed in claim 41, 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; and wherein the tubes of the successive stages are arranged in parallel.
55. The wave energy converter as claimed in claim 41, 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; and wherein the tubes of the successive stages are arranged in parallel; and wherein the higher pressure stages are biased towards being located centrally.
56. The wave energy converter as claimed in claim 41, wherein the water and the air inlets are combined in a combined inlet comprising:
- a rigid tube having a mouth to receive water and air, the mouth having a bottom plate located to cut the top of a wave to take advantage of the momentum of the forward-rotating portion of the water at the top of the wave, and
- buoyancy means to position the rigid tube to receive air and water with the inlet tube sloped downwardly from its front.
57. The wave energy converter as claimed in claim 41, wherein the water and the air inlets are combined in a combined inlet comprising:
- a rigid tube having a mouth to receive water and air, the mouth having a bottom plate located to cut the top of a wave to take advantage of the momentum of the forward-rotating portion of the water at the top of the wave,
- buoyancy means to position the rigid tube to receive air and water with the inlet tube sloped downwardly from its front, and
- wherein the inlet comprises a tapered or curved guide for guiding water into the mouth.
58. The wave energy converter as claimed in claim 41, wherein the water and the air inlets are combined in a combined inlet comprising:
- a rigid tube having a mouth to receive water and air, the mouth having a bottom plate located to cut the top of a wave to take advantage of the momentum of the forward-rotating portion of the water at the top of the wave,
- buoyancy means to position the rigid tube to receive air and water with the inlet tube sloped downwardly from its front, wherein the inlet comprises a tapered or curved guide for guiding water into the mouth, and
- wherein the guide extends downwardly below the mouth.
59. The wave energy converter as claimed in claim 41, 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.
60. The wave energy converter as claimed in claim 41, 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; and
- 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.
61. The wave energy converter as claimed in claim 41, 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; and
- 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; and
- the feedback link includes an air storage tank, and the storage tank is adapted to release air into the riser.
62. The wave energy converter as claimed in claim 41, wherein the water inlet comprises an oscillating water column.
63. The wave energy converter as claimed in claim 41, wherein the water inlet comprises an oscillating water column; and
- wherein the air inlet comprises a one-way valve at an upper end of the oscillating water column.
64. The wave energy converter as claimed in claim 41, wherein the air inlet comprises a bellows.
65. The wave energy converter as claimed in claim 41, wherein the air inlet comprises a bellows; and wherein the inlet comprises a buoy for supporting the bellows.
66. The wave energy converter as claimed in claim 41, wherein the air inlet comprises a floating air trap having an inlet valve and an outlet for pulsed air driven by rising waves.
67. The wave energy converter as claimed in claim 41, wherein the output section comprises a flow restrictor to build pressure of air and water in each tube; and wherein the flow restrictor is an electricity generator such as a turbine.
68. The wave energy converter as claimed in claim 41, wherein the output section comprises an air/water separator.
69. The wave energy converter as claimed in claim 41, wherein the output section comprises an air turbine and a separate water turbine.
Type: Application
Filed: Jul 16, 2009
Publication Date: May 19, 2011
Inventors: Patrick Joseph Duffy (County Dublin), Jocelyn Fitzsimons (Cork)
Application Number: 12/737,433