[0001]    The present invention relates to a device for extracting energy from a fluid flow, and more particularly to a fluid power generator generating air pressure variations that may be used to drive an air turbine. 
         [0002]    Recent years have seen the interest in the development of renewable energy sources increase as concern over the impact of carbon emissions on the environment has been heightened. Whilst focus has been primarily on the development of wind and solar power, these technologies have various disadvantages. Wind power generation is reliant upon the presence of driving wind of a given threshold value to move the propeller at sufficient speed to drive a turbine. Wind power also requires a large area of land dedicated to the production of energy and these large ‘wind farms’ are often unsightly and may pose a hazard to the surrounding wildlife. Solar power also has the disadvantages of providing a non-reliable source of electricity and also suffers from low efficiency and high cost. 
         [0003]    Wave or tidal energy devices can overcome many of the disadvantages listed above. They provide a reliable source of energy as they are driven by the force inherent within tidal and ocean waves and also have the potential to be placed in a large number of areas, particularly in coastal areas with large fetch, such as the western coast of Europe. 
         [0004]    A number of differing techniques have been employed to harness wave, tidal or ocean power. Traditional tidal energy devices have centred on a barrier arrangement that when placed within a tidal system fills with water at high tide and releases the water at low tide through a turbine to generate electricity. Concerns have been raised that the use of conventional barrier type tidal energy devices can prove hazardous to wildlife and boats. Additionally, these devices may only be used after each high tide and do not therefore provide a constant supply of energy. 
         [0005]    One example of a wave energy collector is disclosed within EP 1115976. This device utilises the relative rotational movement between pluralities of segments to drive a hydraulic motor. 
         [0006]    One alternative technique is to use the oscillatory nature of waves to compress a volume of air (an Oscillating Water Column device). By submerging a structure with an air chamber and an underwater aperture, an incident surface wave makes the fluid level within the chamber rise, compressing the volume of air within the air chamber. This (adiabatically) compressed air may then be used to drive a turbine, the rotation of which may be used to power a generator. As the water level falls, the air pressure reduces and air is drawn back into the chamber through the turbine. An example of this type of device is shown within EP 0948716 whereby the parabolic wave is focussed into a chamber wherein the air is compressed and used to drive a unidirectional turbine. Another example of an Oscillating Water Column device has been developed by Wavegen and has been named the ‘Limpet’. 
         [0007]    One inherent problem of these devices is the relatively low energy conversion efficiency, coupled to the varying nature of the size and strength of the incident waves, which leads to an uncertain energy output. These devices are also located on or close to the shore to take advantage of the higher parabolic waves at the shore. This again leads to a variation in the production of energy between high and low tides. Additionally, the above devices focus parabolic ocean waves through structural features, for example an upwardly sloped base or a generally upright wall. These devices are also unsuitable in scenarios of constant flow or current, for example tidal flows; thermohaline induced oceanic currents, for example the North Atlantic Drift and the Gulf Stream; and gravity induced fluid flows, for example within rivers. 
         [0008]    The present invention aims to overcome these problems by providing an improved device for extracting energy from a fluid flow. 
         [0009]    It is a further aim of the present invention to provide an improved water power generator. It is a further aim of the present invention to provide a water power device that requires little maintenance. 
         [0010]    According to the present invention there is provided a device for extracting energy from a fluid flow. The device comprises an air compression chamber and an array of valves, operable to open and close to regulate flow of the fluid through associated valve apertures. The valves are operable to close progressively as the fluid flow is incident thereon, thereby focusing flow of the liquid towards the air compression chamber and compressing air therein, and to open on a return flow of liquid from the compression chamber. 
         [0011]    It is an advantage that the device is configured to focus the energy in a flow of liquid to compress the air in an air compression chamber. The device is configured so that this can occur in a cyclical manner. The progressive closing of the valves focuses the flow of fluid to compress the air in the air compression chamber. The liquid, which then flows back out of the air compression chamber, is allowed to flow through the apertures by the opening of the valves. Another compression cycle can then commence by the progressive closing of the valves. Accordingly the device may be used in any flowing liquid, such as a river, or tidal flow or ocean current, to extract energy in the form of compressed air. 
         [0012]    Embodiments of the invention may further comprise an accumulation chamber for storing compressed air that has been compressed in the air compression chamber. 
         [0013]    Advantageously, the device may further comprise a turbine operable to be driven by the compressed air. A decompression chamber may be positioned downstream of the turbine for enhancing a pressure differential across the turbine during the return flow of liquid from the compression chamber. 
         [0014]    In embodiments of the invention, the valves within the array extend in an upward gradient in the direction of the fluid flow. 
         [0015]    The valves may be flap valves. These flap valves may comprise respective buoyant elements. The buoyant elements may have an angular displacement required to close the flap valves, the angular displacement increasing up the gradient. The buoyancy of the buoyant elements may also increase up the gradient and the buoyant elements may comprise tyres. 
         [0016]    In embodiments of the invention the valves comprise spoiler elements to facilitate the deflection of the fluid flow along the upwardly inclined gradient and/or assist the opening of the valves during the return flow. 
         [0017]    Embodiments of the invention further comprise use of the device as a tidal energy device, to drive a water turbine or to pump water to a higher reservoir. Additional embodiments comprise the use of the device as an oceanic or river flow device 
         [0018]    In final embodiments, multiple devices may be arranged or linked together to form a network of devices positioned to optimise utilisation of the fluid flow. 
     
    
     
         [0019]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a perspective view of a device for extracting energy from a fluid flow, prior to submersion in the fluid; 
           [0021]      FIG. 2  is an end-on view of the device of  FIG. 1 , showing the array of cutaways and valves in detail; 
           [0022]      FIG. 3  is a cross-sectional view through the line A-A in  FIG. 2  after submersion into the fluid flow; 
           [0023]      FIG. 4  is a cross-sectional view as per  FIG. 3  and shows the partial closure of the valves due to the incident fluid flow; and 
           [0024]      FIG. 5  is a cross-sectional view as per  FIGS. 3 and 4  and shows the complete closure of the valves due to the incident fluid flow, and the subsequent upsurge of fluid into the compression space. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  shows a simplified perspective view of a device  10  for extracting energy from a liquid flow. The device comprises a roof  12 , and side walls  14  that form an opening  16  incident to the flow direction. A top portion  20  of the device  10 , below the roof  12 , houses an arrangement of air chambers, as will be described in more detail below. The roof  12 , side walls  14  and additional structural components may be constructed from concrete, although any material capable of producing a stable, water-tight structure may be utilised, for example metals, including steel. A base section  30  extends from the bottom edge of the opening  16 , and this will be described in more detail below. The size of the device may be optimised for efficiency and/or to optimise the capture of the fluid and may be based on characteristics of the incident flow, as will be further described below. 
         [0026]    The base section  30  of the device  10  comprises alternate sloping backwalls  34  and horizontal floors  33 .  FIG. 2  shows an end-on view of the device looking through the opening  16 . An array of apertures  32  on the backwalls  34  are covered by a corresponding array of valves  40  (of which only one column of the array is shown in  FIG. 2 ). Additionally, although the array of apertures  32  is shown with a 6×7 arrangement, it may be appreciated that the number of rows and columns of the array may be varied dependent upon the required focussing effect and size of the device. For example, to utilise oceanic or tidal currents the device  10  may feature an array with as many as 200 or more columns and 2000 or more rows. Additionally, multiple devices  10  may be connected together to form a larger structure. 
         [0027]    To simplify the figures and allow viewing of the apertures  32 , only one column of valves  40  is shown in each figure. The valves  40  are shown as flap valves; however it may be appreciated that other valve types may be employed. The structure of the flap valves  40  is explained in detail below with reference to  FIG. 3-5 . The purpose of these valves  40  is to channel and regulate the flow of liquid in a manner that will be described in greater detail below. The columns of the valves  40  are shown extending along an upwardly extending gradient in the direction of liquid flow so that each row of valves is located both above and behind the lower row. Although the array is shown with a stepped arrangement, any configuration that provides an upwardly extending gradient may be employed, depending upon the orientation of the device with respect to the incident liquid flow or the required liquid channelling. 
         [0028]      FIG. 3  shows a cross-sectional view through the line A-A marked within  FIG. 2 . In this figure, the device has been immersed into a liquid to a level  60 . The valves  40  are open and the level of the liquid  65  within the device is approximately the same as the external level  60 . The top portion  20  includes an air compression chamber  24 , which is open at its bottom so that the liquid level  65  traps the air inside it, an accumulator chamber  22   a , and a decompression chamber  22   b . The pressure of the air trapped within the compression chamber  24  between the roof  12 , side walls  14 , rear wall  18 , chambers  22   a ,  22   b  and the liquid  65  is also approximately the same as the external air pressure. It may be appreciated that the area and volume of the space  24  may vary depending upon the relative dimensions of the constricting components ( 12 ,  14 ,  18 ,  20 ,  22 ) and the height of the water level  65 . 
         [0029]    Within the embodiment shown, the two chambers  20 ,  22  are connected to the roof  12  and sidewalls  14  of the device  10 . These chambers act to store air of differing pressure and are connected to each other via a turbine  50  and piping  52 ,  54 . Flap valves  21 ,  23  interconnect the compression chamber with, respectively, each of the accumulator chamber  22   a , and the decompression chamber  22   b . As the pressure of the air within the compression chamber  24  becomes higher than the pressure in the chamber  22   a , the valve  21  is forced open by the air pressure until the pressure within the chamber  22   a  and the compression chamber  24  are equivalent. Conversely, if the pressure within the chamber  22   b  is greater than the pressure in the compression chamber  24 , then the valve  23  opens until the pressures are equivalent. These chambers  22   a ,  22   b  also act as buoyancy tanks to keep the device floating within the water. As shown in  FIG. 1 , tether means  11  may be employed to secure the device  10  into position and allow the device to face the incident liquid flow. This tether means  11  may take the form of an anchor, mooring ropes, chains or any other anchorage. 
         [0030]    The operation of the device will now be described in relation to  FIGS. 3 ,  4  and  5 . Flow lines are shown for reference only.  FIG. 3  shows the device in the relaxed or initial position. In this position, the valves  40  are open, the water levels  60 ,  65  are approximately level and the pressure of the air within the compression chamber  24  and outside the device are approximately equivalent. An incident flowing liquid or current, represented by the individual flow lines  100  and incident upon the device  10 , flows through the aperture  16  and acts upon the array of valves  40 . The flowing liquid  100  enters the device and acts upon the valves  40 . The valves are arranged so that lowermost row of valves, due to the impulse of the liquid flow  100 , is the first to close against the apertures  32 . Once a valve  40  has closed, the incident liquid flow  100  is deflected in an upwards direction, increasing the impulse of the liquid flow against the second row of valves that are then also closed by the force of the flow  100 . This progressive closing of the valves focuses the flow of the liquid (represented in the figures by the lines of flow  100 ) into the compression chamber  24 , causing the water level  65  within the to rise, and compressing the air within the chamber  24 . This process continues until all the valves  40  are fully closed ( FIG. 5 ). Returning to an intermediate situation ( FIG. 4 ) where (in this representation) 3 of the 7 rows of valves are closed, it is clear that the fluid level  65  within the compression chamber  24  of the device  10  has increased to a level above the external mean level  60 . This increases the air pressure within the compression chamber  24 , closing the valve(s)  23  between the compression chamber  24  and the decompression chamber  22   b  and opening the valve  21  between the compression chamber  24  and the accumulator chamber  22   a.    
         [0031]    As may be seen from  FIG. 4 , the valves  40  close sequentially along the upwardly inclined gradient of the valve array. This sequential closing is achieved by varying the buoyancy and angle of closure for the valves between the rows within the array. In this case, the lower valves have lower buoyancy than the valves within the row directly above. In the current embodiment, the valves comprise car tyres  42  of differing tyre pressure. Additionally, the angle of the backwall  34  with respect to the vertical increases along the upward gradient towards the compression space  24 . Valve  40 ′ has a tyre  42 ′ with a lower pressure and smaller angle between the backwall  34 ′ and the vertical than valve  40 ″, with tyre pressure  42 ″ and backwall  34 ″. 
         [0032]      FIG. 5  shows the end state of the device when all the valves  40  are fully closed. The progressive closing of the valves  40  results in an increase in the level of the water  65  within the compression chamber  24 . The degree of water level  65  increase is dependent upon the number of valves  40  and the impulse of the incident fluid. Although the present embodiment features seven rows of valves, any number of rows may be utilised dependent upon the depth of the fluid and the degree of surge required. The incident flow of the fluid is now concentrated and directed towards the compression chamber  24  (as shown by the lines of flow  100 ), the volume of which has been reduced by the increasing water level  65 . This reduction in volume creates a corresponding increase in the air pressure within the compression chamber  24  and the accumulator chamber  22   a.    
         [0033]    Once the upward surge of water reaches a maximum, the air pressure within the compression chamber  24  rapidly drops and the inlet valve  21  to the accumulator chamber  22   a  closes. At this point there is no net fluid flow within the device  10 . When the device  10  is in this no-flow equilibrium position, both valves  21  and  23  between the compression chamber  24  and the chambers  22   a ,  22   b  are closed. Due to the operation of the valves  21 ,  23  and the relative air pressures of the chamber  24  at varying stages of the operation of the device  10 , the two chambers  22   a ,  22   b  have differing air pressures. Within the embodiment shown, accumulator chamber  22   a  has a greater air pressure than decompression chamber  22   b.    
         [0034]    The two chambers  22   a ,  22   b  are linked by a turbine  50  and inlet and outlet couplings  52 ,  54 . By opening the inlet  52  and outlet  54  couplings to the turbine  50 , the positive pressure air in accumulator chamber  20  is drawn through the coupling  52 , due to the pressure differential between the two bodies of air, into the turbine  50  and through coupling  54  into the decompression chamber  22 . This process drives the turbine  50  and may be used for the generation of electricity via a generator (not shown). Due to the construction of the chambers  22   a ,  22   b  and the method of coupling to the turbine  50 , the chambers may be used to store the varying pressured air over a number of cycles of the oscillatory water level  65 , building up the pressure difference with each cycle. Once a threshold pressure difference is reached, the coupling to the turbine  50  may be opened and the air moved through the turbine  50 . 
         [0035]    When the device  10  is in the no-flow position the water pressure acting upon the front of the valves  40  is the same as the rear of the valves. The valves therefore begin to open due to the buoyancy of the tyres. As the valve closest to the compression space has the highest pressure or buoyancy, this valve opens first. The water level  65  then begins to fall, causing a backward or downward flow of water over the valves. Due to the spoilers  44  on the top of the valves, the downward force of the water acts to open the valves, until all the valves are open, resetting the device to the situation shown in  FIG. 3 . A rail  120  or any other means may be utilised to prevent the valves  40  from opening past a predetermined angle. The device  10  is therefore essentially reset and the process described above is repeated (i.e. the incident fluid flow acts upon, and begins to close, the array of valves). 
         [0036]    As an alternative to the unidirectional turbine  50  described above, the chambers  22   a  and  22   b  could be omitted and a bidirectional flow turbine connected directly to the compression chamber  24 , for example a Wells turbine that is able to rotate in the same direction irrespective to the incident air flow direction. 
         [0037]    Although the device  10  has been explained with reference to a single device operating in isolation, it may be envisaged that multiple devices may be linked or placed together to form a cellular network of devices capable of supplying a larger quantity of energy. These devices may act independently or may share common elements, for example air compression and decompression chambers and/or turbines and generators to maximise the efficiency of the devices. Additionally, in order to maximise the flow of fluid through the devices, the network may be arranged into a “U” or “V” shape to prevent escape of the fluid flow around the outside of the network. Alternatively, the devices may be arranged within a shape akin to that of a “stealth bomber”, creating an area of low liquid pressure behind the structure. Multiple networks may also be linked or arranged together to optimise utilisation of fluid flow depending upon flow conditions. Although the networks of devices have been described in the orientation described above, any orientation may be utilised to suit the particular flow conditions. In addition, the devices may be arranged in series or stacked to increase the amount of energy that is extracted. The number of devices in the stack may be selected to optimise the return in terms of energy extracted in relation to the construction cost. Also, the stacks may be arranged as a series of devices oriented to receive a flow in one direction, with another series oriented to receive flow in the reverse direction. This arrangement is particularly suitable for use in tidal flows and avoids having to turn the devices around when the tide changes direction.
The invention provides a device for extracting energy from a fluid flow. The device has an air compression chamber and an array of valves, operable to open and close to regulate flow of the fluid through associated valve apertures. The valves are operable to close progressively as the fluid flow is incident thereon, thereby focusing flow of the liquid towards the air compression chamber and compressing air therein. The valves also open on a return flow of liquid from the compression chamber.