ADAPTIVE METHOD FOR CONVERSION OF AN EXTERNAL POWER DEMAND INTO A POWER SETPOINT FOR A FUEL CELL SYSTEM BASED ON STACK POWER

Abstract

Method for determining a current setpoint for a fuel cell stack (22), the method comprising: estimating an exchange current density (j0, θ1) and a mass transfer coefficient (c, θ2) based on a relationship between power and electric current of the fuel cell stack (22); converting a power request signal into a current request signal using the exchange current density (j0, θ1) and the mass transfer coefficient; calculating a cell voltage at two predetermined stack current densities to determine a slope (R) of a polarization curve relating a stack current and a stack voltage using the exchange current density (j0, θ1) and the mass transfer coefficient (c, θ2); calculating a change in electric current based on the power request signal, the current request signal, the stack voltage, the stack current, and the calculated slope (R) of the polarization curve;and generating the current setpoint based on the change in electric current.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] This invention relates generally to a system and method for converting a power demand signal for a fuel cell stack into a current setpoint for the stack and in particular to a system and method for converting a power demand signal for a fuel cell stack into a current setpoint for the stack, which takes stack power into account. 2. Description of the state of the art

[0002] Hydrogen is a very interesting fuel because it is clean and can be used to efficiently generate electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device consisting of an anode and a cathode with an electrolyte in between. The anode takes in hydrogen gas, and the cathode takes in oxygen or air. The hydrogen gas is split in the anode to produce free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen 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 therefore conducted through a load to perform work before being sent to the cathode.

[0003] Proton exchange membrane fuel cells (PEMFCs) are a common fuel cell for vehicles. A PEMFC generally comprises a proton-conducting solid polymer electrolyte membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically consist of finely dispersed catalytic particles, usually platinum (Pt), mounted 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 forms a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require specific conditions for effective operation.

[0004] Typically, several fuel cells are combined in a fuel cell stack to generate the desired power output. The fuel cell stack receives a cathode inlet gas, typically an airflow forced through the stack by a compressor. Not all of the oxygen is consumed by the stack, and some of the air is discharged as cathode exhaust, which may include 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] The fuel cell stack comprises a series of bipolar plates positioned between the multiple MEAs in the stack, with the bipolar plates and the MEA positioned between the two end plates.

[0006] The bipolar plates comprise 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 also provided on the cathode side of the bipolar plates, allowing the cathode reactant gas to flow to the respective MEA. One end plate comprises anode gas flow channels, and the other end plate comprises cathode gas flow channels. The bipolar plates and the end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electrical current generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.

[0007] The stack controller must know the electrical current / voltage relationship of the fuel cell stack, known as the polarization curve, to set stack reactant flows according to power requirements. The relationship between the stack's electrical voltage and current is typically difficult to define because it is non-linear and changes depending on many variables, including stack temperature, stack partial pressures, and cathode and anode stoichiometries. Furthermore, the electrical current / voltage relationship changes as the stack degrades over time. In particular, an older stack has lower electrical cell voltages and must supply more electrical current to meet power requirements than a new, non-degraded stack.

[0008] When the fuel cell stack control unit receives a power demand signal from the vehicle's driver, a control algorithm generates a current setpoint for the fuel cell stack. In known systems, this current setpoint is based on the polarization curve, which is assumed to be linear. Typically, only a single polarization curve is used for the fuel cell stack, and this polarization curve is one determined for the beginning of life (BOL) of the fuel cell stack. However, as the stack ages, the electrical voltage across it degrades, requiring a higher stack voltage for a higher stack current. Thus, an increase in power demand necessitates a higher current setpoint. Furthermore, external disturbances to the system cause fluctuations in the electrical voltage across the stack, which manifest as fluctuations in the current setpoint signal. SUMMARY OF THE INVENTION

[0009] According to the teachings of the present invention, a system and method for converting a fuel cell stack power demand signal into a stack current setpoint that takes stack power parameters into account are disclosed. The method comprises obtaining a power / electric current relationship curve of the fuel cell stack to provide stack parameters, including exchange current density and mass transfer coefficient. The method then calculates a slope for the stack using the parameters from the power / electric current relationship curve, which includes calculating a cell voltage at two predetermined stack current densities.Then, in response to the power request signal, the electrical voltage of the stack, the electrical current of the stack, and the calculated slope, the procedure calculates a change in the electrical current and uses the change in the electrical current to update the current setpoint for the stack.

[0010] Further features of the present invention will become apparent from the following description and the attached claims in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. Figure 1 is a graph with stack current on the horizontal axis and stack power on the vertical axis, showing a polarization curve for a fuel cell stack at the beginning of the stack's lifetime and a curve of the power / electric current relationship for a fuel cell stack at the end of the stack's lifetime;

[0012] Fig. Figure 2 is a graph with time on the horizontal axis and stack voltage and current on the vertical axis, showing fluctuations in the electrical voltage and current of the stack;

[0013] Fig. Figure 3 is a block diagram of a fuel cell system that uses an algorithm to convert power into electrical current;

[0014] Fig. Figure 4 is a flowchart showing a process for generating a setpoint of the stack current for a stack power request signal; and

[0015] Fig. Figure 5 is a graph with stack current on the horizontal axis and stack voltage on the vertical axis, showing curves of the relationship between power / electric current for calculating slope based on stack lifetime. DETAILED DESCRIPTION OF THE EXECUTION FORMS

[0016] The following description of the embodiments of the invention, which relates to a system and method for determining a current setpoint for a fuel cell stack, taking into account stack performance parameters, is merely exemplary and is not intended to limit the invention or its applications or uses in any way.

[0017] Fig. Figure 1 is a graph with stacked current on the horizontal axis and stacked power on the vertical axis. A curve is shown. 10 the relationship between power and electric current for a typical fuel cell stack at the beginning of the stack's lifetime and a curve 12 The typical power / electrical current relationship for a fuel cell stack at the end of its lifetime is shown. As can be seen, the curve is 10The relationship is almost linear, with the current setpoint for the fuel cell stack increasing as the required power output increases. However, as shown, a typical power demand (dashed line) would result in a lower current setpoint for the stack at the beginning of its service life and a higher current setpoint for the stack at the end of its service life due to voltage loss.

[0018] Fig. Figure 2 is a graph with time on the horizontal axis and stack voltage and stack current on the vertical axis. The graph line 14 This represents the electrical voltage of the stack over a certain period, showing that external fluctuations occur in the electrical voltage due to many factors. These electrical voltages manifest themselves as a stack current across the line. 16 , which exhibits fluctuations, resulting in a line 18The depicted direct stack flow setpoint fluctuation is provided for.

[0019] As explained in more detail below, the present invention proposes an algorithm that provides a process for generating a more accurate current setpoint for a fuel cell stack based on a power demand signal over the entire lifetime of the stack, which eliminates the fluctuations in the current setpoint that may occur as a result of fluctuations in the external electrical voltage of the fuel cell stack.

[0020] Fig. 3 is a block diagram of a fuel cell system 20 , which is a fuel cell stack 22 includes system parameters in field 24 The system parameters are stored, including the values ​​θ1 and θ2, which are defined below. The system parameters are then passed to a processor. 26 for converting power into electrical current, which also includes a stack power request signal P Stck Req receives. The processor 26 For the conversion of power into electrical current, the power request signal P is converted. Stck Req into a current demand signal I Stck Req using the values ​​θ1 and θ2, where the current demand signal I Stck Req the current setpoint for the fuel cell stack 22 is, as explained in more detail below. The current setpoint signal I Stck Req , which is attached to the fuel cell stack 22 When applied, it generates a stack voltage V and a stack current I, which are fed back to the processor 26 sent for the conversion of power into electrical current.

[0021] Fig. 4 is a flowchart 30 , which is one in the processor 26 executed process for generating the current setpoint signal I Stck Req as shown in the invention. In the field 32The algorithm receives parameters from an estimate of the curve of the power / electric current relationship of the fuel cell stack. 22 In one embodiment, these parameters are the values ​​θ1 and θ2 and are known as the exchange current density and mass transfer coefficient, respectively. A detailed explanation of a system and method for estimating a power / electric current curve, including determining the exchange current density and the mass transfer coefficient, can be found in US patent application serial no. 11 / 669,898, filed on January 31, 2007, entitled "Algorithm For Online Adaptive Polarization Curve Estimation of a Fuel Cell Stack," which was assigned to the acquirer of the application and is incorporated herein by reference.

[0022] At field 34The algorithm for converting power into electrical current calculates the slope R of the curve representing the power / electrical current relationship for the fuel cell stack. 22 based on the lifespan of the stack 22 . Fig. Figure 5 is a graph with electric current on the horizontal axis and stack voltage on the vertical axis, which gives the slope R for the stack. 22 based on stack lifetime shows, with the line 40 the beginning of the stack's lifespan 22 is and the line 42 the end of the stack's lifespan 22 The slope R of the curve representing the power / electric current relationship of the stack is... 22 The cell voltage E is calculated using a voltage prediction algorithm. cell to calculate for two predetermined current densities along the line, for example at 0.1 A / cm 2 and 1.0 A / cm 2In a non-restrictive embodiment, the cell voltage E cell calculated as: where: E cell the cell voltage (V) is, j Stack current density (A / cm³) 2 ) is, R HFR Cell HFR resistance (ohm cm) 2 ) is, E rev a thermodynamically reversible cell potential (V) is, a the background current density from cell short-circuiting / cell crossover (A / cm²) 2 ) is, j 0 (θ1) the exchange current density (A / cm³) 2 ) is, j ∞ a limiting current density (A / cm²) 2 ) is and c (θ2) is the mass transfer coefficient.

[0023] The fuel cell resistance value R HFR defines a high-frequency resistance at the fuel cell stack, which provides an indication of the relative humidity of the stack. The fuel cell resistance value R HFRThis can be a measured value or a model value, as is known to experts. In the model, the fuel cell resistance value R can be represented. HFR as a function of the stack inlet and outlet temperatures and the relative humidity of the cathode gas, which is also a function of the inlet and outlet temperatures, pressure, and flow rate. Therefore, the fuel cell resistance value R represents HFR the stack operating conditions, including temperature, pressure and cathode throughput.

[0024] After obtaining two cell voltage values ​​for two stack current densities, the slope R of the line can be calculated. As stack power degrades over time, the slope R also changes to account for stack degradation.

[0025] At field 36 The algorithm receives the power demand signal P Stck Req , the relationship between electric current / electrical voltage of the stack (polarization curve) and the slope R, and calculates a change in the electric current of the stack. 22 This step involves calculating an increment to the current setpoint calculation, which takes the slope R from the field. 34 , the power demand signal P Stck Req the electricity demand I req , the voltage feedback V t from the pile 22 and the current feedback I t from the pile 22 The system uses time t to calculate a change in the stack current ΔI at the next time step to meet the power requirement. In one embodiment, the following equation is used to calculate the change in current ΔI.

[0026] At field 38 The algorithm for converting power into electrical current calculates the current setpoint signal I Stck req through: I Stck req = I Req(t+1) = I Req(t) + ΔI

[0027] The algorithm for converting power into electrical current updates at the field 36 constantly the current setpoint signal I Stck req at a faster rate than the calculation of the slope R of the stack 22 based on estimates of the curve of the relationship between power and electric current over time.

[0028] The foregoing description discloses and describes only exemplary embodiments of the present invention. A person skilled in the art will readily recognize from such a description, along with the accompanying drawings and claims, that various modifications, adaptations, and alterations can be made to it without deviating from the essence and scope of protection of the invention as set forth in the following claims.

Claims

[1] Method for determining a power setpoint for a fuel cell stack, the method comprising: Estimating a curve of the power / electric current relationship of the fuel cell stack; Providing parameters from the estimation of the curve of the power / electric current relationship; Calculating a slope for the stack using the parameters from the estimation of the power / electric current relationship curve; Calculating a change in electric current in response to a power demand signal, stack voltage, stack current, and the calculated slope; and Generating the target current value based on changes in electric current. [2] Method according to claim 1, wherein the provision of parameters from the estimation of the curve of the power / electric current relationship comprises obtaining an exchange current density and a mass transfer coefficient. [3] Method according to claim 2, wherein calculating a slope for the stack comprises calculating a cell voltage at two predetermined stack current densities. [4] Method according to claim 3, wherein the calculation of the cell voltage uses the following equation: where E cell The cell voltage (V) is, j current density (A / cm²) 2 ) is, R HFR HRF cell resistance (ohm cm) 2 ) is, E rev thermodynamic reversible cell potential (V) is, a background current density from cell short-circuiting / cell crossover (A / cm²) 2 ) is, j 0 Exchange current density (A / cm³) 2 ) is, j ∞ the limiting current density (A / cm²) 2 ) is and c is the mass transfer coefficient. [5] Method according to claim 3, wherein the predetermined current densities are 0.1 A / cm² 2 and 1.0 A / cm 2 be. [6] The method of claim 1, wherein calculating a change in electric current comprises using the following equation: where ΔI is the change in electric current, V t the stacking stress is, I (t) The stack current is R, the slope is R, and P is P. Req This is the power demand signal. [7] Method according to claim 1. where the following equation is used to generate the current setpoint: I Stck req = I Req(t+1) = I Req(t) + ∆I where I Stck req the current setpoint is, I Req where a previous current setpoint is and ΔI is the change in electric current. [8] System for determining a power setpoint for a fuel cell stack, the system comprising: Means for estimating a curve of the power / electric current relationship of the fuel cell stack; Means of providing parameters from the estimation of the curve of the power / electric current relationship; Means to calculate a slope for the stack using the parameters from the estimation of the curve of the power / electric current relationship; Means for calculating a change in electric current in response to a power demand signal, stack voltage, stack current, and the calculated slope; and Means for generating the target current value based on the changing electric current. [9] System according to claim 8, wherein the means for supplying parameters from the estimation of the curve of the power / electric current relationship provides an exchange current density and a mass transfer coefficient. [10] System according to claim 9, wherein the means for calculating a slope for the stack calculates a cell voltage at two predetermined stack current densities.

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