MÜLLER, Helmut (Am Wiesenrain 30, Kirchheim, 73230, DE)
SCHMALZRIEDT, Sven (Gartenstrasse 24, Ostfildern, 73760, DE)
FORD GLOBAL TECHNOLOGIES, LLC (330 Town Center Drive, Suite 800 SouthDearborn, MI, 48126, US)
LIMBECK, Uwe (Beim Schiesswasen 15, Kirchheim, 73230, DE)
MÜLLER, Helmut (Am Wiesenrain 30, Kirchheim, 73230, DE)
SCHMALZRIEDT, Sven (Gartenstrasse 24, Ostfildern, 73760, DE)
Patent claims
1. Method for monitoring the functionality of a pressure sensor (1, 11, 21) in a fuel cell system, in particular on cold starting of a motor vehicle driven by the fuel cell system, the fuel cell system comprising at least one fuel cell (2) with lines for fuel and oxidising a- gent connected thereto, and the pressure sensor (1, 11, 21) being provided for monitoring pressure at a specified location in the lines and/or the fuel cell (2) and producing a measurement signal as a function of the pressure detected thereby, characterized in that the pressure at the specified location is purposefully varied, the measurement signal produced during the variation is compared with a specified value, and the comparison result is evaluated to assess the functionality of the pressure sensor (1, 11, 21) .
2. Method according to Claim 1, characterized in that the pressure at the specified location is purposefully varied, in particular increased, by switching on or purposefully increasing the power of a blower (3) , compressor (5) and/or pump which conveys, in particular circulates, the fuel and/or the oxidising agent into or in the fuel cell system.
3. Method according to one of Claims 1 or 2, characterized in that the functionality of two or more pressure sensors (1, 11, 21) , which are provided for monitoring the pressure at correspondingly two or more specified locations in the lines and/or the fuel cell (2) and, as a function of the pressures detected thereby, produce a corresponding number of measurement signals, is monitored by purposefully varying the pressure at the two or more specified locations, the measurement signals produced during the variation are compared with specified values, and the comparison results are evaluated to assess the functionality of the pressure sensors (1, 11, 21).
4. Method according to Claims 2 or 3, characterized in that a first specified location, whose pressure is monitored by a first pressure sensor (1), is located, seen in the direction of flow, downstream of the blower (3) , compressor (5) or pump, and a second specified location, whose pressure is monitored by a second pressure sensor (11), is located, seen in the direction of flow, upstream of the blower (3), compressor (5) or pump, and in that, by switching on the blower (3) , compressor (5) or pump, the pressure is purposefully increased at the first specified location and purposefully lowered at the second specified location.
5. Method according to one of Claims 2 to 4, characterized in that an anode circuit (4) is provided on the fuel cell (2), via which circuit a feed line for fuel into the fuel cell (2) is connected with a drain line for fuel and/or anode reaction products from the fuel cell (2) , and the purposeful variation of pressure is effected by switching on or purposefully increasing the power of a blower (3) in the anode circuit (4), which blower is provided to recirculate fuel from the drain line into the feed line .
6. Method according to one of Claims 1 to 5, characterized in that the pressure at the specified location is purposefully varied by opening or closing a control member or shut- off member, in particular temporarily over a specified period of time, wherein fuel or oxidising agent is introduced into the lines and/or the fuel cell (2) or drained from the lines and/or the fuel cell (2) via the control member or shut-off member.
7. Method according to one of Claims 1 to 6, characterized in that the specified value is specified in the form of a pressure profile, in particular pressure rise or pressure drop, over time.
8. Method according to Claim 7, characterized in that the pressure profile is recorded prior to monitoring the functionality of the pressure sensor (1) as a nominal value curve during calibration of the fuel cell system, in particular during initial commissioning.
9. Method according to one of Claims 6 to 8, characterized in that pressure is purposefully varied by opening, in particular temporarily opening a pressure control valve (6) for a specified period of time, via which pressure control valve fuel is supplied to an anode of the fuel cell (2) and pressure is controlled in the anode.
10. Method according to one of Claims 1 to 9, characterized in that pressure is purposefully varied by increasing the rotational speed of a compressor (5) , via which oxidising agent is supplied to a cathode of the fuel cell (2) and pressure is in particular controlled in the cathode.
11. Method according to one of Claims 1 to 10, characterized in that monitoring of the functionality of the pressure sensor (s) (1, 11, 21) is effected exclusively by actuating components provided for controlling the operation of the fuel cells (2) in the fuel cell system which are required independently of monitoring functionality. |
Method for monitoring the functionality of a pressure sensor in a fuel cell system
The invention relates to a method for monitoring the functionality of a pressure sensor in a fuel cell system, in particular on cold starting of a motor vehicle driven by the fuel cell system.
Pressure sensors, the functionality of which is to be monitored according to embodiments of the present invention, are used in fuel cell systems for example for controlling the metering of fuel, generally hydrogen. In particular, the hydrogen pressure in the anode or on the anode side of the fuel cell should be kept constant, while hydrogen is electrochemi- cally converted in the fuel cell and "fresh" hydrogen is simultaneously apportioned. When the term fuel cell is used in the present description, it is intended to cover both an individual fuel cell and a stack of fuel cells (fuel cell stack) , as is generally used in fuel cell systems, for example for driving a motor vehicle.
Another field of application of one or more pressure sensors is in establishing the air pressure or, in general, the pressure of the oxidising agent in the cathode or on the cathode side. The present invention for monitoring the functionality
of this pressure sensor or of these pressure sensors may also be used for this purpose.
Differential pressure sensors may also be used in fuel cell systems in order to detect a differential pressure, for example between the anode and the cathode or the anode side and the cathode side. According to one embodiment of the present invention, it is also possible to monitor the functionality of such differential pressure sensors.
Due to condensation during operation of the fuel cell system, water may accumulate in the stated pressure sensors or in lines to which the pressure sensors are connected in pressure- detecting manner, see Fig. 1. When the fuel cell system is shut down in a cold environment, for example in a motor vehicle in winter, and, under very unfavourable conditions, possibly also during operation of the fuel cell system, this water may freeze and decouple the pressure sensor from the operating pressure to be monitored. On restarting the fuel cell system (cold start) or on continued operation of the fuel cell system in the latter-stated case, the pressure sensor measures a pressure, generally the final pressure before freezing, which possibly no longer corresponds to the current operating pressure to be monitored. In this way, erroneous pressure signals from the pressure sensor are processed and reliable pressure monitoring of the fuel cell system is no longer ensured.
System error diagnosis with the assistance of pressure sensors has already been addressed in the development of fuel cell systems, see for example the publication DE 103 92 753 T5, in which it is proposed to diagnose pump or controller failure by pressure sensors, it being possible to achieve a considerable reduction in response time in comparison with
the prior art, which made use of temperature sensors. The problem was also known that such pressure sensors could in general fail, but without there being any awareness of the freezing issue, see for example published patent application JP 2004-259670. Said published patent application proposes, on the basis of redundant provision of all the pressure sensors in a fuel cell system, to reduce the total number of pressure sensors required by monitoring the functionality of two existing absolute pressure sensors with an additional differential pressure sensor. However, in this case too, e- quipment complexity is still very high due to the additional differential pressure sensors, so entailing costly production and servicing of the fuel cell system. Furthermore, the probability of failure of the additional differential pressure sensors is also high.
The object of the present invention is to solve the above- stated problem of unrecognized erroneous pressure detection with pressure sensors in fuel cell systems, in particular due to freezing of the pressure sensors or the lines by means of which the pressure sensors are connected with the location of the pressure to be detected and, as far as possible, in so doing managing without additional components in an existing fuel cell system. Monitoring should additionally be distinguished by elevated reliability.
The object of the invention is achieved by a method having the features of Claim 1. The dependent claims describe advantageous and particularly convenient developments of the method according to the invention.
By means of the method according to the invention, the functionality of one or more pressure sensors in a fuel cell system may be monitored and, when required, suitable countermea-
sures may be initiated. Only a few possible coυntermeasures will be enumerated here. Accordingly, when failure of a pressure sensor is detected or on detection of a frozen pressure sensor on starting the fuel cell system, the start may be terminated or the maximum power output of the fuel cell limited to a value below the closed-loop power level. Alternatively or additionally, the fuel cell may be operated under open-loop control, instead of closed-loop control, as is otherwise conventional. Finally, closed-loop control may be adjusted with the assistance of further existing sensors.
Specifically, the method according to the invention for monitoring the functionality of a pressure sensor in a fuel cell system, in particular on cold starting of a motor vehicle driven by the fuel cell system, wherein the fuel cell system comprises at least one fuel cell (or a fuel cell stack) with lines connected thereto for fuel and oxidising agent (generally hydrogen and air) , and the pressure sensor for monitoring pressure is provided at a specified location in the lines and/or the fuel cell and, as a function of the pressure detected thereby, produces a measurement signal, comprises the method steps that the pressure is purposefully varied at the specified location, that the measurement signal of the pressure sensor produced during the variation is compared with a specified value which is generally produced by measurement or calculation or otherwise specified, and that the comparison result is evaluated to assess the functionality of the pressure sensor.
According to one embodiment, the pressure at the specified location is purposefully varied, for example increased, by switching on or purposefully increasing the power of a blower, compressor or pump, the blower, compressor or pump being provided for the purpose of delivering the oxidising agent
and/or the fuel to the fuel cell system or circulating it/them in a circuit.
According to one advantageous embodiment of the invention, the functionality of a plurality of pressure sensors, for example of two, three, four or more pressure sensors, may be monitored. These pressure sensors are provided for monitoring the pressure or pressures at the corresponding number of specified locations in the lines and/or the fuel cell of the fuel cell system and in each case produce corresponding measurement signals as a function of the pressures detected thereby. According to the invention, the pressure at each individual specified location of different pressure sensors may purposefully be varied mutually independently or dependently, and the measurement signals produced during the variation may mutually independently or dependently be evaluated with comparison results for assessing the functionality of the pressure sensors.
According to one particularly advantageous embodiment, the pressure at the different specified locations of two or more pressure sensors may purposefully be varied by actuating a single unit or individual units of a plurality of units. This measure is described in greater detail in the description of figures below.
Additionally or alternatively to switching on or increasing the power of a pump, compressor or blower, the pressure at the specified location or locations may also purposefully be varied by opening or closing a control member or shut-off member. The control member or shut-off member is provided in the fuel cell system for introduction of fuel or oxidising agent into the lines and/or the fuel cell or discharge thereof from the lines and/or the fuel cell.
The specified value with which the measurement signal produced by the pressure sensor during purposeful variation of pressure at the specified location is compared may also be specified in the form of a pressure profile over time. It is, for example, possible to specify a pressure rise at the specified location or pressure drop at the specified location which is to be expected during proper functioning. The rate of the pressure rise or pressure drop may additionally or alternatively be used as a specified value.
According to one particularly advantageous embodiment, the pressure profile serving as a specified value is recorded with the existing fuel cell system before functionality is monitored. Accordingly, the fuel cell system may for example be operated under specified conditions, under which it may reliably be assumed that the pressure sensor to be monitored is operating properly or has passed corresponding tests, and the pressure profile detected by the pressure sensor, which corresponds to proper functioning of the pressure sensor, may be recorded as a nominal pressure curve, while the unit which is subsequently actuated during monitoring of the functionality of the pressure sensor, is actuated in accordance with the subsequent actuation. The nominal value curve is, so to speak, recorded during calibration of the fuel cell system. Recording particularly advantageously proceeds during initial commissioning of the fuel cell system before the latter is delivered to customers. It is, of course, also possible to have renewed calibrations or recordings of nominal value curves made by specialist personnel or automatically, for example during the course of servicing.
Monitoring of the functionality of one or more pressure sensors according to the invention may advantageously proceed
solely by actuation of units or components in the fuel cell system which are in any event provided or necessary for operation or control of the operation of the fuel cell. Thus, according to one advantageous embodiment, the method according to the invention manages without additional hardware solely by implementing the method in a provided control system or in the form of software.
The following exemplary embodiments are intended to illustrate the invention. In the Figures:
Fig. 1 shows two examples of blockage of a pressure sensor by ice;
Fig. 2 shows an exemplary embodiment of a fuel cell system having three pressure sensors, the functionality of which may be monitored according to the invention solely by actuating existing components and units.
In Fig. 1, a pressure sensor 1 may in each case be seen in cross-section in views a and b. The pressure sensor 1 is screwed with its housing, which in the present case bears an external thread, into an adapter 13. The adapter 13 is mounted externally on a pipe 14, for example welded on.
In representation a, a block of ice 12 preventing proper pressure detection has formed due to freezing of condensation water directly on the membrane or a piston of the pressure sensor 1. In the representation shown in view b, a block of ice 12 is blocking the measurement line in the pressure sensor, which line opens into the pipe 14.
The ice blockage means that the pressure sensor can no longer properly detect the pressure to be detected in the pipe 14 or
produce corresponding measurement signals which correspond to the actual pressure in the pipe 14. Such malfunctioning should be detected by the method according to the invention so that suitable safety measures or countermeasures are initiated.
Fig. 2 shows a fuel cell system with a fuel cell 2 comprising an anode and a cathode. As a rule, the fuel cell 2 will be a fuel cell stack comprising a plurality of individual fuel cells connected in series.
The pressure both in the anode or in the lines connected to the anode and in the cathode or the lines connected to the cathode may be controlled by provided pressure control means.
The pressure sensor 1 on the anode side detects the pressure in the anode, said pressure serving as an input variable for the pressure control valve 6 shown. During operation of the fuel cell 2, the pressure control valve 6 directs more or less fuel (H2) into the fuel cell as a function of the pressure detected by the pressure sensor 1.
In the event of malfunction of the pressure sensor 1, for example in the event of blockage by frozen condensation water, if no countermeasures were taken, the pressure control valve 6 would apportion ever more fuel in order to establish the specified pressure setpoint in the anode. Since the pressure sensor 1 is producing an incorrect measured value, anode pressure would rise in uncontrolled manner.
According to the invention, the problem of blockage of the pressure sensor 1 may be detected during starting of the fuel cell system. According to a first embodiment, in particular on starting of the fuel cell system, the blower 3, which is a
recirculation blower, is switched on (or, in the event that the recirculation blower is already switched on, for example in operating states other than starting, its power could also purposefully be increased) . The position of the pressure control valve 6 may here for example be kept constant. This gives rise to a purposeful pressure rise upstream of the fuel cell 2 which would necessarily be detected if the pressure sensor 1 were operating properly. If the pressure rise is not detected by means of the pressure sensor 1, the pressure sensor 1 is blocked (or otherwise malfunctioning) and a suitable countermeasure may be initiated.
By switching on the blower 3, by means of which fuel or reaction products from a fuel cell 2 drain line in an anode circuit 4 may be recirculated into a fuel feed line into the fuel cell 2, it is possible simultaneously to produce a pressure drop in direction of flow of the fuel downstream of the fuel cell 2 or the anode thereof, which pressure drop may be utilized for testing the functionality of the indicated pressure sensor 11. The pressure sensor 11 is a differential pressure sensor which detects the pressure difference between the anode or anode side and the cathode or cathode side of the fuel cell 2.
An alternative or additional possibility for monitoring the functionality of the pressure sensor 1 consists in producing a purposeful pressure rise in the line by means of which fuel is supplied to the anode of the fuel cell 2 by opening (or further opening) the pressure control valve 6. In particular, the pressure control valve 6 is opened for a specified (short) period of time. Thanks to prior calibration or calculation, the pressure rise in the line whose pressure is being monitored with the pressure sensor 1 is known. This known pressure rise is used as a specified value and compared with
the pressure value actually detected by the pressure sensor 1 on opening of the pressure control valve 6. It is, of course, possible not only to compare two pressure values with one another directly, but a measurement signal in any desired unit, which is produced directly or indirectly by the pressure sensor 1, may be compared with a corresponding specified value.
The purposeful and generally time-limited opening of the pressure control valve 6 may also be utilized for testing the functionality of the pressure sensor 11.
Pressure control in the cathode or on the cathode side of the fuel cell 2 proceeds with the assistance of the pressure sensor 21. The pressure sensor 21 detects the cathode pressure, the detected cathode pressure serving as an input variable for setting the opening cross-section in the throttle valve 7 shown in the cathode exhaust. A blockage or in general a malfunction of the pressure sensor 21 would, if no countermea- sures were taken, lead to a low pressure measurement, such that the throttle valve 7 is closed in order to increase the pressure in the cathode or on the cathode side further. This may result in an uncontrolled rise in pressure.
According to one embodiment of the invention, the problem of blockage (or another malfunction) may, for example again during starting of the fuel cell system, be detected by operating the compressor 5, with which compressed air is supplied to the cathode of the fuel cell 2, with a specified, in particular raised, power, for example by increasing its rotational speed. By increasing the power or rotational speed of the compressor 5, a pressure rise in the line between the compressor 5 and the cathode of the fuel cell 2 is purposefully produced, which pressure rise is detected by the pressure sensor 21 when the pressure sensor 21 is operating prop-
erly. The measurement signal actually produced by the pressure sensor 21 is compared with a specified value, in particular a profile. The profile value may in turn, for example, be produced by previously produced calibrations or measurements or be calculated. On the basis of the comparison, it is possible to determine whether the pressure sensor 21 is functioning as desired.
By increasing the power or the rotational speed or by setting a specified power or rotational speed of the compressor 5, it is also possible to determine the functionality of the pressure sensor 11 by also comparing the value detected by the pressure sensor 11 with a specified value or a specified profile.
Instead of setting or increasing the power or the rotational speed of the compressor 5, the functionality of the pressure sensor 21 and/or of the pressure sensor 11 may alternatively or additionally also be determined by purposefully closing the throttle valve 7 in the cathode exhaust and by comparing the pressure values then measured with a specified value or a specified profile.
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