Abstract:
The invention is an accumulator system in which multiple elastomeric accumulators are attached in series or parallel in order to generate total differential pressure in excess of that generated in a non-series system. Also disclosed is a “stacked” accumulator system. The system stores energy when the accumulators deform from their original shape in response to the flow of a pressurized fluid. The stored energy is available for use when the fluid is released from the accumulators and the accumulators return to their original shape.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/439,128, filed Feb. 3, 2011, entitled “Multiple Accumulator Systems and Methods of Use Thereof” which is hereby incorporated by reference in its entirety. 
         [0002]    Be it known that we, Eric J. Barth, a citizen of the United States, residing at 4601 Park Ave., Nashville, Tenn. 37209; Alexander V. Pedchenko, a citizen of the United States, residing at 513 Cedar Brook Lane, Nolensville, Tenn. 37135; and Wayne J. Book, a citizen of the United States, residing at 1373 Emory Rd. NE, Atlanta, Ga. 30306, have invented a new and useful “Multiple Accumulator Systems and Methods of Use Thereof.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0003]    This invention was made, in part, with Government support under grant number 0540834, awarded by the National Science Foundation&#39;s Engineering Research Center for Compact and Efficient Fluid Power. The United States Government has certain rights in this invention. 
     
    
     REFERENCE TO A MICROFICHE APPENDIX 
       [0004]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    One of the most pressing challenges in the design of an accumulator is the fabrication of a light weight and compact device that may be used in various industries. Hydraulic accumulators are energy storage devices commonly used to provide supplementary fluid power and absorb shock. One particularly interesting recent application of these devices is regenerative braking. Although a theoretically appealing concept, hydraulic regenerative braking is difficult to implement due to some major inherent limitations and non-ideal properties of conventional accumulators. 
         [0006]    Currently available accumulators include gas bladder accumulators and piston accumulators with a gas pre-charge, each of which use gas for energy storage and, therefore, have greater gravimetric energy density than their spring piston counterparts. However, such accumulators present problems to be solved. In these accumulators, a gas, separated by a bladder or a piston, occupies a certain volume of a container which is otherwise filled with a fluid, typically hydraulic fluid. As fluid is forced into this container, the gas inside the separated volume is compressed and energy is stored in this compressed gas. Such accumulators are subject to two serious drawbacks: 1) inefficiency due to heat losses, and 2) gas diffusion through the bladder into the hydraulic fluid. The drawback of inefficiency via heat loss is perhaps addressable through an isothermalizer foam inserted inside the gas bladder, but the gas diffusion issues gives rise to high maintenance costs associated with “bleeding” the gas out of the fluid often. 
         [0007]    What is needed is an accumulator that very efficiently stores energy within a very limited space. While doing so, such an accumulator must be light weight. Conventional accumulators fail to fully address these problems and fail to provide the needed features. 
       SUMMARY OF INVENTION 
       [0008]    The present invention discloses several accumulator systems for storing hydraulic energy. The disclosed system provides a compact and space saving design. That is, the low pressure reservoir of the hydraulic system is combined into the same space as the high pressure accumulator. Use of multiple bladders for storing strain energy in a fluid based system provides the benefits of requiring less space and weight as compared to traditional accumulator systems. The several accumulator systems disclosed provide methods of storing strain energy in a fluid based system so that gas at a pressure greater than that of the fluid does not defuse into the fluid. 
         [0009]    In certain embodiments, the accumulator system includes, a housing having a first end and a second end, a first bladder having a tubular configuration attached to the first end and the second end of the housing, a second bladder having a tubular configuration and a closed end, the second bladder being located within the first bladder and attached to the second end of the housing, a first conduit attached to the first end of the housing on the outside of the first bladder, a first switching valve attached to the first conduit, a pump/motor attached to the first switching valve, a second switching valve attached to the pump/motor, a second conduit attached to the second switching valve and attached to the second end of the housing at a position which is inside of the first bladder and outside of the second bladder, a third conduit attached to the first end of the housing at a position to the inside of the first bladder and attached to the first switching valve, and a fourth conduit attached to the second switching valve and attached to the second end of the housing at a position which is inside of the second bladder. In certain other embodiments, the system further includes a shaft attached to the pump/motor, and a motor attached to the shaft. In still other embodiments, the system further includes a fluid contained within the housing, first bladder, second bladder and each of the conduits. In certain embodiments, the first switching valve and the second switching valve are two position three way valves. 
         [0010]    In other embodiments, the accumulator includes, a first bladder having a tubular configuration with two open ends, a second bladder stacked within the first bladder, the second bladder having a closed end and an open end, a housing having a first end and a second end, the housing surrounding the first bladder and second bladder, wherein the first end and the second end of the housing are attached to the open ends of the first bladder and wherein the second end of the housing is attached to the open end of the second bladder. In certain embodiments, the accumulator further includes a first switching valve operationally attached to the first end of the housing, a pump/motor operationally attached to the first switching valve, and a second switching valve operationally attached to the pump/motor and operationally attached to the second end of the housing. In other embodiments, the second bladder is a length of the housing. In still other embodiments, the first end and the second end of the housing each define a plurality of openings. In yet other embodiments, the accumulator further includes a fluid between each of the bladders and surrounding the bladders within the housing. 
         [0011]    In still other embodiments, the accumulator system includes, a first accumulator, wherein the first accumulator has a low pressure side and a high pressure side, a second accumulator, wherein the second accumulator has a low pressure side and a high pressure side, wherein the low pressure side of the first accumulator is attached to the high pressure side of the second accumulator, and a pump/motor attached to the low pressure side of the second accumulator and the high pressure side of the first accumulator. In certain other embodiments, the accumulator system further includes a plurality of accumulators, wherein each of the plurality of accumulators has a low pressure side and a high pressure side, wherein each of the plurality of the accumulators is operationally attached in a series configuration at a position between the low pressure side of the second accumulator and the pump/motor. In still other embodiments, the accumulator system further includes a third accumulator having a low pressure side and a high pressure side, and a fourth accumulator, wherein the fourth accumulator has a low pressure side and a high pressure side, wherein the low pressure side of the third accumulator is attached to the high pressure side of the fourth accumulator and the pump/motor is attached to the low pressure side of the fourth accumulator and the high pressure side of the third accumulator. In other embodiments, the present invention is a method of manufacturing an accumulator system having a desired working pressure and working volume of fluid, including, providing a plurality of accumulators, each accumulator having a low pressure side and a high pressure side, providing a pump/motor, attaching the pump/motor to the plurality of accumulators so that the plurality of accumulators are attached in series and in parallel, wherein each accumulator acts on a differential pressure between its high pressure side and its low pressure side, incorporating additional accumulators in series in order to increase the working pressure of the accumulator system, and incorporating additional accumulators in parallel in order to increase the working volume of the accumulator system. 
         [0012]    Accordingly, one provision of the invention is to provide an accumulator system having a volumetrically and gravimetrically energy dense design. 
         [0013]    Still another provision of the invention is to provide an accumulator system using multiple bladders for storing strain energy in a fluid based system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIGS. 1A ,  1 B and  1 C are schematic diagrams of embodiments of the present invention. Shown in  FIG. 1A  is a schematic diagram of an embodiment showing two accumulators arranged in series. Also shown are fluid conduits and the pump/motor. Shown in  FIG. 1B  is a schematic diagram of an embodiment of a plurality of accumulators arranged in parallel. Shown in  FIG. 1C  is a schematic diagram of an embodiment of a combined arrangement of accumulators in series and parallel. 
           [0015]      FIG. 2  is a diagram showing a series of cross sections of a portion of an embodiment of a bladder as it progressively expands due to the intake of fluid, as disclosed herein. 
           [0016]      FIG. 3  is a diagram showing the performance of an embodiment of a bladder. Shown therein is the pressure charted against the change in volume. 
           [0017]      FIG. 4  shows three stress-strain curves for three samples of a material called NBR 6212, as further described herein. 
           [0018]      FIG. 5  shows a strain distribution comparison of embodiments of bladders of different thicknesses. 
           [0019]      FIG. 6  is a chart summarizing characteristics of an embodiment of a thin-walled bladder and an embodiment of a thick-walled bladder. 
           [0020]      FIG. 7  is a diagram showing the performance of two different embodiments of accumulators described herein. The top curve is the performance of an embodiment of an accumulator system having two accumulators arranged in series. Shown there is the normalized pressure charted against the normalize change in volume. The bottom curve is the performance of an embodiment of an accumulator system having only a single accumulator. 
           [0021]      FIG. 8  is a schematic diagram of an embodiment of a “stacked” accumulator, as disclosed herein. Shown is a partially expanded outer bladder which is continuing to fill with working fluid. Also shown is the housing, inner bladder, conduits, switching valves and pump/motor. 
           [0022]      FIG. 9  is a schematic diagram of the continued charging of the “stacked” accumulator shown in  FIG. 8 . Now, the outer bladder is fully expanded and the inner bladder is partially filled and continuing to fill with fluid. The arrows show the direction of flow of the working fluid during charging of the embodiment of the accumulator system. 
           [0023]      FIG. 10  is a schematic diagram showing the release of the stored energy from the embodiments of the “stacked” accumulator shown in  FIG. 8 . The inner bladder is releasing its fluid and as the fluid flows out, it is being used by the pump/motor to perform work through the shaft. The arrows show the direction of flow of the working fluid during release of energy from the embodiment of the accumulator system. 
           [0024]      FIG. 11  is a schematic diagram of the continued release of the stored energy from the embodiments of the “stacked” accumulator shown in  FIG. 8 . The inner bladder has forced its fluid out so that it has returned to its unstrained configuration. The outer bladder is partially expanded and is continuing to contract, thereby forcibly releasing the working fluid which is being used by the pump/motor to perform work through the shaft. The arrows show the direction of fluid flow. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    This invention concerns a combined hydraulic accumulator and low pressure reservoir. This invention disclosure is to detail the way in which the hydraulic pressure within the accumulator can be made to exceed the yield stress of the bladder material. This is referred to as a series configuration. A parallel configuration is also described wherein the total volume of displaceable fluid can be specified for a given designable hydraulic pressure. Finally, a stacked configuration is also disclosed. 
         [0026]    Disclosed herein are various arrangements of an accumulator device  10  which includes an elastic bladder  12  contained inside a rigid shroud  14 . Hydraulic fluid  24  occupies the space inside the bladder  12  and outside the bladder  12  within the shroud  14 . A feature of the device  10  is that the low pressure reservoir side  26  may either be open to atmospheric pressure, or sealed as part of a closed hydraulic system. If sealed, the accumulator/reservoir device  10  acts on the differential pressure between the high and low pressure sides as opposed to the gage pressure of the high pressure side. This “differential pressure” embodiment allows this device  10  to be combined either serially or in parallel with repeats of the device, or other hydraulic devices. For example, if the high pressure side is connected to a pump/motor  20  and the low pressure side is connected to the high pressure side of a second device  10  which in turn has its low pressure side connected back to the pump/motor  20 , the two devices  10  combined in series in this manner will have a total differential pressure approximately twice that of each device  10  (assuming that both are identical). In the language of bond graphs, this example is a common flow configuration. A parallel configuration, where the pump/motor  20  feeds several devices  10  in parallel, is a common effort configuration. The common flow configuration (series) allows an arbitrary multiplication (dictated by the number of devices) of the total differential pressure across all devices included. The common effort configuration (parallel) allows an arbitrary multiplication (again dictated by the number of devices) of the flow through the device  10 . 
         [0027]    Referring now to  FIG. 1A , there are shown elastic bladders  12  and  13 , each contained inside a rigid shroud  14 . Hydraulic fluid  24  occupies the space inside and outside of each bladder  12  and  13 . In the example circuit shown, a hydraulic pump/motor  20  is shown connecting the low pressure side  26  of device  10  on the right to the high pressure side (inside the bladder) of device  10  on the left. Conduits  16 ,  18 ,  22  are also shown. Also shown is the seal  28  of each device  10 , which may be used in certain embodiments. The seal  28  may provide a connection of the bladder  12  or  13  to the housing  14  and may have a threaded engagement, or the like, in order to provide a fluid tight junction. In other embodiments of the present invention, any bladder ( 12 ,  13 ,  114 , or  116 ) may be directly or indirectly attached to any housing ( 14  or  112 ), as is well known to those of ordinary skill in the art. As further described herein, fluid  24  exists through out the entire system, although not shown in the conduits, pump/motor, or switching valves in  FIGS. 1A-1C  and  8 - 11 . Regarding any of the fluid tight junctions described herein, connectors for providing such junctions are well known to those of ordinary skill in the art and the materials needed for the same are readily commercially available. Still referring to  FIG. 1A , this example configuration is shown for an application such as regenerative braking whereby the pump/motor  20  is physically connected to the power-train of an automobile. When the pump/motor  20  is absorbing mechanical energy, it acts as a pump and moves hydraulic fluid  24  from the low pressure side  26  of device  10  on the right and into the high pressure side of device  10  on the left, stretching and increasing the volume in the bladders  12  and  13 . Due to the relative incompressibility of the working fluid, the volume outside of bladder  12  is reduced by the same amount as the working fluid that is forced into bladder  13 . When the pump/motor  20  acts as a hydraulic motor to deliver the energy stored in the bladders  12  and  13  (stored in the form of strain energy of the elastic material of the bladder) to shaft work at the output of the motor, the elastic bladders  12  and  13  force high pressure hydraulic fluid  24  through the motor from the high pressure side of the device  10  on the left to the low pressure side  26  of the device  10  on the right. As this occurs, the volume in the bladders  12  and  13  decreases at the same rate at all points in time as the volume of hydraulic fluid  24  increases outside the bladders  12  and  13  in the reservoir space  26 . Not shown in the example configuration are other possible hydraulic machines either in series or in parallel with either the high pressure side or low pressure side of the pump/motor  20 . Hydraulic actuators and valves are possibly connected to either the high or low pressure side as required. A small low pressure reservoir (of the conventional type) may also be connected as needed to the low pressure side—this for example would be needed to account for the asymmetric volume of fluid of a single-rodded hydraulic piston.  FIG. 1B  shows a parallel configuration.  FIG. 1C  shows a device that combines both series and parallel. 
         [0028]    By way of background, with reference to  FIG. 2 , an axisymmetric representation of device  10 , there is shown four simulation frames showing the bladder&#39;s  12  reaction to the flow of fluid (fluid  24  is outside the bladder as shown in  FIGS. 1A-1C ) into its cavity during the charging process. The contours provide a qualitative check against the type of behavior observed during experimentation. The second frame from the left shows bubble formation and the frames to the right of it show progressive bubble propagation along the shroud. These simulation results increase model fidelity since they show the model to have the same bladder behavior as was physically observed. 
         [0029]    By forcing fluid  24  into the bladder  12 , it expands. The pressure inside the bladder  12  remains relatively constant after initial bubble formation. The extended portions of the bladder  12  tend to move from the initial aneurism along the axis of the shroud  14 . Since the pressure is essentially constant, the area under the pressure-volume curve can be greater than conventional compressed gas accumulators. Additionally, the fluid  24  is a liquid, and therefore does not experience loss of energy characteristic to a quickly compressed gas cooling down to its ambient temperature. The energy is stored by straining an elastomeric material. The sizing of the material is optimized and the nominal thickness of the material, among other things, dictates the pressures necessary to expand the bladder  12 . By way of background, toy balloons of a similar geometry exhibit a similar behavior. 
         [0030]    Referring to  FIG. 3 , the accumulator&#39;s cavity&#39;s pressure and fill volume correlation is in good agreement with experimentally recorded data obtained during low pressure prototype testing (in terms of pressure-volume curve shape). The pressure increase seen during the later stages of inflation is due to additional working fluid  24  being forced into the accumulator after the bubble has propagated all the way through the bladder  12 . 
         [0031]    Unfortunately, even the with the unexpected pressure increase achieved by the accumulator towards the end of the inflation process, the operating pressure level of the accumulator resulted in unsatisfactory performance. 
         [0032]    Referring now to  FIG. 4 , the desired performance was based off of the uniaxial stress-strain curve of a nitrile rubber formulation NBR 6212. NBR 6212 is manufactured by the Gates Rubber Corporation of Denver, Colo., and is a material used for illustrative purposes. From the plot, it can be seen that if the material is strained to about 475% (about 125% short of its tensile limit), it would respond with about 13.7 MPa (2000 psi). Its volumetric strain energy density (area under the curve) given those conditions could then be estimated using the following expression— 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       4.75 
                       · 
                       13.7 
                     
                      
                     
                         
                     
                      
                     MPa 
                   
                   ≈ 
                   
                     33 
                      
                     
                         
                     
                      
                     MJ 
                      
                     
                       / 
                     
                      
                     
                       m 
                       3 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Using this as the material&#39;s volumetric energy density, to store the targeted  200  kJ of energy it would require about 6.1 L of material. To compare, the model results show that 18.8 L of NBR 6212 stored about 74.2 kJ, resulting in an effective volumetric energy density of 4 MJ/m 3 . Using this value, about 50 L of material would be required to store 200 kJ. 
         [0033]    The poor performance of the modeled accumulator is due to a tradeoff between holding pressure and strain distribution which occurs in the single bladder elastomeric strain energy hydraulic accumulator design.  FIG. 5  illustrated this tradeoff. When a thin-walled accumulator is charged, its constituent material is strained uniformly throughout the thickness of the bladder  12 . Conversely, when a bladder  12  with a much larger wall thickness is filled, it exhibits a non-uniform strain distribution along its thickness. Since the accumulator is essentially an elastomeric pressure vessel, the equation for the hoop stress in a thick-walled cylindrical pressure vessel can be used to lend some insight: 
         [0000]    
       
         
           
             
               
                 
                   
                     σ 
                     hoop 
                   
                   = 
                   
                     
                       
                         
                           r 
                           i 
                           2 
                         
                          
                         p 
                       
                       
                         
                           r 
                           o 
                           2 
                         
                         · 
                         
                           r 
                           i 
                           2 
                         
                       
                     
                      
                     
                       ( 
                       
                         
                           
                             r 
                             o 
                             2 
                           
                           
                             R 
                             2 
                           
                         
                         + 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where r i  is the inside radius of the pressure vessel, r o  is the outside radius, p is the pressure inside, and R is the radius of interest within the pressure vessel wall. From Equation 2 it can be seen that as R varies from r i  to r o , so does the hoop stress, and therefore, strain. The larger the range of R (i.e., thickness of pressure vessel), the more the strain can vary. This is the reason for the inverse relationship between bladder thickness and consistency of strain in the radial direction that is shown in  FIG. 5 . Since only a portion of the material comprising the thick-walled bladder can be stressed to a certain maximum stress, the bladder&#39;s overall energy density suffers. 
         [0034]    However, simply reducing the wall thickness of the accumulator is not a feasible solution to the energy density problem. Although the reduction results in a more uniform strain distribution, it also severely compromises the extent to which the bladder  12  is capable of pressurizing its contained working fluid  24 .  FIG. 6  shows some key behavior response parameters for the thin and thick-walled bladders  12  shown in  FIG. 5 . The pressure exerted on the fluid  24  by the thicker of the two accumulators is almost an order of magnitude higher than that exerted by its thin-walled counterpart, while the latter actually experiences greater stress values within the material. 
         [0035]    One method for addressing the undesirable tradeoff between the distribution of strain and the pressure to which bladder  12  can be pressurized is to connect bladders  12  and  13  in series, as shown in  FIG. 1A . The setup functions on the same principle as the single bladder  12  accumulator. Working fluid  24  is forced into an elastomeric bladder  12 , which resists expansion, thereby pressurizing the fluid  24  being transferred into it. However, whereas in a single device  10  system, the single bladder  12  provided the resistance to the volumetric expansion, in the series device  10  system, two bladders  12  and  13  are providing the resistance. This is achieved by taking advantage of the fact that bladder expansion occurs based on differential pressure (i.e., the driving mechanism is based on the difference between the pressure inside of each bladder  12  or  13  and the pressure surrounding each bladder  12  or  13 ). 
         [0036]    As fluid  24  enters an embodiment having a single device  10 , it flows into the first bladder  12 . In order for bubble formation and appreciable energy storage to occur, the inside of the bladder  12  needs to see some pressure P expan  greater than that which the outside of the bladder  12  is seeing. In an alternate embodiment having a series of devices  10 , as best seen in  FIG. 1A , the outside of the first bladder  12  in the series is surrounded by working fluid  24  which is in direct contact with the inside of a second bladder  13 . Since the working fluid  24  and the bladders  12  and  13  are incompressible, for the first bladder  12  to expand, it needs to force the working fluid  24  into the second bladder  13 , the outside of which is subjected to working fluid  24  at atmospheric pressure. The second bladder  13 , in turn, also needs to see a pressure difference of P expan  for bubble formation and subsequent energy storage to occur. Thus, the inside of the second bladder  13  in the series will need to reach a pressure of 
         [0000]        P   2nd     —     bubble   =P   expan   +P   atm   (3)
 
         [0000]    to initiate energy storage through bubble propagation. In turn, for the first bladder  12  in the configuration to do the same, its inside pressure will need to reach 
         [0000]        P   1st     —     bubble   =P   expan   +P   2nd     —     bubble =2· P   expan   +P   atm  
 
         [0000]    or almost double the pressure inside of the second bladder  12  because P expan  will be much larger than P atm . 
         [0037]    This multiplicative effect of joining elastomeric accumulators in series does not only affect bubble formation pressure, but multiplies the pressure of a single bladder  12  for all levels of volumetric expansion. Shown in  FIG. 7  is the PV behavior of the series device  10  system, which has two devices  10  connected in series (top curve), as compared to the single device  10  system, which has one device  10  (bottom curve), normalized with respect to change in system volume and holding pressure. Furthermore, provided the compressive strength of the constituent material is sufficient for the first bladder  12  to not fail due to high compressive load, more accumulator devices  10  can be joined in series to linearly increase the pressure presented to the pump. 
       ANOTHER EMBODIMENT OF THE INVENTION 
       [0038]    Although the series device  10  system shows that joining accumulator devices  10  in series allows the use of more uniformly strained bladders  12  and  13  without reducing the maximum pressure achievable by the system as a whole, it requires for the system&#39;s initial working fluid  24  volume to also be multiplied by approximately the number of accumulator devices  10  to be used. This amount of intermediate fluid  24  can be reduced by “stacking” bladders  114  and  116  together such that the space in between the bladders  114  and  116  (as shown in  FIG. 8 ) contains fluid  24 . The embodiment described below expounds this idea. That embodiment describes an extra step of repumping the intermediate fluid  24  through a series of valves. 
         [0039]    The embodiment shown in  FIGS. 8 and 9 , as further described below, achieves similar results with a different emphasis. That embodiment of the present invention also seeks to distribute strain energy within thin walled bladders  114  and  116  to maximize the strain energy density, but with several differences: 1) the embodiment fills bladders progressively instead of filling all bladders simultaneously as in the series configuration described above; and 2) the hydraulic differential pressure of the embodiment described below is maintained as the differential pressure across the bladder currently being filled. If all bladders  114  and  116  are identical, other than the outside bladder  114  being larger, the differential hydraulic pressure from the accumulator side to the reservoir side will be the same with each bladder progressively filled (and would be the same regardless of the number of bladders). With the series configuration embodiment described above, the differential hydraulic pressure from the accumulator side to the reservoir side will be approximately equal to the differential pressure across one of the bladders multiplied by the number of bladders. The primary advantage of the “stacked” embodiment, further described below, is an increase in the systems energy density. Another advantage of the series device  10  system embodiment described above is that the differential hydraulic pressure (from the low pressure side of the hydraulic system to the high pressure side of the system) can be made to exceed the bladder yield stress. 
         [0040]    Referring now to  FIG. 8 , the embodiment shown places a second bladder  116  inside the first bladder  114 . If desired, a third bladder (not shown) can be placed inside the second bladder  116  and so on up to some practical limit. The result is an increase in energy that will be stored in a given volume while still being able to optimize the nominal thickness of the bladder material for the operating parameters of the system  110 . Still referring to  FIG. 8 , there is shown a schematic diagram of the embodiment when the first bladder  114  is being expanded within the housing  112 . As noted above, working fluid  24  is present throughout the system  110 . Since arrows are used to show fluid  24  flow direction, fluid  24  is not shown in the conduits  118 ,  126 ,  128 ,  138 , switching valves  120 ,  124 , and pump/motor  122  in  FIGS. 8-11 . The housing  112  has a first end  140  and a second end  142 . The tubular first bladder  114  having a first end  144  and a second end  146 . This is essentially no different from the previously described design, except to be consistent with the following steps the fluid  24  is shown taken from the housing  112  which surrounds the first bladder  114 , directed by first switching valve  120 , which is a two-position, three-way valve, to the input of the pump/motor  122  which receives the mechanical energy to be stored from the attached energy harvesting device  136 , for example the wheels of a vehicle that is being decelerated. Then, the output of the pump/motor  122  at higher pressure equal to P 1  is directed to the first bladder  114  by a second switching valve, also a two-position three-way valve. Each switching valve is a standard two-position three-way valve, which is readily commercially available and well known to those of ordinary skill in the art. Such switching valves are operated by controllers (not shown) the commercial availability and operation of which are well known to those of ordinary skill in the art. Further, the various conduits used to transport the fluid  24  are conduits which are readily commercially available and well known to those of ordinary skill in the art. Examples of such conduits include stainless steel, or the like. Note that the pump/motor  122  is a variable displacement pump to accommodate the required step up in pressure for a range of torques available from the input shaft  130  to the pump/motor  122 , shown connected to a motor  134 . 
         [0041]      FIG. 9  shows a schematic diagram of operation after the first bladder  114  is fully extended. Next, energy is stored in the second bladder  116 . The second bladder  116  having a first end  148 . When operating at this phase the first switching valve  120  and second switching valve  124  are directing flow of fluid  24  coming from the first bladder  114  through the third conduit  128 , through the fourth conduit  138  and into the second bladder  116 . That is, the fluid  24  goes through the pump/motor  122 , so that the output of the pump/motor  122  is directed into the second bladder  116 . Since high pressure P 1  is on the outside of the second bladder  116 , the pressure necessary to expand the second bladder  116  is P 1 +P 2 . However the pressure boost that the pump/motor  122  must provide need not change, assuming the bladder is appropriately designed. This is because the pump/motor  122  input is at pressure P 1  already. Furthermore, the pressure that must be resisted by the second bladder  116  is also only the difference in the inner and outer pressure, which is P 1 +P 2 −P 1 =P 2 . 
         [0042]    Referring now to  FIGS. 10 and 11 , there is shown the reverse of fluid  24  flow as compared to  FIGS. 8 and 9 . Shown in  FIG. 10  is a schematic diagram at a point in time after the system  110  is fully charged and the direction of fluid  24  flow is reversed. The second bladder  116  forces fluid  24  through conduit  38  and the second switching valve  124  to the pump/motor  122 . The pump/motor  122  uses the fluid  24  flow to perform work through the shaft  130 . Referring now to  FIG. 11 , after the second bladder  116  returns to an unstrained configuration, fluid  24  within the first bladder  114  is forced through conduit  126  and the second switching valve  124  to the pump/motor  122  so that the fluid  24  flow is used to perform work through the shaft  130 . 
         [0043]    If a third bladder (not shown) is placed inside the second bladder  116 , the complexity grows but in a linear manner. In such an embodiment, the first switching valve  120  and the second switching valve  124  are four-way, three-position valves. Such switching valves are readily commercially available and well known to those of ordinary skill in the art. The pressure rating of some components must continue to rise. There will also be a point of diminishing returns as the inside of the various bladders becomes filled with other bladders. In the limiting case, two bladders will hold up to twice as much energy and three bladders will hold up to three times as much energy as a single bladder of the same dimension. 
         [0044]    The material of construction of elastomeric material that may be used for the bladders described herein is readily commercially available and well known to those of ordinary skill in the art. Also, methods and processes for shaping and molding such elastomeric material are well known and readily commercially available. Examples of such elastomeric materials include polyurethane, natural rubber, nitrile rubber or another engineered elastomer or material which is suitable as known to those of ordinary skill in the art. Regarding the dimensions and size of the elastomeric material, in certain embodiments, it may have the shape of a long slender tubular bladder having two open ends or one open end and one closed. 
         [0045]    In certain embodiments of the present invention, the material of construction of the rigid shroud  14  and  112 , also called the housing, is a rigid structural material capable of withstanding the pressures described herein, and providing fluid tight containment of the fluid  24 . Those of ordinary skill in the art are familiar with such materials, which are readily commercially available. Methods of fabricating a housing  14  and  112  of any such material are well known to those of ordinary skill in the art, and such fabrication services are readily commercially available. For example, in certain embodiments of the present invention, the housing  14  and  112  is constructed of steel, carbon fiber, polycarbonate, woven pressure vessel materials, fiberglass, aluminum, or the like. As known to those of ordinary skill in the art, the invention disclosed herein is scalable to accomplish the magnitude of desired energy storage. 
         [0046]    Regarding attachment of fluid conduits  16 ,  18 ,  22 ,  118 ,  126 ,  128  and  138 , they may be constructed of any material which is proper for the use and function described herein. Examples of such conduits are well known to those of ordinary skill in the art, and include hydraulic hose, hydraulic piping, or the like. In certain embodiments, connecting the fluid conduits to the pump/motor  20  or  122  includes standard fluid tight connections known to those of ordinary skill in the art, which are well known and readily commercially available. Connectors for providing such junctions are well known to those of ordinary skill in the art and the materials needed for the same are readily commercially available. 
         [0047]    This patent application expressly incorporates by reference all patents, references, and publications disclosed herein. 
         [0048]    Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all alterations and modifications that fall within the true spirit and scope of the invention.