Anode pressure-based flux estimation in a fuel cell system

Abstract

A method for determining a flux of anode gas from an anode subsystem of a fuel cell system (10), wherein the method comprises: - Defining a time period to determine the flow; - Performing a measurement of the pressure in the anode subsystem at a variety of predetermined time intervals during the time period; - Calculating the slope of a pressure line representing the changes in pressure in the anode subsystem over time using pressure measurements; and - Determining the flux of anode gas from the anode subsystem using the slope of the pressure line, wherein determining the flux of anode gas from the anode subsystem using the slope of the pressure line includes determining whether the pressure line fits a straight line within a predetermined threshold, and if the pressure line fits a straight line within the predetermined threshold, the calculated slope is determined to be valid.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] This invention relates generally to a method for determining the flow from a closed system and in particular to a method for determining the flow of anode gas from an anode sub-system in a fuel cell system, wherein the method determines the slope of a pressure line generated from a plurality of pressure measurements and uses the slope in a flow calculation. 2. Discussion of the state of the art

[0002] Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte between them. The anode receives hydrogen gas, and the cathode receives oxygen or air. The hydrogen gas dissociates at the anode catalyst to produce free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and electrons at the cathode catalyst to produce water. The electrons cannot pass from the anode through the electrolyte. Therefore, they are conducted through a load to perform work before reaching the cathode.

[0003] Proton exchange membrane fuel cells (PEMFCs) are a popular fuel cell for vehicles. A PEMFC generally incorporates a solid polymer electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically contain finely dispersed catalyst particles, usually platinum (Pt), distributed on carbon particles and mixed with an ionomer. The catalyst mixture is applied to opposite sides of the membrane. The combination of the anode catalyst mixture, the cathode catalyst mixture, and the membrane defines a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require specific conditions for effective operation.

[0004] Typically, multiple fuel cells are combined into a fuel cell stack to generate the desired power output. For example, a typical fuel cell stack for a vehicle might contain two hundred or more stacked fuel cells. The fuel cell stack receives a cathode inlet gas, typically an airflow, which is forced through the stack by a compressor. Not all of the oxygen is consumed by the stack, and some air is released as cathode exhaust, which may contain water as a stack byproduct. The fuel cell stack also receives an anode hydrogen inlet gas, which flows into the anode side of the stack.

[0005] A fuel cell stack typically comprises a series of bipolar plates positioned between the multiple MEAs in the stack, with the bipolar plates and MEAs located between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates, allowing the anode reaction gas to flow to the respective MEA. Cathode gas flow channels are also provided on the cathode side of the bipolar plates, allowing the cathode reaction gas to flow to the respective MEA. One end plate contains anode gas flow channels, and the other end plate contains cathode gas flow channels. The bipolar plates and end plates are constructed of a conductive material, such as stainless steel or a conductive composite material.The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also contain flow channels through which a coolant flows.

[0006] It is necessary to accurately determine the flow rate through vent valves, drain valves, and potentially other valves in the anode subsystem of a fuel cell system to know when each valve needs to close, a task well known to experts. Traditional valve opening models work reasonably well but are subject to part-to-part variations because these models assume an effective opening area. Furthermore, the opening model calculation also requires a differential between an inlet pressure and an outlet pressure to determine the flow. For certain known systems, this pressure differential is on the same order of magnitude as the errors of the pressure sensors, which could lead to large estimation errors.

[0007] US patent application US 2011 / 0138883A1, entitled "Injector Flow Measurement for Fuel Cell Applications," filed on December 11, 2009, and registered to the proprietor of this application, discloses a method for determining a flow through a valve in a fuel cell system. An anode subsystem pressure is measured immediately before and immediately after an injector pulse, and the difference between the pressures is determined. This pressure difference, the volume of the anode subsystem, the ideal gas constant, the anode subsystem temperature, the fuel consumed from the reaction of the fuel cell stack during the injection process, and the fuel crossover through the membranes in the fuel cells of the fuel cell stack are used to determine the flow through a valve.The flow determination method in patent application US 2011 / 0 138 883 A1 uses two pressure measurements, which can be affected by noise during pressure acquisition and provides no feedback on the validity of the pressure measurements. Document WO 2004 / 112 179 A2 describes a fuel cell system and a method for operating the fuel cell system, wherein the fuel cell system includes a means for leak detection based on pressure change analysis. Document DE 10 2007 060 712 A1 describes a method for determining fuel flow in a fuel cell stack, wherein the pressure on the anode side of the fuel cell stack is determined before and shortly after an opening event. SUMMARY OF THE INVENTION

[0008] In accordance with the teachings of the present invention, a method for determining the flow of an anode gas out of an anode subsystem is disclosed. The method comprises providing pressure measurements at predetermined sampling times over a predetermined sampling period and using the pressure measurements to calculate the slope of a line that defines a change in pressure from the beginning of the time period to the end of the time period.The slope of the pressure line is then used in a flux equation to determine the amount of gas flowing out of the anode subsystem, which may occur through a valve or system leaks. Determining the flow of anode gas from the anode subsystem using the slope of the pressure line involves determining whether the pressure line fits a straight line within a predetermined threshold. If the pressure line fits a straight line within the predetermined threshold, the calculated slope is determined to be valid.

[0009] Further features of the present invention will become apparent from the following description and the attached claims in conjunction with the attached figures. BRIEF DESCRIPTION OF THE FIGURES Fig. Figure 1 is a schematic block diagram of a fuel cell system; and Fig. Figure 2 is a graph where time is plotted on the horizontal axis and pressure on the vertical axis, illustrating a pressure change over time in an anode subsystem of a fuel cell system. DETAILED DESCRIPTION OF THE EXECUTION FORMS

[0010] Fig. Figure 1 is a schematic block diagram of a fuel cell system 10 with a fuel cell stack 12. Hydrogen gas from a high-pressure hydrogen gas source 14, for example a tank, is supplied to the anode side of the fuel cell stack 12 via an anode inlet line 16. The hydrogen gas from the source 14 is injected into the stack 12 by an injector 20, the injector 20 being intended to represent a single injector or a bank of injectors suitable for the purposes described herein. Anode exhaust from the fuel cell stack 12 is directed into an anode exhaust line 22. A vent valve 24 is arranged in the anode exhaust line 22 and is periodically opened to vent nitrogen from the anode side of the fuel cell stack 12 in a manner well known to those skilled in the art.System 10 further includes an anode recirculation line 34, which directs the anode exhaust gas in the anode exhaust line 22 back to the injector 20, which is located in the anode inlet line 16, while the vent valve 24 is closed. A drain valve 36 is provided in the recirculation line 34 and is configured to periodically drain water from the recirculated anode exhaust gas in a manner well known to those skilled in the art. A pressure sensor 26 is provided in the recirculation line 34 to measure the pressure in the anode subsystem. The location of the pressure sensor 26 is intended to represent any possible suitable location for measuring the pressure at the anode inlet, the anode outlet, or a recirculation line between the anode inlet and the anode outlet.

[0011] System 10 further includes a high-temperature pump 28, which pumps a coolant through a cooling circuit 30 outside the stack 12 and through coolant flow channels within the stack 12 in a manner well known to experts. A temperature sensor 32 measures the temperature of the coolant flowing through the cooling circuit 30 and can be located at any suitable point in the cooling circuit 30, for example, at the inlet of the stack 12, where the coolant is typically coolest, or at the outlet of the fuel cell stack 12, where the coolant is typically hottest. A controller 38 receives a pressure signal from the pressure sensor 26 and a temperature signal from the temperature sensor 32 and controls the duty cycle of the injector 20 as well as the position of the vent valve 24 and the drain valve 36.It is necessary to know how much anode gas flows out of valves 24 and 36 while they are open, so that it is known when valves 24 and 36 should be closed for optimal system performance. The following discussion describes an accurate determination of the flow based on an anode subsystem pressure.

[0012] Fig.Figure 2 is a graph where time is plotted on the horizontal axis and pressure on the vertical axis, illustrating the anode subsystem pressure P on a graph line 42 during and between injection events, indicated by graph points 44 and 46. An injection event at points 44 and 46 is identified by a sharp drop in the anode subsystem pressure P as soon as injector 20 is open, and then by a sharp rise in the anode subsystem pressure P as soon as injector 20 is closed. A pressure P1 is defined immediately after injector 20 is closed at an injection event at point 44, and a pressure P2 is defined immediately before injector 20 is opened at the next injection event at point 46.A drop in pressure P along line 42 between pressures P1 and P2 typically occurs for a variety of reasons, including when a valve is opened and normal pressure loss occurs in the anode subsystem. For example, the vent valve 24 may be opened during this time, the drain valve 36 may be opened during this time, a leak may occur in the closed anode subsystem if valves 24 or 36 are closed, etc.

[0013] As will be discussed in detail below, the slope of line 42 between injection events is used to determine the flow rate from the anode subsystem for all these events. The controller 38 will open the vent valve 24 or the drain valve 36, based on various known system rules and operating modes familiar to those skilled in the art, and use the flow through these valves to know precisely when to close them. Furthermore, the controller 38 can determine whether excessive leakage from the anode subsystem occurs when the anode subsystem is closed while valves 24 and 36 are closed. Each point 40 along line 42 represents a pressure measurement by the pressure sensor 26. These measurements can be taken at any suitable interval, for example, every three milliseconds.

[0014] The slope ̇P of the print line 42 can be calculated along each segment of the line 42, for example with a sampling time between P1 and P2 that is defined: P˙=∑i=1n(ti−t¯)(Pi−P¯)∑i=1n(ti−t¯)2

[0015] Where t i the time at which a pressure measurement is taken, t̅ is an average of the sampling times, P i a determined pressure at a certain sampling time and P̅ein is the mean of all measured pressures during the sampling period.

[0016] To determine for the algorithm whether the pressure measurement data are accurate enough to determine the flux from the anode subsystem, the algorithm uses a pressure approximation value r, as determined by equation (2) below, where the value r determines how close the pressure measurement data are to forming a straight line, and if r is less than a predetermined threshold, the pressure measurement data do not form a line that is straight enough and are therefore not accurate enough, and cannot be used for flux determination. r2=(∑i=1n(ti−t¯)(Pi−P¯)∑i=1n(ti−t¯)2∑i=1n(Pi−P¯)2)2

[0017] Using this method yields a number of variables to determine the validity of the calculated slope ̇Ṗ. The two larger variables are the number of collected pressure measurement points n and the r. 2-value of the linear fit, compared to the data. This is important because the pressure response becomes non-linear after a finite time has elapsed following the closing of injector 20. This decay time varies depending on a number of factors affecting injector 20, such as the supply pressure, supply voltage, coil temperature, etc. The closing time of injector 20 is not modeled, and consequently, if it changes, the algorithm will record an incorrect pressure point, so it is important to ignore the resulting value.

[0018] The slope P of pressure line 42, as calculated by equation (1), can be used to determine the flow through a valve, for example, through the vent valve 24 and the drain valve 36, while these valves are open, or it can be used to determine whether a leak is occurring in the anode subsystem, where a reduction in anode pressure indicates a flow of anode gas out of the anode subsystem over time. Consequently, a diagnostic flag indicating a leak can be set if this outflow from the system is greater than a threshold value that indicates an abnormal stacking operation. This example is, of course, for a flow from the anode subsystem of fuel cell system 10. However, using pressure measurement to determine the slope of the pressure line can be applied to any closed system where pressure can be measured.

[0019] Using the following equation (3), the flux ṅ out of the anode sub-system can be determined as: n˙=P˙⋅VR⋅T−I⋅Ncell2⋅F where ṅ is the flux in mol / sec, V is the volume in the anode sub-system in liters, which is known from the stack design, R is the ideal gas constant (8.315 kPa·L / mol·K), T is the anode sub-system temperature in Kelvin, which is supplied by the stack cooling temperature sensor 32, I is the stack current, N cell the number of fuel cells in the stack 12 and F is the Faraday constant (96485 A·s / mol).

[0020] At low load operation, leakage estimation will be more accurate because the fuel consumption rate is significantly reduced, typically 20 to 100 times lower than at full power, whereas the leakage rate is only slightly reduced, typically 4 to 8 times lower, because the differential pressures that cause leaks are typically lower at low pressure. Accuracy in this measurement improves with longer drop-off times, as the pressure change is increased. This type of extended drop-off time can be applied on a very limited basis, for example, once per load cycle, to limit potential lifetime reduction due to anode undersupply. The amount of leakage gas can be normalized over the drop-off time to obtain an average leakage rate. The drop-off time can be used for normalization, provided that the leak is assumed to occur at the same rate during the injection duration.

Claims

[1] A method for determining a flux of anode gas from an anode subsystem of a fuel cell system (10), the method comprising: - Defining a time period to determine the flow; - Performing a measurement of the pressure in the anode subsystem at a variety of predetermined time intervals during the time period; - Calculating the slope of a pressure line representing the changes in pressure in the anode subsystem over time using pressure measurements; and - Determining the flux of anode gas from the anode subsystem using the slope of the pressure line, wherein determining the flux of anode gas from the anode subsystem using the slope of the pressure line includes determining whether the pressure line fits a straight line within a predetermined threshold, and if the pressure line fits a straight line within the predetermined threshold, the calculated slope is determined to be valid. [2] Method according to claim 1, wherein determining the flow of anode gas from the anode sub-system comprises determining the flow through a venting valve (24) which vents anode gas from the anode sub-system. [3] Method according to claim 1, wherein determining the flow of anode gas from the anode sub-system comprises determining a flow through a drain valve (36) which drains water from the anode sub-system. [4] Method according to claim 1, wherein determining the flow of anode gas from the anode sub-system comprises determining whether there is a leak in the anode sub-system. [5] Method according to claim 1, wherein performing a measurement of the pressure at predetermined time intervals comprises measuring the pressure approximately every three milliseconds during the time period. [6] Method according to claim 1, wherein the anode sub-system has a recirculation line (34) which directs an anode exhaust gas from an outlet of the fuel cell stack (12) to an anode inlet of the fuel cell stack (12). [7] Method according to claim 1, wherein determining the flow of anode gas from the anode sub-system further includes the use of a volume of the anode sub-system and a temperature of the anode sub-system. [8] Method according to claim 7, wherein the fuel cell system comprises a fuel cell stack (12) and wherein determining the flux from the anode sub-system further includes the use of an ideal gas constant, a fuel cell stack current and the number of fuel cells in the stack (12). [9] Method according to claim 1, wherein defining a time period for determining the flow comprises defining the time period between injection events once hydrogen gas is injected into an anode side of the fuel cell stack (12).

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