AU4010599A

AU4010599A - Wave energy conversion method and apparatus

Info

Publication number: AU4010599A
Application number: AU40105/99A
Authority: AU
Inventors: Jack Stein
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-07-29
Filing date: 1999-07-14
Publication date: 2000-02-24
Anticipated expiration: 2019-07-14
Also published as: AU751252B2

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT WAVE ENERGY CONVERSION METHOD AND APPARATUS THE FOLLOWING STATEMENT IS A FULL DESCRIPTION OF THIS INVENTION, INCLUDING THE BEST METHOD OF PERFORMING IT KNOWN TO ME:- The present invention relates to a method and apparatus for wave energy conversion with particular relevance to a deep water, off shore installation.

In Australia an attempt to generate power from the sea was described by S. Firros in Australian patent specification 249,469 (49,783/59) which proposed the construction of a jetty in the Wembley City Beaches area in Perth, Western Australia. The system proposed involved placing floaters in the water attached to two arms pivotally mounted on a shaft suspended from the underside of the jetty. Through a system of pulleys and cables the motion of the floaters was transferred so as to rotate a shaft coupled to an electric generator. Mr. Firros while canvassing the West Australian 400006 Government for a number of years did not succeed in putting his invention to the test.

0: In the United Kingdom various forms of wave energy conversion devices have been 0: "i 0considered. The most intensive development began after the dramatic increase in oil prices in 1973. In 1974 S.H.Salter described a wave energy converter termed the "Salter Duck" in which a cam shaped device oscillates about a spine connecting many devices. One duck is 38 metres long with a width of 22 metres on a 14 metre spine.

This device provided low efficiency.

About the same time P.G.Davies described a device called the "Bristol Cylinder" in which a submerged cylinder was held parallel to the wave fronts. The cylinder is immersed as a wave passes and hydraulic power is developed by large hydraulic pumps supporting the cylinder. The cylinder is 100 metres long and has a diameter of 16 metres.

In 1980 N.B. Bellamy patented a device called the "Sea Clam". This converter has a floating concrete spine with a number of flexible inflatable bags attached to one side and moored at approximately 550 to the incident wave direction. The action of the waves forces air from the bags into a hollow spine through a rectifying air turbine to drive electric generators. This converter is 290 metres long, 13 metres wide and metres high.

None of these British inventions have been commercially developed because of the high capital cost involved in delivering electric power to the sub-station.

oooo In Japan various attempts have also been made. Y.Masuda designed a large floating barge called "Kaimei" which provided pneumatic power conversion. In 1980 power from "Kaimei" was delivered for a short period to the Japanese transmission grid.

However the operation of"Kaimei" was terminated in 1986 due to excessive S •oscillations of the barge under varying weather conditions. This converter was 20 metres long, 12 metres wide and 5 metres deep.

•ooo In 1993, H. Yokozawa developed "The Mighty Whale" where sea waves are trapped in three chambers. The wave energy is converted to an airflow which drives turbo generators. This converter is 60 metres long, 30 metres wide and 12 metres high.

•oo• "The Mighty Whale" appears to be the most efficient device developed so far but this device relies on a pneumatic stage with the consequent increase in the complexity and loss of efficiency of the device.

The present invention seeks to overcome the disadvantages in these prior art devices by providing a method and apparatus for wave energy conversion which provides a more direct conversion of wave energy, is simple and economical to construct and is stable in operation. The present invention is concerned with a wave energy 4 conversion device which is located in deep water in the open ocean, that is, at a sufficient distance from the coastline for the device not to be affected by coastal influences. Deep water is normally characterised by the relation between wave length of waves and the depth of the water when the waves are considered as generated by the interaction of prevailing winds with the sea surface. Under these conditions deep water is where the mean depth of water is greater than or equal to one half the wave length of the waves generated.

The wave length of the waves generated by the wind is given by the equation 1= gt 2 tanh (27d) 27t 1, where 1 is the wave length of the waves generated; h is the wave height measured from crest to trough and t is the wave period.

°o.o°i In deep water this equation reduces to l=gt 2 2t and the wave velocity equation under these conditions is given by v 2 gl/2t tanh (2nd)/1, where d is the mean depth of water and g is the acceleration due to gravity which under deep water conditions (d greater than or equal to 1/2) reduces to 20 v 2 gl/2rc or v=gt/2n.

For a group of travelling waves the rate of transmission of energy or power in watts per metre length of wave crest is given by the equation: p=l/2 gy(h/2) 2 Vg 25 where Vg is the wave group velocity and y is the density of seawater.

In deep water a feature of travelling groups of waves is the successive disappearance of the front waves and the reappearance of new waves at the rear of the train. This results in the group advancing with a velocity that is only half the velocity of any individual wave. Therefore the group velocity is given by the equation: V,=v/2=gt/4n, whereby the power equation can now be written in the form: p=y g2 h 2 t/32t, where g=9.8m/S2and y 1025 kilograms per cubic metre.

This gives therefore the power in a group of travelling waves as p=0.98h 2 t kilowatts per metre.

Any device for wave energy conversion seeks to convert this power into a usable form of energy, for example electrical energy.

According to one aspect of the invention there is provided an apparatus for conversion of wave energy incident thereon into another form of energy including a stable floating platform moored in deep water, said platform having a natural period of oscillation much larger than the period of local seawaves whereby said seawaves only loosely couple to said platform and the amplitude of oscillation of said platform is 15 small in each of its three rotational modes of pitch, roll or yaw, said platform having a first portion maintained substantially submerged below mean sea level and a second portion located above said mean sea level including means for receiving wave crests and generating therefrom said other form of energy.

20 Preferably, the platform is submerged except for a protruding vertically disposed section having turbines for receiving water from the crest of oncoming waves and S. discharging the water after passing through the turbine to a point further along in the direction of propagation of the sea waves. Preferably, the turbine is a vertical axis turbine, for example a Kaplan turbine driving an electric generator.

According to a second aspect of the invention there is provided an apparatus for conversion of wave energy incident thereon into another form of energy including a stable platform in deep water, said platform having a natural period of oscillation much larger than the period of local seawaves whereby said seawaves only loosely couple to said platform and the amplitude of oscillation of said platform is small in each of its three rotational modes of pitch, roll or yaw, said platform having a first portion maintained substantially submerged below mean sea level and a second portion located above said mean sea level including means for receiving wave crests and generating therefrom said other form of energy.

According to a further aspect of the invention there is provided a method of conversion of wave energy into another form of energy including mooring a stable floating platform in deep water, said platform having a natural period of oscillation much larger than the period of local seawaves whereby said seawaves only loosely couple to said platform and the amplitude of oscillation of said platform is small in each of its three rotational modes of pitch, roll or yaw, said platform having a first portion maintained substantially submerged below mean sea level and a second portion located above said mean sea level including means for receiving wave crests and generating therefrom said other form of energy.

15 Embodiments of the invention will now be described with respect to the following S• figures in which: Figure 1 shows an embodiment of the invention in plan; Figure 2 shows the embodiment of figure 1 in elevation; Figure 3 shows section A-A as shown in figure 2 of the embodiment of figure 20 1; Figure 4 shows a graphical analysis of the operation of a device according to the invention; and Figure 5 shows a second embodiment of the invention using a plurality of devices as shown with respect to figures 1-4.

The device according to the invention is shown in figures 1 and 2 being a platform 8 with a submersible portion 10 and superstructure 20 with the mean sea level being indicated at 22. The submersible portion 10 is comprised of a substantially T-shaped cross section 28 as shown more clearly in figure 3 having a horizontal portion or hull 24 and a vertically projecting portion or keel 26. Both these portions 24,26 are made up of steel angles and plates formed into a series of compartments 30 having suitable cross bracing 32 as indicated schematically in the drawing. The portions 24,26 are filled with water sufficient to place the submersible portion 10 under water with the top 36 of the horizontal portion or hull 24 level approximately with the mean sea level 22.

Alternatively the hull and keel of platform 8 can be constructed of plate bulkheads in lieu of steel bracing, similar to oil tanker construction. This construction may be more robust during abnormal storms, although the cost of the platform may increase by approximately The portions 24,26 of the submersible portion 10 are so constructed that they provide the necessary characteristics for the platform 8 to have a large natural period in each of the rotational modes of oscillation of the platform 8 that is in pitch, roll and yaw.

The vertically extending portion 26 also acts as a keel providing stability maintaining the platform 8 upright in the water. The platform 8 is also moored by cables 40 to the 15 end of which are attached drag weights (not shown) allowing some, but not a great o deal of, motion of the platform 8 when impacted by sea waves or the wind. The mooring includes a section of cable 40 attached between the platform 8 and the sea bottom with drag weight or weights attached at the end of the cable 40 resting on or near the sea bottom. The cable between the platform 8 and the drag weights forms a 20 catenary curve allowing limited movement of the platform 8 to be accommodated.

The platform 8 is moored by at least the four corners of the platform 8 for stability.

The superstructure 20 includes a housing 41 extending the length of the platform 8 (along the centre line for stability) housing a series of turbines of the form of the too*: 25 turbine 50 shown in figure 3. Fifteen such turbines are shown in the embodiment of figures 1 and 2 by way of illustration only. Other size or number of turbines may be used. The turbine 50 is arranged with its rotational axis vertical and coupled to an electric generator 42 supported on a thrust bearing 52 immersed in oil. The thrust bearing 52 can be of the pivoted shoe type or of the spring type.

Water flows in through inlet 54 and is directed onto the turbine 50 by a turbine runner 56 consisting of four blades 58 and a number of guide vanes 60 thereby driving the electric generator 42. The water then flows out through exit 62, the water continuing in the general direction 64 in which the inflow occurred.

The inlet 54 is located at an elevation compared to the mean sea level 22 and water from the crest of waves impinging on the platform 8 flows into this inlet 54. The generation of electricity from the turbine 50 relies on the drop of the water entering through the inlet 54 and falling to the mean sea level 22. The width and in particular the height of the inlet 54 determining the volume of water collected and its potential energy. Wave heights in deep ocean vary, for example from 1.5-3.0 metres on average and the height of the inlet 54 is placed approximately 1.5 metres above mean sea level 22 up to a height of 4 metres to accommodate wave heights above these average levels.

S•In this embodiment a large number of small turbines are used to convert the wave 15 energy impinging on the front of the platform into electrical energy. Alternatively, it fe: would be possible to divert the wave energy through a lesser number of turbines •through suitable channeling, although this may add to loss through friction and the redirection of the wave energy.

20 The structure of the platform 8 is made so as to provide a low frequency of oscillation namely by making the horizontal and vertical portions 24,26 long and thin. The cross sectional area facing into the oncoming sea waves for the horizontal portion (hull) 24, at least can be made small to reduce the force imparted to the platform 8. Typically the dimensions of the platform are: length, 60 metres; beam, 10 metres; hull depth, 25 metres and draft (keel height), 6 metres. The hull depth refers to the depth of the horizontal portion 24.

As stated above a platform is designed such that the period of oscillation of the platform is much longer than the period of sea waves which would impinge against the structure. In Australian conditions this may mean waves of a period of 10 seconds having a wave height of 1.5 -3.0 metres. In addition the platform 8 is stablilised by submerging the major portion of the platform 8 and allowing the superstructure 20 to be the only portion of the platform 8 exposed to the sea waves with mooring lines and drag weights further reducing or dampening these oscillations.

The platform is designed such that its natural periodicity is much larger than its forced periodicity.

The equation for determining the frequency of oscillation of a forced oscillatory system driven by an impressed force is md 2 s dt 2 +qds dt+rs=a cos (27tt Pe), where m is the mass of the system, q is the fundamental damping constant, r is the coefficient of elasticity, and p, is the forced periodicity. The system is designed such that q and m are large and r is small.

The natural period, pn, under a transient applied force for a damped system is given by the equation: pn=2c/ (r/m-q 2 /4m 2 12 where m, q, r have the same meaning as in the previous equation, and approximates to pn=2nu (m/r) 2 •g series of curves 70,72,74 as shown in figure 4 corresponding to plotting the amplification factor in the ordinate direction against the ratio of natural periodicity pn o9 to forced periodicity Pe along the coordinate axis shows respectively the value of q2/mr for zero,O.25 and 0.5. It is preferred therefore that in the graph shown in figure 4 the system is designed such that the ratio pn divided by pe is preferably greater than 3. In general, the system can be designed to be critically damped. This system in the 9o** •present context is the combination of the platform, its ballast, the mooring cables and attached drag weights and their associated properties and parameters. In this way the ~platform 8 is relatively immune from oscillating in sympathy with the sea waves and acts as a breakwater having a (relatively speaking) constant orientation with respect to incoming waves. Necessarily the platform will oscillate and move as a result of the impact of the sea waves upon the platform 8. This is minimised both by the form of the platform and by the manner in which the platform is moored with the cables and attached drag weights along the ocean bed.

While the embodiment described with respect to figures 1-4 is a moored platform it is also contemplated that the platform may be secured to the sea bed in the manner of a oil well platform although this would add to the cost and complexity of the construction. If the platform is moored by cables then the platform can be freed from the moorings and taken to port for maintenance or repair as required. In this case the mooring cables will then be supported by buoys to maintain the cables near the surface for resecuring the platform when it has returned from maintenance. The platform is of a size which can be readily accommodated by a dry dock facility.

The space for two chambers 19, 21 at either end of the platform 8 are marked in outline in Figure 2 to indicate that these chambers 19, 21 may be reserved to be used to accommodate a propulsive device to make the platform self-propelled. This is in lieu of the platform 8 being placed in position by a tug (or other independent ocean see going vessel). While this alternative adds to the cost of the platform 8, it provides a 0 Ooe* 15 more versatile apparatus. Otherwise, a further turbine 50 and electric generator 42 oo••I may be accommodated in each of these chambers 19, 21.

As shown in figure 5 a number of platforms such as shown with respect to figures 1-4 can be arranged together, for example in a line. Each platform would be separated be. 20 from its adjacent unit by a gap of the order of 10-20 metres in order to accommodate the movement of each platform with respect to the other and with respect to the mooring cables. The gap is sufficient for this purpose but such that the energy of the waves impinging on the gap is not substantially transferred through the gap to the region behind the platforms.

too* The embodiment shown with respect to figures 1-4 utilizes a vertical axis turbine. A horizontal axis turbine might also be used but is less preferred as the turbine and the platform have to resist more directly the wave energy thereby requiring more mass for the platform and/or turbine. In the embodiment of figures 1-4 the full height of the waves impinging on the structure cannot be utilized. Only the top portion of the waves pass through the inlet opening to drive the turbine, that is, some head is required in order to drive the turbine. In an embodiment where the turbine axis is horizontal the full wave height can be utilized, however, the turbine would be subjected to water moving at different velocities at mean sea level compared to the crest of the wave.

Although the invention has been described above with respect to preferred embodiments thereof alternatives are contemplated within the knowledge of a person skilled in the art. For the purpose of the description herein deepwater is understood to be water at approximately one and a halfkilometres from the shoreline. A typical area that may utilize the invention according to the current description is the Australian coastline south of the 23 0 S parallel, for example from Carnarvon in Western Australia through to Gympie in Queensland or on the Pacific coast of Tasmania where wave heights are consistently one and a half to three metres and the period of the wave is approximately 10 seconds. Materials for construction of the platform are materials which are well known by a person skilled in the art suitable for marine applications.

•coo o•

Claims

1. An apparatus for conversion of wave energy incident thereon into another form of energy including a stable floating platform moored in deep water, said platform having a natural period of oscillation much larger than the period of local sea waves whereby said sea waves only loosely couple to said platform and the amplitude of oscillation of said platform is small in each of its three rotational modes of pitch, roll or yaw, said platform having a first portion maintained substantially submerged below mean sea level and a second portion located above said mean sea level including means for receiving wave crests and generating therefrom said other form of energy.

2. An apparatus for conversion of wave energy incident thereon into another form of energy as claimed in claim 1 wherein said platform is submerged except for a protruding vertically disposed section having turbines for receiving water from the 15 crest of oncoming waves and discharging the water after passing through the turbine to a point further along in the direction of propagation of the sea waves.

3. An apparatus for conversion of wave energy incident thereon into another form of energy as claimed in claim 2 wherein said turbine is a vertical axis turbine driving an 20 electric generator.

4. An apparatus for conversion of wave energy incident thereon into another form of energy including a stable platform in deep water, said platform having a natural period of oscillation much larger than the period of local sea waves whereby said sea waves 25 only loosely couple to said platform and the amplitude of oscillation of said platform is small in each of its three rotational modes of pitch, roll or yaw, said platform having a first portion maintained substantially submerged below mean sea level and a second portion located above said mean sea level including means for receiving wave crests and generating therefrom said other form of energy. An apparatus for conversion of wave energy incident thereon into another form of energy as claimed in claim 4 wherein said platform is submerged except for a protruding vertically disposed section having turbines for receiving water from the crest of oncoming waves and discharging the water after passing through the turbine to a point further along in the direction of propagation of the sea waves.

6. An apparatus for conversion of wave energy incident thereon into another form of energy as claimed in claim 5 wherein said turbine is a vertical axis turbine driving an electric generator.

7. An apparatus for conversion of wave energy incident thereon into another form of energy as claimed any one of claims 1-6 wherein said platform is of a cruciform shape in cross-section with the length being greater than the height or width thereof and the platform is placed with its lengthwise dimension substantially perpendicular to average wave direction.

99.e •15 8. A method of conversion of wave energy into another form of energy including mooring a stable floating platform in deep water, said platform having a natural period °9ooo of oscillation much larger than the period of local sea waves whereby said sea waves only loosely couple to said platform and the amplitude of oscillation of said platform is small in each of its three rotational modes of pitch, roll or yaw, said platform having S 20 a first portion maintained substantially submerged below mean sea level and a second portion located above said mean sea level including means for receiving wave crests 0°0o and generating therefrom said other form of energy. 9. A method of conversion of wave energy into another form of energy as claimed in claim 7 wherein said natural period, Pn, is for a damped system under a transient applied force and is given by the equation: pn=27r/ (r/m-q 2 /4m 2 12 where m, q, r are respectively the mass of the system, the fundamental damping constant, and the co- efficient of elasticity, and wherein said system is designed such that q and m are large and r is small whereby said equation approximates to pn=2n7 (mlr) 1 2 13 An apparatus for conversion of wave energy incident thereon into another form of energy substantially as hereinbefore described with respect to Figures 1-3 or as modified with respect to Figure 11. A method for conversion of wave energy incident thereon into another form of energy substantially as hereinbefore described with respect to Figures 1-4 or as modified with respect to Figure Dated this 14 t h day of July 1999 JACK STEIN PATENT ATTORNEYS FOR THE APPLICANT HALFORD CO e*e *el