Abstract:
A system for energy storage, in particular for mechanical energy, including a system for quasi-isothermal compression of a gas via a hydraulic fluid. The mechanical energy stored is then released by quasi-isothermal expansion of the gas. The system is also configured to store electrical energy, in particular from intermittent sources such as photovoltaic or wind energy. The storage of excess electrical energy can also be considered for use during consumption peaks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a National Stage Application of PCT International Application No. PCT/FR2010/000348 (filed on May 6, 2010), under 35 U.S.C. §371, which claims priority to French Patent Application No. 0902207 (filed on May 7, 2009), which are each hereby incorporated by reference in their respective entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a system for energy storage, in particular for mechanical energy, including a system for quasi-isothermal compression of a gas by means of a hydraulic fluid. The mechanical energy stored is then released by quasi-isothermal expansion of the gas. 
         [0003]    This invention also relates to the storage of electrical energy, in particular from intermittent sources such as photovoltaic or wind energy. The storage of excess electrical energy can also be considered for use during consumption peaks. 
       BACKGROUND OF THE INVENTION 
       [0004]    There are a number of energy storage systems that can be used for applications at different power scales. 
         [0005]    At a small scale, battery and supercapacitor-type electrochemical systems can be used. These systems nevertheless have a certain number of disadvantages. Batteries represent environmental hazards and have a limited lifespan. Supercapacitors have insufficient energy density for most applications. 
         [0006]    At a large scale, the storage of water in a reservoir at elevation is a good option. The water can be released at a chosen time and generate electricity by means of turbines. The main limitation of this technique is the small number of sites that can be adapted without heavy duty and costly work. 
         [0007]    The storage of compressed air in an underground cavity (CAES=Compressed Air Energy Storage) is also an interesting option; it has been envisaged in patents, such as U.S. Pat. No. 4,885,912 to Gibbs &amp; Hill, Inc., U.S. Pat. No. 3,996,741 to George M. Herberg, and in patent applications WO 93 06367 to Arnold Grupping and EP 106 690 to Shell International Research. However, the number of available sites is very limited and economically profitable implementation requires coupling with a combined cycle. This leads to very large plants, with major investments. In addition, this solution involves the consumption of fossil fuels, and is inefficient. 
         [0008]    Finally, another alternative is hydro-pneumatic storage in which the compression of a gas is performed by pumping a liquid. However, this type of technology must be improved in order to increase efficiency and reduce costs. 
         [0009]    Such a system, which uses a liquid piston system as a gas compression device is already known, in particular from document WO 2008 139267 to Ecole Polytechnique Fédérale de Lausanne. A sprayer or a grill integrated in the top portion of the chamber ensures the gas-liquid contact during the compression and expansion phases of the gas so as to maintain quasi-isothermal conditions. In this system, the thermal energy released during the compression phase is discharged into the atmosphere by means of an exchanger. This same exchanger serves to provide calories during the gas expansion phase. 
         [0010]    This type of system, while satisfactory, nevertheless has some non-negligible disadvantages. Indeed, the efficacy of this type of storage remains limited in particular due to the energy loss constituted by the discharge of calories during the gas compression phase. In addition, the stored energy restitution phase is accompanied by a cooling of the liquid associated with the expansion of the gas. It is therefore necessary to expend a non-negligible amount of energy to ensure isothermal expansion of the gas. 
       SUMMARY OF THE INVENTION 
       [0011]    This invention proposes that the disadvantages mentioned above be overcome with a hydro-pneumatic storage system that makes it possible to obtain a high energy efficiency by using a system for storing the thermal energy produced during the gas compression phase, which energy is restored during the gas expansion phase. 
         [0012]    The invention relates to a system for storing energy, in particular mechanical energy, the system including: (a) at least one container containing a hydraulic fluid and a gas; (b) at least one storage chamber containing the hydraulic fluid; (c) compression-expansion means capable, in “compression” mode, of pumping the hydraulic fluid, and, in “expansion” mode, of expanding the hydraulic liquid. In accordance with the invention, (i) the hydraulic fluid and/or the gas contained in the at least one container is in thermal contact with a thermal energy storage medium contained in a chamber; (ii) the container is connected to the at least one storage chamber by lines enabling the hydraulic fluid to be transported from one to the other, across the compression-expansion means; and (iii) the compression-expansion means are configured to pump the hydraulic fluid from the storage chamber to the container, and also configured to expand the hydraulic liquid contained in the container toward the storage chamber, generating mechanical energy. 
         [0013]    The compression-expansion means can be a reversible compression-expansion device, such as a hydraulic pump with pistons also functioning as a piston motor. The compression-expansion means can include means for converting the mechanical energy generated into electrical energy. 
         [0014]    In this mechanical energy storage system, the energy storage is obtained by compression of the gas contained in the at least one container by the hydraulic liquid, which is pumped with the compression-expansion means. 
         [0015]    The at least one container can be constituted by any volume including a suitable surface for exchange with the hydraulic fluid. It can be constituted, for example, by a tube or plate heat exchanger in which it occupies the compartments in heat exchange with those that are occupied by the hydraulic fluid. It can also be constituted by a tube or a plurality of tubes arranged in the hydraulic fluid storage volume. It can in particular be constituted by a spiral tube. 
         [0016]    The gas is a condensable gas, and preferably a gas selected from the group consisting of hydrocarbons, CO 2 , fluorinated hydrocarbons or fluorinated alkanes. It can also be a non-condensable gas such as nitrogen or ambient air. The thermal storage medium can be a phase-change material. 
         [0017]    The at least one container can be located inside the chamber, or it can be located outside the chamber; in this latter case, it advantageously includes a fluid loop that ensures thermal contact between the thermal storage medium of the chamber and the hydraulic fluid contained in the container. 
         [0018]    In a particular embodiment, the system in accordance with the invention includes a first group of containers and a second group of containers, in which the gas is ambient air, and, during the mechanical energy storage phase, the first and second groups of containers alternately function in air compression or air suction. 
         [0019]    In this embodiment, the container can include a contactor for improving the gas-liquid contact, and, in this system: (i.) in the mechanical energy storage phase, the hydraulic fluid stored in the storage chamber is routed by a line to the compression-expansion means then by a line to the container in order to compress the gas; and (ii) in the mechanical energy restitution phase, the gas is expanded by releasing the fluid by a line to the compression-expansion means, then by the line to the storage chamber. 
         [0020]    In the energy storage phase, the contactor enables the gas to be kept quasi-isothermal and the calories to be transferred to the hydraulic fluid, in which a fluid loop enables the calories from the fluid to be transported to the thermal storage medium; in the energy restitution phase, the fluid loop enables the calories stored in the thermal storage medium to be restored to the hydraulic fluid. 
         [0021]    In another particular embodiment, the system in accordance with the invention also includes a device making it possible to provide the thermal storage medium with external thermal energy, such as a solar collector or a heat exchanger running on combustion gases or other external heat sources. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a diagram showing the basic principle of the mechanical energy storage system. 
           [0023]      FIG. 2  is a diagram of a first alternative of the system of  FIG. 1 . 
           [0024]      FIG. 3  is a diagram of a second alternative of the system of  FIG. 1 . 
           [0025]      FIG. 4  is a diagram showing an alternative of the system of  FIG. 2 . 
           [0026]      FIG. 5  is a diagram showing another embodiment of the storage chamber. 
           [0027]      FIG. 6  is a diagram showing another alternative of the system of  FIG. 1 . 
           [0028]      FIG. 7  is a detail view showing a possible implementation of the diagram of  FIG. 1 . 
           [0029]      FIG. 8  diagrammatically shows a plate exchanger, capable of being used in the context of this invention. 
           [0030]      FIG. 9  is a diagram showing another alternative of the system of  FIG. 1 , in which the storage chamber is located at a lower level than the containers. 
           [0031]      FIG. 10  is a diagram showing another embodiment of the invention, in which the thermal storage medium is heated by a solar collector. 
       
    
    
       [0032]    List of reference numbers used in the figures is provided as follows. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 1, 2, 3 
                 Gas 
               
               
                   
                 4, 5, 6 
                 Hydraulic fluid 
               
               
                   
                 7, 8 
                 Hydraulic fluid transfer 
               
               
                   
                   
                 line 
               
               
                   
                  9 
                 Hydraulic fluid 
               
               
                   
                 10 
                 Thermal storage medium 
               
               
                   
                 11 
                 Chamber 
               
               
                   
                 12 
                 Thermal insulator 
               
               
                   
                 13 
                 Storage chamber 
               
               
                   
                 14, 15, 16 
                 Containers 
               
               
                   
                 17, KT1, 
                 Reversible compression- 
               
               
                   
                 KT2 
                 expansion device 
               
               
                   
                 18, 19 
                 Hydraulic fluid transfer 
               
               
                   
                   
                 lines 
               
               
                   
                 20, 30 
                 Compressed gas 
               
               
                   
                 21, 31, 47 
                 Hydraulic fluid 
               
               
                   
                 26, 31, 32, 
                 Hydraulic fluid transfer 
               
               
                   
                 40, 42 
                 line 
               
               
                   
                 35, 36 
                 Storage chamber 
               
               
                   
                 43 
                 Container 
               
               
                   
                 44 
                 Internal lining element 
               
               
                   
                 45 
                 Fluid loop 
               
               
                   
                 48 
                 Gas 
               
               
                   
                 49 
                 Recirculation pump 
               
               
                   
                 51 
                 Balloon 
               
               
                   
                 52 
                 Solar collector 
               
               
                   
                 53 
                 Thermal exchange coil 
               
               
                   
                 54 
                 Compressed air 
               
               
                   
                   
                 storage coil 
               
               
                   
                 60 
                 Plate exchanger 
               
               
                   
                 61, 62 
                 Channel 
               
               
                   
                 63 
                 Flat plate 
               
               
                   
                 64, 65 
                 Corrugated plate 
               
               
                   
                 B1, B1 
                 Containers 
               
               
                   
                 V11, V12, 
                 Valves 
               
               
                   
                 V21, V22 
               
               
                   
                 V41, V42 
                 Valves 
               
               
                   
                 V312 
                 Three-way valve 
               
               
                   
                 V51, V52, 
                 Valves 
               
               
                   
                 V53, V54 
               
               
                   
                 P1 
                 Pump 
               
               
                   
                 T1 
                 Expansion valve 
               
               
                   
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0033]    As illustrated in  FIG. 1 , in accordance with the invention, the mechanical energy is stored in a gas  1 ,  2 ,  3 , which is compressed by means of a hydraulic fluid  4 ,  5 ,  6 . The compressed gas is contained in at least one container  14 ,  15 ,  16 , and preferably in a plurality of containers  14 ,  15 ,  16  connected fluidically to one another by a line  40 . The at least one container  14 ,  15 ,  16  is placed in a chamber  11 , which contains a thermal storage medium  10 , configured to absorb and restore the heat released by the compression of the gas  1 ,  2 ,  3  by keeping it sufficiently isothermal. The chamber  11  is preferably surrounded by a thermal insulator  12 . 
         [0034]    A hydraulic fluid  9  is stored in a storage chamber  13 , and is routed by the line  8  to compression-expansion means preferably constituted by a reversible P-T device  17 . 
         [0035]    The P-T device  17  may be a reversible compression-expansion device  17  configured to either pump the hydraulic fluid  9  by receiving an amount of mechanical energy W, which leads to the compression of the gas  1 ,  2 ,  3 , or the expansion of the fluid  4 ,  5 ,  6  routed by the line  7  by producing an amount of mechanical energy W′. Advantageously, P-T device  17  can have means for converting this mechanical energy into electrical energy. Such a device has a very high efficiency, generally greater than 90%. For example, P-T device  17  can be a hydraulic pump with pistons also functioning as a piston motor. Alternatively, P-T device  17  can be a rotary machine of the deformable rhombus type, known, for example, from U.S. Pat. No. 3,295,505 to Jordan. 
         [0036]    In an alternative embodiment illustrated in  FIG. 2 , the compression-expansion means can be constituted by a circuit that includes, in parallel, a device P 1  configured to pump the hydraulic fluid  9  by receiving an amount of mechanical energy W, and a device T 1  enabling an amount of mechanical energy W to be produced by expanding the fluid  4 ,  5 ,  6  routed by the line  7 . A first pair of valves V 51 , V 52  and a second pair of valves V 53 , V 54  enable the “compression” mode or the “expansion” mode to be selected. 
         [0037]    In all embodiments and alternatives of the invention described herein, these two compression-expansion means can be used indifferently; for the sake of simplicity, the invention will be hereinafter described by calling the compression-expansion means a reversible compression-expansion device  17 . 
         [0038]    A typical embodiment of the system in accordance with the invention is described herein in a simple manner: to store energy, the compression-expansion device  17 , or, as indicated hereinabove, another compression-expansion means, pumps hydraulic fluid  9  through the line  7  into the at least one container  14 ,  15 ,  16 . The level of hydraulic fluid  4 ,  5 ,  6  in each respective one of the containers  14 ,  15 ,  16  rises, and the surface of the fluid acts as a piston and compresses the gas  1 ,  2 ,  3  respectively contained in the containers  14 ,  15 ,  16 . This compression generates heat, which is then transferred to the thermal storage medium  10 . This heat can be restored at the time of the gas expansion; the increase in temperature of the hydraulic liquid  4 ,  5 ,  6  in the “compression” mode is normally low, on the order of several degrees at most. If the gas  1 ,  2 ,  3 , however, is restored to “expansion” mode, it enables the pressure of the gas  1 ,  2 ,  3  to rise significantly. If the compressed gas  1 ,  2 ,  3  is allowed to expand across the line  7  and the compression-expansion device  17  acting in “expansion” mode, the level of hydraulic fluid  9  in the containers  14 ,  15 ,  16  decreases, and the hydraulic fluid  9  causes the energy conversion means of the expansion valve  17  to move in order to generate mechanical energy. This mechanical energy can be converted into electrical energy. The hydraulic fluid  9  is transferred to the line  8  in the storage chamber  13  in which the liquid level rises. 
         [0039]    If the gas  1 ,  2 ,  3  is air, and if the pressure of the air pressure  1 ,  2 ,  3  in the containers  14 ,  15 ,  16  becomes, during expansion of the hydraulic fluid  9 , lower than the atmospheric pressure, it is possible to cause outside air to enter the containers  14 ,  15 ,  16  by means of a valve. 
         [0040]    The hydraulic fluid  4 ,  5 ,  6 ,  9  is generally a liquid, and preferably constituted by an aqueous phase, water or glycolated water in order to avoid the risks of freezing. It can also be an organic phase, such as glycol, a mineral oil, an ester, a vegetable oil or phosphate esters. 
         [0041]    The gas  1 ,  2 ,  3  can be a permanent gas such as air or nitrogen. It can also be another gas such as CO 2  or an organic fluid. 
         [0042]    The thermal storage medium  10  can be constituted by a liquid (aqueous or organic) and/or by a solid phase optionally with a phase change. 
         [0043]    In an alternative embodiment of the method in accordance with the invention, the fluid  1 ,  2 ,  3  may take the form of a condensable fluid, and the compression and expansion are performed on a diphasic fluid. This will be explained hereinbelow. The advantage of this alternative is that it enables a stable pressure to be maintained in the containers  14 ,  15 ,  16 . 
         [0044]      FIG. 3  illustrates a main diagram of an alternative embodiment of the invention. The thermal storage medium  10  is constituted at least partially by the hydraulic fluid  9  used for the compression of the gas  1 ,  2 ,  3 . The volume of hydraulic fluid  9  is easily capable of keeping the air volume within substantially isothermal conditions. Indeed, if the air is, at the outset, at atmospheric pressure (the storage being performed, for example, between atmospheric pressure and 200 to 600 bars), the MCp coefficient of the air for a given volume is 1.2/4200 times lower than the MCp coefficient of the same water volume necessary to displace it. Warming the initial air volume to 100° C. corresponds to an amount of heat that raises the temperature of the water only by 1.2/42=0.03° C. If, for example, the containers  14 ,  15 ,  16  occupy half the volume of the chamber in which they are placed, the level of liquid in the chamber  11  varies between l i  and l h =1.5 l i . 
         [0045]    It is also possible to simultaneously have a solid storage phase  10 , for example, a phase change material that remains stationary, while the hydraulic fluid  9  circulates. The circulation of the hydraulic fluid  9  then makes it possible to ensure the thermal exchanges under good conditions. 
         [0046]    The above arrangement also applies if the gas  1 ,  2 ,  3  is condensable. In this case, if the hydraulic fluid  9  is constituted by an aqueous phase, the fluid  1 ,  2 ,  3  can be constituted by a hydrocarbon or a fluid such as ammonia or CO 2 . This condensable gas must not be miscible with the hydraulic fluid, so that the vapor pressure above the liquid phase resulting from the condensation of the gas  1 ,  2 ,  3  is always the saturation pressure. There is then a triphasic system: two liquid phases (hydraulic liquid  9 +liquid phase resulting from the condensation of the gas  1 ,  2 ,  3 ) and a gaseous phase constituted by the gas  1 ,  2 ,  3 . 
         [0047]    In such an embodiment, during compression and expansion, the pressure in the containers  14 ,  15 ,  16  remains constant, thereby facilitating the operating conditions of the reversible compression-expansion device  17  and makes it possible to avoid a decrease in efficiency of the compression-expansion device  17 . In addition, it is possible in this case to work with a moderate pressure, which reduces the investment costs. 
         [0048]      FIG. 4  illustrates an alternative embodiment of the method in accordance with the invention as illustrated in  FIG. 3 , which differs by the use of an open cycle instead of a closed cycle. The gas used for the energy storage is air taken from the ambient environment by the line  18 . This gas, once compressed, is stored in the storage chamber  35 . This storage chamber  35  can be constituted by a natural or artificial underground cavity. 
         [0049]    The storage system in accordance with the alternative embodiment illustrated in  FIG. 4  works with at least two groups of containers B 1 , B 2 . During the mechanical energy storage phase, the containers B 1  and B 2  alternately function in air compression or in air suction. 
         [0050]    In a first stage, while the first container B 1  suctions the air from the ambient environment by the line  18 , the second container B 2  compresses the air  20  by means of the fluid  21  pumped by the equipment KT 1 . The compressed air  20  is then directed toward the storage chamber  35  by the line  19 . 
         [0051]    In a second stage, while the second container B 2  suctions the air from the ambient environment by the line  26 , the first container B 1  compresses the air  30  by means of the fluid  31  pumped by the equipment KT 1 . The compressed air  30  is then directed toward the storage chamber  35  by the line  19 . 
         [0052]    The insulated chamber  11  makes it possible to store the thermal energy released during the compression of the gas in the thermal storage medium  10 . This energy storage makes it possible to keep the temperature of the first and second containers B 1 , B 2  almost constant during the mechanical energy storage phase. 
         [0053]    During the phase of restitution of the mechanical energy stored by means of the compressed air in the storage chamber  35 , the first and second containers B 1 , B 2  also function alternately. 
         [0054]    In a first stage, the compressed air contained in the storage chamber  35  is directed toward the second container B 2  by the line  19 . The second container B 2  expands the air  20  by means of the fluid  21  expanded by the equipment KT 1 . At the same time, the first container B 1  discharges the air into the ambient environment by the line  18 . 
         [0055]    In a second stage, the compressed air contained in the storage chamber  35  is directed toward the first container B 1  by the line  19 . The first container B 1  expands the air  30  by means of the hydraulic fluid  31  expanded by the equipment KT 1 . At the same time, the second container B 2  discharges the air into the ambient environment by the line  18 . 
         [0056]    The thermal energy stored in the compression phase in the thermal storage medium  10  enables the temperature of the first and second containers B 1 , B 2  to be maintained during the expansion phase. The thermal equilibrium ensuring the isothermal character of the compression and expansion can be achieved by any type of device intended to promote the heat exchange between the hydraulic fluids  21 ,  31  and the thermal storage unit  10  such as a coil (not shown in  FIG. 4 ). The circulation occurring at the time of the compression and expansion can help to standardize the temperatures. Additional circulation or mixing means can be introduced for this purpose. 
         [0057]    It is possible to ensure a constant pressure in the storage chamber  35 , by introducing, in the chamber containing the compressed gas, a variable volume of hydraulic fluid, which volume is regulated so as to keep the pressure constant. The hydraulic fluid can be introduced from a storage chamber  36  at atmospheric pressure. During the step of energy production from the storage, a fraction of the restored energy is used to pump the hydraulic fluid. At the time of the energy storage step, this energy is restored. The system works because the energy needed to compress a liquid from atmospheric pressure to a relatively high pressure P is much lower than the energy needed to compress a gas from atmospheric pressure to pressure P. 
         [0058]    The alternative embodiment illustrated in  FIG. 6  differs from the diagram illustrated in  FIG. 1  by the use of an indirect transfer of the thermal energy released during the compression of the gas toward the chamber  11 . In this alternative embodiment, the possibility of using an internal lining element  44  in the container  43  in order to improve the gas-liquid contact is also presented. For this, a recirculation loop  42  for the hydraulic fluid can also be activated by the use of a recirculation pump  49 . 
         [0059]    In this configuration, in the mechanical energy storage phase, the hydraulic fluid  9  stored in the storage chamber  13  is routed by the line  8  to the pump  17 , then by the line  42  to the container  43  in order to compress the gas  48 . The contactor  44  makes it possible to keep the gas quasi-isothermal and to transfer the calories to the hydraulic fluid  47 . A fluid loop  45  makes it possible to transport the calories from the fluid  47  to the thermal storage medium  10 . 
         [0060]    In the mechanical energy restitution phase, the gas  48  is expanded by releasing the fluid  47  by the line  41  toward the reversible compression-expansion device  17 , then by the line  8  toward the storage chamber  13 . During this phase, the recirculation of the hydraulic fluid  47  activated by the pump  46  makes it possible to keep the temperature of the gas  48  quasi-constant. The fluid loop  45  makes it possible to restore the calories stored in the thermal storage medium  10  to the hydraulic fluid  47 . 
         [0061]      FIG. 7  illustrates an example embodiment of the containers  14 ,  15 ,  16  of  FIGS. 1 and 2 , which can each be made in the form of a tube, preferably wound in a spiral. The use of a tube makes the production of pressurized containers easier and facilitates the heat exchanges with the heat exchange medium  10 . In another alternative embodiment illustrated in  FIG. 5 , the storage chamber  35  is made in the form of one or more straight tubes, stacked or not, connected to one another. In general, the use of tubes is advantageous because a tube is a hollow body capable of resisting a high internal pressure, which has a very simple form and which can easily be produced without welding by extrusion processes. A bundle of straight tubes is particularly suitable for large storage systems. As an example, a new bundle of straight steel tubes with a diameter of 122 cm and a length of 10 meters enables around 105 m 3  of air to be stored; there are nuances in steels enabling such tubes to be produced that resist an internal pressure of over 250 bar. 
         [0062]    As illustrated in  FIG. 8 , the container  14 ,  15 ,  16  can also be constituted by a plate exchanger  60 . A plate exchanger makes it possible to develop a large exchange surface between two thermal media in a restricted volume. Such an exchanger can typically be constituted by a stack consisting of a plurality of flat plates  63  and a plurality of corrugated plates  64 ,  65 , which thereby form two networks of channels  61 ,  62 . In each of the networks of channels, a fluid can circulate. One of the fluids is a hydraulic fluid  4 ,  5 ,  6  with a gas  1 ,  2 ,  3 , and the other fluid is the fluid that constitutes the thermal storage medium  10 . Advantageously, a configuration with a cross-flow or a counter-current is used. The cross-flow alternative is illustrated in  FIG. 8 , in which the channels formed by two adjacent corrugated plates are turned at 90°. 
         [0063]    The alternative embodiment illustrated in  FIG. 9  differs from the diagram illustrated in  FIG. 1  by a particular location of the chamber  11  with respect to the storage chamber  13 . It is indeed possible to combine the principle of hydro-pneumatic storage with that of gravity storage. In this alternative embodiment, the hydraulic fluid  4 ,  5 ,  6  contained in the containers  14 ,  15 ,  16  descends by gravity through the line  7  and to the reversible compression-expansion device  17  in the storage chamber  13 , which is located at a lower level/height with respect to the chamber  11 . 
         [0064]    In this alternative embodiment, during energy storage phases, the pump  17  must provide more mechanical energy W″ in order to raise the hydraulic fluid  9  and compress the gas  1 ,  2 ,  3 . In energy restitution phases, the expansion of the gas  1 ,  2 ,  3  is coupled to the difference in height of the hydraulic fluid  4 ,  5 ,  6  in order to provide a mechanical energy W′″. 
         [0065]      FIG. 10  diagrammatically illustrates another embodiment of the invention in which, before the gas  1 ,  2 ,  3  expansion phase, thermal energy outside the gas is provided. This thermal energy can come from different external sources. Advantageously, such a device includes a solar collector  52  such as a thermal energy source, which is connected to a thermal exchange coil  53  containing a heat transfer fluid and which is immersed in the storage medium  10  contained in a balloon  51 . In this embodiment, the compressed air is also stored inside a coil  54 , and the storage medium  10  is the hydraulic fluid itself. It is obviously possible to produce other embodiments, in which the storage medium  10  is heated by a solar collector  52  or by a heat source with a low temperature difference, with the understanding that it is one of the specificities of the quasi-isothermal system and process in accordance with the invention to be capable of utilizing the calories provided to it with a very low temperature difference. If the storage medium  10  is heated by one degree, this already enables a significant pressure to be created and which can be used in the gas  1 ,  2 ,  3 , which can be converted with a high mechanical energy efficiency by means of the compression-expansion means. 
         [0066]    This embodiment makes it possible, after compression of the gas  1 ,  2 ,  3 , to heat the thermal storage medium  10  by means of the solar collector  52 . This thermal energy is transferred to the gas  1 ,  2 ,  3  by the thermal storage medium  10  and causes an increase in its pressure, which can be converted, with high efficiency, into additional mechanical energy. 
         [0067]    This invention can be better understood with two non-limiting examples of mechanical energy storage described below. 
       Example 1 
       [0068]    Example 1, described in reference to  FIG. 1 , makes it possible to illustrate a first configuration of an implementation of the invention. In this example, the captive gas  1 ,  2 ,  3  is nitrogen contained in 3 1-m 3  cylinders. The total nitrogen mass is 344 kg. It is initially at a pressure of 100 bar and a temperature of 20° C. At time t=0, the pumping of water into the containers  14 ,  15 ,  16  is begun with a flow rate of 1.83 m 3 /h. As the containers  14 ,  15 ,  16  have a limited contact surface with the medium  10 , the gas  1 ,  2 ,  3  is heated substantially during this compression phase. At time t=60, the pressure of the gas is 360 bar and its temperature is 75° C. This step makes it possible to store 9 kWh of mechanical energy. At this time, the system continues to the decompression phase, drawing off an identical flow rate of 1.83 m 3 /h of water from the containers  14 ,  15 ,  16 . At time t=112 min, the gas returns to a pressure of 100 bar and a temperature of 1° C. This second phase makes it possible to restore 7.5 kWh of mechanical energy. The efficiency of the system is therefore 83%. 
       Example 2 
       [0069]    Example 2, described in reference to  FIGS. 1 and 7 , makes it possible to illustrate a second configuration of an implementation of the invention. In this example, the captive gas  1 ,  2 ,  3  is nitrogen contained in 3 wound tubes, as illustrated in  FIG. 7 . Each tube can contain a gas volume of 1 m 3 . The total nitrogen mass is 344 kg. It is initially at a pressure of 100 bar and a temperature of 20° C. At time t=0, the pumping of water into the containers ( 14 ,  15 ,  16 ) is begun with a flow rate of 1.96 m 3 /h. As the containers  14 ,  15 ,  16  have a large contact surface with the medium  10 , the gas  1 ,  2 ,  3  is heated very little during this compression phase. At time t=60 min, the pressure of the gas is 360 bar and its temperature is 40° C. This step makes it possible to store 9.4 kWh of mechanical energy. At this time, the system continues to the decompression phase, drawing off an identical flow rate of 2 m 3 /h of water from the containers  14 ,  15 ,  16 . At time t=120 min, the gas returns to a pressure of 100 bar and a temperature of 17° C. This second phase makes it possible to restore 9 kWh of mechanical energy. The efficiency of the system is therefore 96%. 
         [0070]    Although embodiments have been described herein, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.