Method for operating a fuel cell system

Abstract

The present invention relates to a method of operating a fuel cell system comprising a fuel cell supplied with air compressed by an air compressor driven by an electric motor. [Solution] The operating range of the air compressor can be displayed in a characteristic map (20), which has a surge limit (26) and a choke limit (28) and is stored in a controller of the air compressor driven by an electric motor. In order to extend the life of the fuel cell system, at least one current signal is detected during operation of the air compressor driven by an electric motor and compared with a pre-surge limit value (29) stored in the characteristic map (20), and the air compressor driven by an electric motor is precisely controlled during operation so that the surge limit value also stored in the characteristic map (20) is not reached.

Description

[Technical field]

[0001] The invention relates to a method for operating a fuel cell system comprising a fuel cell supplied with air compressed by an air compressor driven by an electric motor, the operating range of which can be represented in a characteristic map, the characteristic map having surge and choke limits, stored in a controller of the air compressor driven by an electric motor. The invention further relates to such a fuel cell system. [Background technology]

[0002] From US Pat. No. 5,399,543 a fuel cell system is known which comprises a compressor generating an intake air flow, the compressor characteristic map of which is stored in a motor controller which determines the compressor discharge pressure and temperature and, from the compressor speed and the air flow signal from a mass flow meter, determines the point on the compressor characteristic map at which the system operates in order to prevent the compressor from going into a surge state. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 7,771,883 Summary of the Invention [Problem to be solved by the invention]

[0004] SUMMARY OF THE DISCLOSURE It is an object of the present invention to simplify the operation of a fuel cell system. [Means for solving the problem]

[0005] The object is achieved by a method for operating a fuel cell system with a fuel cell to which air compressed by an air compressor driven by an electric motor is supplied, the operating range of which can be displayed in a characteristic map, the characteristic map having a surge limit and a choke limit and stored in a controller of the air compressor driven by an electric motor, in which at least one current signal is detected during operation of the air compressor driven by an electric motor and compared with a pre-surge limit value stored in the characteristic map, and the air compressor driven by an electric motor is controlled appropriately during operation of the air compressor driven by an electric motor in such a way that the surge limit value also stored in the characteristic map is not reached. The air compressor may also be called a compressor. In the claimed method, measures are proactively taken to avoid reaching the surge limit during operation of the air compressor driven by an electric motor, which can significantly increase the service life of the fuel cell system. In that case, the current signal with which the air compressor driven by an electric motor is controlled is changed appropriately in such a way that the surge limit is not reached during operation of the air compressor.

[0006] A preferred embodiment of the method is characterized in that in the stored characteristic map, the pressure ratio is plotted over the current ratio together with the operating point of the air compressor driven by the electric motor. Advantageously, before the fuel cell system with the air compressor driven by the electric motor is used in series operation, a characteristic map is created during testing of the fuel cell system. The characteristic map can effectively reduce the effort of open-loop and closed-loop control during operation of the fuel cell system.

[0007] Another preferred embodiment of the method is characterized in that the presurge limit curve is spaced from the surge limit by a safety distance range, by suitable selection of the magnitude of the safety distance range, it can be achieved with relatively little effort that the presurge limit is never reached or is almost never reached during operation of the air compressor driven by the electric motor.

[0008] Another preferred embodiment of the method is characterized in that a characteristic map is used to determine the position of the operating point of the air compressor driven by the electric motor relative to the surge limit of the air compressor driven by the electric motor, in which case it can be detected with little effort by a simple comparison whether the above-mentioned proactive measures need to be taken.

[0009] Another preferred embodiment of the method is characterized in that the current signal is varied in order to adapt the operating point of the air compressor driven by the electric motor relative to a surge limit, possibly repeatedly or stepwise in order to prevent the surge limit from being reached without a relatively large loss of efficiency.

[0010] Another preferred embodiment of the method is characterized in that the air compressor driven by the electric motor comprises a permanently excited synchronous machine, which is advantageously controlled with different currents, current signals or different frequencies during operation of the air compressor driven by the electric motor in order to prevent reaching the surge limit.

[0011] In a fuel cell system with a fuel cell to which air compressed by an air compressor driven by an electric motor is supplied, the operating range of the air compressor can be displayed in a characteristic map, the characteristic map having a surge limit and a choke limit and stored in a controller of the air compressor driven by an electric motor, the above object is solved alternatively or additionally in that the characteristic map includes a pre-surge limit curve that depends on the current ratio and the pressure ratio of the electric motor drive of the air compressor, so that reaching the surge limit during operation of the fuel cell system can be reliably prevented in a simple manner.

[0012] A preferred embodiment of the fuel cell system is characterized in that the electric motor drive of the air compressor comprises a permanently excited synchronous machine that is controlled at different frequencies depending on the position of the operating point in the characteristic map, so that reaching of the surge limit during operation of the air compressor driven by the electric motor can be avoided without significant open-loop or closed-loop control technical effort.

[0013] The invention further relates to a computer program product comprising a computer program having software means for carrying out the above method when the computer program is executed on a computer, e.g. a controller for an electric motor drive of an air compressor in a fuel cell system.

[0014] A characteristic map for a fuel cell system comprising a fuel cell to which air compressed by an air compressor driven by an electric motor is supplied, the operating range of the air compressor being displayable in the characteristic map, the characteristic map having a surge limit and a choke limit and being stored in a controller of the air compressor driven by an electric motor, in which the above problem is solved alternatively or additionally in that the characteristic map plots the pressure ratio over the current ratio together with the operating point of the air compressor driven by the electric motor and the pre-surge limit curve.

[0015] Other advantages, features and details of the invention will become apparent from the following description in which several embodiments are set forth in detail with reference to the drawings. [Brief description of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a fuel cell system including a fuel cell, an air compressor driven by an electric motor, and a pressure sensor device. [Diagram 2] FIG. 2 is a diagram of a fuel cell system similar to that of FIG. 1 without the pressure sensor device but with a measuring device for measuring the air mass flow supplied to the fuel cell. [Diagram 3] 3 is a diagram of a characteristic map including a surge limit, a choke limit, and a pre-surge limit stored in a controller of the fuel cell system according to FIGS. 1 and 2; FIG. [Figure 4] FIG. 2 is a Cartesian coordinate diagram in which the profile of a current signal is plotted over time. [Diagram 5] FIG. 5 is a diagram similar to that of FIG. 4 in which the current signal has been altered. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In Figures 1 and 2 a fuel cell system 1 is shown diagrammatically comprising a fuel cell 2 and an air compressor 3 driven by an electric motor. The same reference symbols are used to denote the same or similar parts. The fuel cell system 1 is of somewhat similar construction to the fuel cell system disclosed in the patent application WO 2005 / 023363 evaluated at the outset.

[0018] 1 and 2, the air is indicated by the arrow 4, which is supplied to the air compressor 3 via an air filter 5. The air compressor 5 is driven by an electric motor drive 6. The electric motor drive 6 is configured as a permanently excited synchronous machine 8 with a controller 7.

[0019] A valve device 9 is disposed at the air outlet of the fuel cell 2. The above-mentioned features are formed the same in the fuel cell system 1 shown in Figures 1 and 2. Next, differences between the fuel cell system 1 shown in Figures 1 and 2 will be noted.

[0020] The fuel cell system 1 shown in Fig. 1 comprises a pressure sensor device 10 in which the pressure of compressed air supplied by an air compressor 3 to a fuel cell 2 is detected. An arrow 11 indicates that a controller 7 of an electric motor drive 6 is controllably connected to the pressure sensor device 10. An arrow 12 indicates that the controller 7 is controllably connected to a valve device 9.

[0021] In the fuel cell system 1 shown in Fig. 2, instead of the pressure sensor arrangement (10 in Fig. 1) a measuring device 14 is provided, with which the air mass flow is detected between the air filter 5 and the air compressor 3. In Fig. 2, an arrow 15 indicates that the controller 7 of the electric motor drive 6 is controllably connected to the measuring device 14.

[0022] In Fig. 3, a characteristic map 20 for the operation of an air compressor (3 in Figs. 1 and 2) driven by an electric motor is shown in a Cartesian coordinate diagram. On the x-axis 21, the current ratio I vs. I max On the y-axis 22 is plotted the pressure ratio p vs. p max is plotted.

[0023] Three operating points 23, 24, 25 during operation of the air compressor are depicted in the characteristic map 20 as an example. The characteristic map 20 includes a surge limit 26 and a choke limit 28. Furthermore, the characteristic map 20 includes a pre-surge limit curve 27. The pre-surge limit curve 27 is spaced apart from the surge limit 26.

[0024] In figures 4 and 5 two different Cartesian coordinate diagrams are shown. On the x-axis 31 the time is plotted in appropriate time units. On the y-axis 32 the frequency of the current signal per unit time is plotted. In the two diagrams the profiles of two current signals 33, 34 with different frequencies are shown.

[0025] Operation of the air compressor 3, also called compressor, close to its surge limit 26 should be avoided. The control of the electric motor drive 6 of the air compressor 3, the permanently excited synchronous machine 8, is performed using a rotating magnetic field of controlled current, as suggested in Figures 4 and 5. In that case, the current and the permanent magnets of the permanently excited synchronous machine 8 create a mechanical moment to compress the aspirated air 4.

[0026] The operating points 23-25 ​​of the air compressor 3 are determined from the pressure and temperature or directly from the measured air mass flow of the air compressor 3 and the known frequencies of the current signals 33, 34. The respective operating point, for example 24, is then compared with a characteristic map 20 stored in the controller 7.

[0027] If the operating point is located close to the pre-surge limit curve 27, the frequency of the rotating magnetic field can be adapted to maintain an appropriate safety distance relative to the surge limit 26. [Explanation of symbols]

[0028] 1. Fuel cell system 2 fuel cell 3. Air Compressor 4 Arrows, Air 5. Air Filter 6 Electric motor drive unit 7. Controller 8 Permanently excited synchronous machine 9 Valve device 10 Pressure sensor device 11 Arrow 12 Arrow 14 Measuring Equipment 15 Arrow 20 Characteristics Map 21 x-axis 22 y-axis 23 Operating point 24 Operating points 25 Operating points 26 Surge Limit 27 Presage Limit Curve 28 Choke limit 29 Presage Limit 30 Surge Limit 31 x-axis 32 y-axis 33 Current Signal 34 Current Signal

Claims

1. A method for operating a fuel cell system (1) comprising a fuel cell (2) supplied with air (4) compressed by an air compressor (3) driven by an electric motor, the operating range of the air compressor being displayable in a characteristic map (20), the characteristic map having a surge limit (26) and a choke limit (28) and stored in a controller (7) of the air compressor (3) driven by an electric motor, wherein during operation of the air compressor (3) driven by an electric motor, at least one current signal (33, 34) is detected and compared with a pre-surge limit value (29) stored in the characteristic map (20), and during operation of the air compressor (3) driven by an electric motor, the air compressor (3) driven by an electric motor is precisely controlled so that the surge limit value (30), also stored in the characteristic map (20), is not reached, In said stored characteristic map (20), the pressure ratio is plotted against the current ratio of the electric motor drive (6) driving said air compressor (3) together with the operating points (23, 24, 25) of the air compressor (3) driven by said electric motor, 2. The method of claim 1, wherein the air compressor (3) driven by the electric motor comprises a permanently excited synchronous machine (8) which is controlled at different frequencies (33, 34) depending on the position of an operating point (24) in the characteristic map (20) in which the pressure ratio is plotted against the current ratio of the electric motor drive (6) which drives the air compressor (3).

2. 2. The method of claim 1, wherein the presurge limit curve (27) is spaced from the surge limit (26) by a safety distance range.

3. 3. The method according to claim 1 or 2, characterized in that the characteristic map (20) is used to determine a position of an operating point (24) of the air compressor (3) driven by the electric motor relative to the surge limit (26) of the air compressor (3) driven by the electric motor.

4. 4. The method according to claim 3, characterized in that the current signals (33, 34) are varied in order to adapt the operating point (24) of the air compressor (3) driven by the electric motor relative to the surge limit (26).

5. A fuel cell system (1) comprising a fuel cell (2) supplied with air (4) compressed by an air compressor (3) driven by an electric motor, the operating range of the air compressor being displayable in a characteristic map (20), the characteristic map having a surge limit (26) and a choke limit (28) and stored in a controller of the air compressor (3) driven by the electric motor, the characteristic map (20) including a pre-surge limit curve (27) depending on the current ratio and the pressure ratio of an electric motor drive (6) of the air compressor (3), In the stored characteristic map (20), a pressure ratio is plotted against a current ratio of an electric motor drive (6) driving the air compressor (3) together with operating points (23, 24, 25) of the air compressor (3) driven by the electric motor, 2. A fuel cell system comprising: a permanently excited synchronous machine (8) that is controlled at different frequencies (33, 34) depending on the position of an operating point (24) in the characteristic map (20) in which a pressure ratio is plotted over a current ratio of the electric motor drive (6) that drives the air compressor (3).

6. A computer program product comprising a computer program, the computer program having software means for carrying out the method according to any one of claims 1 to 4, when the computer program is run on a computer.

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