GB2282188A - Extracting energy from waves - Google Patents

Abstract

A method and an apparatus for generating electricity from wave energy which comprises a plurality of juxtaposed chambers (10) located along a gradient below sea level. Each chamber (10) is connected to its next adjacent chamber by one-way valve means (12) and bellows or diaphragm members (11) are located above said chambers (10), one for each chamber (10). Wave action, acting upon such members (11), provides a pressure differential between the two most remote chambers (10) which is utilised for generating electricity. A plurality of rows of chambers may be used with interconnections therebetween (figure 4). The chambers may be made from reinforced concrete which may be moulded in situ. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO THE GENERATION OF ELECTRICITY The invention relates to the generation of electricity and more particularly to methods and apparatus for the generation of electricity utilising wave energy.

Many schemes have been proposed for utilising wave energy for generating electricity, some of which are disclosed in report ETSU-R-72 published by The Department of Trade & Industry in 1992 and entitled "A Review of Wave Energy". The majority of the known arrangements are located at or above sea level, so being environmentally unfriendly and being subject to damage by extreme weather conditions, and all are extremely inefficient in their conversion of wave energy.

It is an object of the invention to obviate or mitigate the above disadvantages.

According to one aspect of the invention there is provided a method of generating electricity from wave energy which comprises, locating a plurality of juxtaposed chambers along gradient below sea level, connecting each chamber to its next adjacent chamber by a one-way valve and locating bellows or diaphragm members one above each of said chambers to enclose a volume therewith, whereby wave action, acting upon such members, provides a pressure differential between the two most remote chambers which is utilised for generating electricity.

According to a further aspect of the invention there is provided a method of generating electricity from wave energy which comprises, locating a plurality of juxtaposed chambers along a gradient below sea level, connecting each chamber to its next adjacent chamber by respective one-way valves and locating bellows or diaphragm members above said chambers. each said member enclosing a volume above two adjacent chambers and spanning an oppositely acting one-way valve of each chamber, whereby wave action, acting upon such members, provides a pressure differential between the two most remote chambers which is utilised for generating electricity.

The juxtaposed chambers may be located on or adjacent the sea bed and arranged to extend outwardly from a beach or shoreline generally in the direction of wave motion.

A plurality of rows of juxtaposed chambers may be provided and respective adjacent chambers may be interconnected laterally across the rows.

The row(s) of juxtaposed chambers may be formed of reinforced concrete which may be moulded in situ or may be provided as preformed sections.

Air may be introduced into the chambers nearest the beach or shoreline at low pressure and exhausted at higher pressure at the remote chambers. Such compressed air may be utilised to raise water to a high level water reservoir, for example with peristaltic pipes and may then be utilised for the generation of electricity by conventional hydroelectric means.

According to yet a further aspect of the invention there is provided an apparatus for generating electricity from wave energy which comprises a plurality of juxtaposed chambers located along a gradient below sea level, each chamber being connected to its next adjacent chamber by one-way valve means, and bellows or diaphragm members located above said chambers, one for each chamber, whereby wave action, acting upon such members, provides a pressure differential between the two most remote chambers which is utilised for generating electricity, in use.

Each bellows member may comprise a sheet of strong flexible synthectic plastics material and may be in the order of five meters in diameter.

The juxtaposed chambers may be formed in situ from reinforced concrete or may be provide as preformed sections, and located at or near the sea bed.

The foregoing and further features of the invention may be more readily understood from the following description of some preferred embodiments thereof, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic side sectional view of an apparatus of one embodiment of the invention; Fig. 2 is a schematic side sectional view of an alternative apparatus in a first condition relative to the first wave position; Fig. 3 is a view similar to Fig. 2 in a second condition relative to a second wave position, and Fig. 4 is a plan view of a typical apparatus as shown in Figs. 1, 2 and 3.

Referring now to Fig. 1 of the drawings there is shown schematically an apparatus for utilising wave energy for generating electricity. A plurality of chambers 10 each having an upper surface formed by a diaphragm or bellows 11 are juxtaposed with one-way valves 12 therebetween. The chambers 10 are located at or near the sea bed and the operation of this apparatus is similar to that described in detail in respect of the embodiment of Figs. 2 and 3.

Waves rolling in upon a coast exert varying pressures on points on the sea bed determined by alternate crests and troughs as they pass. Thus if a shallow concrete dish, over which is sealed a strong flexible plastic sheet, is placed on the sea bed and if that dish and sheet contain air, the pressure of that air will vary as the waves roll in over the assembly.

So the arrangement can be made to function as a bellows. As the bellows are well below the surface it will not be subject to the damaging surface turbulence.

Fig. 2 shows a series of four such bellows 11 arranged on a sea bed shelving towards the beach, with waves rolling in above them. Each bellows is about 5 metres in diameter and below and between each is an air chamber 10. Non return air valves 12 connect the various components as shown.

Air is fed into bellows (4) through valve (a) from a low pressure air reservoir (not shown) at a pressure controlled to be between that exerted on the bellows by succeeding wave crests and troughs. Thus, as a trough passes over a bellows, air will enter that bellows. It will be seen (Fig. 2) that wave crest (Z), being over bellows (4), will force the air from that bellows through valve (b) into chamber (D) where it will be trapped by the closing of that non-return valve. The air pressure in chamber (D) will now be that due to the height of the wave crest (Z) above bellows (4). As the waves roll in to the position shown in Fig. 3, where the trough between wave crests (Y) and (Z) is over bellows (3) the water pressure on that bellows is less than the air pressure in chamber (D), so the air in (D) will be transferred to bellows (3).As wave crest (Y) rolls over bellows (3) it forces the air into chamber (C) and, since (C) is at a lower level than (D), that air will be at a correspondingly higher pressure that it was when it was in (3). As successive waves roll in, air is steadily transferred to chambers (B) and then (A) absorbing energy from the waves and increasing the air pressure as it is forced under deeper and deeper water. The air in chamber (A) is then passed into a high pressure air reservoir (not shown) while bellows (4) is fed with air from a low pressure reservoir (not shown), whose pressure is appropriately controlled as mentioned above.

It will be noted that in the above arrangements, where the non-return air valves are installed to pass air to lower levels only, the system will function no matter from which direction the waves approach. The wave length or frequency does not have any relevance to the positioning of the bellows or their size, provided that the wave length is more than twice the bellow's diameter.

Each bellows operates as an independent unit no matter from which direction waves approach or what their frequency is. Even if the crests of the waves lie parallel to the line of the bellows in Figs 2 & 3 the pumping action will function normally, the only difference being that a wave crest will pass over all four bellows simultaneously, forcing the air into all four chambers at the same time; when the next trough passes all four bellows will refill together.

The foregoing is diagrammatic and simplified in order to demonstrate the principle of the bellows action. In fact a bellows battery containing a large number of bellows would be used and a plan view of such a battery is shown at Fig 4. In this arrangement, although each bellows remains individual, the air chambers are joined together horizontally to form air ducts along the length of the battery and running along contours of the sea bed. Thus a constant pressure over the entire length of the duct would be maintained. It is envisaged that the seaward duct would be about 10 metres lower than the in-shore duct, giving a constant pressure difference between a high and a low pressure air reservoir. The actual pressures would vary with the state of the tide, requiring the control of the input pressure to the inshore duct, but the pressure difference would be constant.This is relevant as regards the next function of using that air pressure to pump water to high level water reservoir.

It might be thought that the pressures involved in such an installation would be too great for the plastic sheets to stand and they would split. It is emphasized that, provided enough "slack" is built into those sheets, very little strain would be imposed upon them.

They serve simply as membranes dividing air and water, the pressures of which are equal and opposite.

It is well known that the water within a wave performs a circular or rolling motion. The water at the top of that circle moves in the same direction as the wave crest though slower while the water at the bottom of the circle moves in the opposite direction as the trough passes. So in Fig. 1 with wave moving from the left to right the rolling motion is clockwise and gets weaker as depth increases.

If the only energy outlet is the seas' surface then the rolling motion is as described above, However, if there is another energy outlet on the sea bed as in this case that rolling motion will be as above in the top half of the water but will reverse and increase in strength from that half way level to the sea bed when its strength will be once again that at the surface.

Originally it was intended that the presently proposed bellows battery should be sited around the 10 metre depth contour (ie. between 5m and 15m) where the sum of the losses from the bottom drag and the ratio of bottom pressure to surface amplitude is minimum if no reversal of rolling motion occurs. For example, from the measurements reported in ETSU-R-72 of average bottom drag over four sites on the West of the U.K, bottom drag reduced the energy in a wave on the 10m contour to 38% of its original energy in deep water.

This 38% is further reduced to 32% by the depth of the sea bed (0.85 for a 11.5 second period). However, if the rolling motion does reverse, the Bellows Battery could be sited in much deeper water, say in a depth of 42m at which depth the Bristol Cylinder (described in ETSU-R-72) would be sited.

Compared below is the energy collectable by the presently proposed bellows system and the Bristol Cylinder (which would seem to be one of the more favoured known devices) :- (a) Bristol cylinder (moored at 42m, centre of cvlinder at 14m) Wave energy in deep water 100% Reduced at 42m by bottom drag to 58% Reduced by directionality (35%) to 37.7% Reduced by mixed frequency (50%) to 18.85% Reduced by depth below surface (20%) to 15% (b) Bellows Svstem (sited around 10m. NO reversal of rollina water) Wave energy in deep water 100% Reduced at 10m by bottom drag to 38% Reduced by directionality NIL 38% Reduced by mixed frequency NIL 38% Reduced by depth below surface (15%) to 32% (c) Bellows Svstem (sited around 42m.Reversal of rolling motion present) Wave energy in deep water 100% Reduced at 42m by bottom drag to 58% Reduced by directionality NIL 58% Reduced by mixed frequency NIL 58% Reduced by depth below surface NIL 58% Hence the bellows system would appear to be over twice as efficient as the Bristol Cylinder if no reversal of rolling motion occurs (32% as against 15%), while it is nearly four times as efficient if the reversal does occur (58% as against 15%).

The bellows are each independent and function irrespective of their neighbour's performance so long as those neighbour's are functioning. It might be thought that a wave flowing over the outer bellows lines would give up all its energy to those lines and have none left to activate the inshore bellows lines.

In such a case the outer lines would become starved of air. Those outer lines would indeed be starved of air but only temporarily. Should those outer bellows be empty, the inflowing waves would pass over them, retaining their energy to activate bellows further inshore. Similarly if a particularly small wave has insufficient height to activate the outer bellows line it will flow over those bellows, retaining its energy until it reaches a shallower part of the battery where it can give up its energy. The whole arrangement is self-adjusting in this and other respects.

The presently proposed bellows system has advantages over the other devices currently being developed as listed below: (a) It is uncomplicated and hence more likely to be reliable and less expensive to construct and maintain; (b) It incorporates energy storage in its water reservoir to guard against calm periods; (c) It is away from the damaging surface turbulence and surf; (d) It is not subject to losses from directionality or mix frequency wave patterns; (e) It is capable of expanding to supplant rather than augment conventional generating systems and is competitive with them; (f) It is unobtrusive to other sea users; and (g) It appears to be considerably more efficient in collecting wave energy than the Bristol Cylinder, which is one the more favoured known devices.

Claims (16)

CLAIMS:

1. A method of generating electricity from wave energy which comprises, locating a plurality of juxtaposed chambers along a gradient below sea level, connecting each chamber to its next adjacent chamber by a one-way valve and locating bellows or diaphragm members one above each of said chambers to enclose a volume therewith, whereby wave action, acting upon such members, provides a pressure differential between the two most remote chambers which is utilised for generating electricity.

2. A method of generating electricity from wave energy which comprises, locating a plurality of juxtaposed chambers along a gradient below sea level, connecting each chamber to its next adjacent chamber by respective one-way valves and locating bellows or diaphragm members above said chambers, each said member enclosing a volume above two adjacent chambers and spanning an oppositely acting one-way valve of each chamber, whereby wave action, acting upon such members, provides a pressure differential between the two most remote chrricrs which is utilised for generating electricity.

3. A method as claimed in claim 1 or 2 wherein the juxtaposed chambers are located on or adjacent the sea bed and arranged to extend outwardly from a beach or shoreline generally in the direction of wave motion.

4. A method as claimed in any preceding claim wherein a plurality of rows of juxtaposed chambers are provided.

5. A method as claimed in claim 4 wherein respective adjacent chambers are interconnected laterally across the rows.

6. A method as claimed in any preceding claim wherein the juxtaposed chambers are formed of reinforced concrete moulded in situ.

7. A method as claimed in any one of claims 1 to 5 inclusive wherein the juxtaposed chambers are formed of reinforced concrete and are provided as preformed sections.

8. A method as claimed in claim 3 or any claim appendent thereto wherein air is introduced into the chamber(s) nearest the beach or shoreline at low pressure and exhausted at higher pressure at the remote chamber(s).

9. A method as claimed in claim 8 wherein the compressed air is utilised to raise water to a high level water reservoir and then utilised for the generation of electricity by conventional hydroelectric means.

10. An apparatus for generating electricity from wave energy which comprises a plurality of juxtaposed chambers located along a gradient below sea level, each chamber being connected to its adjacent chamber by oneway valve means, and bellows or diaphragm members located above said chambers, one for each chamber, whereby wave action, acting upon such members, provides a pressure differential between the two most remote chambers which is utilised for generating electricity, in use.

11. An apparatus as claimed in claim 10 wherein each bellows member comprises a sheet of flexible synthetic plastics material.

12. An apparatus as claimed in claim 10 or 11 wherein each bellows member is in the order of five meters in diameter.

13. An apparatus as claimed in claim 10, 11 or 12 weherein the juxtaposed chambers are formed in situ from reinforced concrete.

14. An apparatus as claimed in claim 10, 11 or 12 wherein the juxtaposed chambers are provided as preformed sections of reinforced concrete.

15. A method of generating electricity substantially as hereinbefore described with reference to the accompanying drawings.

16. An apparatus for generating electricity substantially as hereinbefore described with reference to the accompanying drawings.