GB2075127A - Wave energy conversion device - Google Patents

Abstract

A buoyant spine 10 has buttresses 12 to which are secured flexible membranes which co-operate with the spine to form air-pumping chambers, the volume of which varies in response to wave-action. The membranes are attached in a tensioned condition and air-pressure is maintained within the chambers to prevent the membranes creasing. <IMAGE>

Description

SPECIFICATION Tensioned membrane This invention relates to apparatus designed for the conversion of energy from the waves in a body of liquid. Typically, the main use of the apparatus is for the conversion of energy from waves in the sea, but it can be used in other bodies of liquid, and can be used in tidal regions of the sea. For convenience of description however, reference is made hereinafter only to the use of apparatus in the sea.

The theory of wave motion indicates that the particles of water constituting a wave move in orbital paths, the orbit size decreasing (believed exponentially) with increased depth from the sea surface, and the designs of several energy generators are based upon this theory in that such generators have components which are moved by the waves (the energy being derived from the apparatus by virtue of such movement) with the various parts of the component being displaced to match the orbital size of the portion of the water contacting the particular part of the component.

Thus, in one apparatus, the said component is a flap or plate which is hinged to a support member or spine and the apparatus is located in the water. The flap or plate extends upwardly from the hinge and in use will swing back and forth about the hinge relative to the support, under the influence of the wave. The lower portion of the flap has a relatively small displacement, whilst the top portion of the flap has a relatively large displacement, and throughout the height of the flap, the displacements approximate the wave water particle orbit size. The flap or plate operates to open and close relative to the spine and therefore has been referred to and will be referred to in this specification as "the clam".

In another apparatus, the displaced component is a member which rocks under a wave action about a horizontal axis (normally defined by a cylindrical support) when the apparatus is in the water. The component has a iobed portion of progressively and exponentially increasing radius, in a direction from under the rocking axis to above same. As the component is displaced by the wave forces, the said lobe is rocked back and forth, and fact bobs in the water so much so that it is known as "the duck" and the upper portions are displaced more than the lower portion of the lobe, and again the displacements of the lobe portions approximate said water particle orbit size.

In the clam, it has been proposed to place a flexible bag, or bellows, in the space between the spine and the flap. The bag is supplied with air under pressure so that in still water, the flap takes up a position displaced from the spine, such position being achieved when the force on the flap due to the air pressure inside the bag balances th'e force on the flap due to the external water pressure.

A problem in designing the bag arises whilst the pressure of the air inside the bag is constant, the pressure force supplied by the water on the outside of the bag increases progressively with increasing water depth.

Consequently, the water pressure on the outside of the bag at the lower region will be greater than the air pressure inside the bag, whilst the water pressure at the upper region will be less than the air pressure inside the bag, and there will be a neutral, intermediate region at which the air pressure balances the water pressure. This region is referred to herein as the pressure reversal region. The bag tends to distort inwards at the bottom due to the high water pressure, and distort outwards at the top due to the relatively high air pressure. The said pressure reversal region will move cyclically up and down, in relation to the water surface when the apparatus is in uses and under wave loading, which will cause complex flexing in the bag material.

Because of this complex flexing, the design of the bag, as regards thickness and material, is difficult.

Currently, it has been proposed that the bag material should be a braided rubber, and in addition it has been identified that there are two principle criteria which the bag and/or the material must meet, these criteria being as follows: (a) there should be no creasing of the bag material, as this could lead to premature failure; and (b) the contact between the bag material and the rigid surfaces should be rolling contact to avoid slip friction as far as possible, which again could result in bag failure.

The present invention is directed to a novel form apparatus designed as an energy convertor, which again uses a flexible element for containing air under pressure, and which flexes in sympathy with wave loading, but in accordance with the present invention, the said element is a flexible elastic plate or sheet structure tensioned on a support, to define with the support an enclosed cavity, and the flexible element is presented to the waves when in the water, so that it will flex in sympathy with wave loading.

The said tensioning of the element may be very slight, but it does prevent the flexible element from sagging into distorted condition prior to being loaded with the water pressure, and in the preferred example, the element is a sheet of flexible, elastic material forming a membrane stretched between two buttresses which extend upright on a base or support member when the apparatus is in use. The buttresses, base or support member, and the membrane define the enclosed cavity which may be generally rectangular both in horizon tal and vertical cross section, with the buttresses at the top and bottom being curved at the corners in order to ensure that the membrane does not have to curve round the oversharp corners.

The apparatus may comprise a plurality of the said buttresses, spaced horizontally when in use, and a plurality of said membranes stretched between adjacent buttresses.

The or each of the said cavities would preferably have an outlet from which air is displaced as a result of flexing under wave action, and the displaced air could be used for example to drive a turbine or other rotor by which electrical energy could be derived.

If the apparatus is placed in the sea and the cavities are pressurized with air above atmospheric pressure, each membrane will deflect inwards at the bottom because of the greater water pressure, and outwards at the top because of the lower water pressure, but in each case the tension in the membrane would compensate for the difference between the pressures inside and outside the membrane, and as the membrane would always be in tension, there will be no possibility of creasing occurring.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 illustrates in perspective view a portion of a spine for supporting elastic membranes; Figure 2 shows a cross-section of the spine with a membrane attached, and prior to insertion of the spine in water; and Figure 3 shows the same view as Fig. 2, after the apparatus has been placed in water, and the membrane has been flexed and stretched as a result of the external water pressure and internal air pressurization.

Referring to the drawings, in Fig. 1 there is illustrated a rectangular buoyant spine 10, which may be of the order of 300 metres long, 10 metres wide and 1 5 metres deep, the said spine being ballasted to float with some 1.5 metres of freeboard.

At suitable intervals along the face of the spine which is to face the waves (the waveward face) are fixed vertical buttresses 1 2 of the approximate shape as shown in Fig. 1.

Between each pair of buttresses is stretched a flexible elastic membrane 14 (Figs. 2 and 3).

The edges of the membrane are bonded and held to the edge faces of the buttresses and also to the top and bottom corners of the spine so that an air-tight cavity 1 6 (Figs. 2 and 3) is formed between the membrane 14, the buttresses 1 2 and the spine 10.

As shown in Figs. 2 and 3, each cavity 1 6 is pneumatically connected via a duct 1 8 to a self-rectifying air turbine 20. A self-rectifying turbine rotates in the same direction regardless of the side which at any time is the high pressure side. The turbine 20 is coupled to an alternator 22 to pr,Góvide output electrical power when turbine 20 is rotated.

Although not shown, the interior of the cavity 1 6 is coupled to an inlet for pressurising the cavity, the said inlet in its turn being coupled to a compressor which initially pressurises the cavity, an,d tops same up as will be explained.

It is to be mgtioned that the initial tension in the membrane 14 prior to immersion in the water wsid be small, and in particular would be ju--' sufficient to prevent creases from fo,fning in the membrane when it is attached -ás described.

Fig. 2 shows the apparatus before insertion in the water, and if the apparatus is inserted in the water and cavity 1 6 is pressurised with air under pressure as mentioned above, the membrane 14 will distort in that it will deflect inwardly at the bottom, and outwardly at the top, and the central cross-sectional contour will be as shown in Fig. 3. The internal pressure in the cavity will of course be constant, whereas the external water pressure depends upon the depth, but at any point the difference in pressure between the outside and inside of the cavity will be absorbed by the tension in the membrane.There will of course be a neutral point, in the pressure reversal region, as indicated by reference numeral 24, where the internal and external pressures are equal, and the membrane will extend in a straight line between the spaced buttresses 1 2. Above the neutral point 24, the internal air pressure will exceed the external water and atmospheric pressure, and the membrane will bulge outwards, having its maximum deflection at the top where the water pressure over atmospheric becomes zero.

The disposition of the membrane as illustrated in Fig. 3 is related to the apparatus being in still water. The apparatus is for use in water containing waves, and the membrane is arranged to face the oncoming waves. As each wave strikes the membrane 14, the membrane flexes inwards to drive air from the cavity 16, which in turn drives the turbine 20, and electrical power can be taken from the alternator 22, to provide the power output.

The membrane will deflect relatively little below the neutral point 24, as the membrane is already under tension in that section due to the water pressure, but above the neutral point 24 as the membrane 14 deflects inwardly, initially the tension therein will reduce, and then, if the deflection is sufficient, the tension will reach zero value, and when the membrane starts to bulge inwards, the tension force in the membrane will again be increased. As the wave recedes, the pressure in the air duct 25 will exceed the pressure in the cavity, air will be forced back through the turbine into the cavity and the reverse move ments of the membrane will occur.

The deflection of the membrane under wave action will therefore be greatest at the top and smallest at the bottom, which, as explained herein, is the desirable behaviour for wave matching.

By choosing the appropriate buttress spacing, air pressure, and force/strain characteristics of the membrane, the desired response characteristics of the membrane can be achieved.

It may be found necessary to vary the force/strain characteristics of the membrane with depth in order to give the desired load/ deflection response.

The internal air pressure need not necessarily be that required to exactly balance the external force of the water and when the apparatus is in still water. It can be made greater or less than the balancing pressure if this is found to be desirable, the difference between external and internal forces being taken up by tensile forces in the membrane.

Since the membrane is always in tension creasing is at least substantially avoided. By buttress design it can be arranged so that there is no contact between the membrane and the spine, so that abrasion of the membrane is avoided. It may however be desirable, when the apparatus is in still water, to allow the lower part of the membrane to contact and rest against the spine, when the cavity is unpressurised so that the tension in the lower cavity is unpressurised so that the tension in the lower parts of the membrane can be relieved when the cavity is in the unpressurised condition.

Calculations indicate that with a buttress spacing of 1 5 M (49ft), and a maximum membrane deflection of 3 M (lOft), the maximum force in the membrane is of the order of 1-2 tonnes per inch, and the maximum strain is 10-12%. This is within the capabilities of currently available materials.

If it is required to direct pressure air from more than one cavity to a common turbine, intermediate buttresses would have air passages through them, thus interconnecting adjacent cavities.

The tension membrane could also be used in a system providing rectified air flow to a central turbine, as described in our co-pending application No.

The invention can be applied to spine structures which are arranged to be "side on" relative to the wave front, as described, or it can be applied where the structures are arranged in a "downwave" disposition, when the said membranes can be arranged on each side of the spine structure, again as disclosed in our co-pending application.

The invention is also suitable for use on a structure which is sea bed mounted, or is a sea wall' type mounting. In each of the said cases, the supporting structure carrying the buttresses would be fixed relative to the sea bed and would staiid in relatively shallow water.

Also, instead of converting the wave energy to another usable form, such as electrical energy, the invention can be applied to a breakwater arrangement in which the wave energy after conversion to air pressure energy, is simply dissipated by any suitable means.

It will be appreciated that only one form of buttress shape has been described but it is to be mentioned that the buttress shape could be varied based upon design and wave pattern information.

Claims (9)

1. An apparatus for converting the energy in waves in a body of liquid comprising support means, a flexible element for containing air under pressure, and which is carried by the support means so that when the apparatus is in the body of water, the flexible element flexes in sympathy with wave loading, said element being a flexible plate or sheet structure tensioned on the support means, to define with the support an enclosed cavity.

2. An apparatus according to claim 1, wherein the flexible element is a sheet of flexible, elastic material forming a membrane stretched between two buttresses which extend upright on a base or support member when the apparatus is in use, said buttresses and base or support member forming the said support means.

3. An apparatus according to claim 2, wherein the buttresses, base or support member, and the membrane define the enclosed cavity which is generally rectangular both in horizontal and vertical cross-section, with the buttresses at the top and bottom being curved at the corners in order to ensure that the membrane does not have to curve round oversharp corners.

4. An apparatus according to claim 2 or 3, wherein the apparatus comprises a plurality of said buttresses, spaced horizontally when in use, and a plurality of said membranes stretched between adjacent buttresses.

5. An apparatus according to claim 2, 3 or 4 wherein the or each of the said cavities has an outlet from which air is displaced as a result of flexing under wave action, and the support means includes a turbine or other rotor which can be driven by the said displaced air to provide shaft or electrical energy.

6. An apparatus according to claim 5, wherein the turbine or other rotor is of the type which uni-directionally driven regardless of which is side is the high pressure side.

7. An apparatus according to claim 6, wherein each cavity is associated with its own turbine or other rotor.

8. An apparatus according to claim 6, wherein a common turbine or rotor is driven by air displaced from all of the cavities collec tively.

9. An apparatus for the conversion of the energy in waves in a body of liquid, substantially as hereinbefore described, with reference to the accompanying drawings.