Patent Publication Number: US-2022228606-A1

Title: Energy storage systems

Description:
TECHNICAL FIELD 
     The present disclosure relates to energy storage systems and is particularly, although not exclusively, concerned with energy storage systems that provide improved energy storage density. 
     BACKGROUND 
     The use of renewable energy sources, such as solar and wind energy, often results in the amount of energy being generated at a particular time differing from the amount of energy being used at that time. This can result in an energy surplus or deficit on the energy supply grid. It is therefore desirable to store excess energy, when there is a surplus on the energy grid, and to supply the stored energy back to the energy grid, when energy demand exceeds the rate of energy generation. 
     Further, in some situations it may be desirable for energy to be stored and/or supplied at locations that are not connected to the energy supply grid. It may therefore be desirable for a device or system for storing energy to be transportable and able to be installed in a location remote from the power generation facility or power supply grid so that energy can be stored in and/or recovered from the energy storage system or device at the remote location. 
     STATEMENTS OF INVENTION 
     According to an aspect of the present disclosure, there is provided an energy storage system comprising:
         an energy storage device comprising:
           a pressure vessel configured to store pressurised fluid, e.g. at a pressure greater than the pressure outside the pressure vessel or atmospheric pressure; and   one or more resilient elements, wherein the resilient elements comprise a plurality of filaments of resilient material twisted and/or braided to form the resilient elements, wherein the resilient elements are arranged within or about the pressure vessel, and wherein the energy storage device is configured such that storing pressurised fluid within the pressure vessel acts to tension or compress the resilient elements.   
               

     The energy storage system may comprise a compressor configured to pressurise a fluid and supply, e.g. selectively supply, the pressurised fluid to the pressure vessel. 
     The energy storage system may comprise a hydraulic motor in fluidic communication with the energy storage device. Alternatively, the compressor may be a combined hydraulic compressor/motor. The energy storage system may be configured such that pressurised fluid within the pressure vessel can be selectively released from the pressure vessel through the hydraulic motor, e.g. through the operation of a valve, in order to extract energy, e.g. as mechanical power, from the energy storage system. The energy storage system may further comprise a generator configured to convert the extracted energy to electrical energy. 
     One or more of the resilient elements may comprise spheres formed by winding, coiling and/or weaving the twisted or braided filaments of resilient material. The spheres may be arranged within the pressure vessel. 
     Additionally or alternatively, one or more of the resilient elements may be arranged about, e.g. wrapped around, the exterior of the pressure vessel, such that expansion of the pressure vessel due to the introduction of the pressurised fluid acts to tension the resilient elements. The resilient elements may extend around the pressure vessel in a direction with a principle component perpendicular to a longitudinal axis of the pressure vessel. For example, the pressure vessel may be substantially prism shaped, e.g. substantially cylindrical, and may define a longitudinal axis. The resilient elements may extend around the exterior of the pressure vessel in a direction with a principle component in a circumferential direction of the cylindrical shape. 
     The compressor and the pressure vessel may be configured such that introducing the fluid pressurised by the compressor into the pressure vessel causes a perimeter, e.g. circumference, of the pressure vessel to expand, e.g. by a predetermined expansion, thereby tensioning the resilient elements. 
     The energy storage device may further comprise a piston movably arranged within the pressure vessel such that an operative volume of the pressure vessel, e.g. a volume of the pressure vessel able to receive pressurised fluid from the compressor, varies as the position of the piston within the pressure vessel varies. For example, the operative volume may be a volume of the pressure vessel arranged to one side of the piston in fluidic communication with an inlet of the pressure vessel, e.g. a volume between the piston and the inlet. 
     One or more of the resilient elements may be arranged such that varying the position of the piston to increase the operative volume of the pressure vessel acts to tension or compress the resilient elements. For example, the one or more of the resilient elements may be elongate. First ends of the elongate resilient elements may be coupled to the piston and second ends of the elongate resilient elements may be coupled to a wall of the pressure vessel, e.g. such that varying the position of the piston to increase the operative volume of the pressure vessel acts to tension the elongate resilient elements. For example, the second ends of the elongate resilient elements may be coupled to the wall of the pressure vessel adjacent to an inlet of the pressure vessel. 
     The resilient material may be have a tensile strength, e.g. ultimate tensile strength, of greater than or equal to, e.g. approximately equal to, 400 MPa. The resilient material may comprise a natural or synthetic spider silk. 
     According to another aspect of the present disclosure, there is provided an energy storage system comprising:
         a compressor configured to pressurise a fluid within the energy storage system; and   an energy storage device in fluidic communication with the compressor, the energy storage device comprising:
           a pressure vessel arranged to receive, e.g. selectively receive, the pressurised fluid from the compressor; and   one or more resilient elements, wherein the resilient elements comprise a plurality of filaments of resilient material braided to form the resilient elements, and wherein the resilient elements are arranged within or about the pressure vessel; and   
           wherein the energy storage system is configured such that introduction of the pressurised fluid into the pressure vessel places the resilient elements under compression or tension.       

     The filament may comprise a strand or fibre of the resilient material. The resilient material may be a natural or synthetic material, such as a polymer or metal material. Two, three or more than three filaments of the resilient material may be twisted or braided to form the resilient elements. Additionally, the twisted or braided filaments may themselves be braided, woven, coiled and/or wound to form the resilient elements. 
     According to another aspect of the present disclosure, there is provided an energy recovery system comprising:
         an energy storage device comprising:
           a pressure vessel configured to contain a pressurised fluid; and   one or more resilient elements, wherein the resilient elements comprise a plurality of filaments of resilient material twisted and/or braided to form the resilient elements, and wherein the resilient elements are arranged within or about the pressure vessel; and   
           wherein the energy storage system is configured such that storage of the pressurised fluid into the pressure vessel places the resilient elements under compression or tension.       

     According to another aspect of the present disclosure, there is provided an energy storage device for the above-mentioned energy storage system, wherein the energy storage device comprises:
         a pressure vessel;   one or more resilient elements, wherein each of the resilient elements comprises a plurality of filaments of resilient material twisted or braided to form the resilient elements, wherein the resilient elements are arranged within or about the pressure vessel such that the one or more resilient elements are tensioned or compressed when pressurised fluid is introduced into the pressure vessel.       

     According to another aspect of the present disclosure, there is provided an energy storage method comprising:
         providing an energy storage device, the energy storage device comprising:
           a pressure vessel configured to store pressurised fluid; and   one or more resilient elements, wherein the resilient elements comprise a plurality of filaments of resilient material twisted and/or braided to form the resilient elements, wherein the resilient elements are arranged within or about the pressure vessel, and wherein the energy storage system is configured such that storing pressurised fluid within the pressure vessel acts to tension or compress the resilient elements; and   
           introducing pressurised fluid into the pressure vessel and thereby storing potential energy in the energy storage device, e.g. hydraulic potential energy and elastic potential energy.       

     For example, the pressure vessel may be operatively coupled to a compressor for supply pressurised fluid, and the compressor may be operated to supply pressurised fluid to the pressure vessel. 
     The method may further comprise transporting the energy storage device to a location at which energy is to be supplied. The method may comprise operatively coupling the pressure vessel to a hydraulic motor, e.g. at the location at which energy is to be supplied. The method may further comprise releasing the pressurised fluid through the hydraulic motor in order to convert stored energy to mechanical or electrical energy. 
     According to another aspect of the present disclosure, there is provided an energy supply method comprising:
         providing an energy storage device, e.g. within the energy supply system. For example, providing the energy storage device may comprise operatively coupling the energy storage device to the energy supply system. The energy storage device comprises:
           a pressure vessel containing a pressurised fluid; and   one or more resilient elements, wherein the resilient elements comprise a plurality of filaments of resilient material twisted and/or braided to form the resilient elements, wherein the resilient elements are arranged within or about the pressure vessel, and wherein the energy storage device is configured such that the resilient elements are placed in tension or compressor by virtue of the pressured fluid being present within the pressure vessel; and   
           releasing the pressurised fluid from the pressure vessel through a hydraulic motor in order to generate mechanical or electrical energy. For example, the hydraulic motor may be operatively coupled to an electrical generator configured to convert mechanical energy to electrical energy.       

     The pressurised fluid may be released from the pressure vessel at or below a flow rate, e.g. a predetermined flow rate, the flow rate such that a pressure of the fluid within the pressure vessel remains substantially constant, e.g. varies by less than approximately 10% or 20%, whilst the majority of the fluid is release from the pressure vessel. 
     A valve at an outlet of the pressure vessel may be opened to provide a predetermined flow area in order to control the flow rate of pressurised fluid leaving the pressure vessel, e.g. to maintain the flow rate at less than or equal to the predetermined flow rate. 
     To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of an energy storage system according to arrangements of the present disclosure; 
         FIG. 2  is a flow chart illustrating a method of storing energy according to the present disclosure; 
         FIG. 3  is a schematic view of an energy supply system according to arrangements of the present disclosure; 
         FIG. 4  is a flow chart illustrating a method of supplying energy according to the present disclosure; 
         FIG. 5  is a schematic sectional view of an energy storage device for the systems shown in  FIGS. 1 and 3 ; 
         FIG. 6  is a schematic sectional view of another energy storage device for the system shown in  FIGS. 1 and 3 ; 
         FIGS. 7  is a schematic view of another energy storage device for the system shown in  FIGS. 1 and 3 ; and 
         FIG. 8  is a schematic view of another energy storage device for the system shown in  FIGS. 1 and 3 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , an energy storage system  2 , according to the present disclosure, comprises an energy storage device  100 . As described below, the energy storage device  100  is configured to store energy by virtue of a pressurised fluid being stored within the energy storage device  100 . 
     The energy storage system  2  may further comprise a source of working fluid  4  and a compressor  6 . The working fluid may be a substantially incompressible fluid, such as a hydraulic fluid. An inlet  6   a  of the compressor is fluidically coupled to the source of working fluid  4  and an outlet  6   b  of the compressor is fluidically couplable to the energy storage device  100 . The compressor  6  is configured to draw working fluid from the source of working fluid  4 , pressurise the working fluid and deliver the pressurised working fluid to the energy storage device  100  to be stored. 
     In the arrangement shown in  FIG. 1 , the compressor  6  is a rotary compressor, such as a radial flow, mixed flow or axial flow compressor, which operates to pressurise the working fluid by virtue of the rotation of a rotor of the compressor. In other arrangements, the compressor  6  may comprise a reciprocating compressor or any other form of compressor suitable for compressing fluid from the source of working fluid  4  for storage in the energy storage device  100 . 
     A motor  7  may be operatively coupled to the compressor  6  by a shaft  8  for rotatably driving the compressor  6 , in order to pressurise the working fluid. In the arrangement shown in  FIG. 1 , the motor  7  is an electric motor. However, in other arrangements, the motor  7  may be an internal combustion engine, or any other motor  7  for driving the compressor  6 . 
     The energy storage system  2  further comprises a source of energy  9  for the motor  7 . The source of energy  9  may comprise a connection to an electrical supply grid. Additionally or alternatively, the source of energy  9  may comprise a renewal energy source, such as a solar cell or wind turbine. In arrangements in which the motor  7  is an internal combustion engine, the source of energy  9  may comprise a fuel tank. 
     With reference to  FIG. 2 , the energy storage system  2  may be operated according to an energy storage method  200 . The method  200  comprises a first step  202  in which the energy storage device  100  is provided within the energy storage system  2 . In particular, the energy storage device  100  may be operatively coupled to the outlet  6   b  of the compressor  6 . 
     The method  200  further comprises a second step  204  in which pressurised fluid is introduced into the energy storage device  100 , thereby storing energy within the energy storage device, e.g. as hydraulic potential energy and elastic potential energy. Storage of energy by the energy storage device is described in more detail below. 
     In the second step  204 , the motor  7  may use energy from the source of energy  9  to drive the compressor  6 . The compressor  6  may draw working fluid from the source of working fluid  4 , pressurise the working fluid and supply the pressurised working fluid to the energy storage device  100 . 
     After pressurised working fluid has been stored within the energy storage device  100 , the energy storage device  100  may be removed from the energy storage system  2  and may be installed within an energy supply system  300 , as depicted in  FIG. 3 . As described below, the energy supply system  300  is operable to extract stored energy from the energy storage device  100  and supply the extracted energy to energy consuming devices, e.g. local to the energy supply system  300 , or an energy supply grid. 
     The energy supply system  300  may be remote from the location of the energy storage system  2 , e.g. from one or more of the compressor  6 , motor  7  and the source of energy  9  of the energy storage system. One or more components of the energy supply system  300  may be provided in a location in which it is desirable to supply the stored energy. Accordingly, the energy storage device  100  may be transported from the location of the energy storage system  2  to the location of the energy supply system  300 . 
     Alternatively, the energy supply system  300  may be co-located with the energy storage system  2 . Co-locating the energy storage and energy supply systems  2 ,  300  may beneficial enable the energy storage device  100  to be used to store and release energy from and to an energy supply grid, for example, in order to balance fluctuations in supply and demand of energy from the supply grid. 
     As depicted in  FIG. 3 , the energy supply system  300  comprises the energy storage device  100 . The energy supply system  300  may further comprise a hydraulic motor  302  and a reservoir  304  for storing working fluid that has been passed through the hydraulic motor  302 . As shown, an inlet  302   a  of the hydraulic motor is couplable to the energy storage device  100  and an outlet  302   b  of the hydraulic motor is coupled to the reservoir. 
     The hydraulic motor  302  my comprise any machine capable of extracting energy from the pressurised working fluid as it is passed through the hydraulic motor  302  from the energy storage device  100  to the reservoir  304 , e.g. by virtue of a drop in pressure of the pressurised working fluid as it passes through the hydraulic motor  302 , and converting the energy to kinetic energy of an output linkage  308 , e.g. shaft, connected to the hydraulic motor. 
     For example, the hydraulic motor  302  may comprise a hydraulic gear motor or a hydraulic vane motor configured to extract energy from the pressurised working fluid to rotatably drive an output shaft of the hydraulic motor  302 . In other arrangements, the hydraulic motor  302  may be configured to drive an oscillatory movement of the motor output linkage, or drive a linear movement, e.g. reciprocating movement, of the output linkage  308 . 
     The energy supply system  300  may further comprise a generator  306  operatively coupled to the hydraulic motor  302 , e.g. by the output linkage  308 . The generator  306  may be electrically connected to an electrical load  310  to be powered by the energy supply system  300 . 
     In other arrangements, the energy supply system  300  may be for supplying mechanical power directly to a mechanical load, such as a pump, a drill, manufacturing machinery or any other domestic or industrial machinery. The mechanical load may be configured to be driven by a shaft or linkage performing a rotatory, oscillatory or reciprocating movement. In such arrangements, the generator  306  may be omitted and the mechanical load may be driven directly by the hydraulic motor  302 . 
     The energy supply system  300  may comprise a valve  312 , e.g. arranged between the energy storage device  100  and the inlet of the hydraulic motor  302 , configured to control the rate at which the pressurised working fluid is supplied form the energy storage device  100  to the hydraulic motor  302 . The valve  312  may thereby control the mechanical power output by the hydraulic motor, and optionally, the electrical power produced by the generator  306  to be supplied to the electrical load  310 . 
     The energy supply system  300  may further comprise a controller  320  configured to control the operation of the valve  312 , such that a desired power is supplied to the electrical load  310  or the mechanical load. The controller  320  may be configured to determine a power demand of the electrical load  310  or the mechanical load and may control the operation of the valve  312  according to the power demand. For example, the controller  320  may be configured to determine a voltage across the electrical load  310  and may control the operation of the valve to maintain the voltage within predetermined upper and/or lower voltage limits. Alternatively, the controller  320  may be configured to determine a speed, e.g. rotational speed, of a linkage supplying mechanical power to the mechanical load and may control the operation of the valve  312  to maintain the speed within predetermined upper and/or lower speed limits. 
     With reference to  FIG. 4 , the energy supply system  300  may be operated according to an energy supply method  400 . The method  400  may comprise a first step  402 , in which the energy storage device  100  is transported, e.g. from the location of the energy storage system  2 , to the location of the energy supply system  300 . The method  400  comprises a second step  404 , in which the energy storage device  100  is provided, e.g. within the energy supply system  300 . For example, the energy storage device  100  may be operatively coupled to the inlet  302   a  of the hydraulic motor  302 . The energy storage device  100  may be operatively coupled to the hydraulic motor  302  via the valve  312 . 
     The method  400  further comprises a third step  406  in which the pressurised working fluid is released from the energy storage device  100  through the hydraulic motor  302 , e.g. by opening the valve  312 . As described above, releasing the pressurised working fluid through the hydraulic motor  302  generates mechanical power to be supplied to a mechanical load or electrical energy via the generator  306  for the electrical load  310  of the energy supply system  300 . 
     In the arrangements described above, the energy storage system  2  and the energy supply system  300  are distinct systems, with only the energy storage device  100  being common between them. However, in other arrangements, one or more of the components of the energy storage system  2  and energy supply system  300  may be common to both systems. For example, the compressor  6  of the energy storage system  2  may be a hydraulic compressor/motor capable of operating as either a hydraulic compressor, as described with reference to  FIGS. 1 and 2  above, or as a hydraulic motor, as described with reference to  FIGS. 3 and 4  above. Similarly, the motor  7  of the energy storage system  2  may be a motor/generator capable of operating as a motor for driving the compressor  6 , or as a generator, as described with reference to  FIGS. 3 and 4 . 
     When the energy storage device  100  is transported from a location in which the energy is stored to another location in which the energy is to be supplied, the components that are common to both the energy storage system and the energy supply system may be transported together with the energy storage device  100 . 
     In some arrangements, a single system may be configured to perform the functions of both the energy storage system  2  and the energy supply system  300 . Such systems may comprise a combined hydraulic compressor/motor and a combined motor/generator. Alternatively, such systems may comprise both a hydraulic compressor and a hydraulic motor and/or both a motor and a generator. In such arrangements, the first step  402  of the energy supply method may be omitted. 
     Referring to  FIGS. 5, 6, 7 and 8 , different arrangements of energy storage devices  500 ,  600 ,  700 ,  800  according to the present disclosure will now be described. Any of the energy storage devices  500 ,  600 ,  700 ,  800  depicted in  FIGS. 5, 6, 7 and 8  may be used as the energy storage device  100  within the energy storage system  2  and energy supply system  300  described above. 
     Each of the energy storage devices  500 ,  600 ,  700 ,  800  comprises a pressure vessel  510 ,  610 ,  710 ,  800  having one or more walls  512 ,  612 ,  712 ,  812  for containing pressurised fluid stored within the pressure vessel and an inlet  514 ,  614 ,  714 ,  814  through which working fluid can enter and exit the pressure vessel  510 ,  610 ,  710 ,  810 . 
     The pressure vessel may be substantially prism shaped, e.g. substantially cylindrical, and may define a longitudinal axis. The inlet  514 ,  614 ,  714 ,  814  may be arranged at one longitudinal end of the pressure vessel  510 ,  610 ,  710 ,  810 . 
     In one or more arrangements, the pressure vessel  510 ,  610 ,  710 ,  810  has a volume of approximately 50 litres. A volume of 50 litres may provide a balance between energy storage capacity and portability of the energy storage device. In other arrangements, the pressure vessel  510 ,  610 ,  710 ,  810  may have any other desirable volume. Furthermore, a plurality, e.g. any number, of energy storage devices  500 ,  600 ,  700 ,  800  having pressure vessels of any desirable size may be operatively, e.g. fluidically, connected together in series and/or in parallel with one another in order to provide a desired energy storage capacity. 
     The pressure vessel  510 ,  610 ,  710 ,  810  further comprises one or more resilient elements  520 ,  620 ,  720 ,  820  arranged within and/or about the pressure vessel  510 ,  610 ,  710 ,  810 . The resilient elements  520 ,  620 ,  720 ,  820  comprise a plurality of filaments of a resilient material that is twisted and/or braided, and optionally coiled, wound and/or weaved to form the resilient elements  520 ,  620 ,  720 ,  820 . 
     The energy storage devices  500 ,  600 ,  700 ,  800  are configured such that storing pressurised fluid within the pressure vessel  510 ,  610 ,  710 ,  810  acts to tension or compress the associated resilient elements  520 ,  620 ,  720 ,  820 . 
     In the arrangement shown in  FIG. 5 , the resilient elements comprise spheres  520  of resilient material that are arranged within the pressure vessel  510 . Any desirable number of the resilient spheres  520  may be arranged inside of the pressure vessel  510 . For example, a number of resilient sphere  520  may be arranged within the pressure vessel such that the resilient spheres take up approximately 10% of the internal volume of the pressure vessel. 
     The resilient spheres  520  and the walls  512  of the pressure vessel  510  are configured such that, when pressurised working fluid is stored within the pressure vessel  510 , the spheres  520  are compressed. For example, a material and wall thickness of the walls  512  and a material and construction of the resilient spheres  520  may be selected when producing the energy storage device  500 , such that working fluid stored within the pressure vessel at an outlet pressure of the compressor  6 , causes the spheres  520  to be compressed. 
     Energy can thereby be stored by the energy storage device  500  as elastic potential energy relating to the compression of the resilient spheres  520 . The energy storage device  500  is thereby configured to store a greater amount of energy than a pressure vessel of the same size containing working fluid pressurised to the same pressure and not containing the resilient spheres  520 . 
     The resilient spheres  520  comprise a plurality of filaments of material that have been twisted or braided together to form ropes, which have been coiled, wound and/or woven in order to form the resilient spheres  120 . 
     Any desirable number of filaments may be twisted or braided together in order to form the ropes of resilient material. For example, 3 or more than 3 filaments of the material may be twisted or braided to form the ropes. Furthermore, any desirable number of ropes may be coiled, wound and/or woven together, in order to form the spheres. For example, one rope may be coiled to form the sphere. Alternatively, a first rope may be coiled initially and a further rope may be coiled around the first rope to create a sphere of a larger size. Any number of further ropes may be coiled around the sphere, e.g. successively, in order to form a sphere of a desirable size. In one arrangement, 1800 fibres of material are twisted or braded together to form each rope, and 15 ropes are woven together and coiled to form each of the resilient spheres  520 . 
     Forming the spheres from braided ropes of material increased a maximum load capacity of the spheres compared to spheres being formed of a monolithic body of the material. In use of the energy storage and energy supply systems  2 ,  300  described above, the spheres  520  within the energy storage device  500  are compressed and returned to a neutral compression in an alternating fashion as energy is stored and release from the energy storage device over a number of charge cycles of the energy storage device  500 . 
     Increasing the maxing load capacity of the spheres improves the reliability of the energy storage device when subjected to multiple charge cycles. In particular, forming the spheres from the braided ropes makes the sphere more resistant to fatigue failure, since load can be distributed between filaments of the ropes that are less fatigued. Furthermore, cracks or fractures in the material of the spheres arising from fatigue of the material forming the spheres are confined to the particular strand in which the crack or fracture forms. This prevents the fatigue crack from propagating through the sphere, beyond the particular strand, further increasing the life, e.g. maximum number of charge cycles, of the resilient spheres  520 . 
     The resilient spheres  520  may be formed from a material having a high tensile strength, e.g. high ultimate tensile strength. For example, the resilient spheres  520  may be formed from a material having an ultimate tensile strength of greater than or equal to 400 MPa. For example, the resilient spheres  520  may be formed from a high tensile strength steel, fibres of aramid or another high tensile strength polymer or metal material. In one arrangement, the resilient spheres are formed from strands of natural or synthetic spider silk. 
     With reference to  FIG. 6 , in some arrangements, the energy storage device  600  may further comprise a piston  630  arranged within the pressure vessel  610 . The piston  630  may be movably, e.g. slidably, mounted relative to the walls  612  of the pressure vessel. A seal may be formed between the piston  630  and the walls  612  of the pressure vessel, such that a volume, e.g. an operative volume, of the pressure vessel to one side of the piston is in communication with the inlet of the pressure vessel and a remaining volume of the pressure vessel to the other side of the piston is not in communication with the inlet  614  due to the presence of the piston. Movement of the piston  630  within the pressure vessel  610  away from and towards the inlet  614  of the pressure vessel thereby acts to increase and decrease the volume of the pressure vessel  610  in communication with the inlet  614 . 
     As shown in  FIG. 6 , in the energy storage device  600 , the resilient elements comprise elongate cables  620  coupling the piston  630  to walls  612  of the pressure vessel. In particular, first ends  620   a  of the cables  620  are coupled to the piston and second ends  620   b  are coupled to the walls  612  of the pressure vessel. The resilient cables  620  may be formed in a similar way to resilient spheres  520  described above. For example, the resilient cables  620  may be formed from 3 or more strands of resilient material that are twisted or braided together to form a rope. In some arrangements, a plurality of the ropes may be braided or woven to create the resilient cables  620 . The resilient cables  620  may be formed form the same materials as the resilient spheres  520  mentioned above. 
     As depicted, the resilient cables are arranged such that movement of the piston  630  acting to increase the volume of the pressure vessel  610  in communication with the inlet  614  acts to tension the resilient cables  620 . 
     The resilient cables  620  are configured such that when pressurised working fluid from the compressor  6  is introduced into the pressure vessel  610 , the piston  630  is displaced away from the inlet  614 , increasing the volume in which the pressurised working fluid can be stored and tensioning the resilient cables. Energy is thereby stored by the energy storage device  600  as elastic potential energy relating to the extension of the resilient cables  620 , enabling the energy storage device  600  to store a greater amount of energy than a pressure vessel of the same size storing pressurised fluid the same pressure and not comprising the resilient cables  620 . 
     In other arrangements, the resilient cables  620  may additionally or alternatively be arranged on an opposite side of the piston  630  from the inlet  614  such that when the piston  630  is displaced away from the inlet  614 , the resilient elements  620  are compressed. 
       FIG. 7  depicts an arrangement of the energy storage device  700  in which the resilient elements  720  are provided about the pressure vessel  710 . As depicted, the resilient elements are in the form of ropes, which are wrapped around an outer surface of the pressure vessel  710 . The ropes  720  may be formed in the same way as the ropes forming the spheres  520  and cables  620  described above, and may be formed from the same materials mentioned above. 
     The energy storage device  700  is configured such that, when pressurise working fluid from the compressor  6  is introduced into the pressure vessel  710 , the walls  712  of the pressure vessel expand outwards, tensioning the resilient ropes  720  wrapping the pressure vessel. 
     Arranging the resilient elements outside of the pressure vessel may advantageously enable an increased volume of working fluid to be stored within a pressure vessel having a particular volume. Further, the resilient elements arranged outside of the pressure vessel are not exposed to the working fluid and may be easier to access for inspection, maintenance and repair/replacement. 
     In the arrangement shown in  FIG. 7 , the resilient ropes  720  are wound around the pressure vessel  710  to form a spiral of the resilient ropes wrapping the pressure vessel. The resilient ropes  720  therefore extend in a direction with a principle component in a direction perpendicular to the longitudinal axis of the pressure vessel, e.g. in a circumferential direction of the pressure vessel. In other arrangements the resilient ropes may be woven to form a woven arrangement of ropes wrapping the pressure vessel  710 . Energy is thereby stored by the energy storage device  700  as elastic potential energy relating to the extension of the resilient ropes  720 . The energy storage device  700  is therefore capable of storing a greater amount of energy than a pressure vessel of the same size storing pressurised fluid the same pressure and not being wrapped by the resilient ropes  720 . 
     In the arrangement shown in  FIG. 8 , the resilient elements  820  are also in the form of resilient ropes arranged about the pressure vessel  810 . The resilient ropes  820  may be formed in the same way as the resilient ropes  720  and may be formed from the same materials. As depicted, the pressure vessel  810  may comprise first and second flanges  810   a.    810   b.  The flanges may extend outwardly relative to the longitudinal axis of the pressure vessel  810 . The flanges are spaced apart from one another, e.g. in the longitudinal direction of the pressure vessel. First ends  820   a  of the resilient ropes may be coupled to the first flange  810   a  and second ends  820   b  of the resilient ropes may be coupled to the second flange  810   b.    
     The energy storage device  800  is configured such that, when pressurise working fluid from the compressor  6  is introduced into the pressure vessel  810 , the pressure vessel expands, e.g. in the longitudinal direction, such that a distance between the first and second flanges  810   a.    810   b  increases, tensioning the resilient ropes  820 . 
     In  FIG. 8 , two resilient ropes  820  are shown. However in other arrangements, any number of resilient ropes  820  may be provided about the pressure vessel  810 , e.g. spaced circumferentially about the pressure vessel. 
     In the arrangements described above with reference to  FIGS. 5, 6, 7 and 8 , each of the energy storage devices  500 ,  600 ,  700 ,  800  comprises a single form of resilient element. However in other arrangements, the energy storage device may comprise any combination of the resilient elements described above. In other words, the energy storage device may comprise the resilient spheres  520 , the piston  630  and resilient cables  620  and/or the resilient ropes  720 ,  820 . 
     For each of the arrangements described above, due to the presence of the resilient elements  520 ,  620 ,  720 ,  820  within and/or about the pressure vessel  510 ,  610 ,  710 , a greater proportion of the working fluid stored within the pressure vessel of the energy storage device may be released from the energy storage device, e.g. during the energy supply method  400  described above, than from a pressure vessel not comprising the resilient elements. In some arrangements, such as the arrangement depicted in  FIG. 6 , substantially all of the working fluid may be released from the pressure vessel. For example, approximately 90% or more than 90% of the working fluid may be released. 
     Furthermore, due to the presence of the resilient elements  520 ,  620 ,  720  within and/or about the pressure vessel  510 ,  610 ,  710 , the pressure of the working fluid may vary less during release of the working fluid then if the resilient elements were not provided, e.g. due to the resilient elements acting to maintain the pressure of the pressurised fluid as the tension or compression of the resilient elements is relaxed as the fluid is released. Accordingly, a variation in the peak power output of the energy supply system  300  may be reduced, by virtue of the present of the resilient elements. 
     When the pressurised fluid is released from the energy storage device, e.g. during the energy supply method  400 , the pressurised fluid may be realised from the pressure vessel at or below a predetermined flow rate such that a pressure of the fluid within the pressure vessel remains substantially constant whilst the majority of the fluid is release from the pressure vessel. For example, the pressure of the working fluid within the pressure vessel may vary by less than approximately 10% or 20%. 
     The valve  312  of the energy supply system  300  may be opened to provide a predetermine flow area, in order to control the flow rate of pressurised fluid leaving the pressure vessel. 
     The arrangements of the present disclosure detailed above provide a number of advantages over existing energy storage devices and systems. In particular, extracting the energy from the energy storage device using the hydraulic motor can provide a larger peak power output than existing systems, such as battery systems, depending on the size of the valve and power ratings of the hydraulic motor. Further, the hydraulic system is less susceptible to overheating when operating at high powers than existing energy storage systems and does not experience self-discharge or loss of stored energy during transport. 
     Arrangements of the disclosure also provide improved energy storage density compeered to existing systems and improved scalability, as any number of energy storage devices or systems can be connected in series and/or in parallel with one another to provide a desired capacity. Furthermore, the present inventive facilitates the use of renewable energy by providing an improved system for storing and distributing energy generated from renewable sources. 
     In further arrangements, the energy storage system  2  and/or energy supply system  300  may be provided to store and/or supply power in other applications. For example, in some arrangements, a combined energy storage and supply system, according to the present disclosure, may be provided on a motor vehicle in order to supply power for driving the motor vehicle. Mechanical power may be supplied directly to the drive system of the motor vehicle or may be converted into electrical energy supplied to electric motors for driving the motor vehicle. The energy supply and storage system may be configured to store energy recovered during breaking of the motor vehicle, e.g. by pressurising fluid to be stored within the energy storage device  100  provided within the energy storage system on the vehicle. 
     It will be appreciated by those skilled in the art that although the invention has been described by way of example, with reference to one or more exemplary examples, it is not limited to the disclosed examples and that alternative examples could be constructed without departing from the scope of the invention as defined by the appended claims.