Fuel cell system and control method thereof

Abstract

The present invention relates to a fuel cell system and a method for controlling the same, the fuel cell system comprising: a hydrogen gas pressure regulator for regulating a hydrogen gas supply pressure of hydrogen gas supplied to a fuel cell stack; and a controller that determines whether a control mode of the hydrogen supply pressure needs to be controlled according to the state of the fuel cell stack, and that can control the hydrogen pressure regulator so that the hydrogen supply pressure varies within a preset pressure range according to a preset pressure increase rate or a preset pressure decrease rate when the control mode is needed.

Description

TECHNICAL FIELD

[0001] The present application relates to a fuel cell system capable of inducing recirculation of hydrogen gas inside an anode of a fuel cell stack and a control method thereof. BACKGROUND

[0002] A fuel cell vehicle generally employs a fuel cell system composed of a fuel cell stack that generates electric power through electrochemical reactions of fuel and oxidant, and further generates motive power, a fuel supply device that supplies fuel gas to an anode of the fuel cell stack through a fuel gas supply passage, an air supply device that supplies air containing oxygen to a cathode of the fuel cell stack through an oxidant gas supply passage, a thermal management device that controls an operation temperature of the fuel cell stack, a control device that controls the operation of the fuel cell system, and the like.

[0003] In such a fuel cell system, hydrogen gas as fuel is oxidized in an anode (oxidation electrode) of the fuel cell stack to generate hydrogen ions and electrons. The hydrogen ions in the anode move to a cathode (reduction electrode) through an electrolyte membrane, and in the cathode, oxygen in the air is reduced to generate water. After such electrochemical reactions, unspent hydrogen gas and gases such as oxygen, nitrogen, and the like that have permeated from the cathode remain in the anode of the fuel cell stack, and these remaining gases cause uneven distribution of hydrogen gas in the entire anode, and only a local distribution is formed, thereby causing a problem of generating a deviation in hydrogen gas concentration inside the anode.

[0004] In order to solve the deviation in hydrogen gas concentration generated inside the anode, it is necessary to induce recirculation of hydrogen gas inside the anode. However, in order to achieve the recirculation of hydrogen gas, it can be necessary to generate a current or perform a purge operation, or even it can be necessary to configure an additional device such as a recirculation blower.

[0005] However, in the case of generating a current in order to induce the recirculation of hydrogen gas, the fuel cell can be overcharged due to excessive generation of a current under certain operation conditions of the fuel cell stack, or in the case of performing a purge in order to induce the recirculation of hydrogen gas, an adverse effect can be caused on the fuel efficiency of the fuel cell system. In addition, the additional configuration of a device causes a problem of cost increase.

[0006] It is to be specifically noted that the above-described contents as background art are only for the purpose of facilitating the understanding of the technical background, and should not be recognized as belonging to the prior art known to those skilled in the art. SUMMARY

[0007] The present application has been made to solve the above-described problems, and aims to provide a fuel cell system and a control method thereof capable of inducing recirculation of hydrogen gas inside an anode of a fuel cell stack without the need for an additional operation or device, by adjusting a hydrogen gas supply pressure applied to the fuel cell stack.

[0008] The technical problems of the present application are not limited to the above-mentioned technical problems, and other technical problems not mentioned can be clearly understood by those skilled in the art from the following description.

[0009] To achieve the above-mentioned object, a fuel cell system based on the present application includes: a hydrogen pressure regulator for regulating a hydrogen supply pressure of hydrogen supplied to a fuel cell stack; and a controller for judging whether a control mode for controlling the hydrogen supply pressure is required according to a state of the fuel cell stack, and in the case where the control mode is required, the hydrogen pressure regulator can be controlled so that the hydrogen supply pressure is varied within a predetermined pressure range according to a predetermined pressure increase rate or a pressure decrease rate.

[0010] In addition, to achieve the above-mentioned object, a control method of a fuel cell system based on the present application can include: a judging step for judging whether a control mode for controlling a hydrogen supply pressure of hydrogen supplied to a fuel cell stack is required according to a state of the fuel cell stack; and a variation control step for performing control so that the hydrogen supply pressure is varied within a predetermined pressure range according to a predetermined pressure increase rate or a pressure decrease rate in the case where it is judged that the control mode is required.

[0011] According to the fuel cell system and the control method thereof of the present application as described above, in the case where a hydrogen concentration deviation is generated inside an anode of a fuel cell stack, the hydrogen supply pressure can be controlled according to a pressure increase rate or a pressure decrease rate, thereby guiding recirculation of hydrogen inside the anode of the fuel cell stack.

[0012] In addition, the hydrogen supply pressure is controlled according to the pressure increase rate or the pressure decrease rate, thereby guiding the recirculation of hydrogen, and thus it is possible to prevent accumulation or inflow of oxygen to the anode of the fuel cell stack.

[0013] Further, by preventing accumulation or inflow of oxygen to the anode of the fuel cell stack, it is possible to prevent deterioration of an electrode and an electrolyte membrane of the fuel cell stack, to improve durability of the fuel cell stack, and thus it is possible to improve a life expectancy.

[0014] Effects that can be obtained by the present application are not limited to the above-mentioned effects, and other effects not mentioned can be clearly understood by those skilled in the art from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a diagram for explaining a fuel cell system based on an embodiment of the present application.

[0016] Figure 2 is a diagram for explaining variation control of a hydrogen supply pressure based on an embodiment of the present application.

[0017] Figure 3is a flowchart for explaining a control method of a fuel cell system based on an embodiment of the present application.

[0018] Figure 4 is a flowchart for explaining rise control of a hydrogen supply pressure of a controller based on another embodiment of the present application.

[0019] Figure 5 is a flowchart for explaining setting of a pressure rise rate or a pressure fall rate based on an embodiment of the present application.

[0020] Figures 6-7 is a graph for explaining setting of a first maximum pressure rise rate based on an embodiment of the present application.

[0021] Figure 8 is a graph for explaining setting of a second maximum pressure rise rate based on an embodiment of the present application.

[0022] Figure 9 is a flowchart for explaining a control method of a fuel cell system based on another embodiment of the present application in a case where it is possible to detect an oxygen concentration inside an anode of a fuel cell stack.

[0023] BRIEF DESCRIPTION OF DRAWINGS

[0024] 100: fuel cell stack

[0025] 120: hydrogen pressure regulator

[0026] 140: hydrogen purger

[0027] 300: controller DETAILED DESCRIPTION

[0028] In explaining embodiments of the present application, if it is judged that specific explanation of related known structures or functions can hinder understanding of the embodiments of the present application, detailed explanation thereof is omitted. Further, the drawings are used only for more intuitive understanding of the embodiments disclosed in the specification, and the contents thereof do not limit the technical idea involved in the specification, and it is understood that all changed embodiments, equivalent embodiments, and alternative embodiments included in the idea and technical scope of the present application are encompassed.

[0029] In the specification, terms with ordinal numbers such as "first", "second", and the like can be used to describe different components, but the components are not limited by the terms. The terms are used only to distinguish one component from another component.

[0030] It is to be understood that when a part is referred to as being "connected" or "coupled" to another part, it can be directly connected or coupled to the other part or joined to the other part by an intermediate part. In contrast, when a part is referred to as being "directly connected" or "directly coupled" to another part, it is understood that there are no intermediate parts between the two parts.

[0031] The singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise.

[0032] In this specification, the term "comprising" or "having" is understood to mean the presence of the stated feature, number, step, operation, component, part, or combination thereof, but does not exclude the presence of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0033] In addition, the unit (Unit) or control unit (Control Unit) included in the name of the fuel cell control unit (FCU: Fuel Cell Control Unit) or the like is only a naming method for a control device (Controller) that controls a specific function of a vehicle, and does not mean a generic function unit (Generic function unit).

[0034] The controller (Controller) can include a communication device for communicating with other controllers or sensors in order to control the functions assumed thereby, a memory for storing an operating system or a logic instruction and input / output information, and one or more processors for performing judgment, calculation, determination, etc. required for performing control of the functions assumed thereby.

[0035] Hereinafter, embodiments disclosed in the present specification will be described in detail with reference to the accompanying drawings, and the same or similar components are given the same reference numerals regardless of which figure they are shown in, and repetitive descriptions thereof will be omitted.

[0036] An object of the present application is to address a deviation in hydrogen concentration generated inside an anode of a fuel cell stack by inducing recirculation of hydrogen inside the anode. Specifically, the present application aims to induce recirculation of hydrogen inside the anode by repeatedly adjusting a hydrogen supply pressure of the fuel cell stack.

[0037] First, a fuel cell system based on an embodiment of the present application will be described with reference to Figure 1

[0038] Figure 1 is a schematic view for explaining a fuel cell system based on an embodiment of the present application.

[0039] With reference to Figure 1 ​A fuel cell system according to an embodiment of the present application can include a fuel cell stack 100, a hydrogen pressure regulator 120, a hydrogen purger 140, and a controller 300. However, the fuel cell system according to the embodiment of the present application can include more or less components than those shown in the drawing. Figure 1 The fuel cell system shown in the drawing shows only components related to the embodiment of the present application, and an actual fuel cell system can include more or less components.

[0040] The components will be briefly described below.

[0041] The fuel cell stack 100 can be provided with an anode and a cathode, and the hydrogen pressure regulator 120 and the hydrogen purger 140 can be connected to the anode side of the fuel cell stack 100.

[0042] The hydrogen pressure regulator 120 can be provided at the anode inlet side of the fuel cell stack 100, and can regulate the pressure of hydrogen supplied to the anode of the fuel cell stack 100.

[0043] The controller 300 according to the embodiment of the present application, in order to achieve the object of the present application, i.e., in order to induce recirculation of hydrogen within the anode of the fuel cell stack 100, can determine whether or not a control mode for controlling the hydrogen supply pressure supplied to the fuel cell stack 100 needs to be performed. For example, the controller 300 can determine whether or not the control mode needs to be performed according to at least one of the state of the cell voltage of the fuel cell stack 100 and the state of operation of the fuel cell stack 100.

[0044] Specifically, the state of the cell voltage of the fuel cell stack 100 can be determined by determining the voltage of each of a plurality of cells included in the fuel cell stack 100, and determining the difference between the average cell voltage of the fuel cell stack 100 and the minimum cell voltage (hereinafter referred to as "cell deviation DV (Min Cell Diff Voltage)") calculated therefrom. The state of operation of the fuel cell stack 100 can mean that the fuel cell stack 100 is in an idle state or a start state or a shutdown state. However, the above definition is only exemplary and is not necessarily limited to the above explanation.

[0045] The controller 300 can determine that the control mode needs to be performed when the cell deviation (DV) is greater than a first reference difference value or the fuel cell stack 100 is in an idle state, by comparing the cell deviation (DV) with the first reference difference value. At this time, the first reference difference value can mean a value set as a reference of the maximum voltage deviation considering the stability of the operation of the fuel cell stack 100.

[0046] For example, in the case where the hydrogen gas is not distributed uniformly inside the anode of the fuel cell stack 100, the single cell voltage value corresponding to the hydrogen gas deficient portion is smaller than the value measured by the other single cells, and thus a large single cell deviation can be generated. In addition, in the case where the fuel cell stack 100 is in the no-load state, which corresponds to an unfavorable operating condition in which circulation is not formed inside the anode, the hydrogen gas concentration can be non-uniform at each single cell position, or the possibility of generation of oxygen gas on the anode side becomes large.

[0047] Therefore, in the present application, based on the single cell deviation or the no-load state of the fuel cell stack 100, it is determined whether or not hydrogen gas recirculation is required inside the anode of the fuel cell stack 100, and the above-described control mode is executed.

[0048] In the case where it is determined that the control mode is required, the controller 300 can control the hydrogen gas supply pressure (for example, the pressure of the hydrogen gas supplied to the anode) using the hydrogen gas pressure regulator 120 (for example, a hydrogen gas pressure valve or the like). For example, as shown in FIG. 6, the controller 300 can perform variation control of the hydrogen gas supply pressure within a predetermined pressure range according to a predetermined pressure increase rate or a predetermined pressure decrease rate. Figure 2 Figure 2 is a schematic view related to variation control of the hydrogen gas supply pressure.

[0049] At this time, the predetermined pressure range can indicate a range formed by a first pressure as a maximum value (upper limit value) and a second pressure as a minimum value (lower limit value). Furthermore, according to an embodiment, the second pressure indicates a value corresponding to a minimum pressure condition in the operating conditions of the fuel cell stack 100, and the first pressure can indicate a value obtained by increasing a reference pressure difference capable of realizing the recirculation effect of hydrogen gas inside the fuel cell stack 100 on the basis of the second pressure. However, the above-described content is merely exemplary and is not limited thereto.

[0050] In the case where it is determined that the control mode is required, the controller 300 can determine the hydrogen gas supply pressure, and the determination can be achieved by receiving detection information provided by a hydrogen gas pressure sensor provided in the fuel cell system. Alternatively, the hydrogen gas supply pressure can be a target pressure corresponding to a tracking target. Furthermore, in the case where the determined hydrogen gas supply pressure is smaller than the first pressure as the maximum value of the pressure range, the controller 300 can perform control according to the predetermined pressure increase rate until the hydrogen gas supply pressure increases to the first pressure. On the other hand, in the case where the determined hydrogen gas supply pressure is equal to or greater than the first pressure, the controller 300 can perform control according to the predetermined pressure decrease rate until the hydrogen gas supply pressure decreases to the second pressure.

[0051] ​The controller 300 can again control in accordance with the pressure increase rate when the hydrogen supply pressure decreases to the second pressure, until the hydrogen supply pressure increases to the first pressure. That is, the controller 300 according to an embodiment of the present application aims to vary the control (increase control and decrease control) of the hydrogen supply pressure within the predetermined pressure range in order to induce the recirculation of hydrogen within the anode of the fuel cell stack 100 to prevent the generation of a hydrogen concentration bias within the anode.

[0052] On the other hand, in a specific implementation of the fuel cell system, the controller 300 can be a fuel cell controller (FCU) for controlling the fuel cell system. However, this is merely exemplary and is not limited thereto. For example, the controller 300 can be constituted by a fuel cell controller and other controllers, and can be constituted in a manner in which the functions thereof are dispersed among two or more mutually different controllers.

[0053] Hereinafter, the control method of the fuel cell system according to an embodiment of the present application implemented by the controller 300 will be described with reference to the flowchart of FIG. 4. Figure 3

[0054] Figure 3 is a flowchart for explaining the control method of the fuel cell system according to an embodiment of the present application.

[0055] Referring to FIG. 4, Figure 3 , the controller 300 can determine that the control mode of the hydrogen supply pressure is required when the difference between the average cell voltage of the fuel cell stack 100 and the minimum cell voltage (cell bias: DV) is greater than a predetermined first reference difference, or when the fuel cell stack 100 is in an idle state (YES in S301). For example, the controller 300 according to an embodiment of the present application determines whether the cell bias (DV) of the fuel cell stack 100 is greater than the first reference difference and whether the fuel cell stack 100 is in an idle state, and determines that the control mode is required when at least one of the two conditions is satisfied.

[0056] When it is determined that the control mode is required, the controller 300 can confirm the hydrogen supply pressure currently supplied to the fuel cell stack 100 (S302). When the hydrogen supply pressure is less than the first pressure, which is the maximum value of the predetermined pressure range (YES in S302), the controller 300 can control in accordance with the predetermined pressure increase rate until the hydrogen supply pressure increases to the first pressure. When the hydrogen supply pressure is the first pressure or more (NO in S302), the controller 300 can control in accordance with the predetermined pressure decrease rate until the hydrogen supply pressure decreases to the second pressure, which is the minimum value of the pressure range.

[0057] ​The controller 300 can control the hydrogen supply pressure to rise or control the hydrogen supply pressure to fall, according to a result of comparison of the hydrogen supply pressure with the first pressure that is the maximum value of the pressure range (S303, S304).

[0058] Before explaining the control process, the configuration of the fuel cell system 1 according to the embodiment of the application will be described with reference to Figure 5 The pressure rise rate and the pressure fall rate applied at the time of control of the hydrogen supply pressure based on the embodiment of the application will be described.

[0059] Figure 5 is a flowchart for explaining setting of the pressure rise rate or the pressure fall rate based on the embodiment of the application.

[0060] With reference to Figure 5 , the controller 300 can determine the pressure range that is the reference of the variation control of the hydrogen supply pressure (S510). For example, the first pressure that is the maximum value and the second pressure that is the minimum value are determined preferentially, and the pressure range is determined according to the first pressure and the second pressure. Subsequently, the controller 300 can set the initial value of the pressure rise rate or the pressure fall rate in order to control the variation of the hydrogen supply pressure according to the determined pressure range (S520).

[0061] When the initial value of the pressure rise rate or the pressure fall rate is set, the controller 300 performs the variation control of the hydrogen supply pressure according to the set initial value, and in order to confirm whether the hydrogen recirculation is generated in the anode inside the fuel cell stack 100 due to the control, the local hydrogen concentration inside the anode is measured (S530).

[0062] The controller 300 can judge whether the concentration deviation inside the anode is improved according to the measured hydrogen concentration (S540), and if the concentration deviation inside the anode is not improved (NO of S540), it indicates that the hydrogen recirculation is not sufficient, and therefore, the pressure rise rate or the pressure fall rate is changed continuously until the concentration deviation inside the anode is improved (S550), and the variation control of the hydrogen supply pressure is repeated. However, in the process of the variation control of the hydrogen supply pressure in order to generate sufficient hydrogen recirculation in the anode inside the fuel cell stack 100, if a value of the pressure rise rate or the pressure fall rate that is too large is applied and the hydrogen flows in excess, oxygen can be generated in the anode inside due to the difference in the concentration gradient between the anode and the cathode of the fuel cell stack 100. In view of this, the controller 300 can further confirm whether the oxygen concentration of the anode of the fuel cell stack 100 is greater than the reference oxygen concentration (S560), even in the case where the concentration deviation inside the anode is improved (YES of S540).

[0063] The controller 300 can confirm whether the oxygen concentration of the anode of the fuel cell stack 100 is greater than the reference oxygen concentration (S560). In the case where the oxygen concentration of the anode is greater than the reference oxygen concentration (YES in S560), hydrogen recirculation needs to be performed so that the oxygen concentration of the anode of the fuel cell stack 100 falls below the reference oxygen concentration. Therefore, the controller 300, in the case where the oxygen concentration of the anode is greater than the reference oxygen concentration (YES in S560), can change the pressure increase rate or the pressure decrease rate (S550) to perform the fluctuation control of the hydrogen supply pressure.

[0064] When the concentration deviation inside the anode of the fuel cell stack 100 is improved (YES in S540) and the oxygen concentration of the anode of the fuel cell stack 100 is below the reference oxygen concentration (NO in S560), the controller 300 can finally determine the pressure increase rate or the pressure decrease rate corresponding to this time (S570). Subsequently, the controller 300 can perform the fluctuation control of the hydrogen supply pressure based on an embodiment of the present application, reflecting the finally determined pressure increase rate or pressure decrease rate.

[0065] According to Figure 5 The above-described fluctuation control of the hydrogen supply pressure performs the fluctuation control of the hydrogen supply pressure according to the finally determined pressure increase rate or pressure decrease rate, i.e., the pressure increase rate or pressure decrease rate of a certain fixed value.

[0066] However, when the fluctuation control of the hydrogen supply pressure is performed according to the pressure increase rate or pressure decrease rate fixed to a certain value, although hydrogen recirculation can be induced inside the fuel cell stack 100 at the initial stage of the fluctuation control, it can be difficult to continuously induce hydrogen recirculation.

[0067] Thus, returning to Figure 3 The pressure increase rate or pressure decrease rate used by the controller 300 based on an embodiment of the present application can not adopt a fixed value, but can continuously change the pressure increase rate or pressure decrease rate to perform the fluctuation control of the hydrogen supply pressure.

[0068] Specifically, in the increase control of the hydrogen supply pressure, the controller 300 controls the hydrogen supply pressure to increase according to the preset pressure increase rate (S303). For example, the controller 300 can perform control to gradually change the pressure increase rate until the pressure increase rate reaches a preset first maximum pressure increase rate, and to increase the hydrogen supply pressure to a first pressure. At this time, the preset first maximum pressure increase rate can refer to a level of the pressure increase rate at which hydrogen recirculation is induced inside the anode of the fuel cell stack 100, and oxygen does not permeate to the anode side due to the concentration gradient difference between the anode and the cathode, which can be calculated and set in advance through experiments. Regarding the first maximum pressure increase rate set through experiments, reference will be made to Figure 6 andFigure 7 to explain.

[0069] Figures 6-7 is a graph for explaining setting a first maximum pressure increase rate based on an embodiment of the present application.

[0070] Referring to Figure 6 and Figure 7 the process explained can be considered as a process of performing an experiment on a system identically configured as a fuel cell system based on an embodiment of the present application in an ideal state.

[0071] First, referring to Figure 6 , an initial value of the pressure increase rate can be input to the controller 300 (S610), for example, the initial value of the pressure increase rate can be 0 kPa / s. However, this is merely exemplary and is not limited thereto.

[0072] In addition, the controller 300 can first measure the anode internal hydrogen concentration, the single cell deviation (DV) value, and the anode internal oxygen concentration of the fuel cell stack 100 in a state in which the pressure increase rate is not changed according to the initial value of the pressure increase rate (S620). After the first measurement, the controller 300 changes the pressure increase rate (S630) and can perform the increase control of the hydrogen supply pressure according to the changed pressure increase rate. For example, the change of the pressure increase rate based on an embodiment of the present application refers to gradually increasing the pressure increase rate, but this is merely exemplary and is not limited thereto.

[0073] After performing the increase control according to the changed pressure increase rate, the controller 300 can secondarily measure the anode internal hydrogen concentration, the single cell deviation (DV) value, and the anode internal oxygen concentration of the fuel cell stack 100 in a state in which the pressure increase rate is changed (S640).

[0074] The controller 300 can compare the first measured oxygen concentration with the second measured oxygen concentration (S650) and repeatedly perform the processes of S620 to S650 until the first measured oxygen concentration before changing the pressure increase rate is lower than or equal to the second measured oxygen concentration after changing the pressure increase rate.

[0075] Figure 7 is a graph showing the oxygen concentration in the anode of the fuel cell stack 100 changing with the change of the pressure increase rate.

[0076] Referring to Figure 7, in the case where the pressure increase rate is kept at the initial value, it is confirmed that the oxygen concentration inside the anode of the fuel cell stack 100 is excessively high. Thereafter, as the pressure increase rate is changed (for example, increased), it is confirmed that the oxygen concentration inside the anode shows a tendency to gradually decrease, and shows a tendency to again increase in oxygen concentration after a certain time point. The reason for the oxygen concentration to again increase after the certain time point is that, in the case where hydrogen is excessively supplied for hydrogen recirculation inside the anode, a concentration gradient difference (concentration unevenness) is generated between the anode and the cathode of the fuel cell stack 100, thereby causing oxygen on the cathode side to permeate to the anode side. If oxygen again permeates to the anode side, it is difficult to smoothly perform hydrogen recirculation inside the anode due to oxygen accumulated in the anode, and in view of this, by comparing the oxygen concentration before and after the change in the pressure increase rate, the optimal pressure increase rate at which only hydrogen recirculation is generated inside the anode of the fuel cell stack 100 can be found.

[0077] Returning again to Figure 6 , by repeatedly performing the processes of S620 to S650, in the case where the oxygen concentration first measured before the change in the pressure increase rate is lower than the oxygen concentration second measured after the change in the pressure increase rate (NO in S650), the controller 300 sets the pressure increase rate before the change at the time when the condition is satisfied as the first maximum pressure increase rate (S660). For example, the first maximum pressure increase rate based on one embodiment of the present application obtained by experimentally performing the processes of Figure 6 may be 4 kPa / s, but this is merely exemplary and is not limited thereto.

[0078] On the other hand, as another embodiment, the controller 300 separately judges the oxygen concentration first measured before the change in the pressure increase rate and the oxygen concentration second measured after the change, and in the case where the oxygen concentration first measured before the change in the pressure increase rate is lower than the oxygen concentration second measured after the change (NO in S650), it can be judged whether the oxygen concentration second measured has a value close to 0. The controller 300 can repeatedly perform the processes of S620 to S650 until the oxygen concentration second measured has a value close to 0. In this case, the controller 300, in the case where the oxygen concentration first measured before the change in the pressure increase rate is lower than the oxygen concentration second measured after the change (NO in S650) and the oxygen concentration second measured has a value close to 0, can set the pressure increase rate at the time when the condition is satisfied as the first maximum pressure increase rate (S660).

[0079] As yet another embodiment, in the case where the oxygen concentration measured for the first time before the pressure increase rate is changed is lower than the oxygen concentration measured for the second time after the change (NO of S650), the controller 300 changes the pressure increase rate in a gradually decreasing manner while measuring and comparing the oxygen concentration inside the anode of the fuel cell stack 100. At this time, the controller 300 can down-regulate the pressure increase rate by a smaller magnitude than the up-regulation magnitude of the pressure increase rate in the S630 step described above. Even when the pressure increase rate is down-regulated, the oxygen concentration measured for the first time before the pressure increase rate is changed and the oxygen concentration measured for the second time after the change can be compared, and the controller 300 can continuously down-regulate the pressure increase rate until the oxygen concentration measured for the first time is lower than the oxygen concentration measured for the second time. In other words, when the pressure increase rate is up-regulated through the processes of S620 to S650, if the oxygen concentration measured for the first time before the pressure increase rate is changed is lower than the oxygen concentration measured for the second time after the change (NO of S650), the controller 300 can down-regulate the pressure increase rate at the time when the condition is satisfied, and set the pressure increase rate at which the oxygen concentration measured for the first time before the pressure increase rate is changed is lower than the oxygen concentration measured for the second time after the change as the first maximum pressure increase rate (S660).

[0080] Returning again to the S303 step of Figure 3 , the pressure increase rate based on an embodiment of the present application can refer to the speed of pressure increase (kPa / s), and when the pressure increase rate is changed, the controller 300 can gradually change the pressure increase rate according to a preset first pressure increase acceleration. For example, the controller 300 can cause the pressure increase rate to gradually and uniformly change with time by the preset first pressure increase acceleration.

[0081] On the other hand, as another embodiment, the controller 300 can adjust the pressure increase acceleration to gradually change the pressure increase rate in the process of changing the pressure increase rate until the pressure increase rate reaches the first maximum pressure increase rate. This will be described with reference to Figure 4 .

[0082] Figure 4 is a flowchart for explaining the increase control of the hydrogen supply pressure of the controller based on another embodiment of the present application.

[0083] For example, with reference to Figure 4, the controller 300 judges a difference between the first maximum pressure increase rate and the current pressure increase rate (S410), and can adjust the pressure increase acceleration according to the difference between the first maximum pressure increase rate and the current pressure increase rate. Specifically, in a case where the difference is greater than a reference value (YES in S420), the pressure increase rate is gradually changed according to the first pressure increase acceleration (S430), and in a case where the difference is the reference value or less (NO in S420), the pressure increase rate can be gradually changed according to a second pressure increase acceleration that is smaller than the first pressure increase acceleration (S440). The controller 300 controls the hydrogen supply pressure to increase based on the pressure increase rate that is changed at the mutually different pressure increase accelerations according to the result of the judgment in S420 (S450).

[0084] On the other hand, as another embodiment, the controller 300 can also gradually change the pressure increase rate using a pressure increase acceleration that is set in a manner of gradually decreasing as the pressure increase rate approaches the first maximum pressure increase rate, until the pressure increase rate reaches the first maximum pressure increase rate. For example, in order to be able to more finely change the pressure increase rate as the pressure increase rate approaches the first maximum pressure increase rate, a pressure increase acceleration that becomes smaller as the pressure increase rate approaches the first maximum pressure increase rate can be applied, so that the change width of the pressure increase rate is reduced. At this time, if the increase interval of the pressure increase rate before the pressure increase rate reaches the first maximum pressure increase rate is divided into a plurality of subintervals, different pressure increase acceleration values can be applied to each of the subintervals.

[0085] As described in the above-described another embodiment, by finely changing the pressure increase rate, the increase control of the hydrogen supply pressure can be performed finely.

[0086] Returning again to Figure 3 In a case where the hydrogen supply pressure reaches the maximum value of the pressure range, i.e., the first pressure (NO in S302), or the hydrogen supply pressure reaches the first pressure by the increase control of the hydrogen supply pressure described above, the controller 300 can control so that the hydrogen supply pressure decreases to the minimum value of the pressure range, i.e., the second pressure, according to a preset pressure decrease rate (S304). At this time, even if the preset pressure decrease rate is changed, it does not greatly affect the hydrogen recirculation inside the anode of the fuel cell stack 100, so the pressure decrease rate can be a fixed value, rather than a changeable value like the pressure increase rate described above. However, this is merely exemplary, and is not limited thereto.

[0087] Further, when the hydrogen supply pressure reaches the second pressure by the drop control of the hydrogen supply pressure, the controller 300 can control according to the preset pressure increase rate until the hydrogen supply pressure again increases to the first pressure. In other words, the controller 300 according to the embodiment of the present application, in order to induce the hydrogen recirculation inside the anode of the fuel cell stack 100, applies the hydrogen supply pressure so as to continuously supply the hydrogen, however, does not constantly supply the hydrogen supply pressure, but adjusts the pressure intensity in a manner that strengthens the hydrogen supply pressure at times and weakens the hydrogen supply pressure at times. Thereby, it is possible to effectively induce the hydrogen recirculation inside the anode of the fuel cell stack 100.

[0088] As described above, after the variation control of the hydrogen supply pressure, the controller 300 can determine whether the release condition of the control mode is satisfied or not according to at least one of the state of the single cell voltage of the fuel cell stack 100 and the operation state of the fuel cell stack 100. For example, the controller 300 can determine that the release condition of the control mode is not satisfied in a case where the difference between the average single cell voltage and the minimum single cell voltage (single cell deviation, DV) of the fuel cell stack 100 does not satisfy the second reference difference value which is less than the preset, or the fuel cell stack 100 is still in the no-load state ("No" in S305). At this time, the second reference difference value can be similar to the first reference difference value described above, and indicates a value set according to the average single cell voltage deviation considering the operation stability of the fuel cell stack 100.

[0089] In a case where the controller 300 determines that the release condition of the control mode is not satisfied, the controller 300 can preferentially determine whether the pressure increase rate as a control factor at the time of the increase control of the hydrogen supply pressure reaches the first maximum pressure increase rate which is preset (S306). In a case where the pressure increase rate does not reach the first maximum pressure increase rate ("No" in S306), the controller 300 can perform the variation control of the hydrogen supply pressure until the pressure increase rate reaches the first maximum pressure increase rate.

[0090] If the release condition of the control mode is not satisfied, but the pressure increase rate reaches the first maximum pressure increase rate (YES in S306), the controller 300 can change the preset first maximum pressure increase rate to a second maximum pressure increase rate having a value larger than the first maximum pressure increase rate (S307). Although the pressure increase rate reaches the first maximum pressure increase rate, the release condition of the control mode is not satisfied, which indicates that the hydrogen recirculation is not successfully formed inside the anode of the fuel cell stack 100. Therefore, the controller 300 needs to maintain the control mode for the hydrogen recirculation, i.e., needs to continuously perform the variation control of the hydrogen supply pressure, and by changing the first maximum pressure increase rate to the second maximum pressure increase rate larger than the first maximum pressure increase rate, the variation range of the pressure intensity of the hydrogen supply pressure is adjusted to be larger, so that the formation of the hydrogen recirculation can be guided.

[0091] Based on this, the controller 300 reflects the changed second maximum pressure increase rate, and performs the control according to the pressure increase rate or the pressure decrease rate as described above to vary the hydrogen supply pressure (S308).

[0092] On the other hand, the second maximum pressure increase rate can also be calculated and set in advance through experiments like the first maximum pressure increase rate described above. The second maximum pressure increase rate set through experiments will be described with reference to Figure 8 .

[0093] Figure 8 is a diagram for explaining a setting process of the second maximum pressure increase rate based on an embodiment of the present application.

[0094] With reference to Figure 8 , the maximum pressure increase rate R1 that the fuel cell system itself can have is confirmed according to the specification of each component constituting the fuel cell system (S810). In the case where the pressure increase rate is excessively large, the fuel cell system can generate noise, so in order to ensure the marketability of the fuel cell system, the pressure increase rate that can minimize the generated noise needs to be confirmed. Therefore, the maximum pressure increase rate R2 that does not cause a noise problem in the fuel cell system itself can be confirmed within the range of the confirmed maximum pressure increase rate R1 (S820).

[0095] After that, in order to confirm the pressure increase rate that satisfies the above two conditions and can be applied, the maximum pressure increase rate R3 (S830) that is below the confirmed maximum pressure increase rate (R2) and is normally applied in the fuel cell system can be confirmed. For example, the maximum pressure increase rate R3 that is normally applied in the fuel cell system can refer to a pressure increase rate that satisfies the start-up condition of the fuel cell system, but this is merely exemplary and is not limited thereto. Moreover, the controller 300 can set the confirmed maximum pressure increase rate R3 as the second maximum pressure increase rate (S840).

[0096] Returning again to Figure 3 , the controller 300 can reflect the changed second maximum pressure increase rate and cause the hydrogen supply pressure to fluctuate according to the pressure increase rate or the pressure decrease rate in the manner as described above (S308). At this time, similarly to the above S303 step and S304 step, the controller 300 causes the hydrogen supply pressure to increase according to the pressure increase rate changed according to the pressure increase acceleration and causes the hydrogen supply pressure to decrease according to the fixed pressure decrease rate. For example, the controller 300 can gradually change the pressure increase rate until the pressure increase rate reaches the changed second maximum pressure increase rate, and this process can gradually change the pressure increase rate using a third pressure increase acceleration that is smaller than the pressure increase acceleration applied before the pressure increase rate reaches the first maximum pressure increase rate.

[0097] For example, in conjunction with the case described with reference to Figure 4 , the third pressure increase acceleration can be a value that is smaller than the first pressure increase acceleration or the second pressure increase acceleration. However, this is merely exemplary and is not limited thereto. After that, the controller 300 causes the hydrogen supply pressure to increase based on the pressure increase rate that is gradually changed according to the third pressure increase acceleration.

[0098] Moreover, the first maximum pressure increase rate is changed to the second maximum pressure increase rate, and the fluctuation control of the hydrogen supply pressure is performed according to the changed second maximum pressure increase rate, and the controller 300 can again determine whether the state of the fuel cell stack 100 satisfies the release condition of the control mode (S309). Moreover, the controller 300 can determine the hydrogen concentration inside the anode of the fuel cell stack 100 (S310) while determining whether the release condition of the control mode is satisfied. For example, when the hydrogen concentration inside the anode of the fuel cell stack 100 is greater than the preset reference concentration (YES of S310), the fluctuation control of the hydrogen supply pressure is repeatedly performed according to the changed second maximum pressure increase rate until the pressure increase rate reaches the second maximum pressure increase rate.

[0099] However, if the state of the fuel cell stack 100 does not satisfy the release condition of the control mode until the pressure increase rate reaches the changed second maximum pressure increase rate (NO in S309) and the hydrogen concentration inside the anode of the fuel cell stack 100 decreases below the preset reference concentration (NO in S310), the controller 300 can perform a hydrogen purge operation (S311). This case can mean that the controller 300 determines that the hydrogen recirculation inside the anode of the fuel cell stack 100 cannot be further induced by the hydrogen supply pressure variation control and the hydrogen recirculation inside the anode of the fuel cell stack 100 needs to be induced by the hydrogen forced recirculation. For example, in an embodiment of the present application, although it is mentioned that the hydrogen purge operation is controlled by the controller 300 to induce the hydrogen forced recirculation, this is merely exemplary and is not limited thereto.

[0100] By Figure 3 The above-described control method of the fuel cell system based on an embodiment of the present application, which is described above, is a control process in which the controller 300 does not directly detect the oxygen concentration inside the anode of the fuel cell stack 100 when controlling the fuel cell system. If the oxygen concentration inside the anode of the fuel cell stack 100 can be directly detected, the oxygen concentration inside the anode condition can be further included when determining whether the release condition of the control mode is satisfied. Hereinafter, as another embodiment of the control method of the fuel cell system of the present application, the case in which the controller 300 can directly detect the oxygen concentration inside the anode of the fuel cell stack 100 will be described with reference to Figure 9 The case in which the controller 300 can directly detect the oxygen concentration inside the anode of the fuel cell stack 100 will be described.

[0101] Figure 9 is a flowchart for explaining the control method of the fuel cell system in the case in which the oxygen concentration inside the anode of the fuel cell stack based on another embodiment of the present application can be detected.

[0102] Referring to Figure 9 In the case in which the difference between the average cell voltage of the fuel cell stack 100 and the minimum cell voltage (cell deviation, DV) is greater than the preset first reference difference or the fuel cell stack 100 is in an idle state (YES in S910), the controller 300 can determine that the control mode of the hydrogen supply pressure needs to be controlled.

[0103] In the case in which it is determined that the control mode needs to be controlled, the controller 300 can determine the hydrogen supply pressure currently supplied to the fuel cell stack 100 (S920). Also, the controller 300 can perform the variation control of the hydrogen supply pressure according to the determined hydrogen supply pressure (S940). At this time, the S920 step and the S940 step can be performed according to the same procedures as those of the S120 step and the S140 step of the control method of the fuel cell system based on an embodiment of the present application described above. Figure 3The above-described steps S302 to S308 are executed in the same manner as described in the explanation. For example, the controller 300 can control the hydrogen supply pressure to rise to the first pressure according to the preset pressure increase rate in the case where the judged hydrogen supply pressure is less than the first pressure that is the maximum value of the pressure range. The controller 300 can control the hydrogen supply pressure to rise in the same manner as described in the explanation with reference to Figure 3 The controller 300 can control the hydrogen supply pressure to rise in the same manner as described in the explanation with reference to Figure 4 The controller 300 can control the hydrogen supply pressure to rise in the same manner as described in the explanation with reference to steps S410 to S450.

[0104] On the other hand, in the case where the hydrogen supply pressure is the first pressure or more, the controller 300 controls according to the preset pressure decrease rate until the hydrogen supply pressure decreases to the second pressure that is the minimum value of the pressure range. As shown in step S940, the controller 300 can judge whether the release condition of the control mode is satisfied during the execution of the variation control of the hydrogen supply pressure.

[0105] At this time, in the case where the oxygen concentration inside the anode of the fuel cell stack 100 can be detected, the controller 300 can judge the oxygen concentration inside the anode of the fuel cell stack 100, and can judge at least one of the state of the fuel cell stack 100 and the change in the oxygen concentration inside the anode of the fuel cell stack 100 based on the variation of the hydrogen supply pressure, whether the release condition of the control mode is satisfied (S960).

[0106] Specifically, in the case where the oxygen concentration inside the anode of the fuel cell stack 100 can be directly detected, the controller 300 can judge the oxygen concentration inside the anode of the fuel cell stack 100 before the variation control of the hydrogen supply pressure (the increase control or the decrease control of the hydrogen supply pressure) (hereinafter referred to as first oxygen concentration) (S930).

[0107] Further, the controller 300 can judge the oxygen concentration inside the anode of the fuel cell stack 100 after the variation control of the hydrogen supply pressure (hereinafter referred to as second oxygen concentration) (S950). Thus, the controller 300 judges not only whether the difference between the average cell voltage of the fuel cell stack 100 and the minimum cell voltage (cell deviation, DV) is less than the preset second reference difference value or whether the fuel cell stack 100 is in the idle state, but also whether the first oxygen concentration detected before the variation control of the hydrogen supply pressure is less than or equal to the second oxygen concentration detected after the variation control of the hydrogen supply pressure, and judges whether the release condition of the control mode is satisfied based on these judgment results (S960).

[0108] For example, the controller 300 determines that the state of the fuel cell stack 100 satisfies the release condition of the control mode when it is determined that the difference between the average cell voltage of the fuel cell stack 100 and the minimum cell voltage (cell deviation, DV) is smaller than the preset second reference difference value or the fuel cell stack 100 is not in the idle state, and determines that the oxygen concentration inside the anode of the fuel cell stack 100 satisfies the release condition of the control mode in the case where the first oxygen concentration is equal to or lower than the second oxygen concentration (YES in S960). As another example, if the variation control of the hydrogen supply pressure according to the embodiment of the present application is performed when the fuel cell stack 100 is in the idle state, the controller 300 can preferentially determine whether the fuel cell stack 100 is in the idle state, and when the fuel cell stack 100 is not in the idle state, it can directly determine that the release condition of the control mode is satisfied without considering the cell deviation and the change in the oxygen concentration. If the fuel cell stack 100 is still in the idle state, the controller 300 can determine that the release condition of the control mode is satisfied when the cell deviation is smaller than the preset second reference difference value and the first oxygen concentration is equal to or lower than the second oxygen concentration. However, this is merely an exemplary description and is not limited to the above-described embodiment.

[0109] In other words, the controller 300 can determine that the release condition of the control mode is satisfied only when both the state of the fuel cell stack 100 and the change in the oxygen concentration inside the anode of the fuel cell stack 100 satisfy the conditions.

[0110] In the case where the state of the fuel cell stack 100 and the change in the oxygen concentration inside the anode of the fuel cell stack 100 do not satisfy the release condition of the control mode (NO in S960), the controller 300 can perform the variation control of the hydrogen supply pressure until the release condition of the control mode is satisfied. At this time, the variation control of the hydrogen supply pressure can be performed in the same manner as the above-described S304 step described with reference to FIG. 3. Figure 3 Unlike the case where the maximum pressure increase rate is adjusted from the first maximum pressure increase rate to the second maximum pressure increase rate as described above, the controller 300 can fix the maximum pressure increase rate to the second maximum pressure increase rate at the initial stage and control the increase of the hydrogen supply pressure by gradually changing the pressure increase rate until the pressure increase rate reaches the above-described second maximum pressure increase rate. The decrease control of the hydrogen supply pressure according to the pressure decrease rate can be performed in the same manner as the above-described S304 step described with reference to FIG. 3. Figure 3

[0111] ​Further, in a case where neither the state of the fuel cell stack 100 nor the oxygen concentration change inside the anode of the fuel cell stack 100 satisfies the release condition of the control mode (NO in S960), the controller 300 can judge the hydrogen concentration inside the anode of the fuel cell stack 100 (S970), and continue to perform the variation control of the hydrogen supply pressure until the hydrogen concentration inside the anode reaches the preset reference concentration or less.

[0112] On the other hand, in a case where neither the state of the fuel cell stack 100 nor the oxygen concentration change inside the anode of the fuel cell stack 100 satisfies the release condition of the control mode (NO in S960) before the pressure increase rate reaches the second maximum pressure increase rate, and the hydrogen concentration inside the anode of the fuel cell stack 100 is the preset reference concentration or less (NO in S970), the controller 300 can perform the hydrogen purge operation (S980). The step of performing the hydrogen purge operation can be performed in the same manner as the S311 step described with reference to FIG. 3. Figure 3

[0113] As described above, the fuel cell system and the control method thereof according to the present application, in a case where the hydrogen concentration deviation is generated inside the anode of the fuel cell stack, controls the hydrogen supply pressure according to the pressure increase rate or the pressure decrease rate, thereby being able to guide the recirculation of hydrogen inside the anode of the fuel cell stack.

[0114] Further, the hydrogen supply pressure is controlled according to the pressure increase rate or the pressure decrease rate, the recirculation of hydrogen is guided, thereby being able to prevent the accumulation or inflow of oxygen to the anode of the fuel cell stack.

[0115] Still further, by preventing the accumulation or inflow of oxygen to the anode of the fuel cell stack, the deterioration of the electrode and the electrolyte membrane of the fuel cell stack is prevented, the durability of the fuel cell stack is improved, thereby being able to improve the expected life.

[0116] While the present application has been illustrated and described with respect to a particular embodiment thereof, it should not be limited thereto but only by the scope of the claims, and various modifications and changes can be made by those skilled in the art without departing from the scope of the present application.

[0117] ​The present application described above can be implemented by recording a program in a computer-readable code form in a storage medium. The computer-readable medium includes all recording devices capable of storing data that can be read by a computer system. Examples of the computer-readable medium include a hard disk drive (HDD), a solid state drive (SSD), a silicon disk drive (SDD), a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device, etc. Accordingly, the above detailed description should not be understood as limiting in any way but rather be taken as exemplary. The scope of the present application should be determined by reasonable interpretation of the appended claims and any change made within the equivalent range of the present application belongs to the scope of the present application.

Claims

1. A fuel cell system, wherein, The fuel cell system includes: a hydrogen pressure regulator for regulating a hydrogen supply pressure of hydrogen supplied to a fuel cell stack; and a controller configured to determine whether a control mode for controlling the hydrogen supply pressure is required in accordance with a state of the fuel cell stack, and control the hydrogen pressure regulator so that the hydrogen supply pressure is varied within a predetermined pressure range in accordance with a predetermined pressure increase rate or pressure decrease rate when the control mode is required.

2. The fuel cell system of claim 1, wherein The controller is configured to: determine that the control mode is required when a difference between an average cell voltage and a minimum cell voltage of the fuel cell stack is greater than a predetermined first reference difference, or when the fuel cell stack is in an idle state.

3. The fuel cell system of claim 1, wherein The controller is configured to: control the hydrogen pressure regulator so that the hydrogen supply pressure is increased to a first pressure that is a maximum value of the pressure range in accordance with the pressure increase rate when the hydrogen supply pressure is less than the first pressure; control the hydrogen pressure regulator so that the hydrogen supply pressure is decreased to a second pressure that is a minimum value of the pressure range in accordance with the pressure decrease rate when the hydrogen supply pressure is greater than or equal to the first pressure.

4. The fuel cell system of claim 3, wherein The controller is configured to: control the hydrogen pressure regulator so that the pressure increase rate is gradually changed until the pressure increase rate reaches a predetermined first maximum pressure increase rate, and the hydrogen supply pressure is increased to the first pressure.

5. The fuel cell system of claim 4, wherein The controller is configured to: gradually change the pressure increase rate in accordance with a predetermined first pressure increase acceleration.

6. The fuel cell system of claim 4, wherein The controller is configured to: gradually change the pressure increase rate in accordance with the first pressure increase acceleration when a difference between the first maximum pressure increase rate and a current pressure increase rate is greater than a reference value; gradually change the pressure increase rate in accordance with a second pressure increase acceleration that is lower than the first pressure increase acceleration when the difference between the first maximum pressure increase rate and the current pressure increase rate is less than or equal to the reference value.

7. The fuel cell system of claim 4, wherein The controller is configured to: apply a pressure increase acceleration that is set to gradually decrease as the pressure increase rate approaches the first maximum pressure increase rate, and gradually change the pressure increase rate until the pressure increase rate reaches the first maximum pressure increase rate.

8. The fuel cell system of claim 1, wherein The controller is configured to: determine whether a state of the fuel cell stack satisfies a predetermined release condition of the control mode, and control the hydrogen pressure regulator so that the hydrogen supply pressure is varied until the state of the fuel cell stack satisfies the release condition.

9. The fuel cell system of claim 8, wherein The controller is configured to: determine that the release condition of the control mode is satisfied when a difference between the average cell voltage and the minimum cell voltage of the fuel cell stack is less than a predetermined second reference difference, or when the fuel cell stack is not in the idle state.

10. The fuel cell system of claim 8, wherein The controller is configured to: change the first maximum pressure increase rate to a second maximum pressure increase rate that has a value higher than the first maximum pressure increase rate, and re-control the hydrogen pressure regulator so that the hydrogen supply pressure is varied in reflection of the changed second maximum pressure increase rate when the pressure increase rate reaches the predetermined first maximum pressure increase rate without the state of the fuel cell stack satisfying the release condition of the control mode.

11. The fuel cell system of claim 10, wherein, The controller is configured to: gradually change the pressure increase rate until the pressure increase rate reaches the changed second maximum pressure increase rate, and gradually changes the pressure increase rate according to a third pressure increase acceleration which is smaller than the pressure increase acceleration applied before the pressure increase rate reaches the first maximum pressure increase rate.

12. The fuel cell system of claim 10, wherein, The controller is configured to: during the period in which the hydrogen pressure regulator is controlled to reflect the changed second maximum pressure increase rate to cause the hydrogen supply pressure to fluctuate, judge whether the state of the fuel cell stack satisfies the release condition of the control mode or judge the hydrogen concentration inside the anode of the fuel cell stack, and in the case where the state of the fuel cell stack does not satisfy the release condition, when the pressure increase rate reaches the changed second maximum pressure increase rate, or in the case where the hydrogen concentration inside the anode of the fuel cell stack falls below a preset reference concentration, perform a hydrogen purge operation in the fuel cell stack.

13. The fuel cell system of claim 8, wherein, The controller is configured to: further judge the oxygen concentration inside the anode of the fuel cell stack, and judge whether at least one of the state of the fuel cell stack and the change in the oxygen concentration caused by the fluctuation of the hydrogen supply pressure satisfies the release condition of the control mode, and control the hydrogen pressure regulator to cause the hydrogen supply pressure to fluctuate until both the state of the fuel cell stack and the change in the oxygen concentration satisfy the release condition.

14. The fuel cell system of claim 13, wherein, The controller is configured to judge that the state of the fuel cell stack satisfies the release condition when the difference between the average cell voltage of the fuel cell stack and the minimum cell voltage is smaller than a preset second reference difference value, or in the case where the fuel cell stack is not in the no-load state; judge the first oxygen concentration detected before the hydrogen supply pressure changes and the second oxygen concentration detected after the hydrogen supply pressure changes, and in the case where the first oxygen concentration is below the second oxygen concentration, judge that the change in the oxygen concentration satisfies the release condition.

15. The fuel cell system of claim 13, wherein, The controller is configured to control the hydrogen pressure regulator to gradually change the pressure increase rate until the pressure increase rate reaches a preset second maximum pressure increase rate, thereby causing the hydrogen supply pressure to fluctuate.

16. A control method of a fuel cell system, wherein comprises: a judging step of judging whether a control mode of controlling the hydrogen supply pressure supplied to the fuel cell stack is needed, based on the state of the fuel cell stack; and a fluctuation control step of performing control to cause the hydrogen supply pressure to fluctuate within a preset pressure range according to a preset pressure increase rate or pressure decrease rate, in the case where it is judged that the control mode is needed.

17. The control method of the fuel cell system according to claim 16, wherein in the judging step, it is judged that the control mode is needed in the case where the difference between the average cell voltage of the fuel cell stack and the minimum cell voltage is larger than a preset first reference difference value, or in the case where the fuel cell stack is in the no-load state.

18. The control method of the fuel cell system according to claim 16, wherein in the fluctuation control step, when the hydrogen supply pressure is smaller than the maximum value of the pressure range, i.e., the first pressure, the hydrogen supply pressure is controlled according to the pressure increase rate so that the hydrogen supply pressure rises to the first pressure.

19. The control method of the fuel cell system according to claim 16, wherein in the fluctuation control step, when the hydrogen supply pressure is larger than the minimum value of the pressure range, i.e., the second pressure, the hydrogen supply pressure is controlled according to the pressure decrease rate so that the hydrogen supply pressure falls to the second pressure. When the hydrogen supply pressure is equal to or higher than the first pressure, the hydrogen supply pressure is controlled according to a pressure decrease rate so that the hydrogen supply pressure decreases to a minimum value of the pressure range, i.e., a second pressure.

19. The control method of a fuel cell system according to claim 16, characterized in that, the variation control step includes the steps of: determining whether a state of the fuel cell stack satisfies a predetermined release condition of the control mode; variation-controlling the hydrogen supply pressure until the state of the fuel cell stack satisfies the release condition.

20. The control method of a fuel cell system according to claim 19, characterized in that, after the variation control step, the method further includes the steps of: in a case where the state of the fuel cell stack does not satisfy the release condition of the control mode, when the pressure increase rate reaches a predetermined first maximum pressure increase rate, changing the first maximum pressure increase rate to a second maximum pressure increase rate having a value higher than the first maximum pressure increase rate; and variation-controlling the hydrogen supply pressure again while reflecting the changed second maximum pressure increase rate. ​

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