Abstract:
A method for charging and discharging a heat accumulator in a charge cycle and in a discharge cycle is provided. The discharging takes place by means of a steam turbine which has a high-pressure part and a low-pressure part. In order to provide heat to both turbine parts, the heat accumulator is divided into a part-accumulator for the high-pressure part and a part-accumulator for the low-pressure part. Furthermore, a system is provided in which the heat accumulator is divided into two part-accumulators. By operating a turbine with the high-pressure part and low-pressure part, the efficiency and yield of heat from the heat accumulator can be advantageously increased. The system can, for example, be used to temporarily store surplus capacities of a wind plant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2013/066273 filed Aug. 2, 2013, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP12180397 filed Aug. 14, 2012. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for charging and discharging a heat accumulator. The invention relates, moreover, to a system for storing and releasing thermal energy, with a heat accumulator. 
       BACKGROUND OF INVENTION 
       [0003]    The terms “engine” and “working machine” are used in the context of this application in the sense 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 thus operated as a condenser or as a compressor. By contrast, an engine performs work, and in order to perform the work a thermal fluid energy machine converts the thermal energy available in the working gas. In this case, therefore, the thermal fluid energy machine is operated as a motor. 
         [0004]    The term “thermal fluid energy machine” constitutes a generic term for machines which can extract thermal energy from a working fluid or feed thermal energy to the latter. Thermal energy is to be understood as being both heat energy and cold energy. Thermal fluid energy machines (also designated below more briefly as fluid energy machines) may be designed, for example, as piston machines. Hydrodynamic thermal fluid energy machines may preferably also be used, the rotors of which allow a continuous flow of the working gas. Axially acting turbines or condensers are preferably employed. 
         [0005]    The initially indicated principle is described, for example, according to WO 2009/044139 A2. Piston machines are used here in order to carry out the method described above. Moreover, according to U.S. Pat. No. 5,436,508, it is known that, by means of the initially indicated systems for storing thermal energy, surplus capacities during the use of wind power for the generation of electrical current can be intermediately stored in order to retrieve these again, as required. 
       SUMMARY OF INVENTION 
       [0006]    The object is to specify a method for charging and discharging a heat accumulator and a system for carrying out this method, by means of which storage and recovery of energy can take place with comparatively high efficiency, at the same time entailing a comparatively low outlay in terms of components. 
         [0007]    The invention relates to a method for charging and discharging a heat accumulator, in which the following steps are performed preferably alternately. During a charging cycle, the heat accumulator is heated up by a working fluid, a pressure rise being generated in the working fluid, before it runs through the heat accumulator, by a first thermal fluid energy machine, connected up as a working machine, and the working fluid is expanded after running through the heat accumulator. During a discharge cycle, the heat accumulator is cooled by the same or another working fluid, a pressure rise being generated in the working fluid before the latter runs through the heat accumulator and the working fluid, after running through the heat accumulator, being expanded via a second thermal fluid energy machine, connected up as an engine, or via the first thermal fluid energy machine, connected up as an engine. 
         [0008]    The invention relates, moreover, to a system for storing and releasing thermal energy, with a heat accumulator, the heat accumulator being capable of absorbing the stored heat from a charging circuit for a working fluid and of releasing said stored heat to a discharging circuit for another or the same working fluid. In the charging circuit, the following units are connected to one another by means of lines in the order indicated: a first thermal fluid energy machine connected up as a working machine, the heat accumulator, a device for the expansion of the working fluid, in particular a third fluid energy machine, and a first heat exchanger, in particular a cold accumulator. In the discharging circuit, the following units are connected to one another by means of lines in the order indicated: the heat accumulator, a second thermal fluid energy machine connected as an engine or the first fluid energy machine connected up as an engine, the first heat exchanger or a second heat exchanger and a pump. 
         [0009]    The method initially indicated and the system suitable for carrying out the method can be used, for example, in order to convert surplus capacities from the electricity network into thermal energy by means of the charging cycle and to store them in the heat accumulator. This process is reversed, as required, so that the heat accumulator is discharged in a discharging cycle and current can be obtained by means of the thermal energy and can be fed into the network. 
         [0010]    This object is achieved by means of the initially indicated method, according to the invention, in that the discharging cycle is configured as a Rankine process in which the following steps are performed. The working fluid is first conducted through a first line system which runs in the heat accumulator and where it absorbs heat. The working fluid is subsequently expanded via a high-pressure part of the second thermal fluid energy machine (for example, a high-pressure turbine). The working fluid is then conducted through a second line system running in the heat accumulator and once again absorbs heat. Intermediate superheating advantageously takes place. Lastly, the working fluid is expanded via a low-pressure part of the second thermal fluid energy machine (for example, a low-pressure turbine). In the context of the invention, therefore, the fluid energy machine is composed of a high-pressure part and of a low-pressure part. The two parts together are to be interpreted as a fluid energy machine. 
         [0011]    The use of the Rankine process for discharging the heat accumulator has the advantage that the latter can be operated with comparatively high efficiency. 
         [0012]    Particularly due to a two-stage discharge of the heat accumulator, as proposed according to the invention, the heat output of the heat accumulator can advantageously be increased, because the latter can be brought, as a result of discharge via the second line system, to a lower temperature level before it has to be charged again. In the event that, for example when the method is used together with a wind power plant or another regenerative energy source for generating electricity, wind power is unavailable for a relatively long period of time, the resulting electricity outage can be bridged for the longer time by means of the heat stored in the heat accumulator. The second thermal fluid energy machine in this case delivers the energy, for example, in order to drive a generator for the generation of electrical energy. 
         [0013]    According to an advantageous refinement of the invention, there is provision whereby the charging cycle is implemented by means of a heat pump process. Such a process likewise has the great advantage that, with an efficiency of above 100%, it improves the overall efficiency of the method which is composed both of the charging cycle and of the discharging cycle. This is because, when the heat accumulator is being charged, the heat pump process also extracts heat from the surroundings, which heat is available during discharge. 
         [0014]    It is advantageous if nitrogen or dried air is used in the charging cycle. The air has to be dried because water contained in the air would otherwise condense or even freeze in the heat pump process after the cooling of the air and could damage the heat pump used. It is also advantageous if the discharging cycle is operated with steam. Nitrogen, air and steam are working fluids which, when they escape into the environment, are completely neutral and therefore cause no environmental damage. A system can therefore be operated with these working fluids without any environmental risks. 
         [0015]    This also has an effect upon its economic efficiency, since there is no need for increased safety standards to be taken into account. 
         [0016]    Furthermore, the abovementioned object is also achieved by means of the initially indicated system, in that the second thermal fluid energy machine has a high-pressure part and a low-pressure part and two line systems fluidically independent of one another, to be precise a first line system and a second line system, are provided in the heat accumulator, these units being connected to one another by means of lines in the order indicated, specifically the first line system, then the high-pressure part, then the second line system and then the low-pressure part. The abovementioned method can be carried out by means of this arrangement, since such an interconnection of the units affords the precondition for this. Consequently, the advantages explained above are also achieved in the operation of the system and are not explained again in any more detail at this juncture. 
         [0017]    According to one refinement of the invention, there is provision whereby the first line system is accommodated in a first subaccumulator and the second line system is accommodated in a second subaccumulator separated structurally from the first. Structural separation of the two subaccumulators has the effect that these are independent of one another. On the one hand, there is at least substantial thermal independence, since, in the case of structurally separate subaccumulators, heat transfer between these is not possible. Moreover, structurally separated subaccumulators can also be supplied in a simple way by means of two different line systems, since these can have in each case independent connections for the line system. Lastly, it is possible to construct the subaccumulators in a modular manner and thereby offer a construction kit which allows comparatively simple adaptation to different required heat capacities of the subaccumulators used. 
         [0018]    An especial refinement of the system with structurally separate subaccumulators is obtained when the first subaccumulator and the second subaccumulator are arranged in parallel in the charging circuit. This means that both the first subaccumulator and the second subaccumulator are acted upon by the working fluid having the same temperature and therefore the same temperature level is also set in both subaccumulators. Alternatively, it is also possible that the second subaccumulator, which supplies the heat for the low-pressure part of the second thermal fluid energy machine, is brought to a lower temperature level. This is the case when the first subaccumulator is arranged upstream of the second subaccumulator in the charging circuit, that is to say they are connected in series. 
         [0019]    The parallel connection of the subaccumulators has the advantage that the material present in the subaccumulators is utilized optimally in terms of its heat capacity. Moreover, in the parallel connection of the subaccumulators, it is possible especially simply to design these in such a way that the two subaccumulators are simultaneously discharged completely in a discharging cycle and are simultaneously charged completely in a charging cycle. If, however, there is not complete charging or discharging, which, for example when the system is used on a wind power plant, will often happen as a function of the wind, the process can be reversed as often as desired, without the charge ratio of the two subaccumulators thereby being disturbed. 
         [0020]    According to another refinement, it is possible that the first line system and the second line system run in the heat accumulator which is designed as a structural unit. This means that the heat accumulator makes available only one heat stock both for feeding the first line system and for feeding the second line system, that is to say constitutes a unit in structural terms. In this case, the line systems must run independently of one another in this heat accumulator (for example, run parallel). The advantage of this is that construction material can be saved in the construction of the heat accumulator. As a structural unit, the heat accumulator can also advantageously be made more compact, that is to say it also has fewer interfaces, via which heat can be lost to the surroundings. 
         [0021]    If the heat accumulator forms a structural unit, it is advantageous if the first line system is accommodated in a first subregion and the second line system is accommodated in a second subregion separated spatially from the first. Spatial separation in the context of the invention means as substantial thermal separation as possible. Thermal separation exists in a heat accumulator designed as a structural unit when the heat influence zones in the region of the two line systems are as substantially independent of one another as possible. This means that, for example, the first line system can now lie in the front part of the heat accumulator and the second line system can now lie in the rear part of the heat accumulator, the heat accumulator thus having in spatial terms two subregions which differ from the subaccumulators already mentioned above merely in that they are not structurally separated from one another, but instead abut one another at an interface. In this form of construction, the connections for the charging circuit can then also be mounted on the heat accumulator such that the first subregion and the second subregion are arranged in parallel in the charging circuit. The advantages associated with this have already been explained above. 
         [0022]    According to a further refinement of the invention, there is provision whereby the second line system is accommodated together with the first line system in a subregion of the heat accumulator. This means that, in this region, the second line system and the first line system run in the same heat influence zone as the heat accumulator. The advantage of this is that the heat accumulator, at least in the subregion in which the line systems are accommodated together, can be cooled to a temperature level which is still sufficient for supplying the low-pressure part of the turbine. This temperature level lies lower, and therefore a greater part of the heat stored in the heat accumulator can be utilized for energy recovery. It thus becomes possible to utilize the heat accumulator longer for the recovery of energy. This is advantageous particularly when the system is used in the case of regenerative energies, since periods in which the regenerative energies (for example, wind energy) are not available can be bridged. In an emergency, it is also in this case acceptable that the system runs with lower efficiency from the moment when the high-pressure part of the second thermal fluid energy machine is no longer working. This is acceptable in such a case in light of the fact that energy bottlenecks may occur. 
         [0023]    It is especially advantageous if the second line system is accommodated together with the first line system in a plurality of subregions of the heat accumulator, and in this case the second line system can be short-circuited via a bypass line in each of these second subregions. It is thereby advantageously possible, in each of these subregions, to bring the heat level of the heat accumulator to the level required for operating the low-pressure part of the fluid energy machine. If, in the most favorable case, the subregions cover the entire heat accumulator, the entire heat accumulator can consequently be discharged to the lower temperature level. However, in order to ensure operation with high efficiency at the start of the discharging cycle, that is to say when the heat accumulator is still fully charged, at this stage only the second line system in one subregion is connected and the other subregions are bridged via the bypass lines, so that, in the other subregions, the high energy level is available for discharge as long as possible for the purpose of driving the high-pressure part. As a result, the high system efficiency which is desired can be achieved for as long as possible. 
         [0024]    It is especially advantageous if the ratio of the heat capacity of the first subregion to that of the second subregion or second subregions or of the first subaccumulator to the second subaccumulator is adapted to the heat demand caused by the discharging process, in such a way that both subregions or subaccumulators are discharged within the same timespan. This design of the subaccumulators or of the subregions is a precondition for discharging or charging the subregions or subaccumulators always at the same time point. As already mentioned, this process can also be reversed, for example, when the system is used in a wind power plant. The system can then advantageously be operated with the maximum possible efficiency in as many operating states as possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Further details of the invention are described below by means of the drawing. Identical drawing elements or those corresponding to one another are in each case given the same reference symbols in the individual figures and are explained more than once only in so far as differences arise between the individual figures of which: 
           [0026]      FIG. 1  shows a circuit diagram of an exemplary embodiment of the system according to the invention with state variables of the working fluids according to an exemplary embodiment of the method according to the invention, 
           [0027]      FIG. 2  shows diagrammatically a discharging process as an exemplary embodiment of the method according to the invention with intermediate superheating in the T-S graph (that is to say, temperature T as a function of the enthalpy), and 
           [0028]      FIGS. 3 to 6  show various exemplary embodiments of a heat accumulator, such as can be used in a system according to  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0029]      FIG. 1  illustrates the system according to the invention with a heat accumulator  11  and with a cold accumulator  12 . A charging circuit  13  and a discharging circuit  14  are implemented in the system, these circuits being connected to line systems, not illustrated in any more detail, in the heat accumulator  11  and cold accumulator  12  and therefore allowing the charging and discharging of heat or cold into and out of the accumulators. There is, moreover, a heat exchanger circuit  15 . 
         [0030]    First, the charging cycle for the heat accumulator  11  and the cold accumulator  12  is described. The charging of the heat accumulator  11  signifies heating-up of the latter, and the charging of the cold accumulator  12  signifies cooling of the latter. Ambient temperature is to be understood as being the reference with regard to heating and to cooling. During the charging cycle, a wind power plant  16  generates surplus capacities, by means of which an electric motor M can be driven. The motor M has a drive shaft  17 , by means of which a first fluid energy machine  18  and a third fluid energy machine  19  are driven. The first fluid energy machine is a hydrodynamic pump and the third fluid energy machine is a hydrodynamic turbine. The first fluid energy machine  18  condenses the working medium and conducts it through the heat accumulator  11 . The latter is composed of a first subaccumulator  20  and of a second subaccumulator  21  which are connected in series in the charging circuit  13 . In the heat accumulator  11 , the working medium releases the heat which has arisen as a result of condensation. 
         [0031]    Subsequently, the working medium is expanded via the third fluid energy machine  19  and at the same time cools sharply. This cold, while being conducted through the cold accumulator  12 , can be released to the latter. At the same time, the working medium heats up in that it absorbs heat from the surroundings. Subsequently, said working medium can be condensed again by means of the first fluid energy machine  18 . 
         [0032]    In the event of an electricity demand, current is to be generated by a generator G. To drive the generator G, the discharging circuit  14  is set in motion. The working fluid is composed of water which is condensed via a feed pump  22 . 
         [0033]    It is subsequently conducted through the first subregion  20  of the heat accumulator  11  and absorbs the heat energy of the latter. The steam which has occurred is expanded via a high-pressure part HP of a second fluid energy machine  23  and is subsequently conducted into the second subaccumulator  21  where the steam absorbs heat again. This is sufficient to drive the low-pressure part LP of the second fluid energy machine  23 . The second fluid energy machine, in turn, drives the generator G already mentioned. 
         [0034]    After the expansion of the working fluid in the low-pressure part LP of the second fluid energy machine, the working fluid is cooled via a second heat exchanger  24  (condenser). The discharging circuit is subsequently closed in that the liquefied working fluid is fed to the feed pump  22  again. 
         [0035]      FIG. 1  illustrates that the second heat exchanger is connected to the cold accumulator  12  via the heat exchanger circuit  15 . A condenser  25  is driven by means of a motor M 2  and keeps the circuit in motion. In the cold accumulator  12 , the working fluid is cooled in the heat exchanger circuit  15  and therefore absorbs from the second heat exchanger  24  heat which the working fluid makes available in the discharging circuit  14 . 
         [0036]    Alternatively to the illustrated possibility of cooling via a heat exchanger circuit  15 , alternative embodiments may also be envisaged. For example, the heat exchanger  24  may interact with the surroundings (for example, with river water). In this case, the cold energy from the cold accumulator  12  may be utilized in another way, for example for air conditioning systems. It is also conceivable that the working fluid is conducted directly through the cold accumulator  12 . The latter then functions as a heat exchanger, so that the working fluid can release the heat directly to the cold accumulator. 
         [0037]    The states of the working fluid are illustrated in the charging circuit  13  and discharging circuit  14  in each case in circles, these circles designating specific points of the charging circuit  13  or discharging circuit  14 . In each case the prevailing pressure in the working fluid is indicated in bar at top left. The enthalpy is indicated in KJ/kg at top right. The mass flow is indicated in kg/s at bottom left and the temperature in ° C. at bottom right. The circles in the discharging circuit  14  in each case upstream of the second heat exchanger  24  and downstream of the feed pump  22  constitute an exception. Here, the steam content of the working medium is indicated and, before cooling of the heat exchanger, still amounts to 94% and subsequently condenses in the second heat exchanger (this is also designated as a condenser). The steam content upstream of the feed pump is therefore equal to 0. The steam content is indicated by x. 
         [0038]      FIG. 2  illustrates the Rankine process, known per se, in the T-S graph. Reference symbols  1  to  8  in this case refer to characteristic points of the Rankine process and are used in  FIGS. 3 to 5  at the corresponding points of the line system where said states prevail. Condensation by means of the feed pump  22  takes place from 8 to 1. From 1 to 4, the working fluid runs through the first subaccumulator  20 , the steam being superheated for the first time. After a run through the high-pressure part HP, the point  5  is reached, the run-through of the second subaccumulator  21  resulting in further superheating  6  of the working fluid. This is expanded in the low-pressure part LP, by which the point  7  is reached. By heat being released to the second heat exchanger  24 , the working fluid reaches point  8  again. 
         [0039]    In  FIG. 3 , the heat accumulator  11  is produced as a structural unit. A line system  26  of the charging circuit is indicated as a continuous line. The flow direction is indicated by an arrow. The heat accumulator possesses, for example, sand  27  as storage medium. Moreover, a first line system  28  and a second line system  29  run in the heat accumulator  11 . Here, too, the throughflow direction, which lies opposite to the throughflow direction of the line system  26 , is illustrated by an arrow. 
         [0040]    According to  FIG. 3 , it becomes clear that the first line system runs in a first subregion  30  of the heat accumulator  11 . This line system feeds the high-pressure part HP of the second fluid energy machine. The working fluid is subsequently fed into the second line system  29  which lies in a second subregion  31  of the heat accumulator  11 . The subregions  30  and  31  are contiguous to one another at an interface  32 , so that a heat exchange between the first subregion and the second subregion can take place only in this region. As a result, there arise respectively in the region of the first line system  28  and in the second subregion  31  a first heat influence zone  33  and a second heat influence zone  34  which, however, are separated from one another by the interface  32 , and in this case some heat exchange between the heat influence zones can take place solely via the interface. The interface is indicated by dashes and dots, while the heat influence zones are indicated by dashes. 
         [0041]    The heat accumulator  11  according to  FIG. 4  is constructed in a similar way to that according to  FIG. 3 . However, instead of two subregions  30 ,  31  according to  FIG. 3 , there is provision whereby the heat accumulator  11  is composed of the first subaccumulator  20  and of the second subaccumulator  21 . The effect of this is that there is no interface  32 , as shown in  FIG. 3 , between the two subaccumulators, but instead these are separated structurally from one another. The heat influence zones  33 ,  34  are therefore also completely decoupled thermally from one another. A further difference is that the subaccumulators  20 ,  21  are connected in parallel in the charging circuit. In this case, therefore, there are also, for charging, a first line system  35  and a second line system  36  in the first subaccumulator  35  and second subaccumulator  36 . These can consequently be brought to the same temperature level simultaneously during charging. 
         [0042]      FIG. 5  again illustrates a heat accumulator  11  which results in structural unit. Here, only the first line system  28  is present in the first subregion  30  (of course, in addition to the line system  26  for charging). The second line system  29  also runs, in addition to the first line system  28 , in the second subregion  31  of the heat accumulator  11 , with the result that the two line systems share one and the same heat influence zone  36 . 
         [0043]    The embodiment according to  FIG. 5  may be developed according to  FIG. 6 . The heat exchanger  11  according to  FIG. 6  has a first subregion  30 , a second subregion  31  and a third subregion  37 . The first line system runs through the heat exchanger  11  through all three subregions. The second line system runs with a first line section  38  through the subregion  30 , with a second line section  39  through the subregion  31  and with a third line section  40  through the third subregion  37 . These line sections are connected to one another in such a way that there are bypass lines  41  for each line section, so that the line sections can in each case be open to throughflow or bypassed via valves  42 . Thus, by the partial switching of the line sections, the heat accumulator can be brought individually, in each of the subregions  30 ,  31 ,  37 , to the temperature level which is necessary for superheating the working medium upstream of the low-pressure part LP of the second thermal fluid energy machine.