Abstract:
A system for storing and outputting thermal energy and a method for operating the system are provided. The system has a heat accumulator and a cold accumulator. The heat accumulator and the cold accumulator are discharged in two separate discharging circuits, wherein the thermal energy is converted into electrical energy, for example by a generator. The residual heat from the process in the circuit can be advantageously fed to the process in the circuit by a first heat exchanger, whereby the total efficiency is advantageously improved. Furthermore, the heat from the heat accumulator can be advantageously transferred into the first circuit by a waste-heat steam generator. The heat accumulator and the cold accumulator can be charged, for example, with excess energy from the electric network by a motor. Excess energy reserves of alternative energy resources, for example, can thus be stored.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2013/056549 filed Mar. 27, 2013, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP12164473 filed Apr. 17, 2012. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a plant for storing and releasing thermal energy with a heat accumulator and a cold accumulator, wherein the heat accumulator can release the stored energy to a first line in a discharging circuit for a working medium at a suitable release point. In the discharging circuit, the following units are interconnected in the specified sequence by means of this first line: a first thermal fluid energy machine (especially a pump) which is operated as a working machine, a release point (for example a heat exchanger) for heat from the heat accumulator, and a second thermal fluid energy machine (for example a steam turbine) which is operated as a power machine. The described arrangement of the units in the discharging circuit makes it possible for the energy which is stored in the heat accumulator to be released to the working medium and for the second thermal fluid energy machine which is operated as a power machine to be used for driving an electric generator, for example. For storing the thermal energy in the heat accumulator and in the cold accumulator, a charging circuit is conversely required and can be selectively put into effect by means of the first line or by means of another line. The invention also relates to a method which is implemented by the described plant. 
       BACKGROUND OF INVENTION 
       [0003]    The terms power machine and working machine are used within the scope of this application so that a working machine absorbs mechanical work in order to fulfill its purpose. A thermal fluid energy machine which is used as a working machine is therefore operated as a compressor or fluid compression machine. Compared with this, a power machine performs work, wherein a thermal fluid energy machine for performing the work converts the thermal energy which is made available in the working gas. 
         [0004]    In this case, the thermal fluid energy machine is therefore operated as a motor. 
         [0005]    The term “thermal fluid energy machine” constitutes a generic term for machines which can extract thermal energy from a working fluid—a working gas such as air or steam within the context of this application—or can feed thermal energy to this. Both heat energy and cold energy are to be understood by thermal energy. Thermal fluid energy machines (also referred to as fluid energy machines for short in the following text) can be designed as piston machines, for example. Hydrodynamic thermal fluid energy machines, the impellers of which allow a continuous flow of the working gas, can preferably also be used. Axially acting turbines or compressors are preferably used. 
         [0006]    The principle specified in the introduction is described according to WO 2009/044139 A2, for example. In this case, piston machines are used in order to implement the method which is described above. According to U.S. Pat. No. 5,436,508, moreover, it is known that by means of the plants for storing thermal energy which are specified in the introduction over-capacities can also be temporarily stored when wind energy is being utilized for producing electric current in order to retrieve this again when required. 
       SUMMARY OF INVENTION 
       [0007]    An object of the invention is to disclose a plant for storing and releasing thermal energy of the type specified in the introduction (for example conversion of mechanical energy into thermal energy with subsequent storage or conversion of the stored thermal energy into mechanical energy) and to disclose a method for its operation, by means of which a comparatively high level of efficiency with at the same time a justifiable cost of the constructional units being used is possible. 
         [0008]    This object is achieved according to aspects of the invention by means of the plant specified in the introduction by the cold accumulator being able to release the stored cold to a second line at a suitable release point, wherein the second line forms a closed circuit. In this circuit, the following units are interconnected in the specified sequence by means of the second line: downstream of said release point for the cold stored in the cold accumulator (that is to say the point at which the cold accumulator can release the stored cold to the second line), provision is made for a third thermal fluid energy machine (for example a pump) which is operated as a working machine, a heat source is then provided and then a thermal fluid energy machine (for example a steam turbine) which is operated as a power machine. Media which in comparison to the temperature level of the cold accumulator have a higher temperature level are suitable as the heat source. If the cold accumulator in the charged state has a temperature level which lies below the ambient conditions, the environment of the plant (for example flowing water) can be utilized as the heat source. It is especially advantageous, however, if the waste heat or residual heat of another process is utilized, wherein the temperature level of this process lies above the ambient temperature. This process can be a gas turbine circuit, for example. If the gas for this is delivered in liquid form and has first of all to be evaporated, this process can be used for charging the cold accumulator, for example. Other configurations are explained in more detail below. To be named among these is especially the thermal energy accumulator, as has already been explained in the introduction. 
         [0009]    In the first line and in the second line, provision is made in each case for a working medium which conducts a thermodynamic process in the circuit for energy storage or energy generation. In this case, this can be gaseous or liquid. The fluid energy machines have to be optimized to the medium in each case. If this medium is delivered in liquid form then the choice of a pump is especially advantageous. In the case of supply in the gaseous state, hydrodynamic fluid energy machines (turbocompressors) are preferably used. 
         [0010]    It is the fundamental idea of the invention that the heat accumulator and the cold accumulator which are provided in the plant can be used independently of each other in two discharging circuits. As a result of this, it is especially made possible to be able to utilize the waste heat of the discharging circuit which is operated with the heat accumulator in the discharging circuit which is operated with the cold accumulator. As a result of this, the yields of the energy stored in the heat accumulator and in the cold accumulator is advantageously increased, as a result of which the overall efficiency of the plant can be increased. 
         [0011]    According to one advantageous embodiment of the invention, it is provided that the heat source consists of a first heat exchanger which can extract heat from the first line and is arranged between the third fluid energy machine and the fourth fluid energy machine. This arrangement and principle of operation of the first heat exchanger make it possible, as already indicated, that the waste heat of the discharging circuit formed by the first line can be used in the discharging circuit formed by the second line. This temperature level is higher than that of the environment, as a result of which the yield of the release or discharge of the cold accumulator can be advantageously increased. 
         [0012]    Another embodiment of the invention provides that the release point for the heat accumulator is formed by a fifth heat exchanger which can feed heat to the first line and which is connected into a circuit formed by a fourth line. In this circuit, the following units are interconnected: the fifth heat exchanger, a tenth thermal fluid energy machine which is operated as a working machine, and the heat accumulator. With this is provided a configuration in which the heat accumulator is not incorporated directly into the discharging circuit of the first line but is connected to this via a heat exchanger (fifth heat exchanger). This heat exchanger is connected in a circuit to the heat accumulator  14  by means of the fourth line. The fluid energy machine in this case circulates the working medium in the fourth line so that the energy stored in the heat accumulator  14  can be introduced into the heat exchanger. The advantage of this constructional form is that different working media can be used in the first line and in the fourth line. The fifth heat exchanger is especially advantageously designed as a waste heat steam generator, for example. Such heat exchangers are frequently also referred to as waste heat boilers or as HRSGs (heat recovery steam generators). The waste heat steam generator is advantageously operated with water so that steam turbines which are current in the market can be used for generating mechanical energy in the circuit formed by the first line. The heat accumulator  14 , however, can be operated via the fourth line with air, for example, as working medium. This has the advantage that heat accumulators of larger volume can also be inexpensively produced, since possible leaks in the circuit do not constitute a hazard to the environment. 
         [0013]    The waste heat steam generator (that is to say the fifth heat exchanger) and the second thermal fluid energy machine can advantageously also have a plurality of pressure stages. These pressure stages are formed by corresponding pressure stages—which are interconnected by lines in each being provided both in the heat exchanger and in the fluid energy machine. As a result of this, the yield, and therefore the efficiency, of the discharging process can be advantageously further increased. 
         [0014]    According to a specific embodiment of the invention, it is provided that the release point for the cold stored in the cold accumulator consists of a third heat exchanger which can release heat to the second line and is incorporated into a cooling circuit formed by a third line. In this cooling circuit, the following units are interconnected: the third heat exchanger, a fifth thermal fluid energy machine which is operated as a working machine, and the cold accumulator. The fluid energy machine circulates the working medium in the third line. In this way, the cold energy stored in the cold accumulator is released via the third heat exchanger to the second line where work can be performed via the fourth fluid energy machine. This separation of the circuits via the second line and the third line also has the advantage that the circuit which is formed via the second line can be kept as small as possible. In this system, ammonia, for example, can be used as working medium and put into operation under the high technical safety requirements which are associated therewith. In the third line, air, for example, can be used as working medium. This is especially advantageous if the cold accumulator has a large volume on account of the capacity requirements. 
         [0015]    Yet another embodiment of the invention provides that provision is made in the second line between the third thermal fluid energy machine and the first heat exchanger for a fourth heat exchanger which enables a heat input from the environment of the plant into the second line. In this case, consideration has to be given to the fact that the cold accumulator has a temperature level which lies below the atmospheric ambient conditions. Therefore, heat from the environment can be fed to the working medium in a first step before the heat which is made available in the heat accumulator or in the residual heat of the discharging circuit in the heat accumulator is utilized in a second step. The environmental heat is therefore additionally made available to the process, as a result of which the efficiency of the plant can be improved. 
         [0016]    Another object specified in the introduction is also achieved by means of a method for storing and releasing thermal energy via a heat accumulator and a cold accumulator, wherein during the discharging cycle the heat accumulator releases the stored energy to a first line in a discharging circuit for a working medium. In the discharging circuit, the following units are arranged in the specified sequence via a first line and are subjected to throughflow of the working medium in this sequence: a first thermal fluid energy machine (especially a pump) which is operated as a working machine, a release point for heat from the heat accumulator, and a second thermal fluid energy machine (especially a steam turbine) which is operated as a power machine. 
         [0017]    The achieving of the object specifically lies in the fact that the cold accumulator releases the stored cold to a second line, wherein the second line forms a closed circuit in which the following units are operated in the specified sequence via the second line: a third thermal fluid energy machine (especially a pump), which is operated as a working machine, downstream of said release point for the cold stored in the cold accumulator, a heat source, and a fourth thermal fluid energy machine, especially a steam turbine, which is operated as a power machine. By means of the method according to the invention, with which a plant of the type described above can be operated, the already explained advantages of an efficiency increase, and therefore an increase of efficiency of the running processes, are achieved. 
         [0018]    Further details of the invention are described below with reference to the drawing. The same or corresponding elements of the drawing are provided with the same designations in each case and are repeatedly explained only insofar as differences arise between the individual figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In the drawing 
           [0020]      FIGS. 1 and 2  show an exemplary embodiment of the plant according to the invention in the operating states of a charging process ( FIG. 1 ) and discharging process ( FIG. 2 ) in each case as block schematic diagrams, and 
           [0021]      FIG. 3  shows another exemplary embodiment of the plant according to the invention in the operating states of a charging process and discharging process as a block schematic diagram. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]    Based on a plant according to  FIGS. 1 and 2 , the thermal charging and discharging processes of thermal accumulators  12 ,  14 ,  16  shall be explained in more detail. Shown first of all in  FIG. 1  is a two-stage charging process which functions according to the principle of a heat pump. Shown here is an open charging circuit which, however, as indicated by dash-dot lines, could be closed using an optionally provided heat exchanger  17   b . The states in the working gas, which in the case of the exemplary embodiment of  FIG. 1  consists of air, are represented in each case in circles on lines  30 ,  31 . At the top on the left, the pressure in bars is indicated. At the top on the right, the enthalpy in KJ/Kg is indicated. At the bottom on the left, the temperature in ° C. is indicated, and at the bottom on the right the mass flow in Kg/s is indicated. The flow direction of the gas is indicated by arrows in the relevant line. (These arrows and circles are also used in the other figures). 
         [0023]    In the model calculation for the charging circuit of the third line  31  according to  FIG. 1 , the working gas at 1 bar and 20° C. makes its way into a (previously charged) additional heat accumulator  12  and leaves this at a temperature of 80° C. As a result of compression by means of the sixth fluid energy machine  34 , working as a compressor, a pressure increase to 15 bar takes place and consequently also a temperature increase to 540° C. This calculation is based on the following formula 
         [0000]        T   2   =T   1 +( T   2s   −T   1 )/η c   ; T   2s   =T   1 π (K-1)/K ,
 
         [0024]    wherein 
         [0025]    T 2  is the temperature at the compressor exit, 
         [0026]    T 1  is the temperature at the compressor inlet, 
         [0027]    η c  is the isentropic efficiency of the compressor, 
         [0028]    π is the pressure ratio (in this case 15:1), and 
         [0029]    K is the compressibility, which in the case of air is 1.4. 
         [0030]    The isentropic efficiency η c  can be assumed to be 0.85 in the case of a compressor. 
         [0031]    The heated working gas now passes through the heat accumulator  14  where the main part of the available thermal energy is stored. When being stored, the working gas is cooled to 20° C., whereas the pressure is maintained at 15 bar. The working gas is then expanded in two series-connected stages  35   a ,  35   b  of a seventh fluid energy machine  35  so that it arrives at a pressure level of 1 bar. In this case, the working gas is cooled to 5° C. after the first stage and cooled to −100° C. after the second stage. The basis for this calculation is also the formula specified above. 
         [0032]    In the part of the third line  31  which connects the two stages of the seventh fluid energy machine  35   a ,  35   b  in the form of a high-pressure turbine and a low-pressure turbine, provision is additionally made for a water separator  29 . After a first expansion, this enables the air to be dried so that the air moisture which is contained in this in the second stage  35   b  of the seventh fluid energy machine  35  does not lead to icing of the turbine blades. 
         [0033]    In the further process, the expanded and therefore cooled working gas extracts heat from the cold accumulator  16  and is heated to 0° C. as a result. In this way, cold energy is stored in the cold accumulator  16  and can be utilized during a subsequent energy generation. If the temperature of the working gas at the outlet of the cold accumulator  16  and at the inlet of the additional heat accumulator  12  is compared, then it becomes clear why the heat exchanger  17   b  has to be provided for the case of a closed charging circuit. In this case, the working gas can be reheated to an ambient temperature of 20° C., as a result of which heat is extracted from the environment and made available to the process. Such a measure can naturally be dispensed with if the working gas is drawn directly from the environment since this already has ambient temperature. 
         [0034]    So that preheating by means of the additional heat accumulator  12  can be carried out when the charging circuit of the third line  31  is being passed through, an additional circuit is put into effect by means of an additional line  30  by means of which the additional heat accumulator  12  can be charged. The additional heat accumulator  12  has to therefore be connected both to the charging circuit of the third line  31  and to the additional circuit of the additional line  30 . A connection to the third line  31  is effected by means of the valves A, whereas a connection to the additional line  30  is ensured by opening the valves B. During passage through the additional line  30 , the air is first of all directed through an eighth fluid energy machine  36  which operates as a compressor. The compressed air is directed through the additional heat accumulator  12 , wherein the flow direction, corresponding to the indicated arrows, runs exactly opposite to the charging circuit which is formed by the third line  31 . After the air has been brought from ambient pressure (1 bar) and ambient temperature (20° C.) by the compressor to 4 bar and a temperature of 188° C., the air is cooled again to 20° C. by means of the additional heat accumulator  12 . The air is then expanded in two steps by means of the stages  37   a ,  37   b  of a ninth fluid energy machine  37  which operates as a turbine. In this case also, provision is made in the additional line  30  which connects the two stages  37   a ,  37   b  for a water separator  29  which functions in the same way as that which is located in the third line  31 . After expansion of the air via the ninth fluid energy machine  37 , this has a temperature of −56° C. at ambient pressure (1 bar). In case the additional circuit of the additional line  30 —as shown by dash-dot lines—is to be of a closed design, provision therefore has to be made for a heat exchanger  17   c  so that the air can be heated from −56° C. to 20° C. by heat absorption from the environment. 
         [0035]    The circuits of the third line  31  and of the additional line  30  are set in operation independently of each other. Therefore, the sixth and seventh fluid energy machines are mechanically coupled via the shaft  21  having a motor M1 and the eighth and ninth fluid energy machines are mechanically coupled via the other shaft  21  having a motor M2. In the case of over-capacities of the wind power plant  22 , the electric energy can first drive the motor M2 in order to charge the additional heat accumulator  12 . The heat accumulator  14  and the cold accumulator  16  can then be charged by operation of the motor M1 and simultaneous discharging of the additional heat accumulator  12 . The additional heat accumulator  12  can then also be recharged by operation of the motor M2. If all the accumulators are fully charged, an effective discharging cycle for producing electric energy can be initiated (cf.  FIG. 2 ). If the over-capacity of the wind power plant  22  should come to an end, however, without the additional heat accumulator  12  being able to be charged, then the energy available in this can also be replaced by another heat source  41 , or only the heat accumulator  14  is used (cf.  FIG. 2 ). 
         [0036]    According to  FIG. 2 , the plant is now operated with a discharging circuit which is realized by means of a first line  40 . The line  40  constitutes a closed circuit. Water is evaporated and superheated by means of the additional heat accumulator  12 , the heat accumulator  14  and optionally by means of a further heat source  41 , e.g. district heat, via a heat exchanger  42 , and so makes its way via the line  40  (valves C and D are closed) to a third thermal energy machine  43 . This is constructed in two stages consisting of a high-pressure turbine  43   a  and a low-pressure turbine  43   b  which are passed through one after the other. The high-pressure turbine is supplied with steam of a pressure p h . For supplying the low-pressure turbine  43   b , steam with a lower pressure of p 1  is sufficient. This pressure exists in the connecting line  40  between the high-pressure turbine  43   a  and the low-pressure turbine  43   b  or, in specified operating states, also exists in the bypass line  46  after opening of the valve D. The third fluid energy machine  43  drives a generator G via a further shaft  21 . This therefore generates electric current when required while the thermal accumulators  12 ,  14 ,  16  are discharged (Rankine cycle). 
         [0037]    The cold energy stored in the cold accumulator  16  is not made directly available to the circuit formed by the first line  40  but via a first heat exchanger  51 . The first heat exchanger  51  is part of a circuit which is formed by a second line  52 . This circuit itself serves for energy generation which can be produced via a fourth fluid energy machine  53  in the circuit of the second line  52 . The fourth fluid energy machine  53  is connected to a generator G via a shaft  54 . The fourth fluid energy machine  53  also additionally drives a fifth fluid energy machine  55  which is used as a compressor (more about this below). The cold energy from the cold accumulator  16  is therefore primarily used for energy generation in the circuit formed by the second line  52  (for example by means of a Rankine cycle with ammonia). In this case, the circuit formed by the first line  40  profits only indirectly from this cold energy. At the same time, however, the circuit formed by the second line  52  profits from the heat energy which is introduced into this process via the first heat exchanger. The improvement of the overall efficiency of the plant is to be explained by this. 
         [0038]    Downstream of the fourth fluid energy machine, the cold energy from the cold accumulator  16  can be fed again to the second line  52  via a circuit formed by a third line  56  indirectly via a third heat exchanger  57 . For this purpose, the third heat exchanger  57  is provided in the second line. In the second line, a third fluid energy machine in the form of a pump  58  follows after this, as seen in the flow direction. Environmental heat, for example from a flowing medium, can also be fed via a fourth heat exchanger  59  into the working fluid of the second line  52  before this passes through the first heat exchanger  51 . 
         [0039]    As already indicated, the cold energy from the cold accumulator  16  is fed via the third line to the third heat exchanger  57 . In this circuit which is formed by the third line  56  provision is also made for the fifth fluid energy machine which effects a circulation of the working fluid in the third line. The drive is carried out directly by means of the fourth fluid energy machine  53  via the shaft  54 . Alternatively, this circuit which is formed by the third line  56  could even be omitted and instead of the third heat exchanger  57  the cold accumulator  16  is provided directly in the second line  52 . This is indicated by dash-dot lines. In this, the second line  52  in the cold accumulator  16  would be connected directly to a passage system which brings about a surface enlargement in the cold accumulator  16  (more about this below). 
         [0040]    By operating the valves C and D, the efficiency of the plant can be improved in the specified operating states. The valve D lies in a first bypass line  46 , by means of which the high-pressure turbine  43   a  can be bypassed when the valve D is opened. This operating state is advisable if the temperature in the heat accumulator  14  is no longer adequate in order to superheat the steam under high pressure conditions. The latter can be caused as a result of a partial discharging or charging of the heat accumulator  14  which is not yet complete. 
         [0041]    In the extreme case, the heat accumulator  14  is completely emptied, whereas the additional heat accumulator  12  has already been charged. This state can arise, for example, if additional energy could be made available by means of the wind power plant  22  only for a short time, but then an over-demand for electric energy is to be covered. In this case, the valve C can also be connected to a second bypass line  47  in addition to the valve D. In this case, the heat accumulator  14  is bypassed by the bypass line  47  so that the additional heat accumulator  12  can be emptied via the low-pressure turbine  43   b . Therefore, thermal energy is already available in the plant and can be converted into electric energy by means of the generator G with satisfying efficiency. In this case, the cold accumulator  16  is still not charged either since this is charged together with the heat accumulator  14 . For this operating state, a condenser  45  is therefore operated via the valve F. 
         [0042]    Shown in  FIG. 3  is another embodiment of the plant in its general view as a block schematic diagram. Unlike in  FIGS. 1 and 2 , a unitary presentation has been chosen in this case. The circuits formed by the second line  52  and by the third line  56  are designed similar to  FIG. 2  in the main. 
         [0043]    Shown in  FIG. 3 , however, is a simpler system for charging the cold accumulator  16  and the heat accumulator  14  than in  FIG. 1 . The heat accumulator  14  is charged by means of an open circuit which is realized by the line  60 . In this line, ambient air is fed to a compressor  61  via said line  60 , passes through a heat exchanger  62  where the air is heated to 480° C. and this heat is released to the heat accumulator  14  when passing through it. A line  63  also passes through the heat exchanger  62 , forming the circuit by means of which the cold accumulator  16  is cooled. After the working medium in the line  63  has passed through the cold accumulator  16 , this is compressed from ambient conditions to 25 bar via a compressor  64  and heated to 514° C., passes through the heat exchanger  62 , and is then expanded again to 1.1 bar via a turbine  65 . The temperature drops to −121° C. in the process. The working medium in the cold accumulator  16  then absorbs heat again and as a result cools this. The compressor  64  and the turbine  65  are seated on a shaft  66  and can additionally be driven by means of a motor M which is connected to this shaft  66 . 
         [0044]    In the exemplary embodiment according to  FIG. 3 , the heat accumulator  14  is not incorporated directly into the circuit formed by the first line  40 . Rather, a further circuit is formed by a fourth line  67 , in which circuit there is passage through the following units at a constant pressure of approximately 1 bar. After passing through the heat accumulator  14 , the working medium (for example air), heated to 476° C., is fed to a fifth heat exchanger  68 . The heat exchanger  68  releases the heat to the first line  40  and is cooled to 91° C. (more about this below). Working medium then passes through the fourth line  67  to the first heat exchanger  51  so that the residual heat, which was not released to the first line via the fifth heat exchanger  68 , can be released to the second line  52 . The working medium in the further process can be cooled further via a condenser  69 , wherein the condenser  69  is also a heat exchanger which is provided in the first line  40  (more about this below). Via a tenth fluid energy machine  70  in the form of a type of circulating pump, the working medium then makes its way again into the heat accumulator  14  where this is reheated. Instead of the closed circuit shown in  FIG. 3 , the fourth line  67  can also be designed as an open circuit in which the part of the line between the condenser  69  and the tenth fluid energy machine  70  which is detailed by the dash-dot line is omitted. 
         [0045]    The first line  40  forms a circuit by means of which electric current can be generated by a generator G via a shaft  71 . To this end, one circuit is operated with water, wherein the fifth heat exchanger  68  is operated as a multi-stage waste heat steam generator with a high-pressure stage  68   a  and a low-pressure stage  68   b  (Rankine cycle). The water is fed at ambient temperature by means of a feed pump  44   a  at 5.5 bar first of all into the low-pressure stage  68   b  of the fifth heat exchanger  68 . A part of the water leaves this low-pressure stage  68   b  at 4.1 bar and 145° C. in order to be fed (as steam) to the low-pressure stage  43   b  of the second thermal fluid energy machine. Another part of the water is fed in the liquid state by means of a second feed pump  44   b , to the high-pressure stage  68   a  of the fifth heat exchanger  68  and leaves this as steam at 80 bar and 459° C. in order to be fed to the high-pressure stage  43   a  of the second thermal fluid energy machine  43 . Both the fourth and the second thermal fluid energy machines drive a shaft  71  which is connected to a generator G. After expansion of the steam to 0.03 bar at 24° C., this is fed again via the condenser  69  to the feed pump  44   a.    
         [0046]    The construction of the heat accumulator  14 , of the cold accumulator  16  and of the additional heat accumulator in the case of the plant in the figures is the same in each case and is shown in more detail in  FIG. 1  by means of a detailed enlargement based on the cold accumulator  16 . Provided is a container, the wall  24  of which is provided with an insulating material  25  which has large pores  26 . Provision is made inside the container for concrete  27  which functions as a heat accumulator or a cold accumulator. Pipes  28  which extend in parallel are laid within the concrete  27  and the working gas flows through these and in the process releases heat or absorbs heat (depending on the type of operation and type of accumulator). 
         [0047]    The charging and discharging circuits of  FIGS. 1 to 3  can also be combined with each other so that further exemplary embodiments result from this.