Method and system for determining the position of a cathode bypass valve in a fuel cell system

Abstract

A method for determining the position of a cathode bypass valve in a fuel cell system that allows cathode air to bypass a fuel cell stack, the method comprising: Determining a hydrogen concentration in a cathode side of the fuel cell system; Determining a volume flow rate through a compressor that supplies air to the cathode side of the fuel cell system; Determining a volume flow through the cathode of the fuel cell stack; Determining a proportion of a volume flow through the cathode to the total flow through the compressor using the volume flow through the compressor and the volume flow through the cathode; Determining a modeled hydrogen outlet concentration from the fuel cell stack based on the volume flow through the compressor, the proportion of the volume flow through the cathode to the total flow through the compressor, and the hydrogen concentration in the cathode; Determining a desired proportion of volume flow through the cathode to the total flow through the compressor using a desired maximum cathode outlet emission and the modeled hydrogen outlet concentration; and Use the desired fraction of volume flow through the cathode and the total flow through the compressor to determine the position of the bypass valve.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] This invention generally relates to a method and system for determining the position of a cathode bypass valve in a fuel cell system.

[0002] With regard to the state of the art, reference is made to the documents US 2004 / 0018 404 A1 and DE 10 2009 023 882 A1. 2. Description of related technology

[0003] Hydrogen is a very attractive fuel because it is pure and can be used to efficiently generate electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that has an anode and a cathode with an electrolyte in between. The anode absorbs hydrogen gas, and the cathode absorbs oxygen or air. The hydrogen gas is split in the anode to produce free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and electrons in the cathode to produce water. The electrons from the anode cannot pass through the electrolyte and are thus passed through a load where they perform work before being delivered to the cathode.

[0004] Proton exchange membrane fuel cells (PEMFCs) are a popular fuel cell for vehicles. The PEMFC generally features a proton-conducting solid polymer electrolyte membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically comprise finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is applied to opposite sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane defines a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require specific conditions for effective operation.

[0005] Typically, multiple fuel cells are combined in a fuel cell stack to produce the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically an airflow, which is forced through the stack via a compressor. Not all of the oxygen is consumed by the stack, and some of the air is exhausted as a cathode exhaust gas, which may contain water as a stack byproduct. The fuel cell stack also receives an anode hydrogen reactant gas, which flows into the anode side of the stack. The stack also has flow channels through which a cooling fluid flows.

[0006] The fuel cell stack includes a series of bipolar plates positioned between the various MEAs in the stack, with the bipolar plates and MEAs positioned 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 reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates, allowing the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made 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 have flow channels through which a cooling fluid flows.

[0007] The gas permeation rate for state-of-the-art membranes in PEM fuel cells is relatively low compared to the current generated by the fuel cell for power generation. When the fuel cell system is shut down, gas permeation through the membrane continues until the partial pressures of the gas components on both sides of the membrane have equalized. The diffusivity of hydrogen through the membrane from the anode to the cathode is approximately three times the nitrogen rate from the cathode to the anode. Higher hydrogen diffusion rates equalize to a rapid equilibration of the hydrogen partial pressures compared to a relatively slow equilibration of the nitrogen partial pressure. The difference in gas diffusivities causes a drop in the absolute pressure of the anode subsystem until the cathode hydrogen partial pressure reaches the anode hydrogen partial pressure.Typically, the anode side of the fuel cell stack operates at a high hydrogen concentration, such as greater than 60%, and large volumes of hydrogen-rich gas are present outside the stack's anode. As the anode absolute pressure drops, more hydrogen is drawn from the anode subsystem into the stack's anode flow field.

[0008] The net result of hydrogen partial pressure equalization after system shutdown is an increased concentration of hydrogen in the cathode side of the fuel cell stack, at least for a certain period after shutdown. At system startup, the compressor is started, but the concentration of hydrogen leaving the fuel cell stack from the cathode must be limited so that no emissions requirements are violated. Thus, when the fuel cell cathode is filled with fresh air, the hydrogen-rich gas leaving the cathode side of the stack must be diluted. To meet startup time and noise requirements, there is a need to optimize stack cathode filling. Since the cathode flow is limited by the power available to the compressor, the filling process must be robust to changes in the total compressor flow.

[0009] The invention is therefore based on the task of meeting this need.

[0010] Known fuel cell systems typically use a bypass valve that allows cathode air to bypass the fuel cell stack and be routed from the compressor directly to the system outlet. Startup strategies may utilize mechanisms to open the bypass valve so that a significant amount of air does not pass through the cathode of the fuel cell stack and is available at the stack outlet to dilute the hydrogen that may be forced through the cathode side of the stack. Typically, these startup strategies have been overly conservative to avoid exceeding the desired hydrogen emission concentration at any point during startup.Because actual system commissioning must wait for hydrogen emissions to dilute before starting the stack, these conservative commissioning strategies have increased the time it takes for the system to start before ignition. SUMMARY OF THE INVENTION

[0011] The object underlying the invention is achieved by a method having the features of claim 1 and by a system having the features of claim 9.

[0012] According to the teachings of the present invention, a method for controlling cathode air flow to a fuel cell stack at system startup by controlling a stack bypass valve is disclosed. The method includes determining a concentration gradient of hydrogen in a cathode side of the fuel cell system. The method also includes determining a volumetric flow rate through a compressor supplying air to the cathode side of the fuel cell system, determining a volumetric flow rate through the cathode of the fuel cell stack, and determining a proportion of the volumetric flow rate through the cathode to the total flow through the compressor using the volumetric flow rate through the compressor and the volumetric flow rate through the cathode.The method determines a modeled hydrogen outlet concentration from the fuel cell stack based on the volume flow through the compressor, the fraction of volume flow through the cathode to the total flow through the compressor, and the concentration of hydrogen leaving the cathode. The method determines a desired fraction of volume flow through the cathode to the total flow through the compressor using a desired maximum system outlet emission of hydrogen and the modeled cathode hydrogen outlet concentration, and then uses the desired fraction of volume flow through the cathode and the total flow through the compressor to determine the position of the bypass valve.

[0013] Additional features of the present invention will become apparent from the following description and the appended claims taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic plan view of a cathode subsystem in a fuel cell system. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] The following discussion of embodiments of the invention directed to a method for controlling cathode flow during startup of a fuel cell system is merely exemplary in nature.

[0015] As discussed below, the present invention proposes the use and implementation of a cathode hydrogen concentration model to control cathode flow during system startup. This method reduces the cathode startup time of the fuel cell system while maintaining exhaust hydrogen emissions within a low target range.

[0016] Fig.1 is a schematic plan view of a fuel cell system 10 including a fuel cell stack 12. A compressor 14 supplies pressurized air to the cathode side of the fuel cell stack 12 on a cathode input line 16. Cathode exhaust gas is discharged from the fuel cell stack 12 on a cathode exhaust line 18. A pressure sensor 28 measures an ambient pressure in the exhaust line 18. A bypass valve 20 is provided in a bypass line 22 that directly connects the cathode input line 16 to the cathode output line 18 to bypass the stack 12. Thus, selectively controlling the bypass valve 20 determines how much of the cathode air flows through the stack 12 and how much of the cathode air bypasses the stack 12. A compressor flow meter (CFM) 24 is provided in an inlet line 26 to the compressor 14 and measures the air flow through the compressor 14.A gas concentration sensor 30 is also provided in the cathode exhaust line 18 and is applicable to measure the concentration of certain gases, such as hydrogen, leaving the system 10.

[0017] An injector-ejector 32 injects hydrogen gas into the anode side of the fuel cell stack 12 at an anode input line 34 from a hydrogen source 36, such as a high-pressure tank. The anode gas exhausted from the fuel cell stack 12 is recirculated back to the injector-ejector 32 on a recirculation line 38. As is well known in the art, it is periodically necessary to vent the anode exhaust gas to remove nitrogen from the anode side of the stack 12. For this purpose, a vent valve 40 is provided in an anode exhaust line 42. In this non-limiting embodiment, the vented anode exhaust gas is combined with the cathode exhaust gas on line 18 to dilute hydrogen in the anode exhaust gas to below combustible levels. In other system configurations, line 42 may be coupled to cathode input line 16 to provide catalytic cathode combustion.A pressure sensor 44 is provided in the recirculation line 38 and provides a measurement of the pressure in the anode subsystem. Although the pressure sensor 44 is located in the recirculation line 38 in this embodiment, the pressure sensor 44 may be provided at any location in the anode subsystem suitable for taking an accurate pressure reading.

[0018] The system 10 also includes a stack voltage measurement processor 46 that receives voltage signals from one or more of the fuel cells in the stack 12 to provide appropriate voltage measurements.

[0019] The cathode subsystem includes bypass and / or exhaust valves to allow control of the bypass of air around the fuel cell stack 12.

[0020] The control of the various valves can be optimized to maintain the maximum desired cathode emission of hydrogen to fill the cathode side of the stack 12 in the minimum time possible with a specific air flow. Because the cathode subsystem is large, the concentration of hydrogen gas in the cathode subsystem is not uniform. Furthermore, the hydrogen in the fuel cell stack 12 and various lines continues to deplete after the last system shutdown, so the concentration of hydrogen gas not only varies depending on the location but is continuously reduced over time.

[0021] The present invention proposes a method for determining the hydrogen concentration in the cathode subsystem at system startup so that the cathode air can be effectively controlled to flow through the fuel cell stack 12 or around the fuel cell stack 12 on the bypass line 22 as needed to control hydrogen emissions.

[0022] To optimize the concentration of hydrogen leaving the cathode exhaust, the mole fraction of hydrogen is determined across the cathode subsystem. This concentration may be measured using a sensor or by approximation using a gas concentration model. For sensing, concentration measurements may be performed using a gas concentration sensor, which may be an acoustic sensor, or derived from other sensors. Two methods by which the hydrogen concentration may be derived include using the measured pressure drop across an orifice and providing stack voltage measurements during a hydrogen fill of the anode side of the fuel cell stack 12.

[0023] For the measured pressure drop method and for a given orifice size through which a gas flows, the pressure drop across the orifice is a function of the gas composition. At a given flow rate, the pressure drop is lower when the gas is hydrogen compared to when the gas is air, oxygen, or nitrogen. For a gas containing a hydrogen mixture, the pressure drop for a specific molar flow rate is proportional to the hydrogen concentration in the mixture. In this way, the hydrogen concentration in the gas can be determined.

[0024] For the stack voltage measurement method, if a significant stack voltage is measured, as by processor 46, when the stack anode is filled with hydrogen, then the cathode must contain some oxygen. If oxygen exists on the cathode side of the stack 12, it can be assumed that the partial pressure of hydrogen on the cathode side is sufficiently low so as not to limit the cathode filling for emissions reasons.

[0025] In the hydrogen concentration optimization process using a gas concentration modeling method, a highly accurate model or measurement of the cathode hydrogen concentration is desired to minimize system commissioning. Knowledge of the anode hydrogen concentration can be used to model the cathode hydrogen concentration. A fixed calibration of the position-dependent hydrogen concentration in the cathode subsystem can be used instead of the model.

[0026] Once the cathode-side hydrogen concentration has been determined, a valve model for the bypass valve 20 is created to accurately predict the valve position required to achieve a desired flow split between the bypass line 22 and the cathode flow path through the stack 12. For a given exhaust hydrogen emission target, the desired flow split can be determined as a function of the cathode flow.

[0027] An example calculation for the method for controlling the cathode flow split is given as follows: γcathode_flow_%=V˙compressor_mol / sV˙cathode_flow_mol / s=f(kbypass_valve_%,V˙compressormol / s) where k bypass_valve_% represents the percentage that the bypass valve 20 is open in either linear or angular dimensions, V̇ compressor_mol / s is the volume flow of air through the compressor 14, V̇ cathode_flow_mol / sis the volume flow of air through the cathode side of the stack 12 and γ cathode_flow_% is the proportion of the volume flow through the cathode side of the stack 12 to the total flow through the compressor.

[0028] Furthermore, γ measured_H2_% the location and hydrogen concentration in the cathode subsystem, which is determined experimentally, such as: γstack_cathode_H2_outlet_%= f(∫(V˙compressor_mol / s⋅γcathode_flow_%)dt,γmeasured_H2_%) where γ stack_cathode_H2_outlet_% is the modeled hydrogen outlet concentration.

[0029] If the desired cathode outlet emissions γ H2_Max_Outlet are, the desired γ cathode_flow_% be determined as: γcathode_flow_%=γH2_Max_Outletγstack_cathode_H2_outlet_% where γ cathode_flow_% is limited to the range of 42 - 100%, γ H2_Max_Outlet is the desired maximum hydrogen concentration in the cathode outlet and γ cathode_flow_%the desired cathode pitch based on the desired γ H2_Max_Outlet is.

[0030] Using the results from equation (3), equation (1) can be rearranged to obtain the desired position k bypass_valve_% of the bypass valve 20 over time to control the cathode outlet hydrogen percentage during cathode filling.

Claims

[1] A method for determining the position of a cathode bypass valve in a fuel cell system that allows cathode air to bypass a fuel cell stack, the method comprising: Determining a hydrogen concentration in a cathode side of the fuel cell system; Determining a volume flow rate through a compressor that supplies air to the cathode side of the fuel cell system; Determining a volume flow through the cathode of the fuel cell stack; Determining a proportion of a volume flow through the cathode to the total flow through the compressor using the volume flow through the compressor and the volume flow through the cathode; Determining a modeled hydrogen outlet concentration from the fuel cell stack based on the volume flow through the compressor, the proportion of the volume flow through the cathode to the total flow through the compressor, and the hydrogen concentration in the cathode; Determining a desired proportion of volume flow through the cathode to the total flow through the compressor using a desired maximum cathode outlet emission and the modeled hydrogen outlet concentration; and Use the desired fraction of volume flow through the cathode and the total flow through the compressor to determine the position of the bypass valve. [2] The method of claim 1, wherein determining the hydrogen concentration in the cathode side comprises using a concentration sensor. [3] The method of claim 1, wherein determining the hydrogen concentration in the cathode side comprises determining a pressure drop across an orifice in the bypass valve. [4] The method of claim 1, wherein determining the hydrogen concentration in the cathode side comprises measuring the stack voltage when an anode side of the stack is or is being filled. [5] The method of claim 1, wherein determining the concentration of hydrogen in the cathode side comprises using a model to determine the concentration. [6] The method of claim 1, wherein determining the proportion of volume flow through the cathode to the total flow through the compressor comprises using the equation: γcathode_flow_%=V˙compressor_mol / sV˙cathode_flow_mol / s=f(kbypass_valve_%,V˙compressormol / s) where k bypass_valve_%represents the percentage that the bypass valve is open in either linear or angular dimensions, V̇ compressor_mol / s is the volume flow of air through the compressor, V̇ cathode_flow_mol / s is the volume flow of air through the cathode side of the stack and γ cathode_flow_% is the proportion of the volume flow through the cathode side of the stack to the total flow through the compressor. [7] The method of claim 1, wherein determining the modeled hydrogen outlet concentration comprises using the equation: γstack_cathode_H2_outlet_%= f(∫(V˙compressor_mol / s⋅γcathode_flow_%)dt,γmeasured_H2_%) where γ stack_cathode_H2_outlet _ % is the modeled hydrogen outlet concentration, V̇ compressor_mol / s is the volume flow of air through the compressor, γ cathode_flow_% is the proportion of the volume flow through the cathode side of the stack to the total flow through the compressor. [8] The method of claim 1, wherein determining the desired proportion of volume flow through the cathode to the total flow through the compressor comprises using the equation: γcathode_flow_%=γH2_Max_Outletγstack_cathode_H2_outlet_% where γ cathode_flow_% is the proportion of the volume flow through the cathode side of the stack to the total flow through the compressor, γ H2_Max_Outlet is the desired maximum hydrogen concentration in the cathode outlet and γ stack_cathode_H2_outlet_% is the modeled hydrogen outlet concentration. [9] A system for determining the position of a cathode bypass valve in a fuel cell system that allows cathode air to bypass a fuel cell stack, the system comprising: a means for determining a hydrogen concentration in a cathode side of the fuel cell system; means for determining a volumetric flow rate through a compressor that supplies air to the cathode side of the fuel cell system; a means for determining a volume flow rate through the cathode of the fuel cell stack; means for determining a proportion of volume flow through the cathode to the total flow through the compressor using the volume flow through the compressor and the volume flow through the cathode; means for determining a modeled hydrogen outlet concentration from the fuel cell stack based on the volume flow through the compressor, the proportion of volume flow through the cathode to the total flow through the compressor, and the hydrogen concentration in the cathode; means for determining a desired proportion of volume flow through the cathode to the total flow through the compressor using a desired maximum cathode outlet emission and the modeled hydrogen outlet concentration; and a means of using the desired fraction of volume flow through the cathode and the total flow through the compressor to determine the position of the bypass valve. [10] The method of claim 9, wherein the means for determining the hydrogen concentration in the cathode side uses a concentration sensor.

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