METHOD FOR DETERMINING AGING OF A FUEL CELL SYSTEM

Abstract

A method for determining aging of a fuel cell system for a vehicle, comprising identifying a future interval in terms of time or distance during which the fuel cell system is expected to operate at stationary operating conditions, obtaining measurement values pertaining to at least one fuel cell parameter indicative of degradation of the fuel cell system during the identified interval, and determining an aging state of the fuel cell system using the measurement values.

Claims

1. A method for determining aging of a fuel cell system for a vehicle, comprising: during operation of the vehicle, identifying a future interval in terms of time or distance, during which interval the fuel cell system is expected to operate at stationary operating conditions, obtaining measurement values pertaining to at least one fuel cell parameter indicative of degradation of the fuel cell system during the identified interval, and determining an aging state of the fuel cell system using the measurement values.

2. The method of claim 1, wherein the future interval is an interval during which the fuel cell system is expected to be operated or possible to operate at a constant or nearly constant operating power.

3. The method of claim 2, wherein the constant operating power is within a linear power range of the fuel cell system.

4. The method of claim 1, further comprising: determining a reliability of the determined aging state, wherein the reliability is determined in dependence on at least a number of measurement occasions in each one of a predetermined low power operating range, a predetermined medium power operating range, and a predetermined high power operating range of the fuel cell system.

5. The method of claim 1, wherein identifying the future interval comprises predicting an operating power of the fuel cell system during a prediction horizon.

6. The method of claim 5, wherein predicting the operating power comprises: receiving vehicle related information comprising at least one of traffic information for an expected travelling route of the vehicle during the prediction horizon, terrain information for the expected travelling route during the prediction horizon, topographic information for the expected travelling route during the prediction horizon, weather information for the expected travelling route during the prediction horizon, and vehicle gross weight information during the prediction horizon, and using the received vehicle related information for predicting the operating power during the prediction horizon.

7. The method of claim 1, wherein identifying the future interval comprises using machine learning based on historical data relating to an operating power of the fuel cell system.

8. The method of claim 7, wherein the historical data comprises data indicative of the operating power of the fuel cell system as a function of at least geographical location of the vehicle and/or time.

9. The method of claim 7, wherein identifying the future interval comprises determining an expected travelling route of the vehicle, and wherein the historical data comprise data indicative of the operating power of the fuel cell system at one or more previous occasions of travelling along the expected travelling route.

10. The method of claim 1, wherein the future interval has a duration or length corresponding to at least two minutes, preferably at least five minutes, more preferably at least ten minutes.

11. The method of claim 1, further comprising: detecting actual operating conditions and/or an actual operating power of the fuel cell system during the identified interval, wherein the determination of the aging state of the fuel cell system using the measured at least one fuel cell parameter is only carried out when the actual operating conditions and/or the actual operating power fulfill(s) at least one predetermined stability criterion.

12. The method of claim 1, wherein determining the aging state comprises inputting the measured at least one fuel cell parameter to a degradation model describing the aging state of the fuel cell system.

13. A method for controlling power split between a fuel cell system and an electrical energy storage unit to which the fuel cell system is electrically connected, comprising: determining an aging state of the fuel cell system using the method of claim 1, and controlling the power split between the fuel cell system and the electrical energy storage unit in dependence on the determined aging state of the fuel cell system.

14. A computer program comprising program code means for performing the method of claim 1 when the computer program is run on a computer.

15. A computer readable medium carrying a computer program comprising program code means for performing the method of claim 1 when the computer program is run on a computer.

16. A control unit configured to perform the method of claim 1.

17. A vehicle comprising a fuel cell system adapted to deliver power contributing to the propulsion of the vehicle and the control unit of claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0049] In the drawings:

[0050] FIG. 1 is a schematic side view of a vehicle;

[0051] FIG. 2 is a diagram illustrating fuel cell efficiency as a function of operating power;

[0052] FIG. 3 is a flow chart illustrating an embodiment of a method for determining aging of a fuel cell system;

[0053] FIG. 4 is a diagram illustrating fuel cell operating power as a function of time/distance; and

[0054] FIG. 5 is a flow chart illustrating an embodiment of a method for controlling power split between a fuel cell system and an electrical energy storage unit.

DETAILED DESCRIPTION

[0055] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0056] FIG. 1 depicts a side view of a vehicle 100 according to an example embodiment of the invention. The vehicle 100 is here a truck, more specifically a heavy-duty truck for towing one or more trailers (not shown). Even though a heavy-duty truck 100 is shown it shall be noted that the invention is not limited to this type of vehicle but may be used for any other type of vehicle, such as a bus, construction equipment, e.g., a wheel loader and an excavator, and a passenger car.

[0057] The vehicle 100 comprises a power system 10. The power system 10 is here used for powering one or more electric motors (not shown) which are used for creating a propulsion force to the vehicle 100. The power system 10 may additionally or alternatively be used for powering other electric power consumers of the vehicle 100, such as an electric motor for a refrigerator system, an electric motor for an air conditioning system, or any other electric power consuming function of the vehicle 100. The power system 10 comprises a fuel cell system 1 according to an example embodiment of the invention. It further comprises an electrical energy storage unit 2, to which the fuel cell system 1 is electrically connected so that power generated by the fuel cell system 1 can be stored in the electrical energy storage unit 2. The electrical energy storage unit 2 may comprise one or more batteries, such as one or more Li-ion batteries. The power system 10 may further comprise power electronics (not shown) for converting electric energy as necessary within the power system 10 of the vehicle. Such power electronics may e.g. include a DC/DC converter.

[0058] The vehicle 100 further comprises a control unit 30 according to an example embodiment of the invention. The control unit 30 is thus configured for determining aging of the fuel cell system 1 and for controlling power split between the fuel cell system 1 and the electrical energy storage unit 2. Even though an on-board control unit 30 is shown, it shall be understood that the control unit could also be a remote control unit 30, i.e. an off-board control unit, or a combination of an on-board and off-board control unit. The control unit 30 may be configured to control the fuel cell system 1 by issuing control signals and by receiving status information relating to the fuel cell system 1.

[0059] The control unit 30 is an electronic control unit and may comprise processing circuitry which is adapted to run a computer program as disclosed herein. The control unit 30 may comprise hardware and/or software for performing the method according to the invention. In an embodiment the control unit 30 may be denoted a computer. The control unit 30 may be constituted by one or more separate sub-units. For example, the determination of aging and the control of the power split may be performed by different sub-units. In addition, the control unit 30 may communicate by use of wired and/or wireless communication means.

[0060] The fuel cell system 1 comprises one or more fuel cells (not shown), typically several fuel cells. The fuel cells may also be denoted as a fuel cell stack, wherein the fuel cell stack may comprise several hundreds of fuel cells. Further, the fuel cell system 1 is arranged to provide the fuel cells with necessary supply of air and fuel, such as hydrogen. Further, in addition or alternative to what is mentioned in the above, the fuel cell system 1 may comprise various components such as compressors, sensors, pumps, valves and electrical components.

[0061] Fuel cell efficiency η as a function of fuel cell operating power P is schematically shown in FIG. 2 for a fuel cell system at the beginning of life (BoL, solid line) and an aged fuel cell system at the end of life (EoL, dash-dot line), respectively. As illustrated, the fuel cell efficiency 11 reduces over time due to degradation of the fuel cells. It can also be seen in FIG. 2 that the fuel cell efficiency 11 as a function of power P is linear or approximately linear in a power range defined by a low power operating range L, a medium power operating range M, and a high power operating range H, regardless of the aging state of the fuel cell system 1.

[0062] FIG. 3 illustrates a method for determining aging of a fuel cell system 1 according to an example embodiment of the invention. Reference is also made to FIG. 4, illustrating fuel cell operating power P as a function of time t and/or distance x.

[0063] In a first step S1, a future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 during which the fuel cell system 1 is expected to operate at stationary operating conditions, is identified during operation of the vehicle, i.e., during driving of the vehicle. Thus, the future interval may be identified either in terms of time t or distance x. The future interval may be a time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 during which the fuel cell system 1 is expected to be operated or possible to operate at a constant or nearly constant operating power P, preferably within a linear power range of the fuel cell system 1. This is in FIG. 4 illustrated as a first interval dt.sub.1, dx.sub.1, and a second interval dt.sub.2, dx.sub.2, during which the fuel cell system 1 is expected to be operated at a nearly constant operating power P.sub.1 in the low power operating range L and P.sub.2 in the high power operating range H, respectively. The future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 may preferably be a time period or distance corresponding to at least a predetermined duration, such as at least two minutes, five minutes, or ten minutes. Thus, to be identified as a suitable future interval dt.sub.1, dt.sub.2, dx.sub.1, dx.sub.2, the fuel cell system 1 should be expected to operate at stationary operating conditions throughout the predetermined duration. When a future interval is identified in terms of distance, the duration is determined from an expected vehicle speed.

[0064] The first step S1 of identifying the future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 may typically comprise determining an expected travelling route of the vehicle 100, either by identifying that the vehicle is travelling along a known route, previously travelled, or by using data from a route planner or similar.

[0065] The first step S1 may comprise using machine learning based on historical data relating to the operating power P of the fuel cell system 1 to identify the future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2. The historical data may be indicative of the operating power P of the fuel cell system 1 as a function of at least geographical location of the vehicle 100 and/or time. For example, the vehicle 100 may regularly travel a certain route. The historical data may in this case comprise data indicative of the operating power P of the fuel cell system 1 at one or more previous occasions of travelling along the expected travelling route.

[0066] The step S1 of identifying the future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 may additionally or alternatively comprise predicting the operating power P of the fuel cell system 1 during a prediction horizon Δt, Δx, as illustrated in FIG. 4. The step of identifying the future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 may in this case comprise determining the expected travelling route of the vehicle, and further receiving vehicle related information comprising at least one of traffic information for an expected travelling route of the vehicle 100 during the prediction horizon Δt, Δx, terrain information for the expected travelling route, topographic information for the expected travelling route during the prediction horizon Δt, Δx, weather information for the expected travelling route during the prediction horizon Δt, Δx, vehicle gross weight information, etc. The step S1 in this case further comprises using said received vehicle related information for predicting the future operating power P over the prediction horizon Δt, Δx. Battery information indicative of at least one of a current state-of-charge and an energy capacity of the electrical energy storage unit 2 during the prediction horizon Δt, Δx may also be taken into account for predicting the operating power P during the prediction horizon Δt, Δx.

[0067] A combination of machine learning based on historical data and prediction based on e.g. traffic information and weather information may be used for identifying the future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2.

[0068] In a second step S2, measurement values pertaining to at least one fuel cell parameter indicative of degradation of the fuel cell system 1 are obtained during the identified time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2. The at least one fuel cell parameter may be one or more of current, voltage, power, impedance, and efficiency, depending on which method is used for characterizing fuel cell aging.

[0069] Purely by way of example, polarization curves can be used for characterizing aging, in which case fuel cell voltage and current need to be measured. Such measurements may, e.g., be performed using a DC/DC converter of the power system 10. With increasing age, the polarization cell voltage as a function of current decreases. Another way of determining fuel cell aging is by using impedance spectra. Recording of such a spectrum requires pulsing of the fuel cell voltage and a simultaneous measurement of the current response. Yet another method that may be used for characterizing aging is to record efficiency curves by measuring the fuel cell system power P and divide by the actual power provided by the fuel, such as by a hydrogen fuel flow. The efficiency 11 of the fuel cell system 1 decreases with increasing age as illustrated in FIG. 2.

[0070] In a third, optional, step S3, actual operating conditions and/or an actual operating power of the fuel cell system 1 during the identified time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 is/are detected. The operating conditions and/or the operating power P is/are in this case monitored during the course of the identified time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2.

[0071] In a fourth step S4, an aging state of the fuel cell system 1 is determined, using the measurement values obtained in step S2. When step S3 has been carried out, the step S4 may be performed in response to determining that the actual operating conditions and/or the actual operating power fulfill(s) at least one predetermined stability criterion, such as the operating power being within a predefined tolerance range for at least a predetermined time. If the stability criterion(s) is/are not fulfilled, step S4 may be omitted and the method restarts by trying to identify a future time interval dt.sub.1, dt.sub.2 or distance interval dx.sub.1, dx.sub.2 during which the fuel cell system 1 is expected to operate at stationary operating conditions.

[0072] The fourth step S4 may be performed using any of the methods described above in connection with the second step S2. Thus, a degradation model is typically used, describing the aging state of the fuel cell system 1. The measured fuel cell parameter(s) is/are used as input to the degradation model describing the aging state of the fuel cell system 1. In this way, the degradation model can be updated on each occasion the fuel cell system 1 is operated at stationary operating conditions during a sufficiently long time period.

[0073] The method illustrated in FIG. 3 may also comprise an optional fifth step S5 of determining a reliability of the determined aging state, wherein the reliability is determined in dependence on at least a number of measurement occasions in each one of the predetermined low power operating range L, the predetermined medium power operating range M, and a predetermined high power operating range H of the fuel cell system 1, such as a number of measurement occasions within a predetermined time frame or mileage frame of the vehicle 100.

[0074] FIG. 5 illustrates a method for controlling power split between a fuel cell system and an electrical energy storage unit to which the fuel cell system is electrically connected, such as the fuel cell system 1 and the electrical energy storage unit 2 illustrated in FIG. 1. The method comprises a first step S10 of determining an aging state of the fuel cell system 1 by using the method illustrated with reference to FIG. 3. It further comprises a second step S20 of controlling the power split between the fuel cell system 1 and the electrical energy storage unit 2 in dependence on the determined aging state of the fuel cell system 1. Preferably, an aging state of the electrical energy storage unit 2 may also be taken into account.

[0075] It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.