GB2388631A

GB2388631A - Generating electricity from changes in the vertical level of a body of water

Info

Publication number: GB2388631A
Application number: GB0320091A
Authority: GB
Inventors: James Summersell
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-08-28
Filing date: 2003-08-28
Publication date: 2003-11-19
Anticipated expiration: 2023-08-28
Also published as: GB2388631B; GB0320091D0; WO2005021960A1

Abstract

Electricity is generated from a change in the vertical level of a body of water 33, eg due to tides or in a river lock, by floating an object 32 on the water 33, using the resulting vertical motion of the object 32 to operate means for pumping water, and using the pumped water to generate electricity at an electricity generating station 42. The object 32 may float on water in a dock 34 in communication with an ocean and may actuate pumps 36 which deliver water to the electricity generating station directly and via an elevated cylinder 38 which may be pressurised by a weighted piston 40. Thus electricity can be generated even during inactive periods when the direct delivery of water would be insufficient to generate electricity. Two bodies of water with height changes out of phase can be used to maintain generation of electricity.

Description

( M&C Folio No P52837GB METHOD ANl:) APPARATUS FOR GENERATING ELECTRICITY

The present invention relates to a method and apparatus for generating electricity from 5 changes in the vertical level of a body of water A large amount of potential energy is stored in the vast body of ocean water that is made to rise and fall on a daily basis by the operation of ocean tides. The movement of this huge mass of water can be harnessed to generate electricity, and several schemes 10 have been previously proposed.

The ocean tides, bemg the daily rise and fall of ocean levels relative to coastlines, are a result of the gravitational force of the moon and the sun as well as the rotation of the earth. Both the moon and the sun exert a gravitational force on the ocean water, with 15 the moon exerting a larger gravitational force because, although it is smaller in mass, it is far closer to the earth than the sun. The gravitational force causes the ocean water, which maces up 71% of the earth's surface, to bulge along an axis pointing towards the moon, and the tides are produced by the rotation of the earth beneath this bulge of water. This is known as the Lunar Tide. The smaller gravitational effect of the sun 20 results in the same effect of bulging along an axis pointing towards the sun on facing and opposing sides of the earth. This is known as the Solar Tide. The actual tides result from a superposition of the Lunar and Solar Tides.

Figure IA of the accompanying drawings is a schematic diagram illustrating the relative 25 positions of the earth 2, moon 4 and sun 6 during a Neap Tide m which the Lunar Tide 8 is aligned orthogonally to the Solar Tide 10. Figure I B of the accompanying drawings illustrates the relative positions of the earth 2, moon 4 and sun 6 during a Spring Tide m which the Lunar Tide 8 is aligned with the Solar Tide 10, resulting in the maximum tidal range.

( M&C polio No P52837GB Several schemes have been proposed to harness the power of the tides to generate electricity. One of these is illustrated schematically in Figure 2 of the accompanying drawings' comprising a barrage 12 placed across an estuary 13. A passageway 14 for Water is disposed through a lo..,er portion. of the barrage 19, with a t''rhme] 6 located within Ine passageway id. As one lice rises, with ills level a' i; -,e waLei; ' i-isi,g ol,.hc ocean side of the barrage 12, two sluice gates 15 are lifted to allow water to enter through the passageway 14, thus turning the turbine 16 to generate electricity.

When the water 18 on the estuary side of the barrage 12 has risen to its maximum level, 10 the sluice gates 15 are lowered to hold the water 18 and prevent it from flowing back through the passageway 14. When the water 17 on the ocean side of the barrage 12 Is at the low-tide point, the sluice gates 15 are raised to allow the hydrostatic head created by the body of water 18 to force water back through the passageway 14; this movement of water turns the turbine 16 and generates electricity.

The above process is repeated as the tide rises and falls. As there arc two high and two low tides each day, electricity generation from tidal power plants such as that shown in Figure 2 is characterized by periods of maximum generation, with little or no electricity generation in between. Alternatively, the turbine 16 can be used as a pump to pump 20 extra water into the basin 18 behind the barrage 12 during periods of low electricity demand. This water can then be released when demand on the system Is at its greatest, thus allowing the tidal plant to function with some of the characteristics of a "pumped storage" hydroelectric facility.

Other schemes have been proposed that do not rely directly on the tidal movements of 25 the ocean, but rather the disturbances on the surface of the ocean in the form of waves.

Figures 3A and 3B of the accompanying drawings shows just one example of such a scheme, which is used in Wavegen's Limpet and Osprey modules (see wavegen.co.uk).

The wave energy collectors are in the form of a partially submerged shell 20 Into which sea water Is free to enter and leave. As the water enters, with a rising wavecrest 22, the 30 level of water in the chamber 20 rises In sympathy. A column of air, contained above

t ( M&C Folio No P52837G8 the water level, Is compressed by this movement to generate a stream of high velocity air 24 in an exit blowhole. This Is Illustrated in Figure 3A of the accompanying 5 drawings. The reverse situation Is shown m Figure 3B in which a falling wavecrest 28 causes a column of air above the water level in the chamber 20 to he decompressed Llllby gllldiill bitCIII Of lIiil velocity ail -0 GlliClill ill;U file uluwilule. Tl.e air stream is allowed to flow via a pneumatic turbine, thus extracting energy on the system and generating electricity.

Although the movement of ocean water, and therefore its potential for generating electricity, varies throughout the day, ocean tides are a valuable renewable energy source because of their reliability, predictability and dependability. Tidal electricity can be used to displace electricity which would otherwise be generated by fossil fuel fired 15 power plants, thus reducing emissions of greenhouse and acid gasses. Because of this valuable, renewable energy resource, alternative ways of harnessing the power of the ocean tides are always sought.

According to a first aspect of the present invention, there is provided a method of 2() generating electricity from a change in the vertical level of a body of water by floating an object on the water, employing the resulting vertical motion of the object to operate means for pumping water, and employing the pumped water to generate electricity at an electricity generating station.

25 Electricity may be generated from the pressure of the pumped water at the electricity generating station, or equivalently the kinetic energy of the pumped water.

The pumping means may cause water to be pumped from the body of water itself.

( ( M&C Folio No P52837GB The vertical level of the body of water may change in a cyclical manner, with the water rising during at least one part of a cycle and falling during at least one other part of that 5 cycle. The cycles may not repeat exactly. The vertical love] of the body of water may change in a predeterm..in.ed manner, for example zenith the ocean tides.

The pumping means may be operable to cause water to he pumped directly to the electricity generating station for the generation of electricity. In this respect, a cycle 10 may comprise at least one active period in which the vertical motion of the object is sufficient to operate the pumping means to deliver water with a sufficient pressure to the electricity generating station to enable a predetermined electricity generation output, and at least one inactive period in which the vertical motion of the object is not sufficient to operate the pumping means to deliver water with a sufficient pressure to the 15 electricity generating station to enable a predetermined electricity generation output.

For example, an nactivc period may occur around the sum-around point between the rising and falling of the vertical level of the body of water. The pumping means may cause water to be pumped directly to the electricity generating station during at least part of an active period.

In addition to this, or instead of, the pumping means may be operable to cause water to be pumped to a holding reservoir for subsequent delivery to the electricity generating station for the generation of electricity.

25 In this respect, a cycle may comprise at least one active period m which the vertical motion of the object is sufficient to operate the pumping means to deliver water with a sufficient pressure to holding reservoir, and at least one inactive period In which the vertical motion of the object is not sufficient to operate the pumping means to deliver water with a sufficient pressure to the holding reservoir. The pumping means may 30 cause sufficient water to be pumped to the holding reservoir during an active period to maintain a sufficient amount of water in the holding reservoir to enable the generation of electricity from the water in the holding reservoir during at least part of an inactive period. The pumping means may cause a sufficient amount of water to be maintained in

( M&C Follo No P52B37GB the holding reservoir to enable the generation of electncty from the water in the holding reservoir during the whole of the inactive period.

s The hQ!ding reservoir may be located at a,e-.ical height above the e] ectncity generating SI:ation. 1ne vertiC;iai heigni may be hi ill uclivc voter ui-lucf its own weight to the electricity generating station with a sufficient pressure to enable a predetermined electricity generation output. Additional pressure may be applied to the 10 water in the holding reservoir to achieve sufficient pressure of the water delivered to the electricity generating station. A weight pressing down on the water in the holding reservoir is employed to apply the additional pressure. The holding reservoir may be a cylinder and the weight may be a piston acted on by gravity.

15 The body of water may be In a dock In communication with an ocean, the vertical level of the body of water being caused to change by the operation of ocean tides.

The body of water may also be in a lock (for example, a river lock), the vertical level of the body of water being caused to change by water flowing Into or out of the lock.

The vertical level of the body of water may also be controlled by the use of one or more gates across an opening to the dock or lock, as the case may be, which are operated to control the amount of water in the dock or lock, as the case may be.

25 The pumping means may be arranged between the object and an attachment the vertical level of which is not affected, at least as much as the object, by the change in vertical level of the body of water, the vertical motion of the object relative to the attachment operating the pumping means. The attachment may be disposed on the dock or lock side. as the case may be. The object may comprise a projection that overhangs the 30 attachment on the dock or lock side, as the case may be, and the pumping means may be arranged between the attachment and the projection.

M&C Folio No P52S37GB The pumping means may comprise at least one pump that is adapted to pump water for the generation of electricity when the level of the body of watems nsing.

T1- - I rr7Por7C mO'rrmn=P At 1f.Ct renal rennin that is nrtrl to numr, water for 4'' = r -a r-- r --is 1 r line generation OT eiecriciy when the ievei of one Buy us Waite is allh,.

The pumping means may comprise at least one pump that is adapted to pump water for 10 the generation of electricity when the level of the body of water is rising or falling, for example a two-way pump.

At least two such bodies of water may be employed to generate electricity in the manner described above with at least two such bodies of water, the respective cycles associated 1 S with the at least two such bodies of water may be arranged to be out of phase. The phase difference may be controlled so that an active period associated with one body of water at least partially overlaps with an inactive period associated with the other body of water. 90 The phase difference may be controlled so that the respective cycles associated with the at least two such bodies of water do not expenence an inactive period at the same time.

Another type of liquid other than water may be used.

25 According to a second aspect of the present invention, there is provided apparatus for generating electricity from a change in the vertical level of a body of water, comprising an object floating on the water, means for pumping water operated in use by the resulting vertical motion of the object, and means for generating electncty from the pumped water.

M&C Polio No P52S37GB Reference will now be made, by way of example, to the accompanying drawings, n which: s Figures 1A and IB are schematic liagramc for rise in illustrating the operation of ocean , tlUCt,, Figure 2 Is a schematic diagram illustrating one prevously-considered scheme for 10 harnessing the ocean tides to generate electricity; Figures 3A and 3B schematically illustrate one previously-considered scheme for generating electricity from wave movement; 15 figure 4 is a schematic diagram illustrating apparatus for generating electricity according to an embodiment of the present invention; Figure 5 is a graph showing the presence of active and inactive periods for a body of water affected by the tides; and Figure 6 is for illustrating the use of two bodies of water with out-of-phase cycles to mask inactive periods.

An embodiment of the present invention will now be described with reference to Figure 25 4. Apparatus according to an embodiment of the present invention IS provided for generating electricity from a change in the vertical level of a body of water 33 in a basin 34. in this embodiment, the basin 34 is a dock in commumcation with an ocean, so that the vertical level of the body of water 33 Is caused to change by the operation of ocean tides. The apparatus comprises an object 32 floating on the body of water 33, pumps 36, a cylinder 38, a piston 4O, and an electricity generating station 42. The ocean tides cause the vertical level of the body of water 33 to change in a cyclical manner according to the

( i/ M&cC Fobo No P52837GB ocean tides, with the water rising during the part of a cycle between a low tide and a high tide, and falling during the part of a cycle between a high tide and a low tide. The 5 changing vertical level of the body of water 33 results in the vertical motion of the object 39, and this vertical moron of the object is employed to operate the numns 36 The pumps 36 operate under the pressure of the moving object 32 and are employed to pump water 44 from the body of water 33 in the basin 34 to the water cylinder 38. A 10 piston 40 acting under gravity is arranged in the cylinder 38 to apply additional pressure to the water in the cylinder 38. In order to supply water to the cylinder 38, the pumps 36 must supply the water 44 at a pressure that is sufficient to raise both the water above the point of entry into the water cylinder 38 and the piston 40.

15 The water cylinder 38 is located at a vertical height above the electricity generating station 42 so that the hydrostatic head that is developed by virtue of the height of the water in the cylinder 38 above the electricity generating station 42, together with the additional weight of the piston 40, is sufficient to deliver water from the cylinder 38 to the electricity generating station 42 with a sufficient pressure to enable a predetermined 20 electricity generation output. Electricity is generated at the electricity generating station from the pressure of the water 46 acting on turbines in the electricity generating station 42. In this respect, the electricity generating station 42 can be of a type known from existing hydroelectric power generation stations. The mass of the object 32 in this embodiment is of the order of tens of thousands of tons, and the mass of the piston 40 is 25 of the order of tens of tons. After use in the electricity generating station 42, the water is preferably returned to the basin 34.

The pumps 36 are also operable to cause water 45 to be pumped directly to the electricity generating station 42 for the generation of electricity. When water 45 is 30 pumped directly to the electricity generating station 42, the water 45 must be of sufficient pressure to drive the turbines if electricity generation is to be maintained at a predetermined output. The choice as to whether to deliver water to the electricity generating station 42 from the cylinder 38 or directly from the pumps 36 (or a

( M&C Folio No P52837GB combination of both) depends on the available pressure from the pumps 36, which in turn depends on the vertical motion of the object 32 on the body of water 33, as will 5 now be explained.

1 he ocean tides cause the vertical level or the body or water 33 m the basin ro change in a cyclical manner, as shown in Figure S. The cycles comprise active periods A in which the vertical motion of the object 32 is sufficient to operate the pumps 36 to 10 deliver water 45 with a sufficient pressure to the electncty generating station 42 to enable a predetermined electricity generation output. The cycles also comprise inactive periods I in which the vertical motion of the object 32 Is not sufficient to operate the pumps 36 to deliver water 45 with a sufficient pressure to the electricity generating station 42 to enable a predetermined electricity generation output. The predetermined 15 electricity generation output may be, for example, anything greater than zero, or may be any other threshold.

With the object 32 moving up and down with the water 33 as illustrated in Figure 5, the inactive periods I occur around the turn-around points between the rising and falling of 20 the vertical level of the body of water 33. During the inactive periods I, the object 32 Is not moving sufficiently to generate enough pressure in the water 45 delivered to the electricity generating station 42. On the other hand, during the active periods A, the pumps 36 are able to deliver sufficient pressure in the water 45 to generate electricity to a predetermined output level at the electricity generating station 42. In the absence of 95 the water cylinder 38, this would result In an intermittent generation of electricity, with periods I in which no electricity is generated, or electricity is generated at an insufficient level of output.

The use of the water cylinder 38 enables sufficient water to be pumped into the cylinder 30 38 to enable the generation of electricity from the water In the cylinder 38 during at least part of the inactive periods I, and preferably during the whole of the inactive periods I. This enables a continuous supply of electricity from the electricity

l M&C Folio No P52837GB generating station 42. Of course, because of the requirement for water 44 delivered from the pumps 36 to the cylinder 38 to be of a sufcrent pressure to fill the cylinder 38, 5 there are also active and inactive periods associated with the pumping of water into the c,!inder 39. Therefore, suff caret water shnu'!d be pumped into the c!inder 78 uurlng one iiCLIV penous ul Lne cylillue so to InalllLalll ciccLlicicy CIIcldiioll dL predetermined output level during the inactive periods.

10 In this embodiment, where the vertical level of the body of water 33 Is determined by the ocean tides, a cycle lasts for approximately 12 hours, with the time between a high and a low being approximately 6 hours. It can be expected, therefore, that an inactive period, which occurs around the turn-around point between the rising and falling of the tides, would be approximately 9.:S hours long In this embodiment the pumps 36 are two-way pumps which operate to pump water in the same direction when the object 32 rises and when the object 32 falls. Two pumps 36 are used, arranged on opposite sides of the object 32 so as to stabilise the object as it rises and falls, with substantially equal reaction forces between the object 32 and the 20 pump 36 on both sides of the object 32. The object 32 comprises a projection that overhangs an attachment on the basin 34, with the pump 36 arranged between the attachment and the projection.

Although the above embodiment has been described in relation to a body of water 33 in 25 communication with an ocean, so that the basin 34 is a dock, it will be appreciated that an embodiment of the present invention can operate in a similar manner when the body of water is in a lock, for example a river lock, where the vertical level of the body of water 33 is caused to change by water flowing into or out of the lock. [he vertical level of the body of water 33 Is controlled by the use of one or more gates across an opening 30 to the lock which are operated to control the amount of water in the lock. With such an embodiment, the height of the water In the lock can be controlled to any profile, not necessarily that shown in Figure 5. For example, the profile may be saw-shaped; with such a profile, there is a minimal turn-around time between the rising and falling of the

i M&C Folio No P52837GB body of water 33 and therefore the inactive periods described above with reference to Figure 5 will be much shorter. With shorter Inactive periods, the cylinder 38 can he S much smaller so as to maintain a much smaller amount of water for use In generating electricity during the inactive periods.

As an alternative to, or as well as, the use of a water cylinder 38 to maintain the generation of electricity at a predetermined level, two or more bodies of water 33 can be 10 employed in a similar way as described above to generate electricity, as will now be explained with reference to Figure 6.

Figure 6 Is a graph showing the water height profiles associated with the use of two bodies of water to generate electricity. The respective heights of the first and second 15 bodies of water are illustrated as changing in a manner similar to that shown in Figure 5.

However, the rcspectve cycles associated with the two bodies of water are arranged to be out of phase, with the phase being controlled so that an active period associated with one body of water completely covers an inactive period associated with the other body of water. In this way, when the pressure of water from the pumps associated with the 20 first body of water Is Insufficient to dove directly the turbines or fill the cylinder 38, the pressure of water from the pumps associated with the other body of water is sufficient, thereby enabling the generation of electricity to be maintained at a predetermined level.

Even with a body of water 33 in a dock, with the vertical level being affected by the 75 ocean tides, it is possible Lo control the level of the body of water 33 by the use of one or more gates across an opening to the dock which are operated to control the amount of water in the dock This would enable two such docks to be used with the relative water level cycles of the two docks being controlled In a similar manner to that shown in Figure 6. This could be achieved with two docks as follows. As the tide turns from a 30 high point and starts to fall, the gates of the first dock would be closed to hold water In the dock, while the water level in the second dock is allowed to fall. As the water level in the second dock approaches the level associated with an Inactive period, the gates of the first dock would be opened to allow the water level in the first dock to fall. By the

M&C Echo No P52R37GB time the level of water in the first dock reaches a point associated with an inactive period, the water level in the second dock will have started to rise above the pomt associated with the end of the inactive period of that dock. Such a process would be repeated to maintain the generation of electricity above a nredeterrnined output level.

Although the embodiment described above with reference to Figure 4 made use of two pumps, it will of course be appreciated that any number of pumps may be used. The lO more pumps that are used, the more water that can be pumped. Even the use of one pump Is possible, with appropriate means for stabilising the object 32 in the water 33. It will also be appreciated that the use of two-way pumps Is not essential. For example, if two pumps are used, one could be a one-way pump acting when the object 32 is rising, and the other pump could be a one-way pump acting when the object 32 Is falling.

15 Even a single one-way pump would be possible, using an appropriate scheme to maintain the electricity supply above a certain level as described above, If required. The electricity generating station 42 Is preferably located close to the electcty grid for ease of supply thereto.

2() The use of a piston 40 to apply additional pressure to the water in the cylinder 38 is also not essential. If the cylinder 38 is located at a sufficient height, the potential energy of the water itself being convened to kinetic energy by the time it reaches the electricity generating station 42 would be sufficient to apply enough pressure to the turbines of the electricity generating station 42 so as to generate sufficient electricity.

Claims

M&C Folio No P52837GB CI,AIMS:

1. A method of generating electricity from a change in the vertical level of a body of water by floating an object on the water, emnlo,vng the resulting vertical motion of r.. A_, __ _: 1.:_,L _ _.J. -

_ Lila UU,lccL LO L1;laLc; IllL;ail: IV1 Ul11111 wall, alms Glllyll, ills t111- wl TV generate electricity at an electricity generating station 2. A method as claimed in claim 1, wherein electricity is generated from the pressure of the pumped water at the electricity generating station.

3. A method as claimed in claim l or 2, wherein the pumping means cause water to be pumped from the body of water itself.

4. A method as claimed in claim 1, or 3, wherein the vertical level of the body of 15 water changes in a cyclical manner, with the water rising during at least one part of a cycle and falling during at least one other part of that cycle.

5. A method as claimed in claim 4, wherein the vertical level of the body of water changes in a predetermined manner.

6. A method as claimed in any preceding claim, wherein the pumping means are operable to cause water to be pumped directly to the electricity generating station for the generation of electricity.

25 7. A method as claimed in claim 6, when dependent on claim 4, wherein a cycle comprises at least one active period m which the vertical motion of the object is sufficient to operate the pumping means to deliver water with a sufficient pressure to the electricity generating station to enable a predetermined electricity generation output, and at least one inactive period in which the vertical motion of the object is not 30 sufficient to operate the pumping means to deliver water with a sufficient pressure to the electricity generating station to enable a predetermined electncty generation output.

M&C Fobo No P52837GB 8. A method as claimed in claim 7 wherein an inactive period occurs around the turn-around point between the rising and falling of the vertical level of the body of 5 water.

. 1..1. 1 _ 7 _ Cat.1. 1_ _ _. __ _: _ _ _ _ _ A.,., _ _,,,, a. IJICtIIUU old lalillcu li' lalil / Q1 o, Wlll,,llil t11 HU111111 11111 w"._l An, be pumped directly to the electricity generating station during at least part of an active - penod. l O. A method as claimed in any preceding claim wherein the pumping means are operable to cause water to be pumped to a holding reservoir for subsequent delivery to the electricity generating station for the generation of electricity.

15 l 1. A method as claimed in claim 10 when dependent on claim 4 wherein a cycle comprises at least one active period in which the vertical motion of the object is sufficient to operate the pumping means to dehver water with a sufficient pressure to holding reservoir and at least one inactive period In which the vertical motion of the object Is not sufficient to operate the pumping means to deliver water with a sufficient 20 pressure to the holding reservoir.

12. A method as claimed in claim 11 or claim to when dependent on claim 7 wherein the pumping means cause sufficient water to be pumped to the holding reservoir during an active period to maintain a sufficient amount of water in the holding 25 reservoir to enable the generation of electricity from the water in the holding reservoir during at least part of an Inactive period.

13. A method as claimed in claim 12 wherein the pumping means cause a sufficient amount of water to be maintained In the holding reservoir to enable the generation of 30 electricity from the water in the holding reservoir during the whole of the inactive period.

M&C FDI1O NO P52837CB

14. A method as claimed in any one of claims lO to 13, wherem the holding reservoir is located at a vertical height above the electricity generating station.

15. method as claimed in claim 14, wherein the vertical height is sufficient to .It,iIVGi Water Ui-lUi liS OWi-l WelSI-li in t|lG GIoCtiiGIty DeiGiarO aiOi -witty a SuI;iCiCilt pressure to enable a predetermined electricity generation output.

to 16. A method as claimed in claim 14 or 15, wherein additional pressure is applied to the water In the holding reservoir to achieve sufficient pressure of the water delivered to the electricity generating station.

17. A method as claimed In claim 16, wherein a weight pressing down on the water 15 in the holding reservoir is employed to apply the additional pressure.

18. A method as claimed in claim 17, wherem the holding reservoir is a cylinder and the weight is a piston acted on by gravity.

20 19. A method as claimed in any preceding claim, wherein the body of water is in a dock In communication with an ocean, the vertical level of the body of water being caused to change by the operation of ocean tides.

20. A method as claimed m any one of claims l to lS, wherein the body of water is 25 in a lock, the vertical level of the body of water being caused to change by water flowing into or out of the lock.

21. A method as claimed In claim 19 or 20, wherein the vertical level of the body of water is also controlled by the use of one or more gates across an opening to the dock or; 30 lock, as the case may be, which are operated to control the amount of water in the dock or lock, as the case may be.

M&C Fobo No P52837GB 92. A method as claimed In any preceding claim, wherein the pumping means are arranged between the object and an attachment the vertical level of which is not i affected, at least as much as the object, by the change m vcrtcal level of the body of water, the vertical motion of the object relative to the attachment operating the pumping i..eal,. 23. A method as claimed In claim 22, when dependent on claim 10 or 11, wherein 10 the attachment is disposed on the dock or lock side, as the case may be.

24. A method as claimed in claim 23, wherein the object comprises a projection that overhangs the attachment on the dock or lock side, as the case may be, and the pumping means are arranged between the attachment and the projection.

25. A method as clamcd in any preceding claim, wherein the pumping means comprise at least one pump that is adapted to pump water for the generation of electricity when the level of the body of water is rising.

20 26. A method as claimed In any preceding claim, wherem the pumping means comprise at least one pump that Is adapted to pump water for the generation of electricity when the level of the body of water is falling.

27. A method as claimed In any preceding claim, wherein the pumping means 25 comprise at least one pump that is adapted to pump water for the generation of electricity when the level of the body of water Is rising or falling.

28. A method as claimed in claim 27, wherein said at least one pump that is adapted to pump water for the generation of electricity when the level of the body of water is 30 rising or falling is a two-way pump.

29. A method as claimed in any preceding claim, wherein at least two such bodies of water are employed to generate electricity m the manner claimed.

( M&C Foho No P52837GB 30. A method as claimed in claim 29, when dependent on claim 3, wherein the respective cycles associated with the at least two such bodies of water are arranged to he 5 out of phase.

1 i-.,;o" as lai;'cu i; Claim CHUM we'd' ueper,uei-, i, Hawaii;,, Go 1 1, -'.,ci,.,'-.c

phase difference is controlled so that an active period associated with one body of water at least partially overlaps with an inactive period associated with the other body of 1 0 water.

32. A method as claimed m claim 31, wherein the phase difference is controlled so that the respective cycles associated with the at least two such bodies of water do not experience an inactive period at the same time.

33. A method as claimed in any preceding claim, wherein another type of liquid other than water is used.

34. Apparatus for generating electricity from a change m the vertical level of a body 20 of water, comprising an object floating on the water, means for pumping water operated in use by the resulting vertical motion of the object, and means for generating electricity from the pumped water.

35. A method of generating electricity substantially as hereinbefore described with 25 reference to the accompanying drawings.

36. Apparatus for generating electncty substantially as hereinbefore described with reference to the accompanying drawings.