Patent Publication Number: US-2015075210-A1

Title: Method for charging and discharging a heat accumulator and plant for storing and releasing thermal energy, suitable for this method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2013/056659 filed 28 Mar. 2013, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP12164480 filed 17 Apr. 2012. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to method for charging and discharging a heat accumulator, in which the following steps are carried out, preferably in an alternating manner. During a charging cycle, the heat accumulator is heated by means of a working fluid, wherein before passing through the heat accumulator a pressure increase is created in the working fluid by means of a first thermal fluid energy machine which is operated as a working machine, and after passing through the heat accumulator the working fluid is expanded. During a discharging cycle, the heat accumulator is cooled by the same, or another, working fluid, wherein before passing through the heat accumulator a pressure increase is created in the working fluid and after passing through the heat accumulator the working fluid is expanded via a second thermal fluid energy machine which is operated as a power machine, or via the first thermal fluid energy machine which is operated as a power machine. 
     The invention also relates to a plant for storing and releasing thermal energy using a heat accumulator, wherein the heat accumulator can release the stored heat to a charging circuit for a working fluid and to a discharging circuit for another, or the same, working fluid. In the charging circuit, the following units are interconnected in the specified sequence by means of lines: a first thermal fluid energy machine which is operated as a working machine, the heat accumulator, a device for expanding the working fluid, especially a first throttle, and a first heat exchanger. In the discharging circuit, the following units are interconnected in the specified sequence by means of lines: the heat accumulator, a second thermal fluid energy machine which is operated as a power machine, or the first thermal fluid energy machine which is operated as a power machine, the first heat exchanger or a second heat exchanger, and a pump. The method which is specified in the introduction, or the plant which is suitable for implementing the method, can, for example, be used in order to convert over-capacities from the electricity network by means of the charging cycle into thermal energy and to store said thermal energy in the heat accumulator. When required, this process is reversed so that the heat accumulator is discharged in a discharging cycle and by means of the thermal energy electric current can be generated and fed to the network. 
     BACKGROUND OF INVENTION 
     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. In this case, the thermal fluid energy machine is therefore operated as a motor. 
     The term “thermal fluid energy machine” constitutes a generic term for machines which can extract thermal energy from a working fluid 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. 
     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 described above. 
     According to U.S. Pat. No. 5,436,508, it is known, moreover, that by means of the plants specified in the introduction for storing thermal energy over-capacities can also be temporarily stored utilizing wind energy for generating electric current in order to retrieve this again when required. 
     SUMMARY OF INVENTION 
     An object is to disclose a method for charging and discharging a heat accumulator or a plant for implementing this method, by means of which storing and recovery of energy can be carried out with comparatively high efficiency and a comparatively low cost of components is incurred in the process. 
     This object is achieved according to the invention by means of the method referred to in the introduction by both the charging cycle and the discharging cycle being designed as a Rankine process, in which the working fluid is evaporated during the charging cycle via a first heat exchanger and is condensed during the discharging cycle via the first or a second heat exchanger. In the process, the first heat exchanger, and if the second heat exchanger is present, this also, creates, or create, a temperature balance with environment. In this case, it is to be noted that depending on whether separate circuits are provided for the heat exchangers in the case of the discharging cycle or of the charging cycle or both cycles take place in one and the same circuit, one or two heat exchangers can be provided. The same applies to the fluid energy machines. The advantage of using two different fluid energy machines has the advantage that the one can be optimized to the charging cycle and the other to the discharging cycle. As a result of this, especially the aim of an increase in the overall efficiency is achieved. If a single fluid energy machine is used, expectations regarding efficiency certainly have to lowered, for which such a method may be implemented with a less expensive plant since a saving can be made in components. 
     The Rankine process, which synonymously is also referred to as the Clausius-Rankine process, can especially be operated using a steam-heat pump or using a steam turbine. The working medium exists in this case in a gaseous state and a liquid state alternately, as a result of which the specific cyclic working process can be advantageously increased. The features of the process, referred to as Rankine process for short in the following text, are explained in more detail below. 
     A further essential feature of the invention is that an exchange of heat with the environment is provided for the heat exchanger. The environment is to be understood as being a part outside of the running process. The heat exchanger can be used both for absorbing and releasing thermal energy if the running process is set so that the working fluid can be evaporated by absorbing heat from the environment in order to be able to then be compressed via the first fluid energy machine, and in the case of the discharging cycle can be condensed by releasing heat to the environment after the working fluid has performed work via the second fluid energy machine. This can be carried out by choosing ammonia or water as working fluid, for example. In this case, ammonia has the advantage that for example at an ambient temperature of 15° C. a superheating of the ammonia vapor can be ensured. The choice of water as working fluid has the advantage that its use involves low risks for the environment. 
     According to an alternative solution of the problem, it can also be provided in the method specified in the introduction that both the charging cycle and the discharging cycle are designed as the Rankine process, in which the working fluid is evaporated during the charging cycle via a third heat exchanger and is condensed during the discharging cycle via a second heat exchanger. Furthermore, during the charging cycle the third heat exchanger is heated by means of another working fluid with lower boiling point, wherein before passing through the heat exchanger a pressure increase is created in the other working fluid by means of a third thermal fluid energy machine which is operated as a working machine, and after passing through the heat exchanger the other working fluid is expanded. In this case, it is provided according to the invention that the first heat exchanger and the second heat exchanger create a temperature balance with the environment. 
     By means of the alternative solution according to the invention, the advantages already quoted above are achieved. An exchange of heat with the environment is also possible, as a result of which a saving can be made in components. The advantage of ammonia as working fluid being able to be dispensed with and therefore water being able to be used in the circuit of the third heat exchanger without the possibility of superheating having to be dispensed with, can additionally be achieved. This is achieved by means of a two-stage charging cycle, wherein the charging cycle can, for example, be advantageously operated with carbon dioxide in the first heat exchanger which is in communication with the environment. In this case, it also involves a substance the use of which is harmless with regard to risks to the environment. This, however, can already be superheated at lower temperatures, wherein by conducting the Rankine process in the carbon dioxide circuit heating of the third heat exchanger is a carried out. The energy which is made available by the third heat exchanger lies above the temperature level of the environment, however, so that in the water circuit a superheating of the water vapor can be carried out under technically realizable pressure conditions. 
     A particularly advantageous embodiment of the invention is achieved if water is used as a heat transfer medium from the environment. This water can be extracted from a river, for example. This has the advantage that water, especially flowing water, is subjected to smaller temperature fluctuations than the air, for example. As a result, the process can be conducted both in summer and winter within a smaller temperature window. Furthermore, the water can be introduced into the first or the second heat exchanger in a simple manner. 
     An object is otherwise achieved according to the invention by means of the plant specified in the introduction by the first heat exchanger, and if the second heat exchanger is present, this also, ensuring an exchange of heat with the environment. This advantageously enables the plant to make the method specified above realizable. The aforesaid advantages therefore correspondingly apply. 
     The same also applies to the alternative solution of the problem by means of the plant specified in the introduction by the following units being interconnected in an additional circuit in the specified sequence by means of lines: a third thermal fluid energy machine which is operated as a working machine, the third heat exchanger, a device for expanding the working fluid, especially a second throttle, and a first heat exchanger. The first heat exchanger and the second heat exchanger ensure an exchange of heat with the environment of the plant, as a result of which the advantages already mentioned above can be achieved and the plant is especially put in a position to implement one of the previously specified methods. 
     According to an advantageous embodiment of the plant, it can be provided that the charging circuit and the discharging circuit extend through the same lines, at least in certain sections. Meant by this is that there is passage of flow through the lines both in the discharging cycle and in the charging cycle. This can therefore be provided because the plant is always used either only for storing thermal energy or for releasing thermal energy by means of the heat accumulator (corresponds to the charging cycle and to the discharging cycle). This is therefore to be based on the fact that either the state exists in which surplus energy is available for charging the heat accumulator or the requirement arises in which the stored energy from the heat accumulator is to be converted into electric energy, for example. An operation both of the charging cycle and of the discharging cycle at the same time is technically therefore not advisable. By using the same lines, at least in certain sections, a saving in material is advantageously made and the cost of components is further reduced. In particular, it is possible to realize the charging and discharging circuit completely by means of the same line system if the same thermal fluid energy machine is also to be used for charging and discharging. Otherwise, different fluid energy machines and necessary throttles and pumps can be integrated into the circuit by means of suitable bypass lines and valves. 
     It is especially advantageous if the same lines are provided for the charging circuit and the discharging circuit, at least inside the heat accumulator. This substantially reduces the cost for producing the heat accumulator since in this a surface which is as large as possible has to be provided for a transfer of heat through the lines. Also, the volume, which when using two line systems has to be provided in the heat accumulator for one of the line systems, can be filled with the heat accumulator medium in the case of using one line system, as a result of which a more compact constructional form is advantageously possible. 
     If the plants or methods according to the invention are compared with those according to WO 2009/044139 A2, then it becomes clear, moreover, that a saving can be made with regard to the complete cold accumulator. This is achieved by the low temperature level of the respectively running Rankine process lying at ambient level so that the environment can be utilized as a cold accumulator. This additionally has the advantage that the thermal energy which is provided by the environment can be introduced into the process. Furthermore, the component cost of a cold accumulator no longer applies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. In the drawing: 
         FIG. 1  shows an exemplary embodiment of the plant according to the invention as a block schematic diagram, 
         FIGS. 2 and 3  show two exemplary embodiments of the method according to the invention, which can be implemented with a plant according to  FIG. 1 , 
         FIG. 4  shows an alternative exemplary embodiment of the plant according to the invention as a block schematic diagram, and 
         FIGS. 5 and 6  show an exemplary embodiment of the method, which can be implemented with the plant according to  FIG. 4 , wherein the methods are represented in a diagram with the temperature T on the Y-axis and the entropy S on the X-axis. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Apparent in  FIG. 1  is a plant by means of which surplus electric energy of a wind power plant  11  can be converted into thermal energy which can be stored in a heat accumulator  12 . The heat accumulator, in a way not shown in more detail, can comprise a cast body of concrete, for example, in which provision is made for a passage system through which can flow a working medium. The electric energy of the wind power plant  11  is fed to the plant via a motor M. When required, the heat accumulator  12  can be discharged, wherein ultimately electric energy can be generated via a generator G. For converting electric energy into thermal energy, and vice versa, a charging circuit  13  and a discharging circuit  14  are realized in the plant. These are schematically represented as lines in which a working fluid, such as ammonia, can circulate. Via valves  15 , either the charging circuit  13  or the discharging circuit  14  respectively is activated by a connection being made to the heat accumulator  12 . Furthermore, provision is made in the charging and discharging circuits for a first heat exchanger  16  which can be fed with flowing water of a schematically indicated river  17 . Also, valves  18  are provided for the connection of the first heat exchanger  16  so that this can be used in the charging circuit  13  and in the discharging circuit  14 . Instead of valves  18 , a second heat exchanger  19  can also be used, as shown by dash-dot lines. In the region of the heat exchangers, two separate circuits for the charging circuit  13  and discharging circuit  14  are created in this way, as indicated in  FIG. 1 , which makes the valves  18  superfluous. 
     The respective flow directions of the charging circuit  13  and of the discharging circuit  14  are indicated by arrows. Also, typical positions in the charging circuit  13  and discharging circuit  14  are identified by numbers between 1 and 10, wherein these typical positions of the running Rankine processes are also apparent in  FIGS. 2 and 3 . These shall explain in more detail below the conducting of the respective process. 
     Shown in  FIG. 2  are a charging cycle  20  and a discharging cycle  21 , as can be conducted using ammonia as working fluid (R717). In position  1  of the cycle, the working medium is at a pressure of 5 bar. In this case, the boiling temperature of ammonia lies at 4° C. Therefore, the heat of the river water at 15° C. can be used in order to evaporate the working medium in the first heat exchanger  16 . In this way, position  2  is reached. As is to be gathered from  FIG. 1 , the working medium, by means of a first fluid energy machine  22  which is operated as a hydrodynamic compressor, with the aid of the motor M, is brought to a pressure of more than 131 bar, for example 140 bar. In the process, the working medium is heated to 320° C. and reaches position  3 . This heat can then be introduced into the material of the heat accumulator  12 , wherein the latter functions as a heat exchanger. In this process step, the working medium is isobarically cooled to a temperature of less than 30° C., as a result of which position  4  of the cycle is reached. By means of a first throttle  23 , the working medium can be expanded and in this way achieves a pressure of 5 bar again. At this position, the simple component of a throttle is advantageously sufficient. A turbine or the like is not necessary. 
     The discharging cycle  21  proceeds as follows. The condensation pressure can be set at 10 bar so that the boiling temperature of the working medium (also ammonia) lies at 25° C., that is to say above the temperature level of the river at 15° C. In position  5  of the discharging cycle  21 , ammonia exists in the liquid state and is brought to a supercritical pressure via a pump  24 . The working medium is heated by means of the heat accumulator and supercritically brought to position  9 . In this case, the temperature level prevailing in the heat accumulator  12  cannot quite be achieved. Heating to 220° C., for example, is possible. From the working medium in the supercritical state, mechanical energy can be generated via a second fluid energy machine  25  in the form of a turbine and converted into electric energy via the generator G. The mechanical connections between the generator G and the second fluid energy machine  25  and also between the motor M and the first fluid energy machine  22  are designed as shafts  26 . After expansion of the working medium, position  10  is reached. The expanded working medium still exists in a gaseous state and is condensed at 25° C., wherein the river water is heated in the process. 
     Shown in  FIG. 3  is a charging cycle  20  and a discharging cycle  21 , as can be operated with a plant according to  FIG. 1 , using water. In this case, river water at a temperature of 15° C. is again to be used in the first heat exchanger. If the water is to be condensed at a temperature of 25° C., a condensation pressure of 30 mbar is to be expected. An evaporation temperature of the water of 5° C. requires a pressure of 10 mbar. The following parameters for operating the charging cycle result from this. From position  1  to position  2 , the water is evaporated at 10 mbar. Position  3  of the charging cycle is achieved by the water vapor being compressed to 1 bar, wherein the temperature rises to approximately 540° C. The water is then cooled to 99° C. during passage through the heat accumulator  12 , as a result of which the heat accumulator  12  is heated. During this, position  4  is reached. The water vapor is expanded via the throttle  23  to 10 mbar and in the process achieves the temperature of 5° C. again. 
     The charging cycle proceeds through the following positions. In position  5 , the water, which is now fully condensed at 30 mbar, has a temperature of 25° C. By means of the pump  24 , the water is brought to a working pressure and transported through the heat accumulator  12 , absorbs heat in the process, and reaches position  7 . In so doing, the water starts to boil and in the process maintains the temperature in position  7  until this is completely evaporated (position  8 ). In this case, it involves a subcritical evaporation of the water. The temperature level in the heat accumulator  12  then leads to superheating of the water vapor, reaching position  9  at approximately 480° C. Position  10  is then reached by the water vapor being expanded via the second fluid energy machine  25 , the water temperature again achieving a temperature of 25° C. at a pressure of 30 mbar in the process. In the first heat exchanger  16 , the water is then condensed again, as a result of which position  5  of the discharging cycle is reached. 
     Shown in  FIG. 4  is another exemplary embodiment of the plant. This differs from the exemplary embodiment according to  FIG. 1  mainly by the fact that the charging cycle is split into two stages. The heating of the heat accumulator  12  takes place in the charging circuit  13  which, unlike as in  FIG. 1 , is completely separated from the discharging circuit  14 . The charging circuit  13  with the first throttle  23 , the heat exchanger  12  and the first fluid energy machine  22 , and the discharging circuit with the pump  24 , the heat accumulator  12 , the second fluid energy machine  25  and the second heat exchanger  19 , are constructed in a way similar to  FIG. 1 . However, the heat accumulator  12  has two passage systems which are independent of each other for the charging circuit and the discharging circuit in each case, which are not shown in more detail in  FIG. 4 . 
     The essential difference between  FIG. 4  and  FIG. 1 , however, lies in the fact that a third heat exchanger  27  is used in the charging circuit  13 . This is not supplied by river water at ambient temperature for the purpose of heat exchange but is connected to an additional circuit  28 . The additional circuit  28  has the following functions. In addition to the motor M 1 , which supplies the first fluid energy machine  22 , provision is also made for a motor M 2  which via a shaft  26  drives a third fluid energy machine  29 . This is provided in the additional circuit  28  and compresses the working fluid, for example carbon dioxide, which is heated as a result, and releases the heat in the third heat exchanger  27  to the working fluid (for example water) of the charging circuit  13 . The working fluid of the additional circuit  28  is then expanded via a second throttle  30  and condensed via the first heat exchanger  16  which releases its heat to the river  17 . 
     The positions  1 ′,  2 ′,  3 ′ and  4 ′ are inscribed in  FIG. 4  and give the typical positions of the Rankine process which is shown in  FIG. 5 . In this case, it involves an alternative charging cycle  20   a  which represents the first stage of the two-stage charging process according to  FIG. 4 . From position  1 ′ to position  2 ′, the carbon dioxide is evaporated at a temperature of 5° C. and a pressure of 40 bar. In this case, the necessary energy comes from the river  17  and is introduced via the first heat exchanger  16 . Via the third fluid energy machine  29 , the carbon dioxide is compressed to 97 bar and achieves a temperature of 80° C. (position  3 ′). This heat can be released via the third heat exchanger  27  to the charging circuit  13 , wherein a temperature window of between 35 and 80° C. is made available for this. After cooling of the working medium to 35° C., position 4′ is reached, from where a pressure of 40 bar is achieved (position  1 ′) as a result of expansion of the carbon dioxide via the second throttle  30 . 
     The temperature level of 35° C. in the third heat exchanger makes it possible to operate with water as working medium in the charging circuit  13  at a level which differs from that in the case of  FIG. 3 . The condensation can be carried out at 30 mbar at a temperature of 25° C. (position  10  to position  5 ). The temperature is therefore higher than in the case of  FIG. 3 . On the other hand, the discharging cycle can be conducted similar to the case according to  FIG. 3  (position  1  to position  2 ). The temperature level for the discharging cycle is therefore also predetermined by the river  17  at 15° C. 
     In principle, the method according to the invention is not restricted to the working fluids which are specified in the exemplary embodiments. For example, hydrocarbons, such as propane, can also be used. If the exemplary embodiments of the plants according to  FIGS. 1 and 4  are compared, their elements can also be combined with each other. For example, the charging circuit  13  and discharging circuit  14  according to  FIG. 4  can also be realized according to  FIG. 1  with partially the same lines and valves  15 , as a result of which only one passage system has to be provided in the heat accumulator  12 . In this case, however, the second heat exchanger  19  is required in  FIG. 1 , whereas the first heat exchanger  16  according to  FIG. 1  would have to be replaced by the third heat exchanger  27  and the additional circuit  28  with all the components.