Abstract:
A wave-powered pumping device for location in a body of water is described. The pumping device includes a submersible cylinder to be anchored to the bed of the body of water, the cylinder defining a bore. An underwater float acts on the cylinder and is arranged to urge the cylinder into an upright orientation in the water. A surface float is arranged to float at, or close enough to, the surface of the body of water in use to move up and down in the body of water in accordance with wave movement and tidal movement. An elongate member depends from the surface float. The elongate member extends telescopically into the bore of the submersible cylinder to define a pumping chamber within the cylinder. The volume of the pumping chamber varies with wave movement in a pumping cycle to draw fluid into the pumping chamber on an upstroke of the elongate member and to pump fluid out of the pumping chamber on a downstroke of the elongate member. The length of the pumping chamber varies with tidal movement to adjust to changing tidal depth by extending or retracting the elongate member relative to the cylinder while effective pumping cycles continue across a tidal range without needing to move the cylinder with respect to the bed of the body of water. To the extent that the elongate member is retracted into the bore of the cylinder, the elongate member occupies a majority of the cross-sectional area of the bore.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to wave-powered pumping devices. More specifically, the present invention relates to a pumping device for use in tidal waters. 
       BACKGROUND 
       [0002]    Wave-powered pumping devices are known. Typically these devices include a pump that is driven by the vertical displacement of a float located at the surface of a body of water, for example at the surface of the sea. Wave-powered pumping devices may be used to pump water through turbines to generate hydroelectricity, or they may be used to pump water to a reservoir onshore where it is stored for later use, for example to generate hydroelectricity on demand. 
         [0003]    An example of a known pumping device is described in Applicant&#39;s granted patent GB 2453670 B, and shown herein in  FIG. 1 . Referring to  FIG. 1 , the known pumping device includes a pump supported on a submerged platform  30 . The platform  30  is coupled to the sea bed  31  by a chain  28 , and is supported upright in the water by an underwater float  21 . The pump includes a piston  12  arranged for reciprocal motion within a cylinder  9 . A connecting member  5  connects the piston  12  to a weighted float  2  arranged at the surface of the sea. 
         [0004]    In use, the weighted float  2  rises with increasing wave height and falls under gravity as the wave passes. This vertical displacement of the weighted float  2  with passing waves drives the reciprocating movement of the piston  12  and connecting member  5  within the cylinder  9 . The pump is double acting and operates as follows: on an upstroke of the piston  12 , water is drawn into the cylinder  9  through an inlet valve  14  and simultaneously water is expelled from the cylinder  9  through an outlet valve  8  via a manifold  10 ; conversely, on a downstroke of the piston  12 , water is drawn into the cylinder  9  through an inlet valve  7  and simultaneously water is expelled from the cylinder  9  through an outlet valve  13  via the manifold  10 . 
         [0005]    The platform  30  may be raised or lowered in the water to adjust the height of the pump to suit varying tidal conditions. To this end, the platform  30  comprises an air filled column  22 , which is moveable in telescopic relation to a flooded cylinder  23 . As the water level rises in the body of water, for example with a rising tide, the weighted float  2  also rises and lifts the connecting member  5  relative to the cylinder  9 . Once the connecting member  5  reaches its maximum extension from the cylinder  9 , the continuing upward travel of the weighted float  2  with the rising tide causes the air filled column  22  to extend relative to the flooded cylinder  23 . 
         [0006]    At its base, the flooded cylinder  23  includes a suction relief inlet valve  25  and a pressure relief outlet valve  26 . As the air filled column  22  extends with rising tides, water is drawn through the suction relief inlet valve  25  into a chamber between the column  22  and the cylinder  23 . Conversely, to allow the column  22  to retract with falling tides to lower the height of the pump, water is expelled from the chamber through the pressure relief outlet valve  26 . The suction relief inlet valve  25  and the pressure relief outlet valve  26  are set to provide a hydraulic lock to hold the column  22  in a position that allows the pump  9  to operate within its normal stroke. 
         [0007]    Adjusting the height of the pump in the water is important because it ensures that the pump is at the correct height to operate normally, i.e. the weighted float  2  can move up and down, and ensures that the components of the pumping device are protected from extreme loads and forces. For example, if the height of the pump was fixed, then at high tides the pump would be positioned too low in the water. This would result in the equilibrium position of the weighted float  2  being too far from the pump, which would prevent the piston  12  from operating on a full upstroke. 
         [0008]    At high tides, if the equilibrium position of the weighted float  2  is too far from the pump, the connecting member  5  would extend significantly from the pump for long periods of time. In this extended position, the connecting member  5  would not be protected by the cylinder  9 , and would be exposed to severe lateral forces due to wave movement in the water. The connecting member  5  is made of metal and has a small diameter in comparison to the internal diameter of the cylinder  9 . The connecting member  5  extends through the pumping chamber, and hence a small diameter is required in order to maximise the pumping capacity of the device on the upward pumping stroke. As a result of its small diameter, the connecting member  5  could bend or buckle under lateral forces if extended from the cylinder  9  for long periods. Consequently it is necessary to have a height-adjustable platform to prevent the connecting member  5  from being extended from the cylinder  9  for long periods. 
         [0009]    Whilst the system described above with reference to  FIG. 1  works well, the present invention aims to provide an alternative pumping device of simplified construction. 
       SUMMARY OF THE INVENTION 
       [0010]    According to a first aspect of the present invention there is provided a wave-powered pumping device for location in a body of water, the pumping device comprising: a submersible cylinder to be anchored to the bed of the body of water, the cylinder defining a bore; an underwater float acting on the cylinder, the underwater float being arranged to urge the cylinder into an upright orientation in the water; a surface float arranged to float at, or close enough to, the surface of the body of water in use to move up and down in the body of water in accordance with wave movement and tidal movement; and an elongate member depending from the surface float, the elongate member extending telescopically into the bore of the submersible cylinder to define a pumping chamber within the cylinder; wherein: the volume of the pumping chamber varies with wave movement in a pumping cycle to draw fluid into the pumping chamber on an upstroke of the elongate member and to pump fluid out of the pumping chamber on a downstroke of the elongate member; the length of the pumping chamber varies with tidal movement to adjust to changing tidal depth by extending or retracting the elongate member relative to the cylinder while effective pumping cycles continue across a tidal range without needing to move the cylinder with respect to the bed of the body of water; and to the extent that the elongate member is retracted into the bore of the cylinder, the elongate member occupies a majority of the cross-sectional area of the bore. 
         [0011]    The present invention is simpler, cheaper and more effective than prior art systems. Cost savings are at least partly attributable to the use of fewer parts compared to known systems. For example, the present invention does not require a separate height-adjustable platform and associated valves to raise and lower the cylinder in the water to accommodate tidal variations. Instead, the present invention extends or retracts the elongate member telescopically with respect to the cylinder to compensate for tidal changes. At high tides, the elongate member will be extended significantly from the cylinder, whilst at low tides the elongate member will be mainly retracted within the cylinder. 
         [0012]    The elongate member is significantly larger than corresponding elongate members (or connecting rods) of known systems. In contrast to prior art systems, the elongate member of the present invention has a large diameter and occupies the majority of the cross-sectional area of the bore where retracted within the cylinder. 
         [0013]    The large diameter elongate member is able to resist bending or buckling from the strong lateral forces it experiences in the water when extended from the cylinder. The ability to resist deformation in this way allows the elongate member to remain in a highly-extended or exposed position for longer periods than prior art systems. Consequently, the device of the present invention is able to adjust telescopically to tidal changes whereas prior art systems generally utilise a height-adjustable cylinder to prevent the connecting rod from being extended for long periods. 
         [0014]    Typically the elongate member has a diameter that is at least 90% of the diameter of the bore. The diameter of the bore may be in the range 500-1600 mm. Preferably the bore has a diameter of at least 550 mm. The diameter of the elongate member may be in the range 500-1500 mm. It will of course be appreciated that the device may be much larger if it is to be used in very deep waters or in bodies of water having very large waves or tidal ranges. The device may also be scaled up to pump even larger volumes of fluid. Consequently, it is conceivable that the size of the various components could exceed these ranges. 
         [0015]    The elongate member may have a substantially circular cross section and/or a substantially uniform cross-sectional area along its length. The elongate member functions as a piston. A piston head may be provided at a lower end of the elongate member, the lower end being remote from the surface float. The piston head may be a separate part coupled to the elongate member or it may be defined by a closed lower end of the elongate member. In other embodiments, an open-ended elongate member may be employed. An outlet may be provided at an upper end of the elongate member. The outlet may communicate with a generator mounted above the elongate member. 
         [0016]    The device is preferably configured such that the downstroke of the elongate member is the main (or possibly only) working stroke. In embodiments where the elongate member has a closed lower end, the main pumping chamber is below the elongate member when the cylinder is upright. However, where the elongate member is open-ended, the pumping chamber may extend upwardly within the elongate member. In both cases, at least a majority of the pumping chamber is advantageously below an uppermost end of the cylinder when the cylinder is upright. Also, in both cases, the elongate member does not occupy or extend through the pumping chamber and hence there is no need to restrict the size of the elongate member in the present invention. 
         [0017]    To facilitate telescopic tidal adjustment, the elongate member is significantly longer than prior art connecting rods. Similarly, the cylinder is significantly taller than prior art cylinders. Typically, the cylinder and the elongate member would each be ten to twenty metres long, although longer or shorter parts could be manufactured to suit the specific characteristics of a particular body of water. Preferably the cylinder and the elongate member are at least ten metres in length. The device is suitable for use in bodies of water in which tides cause extreme changes in depth. For example, changes of depth of up to twelve metres occur in some bodies of water. To accommodate this tidal range, a fifteen metre cylinder and a fifteen metre elongate member may be used. This would still allow a further three metres of travel for wave-driven reciprocation of the elongate member at high tides. Of course the cylinder and elongate member could be made longer still to accommodate even larger waves at high tides if required. 
         [0018]    The elongate member may be made of metal or reinforced concrete. Alternatively the elongate member may be made from plastics materials. Preferably the elongate member is made of high-density polyethylene (HDPE). The elongate member may be of composite construction. In particular, the elongate member may be made from fibre-reinforced composite materials. For example, the elongate member may be formed from fibre-reinforced plastics materials, such as glass- or nylon-fibre reinforced HDPE. 
         [0019]    Conveniently, the elongate member may be hollow, or may otherwise define an internal cavity. For example, the elongate member may be tubular and have a substantially circular cross-section. Ballast such as aggregate, water, metal or other such dense material may be provided within the cavity to stabilise the elongate member in the water. The ballast increases the weight of the elongate member and assists the downward pumping stroke. 
         [0020]    A clearance region defined between the cylinder and the elongate member within the bore may be annular. The clearance region may have a width that is less than five percent of the diameter of the elongate member. Preferably the width of the clearance region is less than two percent of the diameter of the elongate member, and more preferably less than 1.5 percent. Typically the width of the clearance region is in the range of five to ten millimetres. Preferably, the width of the clearance region is approximately seven millimetres. 
         [0021]    The elongate member is preferably a sliding fit within the bore. Bearings may be provided within the clearance region to facilitate the sliding fit. A first bearing may be mounted to an outer surface of the elongate member. The first bearing may be mounted at a lower end portion of the elongate member. A second bearing may be mounted to an inner wall of the cylinder. The second bearing may be located within an upper end portion of the cylinder. This configuration of bearings helps to maintain the elongate member in concentric relation within the cylinder. 
         [0022]    The device may be configured such that in use, plant life or algae accumulates in the clearance region to lubricate movement of the elongate member within the cylinder. A scraper may be provided at an entrance to the clearance region to remove an excess thickness of plant life or algae during movement of the elongate member within the cylinder. The scraper also prevents barnacles, molluscs etc settling on this part of the cylinder. The device may be configured such that in use, a film of water in the clearance region lubricates movement of the elongate member within the cylinder. 
         [0023]    The surface float may include a buoyant portion and a ballast portion. The buoyant portion may be air filled. The ballast portion may include a tank, which may contain aggregate or water as a ballast. If water is used, the amount of water in the tank may be controlled dynamically to vary the ballast if required. For example, in stormy conditions sufficient water may be allowed into the tank to sink the surface float below the surface of the sea. When conditions are calmer, air may be pumped into the tank from, for example, an accumulator tank, to expel some or all of the water from the tank to raise the surface float once again. Alternately, if desired, the surface float may be sunk below the surface in normal use whilst remaining close enough to the surface to reciprocate in accordance with wave movement. The ballast portion may additionally or alternatively comprise a weight made of concrete or metal e.g. cast iron. 
         [0024]    The cylinder may be anchored to the bed of the body of water by a tether such as a rope, chain, cable etc. Using a tether is convenient and inexpensive when compared to a rigid coupling. However, rigid couplings such as ball joints that enable the cylinder to pivot about an upright position are also contemplated by the invention. The tether allows the height of the cylinder in the body of water to be determined and/or adjusted easily if required. The cylinder may include a flange to which the tether can attach. The cylinder may have a single tethering point to which one or more tethers can attach. Preferably a single tether is used to couple the cylinder to the bed of the body of water. Using a single tether allows the device to move freely about an anchorage point to adjust to the prevailing current. A single tether prevents ‘snatching’, which can be a problem in multiply-tethered systems. 
         [0025]    The underwater float is substantial and of sufficient buoyancy to resist the downward forces of the elongate member and the surface float during a downward pumping stroke. For tethered systems, the underwater float has sufficient buoyancy to ensure that the tether remains taut during a downward pumping stroke. This configuration contrasts with most prior art systems which must be rigidly coupled to the bed of the body of water or rigidly coupled to a fixed platform if the downstroke is used as a working stroke. Consequently, the pumping device of the present invention may be free-standing and self-supporting. In contrast to many prior art systems, the pumping device does not require additional stabilising means to support it upright in the water. 
         [0026]    The underwater float suitably acts on an upper end of the cylinder. In preferred embodiments of the invention, the underwater float is in the form of a jacket secured around the upper end of the cylinder. The jacket may be air filled. In this configuration, where the elongate member is closed-ended, the part of the bore that defines the pumping chamber extends within the cylinder to a level below the underwater float. Similarly, where the elongate member is open-ended, the majority of the pumping chamber would still be below the underwater float. This contrasts with many prior art systems, in which the equivalent bore and pumping chamber are located above an underwater float. Advantageously, the present invention provides a lower centre of gravity, and hence is more stable than free-standing prior art devices. 
         [0027]    The pumping device includes an outlet in communication with the pumping chamber. A pipe or hose may connect to the outlet for channelling pumped fluid to a remote location. Advantageously, the outlet may be provided at a lower end region of the cylinder. Providing the outlet at a low level ensures that the pipe or hose does not pull the cylinder away from its otherwise upright position in the water. In addition, providing the outlet at a low level means that the device is self-flushing: silt or other debris that sinks to the bottom of the cylinder can be flushed out though the outlet on a downward pumping stroke. 
         [0028]    The pumping device may also include an inlet in communication with the pumping chamber. Similarly, the inlet may be provided at the lower end region of the cylinder. At this location, the surrounding water is at a relatively high pressure due to depth. Accordingly, fluid entering the pumping chamber through the inlet assists and accelerates the upward movement of the elongate member on an upstroke. Providing the inlet at a sufficiently low level ensures that this effect is realised for all tidal conditions, including at low tides when the elongate member is retracted within the cylinder to its maximum extent. An additional benefit to providing the inlet at a low level is that this minimises the exposure of the inlet to direct sunlight and hence minimises weed growth and the accumulation of plant life, algae, molluscs, barnacles etc on and around the inlet. 
         [0029]    The pumping device is preferably configured to pump water from the surrounding body of water. In this case, the inlet may communicate with the surrounding water. The pumped water may be used in the generation of hydroelectricity or for desalination for example. Alternatively, the pumping device could be configured to pump other types of fluid, for example oil or gas, by connecting the inlet to an appropriate fluid reservoir. 
         [0030]    Preferably the device is single-acting. However, double-acting devices are also within the scope of the invention. 
         [0031]    According to a second aspect of the present invention, there is provided a method of pumping fluid using a wave-powered pumping device, the pumping device comprising a submerged cylinder urged towards an upright position by buoyancy, an elongate member in telescopic relation within the cylinder, and a float arranged above the cylinder and connected to the elongate member, the float and the elongate member being arranged to reciprocate relative to the cylinder with wave movement and tidal movement in the body of water, wherein the method comprises: pumping fluid during a wave-driven pumping cycle, whereby wave movement in the body of water causes the float and elongate member to reciprocate relative to the cylinder at a frequency and to an extent driven by the frequency and amplitude of waves in the body of water; and adjusting to tidal variation in the body of water during a tidal period by extending or retracting the elongate member telescopically relative to the cylinder while maintaining the wave-driven pumping cycle to pump fluid throughout the tidal period. 
         [0032]    The time between successive wave peaks in the sea may range between seven to twelve seconds; more commonly it is between eight to nine seconds, and typically it is about 8.5 seconds. A wave-driven pumping cycle will have a time span approximately equal to the time interval between successive wave peaks. 
         [0033]    The tidal period, defined herein as the time between successive high tides, is dependent upon the body of water, but is usually approximately twelve and a half hours. When placed in a body of water having this tidal period, the average extension of the elongate member from the cylinder will be at a maximum approximately every twelve and a half hours at the time of high tide. 
         [0034]    The method may include operating the device in bodies of water having a tidal range of up to twelve metres. By tidal range, it is meant the change in average depth of the water between low and high tides. It will be appreciated that the tidal range in a body of water varies with the lunar cycle, with maximum tidal ranges occurring during spring tides, and minimum tidal ranges occurring during neap tides. The cylinder and elongate member are preferably sufficiently long to accommodate a full tidal range, including a spring tide, whilst allowing wave-driven reciprocation to continue even at high tide. Consequently, the method may include extending the elongate member by up to twelve metres (and possibly further during spring tides) from the cylinder to adjust to a high tide, whilst allowing the elongate member to extend further from the cylinder to accommodate wave-driven reciprocation at high tide. In force six conditions, i.e. a strong breeze, the peak to trough height of waves in the sea is typically between three to four metres. In force nine conditions, i.e. a strong gale, the wave height may be as much as seven to ten metres, whilst in force eleven or twelve winds, i.e. violent storms or hurricanes, the wave height may reach sixteen metres. Accordingly, the method may include operating the pumping device in a body of water having waves of this magnitude. 
         [0035]    The method may comprise substantially maintaining the height of the cylinder in the body of water during a tidal period. Maintaining the height of the cylinder may involve maintaining a substantially constant separation between the bed of the body of water and the base of the cylinder. 
         [0036]    The inventive concept encompasses a single-acting wave-powered pumping device comprising a submersible cylinder tethered to the bed of a body of water and supported upright in the water by an underwater float; the cylinder defining a bore within which an elongate piston member is telescopically received; the piston member being connected at an upper end to a surface float arranged above the cylinder so as to reciprocate in the water in accordance with wave movement to drive the piston member within the bore; the piston member being of substantially uniform cross-section along at least that part of its length that is received within the bore in use, and wherein said cross-section occupies a majority of the cross-sectional area of the bore. 
         [0037]    The inventive concept also encompasses a method of pumping fluid using a wave-powered pumping device, the method comprising: submerging a cylinder in a body of water, the cylinder defining a bore; anchoring the cylinder to the bed of the body of water; maintaining the cylinder in a substantially upright orientation using an underwater float; arranging a surface float at, or close enough to, the surface of the body of water so that the surface float moves up and down in the body of water in accordance with wave movement and tidal movement; extending an elongate member telescopically into the bore of the cylinder to define a pumping chamber within the cylinder, the elongate member being connected at an upper end to the surface float; utilising an upstroke of the elongate member to draw fluid into the pumping chamber with increasing wave height; utilising a downstroke of the elongate member to pump fluid out of the pumping chamber with decreasing wave height; and extending or retracting the elongate member relative to the cylinder with tidal movement to vary the length of the pumping chamber in order to adjust the device to changing tidal depth while effective pumping cycles continue across a tidal range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Reference has already been made to  FIG. 1 , which shows a known pumping device, by way of background to the present invention. 
           [0039]    In order that the invention may be more readily understood, reference will now be made, by way of example, to the following figures, in which: 
           [0040]      FIG. 2   a  is a sectional side view of a single-acting pumping device according to a first embodiment of the present invention, in which the device is in a low tide condition; 
           [0041]      FIG. 2   b  is a cross-section taken along the line A-A in  FIG. 2   a;    
           [0042]      FIG. 2   c  corresponds to  FIG. 2   a  but shows the pumping device in a high tide condition; 
           [0043]      FIG. 3  is a sectional side view of a single-acting pumping device according to a second embodiment of the present invention; 
           [0044]      FIG. 4  is a sectional side view of a single-acting pumping device according to a third embodiment of the present invention; and 
           [0045]      FIG. 5  is a sectional side view of a double-acting pumping device according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    Referring to  FIG. 2   a , a pumping device  100  in accordance with a first embodiment of the invention is shown located at sea. By way of an overview, the pumping device  100  includes a tubular cylinder  102 , which is submerged below the surface  104  of the sea. The cylinder  102  is tethered to a concrete block  106  on the sea bed  108  via a chain  110 , and supported substantially upright in the water  112  by an underwater float  114 . The cylinder  102  has a cylindrical bore  116  defined by a circular internal wall  117  of the cylinder  102 . An elongate member  118  is telescopically received within the bore  116 . The elongate member  118  has a lower end  120  connected to a piston head  122 , and an upper end  124  connected to a surface float  126  arranged at the surface  104  of the sea. The piston head  122  and elongate member  118  reciprocate within the bore  116  as the surface float  126  moves up and down in the water  112  driven by wave movement. As shown, the elongate member  118  is at the bottom of its downstroke at low tide. 
         [0047]    The various components of the pumping device will now be described in more detail, still with reference to  FIG. 2   a . It should be appreciated that this drawing is not to scale. 
         [0048]    The surface float  126  has a diameter of approximately ten metres and comprises an air-filled buoyant portion  128  and a ballast portion  130  in the form of a tank containing sea water. The volume of water in the tank may be controlled dynamically to adjust the ballast if required. For example, in stormy conditions sufficient water may be allowed into the tank to sink the surface float  126  below the surface  104  of the sea. When conditions are calmer, air may be pumped into the tank from, for example, an accumulator tank, to expel some or all of the water from the tank to raise the surface float  126  once again. 
         [0049]    The cylinder  102  is approximately fifteen metres long and extends from a closed lower end  132  towards an open upper end  134 . The lower end  132  has the shape of an inverted Y, with arms  136 ,  137  of the inverted Y extending downwards and outwards towards the sea bed  108 . The open upper end  134  defines an entrance  138  to the bore  116 . The entrance  138  faces the surface  104  of the sea when the cylinder  102  is upright as shown. 
         [0050]    The cylinder  102  includes an inlet  140  and an outlet  142 , which are defined by the respective arms  136 ,  137  of the inverted-Y-shaped lower end  132 . The inlet  140  and outlet  142  include respective inlet and outlet valves  144 ,  146 , which communicate with a pumping chamber  148  defined within the bore  116 . An outlet pipe  150  or transfer hose for communicating pumped fluid to a remote location is attached to the outlet branch  137  of the cylinder  102 . A connecting flange  152  is provided between the arms  136 ,  137  of the Y-shaped lower end  132 . An upper end  154  of the chain  110  is attached to the connecting flange  152 , whilst a lower end  156  of the chain  110  is attached to the concrete block  106  on the sea bed  108  to anchor the cylinder  102  to the sea bed  108 . 
         [0051]    The tubular elongate member  118  is approximately fifteen metres long and made from high-density polyethylene (HDPE). An internal cavity  158  of the elongate member  118  contains aggregate  160 , which acts as a ballast to stabilise the elongate member  118  in the water  112 . 
         [0052]    A first guide bearing  162  is mounted externally to the lower end  120  of the elongate member  118 , whilst a second guide bearing  164  is mounted internally within the bore  116  at the open upper end  134  of the cylinder  102 . The first and second guide bearings  162 ,  164  maintain the elongate member  118  and the cylinder  102  in concentric relation, and assist the smooth travel of the elongate member  118  and associated piston head  122  as they reciprocate within the bore  116 . 
         [0053]    Referring to  FIG. 2   b , this shows a cross-section through the cylinder  102  and the elongate member  118  taken along the line A-A in  FIG. 2   a . A narrow clearance region  166  is defined between an external surface  167  of the elongate member  118  and the internal wall  117  of the cylinder  102 . The radial clearance between the cylinder  102  and the elongate member  118 , indicated by arrows  168 , is approximately seven millimetres, which is just large enough to accommodate the first guide bearing  162  shown in  FIG. 2   a . The diameter of the bore  116 , indicated by the double-headed arrow  170 , is 550 mm. The outer diameter of the elongate member  118 , indicated by the double-headed arrow  172 , is 536 mm. Consequently, the elongate member  118  has a cross-sectional area that occupies the majority of the cross-sectional area of the bore  116 . It is important to note that this configuration is in contrast to prior art systems, for example the device shown in  FIG. 1 , which utilises an elongate member  5  of considerably smaller diameter than the diameter of the corresponding bore. Consequently, that elongate member  5  occupies a minority of the cross-sectional area of that bore. 
         [0054]    Referring again to  FIG. 2   a , the underwater float  114  is in the form of a collar that connects around the upper end  134  of the cylinder  102 . The underwater float  114  is of substantial buoyancy, sufficient to support the cylinder  102  upright in the water  112  so that the chain  110  remains taut, even on a vigorous downstroke of the elongate member  118  and associated piston head  122  within the bore  116 . 
         [0055]    The piston head  122  is disc-shaped and lies in a plane orthogonal to a longitudinal axis  174  of the bore  116 . An annular sealing ring (not shown) surrounds the piston head  122  and abuts the internal wall of the cylinder  117  to form a seal between the pumping chamber  148  and the clearance region  166 . 
         [0056]    The pumping chamber  148 , which is of varying volume, is below the piston head  122  when the cylinder  102  is upright as shown in  FIG. 2   a . The volume of the pumping chamber  148  varies in accordance with the swept volume of the cylinder  102  as the elongate member  118  and associated piston head  122  reciprocate within the bore  116 . The swept volume is dependent upon the reciprocal motion of the surface float  126 , which is in turn dependent upon the height of the waves in the water  112  at any given time. 
         [0057]    The operation of the pumping device  100  will now be described with reference again to  FIGS. 2   a  and  2   c.    
         [0058]    Referring first to  FIG. 2   a , in use, as wave height increases at the surface  104  of the sea, the buoyancy of the surface float  126  causes it to move upwards with a rising wave. This upward movement lifts the elongate member  118  and piston head  122  within the bore  116 . During this upstroke, the rising piston head  122  creates a negative pressure in the pumping chamber  148 , which causes the inlet valve member  144  to move away from its seat and water to be drawn into the pumping chamber  148  through the inlet  144 . 
         [0059]    As a wave passes, the surface float  126  falls downwards under gravity, assisted by the weight of the ballast portion  130  and also by the weight of the elongate member  118  and the aggregate  160  contained therein. This downward motion of the surface float  126  drives the elongate member  118  and piston head  122  downwards within the bore  116 . During this downstroke, the piston head  122  pressurises the water in the pumping chamber  148 , which causes the outlet valve member  146  to move away from its seat and water to be expelled from the pumping chamber  148  through the outlet  142 . The water is pumped through the outlet pipe  150  towards hydroelectric turbines or to a reservoir (both not shown) where it may be stored for later use. Alternatively, the pumping device  100  may be used to pump water through a reverse-osmosis desalination system. 
         [0060]    An upstroke followed by a downstroke constitutes one complete cycle of the pumping device  100 . Notably, the pumping device  100  of  FIG. 2   a  is single acting and utilises the downstroke of the elongate member  118  and piston head  122  as the main pumping stroke or main working stroke. This is in contrast to most prior art single-acting pumping devices, which generally utilise the upstroke of the elongate member and piston head as the working stroke; such devices suffer from the disadvantage that the elongate member occupies a portion of the pumping chamber and hence reduces the effective volume of the pumping chamber. By utilising the downstroke as the main pumping stroke in the configuration shown in  FIG. 2   a , the elongate member  118  does not occupy the pumping chamber  148 , and hence the volume of water pumped on the main working stroke corresponds to the entire swept volume of the cylinder  102  by the piston head  122 . 
         [0061]    Also in contrast to many prior art devices, the single-acting pumping device  100  of  FIG. 2   a  does not require a seal between the cylinder  102  and the elongate member  118  at the open upper end  134  of the cylinder  102  because pressure loss at this point is not a concern due to the seal provided by the sealing ring around the piston head  122 . The pumping device  100  is self-lubricating and utilises the surrounding water  112  as a lubricant. In addition, the device  100  is self-flushing: silt or other debris that sinks to the bottom of the cylinder  102  is flushed out though the outlet  142  and the outlet pipe  150  on the downward pumping stroke. 
         [0062]    The ability of the pumping device  100  to self-adjust to rising and falling tides will now be described with reference to  FIG. 2   c . Referring to  FIG. 2   c , as the depth of the water  112  increases with a rising tide, the surface float  126  lifts the elongate member  118  and associated piston head  122  to establish a new equilibrium position in the water  112 . In this high tide equilibrium position, the elongate member  118  is significantly extended from the cylinder  102 . In this extended position, a significant proportion of the elongate member  118  is exposed to lateral forces from wave movement for several hours. However, in contrast to prior art systems, the elongate member  118  is able to withstand these forces due to its large diameter, which is almost as large as the diameter of the cylinder  102 . 
         [0063]    The chain  110  ensures that the separation between the lower end  132  of the cylinder  102  and the sea bed  108  remains substantially constant as the device  100  self-adjusts to rising and falling tides; expressed in other words, the height of the cylinder  102  remains substantially fixed whilst the elongate member  118  adjusts telescopically to rising and falling tides. 
         [0064]    Referring to  FIG. 3 , this shows a pumping device  176  in accordance with a second embodiment of the present invention. The same reference numerals are used in  FIGS. 3 and 2   a  to denote equivalent components. The pumping device  176  is similar in most respects to the pumping device  100  of  FIG. 2   a , except that the pumping device  176  does not include a piston head at the lower end  120  of the elongate member  118 . Instead, the elongate member  118  has an open lower end  120 . In common with the first embodiment, the elongate member  118  is hollow. Consequently, the pumping chamber  148  additionally extends upwards into the elongate interior  178  within the elongate member  118 . 
         [0065]    In this embodiment, a seal  180  is provided between the cylinder  102  and the elongate member  118  at the upper end  134  of the cylinder  102  to prevent water escaping from the clearance region  166 . This embodiment does not include bearings between the elongate member  118  and the internal wall  117  of the cylinder  102 . Instead, a film of sea water in the clearance region  116  lubricates the sliding motion of the elongate member  118  within the bore  116  of the cylinder  102 . Furthermore, algae or plant life accumulates in the clearance region  166 , which acts as an additional lubricant. Whilst an outlet pipe and coupling to the sea bed have been omitted in  FIG. 3 , it will be appreciated that these components may be similar to those shown in  FIG. 2   a.    
         [0066]    Referring to  FIG. 4 , this shows a pumping device  182  in accordance with a third embodiment of the present invention. The same reference numerals are used in  FIG. 4  to denote components that are equivalent to components in  FIGS. 3 and 2   a . The pumping device  182  of  FIG. 4  is similar to the pumping device  176  of  FIG. 3 , in so far as it is bearingless, and has an open-ended elongate member  118  and seal  180  at the upper end  134  of the cylinder  102 . However, the pumping device  182  of  FIG. 4  has been modified so that a surface delivery outlet  184  is provided at the upper end  124  of the elongate member  118 . 
         [0067]    The surface delivery outlet  184  communicates with the elongate interior  178  of the elongate member  118 , which is part of the pumping chamber  148 . A ball-valve element  186  is provided within the surface delivery outlet  184  for controlling the flow of fluid therethrough. The surface delivery outlet  184  may communicate with an onboard generator or other surface equipment. The outlet  142  at the lower end  132  of the cylinder  102  has been blanked off as it is not required in this embodiment. Whilst a coupling to the sea bed has again been omitted in  FIG. 4 , it will be appreciated that an arrangement similar to that shown in  FIG. 2   a  may be employed. 
         [0068]    Referring now to  FIG. 5 , this shows a pumping device  200  in accordance with a fourth embodiment of the present invention. The same reference numerals are used in FIGS.  5  and  2   a  to denote equivalent components. The pumping device  200  is similar in most respects to the pumping device  100  of  FIG. 2   a , except that it has been modified to make it double-acting. Therefore, in contrast to the first embodiment, the pumping device  200  pumps water on both an upstroke and on a downstroke of the elongate member  118  and associated piston head  122 . 
         [0069]    In addition to the main components described above in relation to  FIG. 2   a , the pumping device of  FIG. 5  includes a manifold in the form of a conduit  202  extending parallel and external to the cylinder  102 . The conduit has a lower end  204  that includes a manifold outlet valve  206  in communication with the outlet  142  of the cylinder  102 . An upper portion  208  of the conduit  202  extends through the underwater float  114  and terminates at an upper end  210 , which is substantially flush with an upper surface  212  of the underwater float  114 . The upper end  210  of the conduit  202  communicates with the surrounding sea water  112  via a manifold inlet valve  214 . The upper portion  208  of the conduit  202  also includes a manifold feed channel  216  in communication with the clearance region  166  between the cylinder  102  and the elongate member  118 . A seal  218  is provided between the cylinder  102  and the elongate member  118  at the upper end  134  of the cylinder  102  in this embodiment to prevent water escaping from the clearance region  166 . 
         [0070]    On a downstroke, the descending elongate member  118  and associated piston head  122  force water out of the pumping chamber  148  through the outlet  142  and along the outlet pipe  150  in much the same way as the device  100  of  FIG. 2   a . However, the pumping device  200  of  FIG. 5  also draws water into the sealed clearance region  166  on the downstroke via the manifold inlet valve  214  and through the manifold feed passage  216 . 
         [0071]    On an upstroke, in addition to drawing water into the pumping chamber  148  via the cylinder inlet  140 , the rising elongate member  118  and associated piston head  122  force water out of the clearance region  166 , through the manifold feed passage  216 , down through the conduit  202 , through the manifold outlet valve  206  and cylinder outlet  142  and along the outlet pipe  150 . 
         [0072]    As the clearance region  166  is narrow, the pumping device  200  pumps significantly more water on a downstroke than on an upstroke. However, the contribution of water pumped on the upstroke usefully increases the aggregate volume of the water pumped by the pumping device  200 . 
         [0073]    The fourth embodiment is configured to adjust telescopically to rising and falling tides in the same way as the first embodiment described above. 
         [0074]    Various modifications may be made to the above examples without departing from the scope of the invention as defined in the accompanying claims. For example, whilst the above examples describe coupling the cylinder  102  to the sea bed  108  using a chain  110 , it will be appreciated that the cylinder  102  could be attached to the sea bed  108  in other ways. For example, the cylinder  102  could retained by a pivot coupling. 
         [0075]    Also, whereas some of the embodiments described above include a disc-shaped piston head  122  coupled to the lower end  120  of the elongate member  118 , it will be appreciated that in other embodiments of the invention, the piston head  122  may be integrally formed with the elongate member  118 . For example, the piston head  122  may be defined by the lower end  120  of the elongate member  118 . 
         [0076]    Furthermore, whilst the devices  100 ,  176 ,  182 ,  200  described above are configured to pump water  112  from the body of water, it will be appreciated that other fluids, for example oil or gas, may be pumped by connecting the inlet  140  to an appropriate fluid reservoir.