EP1547184A2

EP1547184A2 - Fuel cell system provided with a cooling circuit

Info

Publication number: EP1547184A2
Application number: EP03788902A
Authority: EP
Inventors: Uli Paulus; Dirk Schröter
Original assignee: DaimlerChrysler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2002-10-01
Filing date: 2003-09-12
Publication date: 2005-06-29
Also published as: JP2006501623A; US20070026267A1; DE10245794A1; WO2004034484A3; WO2004034484A2

Abstract

The invention relates to a method and device for cooling a system which comprises a fuel cell (1). A cooling device (15) of a cooling circuit containing a liquid cooling agent is arranged at least in the fuel cell. Means for separating gas eventually penetrated in the liquid cooling agent from the fuel cell (1) and for transmitting said gas in an oxygen-containing gas flow in the fuel cell are arranged thereoutside in the cooling system.

Description

Fuel cell system with a cooling circuit

The invention relates to a method and a device for cooling a fuel cell system with a fuel cell which has an anode space to which a hydrogen-containing gas is supplied and a cathode space to which an oxygen-containing gas is supplied via an air intake system, a cooling device being arranged at least in the fuel cell which is part of a cooling circuit in which a liquid coolant is moved. In this context, fuel cell is understood to mean both a single and a stack of several fuel cells, which form a stack, a so-called stack.

A power generation system with a fuel cell system is known which has an electrochemical fuel cell which contains an anode compartment and a cathode compartment which are separated by a proton-conducting membrane. Oxygen-containing gas, in particular air, is fed to the cathode compartment and a hydrogen-containing gas is fed to the anode compartment. The fuel cell is designed as a fuel cell stack and contains a cooling device or a heat exchanger which is part of a closed cooling circuit in which a pump moves the coolant water. The cooling circuit contains a reservoir which is connected with an inlet to an outlet of the cooling device of the fuel cell and with an outlet to the pump which feeds the water into the inlet of the cooling device of the fuel cell (EP 0800 708 B1).

Under certain circumstances, during operation of a fuel cell with a cooling device arranged inside it, hydrogen-containing fuel gas and / or air can penetrate come into the coolant. Leakage or diffusion can be the cause of these gases entering the coolant. This is where the invention comes in, which is based on the problem of specifying a method and a device for cooling a fuel cell system with at least one heat exchanger arranged in a fuel cell and through which a liquid coolant flows in a circuit, in which, despite the ingress of gases from the anode, and / or the cathode compartment of the fuel cell, a hazard from the formation of an ignitable mixture of gases is avoided.

The problem is solved according to the invention in a method of the type described in the introduction in that gaseous constituents contained in the liquid coolant are separated off in the cooling circuit outside the fuel cell and are fed to the air intake system via an outlet channel which does not contain any ignition sources for an ignitable mixture. The invention is based on the principle of supplying the gas possibly contained in the coolant to the air intake system, in which it is mixed with a very large air flow. As a result of this mixing, the content of a gas in the air mass flow that possibly originates from the fuel cell and forms an ignitable mixture with oxygen at a certain ratio is reduced to such an extent that an ignitable mixture can no longer arise. No ignitable mixture can form in the liquid coolant due to the solution of the gases or the formation of gas bubbles of the smallest order of magnitude floating in the liquid coolant.

In an expedient embodiment, the coolant emerging from the outlet of the cooling device of the fuel cell is fed to a sedimentation tank, from which gas is discharged at a previously set overpressure value and is fed via the discharge channel to the oxygen-containing gas mass flow of the air intake system and which at a pressure below the overpressure value to avoid the Escape of gas to the discharge channel is closed. When gases penetrate from the fuel cell into the coolant, the way it dissolves also changes due to its heating, so that dissolved gases are converted into floating bubbles of the smallest order of magnitude. This mixture enters the sedimentation tank, in which the gas bubbles are separated. The gas flows upwards into a gas collection area in the calming tank, whereby a gas pressure builds up. When a predetermined pressure value, which is set in advance, is reached, the discharge channel for the discharge of the gas is released and, after the pressure in the stilling tank has dropped below a predeterminable value, is blocked again, so that no coolant can get into the discharge channel.

In a favorable embodiment, any gas that may be present in the coolant is separated from the liquid coolant in front of the calming container and fed into the calming container. In this way, very good degassing of the coolant is achieved with two methods.

Gases from the discharge duct are preferably supplied to the air mass flow in the region of an air filter of the air intake system. This ensures good, even mixing.

In a further expedient embodiment, the exhaust gases from the fuel cell are monitored for the hydrogen content with a hydrogen sensor, the concentration of hydrogen in the exhaust gas being reduced below the limit value when a preset limit value for the gas content is reached by admixing hydrogen-free gas. With this measure, the hydrogen gas content can be reduced to a very low value.

In another favorable embodiment, the exhaust gases from the fuel cell are passed over a catalytic converter which reduces the concentration of hydrogen in the exhaust gases.

After the fuel cell has been switched off and the coolant circulation has been switched off, a compressor in the air intake system for feeding air into the fuel cell is expediently set to run on for a predefinable period of time. This removes any fuel gas or hydrogen that may still be present by purging the air intake system.

In a device of the type described at the outset, the problem is solved according to the invention in that a calming container for the liquid coolant with a gas collection area is connected downstream of the outlet or exit of the cooling device of the fuel cell, a gas outlet valve being arranged at a predetermined gas volume or at the gas collection area Gas pressure in the still tank can be actuated and is connected on the outlet side to the intake system for the oxygen-containing gas via an outlet channel which has no ignition sources for an ignitable gas mixture. The oxygen-containing gas is generally air. With the cooling device according to the invention is prevented that from the fuel cell z. B. by diffusion or leakage into the coolant penetrating gases at ignition sources, where an explosive combustion could possibly take place at a critical concentration. With the cooling device of the type described above, a safe operation of the fuel cell and the cooling circuit is achieved despite the penetration of gases from the fuel cell into the coolant.

A vent line is preferably arranged between the outlet or outlet of the cooling device of the fuel cell and the gas collection area of the calming container. Vent line is also to be understood as a line with which gases, not just air, are separated from a liquid. With this further embodiment and the A very effective degassing of the coolant is achieved in two different ways.

It is particularly expedient if the discharge channel emanating from the sedimentation tank opens into the area of a gas filter in the gas intake system for the oxygen-containing gas, since this results in a uniform mixing of the gases with the oxygen-containing gas, preferably air. Through the mixing, any gas that may have been introduced from the fuel cell via the cooling circuit is reduced to a very great extent in the concentration in the gas stream supplied to the fuel cell. Any fuel gas present in the oxygen-containing gas stream enters the fuel cell, i.e. into the cathode compartment and leaves it with the resulting exhaust gases via the exhaust outlet.

In another preferred embodiment, a sensor for measuring the fuel gas content in the exhaust gas stream is provided in the exhaust gas line for the reaction products of the fuel cell and is connected to a control unit in which a limit value for the fuel gas content in the exhaust gas stream is set and by which a valve in an access to the exhaust gas line can be controlled, which opens an opening in the exhaust gas line for the admixture of air when the set limit value is reached.

In a favorable embodiment, a catalytic converter for reducing the fuel gas in the exhaust gas flow is present in the course of the exhaust gas line. A redundant reduction in the content of fuel gas, in particular hydrogen, is therefore provided in the exhaust gas stream, as a result of which great security is ensured for the reduction of the fuel gas.

The calming container, the gas outlet valve and the discharge channel expediently consist of antistatic materials, as a result of which static charges and the generation of electrical discharges are avoided. The invention is described below with reference to an embodiment shown in a drawing, from which further details, features and advantages result.

The drawing schematically shows a fuel cell system with a fuel cell that has a cooling device or a heat exchanger inside, the cooling device being arranged in a cooling circuit with a moving liquid coolant.

The fuel cell system for the generation of electrical energy contains a fuel cell 1, which is designed in particular as a so-called fuel cell stack with numerous individual fuel cells. The fuel cell 1 contains an anode compartment 2 and a cathode compartment 3, which are separated by a proton-conducting membrane 4. Through an electrochemical reaction between the anode and the cathode, the fuel cell 1 generates an electrical voltage that can be tapped at the output lines 5, 6. Fuel gas, in particular hydrogen, is oxidized at the anode and oxygen, which is contained in particular in the air, is reduced at the cathode. The anode compartment 2 is from a fuel gas source, in particular a hydrogen source, for. B. from a tank 7, in which the hydrogen is stored under pressure, via a feed line 8 in the train of which a pressure control valve 9 is arranged, hydrogen supplied. The pressure of the hydrogen at the inlet of the anode compartment 2 is measured with a pressure sensor, not specified. An intake system for an oxygen-containing gas, in particular air, has an air filter 12 arranged downstream of an opening 10 of an intake duct 11. In the further course of the intake duct 11 there is a compressor 13, from which compressed air, the pressure of which is measured with a corresponding sensor 14, is fed into the anode compartment. The fuel cell 1 has a cooling device 15 or a heat exchanger, through which a liquid coolant, in particular water, mixed with an antifreeze is moved. The inlet or inlet 16 of the cooling device 15 is connected via a line 17 to a cooler or heat exchanger and a coolant pump 18. The outlet 19 or outlet of the cooling device 15 is connected to a ventilation line 20, which draws in the form of small bubbles of admixed or dissolved gases from the coolant which leaves the fuel cell 1 via the outlet 19 and via line 21 into a gas collection area 22 feeds a calming tank 23. The vent line 20 also runs with respect to the coolant to the settling tank 23 and opens below the liquid level into the tank 23, which has an outlet (not specified in more detail) below the liquid level. This output is connected via a line 24 to the suction inlet of the coolant pump 18.

The gas collecting area 22 is connected at an outlet or outlet to a pressure relief valve 25, from which a discharge duct 26 or a pipe runs to the air intake system. In the course of the discharge channel 26, a moisture separator 27 can be arranged, from which separated coolant is fed back into the calming tank 23 via a line 28. The pipe 26 opens into the housing with the air filter 12. The pressure relief valve 25, the discharge channel 26, the moisture separator 27 and the calming tank 23 are made of antistatic materials. A filler line 29 provided with a valve and an outlet line 30 provided with a valve are provided on the calming tank 23. At the cathode compartment 3, the fuel cell 1 has an outlet or outlet 31 for reaction products or gases which, via a pipeline 32, reach a moisture separator 33 which extracts water from the reaction products, which water can be fed back into the fuel cell system or discharged into the atmosphere , examples for example, part of the water can be supplied to the intake air, which should have a certain moisture content when flowing into the fuel cell 1 in order to keep the membrane 4 moist.

A catalyst 34 is connected downstream of the moisture separator 33 in a housing (not designated in any more detail), through which the hydrogen gas contained in the exhaust gas stream is oxidized.

A sensor 35 at the outlet of the housing of the catalyst 34 detects the hydrogen content in the exhaust gas stream and transmits the measured values to a control unit 36. The outlet of the housing of the catalyst 34 is connected to an outlet line 37 which has an inlet line 38 in which one of the control unit 36 actuable valve 39 or a slide is arranged. At the inlet of the inlet line 38 there is a fan 40 which is switched on and off by the control unit 36. The cooler 41 arranged between the coolant pump 18 and the inlet 16 in the course of the line 17 can be supplied with cooling air by a fan 42.

When the coolant flows through the fuel cell 1, the coolant is heated. Hydrogen and / or air can get into the liquid coolant through diffusion and / or leakage. The penetration of hydrogen is particularly critical since it can form an ignitable mixture with oxygen in certain concentrations. The amount of hydrogen and / or oxygen or air that can be penetrated into the coolant cannot be determined. The invention prevents the cooling circuit from being endangered by the penetration of hydrogen and / or air into the coolant.

The coolant, in which hydrogen and / or air may be contained in bubbles of the smallest order of magnitude, passes from the cooling device 15 into the vent line 20. Line known. The coolant hits a bubble separator in the vent line 20. The gases separated in the vent line 20 flow into the gas collection space 22 of the calming container 23. The coolant flows out of the ventilation line 20 into the calming container 23, in which it again encounters a bubble separator, through which gas bubbles in the liquid are excreted. The excreted gas rises upward into the gas collection space 22. The greater the heating of the coolant, the faster the coolant is freed from the gases. The gas escaping from the coolant generates increasing pressure in the calming tank 23. If a pressure limit value set on the pressure relief valve 25 is reached or exceeded, the pressure relief valve 25 opens, as a result of which gas flows into the discharge duct 26 with the moisture separator 27. The gas can contain hydrogen and / or oxygen in various concentrations, which depend on the diffusion or leak rate in the fuel cell 1. Ignition of this gas mixture is avoided due to the antistatic materials of the components with which the gas mixture comes into contact. In the housing of the air filter 12, the gas mixture is mixed with the air stream, the mass of which far exceeds the mass of the gas mixture supplied. Therefore a very strong dilution of the gas mixture is achieved, ie even with hydrogen gas its concentration in the air stream is reduced to a very low value.

Any hydrogen present in the air stream enters the cathode chamber 3 with the air stream and leaves it with the reaction gases via the outlet 31. After the water has been separated off in the moisture separator 33, the remaining reaction gas with a possibly low hydrogen content flows over the catalyst 34, which further increases the hydrogen content is reduced. The hydrogen content of the gas behind the catalytic converter 34 measured by the sensor 35 is compared in the control unit 36 with a stored, predetermined value. If this value is reached or exceeded, then open the control unit 36 net the valve 39 and turns on the fan 40, whereby the exhaust gas flow is mixed with air. By mixing it with air, the hydrogen content in the exhaust gas stream can be reduced to non-critical values that are harmless to the environment.

Before the cooling device is started up, air is pumped into the settling tank 23 via the inlet line 29, as a result of which the tank and the outlet channel 26 are flushed as far as the air filter 12. After the coolant pump 18 has been switched off, the compressor 13 is kept in operation for a short time, as a result of which any hydrogen present in the air intake system is flushed out.

A fuel cell system of the type described above with the cooling device and the cooling circuit is particularly suitable as an energy source in a mobile device such as an electric vehicle.

Claims

claims

1. A method for cooling a fuel cell system with a fuel cell, which has an anode space, to which a hydrogen-containing gas is supplied, and a cathode space, to which an oxygen-containing gas is supplied via an air intake system, wherein a cooling device, which is part of a Cooling circuit in which a liquid coolant is moved, characterized in that gaseous constituents contained in the liquid coolant are separated in the cooling circuit outside the fuel cell and are supplied to the air intake system via an outlet channel which contains no ignition sources for an ignitable gas mixture.

2. The method according to claim 1, since you rchgek characterized in that the coolant emerging from the outlet of the cooling device of the fuel cell is supplied to a settling tank, from which gas is discharged at a previously set overpressure value and is fed via the discharge channel to the oxygen-containing gas mass flow of the air intake system and which is closed at a pressure below the overpressure value to prevent gas from escaping to the discharge duct.

3. The method according to claim 1 or 2, characterized in that possibly existing from the liquid coolant in front of the still tank with a vent line in the coolant Gas is separated and fed into the still tank.

4. The method according to at least one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that

Gases from the discharge duct in the area of an air filter of the air intake system are supplied to the air mass flow.

5. The method according to at least one of the preceding claims, characterized in that the exhaust gases from the fuel cell are monitored with a hydrogen sensor for the hydrogen content and that when a preset limit value of the gas content is reached by admixing hydrogen-free gas, the concentration of hydrogen in the exhaust gas is lowered below the limit.

6. The method according to at least one of the preceding claims, that the exhaust gases of the fuel cell are passed over a catalyst with which the concentration of hydrogen in the exhaust gases is reduced.

7. The method according to at least one of the preceding claims, so that after the fuel cell has been switched off and the coolant circulation has been switched off, a compressor in the air intake system for supplying air to the fuel cell remains on for a predetermined period of time after the fuel cell has been switched off.

8. The method according to at least one of the preceding claims, characterized in that flushing the still tank with air before starting the coolant circuit.

9. Device for cooling a fuel cell system with a fuel cell, which has an anode space, to which a hydrogen-containing gas is supplied, and a cathode space, to which an oxygen-containing gas is supplied via an air intake system, wherein at least in the fuel cell a cooling device is arranged, which is part of a Cooling circuit in which a liquid coolant is moved, characterized in that a calming tank (23) for the liquid coolant with a gas collecting area (22) is connected downstream of the outlet (19) or outlet of the cooling device (15) of the fuel cell (1) is that a gas outlet valve (25) is arranged on the gas collecting area (22), which can be actuated at a predeterminable gas volume or gas pressure in the settling tank (23) and on the output side via an outlet channel (26), which has no ignition sources for an ignitable gas mixture, with the intake system for the oxygen-containing Gas is connected.

10. The device according to claim 9, so that a vent line (20) is arranged between the outlet or outlet (19) of the cooling device (15) of the fuel cell (1) and the gas collecting area (22) of the calming container (23).

11. The device according to claim 9 or 10, so that the outlet channel (26) emanating from the calming container (23) opens into the area of a gas filter (12) in the gas intake system for the oxygen-containing gas.

12. The device according to at least one of claims 9 to 11, characterized in that A sensor (37) for measuring the fuel gas content in the exhaust gas flow is provided in the exhaust line (37) for the reaction products of the fuel cell (1) and is connected to a control unit (36) in which a limit value for the fuel gas content in the exhaust gas flow is connected is set and through which a valve (38) in an access to the exhaust gas line (37) can be controlled, which opens an opening for admixing air to the exhaust gas stream when the limit value is reached.

13. The device according to at least one of claims 9 to 12, so that a catalyst for reducing the fuel gas in the exhaust gas stream is present in the course of the exhaust line (37).

14. The device according to at least one of claims 9 to 13, so that the calming container (23), the gas outlet valve (25) and the gas line channel (26) consist of antistatic materials.

15. The device according to at least one of claims 9 to 14, so that a moisture separator (27) made of antistatic material is arranged in the course of the discharge channel (26).

16. The device according to at least one of claims 9 to 14, g e k e n n z e i c h n e t by the arrangement in a mobile device.