WO2011141691A2

WO2011141691A2 - Tidal or wave energy harnessing device

Info

Publication number: WO2011141691A2
Application number: PCT/GB2011/000707
Authority: WO
Inventors: Alexander George Southcombe
Original assignee: Alexander George Southcombe
Priority date: 2010-05-08
Filing date: 2011-05-06
Publication date: 2011-11-17
Also published as: WO2011141691A3; GB2480110A; GB201007734D0

Abstract

A tidal or wave energy harnessing device (10) comprises a float (12); and a pump arranged to be driven by tidal or wave motion of a body of water. The pump comprises a piston chamber (14), connected to the float (12) and having a first inlet (16), a second inlet (18), a first outlet (20) and a second outlet (22). The pump further comprises a piston (28) comprising a piston head (30) which is movably received within the piston (chamber 14) and which has first and second pressure faces (30a, 30b), the piston (28) being configured to be anchored to a floor supporting said body of water. The piston head (30) is movable relative to the piston chamber (14) as a result of movement of the piston chamber (14) resulting from the tidal or wave motion of said body of water. The first inlet (16) and the first outlet (20) are in fluid communication with the first pressure face (30a) of the piston head (30), and the second inlet (18) and second outlet (22) are in fluid communication with the second pressure face (30b) of the piston head (30). In use, the piston chamber (14) is substantially submerged in the body of water.

Description

TIDAL OR WAVE ENERGY HARNESSING DEVICE

The present invention relates a tidal or wave energy harnessing device. Harnessing the power of the motion of a body of water has long been viewed as a potential source of renewable energy. Known methods of harnessing such power include tidal barrages located at suitable estuarine sites. Tidal barrages generally consist of a dam constructed across the full width of a tidal estuary. The barrage then makes use of the potential energy resulting from the difference in height (also known as the head) between high and low tide.

A known method of operation of tidal barrages is ebb generation (also known as outflow generation) in which the estuary water level is allowed to increase through sluices in the barrage until high tide. At high tide the sluices are closed. In some tidal barrages pumping may occur at this stage, whereby water is pumped from the sea/ocean into the river basin on the other side of the barrage. As the tide level of the sea/ocean decreases, a height difference between the water on either side of the barrier develops. The height difference between the water either side of the barrier may then be used to enable water to flow from the river basin to the sea/ocean via hydroelectric generators (for example turbines) which generate electricity. The tide then begins to rise and simultaneously water flows from the river basin to the sea/ocean. This causes the height difference between the water either side of the barrier to decrease. Once the height difference is sufficiently low, the sluices are opened such that the river basin is filled again. The cycle then repeats itself.

Tidal barrages suffer from various weaknesses. For example, tidal barrages can only be built at specialist sites, e.g. estuaries. The site chosen for the tidal barrage must be such that the tidal range (the difference in height between high tide and low tide) is sufficient for a tidal barrage to be viable. Only sites with a tidal range of about 5 metres are considered viable. Also the site must comprise a large basin area required to store the water and provide sufficient flow to the hydro electric generators. A basin which is many hundreds of square kilometres may be required. Tidal barriers also have high civil infrastructure costs associated with their construction. Tidal barriers may have a negative impact on the area surrounding a chosen site. Various habitats for wildlife, such as mud-flats, may be lost. In addition coastal erosion may be increased in some locations, while additional silting may occur in others. The tidal barrage may also increase the likelihood of flooding. Marine life may also be affected. For example, the tidal barrage may prevent migratory fish from returning to spawn. It is also the case that tidal barrages are not inherently robust structures. It follows that the tidal barrage may be susceptible to storm damage or other detrimental environmental effects such as land subsidence affecting the foundation of the tidal barrage.

Additionally, the power output of the tidal barrage is periodic, the period corresponding to the period between high and low tides. As the periodic nature of the power output of the tidal barrage will not match the demand for power during the course of the day, it is necessary to store the energy so that a continuous smoothed output of power can be provided.

Furthermore, the tidal barrage may present a security risk in that it may constitute a desirable target for terrorist attack.

The present invention seeks to obviate or mitigate at least some of the above mentioned disadvantages. According to a first embodiment of the invention there is provided a tidal or wave energy harnessing device comprising: a float connected to a pump which in use is arranged to float in a body of water and to be driven by tidal or wave motion of said body of water; the pump comprising: a piston chamber in fluid communication with a first inlet, a second inlet, a first outlet and a second outlet; and a piston comprising a piston head which is movably received within the piston chamber; said piston head having a first pressure face in fluid communication with the first inlet and first outlet, and a second pressure face in fluid communication with the second inlet and the second outlet; wherein, in use, the piston chamber is substantially submerged in the body of water and the piston is anchored to a floor supporting said body of water such that movement of the piston head relative to the piston chamber results from movement of the piston chamber as a result of tidal or wave motion of said body of water.

An energy harnessing device according to the invention hence provides an efficient, self-contained means of utilising the energy of the tidal or wave motion of a body of water. It will be appreciated that the energy harnessing device can be used in conjunction with any suitable body of water which undergoes tidal or wave motion. In the case of a sea or ocean, the floor which supports the body of water is commonly referred to as the sea bed or the ocean floor. In the case of an estuary the floor which supports the body of water is commonly referred to as the estuary floor or the river bed. It will also be appreciated that the float may be of any configuration or material which allows the float to displace a mass of water which is greater than its own mass.

Deploying the device so that the piston chamber is substantially submerged in the body of water has a number of advantages, including the fact that it enables the tidal or wave energy to be most efficiently harnessed while protecting the piston chamber from damage caused which might be caused by wind and/or water hitting the side of the chamber. A further advantage is that the environmental impact of the device can be minimised since substantially none of the piston chamber will be visible at the water's surface. It is preferred that the pump itself is substantially submerged. It is most preferred that the piston chamber and/or the pump is submerged just below, i.e. within a few metres of, the surface of the body of water supporting the device. In alternative embodiments, the piston chamber and/or pump may be submerged a greater distance below the water's surface, such as at least around 5 metres or more below the water's surface, although this is most likely to be the preferred option for applications in which the device is utilised in oceans and other relatively deep bodies of water.

The pump may be a double acting force pump. This type of pump can generate a greater output than other known pumps. This is because water is pumped to on outlet when the piston head is moved within the piston chamber in either direction.

The first inlet, the first outlet, the second inlet and the second outlet may each comprise any suitable design of one-way valve. This ensures that the direction of fluid flowing through the device is maintained throughout operation thereby maximising the efficiency with which power can be generated.

The float may at least in part define the piston chamber. The float may be constructed from a material which is suitable to withstand the large pressures which exist within the piston chambers during use. The use of the float to define the piston chamber will also reduce the complexity of the construction of the energy harnessing device. The float may be attached directly to the piston chamber or may be linked via one or more suitable tethers.

The piston may comprise a shaft connected to the piston head, the shaft having an attachment portion configured to anchor the shaft to the floor supporting the body of water. The shaft may be tubular defining a central bore and the energy harnessing device may additionally comprise a securing pillar which is configured to be received by the central bore; the securing pillar being configured at a first end to be attached to the attachment portion of the shaft of the piston member; and being configured at a second end to be secured to the floor supporting the body of water.

The energy harnessing device may additionally comprise an attachment member in the form of a cap; the cap comprising first and second screw threads; the first screw thread being co-operable with a corresponding screw thread of the first end of the securing pillar to thereby secure the cap to the securing pillar; and a second screw thread being co-operable with a corresponding screw thread of the attachment portion of the shaft of the piston to thereby secure the cap to the piston. Alternatively, the attachment portion of the shaft may comprise a first opening and the securing pillar may comprise a corresponding second opening; and the energy harnessing device may additionally comprise a securing pin which is inserted in the corresponding first and second openings, securing the securing pillar to the shaft.

The energy harnessing device may additionally comprise an intermediate mounting structure, the intermediate mounting structure being configured to be secured to the floor supporting the body of water and having an engagement portion, said engagement portion being configured to cooperate with an intermediate attachment portion to secure the shaft and hence the energy harnessing device to the floor supporting the body of water. The intermediate mounting structure may facilitate attachment of the energy harnessing device to the floor which supports the body of water. Furthermore, the intermediate mounting structure may reduce the length of the piston shaft(s) required to anchor the energy harnessing device to the floor which supports the body of water.

The pump may comprise a plurality of piston chambers and a plurality of pistons. In this way it is possible to create a pump with a larger effective piston head area without having to increase the size of the piston heads used. A larger effective piston head area of a pump will result in the pump having a greater power output. Smaller piston heads may be more advantageous than larger piston heads as they are easier to manufacture and transport.

If the pump comprises a plurality of piston chambers, the piston chambers may be arranged in any suitable manner. For example, the piston chambers may be arranged in a plane, into columns and/or rows. In this way, the piston chambers may be arranged in a two dimensional grid pattern.

As mentioned above, the pump may comprise a plurality of piston chambers and corresponding pistons. In a first preferred embodiment, each piston chamber comprises: a first inlet and a first outlet in fluid communication with a first pressure face of a corresponding piston head moveably received within said piston chamber; and a second inlet and a second outlet in fluid communication with a second pressure face of said corresponding piston head. In a second preferred embodiment the pump comprises a first plenum chamber in fluid communication with said first inlet, said first outlet and the first pressure faces of said plurality of piston heads; and a second plenum chamber in fluid communication with said second inlet, said second outlet and the second pressure faces of said plurality of piston heads.

The piston chambers may be arranged into groups, the piston chambers of each group being arranged co-axially above one another; and wherein each piston chamber group is associated with a piston, the piston comprising a plurality of piston heads, each piston head being received by a corresponding piston chamber of the piston chamber group. In this way the piston chambers may be arranged or stacked into columns. The energy harnessing device may comprise a plurality of such columns of piston chambers, arranged in any suitable manner. In this way, the energy harnessing device may comprise a three dimensional array of piston chambers.

At least one of the or each first outlet and the or each second outlet may be connected to a hydroelectric generator.

The energy harnessing device may further comprise a reservoir and a transducer, wherein the first and second outlets of the pump are connected to an inlet of the o

transducer, the transducer having an outlet which is connected the reservoir and wherein the reservoir is connected to the first and second inlets of the pump.

According to a second aspect of the present invention there is provided a method of assembling and locating an energy harnessing device according to the first aspect of the present invention, the method comprising: floating the float on the body of water to a desired location; inserting the piston into its corresponding piston chamber via at least one opening; closing the or each opening with a cover making the or each piston chamber substantially fluid-tight; and anchoring the piston to the floor supporting the body of water.

In this manner, the method of the invention provides a simple way of transporting components of the energy harnessing device representing the first aspect of the present invention to the desired site for their subsequent assembly.

The piston may be inserted into its corresponding piston chamber before the float is floated on the body of water to the desired location. Alternatively, the piston may be inserted into its corresponding piston chamber after the float is floated on the body of water to the desired location; and the at least one opening may be above the surface of the body of water.

In a first preferred embodiment of the second aspect of the present invention, the method may further comprise securing an intermediate mounting structure to the floor supporting the body of water at or near to said desired location; and anchoring the piston to the intermediate mounting structure. In the case where the energy harnessing device comprises an intermediate mounting structure, the method according to the second aspect of the present invention may comprise: securing the intermediate mounting structure to the floor supporting the body of water at the desired location; floating the float on the body of water to a desired location; inserting the piston member into the corresponding piston chamber via at least one opening; closing the or each opening with a cover making the or each piston chamber fluid-tight; and securing the energy harnessing device to the floor supporting the body of water by attaching the intermediate attachment portion to the engagement portion.

The or each opening may be defined at least in part by the float. The energy harnessing device is preferably secured to the floor supporting the body of water when the depth of the body of water in said desired location is a minimum. The proximity of the energy harnessing device to the floor which supports the body of water should aid in anchoring the energy harnessing device.

According to a third aspect of the present invention there is provided a method of smoothing the variation in power output over time of a plurality of tidal or wave energy harnessing devices floating in a body of water undergoing tidal or wave motion, the method comprising: locating at least two of the energy harnessing devices at different locations within the body of water such that there is a phase difference between the tidal or wave motion of the body of water at each of said different locations; and combining the power output of each energy harnessing device.

The phase difference between the tidal or wave motion of the body of water at the different locations of two energy harnessing devices may be approximately a quarter of the period of the tidal or wave motion of the body of water. If the phase difference is approximately a quarter of the tidal or wave motion period, this may result in the power output of one of the energy harnessing devices being a maximum whilst the power output of the other energy harnessing device is a minimum and vice-versa.

Other preferred features of the invention will become apparent from the description below.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic vertical cross section through a platform of an energy harnessing device in accordance with a first embodiment of the present invention; Figure 2 is a schematic vertical cross section through an energy harnessing device comprising the platform of figure 1 and a piston member received by said platform, the platform being in a high tide position relative to the piston member;

Figure 3 is a schematic vertical cross section through the energy harnessing device of figure 2, the platform being in a low tide position relative to the piston member; Figure 4 is a schematic vertical cross section through the energy harnessing device of figures 2 and 3, the piston member being attached to a sea bed and the platform being in a high tide position relative to the piston member;

Figure 5 is a schematic vertical cross section through a second embodiment of the present invention having an extension arrangement in a normal state;

Figure 6 is a schematic vertical cross section through the embodiment of the invention shown in figure 5, the extension arrangement being in an extended state;

Figure 7 is a diagrammatic curve representing the variation in the depth of the sea due to tidal movement over a period of about 36 hours; Figure 8 is a schematic vertical cross section through a third embodiment of an energy harnessing device according to the present invention;

Figure 9 is a schematic horizontal cross section through the energy harnessing device shown in figure 8;

Figure 10 is a schematic vertical cross section through a fourth embodiment of an energy harnessing device according to the present invention, the energy harnessing device being secured to a mounting structure fixed to the sea bed and the platform being in a low tide position relative to the piston members;

Figure 1 1 is a schematic vertical cross section through the energy harnessing device of figure 10, the platform being in a high tide position relative to the piston members; and

Figure 12 is a schematic vertical cross section through a fifth embodiment of an energy harnessing device according to the present invention.

Referring to figures 1-4, an energy harnessing device 10 according to a first embodiment of the present invention is shown submerged in a body of water 1 1. The energy harnessing device 10 comprises a platform 12 which defines a central piston chamber 14. The central piston chamber 14 extends along an axis X. An upper inlet conduit 16 and a lower inlet conduit 18 connect the piston chamber 14 with the exterior of the platform 12 such that fluid may flow therebetween. Likewise, an upper outlet conduit 20 and lower outlet conduit 22 connect the piston chamber 14 with a common outlet 24 such that fluid may flow between the piston chamber 14 and the outlet 24. The inlet conduits 16, 18 and outlet conduits 20, 22 each comprise a one-way valve (not-shown). The one-way valves in the inlet conduits 16, 18 allow fluid flow through the inlet conduits 16, 18 from the exterior of the platform 12 to the piston chamber 14, but do not allow fluid flow through the inlet conduits 16, 18 in the opposite direction. Conversely, the one-way valves in the outlet conduits 20, 22 allow fluid flow through the outlet conduits 20, 22 from the piston chamber 14 to the outlet 24, but do not allow fluid flow through the outlet conduits 20, 22 in the opposite direction back into the piston chamber 14.

Relatively high pressure water passing to the outlet 24 may then be supplied to any suitable known transducer 25 in order to produce useful energy from the water. An example of a suitable transducer 25 is a hydroelectric generator which produces electricity. A known type of hydroelectric generator comprises a turbine, the rotation of which being caused by high pressure water supplied to it. The rotation of the turbine powers the generator. In some embodiments there may be an energy storage means between the outlet 24 and the transducer 25. This is discussed at a later point in more detail.

The platform 12 is designed to be floated in water 11 , as shown in figure 1. The level of the water surrounding the platform is indicated as 26. It follows that when the piston chamber 14 is sealed such that no water may pass from the exterior of the platform 12 into the piston chamber 14, the platform 12 is buoyant. It is well understood in the art that in order to make the platform buoyant, the shape and material composition of the platform must be chosen such that the weight of the water which is displaced by the platform is greater than the weight of the platform 12. Suitable materials from at which at least part of the platform 12 may be made are reinforced concrete and steel. Any low density waterproof or water resistant material may be used to increase the buoyancy of the platform 12. The material of the platform 12, and in particular the part of the platform which defines the piston chamber 14, must be suitable for withstanding the large pressure within the piston chamber during use. Figure 2 shows the platform 12 with a piston 28 installed. The piston 28 comprises a piston head 30 mounted at its centre to a piston shaft 32. The piston shaft 32 is tubular and defines a central bore 34 which extends axially through the piston head 30. The central bore 34 is isolated from piston chamber 14 by the piston shaft 32 such that, in use, no water can pass from the piston chamber 14 into the central bore 34. The piston head 30 is mounted to the piston shaft 32 and has an upper pressure surface 30a and a lower pressure surface 30b which are perpendicular to the piston shaft 32. The piston 28 is shown in an upper position relative to the platform 12, which may correspond to the relative position of the piston 28 and platform 12 when the energy harnessing device 10 is in use and the level of the water 1 1 is a maximum.

Figure 3 shows the platform 12 and installed piston 28 when the piston 28 is in a lower position relative to the platform 12, which may correspond to the relative position of the piston 28 and the platform 12 when the energy harnessing device 10 is in use and the level of the water 11 is a minimum.

Figure 4 shows the energy harnessing device 10 located in position and secured to the sea bed 36. In order to locate the energy harnessing device 10 in position, the platform may be floated (as previously described) into position. The piston 28 is then inserted into the piston chamber 14 from above via an opening (not shown) in the top of the platform 12. After the piston 28 has been inserted into the piston chamber 14 a cover (not shown) is secured over the opening to make the piston chamber 14 fluid tight. In some embodiments of the invention, the piston 28 may be inserted into the piston chamber 14 at a manufacturing location. The energy harnessing device 10 may then be floated into position with the piston 28 in place.

At low tide, when the distance between the piston 28 and the sea bed 36 is at a minimum, a securing pillar 38 is inserted from above into the central bore 34. The securing pillar 38 passes through the central bore 34 of the piston shaft 32 and a first end of the pillar 38 is anchored to the sea bed 36. It will be appreciated that the securing pillar 38 may be secured to the sea bed 36 in any manner as long as the way in which it is secured is capable of withstanding the forces it is subjected to due to movement of the platform 12. Such forces may be due to currents within the sea or due to tidal/wave forces exerted on the platform 12 by the sea. Possible ways of securing the securing pillar 38 to the sea bed 36 include anchoring it by its own weight, screwing the securing pillar into a suitably firm sea bed 36 or embedding a concrete bulb at the end of the securing pillar 38 in a suitably shaped hole in the sea bed 36. It will further be appreciated that the piston 28 may be secured to the sea bed 36 in any appropriate manner.

Once the first end of the securing pillar 38 has been secured to the sea bed 36, a second end of the securing pillar 38 is fixed to the piston shaft 32 by an attachment member 40. The attachment member 40 may for example take the form of a cap which comprises two screw threads: A first screw thread which cooperates with a corresponding screw thread of the securing pillar 38 and a second screw thread which cooperates with a corresponding screw thread of the piston shaft 32. It will be appreciated that any appropriate means may be used for fixing the securing pillar 38 to the piston shaft 32, an example of which is the use of a clamp member to clamp the securing pillar 38 to the piston shaft 32. In a further example, the securing pillar 38 and piston shaft 32 may each comprise a corresponding opening. In order to secure the securing pillar 38 to the piston shaft 32 the corresponding openings are aligned and a pin may then be inserted through both openings.

In some embodiments of the invention, the use of a pin to secure the securing pillar to the piston shaft 32 may be advantageous as it enables a quick release of the piston shaft 32 from the securing pillar 38 (and hence of the energy harnessing device 10 from the sea bed). The quick release of the energy harnessing device 10 from the sea bed may be required, for example, if the sea supporting the energy harnessing device experiences unusual wave heights which exceed the length of the piston chamber 1 .

In some embodiments of the invention, instead of (or in addition to) the provision of a quick release mechanism that enables a quick release of the piston shaft 32 from the securing pillar 38 (and hence of the energy harnessing device 10 from the sea bed), the energy harnessing device may be provided with an alternative way of coping with unusually large wave heights. For example, the energy harnessing device may have an extension arrangement. The extension arrangement may have a normal state and an extended state. The extension arrangement can be used to extend the maximum depth of the sea (i.e. increase the maximum wave height) in which the energy harnessing device can operate. For example, the maximum depth of the sea which the energy harnessing device can operate in when the extension arrangement is in an extended state may be greater than the maximum depth of the sea which the energy harnessing device can operate in when the extension arrangement is in a normal state.

The extension arrangement extends the maximum depth of the sea in which the energy harnessing device can operate by increasing the distance between the sea bed 36 and the piston 28. Increasing the distance between the sea bed 36 and the piston 28 reduces the potential for an increase in the depth of the sea to cause the piston 28 to collide with the base of the piston chamber 14. As a consequence there is also a reduction in the potential for stress exerted on the piston 28, piston shaft 32 and/or securing pillar 38 by the platform 12 due to the force of the sea on the platform 12 and the piston 28 and base of the piston chamber 1 colliding.

Figure 5 shows an embodiment of the present invention having an extension arrangement. The embodiment shown in figure 5 differs from that shown in figure 4 in that the energy harnessing device has an extension arrangement which comprises a generally cylindrical extension member 29. The extension member 29 is arranged such that it is concentric with the piston shaft and the securing pillar. It will be appreciated that figure 5 is a schematic representation and that, as such, the spacing between the piston 28, securing pillar 38 and extension member 29 has been exaggerated to aid clarity.

The extension member 29 is secured at a first end to the securing pillar 38 via the attachment member 40. A rim 31 extends radially outwards from a portion of the extension member 29 that is adjacent the first end of the extension member 29.

The piston shaft 32 of the piston 28 has a rim 35 which extends radially inwards.

Figure 5 shows the extension arrangement in a normal state. In the normal state of the extension arrangement the extension member 29 is secured to the piston shaft 32 by a removable pin 33. The pin 33 secures the extension member 29 to the piston shaft 32 such that the rim 31 of the extension member 29 and the rim 35 of the piston shaft 32 are axially spaced by a distance D. The level of the sea whilst the extension arrangement is in the normal state is indicated by 26. Figure 6 shows the extension arrangement in an extended state. It can be seen that the level of the sea (indicated by 26a) has increased by a distance DS compared to the equivalent level of the sea as shown in figure 5. This increase in the level of the sea may be unexpected and may be caused by unexpected adverse weather conditions, for example a tsunami.

When the extension arrangement is in an extended state the pin 33 has been removed such that the extension member 29 is not secured to the piston shaft 32 by the pin 33. The piston shaft 32 is then free to move axially relative to the extension member 29 in a generally telescopic manner. If the pin 33 is removed and the platform 12 is urged upwards relative to its position when the extension arrangement is in the normal state by increase in the level of the sea (i.e. a change in sea level from that indicated in dashed line by 26 to that indicated by 26a) then the piston shaft 32 will move relative to the extension member 29 (and hence the securing pillar 38 and sea bed (not shown)) in an upward direction such that the rim 35 of the piston shaft 32 moves towards the rim 31 of the extension member, thereby reducing the axial distance between rim 35 and rim 31.

Figure 6 shows the extension arrangement at a limit of extension, whereby the rim 35 of the piston shaft 32 abuts the rim of the extension member 29. However, it will be appreciated that the rim 35 does not have to abut the rim 31 in an extended state of the extension arrangement. It is preferable that the axial distance D between the rims 31 and 35 is greater than the distance DS by which the sea level may increase due to an unexpected increase in the level of the sea. This will prevent the unexpected increase in the level of the sea causing the rims 31 and 35 to abut one another and thereby prevent the piston 28 colliding with the base of the piston chamber 14.

In some embodiments, when the sea level is reduced after the unexpected increase in the level of the sea, then the extension arrangement may be returned to its normal state. In order to achieve this, the extension member 29 is moved axially in a generally telescopic manner relative to the piston shaft 32 until the axial spacing between the rims 31 , 35 is D. The pin 33 can then be reinserted to secure the extension member 29 to the piston shaft 32. In the above described embodiment, the piston shaft 32 is secured to the extension member 29 by a removable pin 33. In other embodiments, this need not be the case. Any appropriate releasable fastening arrangement may be used to releasably secure the piston shaft 32 to the extension member.

The extension member may take any appropriate form. For example, instead of a rigid cylinder, the extension member may include at least one flexible elongate member (e.g. a chain) which is slack in the normal state of the extension arrangement, but which may be extended such that it is substantially taut in the extended state of the extension arrangement.

Either the securing pillar 38 or piston shaft 32 may comprise a plurality of similar holes at different axial positions along their length. In this manner the relative positioning of the securing pillar 38 within the piston shaft 32 can be selected depending on the environment in which the energy harnessing device 10 is to be located (for example, what the average depth of the sea is at that point).

Should the energy harnessing device require moving or servicing, it is possible to reverse the above described procedure so as to detach the piston 28 (and hence the platform 12) from the sea bed 36. The energy harnessing device, whilst floating, can then either be moved to its new location and re-attached or moved to a repair yard for servicing before being returned to its previous position.

Due to the fact that the piston 28 is fixed to the sea bed 36 via the securing pillar 38, if the level (also referred to as the depth) of the sea changes (relative to the sea bed 36), then because the platform 12 floats in the sea, the platform 12 will move relative to the piston 28. The movement of the platform 12 relative to the piston 28 will in turn cause the piston 28 to move within the piston chamber 14 axially (i.e. in the direction indicated by X in figure 1 ).

There are two main types of motion the sea undergoes which cause its depth relative to the sea bed 36 to change. These are wave motion and tidal motion. Tidal motion is caused by the combined effect of the Earth's rotation and the gravitational forces exerted on the sea by the Moon and the Sun. Tidal motion has a period of approximately twelve and a half hours. Wave motion may be caused by various forces including the wind and the Coriolis force. Such waves may be known as surface waves or weather induced waves. Wave motion is less predictable than tidal motion. Wave motion has a period which is dependent on many factors and as such it will be different for different parts of the sea. In addition, successive surface or weather induced waves at the same location can differ in both height and duration. Considerable differences in the characteristics of surface or weather induced waves can occur during calm and rough weather conditions. In general, the period of wave motion is less than 30 seconds. The oscillating change in depth of the sea relative to the sea bed 36 causes the platform 12, which floats at or near the surface of the sea, to undergo reciprocating motion. As the depth of the sea increases, the buoyancy force of the water on the platform 12 causes the platform 12 to move away from the sea bed 36. As the depth of the sea decreases the force of gravity on the platform 12 causes the platform to move towards the sea bed 36. The reciprocating motion of the platform 12 causes the piston 28 to undergo axial reciprocating motion within the piston chamber 14. It will be appreciated that in some embodiments of the invention it is advantageous that the height of the platform 12 (and hence the length of the piston chamber 14 defined by the platform) is chosen such that it is greater than the maximum tidal range at the location at which the energy harnessing device is to be deployed. In this manner, the piston 28 can move freely through the piston chamber 14 throughout the tidal (or wave) cycle. In particular, the piston 28 will not reach a limit of travel within the piston chamber 14 at any point during the tidal (or wave) cycle. This ensures that the entire tidal range (and/or wave range) of the sea contributes to the operation of the energy harnessing device, hence improving the efficiency of the device. It also prevents the platform 12 from putting undue strain on the piston 28. Such undue strain may adversely affect the connection between the piston 28 and the sea bed 36. As discussed above, the energy harnessing device 10 may comprise a quick release mechanism which may facilitate decoupling of the energy harnessing device 10 from the sea bed if the platform 12 puts undue strain on the piston 28 as a result of unexpectedly large tidal or wave movement.

The reciprocating motion of the piston 28 within the piston chamber 14 results in the energy harnessing device 10 undergoing a pumping cycle. As the piston 28 moves from an upper position shown in figure 3 (when the depth of the sea is relatively small) towards a lower position shown in figure 4 (when the depth of the sea is relatively great) the pressure within the piston chamber 14 beneath the piston head 30 will increase. The one way valve in the lower inlet conduit 18 is closed, whereas the one way valve in the lower outlet conduit 22 is opened. Hence, water from the piston chamber 14 below the piston head 30 can flow to the outlet 24 via the lower outlet conduit 22 due to the increased pressure beneath the piston head 30. Also, the pressure within the piston chamber 14 above the piston head 30 will decrease. The one way valve in the upper outlet conduit 20 is closed, whereas the one way valve in the upper inlet conduit 16 is opened. Hence, water flows from the exterior of the platform 12 (i.e. the sea) to the piston chamber 14 above the piston head 30 via the upper inlet conduit 16 due to the pressure above the piston head 30 being lower than the pressure of the sea water to the exterior of the energy harnessing device. As the piston 28 moves from the lower position shown in figure 4 (when the depth of the sea is relatively great) towards the upper position shown in figure 3 (when the depth of the sea is relatively small) the pressure within the piston chamber 14 above the piston head 30 will increase. The one way valve in the upper inlet conduit 16 is closed, whereas the one way valve in the upper outlet conduit 20 is opened. Hence, water from the piston chamber 14 above the piston head 30 can flow to the outlet 24 via the upper outlet conduit 20 due to the increased pressure above the piston head 30. Also, the pressure within the piston chamber 14 below the piston head 30 will decrease. The one way valve in the lower outlet conduit 22 is closed, whereas the one way valve in the lower inlet conduit 18 is opened. Hence, water flows from the exterior of the platform 12 (i.e. the sea) to the piston chamber 14 below the piston head 30 via the lower inlet conduit 18 due to the pressure below the piston head 30 being lower than the pressure of the surrounding water. The pumping cycle then repeats. This type of pump may be referred to as a double acting force pump.

The pumping cycle described above results in water being pumped by the energy harnessing device 10 from the sea surrounding the device 10, through the piston chamber 14 to the outlet 24 at high pressure. Depending on the stage of the pumping cycle, the pressure of the water at the outlet 24 may exceed that which is required for the water to pass from the outlet 24 to the energy transducer 25. The required pressure for the water to pass to the energy transducer 25 may be increased if a nozzle which reduces the cross-sectional area of the outlet 24 is used, for example, to increase the velocity of the pumped water when it reaches the energy transducer 25. The velocity of I /

the water at the energy transducer 25 is dependant amongst other things on the cross- sectional area of the outlet 24 (or of any nozzle which is in the outlet), the surface area of the piston head 30 and the speed at which the piston head 30 moves. Assuming that the piston head 30 is circular, its upper and lower pressure surfaces 30a and 30b will have a surface area A given by:

A = m-² (1 )

where r is the radius of the piston head 30. As the platform 12 is moved by the sea relative to the piston 28 from the lower position shown in figure 4 to the upper position shown in figure 3 (or vice-versa), the volume V of water which will be pumped by the energy harnessing device 10 to the outlet 24 is given by:

V = AL (2)

where L is the distance between the upper and lower positions of the platform 12 (which may be the tidal range or difference between maximum and minimum wave heights). The volume of water which is pumped per second Q is given by:

2 = (3)

where T is the time taken for the platform 12 to move from the lower position to the upper position (or vice-versa). The value of Q is not constant because the speed at which the platform 12 moves between upper and lower positions is not constant. However, if time T is approximated as quarter of the period of the oscillating change in depth of the sea, then Q will approximately correspond to the maximum volume of water which will be pumped per second during the reciprocating motion of the platform. If the water from the outlet 24 is delivered to the energy transducer 25 via a nozzle (not shown) with a cross-sectional area of AN then the velocity S_N of the water delivered through the nozzle is given by: „ ~ - (4)

In order to achieve this velocity of water at the nozzle, the velocity head of the water H_v will be given by:

H_r = - (5)

2g

where g is the acceleration due to gravity.

The power P_gpe of the energy harnessing device 10 in terms of gravitational potential energy of the velocity head is given by: Λ a

P_ipe = pQg _v (6)

where p is the density of the water. The power P_ke of the energy harnessing device 10 in terms of the kinetic energy of the water passing through the nozzle is given by:

Both of the power equations do not take the efficiency of the energy harnessing device into consideration.

The weight of water W_p required to press the amount of water per second Q at the nozzle velocity S_N is:

w_p = _PH_vA_Ng (8)

This weight of water W_P and the weight W_EHD of the energy harnessing device itself must be balanced by the weight W_D of the sea water which is displaced by the platform 12 according to:

W_D = W_{P +} W_EHD (9)

If the cross sectional area Ap of the platform is constant perpendicular to the axis X then the weight WD of the sea water which is displaced is given by:

W_D = pA_pD_v (10)

where D_v is the vertical displacement of the base of the platform from the sea surface 26.

It will be appreciated that because the movement of the piston head 30 within the piston chamber 14 is a direct consequence of the changing depth of the sea, the amount of water pumped to the outlet 24 in any given time (and hence the power output of any transducer 25 attached to the outlet 24) is greater the greater the rate of change of the depth of the sea. Figure 7 shows a schematic curve 41 representing the variation in the depth of the sea due to tidal movement over a period of about 36 hours. The peaks 42 of the curve 41 occur at high tide (i.e. when the depth of the sea is at a maximum) and the troughs 44 of the curve 41 occur at low tide (i.e. when the depth of the sea is at a minimum). As mentioned above, the greatest power output of any transducer attached to the outlet 24 will occur when the rate of change of the depth of the sea is greatest. This condition will typically occur for a period of approximately 3 hours (approximately a quarter of the period of tidal motion) midway between high tide 42 and low tide 44. The portions 45 of the curve 41 where maximum power generation r occurs are indicated between the two bounds 46. It also follows that at the portions of the curve 41 near high tide 42 and low tide 44, because the rate of change of the depth of the sea is very small, the power output of any transducer attached to the outlet 24 will be a minimum and may be very close to zero. It follows that the power output of any transducer attached to the outlet 24 will vary in a periodic manner between its maximum and minimum, the period of the variation in the case of tidal power typically being approximately 6 hours. The total power produced over any given time period is proportional to the tidal range (i.e. the difference between the depth of the sea at high tide and the depth of the sea at low tide).

It will be appreciated that in order for the pumping cycle to operate, the characteristics of the platform 12 must be chosen such that the buoyancy force which acts on the platform 12 to raise the platform 12 away from the sea bed 36 (i.e. so the piston 28 moves relative to the piston chamber 14 towards its lower position, as shown in figure 4) must be greater than the force of gravity on the platform 12, plus any frictional forces which oppose the movement of the platform 12 and piston chamber 14, plus the force required to push the water below the piston head 30 out of the outlet 24 via the lower outlet conduit 22, and plus the force required to create the pressure difference between the piston chamber 14 above the piston head 30 and the water to the exterior of the energy harnessing device, such that water flows from the sea 1 1 to the piston chamber 14 above the piston head 30 via the upper inlet conduit 16. This condition occurs when the depth of the sea is increasing. Similarly, the characteristics of the platform 12 must be chosen such that the force of gravity on the platform 12 whilst the platform 12 descends towards the sea bed 36 (i.e. so the piston 28 moves relative to the piston chamber 14 towards its upper position, as shown in figure 3) must be greater than the force required to push the water above the piston head 30 out of the outlet 24 via the upper outlet conduit 20, plus any frictional forces which oppose the movement of the platform 12 and piston chamber 14, and plus the force required to create the pressure difference between the piston chamber 14 below the piston head 30 and the water to the exterior of the energy harnessing device, such that water flows from the sea 1 1 to the piston chamber 14 below the piston head 30 via the lower inlet conduit 18. This condition occurs when the depth of the sea is decreasing. The characteristics of the platform 12 which must be chosen to meet these conditions include its mass the volume of water it can displace (which is dependent the cross sectional area of platform 12 and the maximum possible vertical displacement of the base of the platform 12 from the sea surface 26), and the cross sectional area of the piston chamber 14.

Theoretical platform 12 (and piston head 30) characteristics which may be used for an energy harnessing device 10 which is used in conjunction with tidal motion of the sea are as follows:

The piston head 30 is circular and has a radius r of 5m. According to equation 1 , this gives each of the surfaces 30a, 30b of the piston head a surface area A of approximately 80m².

The platform 12 has a constant square cross section of approx 12m by 12m giving the platform a cross sectional area A_P of approx 150m². Assuming that the tidal range is 2m (hence piston chamber 1 is preferably at least 2m in length), the distance L between the upper and lower positions of the platform 12 is also 2m. Tidal motion has a period of approximately 12 hours. As previously discussed, when calculating Q according to equation 3, if time T is approximated as quarter of the period of the oscillating change in depth of the sea then Q will approximately correspond to the maximum amount of water which will be pumped per second during the reciprocating motion of the platform. In this case, T would be 3 hours, which is approximately 1 1 ,000 seconds.

The cross sectional area A_N of the nozzle of the outlet 24 is 0.0004 m².

According to equation 2, the volume V of water which will be pumped by the energy harnessing device 10 to the outlet 24 as the platform 12 is moved between the upper and lower positions is approximately 160 m³. According to equation 3, the maximum volume of water which is pumped per second Q by the energy harnessing device is approximately 0.015m³s^"1.

According to equation 4, the maximum velocity S_N of water travelling through the nozzle is approximately 36 ms^"1. According to equation 7, the power P_ke of the energy harnessing device 10 in terms of the kinetic energy of the water passing through the nozzle is approximately 9.7kW.

According to equation 5, the velocity head of the water H_v inside the piston chamber 14 will be approximately 65m. This gives, according to equation 8, a required weight of water W_p of approximately 255kg and, according to equation 10, a vertical displacement D_v of the platform 12 of approximately 0.0017m.

Theoretical platform 12 (and piston head 30) characteristics which may be used for an energy harnessing device 10 which is used in conjunction with wave motion of the sea are as follows:

The piston head 30 is circular and has a radius r of 1 m. According to equation 1 , this gives each of the surfaces 30a, 30b of the piston head a surface area A of approximately 3.14m².

The platform 12 has a constant square cross section of approx 10m by 10m giving the platform a cross sectional area A_P of approx 100m². Assuming that the wave height is 2m (hence piston chamber 14 is preferably at least 2m in length), the distance L between the upper and lower positions of the platform 12 is also 2m. Wave motion has a period of approximately 20 seconds. As previously discussed, when calculating Q according to equation 3, if time T is approximated as quarter of the period of the oscillating change in depth of the sea then Q will approximately correspond to the maximum amount of water which will be pumped per second during the reciprocating motion of the platform. In this case, T would be 5 seconds.

According to equation 2, the volume V of water which will be pumped by the energy harnessing device 10 to the outlet 24 as the platform 12 is moved between the upper and lower positions is approximately 6.3 m³.

According to equation 3, the maximum volume of water which is pumped per second Q by the energy harnessing device is approximately 1.3 m³s^"1. Assuming that it is desired to achieve a nozzle velocity S_N of 35 ms^"1 and according to equation 4, the cross-sectional area AN of the nozzle is approximately 0.037 m².

According to equation 7, the power P_ke of the energy harnessing device 10 in terms of the kinetic energy of the water passing through the nozzle is approximately 800kW.

According to equation 5, the velocity head of the water H_v inside the piston chamber 14 will be approximately 65m. This gives, according to equation 8, a required weight of water W_p of approximately 24 tonnes and, according to equation 10, a vertical displacement D of the platform 12 of approximately 0.24m.

Practical embodiments of the energy harnessing device (whether used in conjunction with tidal motion or wave motion) may have any efficiency of about 60%. In both the tidal and wave examples above the power output of the energy harnessing device is proportional to the square of the nozzle velocity. For this reason, it may be desirable to increase the nozzle velocity in order to increase the power output of the energy harnessing device. Increasing the nozzle velocity in order to increase the power output of the energy harnessing device will also increase the required velocity head of the water (and hence the pressure within the piston chamber 14). If the pressure within the piston chamber 14 is increased so as to increase the power output of the energy harnessing device, it is necessary that the portion of the platform 12 which defines the piston chamber 14 is constructed from a material and in a manner which can withstand the increased pressure.

Furthermore, if the nozzle velocity of the water is increased in order to increase the power output of the energy harnessing device then the energy transducer 25 to be used must be considered. Certain transducers may not be capable of effectively converting the kinetic energy of water with a high velocity to other forms of useful energy (e.g. electrical energy). For example, known transducers which convert the kinetic energy of the water to electrical energy are unable to operate in conjunction with water velocities which exceed 80 ms^"1.

The above examples show that provided appropriate characteristics of the platform 12 and the piston head 30 are chosen, the embodiments of the present invention can £.

operate in locations which have a tidal range or range of wave motion as low as approximately 1m. Consequently, energy harnessing devices according to the present invention can be located at a much greater variety of sites compared to some know tidal/wave power devices (some of which require tidal ranges in excess of 10m).

Due to the stand-alone nature of an energy harnessing device according to the present invention, it is possible to position the device (or a plurality of devices) at a location close to on-land demand or close to an access point to an on-land electricity distribution infrastructure. By positioning a large number of energy harnessing devices according to the present invention at different locations, it is possible to ensure the continuity of power provision in spite of a catastrophic event such as extreme weather conditions or a terrorist attack. This is because such a catastrophic event is likely to be localised and hence only affect a small proportion of the energy harnessing devices. Furthermore, the stand alone nature of an energy harnessing device according to the present invention has less of an environmental impact compared to some known tidal energy devices. For example, compared to tidal barrages, the energy harnessing device according to the present invention will not impede the flow of a body of water (and hence wildlife) from one place to another. Furthermore, energy harnessing devices according to the present invention can be located such that they do not affect shipping routes.

Although the power output of the energy harnessing device 10 varies periodically, it is desirable in some situations for the power output to be fairly constant. For example, an energy harnessing device 10 comprising a hydroelectric generator may be linked to a national electrical distribution infrastructure. The demand over time for power by users attached to the national electrical distribution infrastructure is unlikely to vary such that it mirrors the periodic power output of the energy harnessing device 10 (this is particularly the case in relation to energy harnessing devices which harness the wave motions of the sea because wave motions typically have periods which are less than 30 seconds). Because of this it may be advantageous for the energy harnessing device 10 to supply electricity to the national electrical distribution infrastructure at a constant rate (also known as the energy harnessing device having a constant base load). The energy harnessing device may achieve this in several ways. The exemplified methods below comprise a way of storing at least some of the energy produced by the energy harnessing device 10 whilst it is at its maximum power output and then releasing the stored energy whilst the energy harnessing device is at its minimum power output. Suitable methods include the use of a high level reservoir, the use of water presses, the use of a fly wheel, the use of an electrical battery, or any suitable combination of these methods.

An example of the use of a high level reservoir is where the energy harnessing device 10 is used to pump water to a separate reservoir via the outlet 24. The reservoir is preferably at a height greater than sea level. The water collected in the reservoir can then be allowed to flow to a lower level (for example back to sea level) via a transducer such as a hydroelectric generator. The flow rate of the water from the reservoir to the lower level can be selected such that it flows at a constant rate which is not affected by the periodic variation in the pumping rate of the energy harnessing device.

An example of the use of water presses is where the energy harnessing device 10 is used to pump water into a first reservoir. A piston is linked to the first reservoir and to a second water reservoir. The weight of water in the first reservoir presses down on the piston and the piston causes the water in the second reservoir to be pressurised. The pressurised water from the second reservoir can then be used to supply a transducer at a constant rate.

An example of the use of a fly wheel is where a fly wheel is mechanically linked to the turbine of a hydroelectric generator. When the turbine is rotated by the flow of water from the outlet 24, the flywheel is also rotated. The flywheel has a significant moment of inertia. At the maximum pumping rate of the energy harnessing device, the speed of rotation of the turbine and hence the flywheel increases. This increase in rotational speed of the flywheel results in an increase of rotational energy of the flywheel. Due to the large moment of inertia of the flywheel, when the pumping rate of the energy harnessing device 10 is reduced, the flywheel continues to rotate the turbine and therefore rotational energy stored by the flywheel device is gradually transferred to the turbine.

An example of the use of a battery is where a hydroelectric generator powered by water from the outlet 24 of the energy harnessing device charges an electrical battery. The battery may be located on the energy harnessing device or may be at a separate location such as a fixed platform on the sea or on land close to the energy harnessing device 10. The hydroelectric generator charges the battery at a rate dependent on the pumping rate of the energy harnessing device 10. Electricity can then be taken from the battery as and when it is required. Another method to provide a substantially constant power output from the energy harnessing device is by the use of natural phasing utilising a plurality of energy harnessing devices 10. The energy harnessing devices 10 may be linked to a common output but are positioned at different locations. In one example, the locations of the different energy harnessing devices 10 are chosen such that the tidal motions of the sea at each location are out of phase. That is, the period of tidal motion of the sea at each location is approximately the same, but the times at which high tide (maximum sea depth) and low tide (minimum sea depth) occur at each location differ. As discussed above, the power output of the energy harnessing device 10 varies periodically and is dependent on what part of the tidal cycle the sea at the location of the energy harnessing device is undergoing. By choosing different locations for a plurality of energy harnessing devices 10 such that at a given time the sea at each energy harnessing device 10 is undergoing a different part of the tidal cycle, each energy harnessing device will, at said given time, have a different power output. However, although the power output of each of the energy harnessing devices 10 may be different, the total power output of the plurality of linked energy harnessing devices will be generally more constant over time. This may be termed "base load smoothing". Base load smoothing will occur for any phase difference between the power outputs of the energy harnessing devices. However, in some embodiments it is preferable that the position of at least two of the energy harnessing devices is chosen such that the phase difference between the tidal cycle at each position is approximately a quarter of the period of the tidal motion. As the period of tidal motion is typically approximately 12 and a half hours, an appropriate phase difference between at least two of the energy harnessing devices is just over 3 hours. A phase difference between two energy harnessing devices of a quarter of the period of the tidal motion will result in the power output of one energy harnessing device being a maximum when the power output of the other energy harnessing device is a minimum and vice-versa.

Base load smoothing is more difficult to achieve with wave motion rather than tidal motion. This is because wave motion is less predictable than tidal motion. However, it will be appreciated that, where it is possible to locate a plurality of energy harnessing devices according to the invention at locations such that the wave motion of the sea at each location is out of phase as described above, base load smoothing can be used in relation to wave motion. Figures 8 and 9 show a further embodiment of the invention that comprises a compound cylinder/piston system. This embodiment comprises a floating platform 48 which is similar to that of the previously described embodiment except that it comprises a plurality of similar piston chambers 50, each of which runs in a direction perpendicular to the plane of the platform 48. The embodiment shown in figure 9 includes twelve piston chambers 50 formed in two rows of six. It will be appreciated that in other embodiments of the invention any number of piston chambers 50 may be used in any appropriate layout. Each piston chamber 50 contains a piston 52. Each piston 52 comprises a piston head 54 (equivalent to piston head 30 in the embodiment shown in figure 2) mounted at its centre to a piston shaft 56 (equivalent to piston shaft 32 in the embodiment shown in figure 2). The piston shafts 56 may each be individually connected to the sea bed (in the same manner as the shaft of the previous embodiment i.e. the piston shafts 56 may each be clamped to securing pillars (not shown as separate parts) that connect to the sea bed) or may be connected to one another via an intermediate member and then to the sea bed.

The piston chambers 50 are connected at a position above the piston heads 54 to an upper plenum chamber 58 which is defined by the platform 48. The piston chambers 50 are also connected at a position below the piston heads 54 to a lower plenum chamber 60 which is defined by the platform 48. The upper plenum chamber 58 is provided with an upper inlet conduit 16 and an upper outlet conduit 20 as per the previous embodiment. The lower plenum chamber 60 is provided with a lower inlet conduit 18 and a lower outlet conduit 22 as per the previous embodiment. As before, the inlet conduits 16, 18 and outlet conduits 20, 22 each comprise a one-way valve (not-shown) which functions in the same manner as previously described. The operation of this embodiment of the invention is essentially the same as that of the previous embodiment.

Figures 10 and 11 show a further embodiment of the invention. This embodiment can be likened to a 'stacked' version of the previous embodiment, whereby the floating platform 62 comprises an upper layer of piston chambers 64 (arranged similarly to the previous embodiment) positioned above a lower layer of similarly arranged piston chambers 66. In this way the floating platform 62 comprises an array of piston chambers. Although in the shown embodiment there are two 'stacked' layers of piston chambers, it will be appreciated that other embodiments of the present invention may comprise any number of layers. By stacking piston chambers, larger total piston head surface areas can be employed in platforms of a given cross sectional area. This in turn enables smaller tide/wave ranges to be accommodated by platforms of a given cross sectional area. The upper and lower piston chambers 64, 66 each run in a direction perpendicular to the plane of the platform 62. Each upper piston chamber 64 is aligned above a corresponding lower piston chamber 66. A single piston 67 is received by each upper piston chamber 64 and the corresponding lower piston chamber 66 below it. The piston 67 comprises a central shaft 68 which extends through both the upper piston chamber 64 and the corresponding lower piston chamber 66 below it. An upper piston head 70 and a lower piston head 72 are mounted to the central shaft 68 and are received by the upper piston chamber 64 and lower piston chamber 66 respectively.

The central shafts 68 of each piston 67 extend towards the sea bed 36 and are anchored to the sea bed 36 via an intermediate mounting structure 74. An intermediate attachment portion of each piston 67 (in this case the bottom of shaft 68) is attached to the intermediate mounting structure 74. The intermediate mounting structure 74 may be mounted to the sea bed 36 using any appropriate means. The mounting structure 74 may be secured to the sea bed 36 at a depth below storm level, thus reducing possible strain that may be placed on the mounting structure 74. The mounting structure 74 may be mounted to the sea bed 36 at a desired location for an energy harnessing device before the energy harnessing device is moved to the desired location. The mounting structure 74 thus provides a convenient anchoring for the pistons 67. The pistons 67 and the mounting structure 74 may be provided with suitable cooperating attachments to facilitate the quick and easy attachment of the pistons 67 to the mounting structure 74 (for example, bayonet style fixtures or screw type fixtures). The use of the mounting structure 74 is also advantageous because shorter pistons 67 can be used due to the fact that the mounting structure 74 effectively raises the level of the sea bed 36. Reducing the length of the pistons 67 not only reduces the cost of the pistons 67, but also reduces the strain put on the pistons 67 when the device is in use. It will be appreciated that an intermediate mounting structure similar to that described may be used in conjunction with any of the other described embodiments in order to attach their piston(s) to the sea bed 36.

The piston chambers 64 of the upper layer are connected at a position above the piston heads 70 to a first upper plenum chamber 76 which is defined by the platform 62. The piston chambers of the upper layer 64 are also connected at a position below the piston heads 70 to a first lower plenum chamber 78 which is defined by the platform 62. Similarly, the piston chambers 66 of the lower layer are connected at a position above the piston heads 72 to a second upper plenum chamber 80 which is defined by the platform 62. The piston chambers of the lower layer 66 are also connected at a position below the piston heads 72 to a second lower plenum chamber 82 which is defined by the platform 62. As with the previous embodiment, each plenum chamber 76, 78, 80, 82 has an inlet conduit (84, 86, 88 and 90 respectively) and an outlet conduit (92, 94, 96 and 98 respectively). Each inlet conduit 84, 86, 88, 90 and outlet conduit 92, 94, 96, 98 contains a one-way valve (not shown) which functions in the same manner as the one-way valves previously described. Any of the inlet conduits 84, 86, 88 and 90 may be connected to one another and any of the outlet conduits 92, 94, 96, 98 may be connected to one another. If all of the outlet conduits are connected together they will form a common outlet 24. Each layer of piston chambers/piston heads operates in the same general manner as the embodiment described previously.

The rate at which a pump according to the invention can pump water to the outlet (and hence the power output of the pump) depends, amongst other things, on the surface area of the piston head. The two previously described embodiments both comprise a plurality of piston chambers/piston heads. In these cases, where a number of piston heads work together, the power output of the pump is dependent on the total surface area of all the piston heads. It follows that a pump with a number of smaller surface area cooperating piston heads will have the same power output as a pump with a single larger surface area piston head providing that the total surface area of all the smaller piston heads is equal to the surface area of the larger piston head. The use of a plurality of smaller pistons compared to a single large piston may be advantageous for one or more reasons. For example, smaller pistons are generally easier and cheaper to construct compared to larger pistons. Smaller pistons are also easier to transport to the location where the energy harnessing device is to be constructed. Furthermore, it is generally easier to seal a small piston head within a small piston chamber in a fluid tight manner compared to a large piston head within a large piston chamber. This is because the accuracy with which smaller parts can be made tends to be greater than that for larger parts. The greater the quality of the fluid tight seal between a piston head and a piston chamber, the greater the efficiency (and hence power output) of any pump which comprises the piston head and piston chamber. It follows that an energy harnessing device having smaller pistons/piston chambers (and hence a high quality of fluid tight seal between the piston heads/piston chambers) may have a greater efficiency (and hence power output) compared to that of an energy harnessing device having a larger piston/piston chamber. Furthermore, the greater quality of fluid tight seal possible between a small piston head and a small piston chamber makes it possible for such a piston chamber to withstand higher pressures than a large piston chamber. Due to the fact that small piston chambers can withstand greater pressures than large piston chambers, small piston chambers can be used to produce a larger water velocity head than large piston chambers. It follows that a plurality of smaller pistons/piston chambers having a total piston head surface area A_T are capable of harnessing more power than an equivalent larger piston/piston chamber which has a piston head area Aj.

The previous two embodiments comprise plenum chambers which are linked to the piston chambers such that, in use, water is pumped from the inlet conduits to the piston chambers via at least one plenum chamber, and such that water is pumped from the piston chambers via at least one plenum chamber to the outlet conduits. It will be appreciated that other embodiments of the invention may have a similar layout to those described above, but not comprise any plenum chambers. In this situation each piston chamber (either on its own or in a group with at least one other piston chamber) will have its own inlet conduits and outlet conduits, each inlet conduit and outlet conduit comprising a one-way valve as described above.

The description refers to the energy harnessing device 10 being located in the sea. However, it will be appreciated that the energy harnessing device 10 may be located in any body of water which undergoes motion such that the level (or depth) of the body of water changes (relative to the bed of the body of water). Further examples of such bodies of water include oceans and tidal estuaries. The embodiments of the invention described above may each be described as an open pumping system. In an open pumping system the inlets of the energy harnessing device are open to the sea in which the energy harnessing device is located. In this way, whilst the energy harnessing device undergoes a pumping cycle, it draws in water from the sea in which it is located. The embodiments which are an open pumping system have inlets which are located such that they are each open to the sea water during the period of the pumping cycle for which the pump draws in sea water via that particular inlet. The use of an open pumping system may be advantageous due to the fact that it is simple to construct and also due to the fact that the energy harnessing device can make use of the readily available pumping fluid (i.e. sea water) on which the energy harnessing device is located.

In some embodiments, it may be disadvantageous for the energy harnessing device to be an open pumping system. There may be various reasons for this. The sea water may contain contaminants, such as plant life, animal life or other debris. Referring to figure 4, the points of the platform 12 which are sealed relative to the piston shaft 32 so that the piston chamber 14 is water tight are indicated by S. In some embodiments, the contaminants present within the sea water may contaminate the sealed points of the platform and reduce the effectiveness of the water tight seal at the sealing points S. A reduction in the effectiveness of the water tight seal at the sealing points S may lead to a reduction in the operating efficiency of the energy harnessing device and may therefore be undesirable. A further reason it may be disadvantageous for some embodiments of the energy harnessing device to be an open pumping system is that the sea water may contain salt or other substances which are corrosive to portions of the energy harnessing device which are exposed to the pumped liquid. Furthermore, sea water may have other properties (for example compressibility, density and viscosity) which result in a reduction in the operating efficiency of the energy harnessing device. Figure 12 shows an embodiment of the present invention in which the possible disadvantages of an open pumping system of some embodiments of the invention may be overcome or mitigated.

The energy harnessing device shown in figure 12 comprises a compound cylinder/piston system that is similar to that shown in figures 8 and 9. The same numbering has been used for features of the energy harnessing device shown in figure 12 which are similar to the features of the embodiment shown in figures 8 and 9.

The embodiment of figure 12 comprises a floating platform 48 having two similar piston chambers 50, each of which runs in a direction perpendicular to the plane of the platform 48. Each piston chamber 50 contains a piston 52. Each piston 52 comprises a piston head 54 mounted to a piston shaft 56.

The piston chambers 50 are connected at a position above the piston heads 54 to an upper plenum chamber 58 which is defined by the platform 48. The piston chambers 50 are also connected at a position below the piston heads 54 to a lower plenum chamber 60 which is defined by the platform 48. The upper plenum chamber 58 is provided with an upper inlet conduit 16 and an upper outlet conduit 20. The lower plenum chamber 60 is provided with a lower inlet conduit 18 and a lower outlet conduit 22. As before, the inlet conduits 16, 18 and outlet conduits 20, 22 each comprise a one-way valve (not- shown) which functions in the same manner as previously described.

The embodiment shown in figure 12 differs from the embodiments which have been previously described (for example the embodiment shown in figures 8 and 9) in that it can be referred to as a closed pumping system. As a closed pumping system the inlets 16 and 18 are not open to the sea water on which the energy harnessing device is supported. Instead, the inlets 16 and 18 are connected to a reservoir 100 such that fluid can flow from the reservoir 100 to the inlets 16, 18. In the shown embodiment the reservoir 100 is mounted to the side of the energy harnessing device. This ensures that the reservoir is partially submerged in the sea water during use. At least partially submerging the reservoir 100 in the sea water during use may be beneficial because is minimises the amount of the energy harnessing device which is above water during use. This may reduce the visual pollution produced by the energy harnessing device and may help to protect the reservoir from adverse climatic conditions which occur above the sea. It will be appreciated that in other embodiments of the invention the reservoir may be located in any appropriate position.

The outlets 20, 22 of the energy harnessing device are both connected to a common outlet 24. Fluid is pumped by the energy harnessing device from the outlets 20, 22 via the outlet 24 and a nozzle 102 to a transducer 104. In this case the transducer 104 is an electric turbine which outputs electrical power, but it will be appreciated that any appropriate type of transducer may be used.

Fluid which has passed through the transducer 104 is then fed back to the reservoir 100 so that the fluid can be recycled and thus take part in further pumping cycles of the energy harnessing device.

A vent 106 is provided downstream of the transducer 104. In this case the vent 106 is integral to the reservoir 100, but this need not be the case. The vent ensures that the pressure downstream of the transducer 104 is substantially atmospheric pressure. This maximises the pressure drop (i.e. the difference in pressure) across the transducer 104 to be maximised, thereby increasing the efficiency of the transducer 104 and hence the operating efficiency of the energy harnessing device. The use of a closed pumping system means that a dedicated hydraulic fluid (for example, a fluid other than sea water) may be used. It is this hydraulic fluid which is pumped through the transducer by the energy harnessing device and recycled. The use of a hydraulic fluid other than sea water may be advantageous in certain embodiments. This is because sea water may contain contaminants, may be corrosive or may have sub-optimal fluid properties as discussed above.

It will further be appreciated that a plurality of energy harnessing devices according to the invention may be used in combination with each other, whereby the outlets 24 of each energy harnessing device are connected to one another. Alternatively, if the energy harnessing devices each comprise a transducer, the outputs of each transducer may be connected to each other.

Claims

• CLAIMS:

1. A tidal or wave energy harnessing device comprising:

a float connected to a pump which in use is arranged to float in a body of water and to be driven by tidal or wave motion of said body of water; the pump comprising:

a piston chamber in fluid communication with a first inlet, a second inlet, a first outlet and a second outlet; and

a piston comprising a piston head which is movably received within the piston chamber; said piston head having a first pressure face in fluid communication with the first inlet and first outlet, and a second pressure face in fluid communication with the second inlet and the second outlet; wherein, in use, the piston chamber is substantially submerged in the body of water and the piston is anchored to a floor supporting said body of water such that movement of the piston head relative to the piston chamber results from movement of the piston chamber as a result of tidal or wave motion of said body of water.

2. An energy harnessing device according to claim 1 , wherein the pump is a double acting force pump.

3. An energy harnessing device according to claim 1 or 2, wherein the first inlet, the first outlet, the second inlet and the second outlet each comprise a one-way valve.

4. An energy harnessing device according to any preceding claim, wherein the float at least in part defines the piston chamber.

5. An energy harnessing device according to any preceding claim, wherein the piston comprises a shaft connected to the piston head, the shaft having an attachment portion configured in use to anchor the shaft to the floor supporting the body of water.

6. An energy harnessing device according to claim 5, wherein the shaft is tubular and defines a central bore, the energy harnessing device additionally comprising a securing pillar which is configured to be received by said central bore of the shaft; the securing pillar being configured at a first end to be attached to the attachment portion of the shaft and being configured at a second end to be secured to the floor supporting the body of water.

7. An energy harnessing device according to claim 6, wherein the energy harnessing device additionally comprises an attachment member in the form of a cap, the cap comprising first and second screw threads, the first screw thread being co- operable with a corresponding screw thread of the first end of the securing pillar to secure the cap to the securing pillar and a second screw thread being co-operable with a corresponding screw thread of the attachment portion of the shaft to secure the cap to the piston.

8. An energy harnessing device according to claim 6, wherein the attachment portion of the shaft comprises a first opening and the securing pillar comprises a second corresponding opening; and wherein the energy harnessing device additionally comprises a securing pin which is inserted in both the first and second corresponding openings, securing the securing pillar to the shaft.

9. An energy harnessing device according to any one of claims 5 to 8, additionally comprising an intermediate mounting structure, the intermediate mounting structure being configured to be secured in use to the floor supporting the body of water and having an engagement portion, said engagement portion being configured to cooperate with an intermediate attachment portion to secure said shaft and thereby the energy harnessing device to the floor supporting the body of water.

10. An energy harnessing device according to any preceding claim, wherein the pump comprises a plurality of piston chambers and a plurality of pistons.

11. An energy harnessing device according to claim 10, wherein each piston chamber comprises:

a first inlet and a first outlet in fluid communication with a first pressure face of a corresponding piston head moveably received within said piston chamber; and

a second inlet and a second outlet in fluid communication with a second pressure face of said corresponding piston head.

12. An energy harnessing device according to claim 10, wherein the pump comprises a first plenum chamber in fluid communication with said first inlet, said first outlet and the first pressure faces of said plurality of piston heads; and a second plenum chamber in fluid communication with said second inlet, said second outlet and the second pressure faces of said plurality of piston heads.

13. An energy harnessing device according to any one of claims 10 to 12, wherein the piston chambers are arranged into groups, the piston chambers of each group being arranged co-axially above one another; and wherein each piston chamber group is associated with a piston, the piston comprising a plurality of piston heads, each piston head being received by a corresponding piston chamber of the piston chamber group.

14. An energy harnessing device according to any one of the preceding claims, wherein at least one of the first and second outlets is connected to a hydroelectric generator.

15. An energy harnessing device according to any preceding claim, further comprising a reservoir and a transducer, wherein the first and second outlets of the pump are connected to an inlet of the transducer, the transducer having an outlet which is connected the reservoir and wherein the reservoir is connected to the first and second inlets of the pump.

16. A method of assembling and locating an energy harnessing device according to any preceding claim, the method comprising:

floating the float on the body of water to a desired location;

inserting the piston into its corresponding piston chamber via at least one opening;

closing the or each opening with a cover making the or each piston chamber substantially fluid-tight; and

anchoring the piston to the floor supporting the body of water.

17. A method of assembling and locating an energy harnessing device according to claim 16, wherein the piston is inserted into its corresponding piston chamber before the float is floated on the body of water to the desired location.

18. A method of assembling and locating an energy harnessing device according to claim 15, wherein the piston is inserted into its corresponding piston chamber after the float is floated on the body of water to the desired location; and wherein the at least one opening is above the surface of the body of water.

19. A method of assembling and locating an energy harnessing device according to any of claims claim 16 to 18, the method comprising:

securing an intermediate mounting structure to the floor supporting the body of water at or near to said desired location; and

anchoring the piston to the intermediate mounting structure.

20. A method of assembling and locating an energy harnessing device according to any of claims 16 to 19, wherein the or each opening is defined at least in part by the float.

21. A method according to any one of claims 16 to 20, wherein the energy harnessing device is secured to the floor supporting the body of water when the depth of the body of water in said desired location is a minimum.

22. A method of smoothing the variation in power output over time of a plurality of tidal or wave energy harnessing devices floating in a body of water undergoing tidal or wave motion, the method comprising:

locating at least two of the energy harnessing devices at different locations within the body of water such that there is a phase difference between the tidal or wave motion of the body of water at each of said different locations; and

combining the power output of each energy harnessing device.

23. A method of smoothing the variation in power output according to claim 22, wherein the phase difference between the tidal or wave motion of the body of water at each of said different locations is approximately a quarter of the period of the tidal or wave motion of the body of water.