Methods for operating fuel cell systems

Abstract

The present invention relates to a method for operating a fuel cell system having a fuel cell that supplies air compressed by an electrically driven air compressor to the fuel cell, the operating range of which can be represented in a family of characteristic curves (20) having a surge limit (26) and a choke limit (28) and stored in a control device of the electrically driven air compressor. To extend the service life of the fuel cell system, at least one current signal is detected during operation of the electrically driven air compressor and compared with a pre-surge limit value (29) stored in the family of characteristic curves (20), wherein the electrically driven air compressor is specifically operated such that the surge limit value also stored in the family of characteristic curves (20) is not reached during operation of the electrically driven air compressor.

Description

Technical Field

[0001] This invention relates to a method for operating a fuel cell system having a fuel cell, wherein compressed air, delivered to the fuel cell by an electrically driven air compressor, is supplied with air whose operating range can be represented in a family of characteristic curves having surge limits and stoppage limits, and which are stored in a control device of the electrically driven air compressor. The invention also relates to a fuel cell system of this type. Background Technology

[0002] A fuel cell system with a compressor is known from US Patent 7,771,883 B2, which generates a ladluftstrom of compressed air. The compressor's characteristic curves are stored in the engine's control unit, which determines the compressor's outlet pressure and temperature. The system operates at a position determined by the compressor's rotational speed and airflow signals from a mass flow meter to prevent the compressor from entering a surge state. Summary of the Invention

[0003] The objective of this invention is to simplify the operation of fuel cell systems.

[0004] This task is addressed in a method for operating a fuel cell system having a fuel cell that supplies air compressed by an electrically driven air compressor. The operating range of the air compressor can be represented in a family of characteristic curves, which includes surge limits and choke limits and are stored in a control device for the electrically driven air compressor. During operation of the electrically driven air compressor, at least one current signal is detected and compared with a pre-surge limit value (Vorpumpgrenzwert) stored in the family of characteristic curves. The electrically driven air compressor is selectively manipulated such that the surge limit value, also stored in the family of characteristic curves, is not reached during its operation. The air compressor can also be referred to as a gas compressor. In the claimed method, proactive measures are taken to avoid reaching the surge limit during operation of the electrically driven air compressor. This can significantly increase the lifespan of the fuel cell system. Here, the current signal is selectively modified such that the surge limit is not reached during the operation of the air compressor by controlling the electrically driven air compressor through the current signal.

[0005] A preferred embodiment of this method is characterized by plotting the pressure ratio as a function of the current ratio, along with the operating point of the electrically driven air compressor, within a stored characteristic curve graph. Advantageously, this family of characteristic curves is created during testing of the fuel cell system before it is used with the electrically driven air compressor operating in series. The family of characteristic curves effectively reduces the control and regulation overhead during the operation of the fuel cell system.

[0006] Another preferred embodiment of the method is characterized in that the pre-surge limit curve is separated from the surge limit by a safe distance range. By appropriately selecting the size of the safe distance range, it is possible to achieve, with relatively little overhead, that the pre-surge limit is never or almost never reached during the operation of the electrically driven air compressor.

[0007] Another preferred embodiment of the method is characterized by using a family of characteristic curves to determine the location of the operating point of the electrically driven air compressor relative to its surge limit. Then, it can be determined, with low overhead and simple comparison, whether the aforementioned forward-looking measures are necessary.

[0008] Another preferred embodiment of the method is characterized by altering the current signal to match the operating point of the electrically driven air compressor relative to the surge limit. This can be done iteratively or incrementally, if necessary, to prevent reaching the surge limit without significant efficiency loss.

[0009] Another preferred embodiment of the method is characterized in that the electrically driven air compressor includes a permanently excited synchronous motor. Advantageously, the permanently excited synchronous motor is operated with different currents, current signals, or different frequencies to prevent surge limits from being reached during operation of the electrically driven air compressor.

[0010] Alternatively or additionally, the above-described task is addressed in a fuel cell system having a fuel cell that supplies air compressed by an electrically driven air compressor. The operating range of the air compressor can be represented in a family of characteristic curves, which includes surge limits and choke limits and is stored in the control unit of the electrically driven air compressor. This family of characteristic curves includes a pre-surge limit curve, which depends on the current ratio and pressure ratio of the electric drive of the air compressor. Thus, reaching the surge limit during operation of the fuel cell system can be reliably prevented in a simple manner and method.

[0011] A preferred embodiment of this fuel cell system is characterized in that the electric drive of the air compressor includes a permanently excited synchronous motor that is operated at different frequencies according to the position of the operating point in a family of characteristic curves. This prevents surge limits from being reached during the operation of the electrically driven air compressor without requiring significant overhead in control or regulation technologies.

[0012] The present invention also relates to a computer program product having a computer program having software units that, when implemented on a computer, perform the aforementioned methods. For example, the computer is a control device for an electric drive of an air compressor in a fuel cell system.

[0013] Alternatively or additionally, the task described above is addressed in a family of characteristic curves for a fuel cell system having a fuel cell that supplies air compressed by an electrically driven air compressor to the fuel cell, the operating range of which can be represented in the family of characteristic curves having surge limits and choke limits and stored in the control device of the electrically driven air compressor: plotting the pressure ratio as a function of the current ratio, along with the operating point of the electrically driven air compressor, and along with the pre-surge limit curve, in the characteristic curves. Attached Figure Description

[0014] Other advantages, features and details of the invention will become apparent from the following description, in which various embodiments are described in detail with reference to the accompanying drawings.

[0015] The attached diagram shows:

[0016] Figure 1 A schematic diagram of a fuel cell system is shown, which includes a fuel cell, an electrically driven air compressor, and a pressure sensor device.

[0017] Figure 2 It shows the relationship with Figure 1 A similar fuel cell system, which does not have a pressure sensor device, but has a measuring device for measuring the mass flow of air supplied to the fuel cell;

[0018] Figure 3 A family of characteristic curves with surge limit, choke limit, and pre-surge limit is shown, stored in [the database]. Figure 1 and Figure 2 In the control device of the fuel cell system;

[0019] Figure 4 A Cartesian coordinate graph is shown, which plots the change of the current signal over time; and

[0020] Figure 5 It shows the relationship with Figure 4 A similar diagram is shown, which features a changing current signal. Detailed Implementation

[0021] exist Figure 1 and Figure 2 The figure schematically illustrates a fuel cell system 1 having a fuel cell 2 and an electrically driven air compressor 3. The same reference numerals are used to indicate the same or similar parts. The fuel cell system 1 is constructed in the same or similar manner as the fuel cell system disclosed in the opening evaluation of U.S. Patent 7,771,883 B2.

[0022] via arrow 4 Figure 1 and Figure 2 The image shows the air supplied to the air compressor 3 via the air filter 5. The air compressor 5 is driven by an electric drive 6. The electric drive 6 is implemented as a permanently excited synchronous motor 8 with a control device 7.

[0023] A valve device 9 is arranged at the air outlet of fuel cell 2. The aforementioned features are... Figure 1 and Figure 2 The same implementation is shown in fuel cell system 1. The following discussion focuses on... Figure 1 and Figure 2 The differences between the fuel cell systems 1 shown.

[0024] Figure 1 The fuel cell system 1 shown includes a pressure sensor device 10, which detects the pressure of the compressed air supplied to the fuel cell 2 by the air compressor 3. Arrow 11 indicates that the control device 7 of the electric drive 6 is connected to the pressure sensor device 10 for control purposes. Arrow 12 indicates that the control device 7 is connected to the valve device 9 for control purposes.

[0025] exist Figure 2 In the fuel cell system 1 shown, instead of a pressure sensor device ( Figure 1 A measuring device 14 is provided in section 10) to detect the air mass flow between the air filter 5 and the air compressor 3. (See arrow 15.) Figure 2 The diagram shows that the control device 7 of the electric drive 6 is connected to the measuring device 14 in terms of control.

[0026] exist Figure 3 In the Cartesian coordinate diagram, an air compressor used for operation of an electric drive is shown. Figure 1 and Figure 2 The characteristic curve family 20 of section 3). Plot I relative to I on the x-axis 21.max The current ratio. P is plotted on the y-axis relative to p. max The pressure ratio.

[0027] In characteristic curve family 20, three operating points 23, 24, and 25 during the operation of the air compressor are exemplaryly marked. Characteristic curve family 20 includes surge limit 26 and choke limit 28. Characteristic curve family 20 also includes pre-surge limit curve 27. Pre-surge limit curve 27 is spaced apart from surge limit 26.

[0028] exist Figure 4 and Figure 5 Two additional Cartesian coordinate plots are shown. Time is plotted in appropriate time units on the x-axis (31). The frequency of the current signal per unit time is plotted on the y-axis (32). The two graphs illustrate the variation of two current signals (33, 34) with different frequencies.

[0029] Operation of the air compressor 3 (also known as the compressor) near the surge limit 26 should be avoided. The control of the permanently excited synchronous motor 8 of the electric drive 6 of the air compressor 3 is achieved by means of a regulated rotating field for current, as shown in... Figure 4 and Figure 5 As indicated in the diagram. Then, the current of the permanently excited synchronous motor 8 and the permanent magnet generate mechanical torque for compressing the drawn-in air 4.

[0030] The operating points 23 to 25 of the air compressor 3 are determined by pressure and temperature, or directly by the measured air mass flow of the air compressor 3 and the known frequencies of the current signals 33 and 34. The corresponding operating point (e.g., 24) is then compared with the family of characteristic curves 20 stored in the control device 7.

[0031] If the operating point is near the pre-surge limit curve 27, the frequency of the rotating field can be matched to maintain an appropriate safe distance from the surge limit 26.

Claims

1. A method for operating a fuel cell system (1), the fuel cell system having a fuel cell (2), supplying air (4) compressed by an electrically driven air compressor (3) to the fuel cell, the operating range of the air compressor being representable in a family of characteristic curves (20), the family of characteristic curves having a surge limit (26) and a choke limit (28) and stored in a control device (7) of the electrically driven air compressor (3), wherein, At least one current signal (33, 34) is detected during the operation of the electrically driven air compressor (3) and the at least one current signal is compared with a pre-surge limit value (29) stored in the family of characteristic curves (20), wherein the electrically driven air compressor (3) is manipulated in such a targeted manner that the surge limit value (30) also stored in the family of characteristic curves (20) is not reached during the operation of the electrically driven air compressor (3), wherein the pressure ratio varying with the current ratio is plotted in the family of characteristic curves (20), together with the operating points (23, 24, 25) of the electrically driven air compressor (3), wherein the family of characteristic curves (20) includes a pre-surge limit curve (27) which depends on the current ratio and pressure ratio of the electric drive (6) of the air compressor (3), and the pre-surge limit curve (27) is spaced from the surge limit (26) by a safe distance range.

2. The method according to claim 1, characterized in that, The family of characteristic curves (20) is used to determine the position of the operating point (24) of the electrically driven air compressor (3) relative to the surge limit (26) of the electrically driven air compressor (3).

3. The method according to claim 2, characterized in that, The current signals (33, 34) are changed to match the operating point (24) of the electrically driven air compressor (3) relative to the surge limit (26).

4. The method according to any one of the preceding claims, characterized in that, The electrically driven air compressor (3) includes a permanently excited synchronous motor (8).

5. A fuel cell system (1) having a fuel cell (2) supplying air (4) compressed by an electrically driven air compressor (3) to the fuel cell, the operating range of which can be represented in a family of characteristic curves (20) having a surge limit (26) and a choke limit (28) and stored in a control device of the electrically driven air compressor (3), wherein, Plot the pressure ratio as a function of the current ratio in the stored family of characteristic curves (20), together with the operating points (23, 24, 25) of the electrically driven air compressor (3), wherein the family of characteristic curves (20) includes a pre-surge limit curve (27) which depends on the current ratio and pressure ratio of the electric drive (6) of the air compressor (3), and the pre-surge limit curve is spaced at a safe distance from the surge limit (26).

6. The fuel cell system according to claim 5, characterized in that, The electric drive (6) of the air compressor (3) includes a permanently excited synchronous motor (8) that operates at different frequencies (33, 34) according to the position of the operating point (24) in the family of characteristic curves (20).

7. A computer program product having a computer program, the computer program having a software unit that, when the computer program is implemented on a computer, is used to perform the method according to any one of claims 1 to 4.

8. A family of characteristic curves (20) for a fuel cell system (1) having a fuel cell (2) supplying air (4) compressed by an electrically driven air compressor (3) to the fuel cell, the operating range of the air compressor being expressible in the family of characteristic curves (20), the family of characteristic curves having a surge limit (26) and a choke limit (29) and stored in a control device (7) of the electrically driven air compressor (3), wherein, Plot the pressure ratio as a function of the current ratio in the family of characteristic curves (20), along with the operating points (23, 24, 25) of the electrically driven air compressor (3), and along with the pre-surge limit curve (27), which depends on the current ratio and pressure ratio of the electric drive (6) of the air compressor (3), and the pre-surge limit curve is separated from the surge limit (26) by a safe distance.

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