Fuel cell system and method for controlling it

Abstract

Fuel cell system for a vehicle, comprising: a compressor (200) configured to compress air to be supplied to a fuel cell stack (100); a valve (600) installed in an air supply pipe (300) connected from the compressor (200) to the fuel cell stack (100); a bypass pipe (700) branching off from the valve (600) and connected to an outlet pipe of the fuel cell stack (100); and a control (800) configured to increase the pressure of the air supplied by the compressor (200) and to release a portion of the air through the bypass pipe (700) by actuating the valve (600) in response to determining the pressure of the air supplied by the compressor (200) to increase to a pressure above a point on a pump curve when the measured moisture content of an electrode membrane indicates a dry condition.

Description

TECHNICAL FIELDThe present invention relates to a fuel cell system and a method for controlling the fuel cell system that controls the humidity of air supplied to a fuel cell stack.BACKGROUNDA polymer electrolyte membrane fuel cell system is widely used for vehicles. In the polymer electrolyte membrane, water is used for transferring hydrogen ions (H +) to a membrane disposed between an air electrode and an anode, and a predetermined water level is maintained within the electrolyte membrane, which is related to the performance of a fuel cell.Under a dry condition in which less water than the predetermined level is supplied, the water content in the electrolyte membrane decreases, which increases durability of an electrode and deteriorates performance of a fuel cell. Under a flooding condition in which a larger amount of water than the predetermined level is supplied, water vapor may condense in channels through which air and hydrogen gas are supplied to an air electrode and an anode, respectively, and reaction gases may not be supplied to the electrodes, thereby causing a significant drop in a cell voltage of the fuel cell.In the related art, a conventional method is provided for adjusting pressures and overcharge ratios of air and hydrogen gas supplied to a fuel cell according to the water content in an electrode of a fuel cell or the humidity in the supplied air. Although the method of adjusting the air pressure is effectively used, the effect may be insufficient because a range of the pressure of the air supplied by a compressor or a blower may be limited based on pumping and throttling conditions and air flow rates.Further, in a conventional method for controlling the water content in an electrode membrane, air pressure may be increased and overcharge ratios may be reduced in dry conditions of the fuel cell stack, and air pressure may be decreased and overcharge ratios may be increased in flooding conditions including a high water content in the fuel cell stack.Such control can be performed based on the following assumptions. Since the saturated vapor pressure of water depends on a temperature, a partial pressure of the vapor may be constant at a specific temperature and humidity. Consequently, when the pressure of the supplied air is increased, the amount of water per unit mass of air for maintaining the predetermined humidity level may decrease. As such, the humidity of the supplied air may increase with a reduced amount of water. On the other hand, the amount of water for maintaining the predetermined humidity level may increase as the pressure of the supplied air is decreased.FIG. 1 shows a performance curve of an exemplary compressor in the related art. A compressor is used for supplying air to an air electrode of a fuel cell. The compressor can control the pressure and the flow rate of the air only within operable ranges due to pumps and throttles. The operable ranges are directed to a range below the pump curve and above the throttle curve in FIG. 1. As illustrated in FIG. 1, according to the related art, the control of the pressure and the flow rate of the air within the operable ranges may be performed when the water content in a fuel cell is adjusted according to the pressure and the flow rate of the supplied air. Therefore, the effect may be insufficient. When the flow rate of air supplied to a fuel cell stack is decreased, the air pressure cannot be increased to pressures beyond points of the pumping curve of FIG. 1 due to the limitation of a system. Further, the air pressure can be controlled within a limited pressure range under the conditions of the operable range below the pumping curve, because a minimum operating curve can be determined by a back pressure applied to parts connected to a pipe extended to an air electrode.From WO 2013 / 129 521 A1, a fuel cell system for a vehicle is configured including: a compressor configured to compress air to be supplied to a fuel cell stack; a valve installed in an air supply pipe connected from the compressor to the fuel cell stack; a bypass pipe branching at the valve and connected to an outlet pipe of the fuel cell stack; and a controller configured to increase a pressure of the air supplied from the compressor and discharge a portion of the air through the bypass pipe by operating the valve in response to determining to increase the pressure of the air supplied from the compressor to a pressure above a point on a pumping curve.US 2005 / 0 112 424 A1 moreover discloses a fuel cell system including a compressor configured to compress air to be supplied to a fuel cell stack, a valve installed in an air supply pipe connected from the compressor to the fuel cell stack, a bypass pipe branching at the valve and connected to an outlet pipe of the fuel cell stack, and a humidifier.The foregoing is intended merely to aid in understanding the background of the present invention and is not intended to mean that the present invention is within the scope of the related art already known to those skilled in the art.OVERVIEWIt is an object of the present invention to provide a fuel cell system and a method for controlling the fuel cell system, which control the humidity of air to be supplied to a fuel cell stack in the fuel cell system more quickly and accurately.The object is achieved by a fuel cell system having the features of claim 1 and a method for controlling the same having the features of claim 4.In one aspect, a fuel cell system for a vehicle may include: a compressor configured to compress the air to be supplied to a fuel cell stack; a valve installed in an air supply pipe connected from the compressor to the fuel cell stack; a bypass pipe branching at the valve and connected to the outlet pipe of the fuel cell stack; and a controller configured to increase a pressure of the air supplied from the compressor and discharge an air fraction via the bypass pipe by operating the valve, particularly when it is required to increase the pressure of the air supplied from the compressor to a pressure level above a point on a pumping curve, when the measured moisture content of an electrode membrane indicates a dry condition by the measured water content.The fuel cell system may further include humidification means that may be installed downstream of the valve of the air supply pipe. The humidifier may include: a supply portion that may be connected to the compressor and an air supply pipe to supply the supplied air with moisture and configured to supply the humidified air to a fuel cell stack; and an outlet portion that may be configured to receive air discharged from the fuel cell stack, supply moisture from the received air to the supply portion, and discharge the air to an outlet pipe.In response to determining to increase a target pressure of a compressed air to a pressure above a point on the compressor pumping curve, when air is supplied to the fuel cell stack at a controlled flow rate, the controller may be configured to actuate the compressor to supply air from the compressor at a flow rate greater than the controlled flow rate and actuate the valve to exhaust a portion of air through the bypass pipe. As such, both the set flow rate of the air supplied to the fuel cell stack and the target pressure of the compressed air may be satisfied. As used herein, the term "target pressure" refers to a pressure of the supplied air to maintain the predetermined moisture level. Another aspect of the present invention provides a method of controlling the fuel cell system. The method may include: comparing a target pressure of compressed air supplied from the compressor with a pressure of the surge line of the compressor; and discharging a portion of the air through a bypass pipe by operating the valve when the target pressure is higher than the pressure of the surge line.During the discharge of the air, a flow rate of the compressed air supplied from the compressor may be increased to a level that a pressure of the compressor may reach the target pressure, and an air fraction may be discharged through the bypass pipe by opening the valve by a degree corresponding to a difference between a set flow rate of the air supplied to the fuel cell stack and a flow rate of the compressed air supplied from the compressor.The method may further include: calculating the set flow rate of the air supplied to the fuel cell stack and the target pressure of the compressed air supplied from the compressor; and the target pressure may be compared with a limit pressure. As used herein, the term "limit pressure" refers to a pressure value or point on the pump curve at the controlled flow rate when comparing the pressures in FIG. 1.During the exhaust of the air, when the target pressure is higher than the threshold pressure, the valve for exhausting an air fraction through the bypass pipe may be operated. Further, during the discharge of the air, the flow rate of the air supplied from the compressor may be increased to the level that the limit pressure may reach the target pressure, and an air fraction may be discharged through the bypass pipe by opening the valve by a degree corresponding to a difference between the set flow rate of the air supplied to the fuel cell stack and the flow rate of the compressed air supplied from the compressor.According to various exemplary embodiments of the present invention, the fuel cell system, and the method for controlling the fuel cell system having the structures described above, the water content in an electrode membrane of a fuel cell can be effectively adjusted by varying the pressure and the flow rate of air supplied to a fuel cell. In addition, durability and performance of a fuel cell can be improved by maintaining a condition in which the predetermined moisture level can be maintained in an electrode membrane. In addition, even under a substantially high temperature operating condition in which operations of a fuel cell system may be restricted, the performance of the fuel cell can be maintained by effectively adjusting the humidity of the air supplied to the fuel cell. Further, performance under a severe operating condition such as a condition having substantially high temperature, high speed, and climbing ability can be satisfied.BRIEF DESCRIPTION OF THE DRAWINGSThe above and other objects, features and other advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 shows a performance curve of an exemplary compressor in the related art; FIG. 2 illustrates an exemplary fuel cell system for a vehicle according to an exemplary embodiment of the present invention; FIG. 3 shows an exemplary method for controlling an exemplary fuel cell system for a vehicle according to an exemplary embodiment of the present invention; and FIG. 4 is a power curve of an exemplary compressor of an exemplary fuel cell system for a vehicle according to an exemplary embodiment of the present invention.DETAILED DESCRIPTIONThe terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a / an" and "the / s" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one / more other features, integers, steps, operations, elements, components, and / or groups thereof. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.It will be understood that the term "vehicle" or "vehicular" or other similar term as used herein includes motor vehicles in general such as passenger automobiles including off-road vehicles (SUV), buses, trucks, various commercial 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., fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle having two or more sources of power, such as both gasoline-powered and electric-powered vehicles.Although an example embodiment will be described as using a plurality of units to perform the example process, it will be appreciated that the example processes may also be performed by one module or a plurality of modules. In addition, it is understood that the term controller / controller refers to a hardware device including a memory and a processor. The memory is configured to store the modules, and the processor is specifically configured to execute the modules to perform one or more processes described below.In addition, the control logic of the present invention may be embodied as non-transitory computer readable media on a computer readable medium containing executable program instructions executed by a processor, controller / controller, or the like. Examples of computer readable media include, but are not limited to, read-only memories, random access memories, compact disc read-only memories (CD-ROMs), magnetic tapes, floppy disks, flash drives, smart cards, and optical data storage devices. The computer readable storage medium may also be distributed in network coupled computer systems such that the computer readable medium is stored and executed in a distributed manner, e.g., by a telematics server or a controller area network (CAN).Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.FIG. 2 illustrates an exemplary fuel cell system for a vehicle according to an exemplary embodiment of the present invention; FIG. 3 illustrates an exemplary method of controlling an exemplary fuel cell system for a vehicle according to an exemplary embodiment of the present invention; and FIG. 4 illustrates a power curve of an exemplary compressor of an exemplary fuel cell system for a vehicle according to an exemplary embodiment of the present invention.In FIG. 2, an exemplary fuel cell system for a vehicle may include: a compressor 200 configured to supply air; a humidifier 400 that may include a supply portion 420 connected to the compressor 200 and an air supply pipe 300 and configured to supply the supplied air with moisture and supply the humidified air to a fuel cell stack, and an outlet portion 440 configured to receive an air discharged from the fuel cell stack 100, discharge moisture from the received air to the supply portion 420, and discharge the remaining air to the outlet pipe 320; a valve 600 installed in the air supply pipe 300 connected between the compressor 200 and the humidifier 400; and a bypass pipe 700 branching at the valve 600 and connected to the outlet pipe 320.In certain example embodiments, a fuel cell vehicle may include the basic configuration of an example fuel cell as illustrated in FIG. 2. In the present exemplary embodiment, a valve, a bypass pipe, and a controller 800 configured to operate the valve may be further included in the basic configuration.Specifically, a three-way valve and a bypass pipe may be connected to an air supply pipe through which air can be supplied to an air electrode of a fuel cell, thereby providing a desired portion of the air supplied from the compressor to the fuel cell stack by bypassing. As such, the pressure and flow rate of the air supplied to the fuel cell stack can be adjusted and pumping and throttling conditions of the compressor can be avoided, so that more effective and rapid control of the humidity of the air supplied to the fuel cell can be provided.The configuration of an exemplary fuel cell system implementing the above-mentioned features is illustrated in FIG. 2. The system may further include: a three-way valve configured to be a pipe connection between a rear end of a compressor and an inlet of an air electrode of a fuel cell stack; and a bypass pipe branching at the three-way valve and connected to a pipe connected between an outlet of the air electrode of the fuel cell stack and a humidifier. By using the three-way valve and the bypass pipe, a flow of the air supplied from the air compressor can be divided into an air flow flowing to the inlet and an air flow flowing to the outlet of the air electrode of the fuel cell stack. In addition, the pressure of the air compressed and supplied by the compressor may be increased while supplying air to a fuel cell stack at a reduced flow rate. In an exemplary embodiment illustrated in FIG. 2, the bypass pipe may be connected to the rear end of the humidifier disposed on the outlet side of the air electrode. Alternatively, when the bypass pipe is connected to a pipe between the outlet of the air electrode of the fuel cell stack and the humidifier, the humidity of the air on the outlet side of the humidifier can be decreased, thereby decreasing the performance of the humidifier. Thus, since the bypass pipe may be installed to avoid a dry condition in the fuel cell stack by increasing the pressure of the air to be supplied to the air electrode according to the invention, the bypass pipe may be installed as illustrated in FIG. 2.In addition, the compressor 200 configured to supply air may be included. The humidification device 400 may include the supply portion 420 and the outlet portion 440. The supply portion 420 may be connected to the compressor 200 and the air supply pipe 300 and configured to supply the supplied air with moisture and then supply the humidified air to the fuel cell stack 100. The outlet portion 400 may be configured to receive the air discharged from the fuel cell stack 100, discharge moisture from the received air to the supply portion 420, and discharge the discharged air to the outlet pipe 320. As such, the humidifier 400 may be configured to supply the air to be supplied to the air electrode with moisture obtained from the discharged gas. In other words, the humidifier 400 may be configured to provide the humidified air to be supplied to the fuel cell stack 100.The valve 600 may be installed in the air supply line 300 connected between the compressor 200 and the humidifier 400. The bypass pipe 700 may branch at the valve and be connected to the outlet pipe. When the valve 600 is opened, an air flow may be divided into a flow flowing to the humidifier and an air flow flowing to the outlet pipe. When it is determined that the pressure of the air supplied by the compressor is to be increased to a predetermined pressure level above a point on a surge curve, the controller 800 may perform the control of exhausting an air fraction through the bypass pipe by controlling the valve 600.Specifically, in response to determining to increase a target air pressure to a pressure above a point on the compressor pumping curve under a condition where air is supplied to the fuel cell stack at a set air flow rate, the controller 800 may be configured to actuate the compressor to supply air through the compressor at a flow rate higher than the controlled flow rate. In addition, the controller 800 may be configured to operate the valve to discharge a portion of the air supplied from the compressor through the bypass pipe, thereby satisfying both the set flow rate of the air supplied to the fuel cell stack and the target pressure of the air supplied from the compressor.FIG. 3 illustrates processes of an exemplary method executed by a controller for controlling an exemplary fuel cell system according to an exemplary embodiment of the present invention. The method for controlling the fuel cell system may include: comparing the target pressure of the air supplied from the compressor with a pressure of the surge line of the compressor (S 300); and discharging air through the bypass pipe by operating the valve when the target pressure is higher than the pressure of the surge line (S 500).Specifically, in the process of exhausting air through the bypass pipe, a flow rate of the air supplied from the compressor may be increased (S 400) to cause the pressure of the air supplied from the compressor to reach the target pressure, and then exhaust air through the bypass pipe by opening the valve by a degree corresponding to a difference between the set flow rate of the air to be supplied to the fuel cell stack and the flow rate of the air supplied from the compressor. When the target pressure is less than the limit pressure, the compressor may operate normally at pressures below the surge line.Further, the method may include calculating the adjusted flow rate and the target pressure. In this calculation, the set flow rate may be a flow rate of the air supplied to the fuel cell stack, and the target pressure may be a pressure of the compressed air supplied from the compressor. In the comparison step (S 300), the calculated target pressure may be compared with a limit pressure corresponding to a point on the pumping curve at the set flow rate (S 200). In the bypass step (S 500), when the target pressure is higher than the threshold pressure, the valve may be adjusted such that air may be discharged through the bypass pipe.This control may be performed when the fuel cell system is operated under a dry condition. Alternatively, such control may be performed when it is determined that the pressure of the air supplied from the compressor is increased to a pressure above a point on the pumping curve, because a water content sensor that measures the water content in the electrode membrane of the fuel cell stack indicates that the electrode membrane is dry.In the bypassing step, the flow rate of the air supplied from the compressor may be increased until the limit pressure reaches the target pressure and air is discharged through the bypass pipe by opening the valve by a degree corresponding to a difference between the controlled flow rate and a flow rate of the air supplied from the compressor. According to the control method described above, air can be compressed to have a desired increased pressure before being supplied to the air electrode, and further at the same time, the flow rate can be reduced to a desired level. Consequently, a dry condition can be managed effectively and a dry condition cannot occur.In response to determining to perform the control to increase the pressure of the air supplied from the compressor to a pressure above a point on the pump voltage due to the fuel cell system operating in a dry condition or a water content sensor measuring the water content in the electrode membrane of the fuel cell stack indicating that the electrode membrane is dry, a sufficient amount of air may be supplied by the compressor at an increased flow rate within a range to avoid pumping. Specifically, a desired amount of air required by the fuel cell may be supplied to the inlet side of the air electrode of the fuel cell stack by operating the three-way valve, and the air supplied by the compressor may be discharged to the outlet side of the fuel cell stack through the bypass pipe to increase the air pressure.Further, since the power consumed by the compressor can be obtained by a combination of the air flow rate and a pressure difference between pressures at the inlet and outlet of the compressor, and the conditions of the pumping curve can result in a minimum amount of air used at the same pressure, the control along the pumping curve can be a more effective manner for adjusting the flow rate of the air used to maintain the target pressure.According to various exemplary embodiments of the present invention, the method of returning the fuel cell stack from the dry condition may increase the air flow rate to increase the air pressure. Although the method may increase power consumption by the compressor, the method may be used for currently available fuel cell vehicles when a temperature of a fuel cell stack of the vehicle rises and an electrode membrane thereof may dry under a super pump condition of about 40° C.In the related art, many vehicle manufacturers have used a conventional method for increasing the pressure of air supplied to an air electrode during high temperature operation using a compressor despite the inefficiency of the method. However, since the upper limit of the pressure control range of the compressor may be the value of the surge curve, the operation of the pressure control may be insufficient at a low output level of a fuel cell, and the effect of using the pressure control method may be limited due to the limited operable range, and the conventional method has not provided an appropriate solution. However, in various exemplary embodiments of the present invention, methods may provide the compressor to supply air at the higher pressure that may be generated by the compressor, regardless of the output level of the fuel cell, under a boundary condition for operations of a system, such as an over-pumping condition, and thereby provide an effective solution for drying in the fuel cell stack.FIG. 4 is a power curve of an exemplary compressor of an exemplary fuel cell system according to an exemplary embodiment of the present invention. Consequently, the ranges of the flow rate and the pressure of air supplied to an air electrode of a fuel cell can be expanded using the piping structures of the present invention. In the prior art section, conventional technologies can adjust the flow rate and pressure of air only within an operable range without using bypass. In contrast, according to exemplary embodiments of the present invention, in response to determining to increase the pressure of air supplied from a compressor to a pressure above a point on the pumping curve even under a condition where a flow rate of air supplied to a fuel cell stack is decreased, a sufficient amount of air may be supplied at a target pressure by the compressor. Specifically, a desired amount of air required by a fuel cell stack may be supplied to an inlet side of an air electrode of a fuel cell stack using a three-way valve, and the remaining air may be discharged to an outlet side of the fuel cell stack through a bypass pipe. Consequently, the ranges of the pressure and the flow rate of air supplied to a fuel cell can be expanded to an operable range with bypass utilization.According to various exemplary embodiments of the present invention, the fuel cell system, and the methods for controlling the fuel cell having the structure described above, the water content of an electrode membrane of the fuel cell can be effectively adjusted by varying the pressure and the flow rate of air supplied to the fuel cell.In addition, durability and performance of a fuel cell can be improved by maintaining a condition in which an optimum water content is supplied in an electrode membrane. In addition, even under an operating condition having a substantially high temperature for operating the fuel cell system, the performance of a fuel cell can be effectively maintained by adjusting the humidity of the air supplied to the fuel cell. For example, performance of the fuel cell can be maintained even under a severe operating condition such as a high temperature, high speed, and climbing capability.

Claims

A fuel cell system for a vehicle, comprising: a compressor (200) configured to compress air to be supplied to a fuel cell stack (100); a valve (600) installed in an air supply pipe (300) connected from the compressor (200) to the fuel cell stack (100); a bypass pipe (700) branched at the valve (600) and connected to an outlet pipe of the fuel cell stack (100); and a controller (800) configured to increase a pressure of the air supplied from the compressor (200) and exhaust a portion of the air through the bypass pipe (700) by operating the valve (600) in response to determining to increase the pressure of the air supplied from the compressor (200) to a pressure above a point on a pumping curve when the measured moisture content of an electrode membrane indicates a dry condition by the measured water content.The fuel cell system for a vehicle according to claim 1, further comprising: a humidifier (400) installed downstream of the valve (600) of the air supply pipe (300), comprising: a supply portion (420) connected to the compressor (200) and the air supply pipe (300) to supply the supplied air with moisture and supply the humidified air to the fuel cell stack (100); and an outlet portion (4400) configured to receive the air discharged from the fuel cell stack (100), supply moisture from the received air to the supply portion (420), and discharge the discharged air to an outlet pipe.The fuel cell system for a vehicle according to claim 1, wherein the controller (800) is configured to, in response to determining to increase a target pressure of the compressed air to the pressure above a point on the pumping curve of the compressor (200) under a condition where the air is supplied to the fuel cell stack (100) at a set air flow rate, operate the compressor (200) to supply the air from the compressor (200) at a flow rate higher than the set flow rate, and operate the valve (600) to discharge a portion of the air through the bypass pipe (700) to satisfy both the set flow rate of the air supplied to the fuel cell stack (100) and the target pressure of the air compressed by the compressor (200).A method for controlling the fuel cell system according to claim 1, comprising: comparing, by the controller (800), the target pressure of the compressed air to be supplied to the fuel cell stack (100) with the pressure of the pumping curve of the compressor (200); and discharging, by the controller (800), the air fraction through the bypass pipe (700) by operating the valve (600) when the target pressure is higher than the pressure of the pumping curve.The method according to claim 4, wherein during the discharging of the air, a flow rate of the air supplied from the compressor (200) is increased such that the pressure of the air supplied from the compressor (200) reaches the target pressure, and a portion of the air supplied from the compressor (200) is discharged through the bypass pipe (700) by opening the valve (600) by a degree corresponding to a difference between the controlled flow rate of the air supplied to the fuel cell stack (100) and the flow rate of the air supplied from the compressor (200).The method of claim 4, further comprising: calculating the set flow rate of the air supplied to the fuel cell stack (100) and the target pressure of the compressed air supplied to the fuel cell stack (100), wherein during the comparing, the target pressure is compared with a limit pressure corresponding to a point on the pumping curve at the set flow rate.The method of claim 6, wherein during the exhausting of the air, the air is exhausted through the bypass pipe (700) by operating the valve (600), when the target pressure is higher than the threshold pressure.The method according to claim 6, wherein during the exhausting of the air, the flow rate of the air supplied from the compressor (200) is increased such that the limit pressure reaches the target pressure, and a portion of the air is exhausted through the bypass pipe (700) by opening the valve (600) by a degree corresponding to a difference between the set flow rate and the flow rate of the air supplied from the compressor (200).

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