Starting up a fuel cell system using voltage profile control

Abstract

Fuel cell system (12), comprising: a fuel cell stack (14) with a plurality of fuel cells (15) and a stack voltage (Vs), a DC-DC boost converter (30); and a controller (50B) configured to control the stack voltage (V S ) via the DC / DC boost converter (30) during the start-up of the fuel cell stack (14), and is programmed with several calibrated voltage profiles, each having a corresponding size and rate of change; wherein the controller (50B) is configured to detect an air start of the fuel cell stack (14) in response to a requested start-up of the fuel cell stack (14) and in response to the detected air start a predetermined voltage profile of the stack voltage (V S) during an actual start-up of the fuel cell stack (14) via the control of the DC / DC boost converter (30) using the multiple calibrated voltage profiles.

Description

INTRODUCTION

[0001] A fuel cell stack is an electrochemical device capable of generating electricity from a paired oxidation / reduction reaction occurring in a stack of identically configured fuel cells. Each fuel cell contains an electrolyte that serves to transport charged particles between oppositely charged electrodes. A platinum-based or other suitable catalyst accelerates the catalytic process at the electrodes. Hydrogen and oxygen are the basic fuels of the oxidation / reduction reaction, allowing the formation of water as an inert byproduct of the reaction. As a result, fuel cells can be used as an alternative source of clean energy in vehicle propulsion systems, power plants, and other mobile or stationary systems.

[0002] In a fuel cell system, stack startup is defined as the elapsed time between the initiation of the paired oxidation / reduction reaction and a later time at which the stack finally reaches a stable output voltage. Thus, the fuel cell system begins outputting usable power levels upon completion of the startup process. Certain issues can prevent a given fuel cell system from experiencing a successful startup, such as an "air start" event or a sudden increase in cell voltage. An air start event occurs when negligible hydrogen levels remain in the stack anode after an extended off-time period, such as when a fuel cell vehicle remains in an off-state over a weekend. An air start can result in a negative anode potential and potentially stack degradation.Voltage spikes above 900 mV can lead to oxidation and corrosion of the catalyst, which in turn can reduce the available surface area of ​​the catalyst and impair the efficiency of the fuel cell.

[0003] Document DE 10 2016 212 683 A1 discloses a method for controlling the startup of a fuel cell stack, in which hydrogen is supplied to the fuel cell stack. If this causes the fuel cell voltage to rise, air is introduced, maintaining the fuel cell voltage at a predetermined value.

[0004] The document DE 10 2016 110 451 A1 discloses a starting method for a fuel cell system that uses a DC-DC converter for voltage limitation in order to reduce half-cell voltages. SUMMARY

[0005] Disclosed herein is a control method for controlling the output voltage of a fuel cell stack, ie, a stack voltage, during a stack startup process according to predetermined voltage profiles, each profile having a corresponding magnitude and rate of change. The method includes using a direct current-to-direct current (DC / DC) boost converter to control the stack voltage to the predetermined voltage profiles, thereby substantially preventing a negative potential of the stack anode and reducing the occurrence of catalyst oxidation.

[0006] A fuel cell system includes a fuel cell stack having a plurality of fuel cells and a stack voltage, as well as a DC / DC boost converter and a controller. The controller is configured to regulate the stack voltage during fuel cell stack startup via the DC / DC boost converter and is programmed with calibrated voltage profiles, each having a corresponding magnitude and rate of change. The controller is also configured to detect an air start of the fuel cell stack in response to a requested fuel cell stack startup. In response to the detected air start, the controller enforces predetermined voltage profiles during an actual fuel cell stack startup via stack voltage boost converter regulation, following calibrated voltage profiles.

[0007] The calibrated voltage profiles include a first voltage profile corresponding to an anode pressurization stage and a second voltage profile corresponding to a cathode fill stage, wherein the second voltage profile has a rate of change that is smaller than that of the first voltage profile.

[0008] The controller is programmed to detect the air start of the fuel cell stack using an output from a gas concentration estimation (GCE) model. The output of the GCE model in this embodiment may be an estimated oxygen level in an anode plate of the fuel cell stack. The controller may receive, as a set of inputs, an off-time, a temperature, and a pressure value of the fuel cell stack, with the output of the GCE model being determined as a function of the set of inputs.

[0009] The DC / DC boost converter may include a semiconductor switch and a gate drive circuit, wherein the controller applies or enforces the voltage profiles by controlling a duty cycle of the semiconductor switch via the gate drive circuit during a voltage recovery mode of operation.

[0010] The controller may include a microprocessor that is part of the DC / DC boost converter and that controls the duty cycle of the semiconductor switch during the voltage recovery mode of operation. The controller is configured to enforce the voltage profiles after the startup is complete by controlling the DC / DC boost converter in a current control mode.

[0011] The fuel cell system may include a high-voltage battery, an inverter module electrically connected to the battery, an axle, and an electric motor operable to output torque to the axle. In one non-limiting embodiment of the motor vehicle, a road wheel may be rotatably connected to the axle.

[0012] A vehicle includes the fuel cell stack and the DC / DC boost converter, a battery pack connected to the fuel cell system and having a DC output voltage, a power inverter module (PIM) electrically connected to the battery pack and configured to convert the DC output voltage to an AC voltage, an electric motor electrically connected to the PIM and excited by the AC voltage to produce an output torque, a drive wheel connected to the electric motor and driven by the output torque to propel the vehicle, and a controller.

[0013] The controller is configured to perform a method comprising detecting an air start of the fuel cell stack in response to a requested startup of the fuel cell stack, wherein the controller is programmed with calibrated voltage profiles, each having a corresponding magnitude and rate of change. The method also includes forcing a predetermined voltage profile of the stack voltage in response to the detected air start during an actual startup of the fuel cell stack, including closed-loop operation of the DC / DC boost converter, using the plurality of calibrated voltage profiles.In one embodiment, the calibrated voltage profiles include a first voltage profile corresponding to an anode pressurization stage and a second voltage profile corresponding to a cathode fill stage, wherein the second voltage profile has a rate of change that is smaller than that of the first voltage profile.

[0014] The foregoing summary is not intended to represent every embodiment or aspect of the present disclosure. Rather, the foregoing summary merely illustrates some of the novel aspects and features set forth herein. The above-listed features and advantages, as well as other features and advantages of the present disclosure, will be readily apparent from the following detailed description of the illustrated embodiments and representative modes for carrying out the present disclosure, taken in conjunction with the accompanying drawings and the appended claims. Furthermore, the present disclosure expressly includes all combinations and subcombinations of the foregoing elements and features set forth above and below. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of an exemplary vehicle having a fuel cell stack and a controller configured to control an output voltage of the stack, ie, a stack voltage, to predetermined voltage profiles during a stack startup operation. Fig. 2A and Fig. 2B are representative voltage profiles for a hydrogen start and an air start, respectively. Fig. 3 is a flowchart illustrating an exemplary method for controlling the stack voltage of the fuel cell system of Fig. 1 during startup using predetermined voltage profiles.

[0015] Various modifications and alternative forms are susceptible to the present disclosure, and some exemplary embodiments are illustrated herein by way of example only with reference to the drawings. It should be understood, however, that the novel aspects of this disclosure are not limited to the particular forms illustrated in the accompanying drawings. Rather, this disclosure encompasses all modifications, equivalents, combinations, subcombinations, permutations, groupings, and alternatives consistent with the spirit and scope of the disclosure as defined by the appended claims. DETAILED DESCRIPTION

[0016] Referring to the drawings, wherein like reference numerals refer to like features throughout the several views, an exemplary fuel cell system 12 is schematically illustrated in Fig. 1. The fuel cell system 12 includes a fuel cell stack 14 that can be adapted for use as a DC power supply in a vehicle 10, such as a motor vehicle as shown, or a boat or other marine vehicle, a spacecraft, a robot, or other mobile platforms. Alternatively, the fuel cell stack 14 can be used as a power supply as part of a stationary power plant (not shown) or in other systems requiring onboard DC power generation. For illustrative purposes, the fuel cell stack 14 is Fig. 1 is described below in connection with a drive function on board the vehicle 10, without limiting the fuel cell stack 14 to such applications.

[0017] The fuel cell stack 14 includes a plurality of fuel cells 15 arranged between respective anode and cathode plates 14A and 14C. While the specific shape of the fuel cell stack 14 and its constituent fuel cells 15 may vary with the intended application of the fuel cell system 12, according to one exemplary embodiment, each of the fuel cells 15 may be a polymer electrolyte membrane / proton exchange membrane (PEM) fuel cell, which delivers a relatively high power density with low weight and volume. PEM-type fuel cells utilize a catalyst in the form of platinum or a platinum alloy and a solid electrolyte polymer material (not shown).The fuel cells 15, when configured as PEM fuel cells, operate using a supply of gaseous hydrogen (arrow H2) from a hydrogen storage tank 18 and oxygen (arrow O2), with the oxygen (arrow O2) typically being supplied from ambient air via a compressor 16 (arrow A).

[0018] In the exemplary fuel cell system 12 of Fig. 1, a stack bypass line 19 is routed around the fuel cell stack 14. A bypass valve 17, disposed within the bypass line 19, has a variable open / closed position that can be automatically adjusted, e.g., via a controller 50B. Control of the bypass valve 17 allows the controller 50B to regulate the operation of the fuel cell stack 14, for example, via an air control signal (arrow CCA), by adjusting an amount of oxygen (arrow O2) flowing through or bypassing the fuel cell stack 14. Oxygen-containing air discharged from the bypass valve 17 eventually returns to the cathode outlet (arrow CEX), is expelled from the cathode plate 14C, before being vented from the fuel cell stack 14. Anode exhaust gas (arrow AEX) is similarly vented from the anode plate 14A.The controller 50B may alternatively control the operation of the fuel cell stack 14 via an electrical control signal (arrow CCE), ie, via a voltage or current control of the DC / DC boost converter 30, as indicated below.

[0019] The fuel cell system 12 from Fig. 1 further includes a voltage monitoring circuit (VMC) 20. The VMC 20 may be configured to monitor a voltage level of the fuel cell stack 14, referred to below as a stack voltage, to measure minimum and maximum cell voltages of the individual fuel cells 15, and to calculate an average cell voltage. Such collective voltage control values ​​(arrow VC) may be used by the controller 50B to control the operation of the fuel cell stack 14 in response to changing power demands, such as an increased driver throttle request in the illustrated embodiment of the vehicle propulsion system.

[0020] The exemplary vehicle 10 includes a DC / DC boost converter 30 disposed between the fuel cell system 12 and a high-voltage system (HV system) 40, the latter including a battery 31. The battery 31 used in the embodiment of Fig. 1 includes a semiconductor switch Q1 and a diode (D2), the former being illustrated here as an example of an insulated gate bipolar transistor (IGBT). The semiconductor switch Q1 has an on / off state controlled via a gate drive circuit (GD) 26 using a gate drive signal (arrow CCG) from another controller 50A or from the controller 50B in a single controller embodiment. A magnitude and direction / sign of a stack current (arrow IS) from the fuel cell stack 14 may be measured by a current sensor (SC) and communicated to the controller 50A, where the measured magnitude and direction of the stack current (arrow IS) is used to control the on / off state of the semiconductor switch Q1.

[0021] The intended function of the boost converter 30 of Fig. 1 is to increase an input voltage, here a stack voltage (VS), to a higher or "boosted" output voltage level (VBO). The voltage boost operation may be achieved via pulse width modulation or other switching control of the semiconductor switch Q1, as is known in the art. During a period of time when the semiconductor switch Q1 is turned on and thus conducting, the stack current (arrow IS) flows to an inductor L1 having primary (1) and secondary (2) windings, via the semiconductor switch Q1, and back into the fuel cell stack 14. Energy is stored in the inductor L1 as the voltage across the inductor L1 increases. A diode (D1) and a capacitor (C1) may be arranged downstream of the inductor L1 as shown, with the capacitor C1 being arranged in parallel with the HV system 40.

[0022] When the semiconductor switch Q1 has received a turn-off command from the controller 50A, the energy stored in the inductor L1 causes the stack current (arrow IS) to flow to an output load, which in the particular embodiment of Fig. 1 is the HV system 40. The voltage at the output of the boost converter 30 (VBO), which forms the electrical potential across the capacitor C1, is the sum of the stack voltage (VS) and a voltage across the inductor L1. An off / on cycle of the semiconductor switch Q1 occurs cyclically, with the total cycle time defined as the sum of the off and on times. The on-time of the semiconductor switch Q1 can therefore be defined as TON / TSWITCH, where TON is the on-time of the switch Q1 and TSWITCH is the total cycle time.

[0023] As in the Fig. 1, the controllers 50A and the controller 50B, which may be separate task-specific microprocessor-based devices as shown or a single device, may be programmed with instructions embodying the present method 100. The method 100 is used to distinguish an air start from a hydrogen start using different calibrated voltage profiles, e.g., based on the output of a gas concentration estimation model 55, and to use the boost converter 30 to control the fuel cell system 12 to voltage profiles during startup of the fuel cell stack 14. During normal operation, the controllers 50A, 50B control the duty cycle of the semiconductor switch Q1 as a function of the stack current (arrow IS). During execution of the method 100 of Fig. 3, the controllers 50A, 50B proceed to control the duty cycle of the semiconductor switch Q1 as a function of the stack voltage (VS), particularly during a voltage recovery mode, in order to accurately regulate the boosted output voltage (VBO).

[0024] Still referring to Fig. 1, when the fuel cell system 12 of Fig. 1, a device powers a multi-phase electric machine (ME) 34, the magnitude of the alternating voltage (VAC) of the electric motor 34 may exceed the available direct current (VDC) output of the fuel cell stack 14. The boost converter 30 thus acts to increase the voltage level supplied to the electric motor 34. Additionally, a power inverter module (PIM) 32 may be electrically connected to the HV system 40 and used to convert a direct current voltage on a high-voltage DC bus 33 to an alternating current level suitable for energizing individual phase windings 35 of the electric motor 34, e.g., a traction motor used to propel the vehicle 10.The output torque (arrow TO) of the electric motor 34 may be provided to an output member 36 to perform work on board the vehicle 10, such as rotating a set of road wheels 39, only one of which is shown for simplicity, to propel the vehicle 10 when the vehicle 10 is optionally configured as a motor vehicle.

[0025] Although omitted for simplicity, as part of the fuel cell system 12 of Fig. 1, an open / closed set of switches or switching devices 21 may be controlled via controller 50A or 50B to electrically isolate the fuel cell stack 14 from the rest of the fuel cell system 12, for example, when the vehicle 10 is not running or during a possible high-voltage electrical fault. Additionally, and also omitted for simplicity of illustration, lower voltage auxiliary power may be provided to an auxiliary battery via an auxiliary power module, such as a DC / DC step-down converter, which may be used to reduce voltage levels on the high-voltage bus 33 and step it down to voltage levels suitable for storage in the auxiliary battery, e.g., 12-15 VDC.

[0026] Referring to Fig. 2A, a hydrogen start of the fuel cell stack 14 is defined herein as a start-up event in which an initial concentration of oxygen in the stack anode 14A of Fig. 1 is at or near zero, with an initial hydrogen concentration exceeding zero. During a hydrogen start-up, no significant changes in the stack voltage (VS) should occur during the anode pressurization phase of stack startup. The introduction of air (arrow A) into the cathode plate 14C of Fig. 1 causes the stack voltage (VS) to rise. A maximum voltage level (V1) that the fuel cell stack 14 is allowed to reach is limited to minimize voltage degradation. The maximum allowable voltage level (V1) is similar to the maximum / runtime voltage suppression level (VMAX) of the stack 14, but can be set to a slightly lower level.

[0027] The hydrogen ramp-up profile for a given hydrogen start-up begins at or near 0 VDC. It is expected that the controller 50B can operate the stack 14 near 0 VDC up to the maximum allowable voltage (V1) for a relatively short duration when air (arrow A) is drawn from the cathode plate 14C by Fig. 1 occurs. Once the stack voltage (VS) reaches the level of V1, it is subsequently possible to reach the level of VMAX at a slower rate.

[0028] Fig. Figure 2B shows a generic profile for an air start. An air start begins with a non-negligible oxygen concentration in the anode plate 14A of Fig. 1. In this situation, the stack voltage (VS) increases rapidly during the anode pressurization of the start-up, requiring a lower stack voltage limit (V1) and a higher stack current to adequately mitigate the potential voltage degradation, which is primarily due to cathode carbon corrosion. The duration of the corrosion event is equivalent to the time required for hydrogen gas to completely fill and displace an initial gas volume of the fuel cell stack 14, typically between 100 and 500 ms. After the anode pressurization stage is completed, the stack voltage (VS) will reinforce the hydrogen start-up voltage limit (V2). The exemplary start-up event of Fig. 2B will end in a similar way to the hydrogen launch event.

[0029] Fig. 3 shows an exemplary embodiment of the above-mentioned method 100. Aspects of the method 100 may be implemented by computer-executable instructions, i.e., software applications or application programs executed by the controller 50A and / or 50B. The software may be stored in a variety of memories (M) of the controller 50A and / or 50B, including computer-readable data such as CD-ROM, magnetic disk, bubble memory, and semiconductor memories, such as various types of RAM or ROM. Furthermore, aspects of the method 100 may be practiced in distributed computing environments in which tasks are performed by remote processing devices connected via a communications network, e.g., a CAN (Controller Area Network) bus (not shown). Although with reference to Fig. 3 a specific algorithm is described, other methods for implementing the exemplary machine-readable instructions may be used within the scope of the disclosure.

[0030] In general, using method 100, controllers 50A and 50B operate DC / DC boost converter 30 from Fig. 1 in a regular current control mode through the first several stages of startup, i.e., stack discharge, low-voltage power limiting, and auxiliary power management. Based on a decision by the gas concentration estimation model (GCE model) 55, a decision is made as to whether the current start event is an air start or a hydrogen start. For a hydrogen start, the voltage profile information from the GCE model 55 includes a voltage setpoint and a voltage slew rate. If the start event is an air start, the voltage profile from the GCE model 55 comprises three segments, each with a different voltage setpoint and slew rate.The DC / DC boost converter 30 may be instructed to operate in a voltage control mode during the anode pressurization, cathode filling, and voltage stabilization stages, and then return to the regular / current control mode when the final stage, voltage stabilization, is complete.

[0031] In general, the method 100 includes detecting an air start of the fuel cell stack 14 via the controller 50B in response to a requested start-up of the fuel cell stack 14, wherein the controller 50B is programmed with a plurality of calibrated voltage profiles, each having a corresponding magnitude and rate of change. Then, in response to the detected air start, the method 100 includes forcing a predetermined voltage profile of the stack voltage during an actual start-up of the fuel cell stack 14, including regulating operation of the DC / DC boost converter 30 using the calibrated voltage profiles.

[0032] In an exemplary embodiment beginning with step S102, the controller 50B wakes up upon request for the startup of the fuel cell system 12 from Fig. 1, e.g., a requested ignition-based start of the Fig. 1 and begins to communicate with the fuel cell system 12. The method 100 then proceeds to step S104.

[0033] Step S104 includes controlling the boost converter 30 from Fig. 1 in a regular / current control mode by the operation of the controller 50A. As is known in the art, the operation of a fuel cell stack, such as the stack 14 of Fig. 1 according to several stages: stack discharge, low-voltage power limitation, auxiliary power management, anode pressurization, cathode filling, and voltage stabilization. Step S104 comprises controlling the stack discharge, the low-voltage power limitation, and auxiliary power management stages by instructing the boost converter 30 to operate in a standard mode, i.e., in a current control mode in which the duty cycle to the semiconductor switch (arrow S1) is Fig. 1 is determined as a function of the stack current (arrow IS) measured by the sensor (SC). The method 100 then proceeds to step S106.

[0034] Step S106 may include determining whether an air start event is present. For example, the GCE model 55 described above with reference to Fig. 1, may be used to estimate whether oxygen (arrow O2) remains in the anode plate 14A. In an exemplary embodiment, the GCE model 55 may consider downtime, temperature of the environment and / or the fuel cell stack 14, pressures of the oxygen and hydrogen within the fuel cell stack 14, and known characteristics of the fuel cell stack 14 such as size, materials, and the like, with the controller 50B outputting an estimated oxygen level. Once the controller 50B determines that oxygen is present in the anode plate 14A, the controller 50 may register that an air start is active. The method 100 proceeds to step S107 if an air start is detected. Otherwise, the controller 50B determines that a hydrogen start is active and instead proceeds to step S108.In any case, the controller 50B automatically switches the operating mode of the boost converter 30 from current control to voltage control so that the duty cycle of the boost converter 30 is controlled as a function of the stack current (VS).

[0035] Steps S107, S109, and S111 include controlling the anode pressurization, cathode filling, and voltage stabilization stages via the controller 50A and the boost converter 30 in a manner suitable to minimize the influence of the detected air start and then continuing the process with step S114. Similarly, steps S108, S110, and S112 are executed before continuing with step S114 and include controlling the anode pressurization, cathode filling, and voltage stabilization states in a manner suitable for the hydrogen start. The operating differences can be determined by comparing the trajectories of the Fig. 2A and Fig. 2B can be seen.

[0036] When a hydrogen start is controlled, as in Fig. 2A, the hydrogen start begins at or near 0 VDC at t0. When air is introduced into the fuel cell stack 14 from Fig. 1 from the cathode plate 14C, the stack voltage (VS) is allowed to rise to the level of (V1) at a faster rate between t1 and t2. Once the stack voltage (VS) has reached the level of (V1), it is then allowed to gradually reach VMAX at a reduced rate between t2 and t3.

[0037] When an air start is controlled as in Fig. 2B, the controller 50B controls the boost converter 30 to achieve several different voltage increase rates, ie, K1, K2 and K3. Step S107 controls the anode pressurization with the voltage increase to the voltage level V1 at a first rate (K1) between t0 and t1. Then, step S109 controls the cathode fill stage to a higher voltage level V2 at second and third rates (K2 and K3), the second rate (K2) as shown in Fig. 2, is less than the first rate (K1). Upon reaching the higher voltage level (V2), the controller 50B may slowly decrease the stack voltage (VS) until the maximum voltage (VMAX) is reached. Then, at step S114, control of the boost converter 30 may return to current control for steady-state operation of the fuel cell stack 14.

Claims

[1] Fuel cell system (12) comprising: a fuel cell stack (14) having a plurality of fuel cells (15) and a stack voltage (Vs), a DC-DC boost converter (30); and a controller (50B) configured to control the stack voltage (V S ) via the DC / DC boost converter (30) during start-up of the fuel cell stack (14), and is programmed with a plurality of calibrated voltage profiles, each having a corresponding magnitude and rate of change; wherein the controller (50B) is configured to detect an air start of the fuel cell stack (14) in response to a requested start-up of the fuel cell stack (14) and, in response to the detected air start, to generate a predetermined voltage profile of the stack voltage (V S) during an actual start-up of the fuel cell stack (14) via the control of the DC / DC boost converter (30) using the plurality of calibrated voltage profiles. [2] The fuel cell system (12) of claim 1, wherein the calibrated voltage profiles comprise a first voltage profile corresponding to an anode pressurization and a second voltage profile corresponding to a cathode fill stage, the second voltage profile having a rate of change less than that of the first voltage profile. [3] The fuel cell system (12) of claim 1, wherein the controller (50B) is programmed to detect the air start of the fuel cell stack (14) using an output of a gas concentration estimation (GCE) model (55), wherein the output of the GCE model (55) is an estimated oxygen level in an anode plate (14A) of the fuel cell stack (14). [4] The fuel cell system (12) of claim 3, wherein the controller (50B) is configured to receive as a set of inputs each of an off-time, a temperature, and a pressure value of the fuel cell stack (14), and wherein the output of the GCE model (55) is determined as a function of the set of inputs. [5] The fuel cell system (12) of claim 1, wherein the DC / DC boost converter (30) comprises a semiconductor switch (Q1) and a gate drive circuit (26), and the controller (50A, 50B) is configured to apply the voltage profiles by controlling a duty cycle of the semiconductor switch (Q1) via the gate drive circuit (26) during startup of the fuel cell stack (14). [6] The fuel cell system (12) of claim 5, wherein the controller (50A) comprises a microprocessor that is part of the DC / DC boost converter (30) and that controls the duty cycle of the semiconductor switch (Q1) during startup of the fuel cell stack (14). [7] The fuel cell system (12) of claim 1, wherein the controller (50B) is configured to enforce voltage profiles after the startup process is completed by controlling the DC / DC boost converter (30) in a current control mode. [8] Fuel cell system (12) according to claim 1, further comprising: a battery (31); an inverter module (PIM) (32) electrically connected to the battery (31), an axle (36), and an electric motor (34) operable to output a torque (T O ) to the axle (36). [9] The fuel cell system (12) of claim 8, further comprising a road wheel (39) rotatably connected to the axle (36).

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