Patent Application
20180226667
The invention relates to a method for operating a fuel cell system during a heating phase or another transient operating phase of a fuel cell system. The invention further relates to a method for adjusting a relative humidity of a cathode operating gas during a heating phase or another transient operating phase. The invention furthermore relates to a fuel cell system configured for carrying out the method and to a corresponding vehicle.
Fuel cells use the chemical conversion of a fuel with oxygen into water in order to generate electrical energy. For this purpose, fuel cells contain the so-called membrane electrode assembly (MEA) as a core component, which is an arrangement of an ion-conducting (usually proton-conducting) membrane and of a catalytic electrode (anode and cathode), respectively arranged on both sides of the membrane. The electrodes generally comprise supported precious metals, in particular platinum. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane electrode assembly, on the sides of the electrodes facing away from the membrane. Generally, the fuel cell is formed by a plurality of MEAs arranged in the stack, the electrical power outputs of which MEAs add up. Bipolar plates (also called flow field plates or separator plates), which ensure a supply of the individual cells with the operating media, i.e., the reactants, and which are usually also used for cooling, are generally arranged between the individual membrane electrode assemblies. In addition, the bipolar plates also ensure an electrically conductive contact to the membrane electrode assemblies.
During operation of the fuel cell, the fuel (anode operating medium), particularly hydrogen H2 or a gas mixture containing hydrogen, is supplied to the anode via an open flow field of the bipolar plate on the anode side, where electrochemical oxidation of H2 to protons H+ with loss of electrons takes place (H2→2 H++2 e−). A (water-bound or water-free) transport of the protons from the anode chamber into the cathode chamber is effected via the electrolyte or the membrane, which separates the reaction spaces from each other in a gas-tight manner and electrically insulates them. The electrons provided at the anode are guided to the cathode via an electrical line. The cathode receives, as cathode operating medium, oxygen or a gas mixture containing oxygen (such as air) via an open flow field of the bipolar plate on the cathode side so that a reduction of O2 to O2− with gain of electrons takes place (½ O2+2 e−→O2−). At the same time, the oxygen anions react in the cathode chamber with the protons transported across the membrane to form water (O2−+2 H+→H2O).
Polymer electrolyte membranes of fuel cells require some moisture to provide good ionic conductivity and hence high power density to the fuel cell. There is also the risk of damage to the membrane if it dries out too much. In order to keep the membrane moist, the cathode operating gas, mostly air, is actively humidified. Widely used for this purpose is the use of humidifiers, in particular membrane humidifiers, which work with steam-permeable flat or hollow-fiber membranes. In doing so, the cathode operating gas to be humidified is guided on one side of the membrane and a relatively moist gas is guided on the other side of the membrane so that water vapor passes from the moister gas to the cathode operating gas. As the moist gas, the cathode exhaust gas is mostly used, which is loaded with the product water formed as a result of the reactions taking place in the fuel cell.
DE 10 2007 026 331 A1 discloses a control system for a fuel cell stack in which the cathode exhaust gas is passed through a humidifier to humidify the cathode inlet air. For example, to keep the relative humidity of the cathode inlet air above a predetermined setpoint, the stack cooling fluid temperature is reduced.
DE 10 2006 022 863 A1 discloses an operating strategy for controlling a degree of hydration of membranes in fuel cells. For this purpose, a relative inlet and outlet target humidity for the cathode gas supplied to and removed from the fuel cell stack is first of all selected such that a desired hydration state is ensured for the membrane. Furthermore, a water mass balance is carried out for the cathode flow path. Subsequently, the inlet and outlet setpoint temperatures for the cathode gas are determined in order to achieve the relative inlet and outlet target humidity. In order to adjust the determined inlet and outlet setpoint temperatures for the cathode gas, the inlet and outlet setpoint temperatures for the coolant are set to the appropriate setpoints for the cathode gas and these coolant setpoint temperatures are adjusted by appropriate control of the coolant system.
A difficulty in adjusting a desired relative humidity of the cathode operating gas is in the warm-up phases of the fuel cell stack when the ambient air drawn in as the cathode operating gas is cold and the cold line system, due to its thermal inertia, also does not permit rapid heating of the cathode operating gas. The present inventor has found that in such situations, the adjusting of a desired humidity of the cathode operating gas is only possible very imprecisely and the target humidity in the stack is often not reached.
The invention is based on the object of providing a method for operating a fuel cell system and a corresponding fuel cell system, which permits improved accuracy of adjusting a desired relative humidity of the cathode operating gas in warm-up phases or other transition phases.
This object is achieved by a method for operating a fuel cell system during a heating phase or during another transient operating phase, by a method for adjusting a relative humidity of a cathode operating gas during a heating phase or another transient operating phase and by a corresponding fuel cell system.
The term “transient operating phase” is understood to mean any operating phase of the fuel cell system in which the fuel cell stack is outside its setpoint temperature, i.e., the cooling system is required to heat the stack from a currently prevailing stack temperature to a higher temperature or to cool it to a lower temperature.
The method according to the invention for operating a fuel cell system during a heating phase or another transient operating phase relates to a fuel cell system comprising a fuel cell stack with anode and cathode chambers separated by polymer electrolyte membranes, and a cathode supply for supplying the cathode operating gas into the cathode chambers and removing a cathode exhaust gas from the cathode chambers, and a cooling system for controlling the temperature of the fuel cell stack. The method has the following steps:
determining an inlet temperature (TG,actual) of the cathode operating gas at the inlet of the fuel cell stack,
setting a coolant setpoint temperature (TCOOL,setpoint) at the inlet of the fuel cell stack to a value which is equal to or less by a predetermined amount than the inlet temperature (TG,actual) of the cathode operating gas, and
controlling the cooling system so that a coolant temperature (TCOOL,actual) prevailing at the inlet of the fuel cell stack at least approximates the coolant setpoint temperature (TCOOL,setpoint).
According to the present invention, the coolant temperature prevailing at the inlet of the fuel cell stack (hereinafter also referred to as coolant inlet temperature or actual coolant temperature) is actively controlled during the heating phase or transient operating phase of the fuel cell stack on the basis of the inlet temperature of the cathode operating gas currently prevailing at the inlet of the fuel cell stack (hereinafter also actual cathode gas temperature). The coolant inlet temperature is thus adapted to the actual cathode gas temperature. This has the consequence that the temperature of the cathode operating gas does not change substantially over the flow fields of the cathode chambers of the fuel cell stack, which is temperature-controlled to the coolant setpoint temperature. This has the effect that the relative humidity of the cathode operating gas also does not change as a result of a temperature change, in particular does not decrease as a result of heating. Namely, the present inventor has observed that in conventionally operated fuel cells, the coolant and thus also the fuel cell stack are heated faster than the cathode operating gas during a heating phase. As a result, the temperature of the cathode operating gas rises after entering the stack, reducing the relative humidity within the cathode chambers. Consequently, a sufficient humidity of the membranes of the fuel cell stack cannot be ensured. By the method according to the invention, however, heating of the incoming cathode operating gas and the concomitant decreasing relative humidity are prevented. The method according to the invention thus enables a more reliable humidification of the membranes of the fuel cell stack during heating phases or under transient conditions.
As already mentioned, the coolant setpoint temperature at the stack inlet is set to a value that is equal to or less by predetermined amount than the actual cathode gas temperature. In order to achieve the lowest possible temperature change of the cathode operating gas within the stack, this amount is to be chosen as small as possible. In particular, the amount is at most 10 Kelvin, preferably at most 7 Kelvin and more preferably at most 5 Kelvin.
The coolant temperature (actual coolant temperature) prevailing at the inlet of the fuel cell stack may be controlled by various means in order to approximate it to the coolant setpoint temperature (and thus to the actual cathode gas temperature). In one embodiment of the method, this is done by influencing a cooling capacity of a cooler arranged in the cooling system. Depending on the design of the cooler, this can be done, for example, by influencing a speed of a fan of the cooler. Alternatively or additionally, the coolant temperature is adjusted by influencing a bypass opening of a cooler bypass line bypassing the cooler. In this way, a volume flow of the coolant flowing through the cooler or the bypass line can be regulated. Alternatively or additionally, the coolant temperature is adjusted by influencing a power of a conveyor, for example a coolant pump, of the cooling system. The aforementioned measures allow a precise and rapid adjusting of a desired target temperature of the coolant and can be used individually or in combination with each other.
Another aspect of the invention relates to a method for adjusting a relative humidity of a cathode operating gas of the fuel cell system described above during a heating phase or another transient operating phase. The method has the following steps:
determining an inlet temperature (TG,actual) of the cathode operating gas at the inlet of the fuel cell stack,
setting a coolant setpoint temperature (TCOOL,setpoint) at the inlet of the fuel cell stack to a value which is equal to or less by a predetermined amount than the inlet temperature (TG,actual) of the cathode operating gas,
controlling the cooling system so that a coolant temperature (TCOOL,actual) prevailing at the inlet of the fuel cell stack at least approximates the coolant setpoint temperature (TCOOL,setpoint),
setting a setpoint value for the relative humidity (RHsetpoint) of the cathode operating gas at the inlet of the fuel cell stack as a function of the cathode inlet temperature (TG,actual) of the cathode operating gas at the inlet of the fuel cell stack,
controlling the cathode supply so that a relative humidity (RHactual) of the cathode operating gas prevailing at the inlet of the fuel cell stack at least approximates the setpoint value for the relative humidity (RHsetpoint).
The first three steps correspond to the above-explained method for operating the fuel cell system; the explanations in this respect apply accordingly.
The method according to the invention allows a particularly precise and reliable adjustment of the relative humidity of the cathode operating gas during the heating phase or another transient operating phase of the system. The adaptation according to the invention of the coolant inlet temperature in the stack to the currently prevailing inlet temperature of the cathode operating gas prevents a temperature change of the cathode operating gas, in particular a heating. As a result, the relative humidity of the cathode operating gas adjusted at the stack inlet can also be maintained within the cathode chambers. A decrease in relative humidity within the stack due to a temperature increase of the cathode operating gas is avoided and the polymer electrolyte membrane of the fuel cell stack can be reliably humidified.
The setting of the setpoint value for the relative humidity of the cathode operating gas as a function of the cathode inlet temperature can be effected, in particular, by using characteristic diagrams which map the relative humidity as a function of the temperature. In addition, the setpoint value can be determined as a function of further parameters, in particular the pressure of the cathode operating gas at the stack inlet.
The relative humidity of the cathode operating gas depends on its pressure, its temperature, the humidity originally present in the cathode operating gas, in particular in the ambient air, and a humidity actively supplied in a humidifier. With the exception of the original humidity content, all other parameters can be influenced in order to affect the relative humidity of the cathode operating gas at the stack inlet. According to one embodiment, adjusting the relative humidity of the cathode operating gas at the inlet of the fuel cell stack is effected by influencing the cathode pressure of the cathode operating gas. The cathode pressure may be accomplished, for example, by varying a compressor power of the cathode supply, by controlling an exhaust flap in a cathode exhaust path, or by suitable control of other throttles or valves of the cathode supply.
According to further embodiments of the invention, the adjustment of the relative humidity of the cathode operating gas at the stack inlet occurs by influencing an opening of a humidifier bypass line. The proportion of the cathode operating gas or the cathode exhaust gas which bypasses a humidifier arranged in the cathode supply or flows through it can be regulated in this way. By this measure, the additional amount of water vapor introduced into the cathode operating gas is regulated.
In other embodiments of the invention, adjusting the relative humidity of the cathode operating gas at the stack inlet occurs by affecting the cathode inlet temperature of the cathode operating gas. For example, the temperature can be controlled by appropriately arranged heat exchangers or heating elements. Likewise, a heat exchange, in particular a preheating of the cathode operating gas, by the warmer cathode exhaust gas takes place in the humidifier. In this respect, by influencing the opening of the humidifier bypass line not only the supply of water vapor but also the temperature can be influenced.
All of the aforementioned measures for adjusting the relative humidity of the cathode operating gas can advantageously also be used in combination.
Another aspect of the invention relates to a fuel cell system comprising a fuel cell stack having anode and cathode chambers separated by polymer electrolyte membranes; a cathode supply for supplying the cathode operating gas into the cathode chambers and discharging a cathode exhaust gas from the cathode chambers; a cooling system for controlling the temperature of the fuel cell stack to a setpoint temperature; and a control device configured to carry out the method according to the invention for operating the fuel cell system and/or the method according to the invention for adjusting a relative humidity of the cathode operating gas.
Preferably, the cathode supply furthermore comprises a humidifier configured to be flowed through by the cathode operating gas and the cathode exhaust gas such that a water vapor transfer occurs from the cathode exhaust gas to the cathode operating gas. As a result, an active supply of water to the cathode operating gas supplied to the fuel cell stack is made possible so that high relative humidities can be adjusted.
Another aspect of the invention relates to a vehicle having a fuel cell system according to the invention. The vehicle is preferably an electric vehicle in which an electrical energy generated by the fuel cell system serves to supply an electric traction motor and/or a traction battery.
The various embodiments of the invention mentioned in this application may be combined advantageously with one another unless stated otherwise in individual cases.
The invention is explained below in exemplary embodiments in reference to the respective drawings. The following is shown:
The fuel cell system 100 comprises as core component a fuel cell stack 10, which comprises a plurality of individual cells 11, which are arranged in the form of a stack and which are formed by alternately stacked membrane electrode assemblies (MEAs) 14 and bipolar plates 15 (see detailed view). Each individual cell 11 thus comprises, in each case, an MEA 14 which has an ionically conductive polymer electrolyte membrane (not shown in detail) as well as catalytic electrodes arranged on both sides thereof, namely an anode and a cathode which catalyze the respective partial reaction of the fuel cell conversion. The anode and cathode electrodes comprise a catalytic material, for example platinum, which is supported on an electrically conductive carrier material with a large specific surface, for example a carbon-based material. An anode chamber 12 is thus formed between a bipolar plate 15 and the anode, and the cathode chamber 13 between the cathode and the next bipolar plate 15. The bipolar plates 15 serve to supply the operating media into the anode and cathode chambers 12, 13 and also establish the electrical connection between the individual fuel cells 11. Furthermore, the bipolar plates 15 serve the passage of a coolant for the fuel cell stack 10.
In order to supply the fuel cell stack 10 with the operating media, the fuel cell systems 100 comprise an anode supply 20 on the one hand and a cathode supply 30 as well as a cooling system 40 on the other hand.
The anode supply 20 of the fuel cell system 100 shown in
In the anode exhaust gas line 22 of the fuel cell system 100 shown in
The cathode supply 30 of the fuel cell system 100 shown in
The fuel cell system 100 shown in
The cathode supply 30 furthermore comprises a humidifier bypass line 38 which connects the cathode supply line 31 to the cathode supply line 31 so that the humidifier 37 upstream of the fuel cell stack 10 is not flowed through by the cathode operating gas. An adjusting means (humidifier bypass valve) 39 arranged in the humidifier bypass line 38 serves to control the amount of the cathode operating gas bypassing the humidifier 37. Alternatively or additionally, the cathode supply 30 may comprise another humidifier bypass line connecting the cathode exhaust gas line 32 to the cathode exhaust gas line 32 so that the humidifier 37 downstream of the fuel cell stack 10 is not flowed through by the cathode exhaust gas (not shown).
For cooling the fuel cell stack 10, the fuel cell system 100 shown in
All adjusting means 24, 26, 39 of the fuel cell system 100 can be designed as controllable or non-controllable valves or throttles. Additional adjusting means may be arranged in the lines 21, 22, 31 and 32 in order to be able to isolate the fuel cell stack 10 from the environment after turning off the system.
The fuel cell system 100 of
If a conventional fuel cell system is operated in a conventional manner during a heating phase, the polymer electrolyte membranes of the membrane electrode assemblies 14 of the fuel cell stack 10 may be undersupplied with humidity. This will be explained with reference to the curves of various operating parameters shown in
In order to achieve the setpoint value of the relative humidity RHsetpoint, the humidifier bypass valve 39 is first completely closed in accordance with the conventional procedure according to
A corresponding control module 60 of the control unit 50 for controlling the coolant temperature of the cooling circuit 40 is shown in
In block 71, the control unit 50 reads in various measurands provided by the various sensors. Specifically, the inlet temperature of the cathode operating gas TG,actual, the inlet temperature of the coolant TCOOL,actual and the relative humidity RHactual of the cathode operating gas at the stack inlet are detected. In block 72, the coolant setpoint temperature at the stack inlet is determined. In this case, the coolant setpoint temperature TCOOL,setpoint is set to a value which is equal to or less by a predetermined amount, for example by not more than 5 Kelvin, than the inlet temperature TG,actual of the cathode operating gas at the stack inlet. In block 73, the cooling system 40 is controlled such that the coolant temperature TCOOL,actual prevailing at the inlet of the fuel cell stack 10 is approximating the coolant setpoint temperature TCOOL,setpoint. For this purpose, the control module 60, shown in
In block 74, the setpoint value for the relative humidity RHsetpoint of the cathode operating gas at the inlet of the fuel cell stack 10 is determined. This is done as a function of the cathode inlet temperature TG,actual and possible further parameters, such as the pressure pG,actual. In block 75, the cathode supply 30 of the fuel cell system 100 is controlled such that a relative humidity RHactual of the cathode operating gas present at the inlet of the fuel cell stack 10 approximates the setpoint value RHsetpoint. For this purpose a control module for controlling the humidifier bypass valve 39 can, for example, be used.
By means of the method 70 according to the invention as illustrated in
German patent application no. 10 2017 102354.2, filed Feb. 7, 2017, to which this application claims priority, is hereby incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 102 354.2 | Feb 2017 | DE | national |
