Abstract:
A system for storing and outputting thermal energy and a method for operating the system are provided. The system operates according to the Brayton cycle, wherein a heat accumulator is charged by a compressor and a cold accumulator is charged by turbines. The cycle is reversed for discharging. In addition, the cold accumulator supplies a cooling circuit, which provides the cooling for a superconducting generator by a cooling unit. A favorable generator weight can thereby be advantageously achieved in particular for wind turbines, because the generators are limited regarding the weight thereof due to being housed in the nacelle of the wind power plant. Thus, advantageously higher power can be converted in the wind power plant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2013/056735 filed Mar. 28, 2013, and claims the benefit thereof. The International application claims the benefit of German Application No. DE 102012206296.3 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 thermal energy which has a circuit for a working gas. In the circuit in this case the following units are interconnected in the specified sequence by means of a line for the working gas: a first thermal fluid energy machine, a heat accumulator, a second thermal fluid energy machine and a cold accumulator. 
         [0003]    The first thermal fluid energy machine is operated as a working machine and the second thermal fluid energy machine is operated as a power machine, as seen in the flow direction of the working gas from the heat accumulator to the cold accumulator. 
         [0004]    Furthermore, the invention relates to two methods for operating this plant. In one method for storing thermal energy, passage through the circuit is in the direction of the heat accumulator to the cold accumulator, which corresponds to the sequence of the modular units specified above. According to a further method, to which the invention also refers, stored thermal energy from the plant can also be converted into mechanical energy, for example. In this case, passage through the units is in the reverse sequence, in other words the flow direction of the working gas is reversed. This working gas then passes first of all through the cold accumulator and then through the heat accumulator, wherein in this case the first thermal fluid energy machine is operated as a power machine and the second thermal fluid energy machine is operated as a working machine. 
       BACKGROUND OF INVENTION 
       [0005]    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 used 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. In this case, the thermal fluid energy machine is therefore operated as a motor. 
         [0006]    The term “thermal fluid energy machine” constitutes a generic term for machines which can extract thermal energy from a working fluid—a working gas 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 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. 
         [0007]    The principle specified in the introduction is described according to US 2010/0257862 A1, 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 
       [0008]    An object of the invention is to disclose a plant for storing thermal energy of the type specified in the introduction and methods for conversion of thermal energy (for example conversion of mechanical energy into thermal energy with subsequent storage or conversion of the stored thermal energy into mechanical energy) by means of which a utilization of the stored thermal energy which is as efficient as possible is enabled. 
         [0009]    This object is achieved according to the invention by means of the plant specified in the introduction by the cold accumulator being able to be connected into a circuit for a cooling medium, which differs from the aforesaid circuit, by the following units being interconnected in the specified sequence by means of a line for a cooling medium: the cold accumulator, a cooling unit and a cold consumer which is to be cooled. The cooling medium is normally different from the working gas which accounts for the fact that the cooling circuit is different from the circuit. In order to avoid cleaning during a change of the operating state, it is particularly advantageous if in the cold accumulator the passages used for heat transfer also form two passage systems and if in this respect each of the passage systems can be connected to one of the circuits. The cooling circuit therefore uses the one passage system, whereas the charging circuit uses the other passage system. The charging circuit, however, can share the passage system in the cold accumulator—and possibly also other parts of the line associated with this—with a discharging circuit (more about this below). Whereas the charging circuit is responsible for storing the thermal energy, the thermal energy can be released again to the working gas via a discharging circuit. 
         [0010]    The cooling unit is required in order to be able to set the necessary temperature level for the cold consumer which is to be cooled because the storage temperature of the cold accumulator is higher than the necessary temperature level. However, by means of the cold accumulator precooling of the cooling medium can be carried out so that a smaller temperature difference has to be overcome in the cooling unit. This also advantageously reduces the power demand for the cooling unit. Process cold, which accumulates anyway in the plant for storing thermal energy, can be utilized. This is admittedly no longer available during discharging for the purpose of releasing thermal energy, for which this does not have to be produced separately, however, for operation of the cold consumer which is to be cooled. The overall energy balance of the plant for storing and releasing thermal energy and of the cold consumer is advantageously improved as a result. 
         [0011]    The charging circuit (and discharging circuit) can be operated as an open or closed circuit (more about this below). In the case of an open circuit, air forms the working gas which can be extracted from the atmosphere and then fed again to this. As a cooling unit, any form of unit can be used. The use of a thermosiphon, which advantageously achieves comparatively low temperature levels, is particularly advantageous. 
         [0012]    The cold consumer is equipped according to a specific embodiment of the invention with a superconducting component. As cooling medium, nitrogen can be used here, especially when high-temperature superconductors, for example Bi2223 or YBCO, are being used. This has to be brought to a temperature level of approximately 50 to 60 K. A precooling via the cold accumulator to approximately 180 K simplifies the cooling process and reduces the power consumption on the cooling unit. 
         [0013]    According to another embodiment of the invention, it is provided that the electric machine is a generator which in particular can be installed in a wind power plant. This application offers particular advantages since electric machines can be constructed with superconducting components (especially the winding of an electrically excited rotor in a synchronous generator) with a lower mass. The mass of the generator, however, constitutes the limiting factor in the design of wind power plants since the generators have to be installed at great height in the nacelle of the wind power plant. In the case of conventional generators, the mass of the applied generators increases more quickly, however, than the output, wherein approximately a power of 1.6 lies within this ratio. Therefore, at present the economical limit for an increase of the generator output in wind power plants is approximately 6 MW. On the other hand, the construction of wind power plants exposed to strong winds requires the installation of a larger generator capacity in the nacelle. This can be achieved according to the invention by using generators with superconducting components. If the wind power plant is coupled to the plant according to the invention of storing and releasing thermal energy, then this has the advantage that the cold accumulator can be expediently used in order to minimize the losses which become necessary on account of the required cooling of the superconducting generator windings. At the same time, this plant for storing thermal energy can also be used in order to temporarily store, in a known manner per se, over-capacities in the electricity network and to convert them again into electric current, by releasing the thermal energy, in the event of consumption peaks in electric energy. It therefore involves the utilization of a synergy effect which altogether increases the efficiency during operation of the plant especially wind power plants. However, the plant can also be operated for example with pump storage power plants or with conventional power plants, such gas-turbine power plants. 
         [0014]    It can furthermore be advantageously provided that the electric machine is a motor which is mechanically coupled to the first thermal fluid energy machine. This fluid energy machine has to be operated specifically during the charging process of the cold accumulator and of the heat accumulator (possibly also of an additional low-temperature heat accumulator) in order to bring the thermodynamic charging process into operation. It is particularly advantageous to also construct this motor as an electric machine with a superconducting winding if the infrastructure for cooling this machine is available on account of using a superconducting generator, for example in the wind power plant. With this, a further efficiency increase for the plant is possible. 
         [0015]    It can have an equally efficiency-increasing, advantageous effect if a further generator with superconducting components (e.g. the winding) is used as an electric machine. This is then coupled to the first fluid energy machine and used if in times of increased energy demand the heat accumulator and the cold accumulator are to be discharged. It is alternatively also possible that the generator is connected to a third thermal fluid energy machine, wherein the third thermal fluid energy machine is connected in parallel with the first thermal fluid energy machine and a fourth thermal fluid energy machine is connected in parallel with the second thermal fluid energy machine in the charging and discharging circuit. In this case, a valve mechanism is provided in each case between the first and the third thermal fluid energy machines and/or between the second and the fourth thermal fluid energy machines. By switching of the valve mechanism, the one or the other fluid energy machine can now be advantageously selected in each case, depending on the flow direction of the working gas. This has the advantage that the respective fluid energy machine being used can be optimized to the operating state which is to be selected. Since when using only two fluid energy machines both have to be used both as a working machine and as a power machine, depending on the flow direction, only one constructional compromise can be selected without the provision of additional fluid energy machines. Since, however, both in the thermal charging operation and in thermal discharging operation the aim is a highest possible level of efficiency, the parallel connection of fluid energy machines allows both the method for storing the thermal energy and the method for conversion of the thermal energy to be undertaken with optimum efficiency. Instead of using valves, separate lines can also be provided for the charging circuit and the discharging circuit. The configuration has the advantage that the fluid energy machines being used in each case can be optimized to the respective operating state during the charging process and the discharging process. As a result of this, an increase of efficiency of the system is achieved. This, however, is at the cost of higher investment costs of the plant. An economical assessment has to be made in this case. 
         [0016]    By choice, the working gas can be conducted in a closed circuit or an open circuit (this applies both to the charging circuit and to the discharging circuit, but not to the cooling circuit). An open circuit always uses the ambient air as working gas. This is drawn from the environment and at the end of the process is also released into this again so that the environment closes the open circuit. A closed circuit also allows the use of a working gas which is other than ambient air. This working gas is conducted in the closed circuit. Since an expansion into the environment with simultaneous adjustment of the ambient pressure and the ambient temperature does not apply, the working gas, in the case of a closed circuit, has to be conducted through a heat exchanger which allows a release or absorption of the heat of the working gas into or from the environment. 
         [0017]    In addition, it can be provided that a low-temperature heat accumulator is additionally provided in the circuit upstream of the first fluid energy machine. This heat accumulator is referred to as a low-temperature heat accumulator because the temperature level which is reached as a result of storing the heat principally lies below the temperature level of the heat accumulator. Heat is defined in relation to the ambient temperature of the plant. Everything above ambient temperature is heat, whereas everything below the ambient temperature is cold. Therefore, it also becomes clear that the temperature level of the cold accumulator lies below the ambient temperature. 
         [0018]    The use of the low-temperature heat accumulator has the following advantages. If the plant for storing thermal energy is used, then the low-temperature heat accumulator is passed through before passage through the first fluid energy machine which in this case works as a working machine (compressor). As a result of this, the working gas is already heated above ambient temperature. This has the advantage that the working machine has to absorb a lower level of power in order to achieve the required temperature of the working gas. In concrete terms, the heat accumulator is to be heated to above 500° C., which can advantageously also be carried out, subsequent to the preheating of the working gas, using technically available thermodynamic compressors which allow compression of the working gas to 15 bar. Therefore, recourse may advantageously be had to components for the modular units of the plant which are obtainable on the market without costly modifications. 
         [0019]    The achieving of an object is additionally managed by means of the method referred to in the introduction for storing and releasing thermal energy by the cold accumulator being connected, when required, into a cooling circuit which differs from said circuit, wherein in the cooling circuit cooling medium flows through the following units in the specified sequence: the cold accumulator, a cooling unit and a cold consumer which is to be cooled. By means of this method, the advantages explained above for the plant according to the invention are achieved, wherein the method can be implemented with the aforesaid plant. As a particularly suitable cooling medium, especially for superconducting components, nitrogen can be used. This exists in liquid form at the temperatures which are required for cooling these superconducting components and can be brought to the required temperature level in a thermosiphon, for example, as the cooling unit. The precooling via the cold accumulator reduces the energy expenditure in this case during operation of the cooling unit. This can also be of smaller dimensions. This make this solution particularly economical. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    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 in this and are repeatedly explained only insofar as differences arise between the individual figures. 
           [0021]    In the drawing: 
           [0022]      FIG. 1  shows an exemplary embodiment of the plant according to the invention as a schematic diagram, and 
           [0023]      FIGS. 2 and 3  show exemplary embodiments of the method according to the invention (Brayton process) with reference to further schematic diagrams. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0024]    A plant for storing thermal energy according to  FIG. 1  has a line  11  by means of which a plurality of units are interconnected in such a way that a working gas can flow through these. The working gas flows through a low-temperature heat accumulator  12  and then through a first thermal fluid energy machine  13  which is designed as a hydrodynamic compressor. The line then continues to a heat accumulator  14 . This is connected to a second thermal fluid energy machine  15  which is designed as a hydrodynamic turbine. From the turbine, the line  11  leads to a cold accumulator  16 . The cold accumulator  16  is connected to the low-temperature heat accumulator  12  by means of the line  11 , wherein in this line section provision is also made for a heat exchanger  17  via which the working gas can release heat to the environment or absorb heat from the environment (depending on the type of operation). 
         [0025]    In  FIG. 1 , a closed circuit for the working gas is provided in this respect. However, it is equally conceivable that the line section between the cold accumulator  16  and the low-temperature heat accumulator  12  together with the heat exchanger  17  are dispensed with, in a way not shown. In this case, the circuit via the environment would be closed, wherein the working gas, which in this case comprises ambient air, would be drawn in at the low-temperature heat accumulator  12  and be blown out again into the environment downstream of the cold accumulator  16 . 
         [0026]    Also provided in  FIG. 1  is a third thermal fluid energy machine  18  in the form of a hydrodynamic turbine and a fourth thermal fluid energy machine  19  in the form of a hydrodynamic compressor. It is also to be noted that the first hydrodynamic fluid energy machine  13  in the line  11  is connected in parallel with the third hydrodynamic machine  18  and the second fluid energy machine  15  in the line  11  is connected in parallel with the fourth fluid energy machine  19 . Valve mechanisms  20  by opening and closing ensure that flow only passes through the first and second fluid energy machines or the third and fourth fluid energy machines in each case. The first and second fluid energy machines  13  and  15  are mechanically intercoupled via a first shaft  21  and are driven by an electric motor M which is fed from a wind power plant  22  as long as there is no demand for the generated electric energy in the electricity network. During this operating state, the heat accumulator  14  and the cold accumulator  16  are charged, as is explained in more detail later. If the demand for electric energy is greater in relation to the currently generated quantity of electric energy, then the electric current generated by the wind power plant  22  is fed directly into the network. The plant additionally supports power generation in another operating state by the heat accumulator  14  and the cold accumulator  16  being discharged and a generator G1 being driven by the fluid energy machines  18  and  19  via a second shaft  23 . The second shaft  23  is mechanically coupled to the third fluid energy machine  18  and to the fourth fluid energy machine  19  for this purpose. 
         [0027]    The construction of the low-temperature heat accumulator  12 , of the heat accumulator  14  and of the cold accumulator  16  in the plant according to  FIG. 1  is the same in each case and is explained in more detail by means of a detail 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 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). 
         [0028]    The cold accumulator  16  also supplies a further line  31  with the stored cold. For this line  31 , a passage system—not shown in more detail—is provided in the cold accumulator  16  and is independent of another passage system (not shown either) which is connected to the line  11 . The line  31  is part of a cooling circuit by means of which a cooling medium, such as nitrogen, can be precooled. By means of a pump  32 , this cooling medium is circulated in the cooling circuit and also pumped through a cooling unit in the form of a thermosiphon, which is not shown in more detail. Via different valves  34 , bypass lines  35 , which are connected to heat exchangers  36  in each case, can be connected into the cooling circuit. The heat exchangers  36  in each case lead to the motor M, to the generator G1 and to a generator G2 in the wind power plant  22 . These generators are provided with superconducting components, especially windings, comprising high-temperature superconductors. The cooling medium is sufficient to hold these windings at a temperature level which the superconducting properties maintain. 
         [0029]    Shown in  FIG. 1  is a variant of the cooling circuit in which the cooling unit is arranged outside of the wind power plant  22 . In order to keep the paths of the line  31  to be insulated as short as possible, the cooling unit, however, is to be arranged in the direct proximity of the wind power plant  22  and of the motor M and also of the generator G1. Therefore, the cold accumulator  16  should also be arranged in the vicinity of the wind power plant  22 . Such a cold accumulator  16  will advantageously be allocated in each case to only one wind power plant  22 , or to a few wind power plants  22 , of a wind park. On the other hand, the losses on account of transporting the cold in the line  31  or the cost of thermal insulation would be too high. 
         [0030]    With reference to the plant according to  FIGS. 2 and 3 , the thermal charging and discharging process shall be explained in more detail. Shown first of all in  FIG. 2  is the charging process which functions according to the principle of a heat pump. Shown in  FIGS. 2 and 3 , in contrast to  FIG. 1 , is an open circuit which, however, as indicated by dash-dot lines, could be closed using the optionally provided heat exchanger  17 . The states in the working gas, which in the case of the exemplary embodiment of  FIGS. 2 and 3  comprises air, are shown in each case in circles on the lines. 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 is the temperature in ° C., 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 line  11 . 
         [0031]    In the model calculation the working gas at 1 bar and 20° C. makes its way into the (previously charged) low-temperature heat accumulator and leaves this at a temperature of 80° C. As a result of compression by means of the first fluid energy machine  13 , working as a compressor, a pressure increase to 15 bar takes place and consequently also a temperature increase to 547° C. This calculation is based on the following formula 
         [0000]        T   2   =T   1 +( T   2   −T   1 )/η c   ;T   2s   =T   1 π (K-1)/K ,
 
         [0000]    wherein 
         [0032]    T 2  is the temperature at the compressor exit, 
         [0033]    T 1  is the temperature at the compressor inlet, 
         [0034]    η c  is the isentropic efficiency of the compressor, 
         [0035]    π is the pressure ratio (in this case 15:1) and 
         [0036]    K is the compressibility, which in the case of air is 1.4. 
         [0037]    The isentropic efficiency η c  can be assumed to be 0.85 in the case of a compressor. 
         [0038]    The heated working gas now passes through the heat accumulator  14  where the main part of the available thermal energy is stored. While being stored, the working gas is cooled to 20° C., whereas the pressure (apart from flow-inducted pressure losses) is maintained at 15 bar. The working gas is then expanded in two series-connected stages  15   a ,  15   b  of a second fluid energy machine so that it arrives at a pressure level of one bar. In the process, the working gas is cooled to 5° C. after the first stage and cooled to −114° C. after the second stage. The basis for this calculation is also the formula specified above. 
         [0039]    In the part of the line  11  which connects the two stages  15   a ,  15   b  of the second fluid energy machine 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  15   b  of the second fluid energy machine  15  does not lead to icing of the turbine blades (necessary only for the case of an open circuit). 
         [0040]    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 heat accumulator  12  is compared, then it becomes clear why the heat exchanger  17  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. 
         [0041]    For the cooling, an embodiment which deviates from the variant in  FIG. 1  is shown in  FIG. 2 . The motor M and the generator G1 do not have any superconducting components in this case. Only the generator G2 in the wind power plant  22 , which on account of its installed height in the nacelle of the wind power plant is to have a mass which is as low as possible, utilizes the advantages which are associated with superconducting windings and their smaller, necessary conductor cross sections. The line  31  therefore leads without bypass lines directly to the wind power plant  22 . The cooling unit  33  is also accommodated in the nacelle of the wind power plant  22  so that the paths of the cooling medium can be advantageously minimized, at least at low temperature level. 
         [0042]    By means of  FIG. 3 , the discharging cycle of the heat accumulator  14  and of the cold accumulator  16  can be understood, wherein electric energy is generated at the generator G1. Unlike as in  FIG. 1 , in  FIG. 3  the first fluid energy machine  13  and the second (two-stage) fluid energy machine  15  are used both in the charging cycle and in the discharging cycle. This does not impair the functioning principle of the plant but, however, is at the cost of lower efficiency. Therefore, the higher investment cost when additionally using a third and a fourth fluid energy machine is to be balanced against the gain in efficiency which is achieved when using four fluid energy machines by each being able to be optimized to the corresponding operating state. Also shown, by dash-dot lines again, is the alternative of a closed circuit. The water separator  29  is not shown in the representation according to  FIG. 3  since this is not used. 
         [0043]    The working gas is directed through the cold accumulator  16 . In the process, it is cooled from 20° C. to −92° C. This measure serves for reducing the power consumption in order to operate the second fluid energy machine which works as a compressor. The power consumption is reduced correspondingly by the factor of the temperature difference in Kelvin, that is to say 293K/181K=1.62. In the example, the compressor compresses the working gas to 10 bar. During this, the temperature rises to 100° C. A compression of up to 15 bar would also be technically acceptable. The compressed working gas passes through the heat accumulator  14  and is consequently heated to 500° C., wherein the pressure reduces slightly to 9.8 bar. The working gas is then expanded by means of the first fluid energy machine which therefore works as a turbine in this operating state. An expansion to 1 bar is carried out, wherein at the outlet of the first fluid energy machine a temperature of 183° C. still prevails in the working gas. 
         [0044]    In order to be able to also utilize this residual heat, the working gas is then directed through the low-temperature heat accumulator and additionally cooled to 130° C. as a result. This heat has to be stored in order to serve for preheating of the working gas to 80° C. in a subsequent charging process of the heat accumulator  14  and of the cold accumulator  16  (as already described above). The low-temperature heat accumulator therefore works as a temporary store and is always charged especially when the two other accumulators, i.e. the heat accumulator  14  and the cold accumulator  16 , are discharged, and vice versa. As already mentioned, the functioning principle of the plant and of the method is not limited, however, if the low-temperature heat accumulator is omitted.