Abstract:
A compressed-air energy-storage system is described. The system includes a first compressor arrangement for compressing an air stream; a thermal energy storage unit, where through compressed air from the first compressor arrangement exchanges heat against a heat accumulation means; an air storage device arranged and configured for receiving and accumulating compressed air from the thermal energy storage unit; at least one expander for receiving compressed air from the air storage device and producing useful power therefrom. A further compressor arrangement is located between the thermal energy storage unit and the air storage device.

Description:
BACKGROUND 
       [0001]    Embodiments of the present invention relate to compressed-air energy-storage systems, also shortly called CAES systems. The disclosure also refers to methods for energy storage and electric energy production using CAES systems. More specifically embodiments of the present invention concern so-called adiabatic CAES systems (ACAES) also named advanced adiabatic CAES systems (AACAES). 
         [0002]    CAES power plants or systems are commonly used as a means for optimizing the exploitation of energy. As known to those skilled in the art, the electric power required from the electric distribution grid varies with peaks of electric power requests during the day and reduced power request at nighttime. Large steam power plants or renewable power plants produce an amount of power, which cannot be varied at will. This causes an excess of power available on the electric distribution grid at nighttime and a power shortage during peak hours. Small power plants using gas turbines, especially aeroderivative gas turbines, have been implemented in order to cover the peak power requests. These plants can be turned on and shut down according to the variable power requirement during the 24 hours. This notwithstanding, further measures must be met, in order to store the energy in excess produced at night time and recover the stored energy to increase the electric power production during peak hours. One of the means used for that purpose is the CAES technology. These systems typically include a compression train having one or more compressors, which are driven by electric power from the grid during night time, i.e. when less power is required than available on the grid. Excess power available from the grid is thus transformed into pressure energy of the stored compressed air. 
         [0003]    The compressed air is then exploited during the day to cover peak power requests from the grid, expanding the compressed air to a suitable pressure and burning an air/fuel mixture in a combustion chamber to generate combustion gases, which are expanded in a turbine for power generation. 
         [0004]    In order to reduce the environmental impact of these plants, so-called adiabatic or advanced-adiabatic compressed-air energy-storage systems (ACAES or AACAES) have been developed. ACAES or AACAES systems do not make use of fossil fuel to convert the accumulated energy into electric power. Rather, they store heat generated by the process of air compression and recover the heat to increase the air temperature before expanding the compressed air through one or more expanders. 
         [0005]    In  FIG. 1  an AACAES or ACAES system according to the current art is schematically represented. The ACAES system is labeled  100  as a whole. The system includes a compressor train  101  which, in the exemplary embodiment shown in  FIG. 1 , has three serially arranged compressors  103 ,  105 ,  107 , having a common shaft line  109 . Air entering the first compressor  103  at the compressor inlet  1031  is sequentially compressed at increasing pressure values and finally delivered at the outlet  107 E of the last compressor  107 . Between at least two sequentially arranged compressors, in the example between compressor  103  and compressor  105 , an intercooler  111  is arranged. The intercooler is a heat exchanger wherein partially compressed air delivered from the upstream compressor is cooled before entering the next compressor, so that the volume of the air being processed is reduced by removing heat therefrom. Heat is removed by means of a heat exchange against ambient air, water or any other cooling medium. By removing heat from the partially compressed air the amount of mechanical power required to drive the compressor train  101  is reduced. 
         [0006]    Compressed air exiting the compressor train at  107 E flows through a thermal energy storage arrangement  113 , wherein heat is removed from the compressed air flow and stored in a suitable thermal energy storage medium, for example a solid heat storage medium, such as rock, or a liquid heat storage medium such as oil, compressed water or glycol. In some known embodiments heat is stored in a heat storage medium which is selected so as to undergo a phase transformation from solid into liquid thus accumulating thermal energy in the form of latent liquefaction heat. 
         [0007]    The cooled air is finally delivered through a safety cooler  115  and stored in an air storage device, for example a cavern  117 . In the schematic of  FIG. 1  exemplary pressure and temperature values of the air stream are indicated. These values are given by way of example only. Air exiting the first compressor  103  may have a pressure value of 7 bar and a temperature of 250° C. and is cooled down to 180° C. in the intercooler  111  before entering the second compressor  105 . The air pressure is boosted up to 28 bar by the second compressor  105  and achieves a temperature of 450° C. before being processed by the last compressor  107  or compressor train  101 , at the delivery side whereof the air may achieve a pressure of 65 bar and a temperature of 650° C. After cooling in the thermal energy storage unit air may have a temperature of 70° C. and substantially the same pressure as at the inlet side of the thermal energy storage unit, if pressure drop across the thermal energy storage unit is negligible. 
         [0008]    The compressor train  101  can be driven by a reversible electric machine  119  which is selectively connectable with the shaft line  109  through a first clutch  121 . The reversible electric machine  119  operates in the motor mode when excess power is available from an electric power distribution grid G. For example the reversible electric machine  119  can operate in the motor mode at night time so that electric power from the grid G is converted into thermal power accumulated in the thermal energy storage unit and in pressure energy stored in the form of compressed air in the compressed air storage device  117 . When no power is available from the electric power distribution grid G, the first clutch  121  can be disengaged and the reversible electric machine  119  can remain at still stand. 
         [0009]    If additional power is required from the electric power distribution grid G, the reversible electric machine  119  can be switched in the generator mode and connected, through a second clutch  123 , to an expander  125 . Compressed air from the air storage device  117  can then be delivered through the thermal energy storage unit to the expander  125 . The compressed air from the air storage device  117  is heated up to for example 650° C. in the thermal energy storage unit by exchanging heat with the heat storage medium of the thermal energy storage unit  113 . Compressed and heated air is expanded in the expander  125 , which converts at least part of the power available in the compressed and heated air flow into useful mechanical power, which drives the reversible electric machine  119 , thus producing electric power that is finally injected into the electric power distribution grid G. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The efficiency of a compressed-air energy-storage system is improved by splitting the compression phase in a first compression step and a second compression step. When the CAES system is operated in the air-compression mode, heat is removed from the partially compressed air after the first compression step and prior to the second compression step. Heat removed from the partially compressed air is accumulated in a thermal energy storage unit, wherefrom thermal energy is recovered when the CAES system is operated in the energy-production mode. 
         [0011]    Intercooling of the air stream between two sequentially arranged compressors can be omitted and replaced by a heat removal and storage phase, so that more thermal energy is available for subsequent re-use in the energy-production mode of the CAES system. 
         [0012]    Power required for driving the compressors is reduced with respect to the current art arrangements. 
         [0013]    In some embodiments, an intercooler can still be provided, e.g. between subsequently arranged compressors of a first compressor arrangement, in combination with a thermal energy storage unit arranged between the first compressor arrangement and a second compressor arrangement. 
         [0014]    According to some embodiments, therefore, the subject matter disclosed herein specifically concerns a compressed-air-energy-storage system comprising: a first compressor arrangement for compressing an air stream; a thermal energy storage unit, through which compressed air from the first compressor arrangement exchanges heat against a heat accumulation means; an air storage device arranged and configured for receiving and accumulating compressed air from the thermal energy storage unit; at least one expander for producing useful power from the compressed and heated air. The system additionally includes a further compressor arrangement located between the thermal energy storage unit and the air storage device. The heat accumulation means can be any device, substance, arrangement, or the like, which is capable of accumulating and storing thermal energy removed from the partly compressed air flow and deliver the accumulated thermal energy to a cold stream of air or the like. 
         [0015]    The expander can include an air expander, wherein compressed and heated air is expanded for producing mechanical power. An expander can also include a gas turbine, wherein combustion gases, generated by the combustion of compressed and heated air mixed with fuel. In general, unless differently indicated, in the context of the present disclosure and appended claims, the term expander shall be construed as a machine capable of converting at least part of heat and pressure energy of a compressible fluid flow into useful mechanical energy by expansion of the flow. 
         [0016]    In some embodiments, compressed and heated air delivered by the thermal energy storage unit during the energy producing operation mode of the CAES system can be entirely expanded in at least one or more expanders, which convert the pressure and thermal energy of the air flow into useful mechanical energy without the use of fuel. The useful mechanical energy can be in turn converted into electric energy by an electric generator or a reversible electric machine. In this case the CAES system is a so-called adiabatic, or advanced adiabatic CAES system (AACAES or ACAES system). 
         [0017]    In other embodiments, fuel can be added to the heated and compressed air stream, and the fuel and air mixture can be ignited to generate hot and pressurized combustion gases, which can be expanded in one or more turbines, for producing useful mechanical power. A combination of one or more air expanders and one or more gas turbines can also be envisaged. Hot and pressurized air can be partly expanded in the air expander(s) and the partly expanded, still hot air can then be mixed with a fuel and ignited to generate hot and pressurized combustion gases, which are expanded in one or more gas turbines. 
         [0018]    In both situations there may be benefits of overall system efficiency that can be achieved by the arrangement disclosed herein. 
         [0019]    According to some embodiments, the first compressor arrangement includes a plurality of sequentially arranged compressors, with no intercooler arranged therebetween. The second compressor arrangement can include one or more sequentially arranged compressors. A safety heat exchanger can be arranged between the last compressor and the compressed-air storage device. 
         [0020]    In some embodiment, a first electric machine can be provided for driving the first compressor arrangement and a second electric machine can be provided for driving the further compressor arrangement. A speed manipulation device can be provided for one of the electric machines or both the electric machines. 
         [0021]    In some embodiments, for example, a gearbox can be provided between the first electric machine and/or the second electric machine, and at least one of the compressors of the first compressor arrangement or of the further compressor arrangement. 
         [0022]    In some embodiments, a variable frequency driver can be provided for one or both the electric machines, so that the rotary speed thereof can be changed, e.g. depending upon the operating conditions, such as the suction and delivery pressure. 
         [0023]    In other embodiments a single electric machine can be used for driving into rotation all the compressors of the system. Suitable gearbox(es) can be provided for modifying the rotary speed of the compressors. 
         [0024]    At least one of the electric machines used for driving the compressor(s) can be a reversible electric machine, which can be selectively drivingly connected to the compressor(s) or the expander. In other embodiments, separate electric machines can be used to respectively drive the compressor(s) and convert the mechanical energy generated by the expander. 
         [0025]    According to a further aspect, the present disclosure also relates to a method for operating a compressed-air energy-storage system. The method includes the steps of: partly compressing a flow of air at a first pressure; removing heat from the partly compressed air and storing thermal energy; further compressing the partly compressed and cooled air at a second pressure; and storing the further compressed air in an air storage device. 
         [0026]    The method can further include the steps of: heating the compressed and cooled air through stored thermal energy; and expanding heated compressed air in at least one expander and producing useful energy therefrom. 
         [0027]    Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
         [0028]    As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of embodiments of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    A more complete appreciation of the disclosed embodiments of the present invention and many of the potential benefits thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
           [0030]      FIG. 1  schematically illustrates an AACAES or ACAES system according to the current art; 
           [0031]      FIG. 2  schematically illustrates an ACAES system according to the present disclosure; 
           [0032]      FIGS. 3, 4 and 5  illustrate schematically three exemplary embodiments of the compressor trains used in ACAES system according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit embodiments of the present invention. Instead, the scope of the invention is defined by the appended claims. 
         [0034]    Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0035]    In the following description embodiments of the subject matter will be described in connection with ACAES or AACAES systems. It shall be understood, however, that embodiments may also relate to a CAES system where compressed air is partly expanded and subsequently mixed with fuel to produce compressed combustion gases which are expanded in a gas turbine to produce further useful power. 
         [0036]    Referring to  FIG. 2 , an exemplary embodiment of an ACAES system according to the present disclosure is schematically shown and labeled  1  as a whole. The ACAES system  1  can include a compressor or a compressor train  3  which compresses ambient air using excess electric power available from an electric power distribution grid G. 
         [0037]    In some embodiments the compressor train  3  includes a plurality of sequentially arranged compressors  5 ,  7  and  9 . In other embodiments, not shown, a single compressor can be provided. In yet further embodiments, as will be discussed later on, more than one compressor train  3  can be provided in parallel, to process a larger air flow rate. 
         [0038]    The compressors  5 ,  7  and  9  can be connected by a single shaft line  11 . 
         [0039]    Ambient air is ingested by the first compressor  5 , processed and compressed up to a first pressure value and then delivered to the second compressor  7  without intercooling. The second compressor  7  processes the air stream up to a second pressure value and delivers the compressed air stream to the inlet of the third compressor  9  without intermediate intercooling. A different number of compressors can be provided in the train, for example two or more than three compressors or compressor stages. In any event, in embodiments, no intercooler is provided between sequentially arranged compressors of the compressor train  3 . 
         [0040]    Exemplary pressure and temperature values at the suction side and delivery side of each compressor  5 ,  7  and  9  are indicated in  FIG. 2 . Those values shall be understood as being merely exemplary and do not limit the scope of the present disclosure. 
         [0041]    Inlet air may have a temperature of 15° C. and a 1 bar pressure (absolute) and in some embodiments will exit first compressor  5  at 250° C., and at a pressure of e.g. approximately 7 bar. The air inlet conditions at the suction side of the second compressor  7  are substantially the same as at the delivery side of the first compressor  5 . Air exiting the second compressor  7  may have a pressure of around 26-27 bar, e.g. 26.8 bar and a temperature of around 500-550 ° C., for instance 534° C. In the last compressor  9  the air pressure is increased up to around 40-45 bar, e.g. about 42.5 bar and will have a temperature of around 600-680° C., e.g. about 650° C. 
         [0042]    The ACAES system  1  further includes a thermal energy storage unit  13  through which the compressed air stream delivered by the compressor train  3  flows. The thermal energy storage unit  13  can contain a heat storage medium, for example a solid or a liquid heat storage medium or a heat storage medium which undergoes a phase transformation, e.g. from solid to liquid, thus accumulating latent liquefaction heat. 
         [0043]    Air is delivered at an exit side of the thermal energy storage unit  13  at substantially the same pressure achieved at the delivery side of the last compressor  9  or at a slightly lower pressure, due to the pressure drop across the thermal energy storage unit  13 . Due to heat removal from the compressed air flow, the temperature of the air delivered from the thermal entry storage unit  13  is substantially lower than the temperature at the delivery side of compressor  9 . By way of example only, the air flow exiting the thermal energy storage unit  13  may have a temperature of around 70-90° C., e.g. about 80° C. and a pressure of around 40-45 bar, e.g. about 42 bar. 
         [0044]    Downstream of the thermal energy storage unit  13  a further compressor arrangement  15  is provided. In the schematic representation of  FIG. 2  the further compressor arrangement  15  is driven by a separate electric motor  16 . Air exiting the thermal energy storage unit  13  is ingested by the further compressor arrangement  15  and delivered to an air storage device, such as a cavern or the like, schematically shown at  17 . 
         [0045]    In some embodiments a safety cooler  19  can be arranged between the delivery side of the further compressor arrangement  15  and the air storage device  17 . 
         [0046]    Exemplary pressure and temperature values of the air stream delivered by the further compressor arrangement  15  are indicated in  FIG. 2 . In some embodiments a pressure of around 60-70 bar, e.g. about 65 bar can be achieved, while the air temperature can be increased to around 110-130° C., e.g. about 120° C. at the delivery side of the further compressor arrangement  15 . After cooling in the safety cooler  19  the air flow will have a temperature around 40-60° C., e.g. about 50° C. 
         [0047]    The AACAES system  1  of  FIG. 2  further includes at least one expander  21 , wherein compressed and heated air can be expanded to produce useful mechanical energy during daytime. The expander  21  can be put in fluid communication through a compressed air duct  31  with the air storage device  17 . 
         [0048]    In some embodiments a single reversible electric machine  23  can be selectively engaged with the shaft line  11  of the compressor train  3  and with a shaft  25  of the expander  21 . A first clutch  27  can be provided for connecting the reversible electric machine  23  to the shaft line  11 . A second clutch arrangement  29  can be provided for drivingly connecting the expander  21  to the reversible electric machine  23 . 
         [0049]    The AACAES system  1  described so far operates as follows. 
         [0050]    If excess electric power is available from the electric power distribution grid G, the reversible electric machine  23  is switched in the motor mode and is operated to drive the compressor train  3  by engaging the first clutch  27 . Electric power from the grid G is also used to drive the motor  16 . Ambient air is thus sequentially compressed by the first, second and third compressors  5 ,  7 ,  9  of the compressor train  3  and delivered through the thermal energy storage unit  13 , where heat is removed from the compressed air flow. Cooled air exiting the thermal energy storage unit  13  is further compressed in the further compressor arrangement  15 , cooled in the safety cooler  19  and stored at high pressure in the air storage device  17 . 
         [0051]    When the maximum pressure in the air storage device  17  has been achieved and/or if no power is available from the electric power distribution grid G, the reversible electric machine  23  and the electric motor  16  are stopped and the first clutch  27  can be disengaged. 
         [0052]    If supplemental power is required by the electric power distribution grid G, pressure energy is available in the air storage device  17  and heat energy is available in the thermal energy storage unit  13 , the reversible electric machine  23  can be switched in the generator mode and the clutch  29  can be engaged. Compressed air from the air storage device  17  flows through the thermal energy storage unit  13 , is heated exploiting the thermal energy accumulated in the thermal energy storage unit  13  and is finally delivered to the expander  21 . A compressed air delivery line  18  can be provided for delivering compressed air from the air storage device  17  to the thermal energy storage unit  13 . 
         [0053]    The expander  21  converts power from the compressed and heated air flow into mechanical power available on the shaft  25  which, through clutch  29 , drives the reversible electric machine  23 , which is operating in the generator mode, so that mechanical power is finally converted into electric power and injected in the electric power distribution grid G. 
         [0054]    In other embodiments, not shown, two different electric machines can be provided for driving the compressor train  3  and for converting mechanical power from the expander  21  into useful electric power. In that case, clutches  27  and  29  can be omitted. A first electric machine continuously operating in the motor mode can be stably connected to the shaft line  11  and a second electric machine, continuously operating in the generator mode, can be stably connected to the shaft  25  of the expander  21 . 
         [0055]      FIG. 3  schematically illustrates more details of a possible embodiment of the compressor arrangements of the ACAES system  1  of  FIG. 2 . The same reference numbers indicate the same components as illustrated in  FIG. 2 . 
         [0056]    In the embodiment illustrated in  FIG. 3 , a speed manipulating device, e.g. a gear box  35  is provided between the reversible electric machine  23  and the shaft line  11 . The gear box  35  provides the correct transmission ratio between the electric machine  23  and the shaft line  11 . 
         [0057]    In some embodiments, as shown by way of example in  FIG. 3 , the electric motor  16  which drives the further compressor arrangement  15  can be provided with a variable frequency driver  37 , which can modify the AC current frequency from the electric power distribution grid G so that the rotary speed of the electric motor  16  and thus rotary speed of the further compressor arrangement  15  can be adjusted within a rather wide range according to operating conditions of the system. 
         [0058]    In other embodiments, not shown, also the reversible electric machine  23  can be powered by a variable frequency driver. In some embodiments, the gear box or other speed manipulating device  35  can be omitted. 
         [0059]    In the exemplary embodiments of  FIGS. 2 and 3  the further compressor arrangement  15  is driven by a separate electric motor  16 , different from the reversible electric machine  23  which drives the compressor train  3 . This, however, is not mandatory and should not be considered as a limiting feature of the present disclosure. 
         [0060]    By way of example, in  FIG. 4  a further embodiment is shown, wherein a single electric machine is used to drive the entire set of compressors, namely compressors  5 ,  7  and  9  forming the compressor train  3  as well as the further compressor arrangement  15 . In the exemplary embodiment of  FIG. 4  a reversible electric machine  23  is provided for that purpose and the electric motor  16  can be omitted. A double clutch arrangement made up of the first clutch  27  and the second clutch  29  can be provided for selectively connecting the reversible electric machine  23  to the compressor arrangement  3 ,  15  and to the expander  21 . In other embodiments, as mentioned above, the electric machine  23  can be operated in the motor mode only and can be used to drive the compressors, while a separate electric generator is connected to the expander  21  for converting the mechanical power generated by the expander  21  into electric power. In that case the clutch  29  would be omitted. Also clutch  27  could be omitted and the motor  23  could be permanently connected to the shaft line of the compressors. 
         [0061]    Further referring to  FIG. 4 , in some embodiments a first gear box, or other speed manipulating device  35 , can be provided between the electric machine  23  and the compressor train  3 . In some embodiments a further gear box, or other speed manipulating device  36 , can be provided between the compressor train  3  and the further compressor arrangement  15 . In this way the compressor train  3  can rotate at a rotary speed different from the rotary speed of the further compressor arrangement  15 . 
         [0062]    In other embodiments, not shown, the motor  23  or the reversible electric machine  23  can be driven through a variable frequency driver such as the variable frequency driver  37  shown in  FIG. 3 . This would allow the rotary speed of the reversible electric machine or motor  23  to be modified according to needs within a rather wide range. One or both gear boxes  25  and  37  could be omitted. In some embodiments, the reversible electric machine  23  can be driven by a variable frequency driver  37  at a speed corresponding to the rotary speed of the compressor train  3 , while the gear box  36  is maintained so that the further compressor arrangement  15  can be driven at a rotary speed different from the rotary speed of the electric machine  23  and the compressor train  3 . 
         [0063]      FIG. 5  illustrates a further embodiment of the compressor arrangements which can be used in the ACAES system  1  of  FIG. 2 . 
         [0064]    In this embodiment three different compressor trains  3 A,  3 B and  3 C are provided. Each compressor train  3 A,  3 B and  3 C can be made up of a plurality of compressors, for example three compressors. In  FIG. 5  the compressor train  3 A includes compressors  5 A,  7 A and  9 A; compressor train  3 B includes compressors  5 B,  7 B and  9 B; compressor train  3 C includes compressors  5 C,  7 C and  9 C. The three compressor trains can be arranged in parallel, i.e. the delivery side of the most downstream compressors  9 A,  9 B,  9 C can be connected to one another to deliver a single stream of compressed air through the thermal energy storage unit  13 . 
         [0065]    Each compressor train  3 A,  3 B and  3 C can include an electric motor or a reversible electric machine  23 A,  23 B and  23 C, respectively. A clutch  27 A,  27 B and  27 C can be provided for connecting each electric motor or reversible electric machine  23 A- 23 C to the respective shaft line  11 A,  11 B,  11 C of the respective compressor train  3 A- 3 C. 
         [0066]    In some embodiments, respective gear boxes or other speed manipulating devices  35 A,  35 B,  35 C can be provided between each electric motor or reversible electric machine  23 A- 23 C and the respective shaft line  11 A- 11 C of each compressor train  3 A- 3 C. 
         [0067]    One, two or all the reversible electric machines  23 A- 23 C can be selectively connected through respective second clutches  29 A- 29 C to respective expanders (not shown) similar to expander  21 . 
         [0068]    In other embodiments, the electric machines  23 A- 23 C can be operated in the motor mode only, and one or more electric generators can be connected to one or more expanders for power conversion purposes. In some embodiments a single expander and a single generator can be provided for processing the hot compressed air and produce useful mechanical power therefrom. 
         [0069]    In further embodiments, not shown, the electric machines  23 A,  23 B and  23 C can be driven by a variable frequency driver, to adjust their rotary speed according to the operating conditions. 
         [0070]    A single further compressor arrangement  15 , which in turn can include a single compressor or a compressor train, can be combined with the three compressor trains  3 A- 3 C. In some embodiments, similarly to the embodiment of  FIG. 3 , a separate electric motor  16  can be provided for driving the further compressor arrangement  15 . In some embodiments the electric motor  16  can be driven by a variable frequency driver as already disclosed in connection with  FIG. 3 . 
         [0071]    The novel arrangements described above improve the total efficiency of the ACAES system over similar systems of the current art. More specifically, as will be shown by the following example, under the same flow rate conditions and achieving the same temperature of the thermal energy storage medium and the same air pressure in the air storage device, the total power consumption of a system according to the present disclosure is less than the power consumption of the system according to the current art. 
       EXAMPLE 
       [0072]    System efficiency of embodiments of the invention can be better appreciated by comparing the efficiency of an AACAES system according to  FIG. 1  (state of the art) with the efficiency of a system according to  FIG. 2  using the same turbomachinery. 
         [0073]    An ACAES system according to  FIG. 1 , using the following compressors: 
         [0074]    compressor  103 : Axial  6  stages compressor 
         [0075]    compressor  105 : MCL 805 
         [0076]    compressor  107 : BCL605 
         [0077]    with a flow rate of 101.5 kg/s will require a power of 75.87 MW at mid compression, i.e. a mid point between the starting of compression to end of compression, the point corresponding to the design point of the compressors used. It shall be noted that while the ACAES system is operating in the compression phase, the discharge pressures of each compressor varies from the starting value to a final value, corresponding to a condition when the maximum pressure in the cavern  17  has been achieved. 
         [0078]    In a system according to  FIG. 2 , using the following compressors: 
         [0079]    compressor  5 : Axial  6  stages compressor 
         [0080]    compressor  7 : MCL 805 
         [0081]    compressor  9 : BCL603 
         [0082]    compressor  15 : BCL351 
         [0083]    with a flow rate of 101.5 kg/s will require a power of 73.17 MW at mid compression. 
         [0084]    Thus, the system according to the present disclosure has a consumption which is 2.7 bar lower than the prior art system. Additionally, the air/air intercooler  111  is removed, thus reducing the cost of the system and the footprint thereof. 
         [0085]    While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. Different features, structures and instrumentalities of the various embodiments can be differently combined.