Method for operating a fuel cell system

Abstract

Method for operating a fuel cell system in a reduced power output mode, wherein the fuel cell system comprises a fuel cell stack (BS) with at least one fuel cell (BZ) with an anode (A), a cathode (K) and a proton exchange membrane, anode and cathode inlets and anode and cathode outlets and a hydrogen and air supply, wherein the air supply to the cathode (K) is interrupted when switching to the reduced power output mode, characterized in that When switching to the reduced power output mode, an electrical voltage (U) of the fuel cell stack (BS) is additionally reduced by means of a current pulse.

Description

[0001] The invention relates to a method for operating a fuel cell system in a reduced power output mode according to the preamble of claim 1.

[0002] Fuel cell systems are used as an energy source in many applications, for example in vehicles for propulsion or other components. The most common are proton exchange membrane (PEM) fuel cells, in which the anode of the fuel cell is supplied with hydrogen as fuel and the cathode with oxygen or air as the oxidant. The anode and cathode are separated by a proton-permeable, electrically non-conductive membrane. The electrochemical reaction of hydrogen and oxygen to form water generates electrical energy, which is tapped by electrodes at the anode and cathode. This reaction can only be maintained if the resulting current is discharged from the fuel cell. Several individual fuel cells connected electrically in series are combined to form a fuel cell stack.

[0003] Typically, especially in mobile applications, air is supplied to the cathode of the fuel cell as an oxygen-rich gas, rather than pure oxygen. Therefore, references to air in the following also refer to pure oxygen or a gas with a different oxygen content.

[0004] DE 103 32 129 A1 discloses an operating system for a vehicle with a fuel cell, in which the fuel cell is operated in three different energy generation modes: a normal energy generation mode, an idle stop mode, and an idle charge mode. In normal energy generation mode, the operating system adjusts the current generated by the fuel cell depending on the respective power requirements for the drive motor and for additional devices. In idle stop mode, the operating system stops the energy generation of the fuel cell, i.e. the operating system sets the current to be generated by the fuel cell to zero. In idle charge mode, the operating system adjusts the current generated by the fuel cell according to the optimal efficiency of the fuel cell in order to charge an energy storage device.The control of the power generation of the fuel cell is achieved by controlling the respective amount of reaction gases containing oxygen and hydrogen.

[0005] Furthermore, US 2002 / 0 182 456 A1 discloses a method for shutting down a PEM fuel cell system, wherein an external load is disconnected from the grid and the supply of cathode air is stopped. The hydrogen still circulating in the anode circuit and the residual oxygen content in the cathode circuit continue to react electrochemically, with the resulting current being diverted via an additional load resistor. The oxygen concentration at the cathode continuously decreases until a lower voltage value is finally reached. The cathode circuit has a recirculation line with a circulation pump that operates until the lower voltage limit is reached.

[0006] In addition, JP H11-26 003 A describes a fuel cell with a proton-conducting electrolyte in which, when changing from normal to idle operation, the air supply is first interrupted by closing a valve and the residual amount of adsorbed oxygen remaining in the cathode, after purging with an inert gas, reacts electrochemically by discharging via a resistor R.

[0007] DE 100 59 393 A1 also describes a direct current supply device and a method for shutting down a fuel cell block, in which, in a standby state of the fuel cell block, a current resulting from a residual gas reaction can be dissipated by connecting a discharge resistor. The resistance value of the discharge resistor can be controlled by a control device.

[0008] The object of the invention is to propose a method for operating a fuel cell system in a reduced power output mode which does not have a negative effect on the service life of the fuel cell system.

[0009] The object is achieved by a method having the features of claim 1. Accordingly, when switching to the reduced power output mode, the air supply to the cathode is interrupted and an electrical voltage of the fuel cell stack is reduced by means of a current pulse.

[0010] This process has the advantage that the current pulse initiates increased consumption of the oxygen in the cathode, thus significantly reducing the electrical voltage of the fuel cell stack in a short period of time. This prevents high voltage potentials in the fuel cell from occurring during continued operation at reduced power output, which in extreme cases can even mean zero watts, which would in turn reduce the service life of the fuel cell.

[0011] The method according to the invention also leads to a reduction in consumption and noise in a reduced power output mode.

[0012] The electrical voltage of the fuel cell stack is advantageously controlled by a DC-DC converter or an adjustable electrical load. Especially when controlled by a DC-DC converter, the electrical voltage can be brought below an upper voltage limit of a working range within the shortest possible time.

[0013] Advantageously, the electrical voltage of the fuel cell stack is regulated by means of an adjustable electrical load, for example a coolant pump, such that it remains below an upper voltage limit and above a lower voltage limit of an operating range. This can prevent degradation of the fuel cell. Operation of the coolant pump advantageously leads to a uniform temperature distribution in the fuel cell. To keep the electrical voltage of the fuel cell stack above the lower voltage limit of the operating range, oxygen is added again before the lower voltage limit is exceeded. Alternatively, an energy storage device can be used as an adjustable electrical load, which optimizes operation from an energetic perspective.

[0014] If the hydrogen supply to the anode is not interrupted when switching to reduced power output mode, this is beneficial for a return to normal power output mode. It also prevents potential shifts within the fuel cell, which would negatively impact the service life of the electrodes.

[0015] Recirculating the depleted cathode air into the cathode via a cathode recirculation circuit advantageously prevents local oxygen deficiencies in the cathode. Alternatively, omitting the cathode air recirculation simplifies the fuel cell system and increases energy efficiency.

[0016] Further advantages of the invention will become apparent from the description and the drawings. Specific embodiments of the invention are illustrated in simplified form in the drawings and explained in more detail in the following description. Fig. 1 a schematic representation of a fuel cell system, Fig. 2 shows a time profile of an electric current or an electric voltage of a fuel cell stack, an anode pressure and a differential pressure between anode and cathode during a change according to the invention to a mode of reduced power output, and Fig. 3 shows a time profile of an electric current or an electric voltage of a fuel cell stack and a differential pressure between anode and cathode in an inventive control of the electric voltage of the fuel cell stack within a working range.

[0017] Fig. 1 shows a fuel cell system with a fuel cell stack BS, wherein for reasons of clarity only one fuel cell BZ is shown. The fuel cell BZ comprises an anode A and a cathode K. Hydrogen W or hydrogen-rich gas is supplied to the anode A from a hydrogen tank (not shown), preferably a pressure tank, or a reformer system. When using a hydrogen tank, the fuel cell system has an anode recirculation circuit AR through which hydrogen W can be conducted from an anode outlet to an anode inlet. The anode recirculation circuit AR has means for controlling the recirculation quantity. Furthermore, the recirculation quantity or the quantity of hydrogen W released by the fuel cell system can be controlled by a controllable valve V1. When using a reformer system, the anode-side recirculation can be dispensed with.

[0018] The amount of oxygen supplied to the cathode K, preferably as air L, can be controlled by an adjustable three-way valve V2 or by a compressor KO.

[0019] Advantageously, the amount of hydrogen W supplied to the anode A is controlled depending on the delivery rate of the compressor KO.

[0020] The depleted cathode air KA can be fed back to the cathode K via a cathode recirculation circuit KR. This is controlled by both the adjustable three-way valve V2 and an adjustable valve V3.

[0021] The cooling of the fuel cell BZ or a uniform temperature distribution in the fuel cell BZ is achieved by a coolant circuit KK, which includes a coolant pump KP.

[0022] The electrical energy generated by the electrochemical reaction of hydrogen W and oxygen to form water is tapped by an anode electrode EA and a cathode electrode EK. The fuel cells BZ of the fuel cell stack BS are electrically connected in series and electrically connected to a DC-DC converter (not shown), which regulates the electrical voltage and current of the fuel cell stack BS.

[0023] The DC-DC converter is preferably electrically connected to the coolant pump KP, the compressor KO, other electrical consumers and an electrical energy storage device (not shown).

[0024] In Fig. 2 shows a time profile of an electrical current I or an electrical voltage U of a fuel cell stack BS, hereinafter referred to as stack current I or stack voltage U, an anode pressure pA and a differential pressure dp between anode A and cathode K of a fuel cell BZ during a change according to the invention to a mode of reduced power output.

[0025] Reduced power output mode is understood to mean operation of the fuel cell system in which the fuel cell stack BS only has to generate electrical current I below a normal power requirement, since, for example, a vehicle is in idle mode, for example when the vehicle is stationary, or in overrun mode, i.e. the vehicle does not have to be driven by the drive motor.

[0026] At a time t1 according to Fig. 2, the fuel cell system switches to a reduced power output mode. First, the compressor KO is switched off, and the air supply to the cathode K is interrupted. A control of the DC-DC converter then generates a current pulse I, which leads to increased consumption of the oxygen in the cathode K and a very short drop in the stack voltage U. A current pulse is understood here as a short-term increase in the stack current I. The stack current I is preferably fed to an electrical consumer, particularly preferably the coolant pump KP, or to an electrical energy storage device.

[0027] As the reduced power output mode continues, the stack current I remains almost constant at a low level. The stack voltage U decreases continuously.

[0028] Due to an interruption of the hydrogen supply and the consumption of the hydrogen W present in the anode A, the anode pressure pA also continuously decreases. The hydrogen pressure can be maintained above the air pressure level L at the cathode K by means of a control device.

[0029] It is also possible not to interrupt the hydrogen supply to anode A when switching to reduced power output mode.

[0030] To prevent current from flowing into the fuel cell stack BS during operation in reduced power output mode, which could lead to damage to the fuel cell stack BS, a current flow from the fuel cell stack BS is ensured. In this case, the stack current I is also preferentially fed to the coolant pump KP, an electrical consumer, or an electrical energy storage device. Likewise, air flowing into the cathode K, for example, due to leaks, can be consumed.

[0031] At a time t2, the operation in the reduced power output mode is terminated and the stack current I and the stack voltage U rise again to a normal level as before the operation in the reduced power output mode.

[0032] Fig. 3 shows a time course of a stack current I or stack voltage U and a differential pressure between anode A and cathode K in an inventive control of the stack voltage U within an operating range limited by an upper and a lower voltage limit value.

[0033] At a time t3 according to Fig. 3 the fuel cell system changes according to the procedure in Fig. 2 into a reduced power output mode. Here, the compressor KO is also initially switched off and the air supply to the cathode K is interrupted. A current pulse I is then generated by regulating the DC-DC converter, which leads to increased consumption of the oxygen in the cathode K and a drop in the stack voltage U.

[0034] At time t4, the control of the DC-DC converter is switched in such a way that the stack voltage U drops as slowly as possible, but at the same time a backflow of the current I into the fuel cell stack BS is prevented.

[0035] As the reduced power output mode continues, the stack current I remains almost constant at a low level. This leads to a continuous reduction in the stack voltage U.

[0036] Before the stack voltage U falls below the lower voltage limit of the working range at a time t5, the compressor KO is reactivated and air L is supplied to the cathode K.

[0037] Thus, the stack voltage U rises again up to the upper voltage limit of the operating range until a time t6, where the process is repeated according to the time t3 until the operation in the reduced power output mode is terminated.

Claims

[1] Method for operating a fuel cell system in a reduced power output mode, wherein the fuel cell system comprises a fuel cell stack (BS) with at least one fuel cell (BZ) with an anode (A), a cathode (K) and a proton exchange membrane, anode and cathode inlets and anode and cathode outlets and a hydrogen and air supply, wherein the air supply to the cathode (K) is interrupted when switching to the reduced power output mode, characterized by , that When switching to the reduced power output mode, an electrical voltage (U) of the fuel cell stack (BS) is additionally reduced by means of a current pulse. [2] Method according to claim 1, characterized bythat the electrical voltage (U) of the fuel cell stack (BS) is regulated by means of an adjustable electrical load (KP) in such a way that it is within an operating range limited by an upper and a lower voltage limit value. [3] Method according to claim 1 or 2, characterized by that the electrical voltage (U) of the fuel cell stack (BS) is regulated by a DC-DC converter. [4] Method according to one of the preceding claims, characterized by that the electrical voltage (U) of the fuel cell stack (BS) is regulated by means of an adjustable electrical load (KP). [5] Method according to one of the preceding claims, characterized by that when switching to the reduced power output mode, the hydrogen supply to the anode (A) is not interrupted. [6] Method according to one of the preceding claims, characterized bythat the depleted cathode air (KA) is fed back to the cathode (K) via a cathode recirculation circuit (KR). [7] Method according to one of the preceding claims, characterized by that oxygen is supplied to the cathode (K) again before the electrical voltage (U) of the fuel cell stack (BS) falls below a lower voltage limit value.

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