Operation control method of a fuel cell system

Abstract

A method for controlling operation of a fuel cell system in which hydrogen purging is performed by opening a hydrogen purge valve (2) to discharge anode exhaust gas of a fuel cell stack through a cathode outlet of the fuel cell stack, the method comprising: Determining, by a controller, whether the fuel cell system enters an idle stop in which an air supply to the fuel cell stack stops; Lowering, by the controller, an intermediate circuit terminal voltage controlled by a DC-DC converter (21) to a first target voltage by controlling an operation of the DC-DC converter (21) connected to an intermediate circuit terminal (12) that outputs a generation power of the fuel cell stack upon entering the idle stop; and Lowering, by the controller, a voltage of the fuel cell stack to reduce an oxide of platinum, which is a catalyst of a fuel cell, by allowing the anode off-gas to flow back into a cathode of the fuel cell stack by performing the hydrogen purge in an idle stop state of opening the hydrogen purge valve after the voltage of the intermediate circuit terminal (12) has dropped.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for controlling the operation of a fuel cell system. More specifically, the present disclosure relates to a method for controlling the operation of a fuel cell system that reduces platinum oxide generated by oxidizing platinum, which is a catalyst of a fuel cell, before elution while a fuel cell system is operating, in order to restore catalyst performance and prevent platinum from being lost, and to increase the durability performance of a fuel cell stack. BACKGROUND

[0002] A fuel cell, as a power generation device that converts chemical energy of a fuel into electrical energy by electrochemically reacting fuel gas and oxidizing gas, can be widely used as industrial, household and vehicle power supply, and even used to power small electrical / electronic products and portable devices.

[0003] Currently, the most widely studied vehicle fuel cell is a proton exchange membrane fuel cell or polymer electrolyte membrane fuel cell (PEMFC) with a high power density.

[0004] The PEMFC uses hydrogen as the fuel gas and oxygen or air containing the oxygen as the oxidizing gas.

[0005] The fuel cell comprises a plurality of cells that generate electrical energy by reacting the fuel gas and the oxidizing gas, and is generally used in a stack in which the cells are stacked and mounted in series to meet a required power level.

[0006] Even in the case of the fuel cell mounted in a vehicle, since high output power is required, hundreds of cells that generate the electrical energy individually are stacked in the stack to meet such a requirement.

[0007] Herein, when describing a unit cell configuration of the PEMFC, the PEMFC is configured to include a membrane electrode assembly (MEA) in which catalyst electrode layers are attached to both sides of a membrane based on a polymer electrolyte membrane in which hydrogen ions move, a gas diffusion layer (GDL) that supplies the fuel gas and the oxidizing gas as the reactant gas to the MEA and transmits the generated electrical power, a seal and a connecting mechanism for maintaining airtightness and an appropriate joining pressure of the reactant gas and cooling water, and a partition plate (bipolar plate (BP)) that moves the reactant gas and the cooling water.

[0008] Herein, the MEA is configured to comprise the polymer electrolyte membrane capable of moving the hydrogen ions, and a cathode and an anode as electrode layers to which the catalyst is applied, which allows the hydrogen as fuel gas and the air (alternatively oxygen) as oxidant to react on both surfaces of the electrolyte membrane.

[0009] The gas diffusion layers for uniformly distributing the fuel gas and the oxidizing gas are stacked in an outer part of the MEA, that is, the outer parts of the cathode and the anode, and the BP is arranged in the outer part of the gas diffusion layer, which provides a flow path through which the reaction gas and the cooling water flow and supplies the reaction gas to the gas diffusion layer.

[0010] A gasket for sealing fluids and the like are stacked so as to be interposed between the unit cell forming components, and the gasket may be provided while being integrally molded to the MEA or BP.

[0011] A plurality of cells are stacked using such a configuration as the unit cell, and thereafter, end plates for supporting / holding the cells are coupled to an outermost part, and the end plates and the cells are connected together using a stack connection mechanism while the cells are stacked and arranged between the end plates to construct the fuel cell stack.

[0012] The fuel cell system mounted in the fuel cell vehicle includes the fuel cell stack and devices for supplying the reaction gas to the fuel cell stack.

[0013] That is, the fuel cell system includes the fuel cell stack that generates electrical energy from the electrochemical reaction of the reaction gas, a hydrogen supply device that supplies hydrogen as fuel gas to the fuel cell stack, an air supply device that supplies oxygen-containing air as oxidizing gas to the fuel cell stack, a heat and water management system that controls an operating temperature of the fuel cell stack and performs heat and water management functions, and a fuel cell system controller that controls an overall operation of the fuel cell system.

[0014] In the general fuel cell system, the hydrogen supply device may include a hydrogen storage unit (hydrogen tank), a regulator, a hydrogen pressure regulating valve, a hydrogen recirculation device, and the like; the air supply device may include an air blower or an air compressor, a humidifier or a humidifying device, and the like; and the heat and water management system may include a water separator, an electric water pump (cooling water pump), and a water tank, a radiator, and the like.

[0015] In such a configuration, the pressure of the high-pressure hydrogen supplied from the hydrogen tank of the hydrogen supply device is reduced in pressure to a predetermined pressure in the regulator and then supplied to the fuel cell stack, and in this case, the pressure-reduced hydrogen is pressure-regulated according to an operating condition of the fuel cell stack to be supplied to the fuel cell stack while controlling a supply amount.

[0016] In the fuel cell stack, hydrogen that does not undergo reaction but remains is discharged through an outlet of an anode (hydrogen electrode) of the stack or returned to an inlet of the anode of the stack through the hydrogen recirculation device.

[0017] The hydrogen recycling device is a device capable of increasing the reliability of hydrogen supply and improving a lifetime of the fuel cell, and various recycling methods are provided, although a method using an ejector, a method using a blower, a method using both the ejector and the blower, and the like are known.

[0018] The hydrogen recirculation device recirculates unreacted hydrogen that cannot be used and remains in the anode of the fuel cell stack to the anode (hydrogen electrode) of the stack through a recirculation line to promote reuse of the hydrogen.

[0019] In the fuel cell, as foreign materials / impurities including nitrogen, water and steam flowing to the anode through the electrolyte membrane in the stack increase, the amount of hydrogen in the anode decreases and thus the reaction efficiency deteriorates, and as a result, hydrogen purging may be performed by opening a hydrogen purge valve attached to an exhaust pipe / outlet pipe of the stack anode.

[0020] That is, the hydrogen purge valve for hydrogen purge is attached to a pipe at the outlet of the anode of the fuel cell stack to allow the foreign materials including nitrogen, water, and the like to be discharged and removed together with the hydrogen in the anode of the stack, and as a result, a hydrogen concentration in the anode is controlled and maintained at an appropriate level to increase a hydrogen utilization rate.

[0021] When the foreign materials in the fuel cell stack are removed, there are advantages such as controlling the appropriate hydrogen concentration in the anode, increasing the hydrogen utilization rate, and improving a gas diffusion rate and reactivity.

[0022] Meanwhile, the durability of the fuel cell stack is a very important element in ensuring the marketability of the fuel cell vehicle.

[0023] Accordingly, various efforts to increase the durability of the fuel cell stack while preventing stack deterioration are being made in competition, and various causes of stack deterioration are also being investigated.

[0024] One cause that constitutes a large part among them is the loss of platinum (Pt), which is used as a catalyst that activates the reaction of oxygen and hydrogen in the stack.

[0025] If the loss of platinum in the MEA continues and accelerates, the reactivity of the hydrogen and oxygen deteriorates, and as a result, the performance of the stack deteriorates.

[0026] If the operation of the fuel cell continues, the platinum will be continuously oxidized, and it is almost impossible to detect a phenomenon in which the oxidized platinum forms PtO or PtO 2 is eluted, and as a result the platinum disappears or is lost.

[0027] As a result, an oxidation rate of platinum is delayed or the oxidized platinum is quickly reduced again before the oxidized platinum is eluted to delay a loss rate of platinum.

[0028] It is important to maintain a potential to activate the corresponding reaction for retarding the oxidation rate of platinum and reducing the platinum, and a potential range in which the platinum can be oxidized and a potential range in which a platinum oxide can be reduced are distinguished to some extent based on the potential of the cells of the fuel cell stack.

[0029] More specifically, an oxidation reaction or a reduction reaction of platinum occurs according to the potential of the cells of the fuel cell stack.

[0030] For example, in the cells of the fuel cell stack, in an oxidation section (a voltage increase direction section) up to a certain potential or higher, for example, 0.8 V or higher, the oxidation reaction of platinum mainly occurs, and in a reduction section (for example, 0 to 0.8 V, a voltage decrease direction) of 0.8 V or lower, the reduction reaction of platinum mainly occurs.

[0031] When an operating mode between 0.7 to 1.0 V is applied, the oxidation of platinum occurs more frequently than the reduction of platinum, and if the operation continues within such a potential range, the platinum loss occurs, wherein, since the platinum is mixed with water (H 2 O) a reaction occurs, chemical dissolution occurs and the platinum is thus eluted.

[0032] It is experimentally and theoretically proved that with decreasing potential, the reduction reaction proceeds more actively, and when a voltage distribution of actual running vehicles is analyzed, the oxidation section in which the average cell voltage is 0.8 V or more usually occupies 50% or more in a whole section, and as a result, it is very important to increase the frequency of the voltage decreasing direction (reduction section) in order to increase the possibility of reducing the platinum oxide to reduce the loss of platinum.

[0033] However, in the existing fuel cell hybrid system, the control degree of freedom for the stack voltage is lower, and there is a high probability that a range of the stack voltage that is primarily controlled will be a voltage range in which the phenomenon in which the platinum is oxidized is dominant, and as a result, the existing fuel cell hybrid system is very disadvantageous in terms of the durability of the stack.

[0034] The above information disclosed in this Background section is provided only to enhance the understanding of the background of the invention and may accordingly contain information that does not constitute prior art already known to a person of ordinary skill in the art in this country. SUMMARY OF REVELATION

[0035] The present disclosure has been made in an effort to solve the above-described problems associated with the prior art.

[0036] Accordingly, the present disclosure provides a method for controlling the operation / driving of a fuel cell system that reduces platinum oxide generated by oxidizing platinum, which is a catalyst of a fuel cell, before elution while operating a fuel cell system to restore catalyst performance and prevent platinum from being lost and to increase the durability performance of a fuel cell stack.

[0037] In one aspect of the present disclosure, a method for controlling the operation of a fuel cell system in which hydrogen purging is performed by opening a hydrogen purging valve to discharge anode off-gas of a fuel cell stack through a cathode outlet of the fuel cell stack includes: determining, by a controller, whether the fuel cell system enters an idle stop in which air supply to the fuel cell stack stops; lowering, by the controller, a DC link terminal voltage controlled by a DC-DC converter to a first target voltage by controlling an operation of the DC-DC converter connected to a DC link terminal that outputs generation power of the fuel cell stack at the time of entering the idle stop;and lowering, by the controller, a voltage of the fuel cell stack to reduce an oxide of platinum, which is a catalyst of a fuel cell, by allowing the anode off-gas to flow back into a cathode of the fuel cell stack by performing the hydrogen purge in an idle stop state of opening the hydrogen purge valve after the voltage of the DC link terminal has dropped.;

[0038] As a result, by the method for controlling the operation of a fuel cell system according to the present disclosure, a voltage of a fuel cell stack is reduced by a method that performs hydrogen purging at the time of entering an idle stop of the fuel cell system to reduce a platinum oxide generated by oxidizing platinum, which is a catalyst of the cells of the fuel cell stack, before elution.

[0039] The stack voltage reducing control and platinum oxide reducing processes of the present disclosure can be easily performed by merely configuring control logic in a controller without adding separate hardware, and effects of restoring catalyst performance, preventing platinum loss, and increasing durability performance of the fuel cell stack are provided.

[0040] In order to prevent the hydrogen consumption from being increased due to the hydrogen purging, control is performed for shortening and compensating / equalizing a purging time at the time of performing the hydrogen purging during normal operation of the fuel cell system as much as a hydrogen purging time during the idle stop to perform the control for reducing the platinum and lowering the stack voltage while maintaining the same level as in the prior art without increasing the hydrogen purging time and a purging amount as a whole.

[0041] Further refinements and embodiments of the invention are explained below.

[0042] It is understood that the term "vehicle" or "vehicular" or other synonymous terms as used herein include motor vehicles in general, such as passenger cars including sports utility vehicles (SUVs), buses, trucks, various utility vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles, and other alternative fuel vehicles (e.g., fuel derived from sources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, such as both gasoline-powered and electric-powered vehicles.

[0043] The above and other features of the invention are explained below. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The above and other features of the present disclosure will now be described in detail with reference to embodiments thereof illustrated in the accompanying drawings, which are given below for illustrative purposes only and thus do not constitute a limitation of the present invention. In the figures: Fig. 1 and Fig. 2 diagrams illustrating a high-voltage grid configuration of a conventional fuel cell-battery hybrid system; Fig. 3 is a diagram illustrating a pull-down pattern / pull-down model of a fuel cell voltage during an idle stop in the conventional fuel cell system; Fig. 4 is a diagram schematically illustrating movement paths of air and hydrogen in the conventional fuel cell system; Fig. 5 is a diagram illustrating a timing chart comparing and showing states wherein hydrogen purging in the present disclosure and the hydrogen purging in the prior art are performed; Fig. 6 is a diagram comparing and illustrating voltage behavior in an idle stop mode in the prior art and the present disclosure; Fig. 7 is a flowchart illustrating a method for controlling the operation of a fuel cell system according to an embodiment of the present disclosure; and Fig. 8 is a flowchart illustrating in more detail a hydrogen purging process during normal operation and idle stop of the fuel cell system.

[0045] The reference numerals shown in the drawings include a reference to the following elements, as explained below: It should be noted that the above drawings are not necessarily to scale and represent a somewhat simplified representation of various preferred features intended to illustrate the principles of the disclosure. The specific design features of the present disclosure as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes, will be determined in part by the dedicated application and the operating environment.

[0046] In the figures, reference numerals refer to the same or equivalent parts of the present disclosure throughout the several figures of the drawing. DETAILED DESCRIPTION

[0047] Reference will now be made in detail to the various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in connection with exemplary embodiments, it is to be understood that the present description is not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents, and other embodiments that may be included within the spirit and scope of the invention as defined by the appended claims.

[0048] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings so that a person skilled in the art can easily implement / carry out the embodiments of the present disclosure. However, the present disclosure is not limited to the embodiments described below, but may be implemented / realized in other forms.

[0049] In the description, unless expressly stated otherwise, the word "comprise" and variations such as "comprises" or "comprising" are understood to include / imply the inclusion of the elements mentioned, but not the exclusion of any other elements.

[0050] The present disclosure has been made in an effort to provide a method for controlling a fuel cell system that reduces platinum oxide produced by oxidizing platinum, which is a catalyst of a fuel battery cell, before elution while operating a fuel cell system, to restore catalyst performance and prevent the platinum from being lost and to increase durability performance of a fuel cell stack.

[0051] As is already known, a unit cell of a polymer electrolyte membrane fuel cell includes a membrane electrode assembly (MEA) in which catalyst electrode layers are attached to both sides of a membrane based on an electrolyte membrane as a primary component, and the catalyst electrode layers attached to both sides of the membrane become a cathode electrode to which oxygen gas (air) is supplied and an anode electrode to which fuel gas (hydrogen) is supplied.

[0052] Platinum (Pt) is often used as a catalyst of the catalyst electrode layer, and the platinum, which is a noble metal catalyst, may be oxidized according to a cell potential while a fuel cell system is operated, and further, a platinum loss phenomenon may occur in which a generated oxide PtO or PtO 2 is eluted and lost.

[0053] When the loss of platinum occurs, the catalyst performance deteriorates and the durability of a stack deteriorates (the stack degrades), and as a result, it is necessary to restore the catalyst performance by reducing oxidized platinum catalyst particles before eluting the oxidized platinum catalyst particles.

[0054] Accordingly, the present disclosure includes a method for controlling operation of a fuel cell that reduces oxidized platinum during operation of the fuel cell system.

[0055] As a method for limiting the upper limit voltage of the fuel cell stack, methods for retarding an oxidation rate of platinum are proposed, however, a more positive operation control method for reducing the already oxidized platinum is required, and this method becomes a method that can improve the durability performance of the stacks.

[0056] The oxidation and reduction of platinum occurs continuously during the operation of the fuel cell system, but the degrees of occurrence of the oxidation and reduction reactions vary depending on their potentials.

[0057] That is, when the potential increases, the oxidation reaction predominates, and when the potential decreases, the reduction reaction predominates.

[0058] Accordingly, the stack voltage must be lowered to the potential at which the reduction reaction is predominant to compensate for the oxidation of platinum, and in the case of the existing fuel cell hybrid system, that is, a non-booster type fuel cell hybrid system, it is difficult to operate the system by lowering the stack voltage to a desired range.

[0059] Although the system is operated by lowering the stack voltage to the desired range, such an operating condition becomes a state as if the system is operated by lowering the efficiency of the fuel cell, and as a result, the efficiency may deteriorate.

[0060] Therefore, it is required to operate the fuel cell system by reducing the voltage of the stack without deteriorating the efficiency of the fuel cell system, and to this end, the present disclosure shows a method that operates the fuel cell system by intermittently and positively reducing the stack voltage to a level for reducing the platinum oxide according to a situation by using an idle stop state to temporarily stop generation of the fuel cell.

[0061] Consequently, the platinum oxide is reduced again without deteriorating the operating efficiency of the fuel cell system to prevent the loss of platinum and improve the catalyst performance and durability performance of the stack.

[0062] Booster type and non-booster type fuel cell hybrid systems are described below to facilitate understanding of the present disclosure.

[0063] When only one fuel cell is used as a power source (electrical power source) of a vehicle in a vehicle fuel cell system, the fuel cell may operate in an operating range / drive range where the efficiency is low because the fuel cell must bear all the loads in the vehicle, and as a result, the performance may deteriorate in the other range where the efficiency of the fuel cell is low.

[0064] When a basic load is applied to the fuel cell, the output voltage of the fuel cell drops rapidly instantly, and as a result, a drive motor for driving the vehicle may not be supplied with sufficient electrical energy, thereby deteriorating the performance of the vehicle.

[0065] Since the fuel cell generates electrical energy through an electrochemical reaction, it is difficult to cope with rapid load fluctuations, and most importantly, since the fuel cell has a unidirectional power characteristic (only the output / discharge is available), the regenerative energy generated by a motor cannot be recovered when the vehicle is braking or coasting, and as a result, the system efficiency deteriorates.

[0066] As a countermeasure to overcome the disadvantages, an energy storage device, which is a separate auxiliary power source for driving a drive motor 30, and a high-voltage component 40, for example, a chargeable / dischargeable high-voltage battery 20 or a supercapacitor (supercap), are mounted in addition to the fuel cell 10, which is a primary power source, as shown in FIG. Fig. 1 and Fig. 2 is shown in .

[0067] Fig. 1 and Fig. 2 show diagrams illustrating a high-voltage grid configuration of a fuel cell-battery hybrid system having the fuel cell 10 as the primary power source and the high-voltage battery 20 as the auxiliary power source. Fig. 1 represents a boost-type fuel cell hybrid system with a booster / amplifier and Fig. Figure 2 represents a non-boost type fuel cell hybrid system without the booster / amplifier.

[0068] As in Fig. 1 and Fig. 2, the fuel cell 10 and the motor (drive motor) 30 for driving the vehicle are connected to each other through an intermediate circuit terminal 12, and the high-voltage battery (main battery) 20 is connected to the intermediate circuit terminal 12 via a DC conversion device for converting the electrical energy, that is, a DC-DC converter (DC-DC converter) 21.

[0069] By using the fuel cell 10 and the high-voltage battery 20 as an electric power source (energy source), high-voltage loads in the vehicle are connected to the intermediate circuit terminal 12, which is connected in parallel with the energy sources to the two energy sources.

[0070] As a high-voltage load, the motor 30, which becomes a drive source for driving the vehicle, that is, the drive motor 30 is connected to the intermediate circuit terminal 12 via an inverter 31, and the inverter 31 converts a direct current (DC power) of the fuel cell or the battery 20 into a 3-phase alternating current (3-phase AC power) and applies the 3-phase alternating current to the motor 30 to drive the motor.

[0071] In such a configuration, the battery 20 is connected to the motor 30 to be charged and discharged, and as a result, a motor drive power is supplied from the battery through the DC-DC converter 21 at the time of driving the motor (in the drive mode), and in this case, the inverter 31 converts the electric energy of the battery into a 3-phase alternating current and applies the 3-phase alternating current to the motor.

[0072] The electrical energy generated by the motor 30, which serves as a generator, is transferred to the battery 20 through the DC-DC converter 21 for charging the battery when the motor is fed back during vehicle braking or coasting (in a generator mode / operation).

[0073] The high-voltage drive component 40 of the fuel cell system, which must be operated / driven when operating the fuel cell, that is, a balance of plant (BOP) device such as an air blower or a recirculation blower, is connected to the fuel cell 10 and the battery 20, which become the energy sources, through the DC link terminal 12.

[0074] The BOP 40 is operated by receiving / absorbing a generated power of the fuel cell 10 or charging power of the battery 20.

[0075] In the booster type fuel cell hybrid system, the booster 11 is mounted on an output terminal of the fuel cell, which converts and controls the electric power output from the fuel cell 10, and in the non-booster type fuel cell hybrid system, the booster is not provided.

[0076] One of the differences between the two systems is a degree of freedom in fuel cell voltage regulation.

[0077] That is, since the booster system can increase the voltage of the fuel cell 10 using the booster 11, even though the voltage of the fuel cell is kept very low, the booster system can maintain a voltage at which the BOP 40 or the DC-DC converter 21 and the inverter 31 for driving the motor can normally operate while the fuel cell system is operating.

[0078] However, in the non-booster system, since the voltage of the fuel cell 10 directly becomes the control voltage of the BOP 40, the DC-DC converter 21, the inverter 31, and the like, the voltage of the fuel cell must be maintained at a certain voltage or higher in order for the fuel cell system to operate.

[0079] If the BOP, DC-DC converter, inverter, and the like are designed to operate at a low voltage, a range of a required voltage (current) specification is excessively expanded, which may cause increasing costs, and as a result, the non-booster system is generally set up / configured such that the components can only operate within a predetermined voltage range.

[0080] In this case, there is a limitation that the voltage of the fuel cell must be kept at a certain voltage or higher.

[0081] Consequently, the booster system is more advantageous in terms of the degree of freedom of control of the voltage of the fuel cells, that is, in terms of operating / driving at a voltage at which the durability of the stack deteriorates less.

[0082] In the non-booster type fuel cell hybrid system, it is very important to develop a method (a voltage sweep method) that can reduce the controllable voltage to maintain the durability performance of the stack.

[0083] In the case of the non-booster type fuel cell hybrid system, the voltage (stack voltage) of the fuel cell must not drop to a lower limit acceptable in the system (e.g., 250 V) or lower in order to ensure the functionality of the high-voltage electrical components, i.e., the BOP, the DC-DC converter, the inverter, and the like, which become the electrical loads of the fuel cell.

[0084] As a unique condition in which the stack voltage is allowed to drop to the lower limit acceptable for the system or less, when the vehicle stops, the vehicle undergoes downhill driving in which the vehicle is driven downhill, or a required output is very small or other fuel cells do not need to output power, the fuel cell may be kept in a driving state of an idle mode to cut off air supply by stopping operation of an air supply device, that is, operation of an air blower or air compressor.

[0085] In the present specification, the fuel cell operating state and mode under such a condition are collectively referred to as an idle stop state and mode.

[0086] Fig. 3 is a diagram illustrating a pull-down pattern / pull-down model of a fuel cell voltage during an idle stop in the conventional fuel cell system.

[0087] When the operation mode of the fuel cell system enters the idle stop mode, no air is supplied to the fuel cell stack and, moreover, as shown in Fig. 3, the voltage of the intermediate circuit terminal 12, which is a high-voltage terminal, is maintained at a predetermined level by the DC-DC converter.

[0088] In this case, the voltage of the stack decreases to a voltage controlled / regulated by the DC-DC converter while the high voltage battery is being charged, and thereafter, when the air in the stack is sparse, the voltage of the stack decreases to the voltage below the voltage controlled by the DC-DC converter.

[0089] As from Fig. 3, when the fuel cell voltage gradually decreases at the time of entering the idle stop mode and a long time elapses, the fuel cell may reach a potential for reducing the platinum.

[0090] However, the time is fluid according to a state of the stack, but a time of about 20 seconds is required until the voltage decreases to 0.7 V or less at which a reduction reaction of PtO is activated, and a time of 40 seconds to 1 minute is required for 0.4 V or less at which the reduction of PtO 2 is activated.

[0091] In an actual driving section / driving section of the vehicle where the non-booster type fuel cell hybrid system is applied, a frequency at which the voltage decreases to a range where the reduction of PtO and PtO 2 is activated, not high.

[0092] A situation in which the drive mode frequently reaches the idle stop state may sometimes occur, but there are many cases in which a driver accelerates the vehicle again in the case of a conventional drive pattern / driving pattern, and as a result, in fact, a situation in which the stack voltage drops / decreases does not normally occur before the vehicle is stopped for a long time.

[0093] Accordingly, in a situation where a positive drop or a positive decrease in the voltage is impossible in a drive section where an actual load exists, the idle stop state of the fuel cell system must be positively used.

[0094] A method for positively lowering the voltage of the stack in the idle stop state includes 1) a method for lowering a DC-DC control voltage (intermediate circuit terminal voltage controlled by the DC-DC converter) to the voltage of the lower limit acceptable for the system or 2) a method for performing hydrogen purging.

[0095] In this case, lowering the DC-DC control voltage to the range where the reduction of PtO can actively occur is beneficial in terms of stack durability.

[0096] Voltage reduction using hydrogen purging must be performed in parallel with the nature of the fuel cell system configuration and can only be applied by adding a function to recover the amount of consumed hydrogen by purging the hydrogen.

[0097] As a result, the present disclosure presents a method that can effectively reduce the stack voltage in the idle stop to compensate for the resulting reduction in efficiency, particularly a method that can only reduce the voltage without an increase in hydrogen consumption.

[0098] Fig. 4 is a diagram schematically illustrating movement paths of air and hydrogen in the fuel cell system, and hydrogen supplied by a hydrogen supply device is supplied to an anode of the stack through an ejector 8 and an anode inlet manifold 4 of a stack 10a while being mixed with anode off-gas returned through a return pipe 9, that is, recirculation gas containing hydrogen.

[0099] At the same time, the air supplied by the air supply device is humidified and then supplied to a cathode of the stack through a cathode inlet manifold 5 of the stack 10a.

[0100] In the hydrogen purge, the anode exhaust gas discharged through an anode outlet manifold 6 of the stack 10a is mixed with the cathode exhaust gas discharged through a cathode outlet manifold 7 of the stack and then discharged to the outside.

[0101] As is already known, in order to control and maintain a hydrogen concentration in the anode of the stack 10a at a predetermined level or higher during operation of the fuel cell, a process is required that removes gas and moisture at the anode, thereby decreasing the hydrogen concentration, and injects new hydrogen, and this process is called hydrogen purging.

[0102] The hydrogen purge may be performed periodically based on an estimate of the hydrogen concentration in the anode, or the hydrogen purge may be performed whenever a current integration value obtained by integrating the current generated by the stack reaches a predetermined reference value.

[0103] With reference to Fig. 4, in hydrogen purging, the gas and moisture discharged from the anode outlet enter the cathode outlet manifold 7 and are then discharged to the outside while being mixed with the cathode off-gas, and the anode off-gas (purge gas) containing the hydrogen is diluted with the air discharged from the cathode of the stack 10a to discharge the anode off-gas diluted with the air to the outside while reducing the hydrogen concentration in the gas (the anode off-gas must meet specifications or regulations on the concentration of leaked gases).

[0104] However, when the flow of air discharged from the cathode of the stack 10a is a predetermined amount or more, the anode exhaust gas can be normally diluted with the cathode exhaust gas and then discharged, but when the flow of air discharged from the cathode is smaller than the predetermined amount, the anode exhaust gas containing the hydrogen may flow back into the cathode of the stack.

[0105] When such a phenomenon occurs, the voltage of the stack decreases rapidly, while a direct response (H 2 + O 2 → H 2 O) of hydrogen and oxygen in the cathode.

[0106] However, if such a phenomenon occurs when the fuel cell normally needs to generate output power, there may be a problem in maintaining the normal output power of the fuel cell.

[0107] However, if such a phenomenon is appropriately used in a situation where the stack voltage needs to be reduced, such a phenomenon can significantly contribute to improving (reducing oxidized platinum) the durability of the stack.

[0108] When hydrogen is directly injected into the cathode of the stack using hydrogen purge while the air supply to the cathode of the stack stops in the idle stop mode, the stack voltage can be lowered to a desired range within a short time.

[0109] In this case, since the concentration of oxygen in the cathode is lower, a voltage drop effect increases greatly.

[0110] Accordingly, in the present disclosure, the voltage drop control is performed using the hydrogen purge during the idle stop, and the concentration of oxygen (air) in the cathode of the stack becomes sparse to a certain extent, that is, it is verified that the stack voltage is lower than a predetermined voltage, and thereafter, the stack voltage is lowered to the potential or less at which the reduction of PtO 2 takes place.

[0111] In this case, it may be more effective to close an air shutoff valve 3 at the cathode outlet of the stack to maximize the voltage drop effect even with a small amount of hydrogen purge.

[0112] However, if hydrogen purging is used for the purpose of lowering the stack voltage, more hydrogen may be lost than the hydrogen under the existing operating condition.

[0113] The loss of hydrogen causes a reduction in system efficiency and fuel efficiency, but one purpose of hydrogen purging is to enable the stack to provide normal performance by maintaining the hydrogen concentration in the anode of the stack at a predetermined level or higher, in which hydrogen purging is inevitably carried out even though the hydrogen is lost.

[0114] In the present disclosure, an effect of increasing the hydrogen concentration in the anode of the stack is still effective even when the hydrogen purge is performed for the purpose of reducing the stack voltage during the idle stop.

[0115] Accordingly, in the present disclosure, when the hydrogen purge for the purpose of lowering the voltage is performed during the idle stop, it can compensate / balance the purge time at which the hydrogen purge for the purpose of lowering the voltage has been performed at the time of reaching a normal hydrogen purge timing.

[0116] That is, during normal operation of the fuel cell system, normal hydrogen purging is performed to control the hydrogen concentration in the anode of the stack. The hydrogen purging is performed by reducing the time from the time when the previous hydrogen purging is performed to lower the voltage until the normal hydrogen purging time is reached. Thus, it is possible to maintain the hydrogen concentration while maintaining the entire hydrogen purging time, achieving the purpose of voltage drop without unnecessary waste of hydrogen.

[0117] As such, in the present disclosure, the hydrogen purge performed for lowering the voltage of the stack during the idle stop and the normal hydrogen purge for controlling the hydrogen concentration in the anode of the stack during normal driving are performed separately, and during the hydrogen purge in the normal driving, the hydrogen purge can be performed by reducing the time by the hydrogen purge time for lowering the voltage during the idle stop.

[0118] Herein, the hydrogen purging performed during normal operation of the fuel cell system means general hydrogen purging in which anode off-gas of the stack is discharged to the outside while being mixed with the cathode off-gas by opening an air shutoff valve 3.

[0119] It is inefficient to lower the voltage by performing hydrogen purge every time the idle stop operation is entered.

[0120] If the number of purge times performed during idle stop is greater than the number of hydrogen purge operation times during normal operation, the above-described compensation control of purge time reduction cannot be performed, and thus unnecessary hydrogen loss may be caused.

[0121] Accordingly, the voltage drop using hydrogen purging can be performed only when the idle stop mode is maintained under a certain condition (for example, a certain time or a stack voltage state).

[0122] Fig. 5 is a timing chart showing a comparison of states of hydrogen purging in the present invention and hydrogen purging in the prior art, and as is known, hydrogen purging is performed by opening the hydrogen purging valve 2 mounted in a stack anode exhaust line, and the known general hydrogen purging can be set to be performed whenever a current integration value obtained by integrating a current generated in the stack reaches a predetermined reference current integration value.

[0123] Such general hydrogen purging is performed to improve the utilization efficiency of hydrogen while maintaining a hydrogen concentration in the anode at a suitable level by purging impurities such as nitrogen, water, and water vapor present in the anode of the stack to the outside.

[0124] In the present disclosure, the hydrogen purge valve 2 is opened in the idle stop section in which the air supply of the fuel cell stack is cut off to perform the hydrogen purge for the purpose of lowering the stack voltage, and at the next general hydrogen purge timing, the hydrogen purge is performed only for the time reduced and compensated by the time of the hydrogen purge for the voltage drop performed during the previous idle stop.

[0125] That is, after the hydrogen purge for voltage drop is performed during the idle stop, when the integrated current value reaches the reference current integration value during a normal operation for performing the normal hydrogen purge, the purge time is reduced by the hydrogen purge time for voltage drop that was previously performed.

[0126] Fig. 6 is a diagram illustrating a comparison of the voltage behavior in an idle stop mode in the prior art and the present invention, where V4 represents a DC-DC control voltage in a conventional idle stop and V2 represents a DC-DC control voltage during the idle stop in the present invention.

[0127] As in Fig. As shown in Figure 6, V2 can be set to a voltage value in a potential range in which a reduction reaction of PtO can occur in the fuel cell.

[0128] V3 represents a potential at which the reduction of PtO as platinum oxide can start, and V1 represents a potential at which the reduction of PtO 2 than another platinum oxide can begin.

[0129] V3 is a reference potential at which the reduction of PtO is controlled and is a voltage value not used in an actual control process, as described below.

[0130] V1 is a reference potential at which the reduction of PtO 2 is controlled, which is a voltage value preset and inputted in a controller (which may be a controller of the fuel cell system in the present invention) and used to determine whether to perform hydrogen purging for lowering the voltage of the stack as described below.

[0131] A stack voltage range lower than V3 is a potential range in which reduction of PtO can occur, and a stack voltage range lower than V1 is a potential range in which reduction of PtO 2 can occur.

[0132] As shown in the drawing, in the present invention, when the fuel cell system enters the idle stop mode, a DC link terminal voltage controlled by the DC-DC converter 21, that is, a DC-DC control voltage is set to a lower level than that of the prior art, and in particular, the DC-DC control voltage is set to be low to the level at which the reduction of PtO can actively occur.

[0133] Accordingly, when the fuel cell system enters the actual idle stop mode, the controller controls the driving of the DC-DC converter 21 in a state where the air supply to the fuel cell stack is stopped to increase the DC-DC control voltage (intermediate circuit terminal voltage) to the voltage V2 of Fig. 6, where the reduction of PtO can actively occur, allowing the stack voltage to drop rapidly and abruptly to voltage V2.

[0134] As such, when the stack voltage drops to the set V2, which is the DC-DC control voltage during idle stop, the stack voltage continues to drop from V2 as the air in the stack becomes sparse.

[0135] Thereafter, while maintaining the idle stop mode of the fuel cell system and then stopping the supply of air, the voltage of the stack decreases to a predetermined voltage V1, the controller controls the operation of the hydrogen purge valve 2 to perform the hydrogen purge, and thus the voltage of the stack drops rapidly and rapidly to the voltage to the level where a reduction of PtO 2 can occur actively.

[0136] As described above, in the present invention, when the fuel cell system enters the idle stop, the DC-DC converter 21 is controlled to reduce the DC-DC control voltage to a range (voltage V2) where the reduction of PtO is active, and thus the voltage of the stack quickly decreases to the DC-DC control voltage V2 to induce the reduction reaction of PtO.

[0137] Then, the idle stop state is held for a long time, the air supply is stopped, and the hydrogen purge is performed only under a condition that the idle stop holding time exceeds a set time from the time when the idle stop is entered, and a condition that the voltage of the stack is reduced to a preset V1 (a potential range at which a reduction of PtO 2 may occur) drops, carried out to induce the reduction of PtO 2and PtO is more active.

[0138] Fig. 7 is a flowchart illustrating a method for controlling the operation of a fuel cell system according to an embodiment of the present disclosure, and a control process will be described below with reference to Fig. 7 described.

[0139] In the present invention, the voltage drop and compensation process of the stack is largely divided into three processes.

[0140] That is, the control process according to the present invention includes a process for reducing the DC-DC control voltage (the intermediate circuit terminal voltage regulated by the DC-DC converter) when entering the idle stop to the voltage at which the reduction of PtO can actively occur, a process for reducing the voltage of the stack to the voltage level at which the reduction of PtO 2can actively occur by performing the hydrogen purge for lowering the voltage for a predetermined time when the idle stop state is maintained for a predetermined time and the voltage of the stack is lower than the DC-DC control voltage by a predetermined level or more, and a process of performing the hydrogen purge only for a time obtained by subtracting the hydrogen purge time for lowering the voltage from a predetermined hydrogen purge time when a normal hydrogen purge time comes.

[0141] With reference to Fig. 7, when the controller confirms entry into the idle stop mode during normal operation of the fuel cell system (S1), the controller controls the operation of the DC-DC converter 21 to reduce a voltage regulated by the DC-DC converter 21, that is, the DC-DC control voltage to a level of a first set voltage V2 (S12 and S13).

[0142] Next, when the time has elapsed so that the idle stop holding time exceeds the first set time T1, the controller checks whether the voltage of the stack in the idle stop state (the air supply stop state to the stack) drops / decreases to be reduced to a second set voltage V1 or lower (V1 <V2) (S14 und S15).

[0143] Herein, when the voltage of the stack is lower than the second set voltage, the controller closes the air shutoff valve 3 attached to a cathode outlet side of the stack or reduces an opening amount (S16), and then the hydrogen purge for lowering the voltage of the stack is performed by opening the hydrogen purge valve 2 for the second set time (B seconds) (S17).

[0144] As such, when the hydrogen purge is performed, the voltage of the stack drops rapidly to reach a level where the reduction of PtO 2 can occur actively.

[0145] Thereafter, the idle stop mode is released and the normal operation of the fuel cell system is performed (S18), and then the controller opens the hydrogen purge valve 2 when the integrated current value obtained by integrating the current generated in the stack reaches a predetermined reference current integration value to perform normal hydrogen purge for controlling the hydrogen concentration in the anode of the stack (S19 and S20).

[0146] At this time, when the hydrogen purge for lowering the voltage of the stack is performed in the idle stop immediately before setting the general hydrogen purge time, the hydrogen purge is performed by opening the hydrogen purge valve 2 by a time (AB seconds) obtained by subtracting the hydrogen purge time (B seconds as the second set time) performed for the previous lowering of the voltage of the stack from the third set time (A seconds) for the general hydrogen purge.

[0147] The first set time T1 is a reference value of the elapsed time after entering the idle stop mode, which is capable of performing hydrogen purging for stack voltage reduction, and is a time preset in the controller. As described above, in the present invention, hydrogen purging for stack voltage reduction can be performed under the condition that the time from the time when the idle stop mode is entered exceeds T1.

[0148] The second set time B is a time (B < A) preset in the controller to open the hydrogen purge valve 2 during hydrogen purge for lowering the stack voltage during idle stop, and the third set time A is a time preset in the controller to open the hydrogen purge valve 2 during hydrogen purge during normal operation for controlling the hydrogen concentration in the anode of the stack.

[0149] Meanwhile, Fig. 8 is a flowchart illustrating in more detail the hydrogen purging process during normal operation of the fuel cell system and during idle stop.

[0150] Fig. 7 only represents a basic situation in which the purge time during hydrogen purge during normal operation is reduced and compensated after the hydrogen purge is performed for stack voltage lowering control during idle stop of the fuel cell system.

[0151] On the other hand, Fig. 8 shows a more detailed control process including a case where the voltage drop control condition is satisfied again during the idle stop (before) the hydrogen purging is performed in the normal operation of the fuel cell system, that is, a situation where the idle stop is repeatedly entered and released.

[0152] When the situation occurs where the entry into and release of the idle stop is repeated and the control condition of lowering the stack voltage is repeatedly satisfied during the idle stop, the hydrogen purge for lowering the stack voltage is only performed up to two consecutive operations / events, but the hydrogen purge for lowering the stack voltage is not performed in the case of three consecutive operations / events.

[0153] In the case of the two consecutive operations, first the hydrogen purge is performed for B seconds and second the hydrogen purge is performed for AB seconds.

[0154] In the state where the number of hydrogen purge times for stack voltage lowering is reset (there is no history of performing hydrogen purge for stack voltage lowering after the previous normal hydrogen purge), the hydrogen purge is performed for a general set time when the normal hydrogen purge is performed, that is, the third set time of A seconds, whereas when the hydrogen purge for stack voltage lowering is performed once (B seconds) immediately before the hydrogen purge, the hydrogen purge is performed for AB seconds during the normal hydrogen purge.

[0155] When the normal hydrogen purging needs to be performed by satisfying the hydrogen purging condition during the normal operation of the fuel cell system, if the hydrogen purging for lowering the stack voltage has already been continuously performed two or more times immediately before the hydrogen purging, the normal hydrogen purging is skipped once at that time even though the hydrogen purging condition is satisfied during the normal operation.

[0156] This is because it can be assumed that the hydrogen concentration in the anode of the stack is already sufficient to lower the stack voltage due to the previous hydrogen purge, so the normal hydrogen purge is skipped once. Thus, the total hydrogen purge time and purge amount are not increased but maintained at the same level to perform the stack voltage reduction control.

[0157] Steps S11 to S16 in Fig. 8 are the same as in Fig. 7 described.

[0158] However, when describing steps S17-1 to S17-5 in Fig. 8, without the hydrogen purging process during normal operation of the fuel cell system, when the hydrogen purging conditions for lowering the stack voltage during the idle stop are continuously met, first (when N = 0) the hydrogen purging is performed for B seconds (S17-1 and S17-2), and second (when N = 1) a hydrogen purging is performed for AB seconds (S17-3 and S17-4).

[0159] If two or more continuous conditions are met, hydrogen purging is not performed from the third onwards (S17-5).

[0160] When the hydrogen purging process is described, when the fuel cell system is normally operated, if the hydrogen purging needs to be performed by satisfying the hydrogen purging condition during the normal operation, if the hydrogen purging is not performed at the idle stop immediately before the hydrogen purging (N=0), the hydrogen purging is performed for B seconds (S21, S22 and S23).

[0161] On the other hand, when the hydrogen purge is performed once at the idle stop immediately before the hydrogen purge (N=1), the hydrogen purge is performed for AB seconds (S24 and S25), and when the hydrogen purge is performed twice or more at the idle stop immediately before the hydrogen purge, one hydrogen purge is skipped during the normal operation and the integrated current value is reset (S26).

[0162] The invention has been described in detail with reference to embodiments thereof. However, those skilled in the art will recognize that changes may be made in these embodiments without departing from the principles and teachings of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

[1] A method for controlling operation of a fuel cell system in which hydrogen purging is performed by opening a hydrogen purge valve (2) to discharge anode exhaust gas of a fuel cell stack through a cathode outlet of the fuel cell stack, the method comprising: Determining, by a controller, whether the fuel cell system enters an idle stop in which an air supply to the fuel cell stack stops; Lowering, by the controller, an intermediate circuit terminal voltage controlled by a DC-DC converter (21) to a first target voltage by controlling an operation of the DC-DC converter (21) connected to an intermediate circuit terminal (12) that outputs a generation power of the fuel cell stack upon entering the idle stop; and Lowering, by the controller, a voltage of the fuel cell stack to reduce an oxide of platinum, which is a catalyst of a fuel cell, by allowing the anode off-gas to flow back into a cathode of the fuel cell stack by performing the hydrogen purge in an idle stop state of opening the hydrogen purge valve after the voltage of the intermediate circuit terminal (12) has dropped. [2] The method of claim 1, further comprising: Closing, by the controller, an air shut-off valve (3) at the cathode outlet of the fuel cell stack or reducing an opening degree of the air shut-off valve (3) before performing the hydrogen purging after lowering the voltage of the intermediate circuit terminal (12). [3] The method of claim 1, wherein the first target voltage is a voltage value in a potential range in which a reduction reaction of PtO as the oxide of platinum can occur in cells of the fuel cell stack. [4] The method according to claim 1, wherein the controller performs a step of performing the hydrogen purge to lower the voltage of the fuel cell stack when a predetermined fuel cell operating condition is satisfied after lowering the DC link terminal voltage while the fuel cell system enters the idle stop. [5] The method of claim 2, wherein a certain fuel cell operating condition includes a condition in which an idle stop maintenance from the time when the fuel cell system enters the idle stop is more than a first set time. [6] The method of claim 5, wherein a particular fuel cell operating condition is a condition in which the voltage of the fuel cell stack, to be low, drops to a second target voltage or less, and wherein the second target voltage is less than the first target voltage. [7] The method according to claim 6, wherein the second target voltage is a voltage value at which a reduction reaction of PtO 2 as the oxide of platinum in cells of the fuel cell stack begins. [8] The method of claim 2, wherein a particular fuel cell operating condition comprises a condition in which the voltage of the fuel cell stack drops to a second target voltage or less, the second target voltage being less than the first target voltage. [9] The method according to claim 8, wherein the second target voltage is set in the controller as a voltage value in which a reduction reaction of PtO 2as the oxide of platinum in cells of the fuel cell stack begins. [10] The method according to claim 1, wherein a purge time of opening the hydrogen purge valve (2) at the time of performing the hydrogen purge for lowering the voltage of the fuel cell stack is set as a second set time. [11] The method of claim 10, further comprising: Performing, by the controller, hydrogen purging during normal operation, which allows the anode exhaust gas of the fuel cell stack to be discharged to the outside while being mixed with cathode exhaust gas when a hydrogen purging condition is satisfied after the idle stop state is released and the fuel cell system is switched to a normal operation state. [12] The method according to claim 11, wherein a hydrogen purge time in the controller when the hydrogen purge valve (2) opens during normal operation of the fuel cell system is set as a third set time, and a first hydrogen purge is performed during normal operation for a time obtained by subtracting the second set time from the third set time after the idle stop state is released and the fuel cell system is switched to a normal operation state. [13] The method according to claim 12, wherein the controller performs the hydrogen purge for lowering the stack voltage when a certain fuel cell operating condition is satisfied after the DC link terminal voltage is lowered while the fuel cell system enters the idle stop, and performs the hydrogen purge for the third set time when the hydrogen purge for lowering the stack voltage is not performed in the previous idle stop entry state at the time of performing the first hydrogen purge during the normal operation after the idle stop state is released and the fuel cell system is switched to a normal operation state. [14] The method according to claim 11, wherein the controller opens the air shutoff valve (3) at the cathode outlet of the fuel cell stack to discharge the anode exhaust gas of the fuel cell stack to the outside while being mixed with cathode exhaust gas in the hydrogen purge during normal operation. [15] The method according to claim 10, wherein a hydrogen purge time in the controller when the hydrogen purge valve (2) is opened during normal operation of the fuel cell system is set as a third set time, and the controller performs hydrogen purge for a second lowering of the stack voltage for a time obtained by subtracting the second set time from the third set time when the hydrogen purge for lowering the stack voltage is sequentially repeatedly performed in the idle stop state during normal operation of the fuel cell system without the hydrogen purge. [16] The method of claim 15, wherein the controller performs the hydrogen purging in the idle stop state up to a maximum of two times in succession. [17] The method according to claim 1, wherein the controller skips the hydrogen purging once during a normal operation even though the determined hydrogen purging condition is subsequently satisfied while the fuel cell system is switched to a normal operation state when the hydrogen purging for lowering the stack voltage is sequentially repeatedly performed in the idle stop state without the hydrogen purging during the normal operation of the fuel cell system.

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