Method for symmetrizing an energy storage system

Abstract

In a method for balancing an energy storage system, a capacitance of capacitive storage modules of a series circuit of capacitive storage modules is determined. The capacitive storage modules are connected to a balancing device to allow control of a charge of each of the capacitive storage modules via a flow of current between the balancing device and the capacitive storage modules. For each of the capacitive storage modules a module charge is determined from a voltage of the capacitive storage module and a predefined balancing voltage. A reference charge is determined from the module charges of the capacitive storage modules, and a balancing charge is determined for each of the capacitive storage modules from the reference charge and the module charge of the capacitive storage module. The charge of the capacitive storage modules is controlled by exchanging the balancing charge between the capacitive storage module and the balancing device.

Claims

1. A method for balancing an energy storage system, comprising: determining a capacitance of individual capacitive storage modules of a series circuit of at least two capacitive storage modules, with the at least two capacitive storage modules connected to a balancing device to allow control of a charge of each of the capacitive storage modules via a flow of current between the balancing device and the capacitive storage modules; determining for each of the individual capacitive storage modules a module charge required to change the individual capacitive storage module from a voltage of the capacitive storage module to a predefined balancing voltage; determining a reference charge from the module charges of the capacitive storage modules, wherein the reference charge is formed from a maximum value of all the module charges or from an average value of all the module charges; determining for each of the individual capacitive storage modules a balancing charge from a difference between the reference charge and the module charge of the capacitive storage module; and controlling the charge of the capacitive storage modules by exchanging the balancing charge between the capacitive storage module and the balancing device.

2. The method of claim 1, wherein the capacitive storage modules have each capacitor, a double-layer capacitor or a lithium capacitor.

3. The method of claim 1, wherein the charge is the same for the capacitive storage modules, with a same current flowing through the capacitive storage modules in the series circuit.

4. The method of claim 1, wherein the balancing voltages of the individual capacitive storage modules are predefined independently of one another.

5. The method of claim 1, wherein the capacitance of each capacitive storage module is determined by a change in the voltage of the capacitive storage module and in the current through the series circuit of the capacitive storage modules.

6. The method of claim 1, further comprising connecting the capacitive storage modules to a resistor, and determining the capacitance of each capacitive storage module from the voltage of the capacitive storage module or from the current through the resistor.

7. The method of claim 1, wherein the balancing charge for the capacitive storage modules is determined from a difference between the reference charge and the module charge of the capacitive storage module.

8. The method of claim 1, wherein the balancing charge for the capacitive storage modules is determined as a function of a predefined tolerance band of the balancing voltage.

9. The method of claim 1, wherein the charges of the capacitive storage modules are exchanged between the capacitive storage modules via the balancing device.

10. The method of claim 1, wherein the balancing voltage of the capacitive storage modules is predefined as a function of a predefined maintenance interval and/or of the capacitance of the capacitive storage modules.

11. The method of claim 1, wherein the capacitance is determined cyclically.

12. A control device, comprising an input receiving a voltage of at least two capacitive storage modules of a series circuit of capacitive storage modules, with the at least two capacitive storage modules connected to a balancing device, said control device configured to generate a control command as a function of the received voltage to allow control of a charge exchange between the capacitive storage modules and the balancing device, determine for each of the individual capacitive storage modules a module charge required to change the individual capacitive storage module from a voltage of the capacitive storage module to a predefined balancing voltage; determine a reference charge from the module charges of the capacitive storage modules, wherein the reference charge is formed from a maximum value of all the module charges or from an average value of all the module charges; determine for each of the individual capacitive storage modules a balancing charge from a difference between the reference charge and the module charge of the capacitive storage module; and control the charge of the capacitive storage modules by exchanging the balancing charge between the capacitive storage module and the balancing device.

13. An energy storage arrangement, comprising: an energy storage system including a series circuit of at least two capacitive storage modules, each said capacitive storage module including a capacitor, a double-layer capacitor or a lithium capacitor; a balancing device connected to each of the capacitive storage modules such as to allow control of a charge of each capacitive storage module via a flow of current between the balancing device and the capacitive storage module; and a control device comprising an input receiving a voltage of at least two capacitive storage modules of the series circuit of the at least two capacitive storage modules, said control device configured to generate a control command as a function of the received voltage to allow control of a charge exchange between the capacitive storage modules and the balancing device, determine for each of the individual capacitive storage modules a module charge required to change the individual capacitive storage module from a voltage of the capacitive storage module to a predefined balancing voltage; determine a reference charge from the module charges of the capacitive storage modules, wherein the reference charge is formed from a maximum value of all the module charges or from an average value of all the module charges; determine for each of the individual capacitive storage modules a balancing charge from a difference between the reference charge and the module charge of the capacitive storage module; and control the charge of the capacitive storage modules by exchanging the balancing charge between the capacitive storage module and the balancing device.

14. The energy storage arrangement of claim 13, wherein both ends of the series circuit are implemented as terminals of the energy storage system.

15. The energy storage arrangement of claim 13, wherein the balancing device includes a resistor.

16. The energy storage arrangement of claim 13, wherein the balancing device includes a current source.

17. The energy storage arrangement of claim 13, wherein the series circuit of the least two capacitive storage modules includes at least one other storage module.

18. The energy storage arrangement of claim 13, wherein at least one of the capacitive storage modules includes a series circuit and/or parallel circuit of storage cells.

19. The energy storage arrangement of claim 13, further comprising at least one constructional unit including at least one capacitive storage module and at least a part of the balancing device.

20. A vehicle, comprising an energy storage arrangement, said energy storage arrangement comprising an energy storage system including a series circuit of least two capacitive storage modules, each said capacitive storage module including a capacitor, a double-layer capacitor or a lithium capacitor, a balancing device connected to each of the capacitive storage modules such as to allow control of a charge of each capacitive storage module via a flow of current between the balancing device and the capacitive storage module, and a control device comprising an input receiving a voltage of at least two capacitive storage modules of the series circuit of the at least two capacitive storage modules, said control device configured to generate a control command as a function of the received voltage to allow control of a charge exchange between the capacitive storage modules and the balancing device, determine for each of the individual capacitive storage modules a module charge required to change the individual capacitive storage module from a voltage of the capacitive storage module to a predefined balancing voltage; determine a reference charge from the module charges of the capacitive storage modules, wherein the reference charge is formed from a maximum value of all the module charges or from an average value of all the module charges; determine for each of the individual capacitive storage modules a balancing charge from a difference between the reference charge and the module charge of the capacitive storage module; and control the charge of the capacitive storage modules by exchanging the balancing charge between the capacitive storage module and the balancing device.

21. The vehicle of claim 20, constructed in the form of a bus or a rail vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described and explained in greater detail with reference to the exemplary embodiments illustrated in the accompanying drawings in which:

(2) FIG. 1 shows a first exemplary embodiment of an energy storage system,

(3) FIG. 2 shows a block diagram for the carrying-out of the method,

(4) FIG. 3 shows another block diagram for the carrying-out the method,

(5) FIG. 4, FIG. 5 show diagrams illustrating the electrical behavior of the storage modules,

(6) FIG. 6 to FIG. 8 show relationships between voltages and charges of the storage modules for different exemplary embodiments,

(7) FIG. 9 shows an exemplary embodiment of a storage module and parts of a balancing device,

(8) FIG. 10 shows another exemplary embodiment of a storage module and parts of a balancing device, and

(9) FIG. 11 shows an energy storage arrangement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(10) FIG. 1 shows an energy storage system 1 which comprises capacitive storage modules 2 and a balancing device 4. The capacitive storage modules 2 are connected in series. Each of these capacitive storage modules is connected to the balancing device 4. Both ends of the series circuit are brought out as terminals of the energy storage system 1.

(11) FIG. 2 shows a block diagram for determining the balancing charge Q.sub.sym_i. The block diagram shows means 31 for determining the capacitance means 32 for determining the module charge Q.sub.eq_i, means 33 for determining the reference charge Q.sub.ref, and means 34 for determining the balancing charge Q.sub.sym_i. The calculation steps of the means 31, 32, 34 for determining the capacitance C.sub.i, the module charge Q.sub.eq_i and the balancing charge Q.sub.sym_i are carried out for each of the individual capacitive storage modules 2. The subscript i indicates that the calculation relates to the ith capacitive storage module 2 of the at least two capacitive storage modules 2. The means 33 for determining the reference charge Q.sub.ref requires the module charges Q.sub.eq_i of the individual storage modules in order to determine the reference charge Q.sub.ref. The reader is referred at this point to FIG. 3 and the corresponding calculation of the reference charge Q.sub.ref. The module charge Q.sub.eq_i is determined using the means 32 for determining the module charge Q.sub.eq_i. The capacitance C.sub.i of the respective capacitive storage module 2, the voltage U.sub.i of the capacitive storage module 2 and the balancing voltage U.sub.eq_i of the respective capacitive storage module 2 are used as input variables for this purpose. The capacitance C.sub.i can be obtained, for example, from the energy storage system's state monitoring facility which monitors the state of aging of the individual capacitive storage modules 2. In the exemplary embodiment shown in FIG. 2, the capacitance C.sub.i of the respective storage module 2 is determined by means 31 for determining the capacitance C.sub.i. Here the voltage U.sub.i of the capacitive storage module 2 and the current i through the series circuit of storage modules 2 are specified as input variables. Alternatively, the current i through the series circuit of capacitive storage modules 2 can be dispensed with if the capacitance C.sub.i is determined via the change of the voltage U.sub.i over time during discharging of the capacitive storage module 2 via a resistance of known resistance value.

(12) The module charge Q.sub.eq_i is then supplied to the means 34 for determining the balancing charge Q.sub.sym_i. Another input variable required by the means 34 for determining the balancing charge Q.sub.sym_i is the reference charge Q.sub.ref. The reference charge Q.sub.ref is determined on the basis of the individual module charges Q.sub.eq_i, as shown in FIG. 3. In the exemplary embodiment shown, the balancing charge Q.sub.sym_i is determined by taking the difference between the reference charge Q.sub.ref and the module charge Q.sub.eq_i of the respective capacitive storage module 2. The result is the balancing charge Q.sub.sym.

(13) FIG. 3 shows the determining of the reference charge Q.sub.ref as a function of the individual module charges Q.sub.eq_i. The module charges Q.sub.eq_i of the series-connected capacitive storage modules 2 are used as input variables. In general, the number of capacitive storage modules 2 connected in series is denoted by n in this exemplary embodiment. It can be seen that in this exemplary embodiment the module charge of all the series-connected capacitive storage modules 2 is used for determining the reference charge Q.sub.ref. The reference charge Q.sub.ref can be determined as a function of the input variables of the module charges Q.sub.eq_i. It has been found to be particularly advantageous for the reference charge Q.sub.ref to be determined by the maximum value of the module charge Q.sub.eq_i. Alternatively, it has also been found to be advantageous to use the average of the individual module charges Q.sub.eq_i for determining the reference charge Q.sub.ref.

(14) FIG. 4 shows the behavior of the voltage U.sub.i of different capacitive storage modules 2 when charges Q.sub.i are added. The voltage U.sub.i increases as a function of the capacitance C.sub.i of the individual capacitive storage module 2 as charge Q.sub.i is introduced. The smaller the capacitance C.sub.i, the greater the increase in the voltage U.sub.i when a corresponding charge Q.sub.i is involved. The charge Q.sub.i results from the current i through the storage module. A corresponding current continuously increases the charge Q.sub.i of the storage module. The charge Q.sub.i is derived from the integral of the current i over time. The exemplary embodiment shown in FIG. 4 represents an energy storage system which is balanced in the operating state U.sub.i=0. In this example, the balancing voltage U.sub.eq_i is 0V. It can be seen that, without the intervention of a balancing device as the introduced charge a increases, i.e. with the flowing of current i, the voltages U.sub.i diverge because of the different capacitances C.sub.i of the individual capacitive storage modules 2.

(15) FIG. 5 shows a corresponding energy storage system 1 in which, in contrast to FIG. 4, the balancing voltage U.sub.eq_i corresponds to the maximum voltage U.sub.max. To avoid repetitions in respect of corresponding elements of the diagram, the reader is referred to the description relating to FIG. 4 and the reference characters introduced there.

(16) FIG. 6 shows the relationship between charges and voltages of the individual capacitive storage modules 2 for the balancing method. The charges of the individual capacitive storage modules 2 are indicated on the horizontal axis. The voltages U.sub.i of the individual capacitive storage modules 2 are indicated on the vertical axis. The voltages U.sub.i of the individual capacitive storage modules 2 and the corresponding balancing voltage U.sub.eq_i are indicated. In the event that the capacitance C.sub.i of the individual capacitive storage modules 2 is of equal size, the length of the arrows constitutes a measure for the corresponding charge. In general, however, the capacitance C.sub.i of the individual capacitive storage modules 2 differs, so that the length of the arrows is then not a measure for the corresponding module charge Q.sub.eq_i. The greater the capacitance C.sub.i of a capacitive storage module 2, the more charge is required to vary the corresponding voltage of the storage module.

(17) In this exemplary embodiment, the balancing shall take place using the balancing voltage U.sub.eq_i for the individual capacitive storage modules 2. For explanation of the principle, as can be seen from the diagram, the balancing voltage U.sub.eq_i is set the same for all the capacitive storage modules 2. In general, the balancing voltage U.sub.eq_i of the individual storage modules 2 can be set independently of one another. In the present operating state, the individual capacitive storage modules 2 have different voltages U.sub.i. In a first step, the charges Q.sub.eq_i required to bring the individual capacitive storage modules 2 to the voltage U.sub.eq_i must be determined.

(18) The reference voltage Q.sub.ref is determined from the individual module charges Q.sub.eq_i. In this exemplary embodiment, the reference charge U.sub.ref is determined from the maximum value of the individual module charges Q.sub.eq_i. In this example, the module charge Q.sub.eq_3 therefore constitutes the reference charge Q.sub.ref. The reference charge Q.sub.ref influences the voltage of the individual capacitive storage modules 2 during charging/discharging of the energy storage system 1 by the current i of the series circuit. The then resulting unequal voltages must be equalized by the balancing charges Q.sub.sym_i. This is shown in the diagram in that the effect of the reference charge Q.sub.ref is influenced by the balancing charge Q.sub.sym_i such that the voltages of the individual capacitive storage modules 2 assume the balancing voltage U.sub.eq_i. This exemplary embodiment is particularly suitable for a balancing device 4 which can remove charge from the individual capacitive storage modules 2 by means of one or more resistors. Selecting the reference charge Q.sub.ref as the maximum of the individual module charges Q.sub.eq_i produces only balancing charges Q.sub.sym_i which point downward in FIG. 6. These downward pointing balancing charges represent charges which are to be removed from the capacitive storage modules 2. This can take place in a simple manner by means of a resistor. In this exemplary embodiment, it is not necessary for charges to be supplied to a capacitive storage module 2. It can be seen from the diagram that there is no balancing charge Q.sub.sym_i which constitutes an upward pointing arrow in FIG. 6.

(19) FIG. 7 shows another exemplary embodiment for the balancing method. In this example, a tolerance band U.sub.eq_i for the balancing voltage is specified in addition to the balancing charge U.sub.eq_i. To avoid repetitions in respect of corresponding elements of the diagram, the reader is referred to the description relating to FIG. 6 and the reference characters introduced there. From the predefinition of the tolerance band U.sub.eq_i, the individual balancing charges Q.sub.sym_i can come out smaller in absolute terms. The electrical losses resulting from the flow of current during balancing, in particular by the resistor, are therefore reduced. In the case of the second capacitive storage module 2, the balancing device 4 does not need to be active, as, due to the reference charge Q.sub.ref, the voltage of the second capacitive storage module 2 is already within the tolerance band U.sub.eq_i for the balancing voltage U.sub.eq_i. This exemplary embodiment is also particularly suitable for balancing by means of one or more resistors.

(20) FIG. 8 shows another diagram illustrating the electrical behavior of storage modules. The reference charge Q.sub.ref is here the average of the individual module charges Q.sub.eq_i. This produces both balancing charges Q.sub.sym_i, whereby charge must be removed from the capacitive storage module 2, and balancing charges Q.sub.sym_i, whereby charge must be added to the capacitive storage module. The balancing charges Q.sub.sym_i to be removed are indicated in the diagram by the corresponding balancing charge Q.sub.sym_i being represented by a downward pointing arrow. This is the case for the capacitive storage modules having the number 1 and the number 2. In the case of the capacitive storage modules having the number 3 and the number n, the upward pointing arrow of the balancing charge Q.sub.sym_i shows that charge must added to these corresponding capacitive storage modules 2. To avoid repetitions in respect of corresponding elements of the diagram, the reader is once again referred to the description relating to FIGS. 6 and 7 and the reference characters introduced there. The example shown in FIG. 8 is particularly suitable for a design comprising current sources for balancing the individual capacitive storage modules 2. Here it has been found particularly advantageous for the current source to be implemented as a bidirectional current source. The reason for this is that charge not only has to be removed from the individual capacitive storage modules 2 but charge also has to be added to the individual capacitive storage modules 2. A suitable energy source is necessary for this purpose. It has been found to be particularly advantageous to use the method described here when the possibility of charge reversal of individual capacitive storage modules 2 is present. For this charge reversal, charge of individual capacitive storage modules 2 which are to be discharged is fed to other capacitive storage modules 2 to which a corresponding balancing charge Q.sub.sym_i must be added. In the event that the charge to be released and the charge to be received by the capacitive storage modules 2 are of equal size in total, no energy has to be supplied to the balancing device 4 from outside. The corresponding supplying with charge can take place by charge reversal between the individual capacitive storage modules 2. As a result, no electrical losses or at least only small electrical losses therefore arise during balancing.

(21) FIG. 9 shows an exemplary embodiment of a capacitive storage module 2 and parts of a balancing device 4. The capacitive storage module 2 comprises a series circuit of storage cells 3. The storage cells 3 are capacitors, for example. The voltage U.sub.i is present at the terminals of the capacitive storage module. The current i flows through the series circuit and therefore also the capacitive storage module 2 shown in FIG. 9. In the exemplary embodiment shown, the capacitive storage module 2 is connected to a resistor 5 and a final control element 7 of the balancing device 4. Here the final control element 7 is a switch. It has been found to be advantageous to provide for each capacitive storage module 2 a corresponding resistor 5 and a final control element 7. Charge can be removed from the capacitive storage module 2 via the final control element 7, the switch, by means of the resistor 5. The final control element 7 is controlled by a corresponding closed-/open-loop control system which is not shown in this exemplary embodiment.

(22) FIG. 10 shows another exemplary embodiment of a storage module and parts of a balancing device 4. To avoid repetitions in respect of corresponding elements of the diagram, the reader is referred to the description relating to FIG. 9 and the reference characters introduced there. In this example the balancing device 4 has a current source 6 which is simultaneously used as a final control element 7. This current source is electrically connected to the capacitive storage module 2. In contrast to the arrangement with resistor, in this design of the capacitive storage module 2, charge is not only removed but also added. In addition, the current between capacitive storage module 2 and current source 6 can be predefined independently of the voltage U.sub.i of the capacitive storage module 2. It has been found to be advantageous for each capacitive storage module 2 to be connected to a respective current source 6. If charge exchange between the current sources 6 is additionally possible, an exchange of charges between individual capacitive storage modules 2 can hereby take place. This constitutes the possibility of implementing a charge reversal circuit. This enables electrical losses in the energy storage system 1 to be reduced.

(23) FIG. 11 shows an energy storage arrangement 12. This comprises an energy storage system 1 and a control device 10. The energy storage system 1 again comprises a series circuit of capacitive storage modules 2. This series circuit additionally has other storage modules 21. These other storage modules 21 can likewise be additional capacitive storage modules 21 or also batteries, flywheel storage devices or other electrical or chemical energy storage devices. In principle, all energy storage devices can be used as another storage module 21. The capacitive storage modules 2 are in each case electrically connected to the balancing device 4. The control of the charge exchange between capacitive storage module 2 and balancing device 4 is performed by the control device 10. For this purpose, state variables are transmitted from the capacitive storage modules 2 to the input 11 of the control device 12. From this information, control commands for at least one final control element 7 of the balancing device 4 are determined. The final control element 7 can be a switch which enables current to flow through a resistor 7, or a current source 6. The control device 10 can be disposed both inside the energy storage system 1 or, as shown, outside the energy storage system 1. It has been found to be particularly advantageous for the voltage U.sub.i of the capacitive storage modules to be transmitted to the input 11 of the control device 10 as a state variable. This enables the described balancing method to be carried out reliably. In addition, it has further been found advisable to make the current i through the series circuit of the storage modules 2, 21 available to the control device 10. For this purpose a corresponding measured value of the current i must be supplied to the input 11 of the control device 10. This can be used to monitor the functional capability of the energy storage system or also to determine the capacitance C.sub.i of the individual capacitive storage modules.

(24) Although the invention has been illustrated and described in detail by the preferred exemplary embodiments, the invention is not limited solely to the examples disclosed and other variations may be inferred therefrom by persons skilled in the art without departing from the scope of protection sought for the invention.