Abstract:
A wave energy device in the form of a self buoyant heave-resistant vessel ( 10 ) comprising two or more discrete flow paths into which water can be urged by wave action and a bi-directional energy extractor in fluid communication with said path(s) and atmosphere, arranged to extract energy from air movements in said path(s) caused by said wave action.

Description:
FIELD OF THE INVENTION 
   The Invention relates to a wave energy device. 
   BACKGROUND OF THE INVENTION 
   Wave motion in bodies of water has long been recognised as a source of renewable energy. The invention aims to provide a device for exploiting this energy resource. 
   SUMMARY OF THE INVENTION 
   According to one aspect, the invention provides a wave energy device in the form of heave-resistant vessel comprising two or more flow paths into which water can be urged by wave action and an energy extractor in fluid communication with said path(s) and arranged to extract energy from air movements in said path(s) caused by said wave action. 
   According to another aspect of the invention, there is provided a wave energy device in the form of a heave-resistant vessel comprising one or more flow paths into which water can be urged by wave action and an energy extractor in fluid communication with said path(s) and arranged to extract energy from air movements in said path(s) caused by said wave action. 
   In a preferred embodiment, there is a plurality of flow paths, as least some of which have different lengths. In this way, flow paths can have different frequencies in order to allow the device to extract energy from waves of different periodicity in an efficient manner. 
   The or each flow path may be in communication with the atmosphere, at some point beyond the energy extractor. 
   In one embodiment, there is a plurality of flow paths comprised of a group of chambers. The chambers may be uniform or dissimilar. The chambers may be polygonal, rounded or indeed circular in cross-section. 
   In some embodiments, the wave energy entrance to at least one of the flow paths is shaped to enhance the energy extraction process. Preferably, this shaping is applied to the part of the flow path that is adjacent to the wave entrance of the flow path and takes the form of a smooth flaring of the flow path as it extends towards the wave entrance. Such shaping can reduce the turbulence created in water exchanged with the flow path under wave action. 
   In certain embodiments, two or more flow paths may combine before communicating with the energy extractor. 
   The heave-resistance of the vessel can be achieved in various ways. For example, the vessel may be tethered under tension to, say, the sea bed. If necessary or desired, the tethering system can have sufficient elasticity to allow the vessel to rise and fall with tidal activity. As one alternative to the use of tethers, or possibly in addition to the use of tethers if necessary, the heave-resistance of the vessels may be achieved or aided by the design of the vessel. This is to say, the footprint which the vessel presents to the water or, to state it another way, the cross-sectional area occupied by the vessel in plane of the water surface may be kept low in order to reduce an increase in buoyancy of the vessel due to water rising around the vessel from wave action. If necessary or desired, it is also possible to fit the vessel with a floatation aid. 
   Preferably, said heave resistance is provided by a positive buoyancy and tethering arrangement. 
   The energy extractor is preferably turbine-based for converting the energy in the air movements into electrical energy. However, the energy extractor may be arranged to perform other types of energy extraction. For example, the energy extractor may simply convert the air movements into mechanical energy for the use by some other system or the energy extractor could be arranged to convert the energy in the air movements into hydraulic energy. 
   Preferably, there is a plurality of flow paths, at least some of which have different flow cross-sectional areas. 
   Preferably, there is a plurality of flow paths, at least some of which have different internal flow volumes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By the way of example only, certain embodiments of the invention will now be described with reference to the accompanying figures in which: 
       FIG. 1  is a side elevation of a wave energy device; 
       FIG. 2  is a simplified view of the device of  FIG. 1  taken from the sea bed; 
       FIG. 3  illustrates the thickening of the chamber walls in the device of  FIG. 1 ; 
       FIG. 4  is a schematic diagram illustrating the principle of operation of the device of  FIG. 1 ; and 
       FIG. 5  illustrates an alternative deployment of the device shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a wave energy device  10  deployed in the sea and anchored to the sea bed  12  by tethers  14 . The wave energy device  10  is arranged to float such that part of its structure protrudes from the surface of the water. In  FIG. 1 , the mean sea level is indicated  16 . Wave crests will protrude above the mean sea level  16  and wave troughs will dip below this level. 
   The main structural element of the wave energy device  10  is a group of elongated chambers that extend in parallel to one another and whose walls are structurally linked or common and thus lend rigidity to the wave energy device  10 . The chambers extend vertically from the vicinity of a turbine  18  located above the water surface to differing depths below the water surface. The upper ends of the chambers are ducted into the turbine  18  and the lower ends of the chambers extend, by differing amounts, from a buoyancy jacket  20  which shrouds the group of chambers. The lower ends of the chambers protruding from the buoyancy jacket  20  are generally indicated  22  in  FIG. 1 . 
   The lower ends of the chambers are open to the water. The upper ends of the chambers lead into the ducting which places the chambers in fluid communication with the turbine  18 . The turbine  18  opens into the atmosphere and places the ducting, and hence the chambers, in fluid communication with the atmosphere. 
   In the wave energy device  10  shown in  FIG. 1 , there are six chambers in the group. The chambers, when viewed from below towards their open ends, appear as equal segments of a hexagon, as shown in  FIG. 2 . The six chambers have different lengths. Chamber  24  is the shortest and each successive chamber in the clockwise direction is longer than the preceding one, the sequence terminating with chamber  26 . 
   The wave energy device  10  is designed such that its overall density would cause it to float relative to the mean sea level  16  in the position shown in  FIG. 1 . Moreover, the wave energy device  10  is designed so that the effect of wave action upon its buoyancy is minimised. Essentially, the advent of a wave crest at the side of the wave energy device is a temporary increase in the water level in the vicinity of the device  10 . This increase produces a temporary submersion of an additional part of the volume of the device  10 , leading to an additional upward force on the device  10 . The device  10  can be designed such that the amount of additional volume that is submerged by wave action is reduced to an extent sufficient to make the aforementioned additional upward force negligible relative to the weight of the device  10 , therefore producing the result that the wave action has little effect on absolute vertical position of the wave energy device  10 . 
   By way of illustration of this principle, reference can be made to  FIG. 3  in which the walls of the cylindrical chambers have been thickened (without changing the chamber&#39;s external cross-sectional dimensions). In  FIGS. 2 and 3 , the shaded, i.e. solid, areas represent the cross-sectional area of the chamber walls in the plane of the mean water level  16 . The thicker walls in the embodiment of  FIG. 3  provide for increased buoyancy of the device  10 . It will be apparent that a temporary increase in water level due to the presence of a wave crest at the device  10  would lead to a significantly larger increase in water displacement in the case of a device having a greater cross-sectional area in the plane of the mean water level leading to a greater upward force in that case, that is to say the upward force is dependent upon the overall flow cross-sectional area of the successive chambers at the plane of the mean water level. 
   In the embodiments of  FIGS. 2 and 3  the cross-sectional areas of the successive chambers are substantially the same. However, in an alternative embodiment the flow cross-sectional area of the chambers may be different. For example, the flow cross-sectional of one or more of the chambers may be lesser or greater than the remainder. In different embodiments all the chambers may have different flow cross-sectional areas. As described above, the wave energy device  10  is designed to heave (i.e. move vertically) only minimally in response to wave action. Accordingly, as a wave passes through the device  10 , the water level will rise and fall in the chambers. As the water level rises in any given chamber, air will be pushed out of the chamber, through the ducting and through the turbine into the atmosphere. When the water level falls in any given chamber, air will be drawn down into the chamber from the external atmosphere via the turbine  18  and the ducting. Therefore, the passage of waves through the device  10  leads to an exchange of air through the turbine  18 .  FIG. 4  provides a schematic illustration of this process. 
   In  FIG. 4 , only those elements of the wave energy device  10  that are necessary for understanding the passage of the air flow through the turbine are shown. For example, in  FIG. 4 , only one chamber,  26 , is shown and the ducting between the chamber and the turbine  18  is omitted for the sake of clarity. The turbine includes a fan with a large number of vanes. In  FIG. 4 , the axis of rotation of this fan is indicated by the dashed line and the tips of the vane on the half of the fan&#39;s circumference that faces towards the reader are indicated by the row of curved lines, of which  30  is an example. 
   As a wave crest approaches the device  10 , the water level  32  within chamber  26  rises causing air to be expelled from the chamber  26  through the turbine  18  in the direction indicated by the arrow A. This causes the fan to rotate in the sense in which the vane tips shown in  FIG. 4 , such as  30 , move from right to left. When the crest of the wave has passed by, the water level  32  within the chamber  26  drops, reducing the air pressure in the space above the water in chamber  26 , causing air to be drawn into chamber  26  through the turbine in the direction indicated by arrow B. Although this air flow is in the opposite direction to before, the fan will rotate in the same sense as before because of special shaping applied to its vanes. The chambers vent independently into the turbine  18 . Thus, it is possible that one chamber could be driving the turbine  18  by drawing air down into the device whilst, at the same time, another chamber is expelling air through the turbine. However, it is possible that some chambers join or merge so that they act on the turbine in unison. The fan of the turbine  18  is mechanically coupled to a generator so that the rotational energy of the fan can be converted into electricity. 
   It will be apparent that the chamber  26  will resonate with a water wave of particular wavelength of period. The precise wavelength that will resonate with the chamber  26  depends upon the length of the chamber, When the chamber  26  is in resonance, the energy extractor from the chamber  26  by the turbine  18  will be at a maximum. However, water waves will, in practice, contain a spectrum of continuously varying wavelengths. To cater for this, the chambers of the wave energy device  10  each have a different length as described earlier. The provision of various chamber lengths increase the probability that at least some part of the wave energy device  10  will be operating under resonant conditions at any given time. 
     FIG. 5  shows an alternative way of deploying the wave energy device  10 . In  FIG. 5 , the wave energy device  10  is provided with positive buoyancy and it is restrained by taut tethers  14   a  which hold the device  10  in the position relative to the mean water level  16  that is shown in  FIG. 5 . The tethers  14   a  prevent heave motion of the wave energy device  10  and wave action causes the water level to rise and fall in the chambers  26  as in the deployment shown in  FIG. 1 . The tethers  14   a  are, to a certain extent, elastic so as to accommodate any rise and fall of the mean water level  16  due to, for example, tidal action.