Abstract:
A fuel cell system including: a fuel cell stack; a cathode gas supply source including a cathode supply path for feeding cathode operating gas into the fuel cell stack and a cathode exhaust gas path for discharging cathode exhaust gas out of the fuel cell stack, and a fuel cell cooling system for cooling the fuel cell stack including a coolant path into which the fuel cell stack is integrated so as to transfer heat is provided. It is provided that the cathode gas supply source also includes a turbine arranged in the cathode exhaust gas path, and the coolant path of the fuel cell cooling system in the flow path of the cathode exhaust gas downstream from the turbine is in heat-transferring contact with the cathode exhaust gas path. This achieves a cooling of the coolant by the cathode exhaust gas and a heating of the cathode exhaust gas by the coolant.

Description:
[0001]    This claims the benefit of German Patent Application DE102014227014.6, filed Dec. 29, 2014 and hereby incorporated by reference herein. 
         [0002]    The invention relates to a fuel cell system as well as to a vehicle having such a fuel cell system. 
       BACKGROUND 
       [0003]    Fuel cells utilize the chemical reaction of a fuel with oxygen to form water in order to generate electric energy. For this purpose, the core component of fuel cells is the so-called membrane electrode assembly (MEA), which consists of an ion-conductive (usually proton-conductive) membrane and an electrode (anode and cathode) arranged on each side of the membrane. Moreover, it is also possible to arrange gas diffusion layers (GDL) on both sides of the membrane electrode assembly on the sides of the electrodes facing away from the membrane. As a rule, the fuel cell is formed by a plurality of stacked MEAs whose electric outputs are cumulative. As a rule, there are bipolar plates (also called flow field plates) arranged between the individual membrane electrode assemblies and they ensure that the individual cells are supplied with the operating media, in other words, the reactants, in addition to which they also serve for cooling purposes. Moreover, the bipolar plates establish an electrically conductive contact with the membrane electrode assemblies. 
         [0004]    During operation of the fuel cell, the fuel, especially hydrogen H 2  or a gas mixture containing hydrogen, is fed to the anode via a flow field of the bipolar plate that is open on the anode side, where an electrochemical oxidation of H 2  to form H +  takes place while electrons are released. A (hydrous or anhydrous) transport of the protons H +  from the anode space into the cathode space takes place via the electrolytes or the membrane which separates the reaction spaces and electrically insulates them from each other in a gas-tight manner. The electrons provided on the anode are fed to the cathode via an electric line. Oxygen or a gas mixture containing oxygen (for instance, air) is fed to the cathode via a flow field of the bipolar plate that is open on the cathode side so that a reduction of O 2  to form O 2−  takes place while electrons are picked up. At the same time, the oxygen anions react in the cathode space with the protons that have been transported via the membrane, a process in which water is formed. 
         [0005]    In order to supply the fuel cell stack with its operating media, in other words, the reactants, the fuel cell stack has, on the one hand, an anode supply source and, on the other hand, a cathode supply source. The anode supply source comprises an anode supply path for feeding an anode operating gas into the anode spaces of the stack, and an anode exhaust gas path for discharging an anode exhaust gas out of the anode spaces. By the same token, the cathode supply source comprises a cathode supply path for feeding a cathode operating gas into the cathode spaces, and a cathode exhaust gas path for discharging a cathode exhaust gas out of the cathode spaces of the fuel cell stack. 
       SUMMARY OF THE INVENTION 
       [0006]    During the operation of the fuel cell, the fuel cell reaction generates heat, which is why the fuel cell stack is integrated into a cooling circuit that discharges the waste heat via a coolant. The coolant is cooled, for example, by means of an air cooler; in the case of a vehicle, this is normally a radiator. Moreover, the waste heat is partially transported away by the exhaust gases of the fuel cell. Another problem is the product water that is generated during the fuel cell reaction, most of which is transported away via the cathode exhaust gas. At low ambient temperatures, it is necessary to prevent freezing of condensed water in the fuel cell and in the exhaust gas lines. 
         [0007]    Japanese patent application JP 2009-076216 A discloses the approach of condensing the product water contained in the cathode exhaust gas and then using it to cool the fuel cell stack. 
         [0008]    It is likewise a known procedure to utilize the waste heat contained in the cathode exhaust gas by employing heat exchangers. European patent application 1 306 916 A1 describes the technique of integrating the heat exchanger of a fuel cell cooling circuit into the cathode exhaust gas path of the fuel cell stack in order to cool the cathode exhaust gas before it is conveyed to a condenser for purposes of recovering the product water. 
         [0009]    In the case of solid oxide fuel cells (SOFC), which are operated at very high temperatures, it is also a known procedure to utilize the waste heat in the cathode exhaust gas by employing heat exchangers. For instance, international patent application WO 2013/039022 A1 describes controlling the temperature of a secondary battery in that the cooling takes place by means of the cathode supply air of the fuel cell and the battery is heated by means of the hot cathode exhaust gas. If the battery is being discharged in an exothermic process, the cooling takes place via the cathode supply air, and if the battery is being charged in an endothermic process, the cathode exhaust gas is heated up. The heat transfer takes place via an air heat exchanger. 
         [0010]    It is a challenge to provide lower coolant temperatures in order to cool the fuel cell stack as well as other components of the system, for example, the electric motor of an electric turbocharger or its power electronics. Thus, for instance, the intake air of fuel cell stacks at high temperatures is often cooled over large cooling surfaces or by auxiliary coolers which are accommodated, for example, in the wheel wells of the vehicle. This creates the need for a large installation space. In order to limit the cooling load of the fuel cell stack, it is also a known procedure to lower the fuel cell output and/or to change several operating parameters (for example, reducing the operating pressure or the mass flows of the operating gas). The reduction in output, however, goes hand in hand with undesired output losses in vehicles, whereas shifting the operating point entails a deterioration of the efficiency as well as a reduction of the service life of the stack. 
         [0011]    Auxiliary aggregates of the fuel cell system are also integrated into external cooling circuits that are at a low temperature, or else components are employed that have a better temperature resistance, as a result of which the cooling problem, however, is merely shifted or the design freedom is restricted. 
         [0012]    The present invention provides a fuel cell system, especially for a vehicle, with which the drawbacks of the state of the art are at least partially eliminated. 
         [0013]    The present invention provides a fuel cell system including:
       a fuel cell stack,   a cathode gas supply source, comprising a cathode supply path for feeding cathode operating gas into the fuel cell stack and a cathode exhaust gas path for discharging cathode exhaust gas out of the fuel cell stack, and   a fuel cell cooling system for cooling the fuel cell stack, comprising a coolant path into which the fuel cell stack is integrated so as to transfer heat.       
 
         [0017]    The fuel cell system according to the invention is characterized in that the cathode gas supply source also comprises a turbine arranged in the cathode exhaust gas path, and the coolant path of the fuel cell cooling system in the flow path of the cathode exhaust gas downstream from the turbine is in heat-transferring contact with the cathode exhaust gas path. The turbine allows a relief of the cathode exhaust gas and thus a cooling of the cathode exhaust gas. The relatively low exhaust gas temperature makes it possible to cool the coolant by means of the cathode exhaust gas and to heat the cathode exhaust gas by means of the coolant. The turbine is preferably mechanically connected to a compressor arranged in the cathode supply path in order to assist or effectuate its drive. Therefore, the expression “heat-transferring contact of the coolant path with the cathode exhaust gas path” includes a heat-transferring contact between the coolant and the cathode exhaust gas while excluding an exchange of material. 
         [0018]    Therefore, the integration of the cathode exhaust gas path into the coolant path of the fuel cell cooling system leads to a heat transfer between the coolant and the cathode exhaust gas, so that, owing to the cool exhaust gas, the temperature of the coolant is lowered. Even if this measure for cooling the coolant is not sufficient as the sole cooling measure in the fuel cell cooling system, thanks to the lesser cooling requirements, the invention nevertheless allows a smaller dimensioning of an auxiliary or main cooler of the cooling system, for instance, a vehicle radiator. Moreover, if the system has been at a standstill for a prolonged period of time, especially at low ambient temperatures, the coolant can be brought to the operating temperature by means of the exhaust gas flow, which heats up considerably faster. On the other hand, the invention achieves an increase in the temperature of the cathode exhaust gas, as a result of which condensation of the gaseous product water in the cathode exhaust gas is prevented or else water that has already condensed even evaporates. Consequently, the additional evaporation energy can be employed to attain further cooling of the coolant. At the same time, the fraction of liquid water in the cathode exhaust gas is reduced and it might be possible to dispense with additional measures and components in order to drain the liquid water or to prevent condensation. As a result, the invention makes it possible to reduce the energy demand and thus to increase the efficiency of the entire system. 
         [0019]    In a preferred embodiment of the invention, the cathode gas supply source also comprises a humidifier, especially a membrane humidifier, through which the cathode exhaust gas flows. In this case, the coolant path of the fuel cell cooling system is in the flow path of the cathode exhaust gas, preferably downstream from the humidifier in heat-transferring contact with the cathode exhaust gas path. The use of humidifiers, especially of membrane humidifiers, in order to humidify a cathode operating gas that is to be fed to a fuel cell stack is known in the state of the art. In this process, the cathode operating gas that is to be humidified and fed to the fuel cell is conveyed via a water vapor-permeable membrane through which the more humidified cathode exhaust gas flows from the other side of the membrane. Humidity is transferred to the cathode operating gas via the membrane, so that the polymer electrolyte membrane of the fuel cell stack achieves the humidity it needs. Owing to the arrangement of the humidifier upstream from the heat-transferring contact, the temperature is reduced further and the relative humidity of the catalyst exhaust gas is achieved. 
         [0020]    Especially preferably, the humidifier is arranged upstream from the turbine in the cathode exhaust gas path. In this manner, the original high humidity of the cathode exhaust gas as it enters the humidifier is utilized and the humidifying potential for the cathode operating gas in increased. 
         [0021]    According to a preferred embodiment of the invention, aside from the fuel cell stack, at least one additional component that is to be cooled is integrated into the coolant path of the fuel cell cooling system. In this context, the expression “integrated into the cooling system” refers to an arrangement such that the component that is to be cooled is in heat-transferring contact with the coolant path, so that heat is transferred from the component to the coolant. The integration of an additional component that is to be cooled into the fuel cell cooling system reduces the complexity of the system and the cooling potential is optimally utilized. The additional component that is to be cooled is preferably the electric motor of an electric compressor, especially for the cathode operating gas and/or the power electronics of such an electric motor, since these components are often very temperature-sensitive when exposed to intense heat. 
         [0022]    According to one embodiment of the invention, the at least one additional component that is to be cooled and the heat-transferring contact with the cathode exhaust gas path are connected in series in the flow path of the coolant, in other words, they are connected one after the other. This embodiment is technically simple to implement since it entails simple piping work and only requires a small amount of space. 
         [0023]    Preferably, the at least one additional component that is to be cooled is arranged in the flow path of the coolant downstream from the heat-transferring contact with the cathode exhaust gas path. As a result, the component is exposed to a very low coolant inlet temperature, so that particularly temperature-sensitive components can be effectively cooled. 
         [0024]    Preferably, the fuel cell stack and the heat-transferring contact with the cathode exhaust gas path are connected in parallel to each other in the flow path of the coolant. In comparison to a series connection, this embodiment has the advantage that it is technically simpler to implement in terms of the line configuration, and it requires less space. 
         [0025]    As an alternative, however, the fuel cell stack and the heat-transferring contact with the cathode exhaust gas path can be likewise connected in series, that is to say, one after the other, in the flow path of the coolant, and especially to arrange the fuel cell stack upstream from the heat-transferring contact with the cathode exhaust gas path. This embodiment allows a reduction in the temperature level throughout the cathode exhaust gas in the entire cooling circuit and thus translates into a particularly effective cooling of the components integrated into it. 
         [0026]    Preferably, the heat-transferring contact of the coolant path with the cathode exhaust gas path is effectuated by means of a heat exchanger. Heat exchangers have a large heat exchange surface area between the appertaining media and have become well-established in a wide array of configurations. 
         [0027]    According to another advantageous embodiment of the invention, a heat exchanger arranged in the cathode supply path for purposes of cooling or heating the cathode operating gas (depending on the operating point of the fuel cell) is integrated into the coolant path of the fuel cell cooling system. In this manner, simple means can achieve cooling of the cathode operating gas that has heated up due to the compression before entering the fuel cell stack, especially at operating points associated with a greater load. At operating points in the lower load range, in contrast, when the compressed air is cooler than necessary, the heating takes place by means of the heat exchanger. 
         [0028]    Another aspect of the present invention relates to a vehicle that has a fuel cell system according to the present invention. In this context, the fuel cell system especially serves to supply an electric drive aggregate of the vehicle or to charge a battery. 
         [0029]    The various embodiments of the invention cited in this application can be advantageously combined with each other, unless otherwise indicated in individual cases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The invention will be described below by means of embodiments on the basis of the accompanying drawings. The following is shown: 
           [0031]      FIG. 1  a flow chart of a conventional fuel cell system; 
           [0032]      FIG. 2  a flow chart of a fuel cell system according to an embodiment of the invention; and 
           [0033]      FIG. 3  a flow chart of a fuel cell system according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  shows a conventional fuel cell system designated in its entirety by the reference numeral  100 ′. The fuel cell system  100 ′ is part of a vehicle, especially an electric vehicle, which has an electric traction motor that is supplied with electric energy by the fuel cell system  100 ′. 
         [0035]    As its core component, the fuel cell system  100 ′ comprises a fuel cell stack  10  that has a plurality of stacked individual cells  11 , each of which comprises an anode space  12  as well as a cathode space  13  that are separated from each other by an ion-conductive polymer electrolyte membrane  14  (see detail cutout view). The anode and cathode spaces  12 ,  13  each have a catalytic electrode, namely, the anode or the cathode, which catalyze the appertaining partial reaction of the fuel cell reaction. Between two such membrane electrode units, there is also a bipolar plate that serves to feed the operating media into the anode and cathode spaces  12 ,  13  and that also establishes the electric connection between the individual fuel cells  11 . 
         [0036]    In order for the fuel cell stack  10  to be supplied with the operating gases, the fuel cell system  100 ′ has an anode supply source  20  on the one hand, and a cathode supply source  30  on the other hand. 
         [0037]    The anode supply source  20  comprises an anode supply path  21  that serves to feed an anode operating gas (the fuel), for example, hydrogen, into the anode space  12  of the fuel cell stack  10 . For this purpose, the anode supply path  21  connects a fuel reservoir  23  to an anode inlet of the fuel cell stack  10 . The anode supply source  20  also comprises an anode exhaust gas path  22  that discharges the anode exhaust gas out of the anode spaces  12  via an anode outlet of the fuel cell stack  10 . Moreover, the anode supply source  20  can have a fuel recirculation line that connects the anode exhaust gas path  22  to the anode supply path  21 . The recirculation of fuel is a conventional process to return and to utilize the fuel, which is usually employed more than stoichiometrically. 
         [0038]    The cathode supply source  30  comprises a cathode supply path  31  that feeds a cathode operating gas containing oxygen, especially air, into the cathode spaces  13  of the fuel cell stack  10 . The cathode supply source  30  also comprises a cathode exhaust gas path  32  that discharges the cathode exhaust gas (especially the exhaust air) out of the cathode spaces  12  of the fuel cell stack  10  and, if applicable, conveys it to an exhaust gas system (not shown here). In order to convey and compress the cathode operating gas, there is a compressor  33  in the cathode supply path  31  which especially is an electric turbocharger (ETC) that is operated by an electric motor  34  having power electronics  35 . Optionally, as shown here, the compressor  33  can be driven with the assistance of a turbine  36  that is arranged in the cathode exhaust gas path  32 . In this context, the compressor  33  and the turbine  36  are connected to each other via a shared shaft. Downstream from the compressor  33 , a heat exchanger (intercooler)  37  is integrated into the cathode supply path  31  and, as a function of the operating point, it serves to cool or heat the cathode operating gas that has been compressed by the compressor  33 . 
         [0039]    Moreover, the cathode supply source  30  has a humidifier  38  that humidifies the cathode operating gas. In this process, the cathode operating gas that is to be humidified as well as the more humidified cathode exhaust gas are conveyed on both sides through a water vapor-permeable membrane so that the water vapor diffuses out of the humidified cathode exhaust gas through the membrane into the dry cathode operating gas, thereby humidifying it. The humidifier  38  is arranged, on the one hand, in the cathode supply path  31  between the compressor  33  and the cathode input and, on the other hand, in the cathode exhaust gas path  32  between the turbine  36  and the cathode output of the stack  10 . 
         [0040]    For the sake of clarity, several other details of the anode and cathode supply sources  20 ,  30  are not shown in the simplified  FIG. 1 . For instance, the humidifier  38  can be bypassed by an appropriate bypass line on the side of the cathode supply path  31  as well as on side of the cathode exhaust gas path  32 . There can also be a turbine bypass line that bypasses the turbine  36  on the side of the cathode exhaust gas path  32 . Furthermore, there can be a wastegate line that connects the cathode supply line  31  to the cathode exhaust gas line  32 . Moreover, a water separator can be installed in the anode and/or cathode exhaust gas path  22 ,  32  in order to condense and drain the product water generated by the fuel cell reaction. Finally, the anode exhaust gas line  26  can open into the cathode exhaust gas line  32  so that the anode exhaust gas and the cathode exhaust gas are discharged via a shared exhaust gas system. 
         [0041]    The fuel cell system  100 ′ shown in  FIG. 1  also comprises a fuel cell cooling system which is designated in its entirety by the reference numeral  40  and which is integrated into the fuel cell stack  10  in a heat-exchanging manner. The coolant that is circulating in the coolant path  41  is conveyed by a coolant pump  42  driven by an electric motor. The coolant, which especially is water, a water-alcohol mixture or a water-ethylene glycol mixture, is conveyed via a main cooler  43  which, in case of a vehicle, is normally a radiator equipped with an air fan. The main cooler  43  can be bypassed by a cooler bypass line  44 , whereby a valve  45  controls the mass flows of the coolant through the main cooler  43  and the bypass line  44 . 
         [0042]    The main cooling circuit  41 . 1  of the coolant path  41  surrounds the fuel cell stack  10 . Moreover, downstream from coolant pump  42 , the coolant path  41  branches off so that, in a cooling circuit side branch  41 . 2 , it passes through the heat exchanger  37  situated in the cathode flow path  31 , as a result of which a heat-transferring contact between the coolant and the cathode operating gas takes place. Another cooling circuit side branch  41 . 3  integrates the power electronics  35  as well as the electric motor  34  of the electric turbocharger  33 . In the embodiment of the state of the art shown, the components, namely, the fuel cell stack  10 , the heat exchanger  37  as well as the power electronics  35  or the electric motor  34  are connected in parallel in the various branches  41 . 1 ,  41 . 2  or  41 . 3  of the coolant path  41 . 
         [0043]      FIGS. 2 and 3  each show a fuel cell system  100  according to several embodiments of the present invention. Here, equivalent elements are designated with the same reference numerals as in  FIG. 1  and they exhibit the properties and functions described there. 
         [0044]    In accordance with the fuel cell system  100  according to the invention shown in  FIGS. 2 and 3 , the cathode gas supply source  30  has a heat exchanger  39  which is arranged in the cathode exhaust gas path  32 . The heat exchanger  39  is configured in such a way that the cathode exhaust gas path  32  or the cathode exhaust gas is in heat-transferring contact with the coolant path  41  or with the coolant of the fuel cell cooling system  40 . Consequently heat is transferred inside the heat exchanger  39  between the coolant and the cathode exhaust gas. In particular, the coolant is cooled in the heat exchanger  39  by the cathode exhaust gas, or else the cathode exhaust gas is heated by the coolant. The heat exchanger  39  is situated downstream from the turbine  36  in the flow path of the cathode exhaust gas. Moreover, if the system has been at a standstill for a prolonged period of time, especially at low ambient temperatures, the coolant can be brought to the operating temperature by means of the exhaust gas flow that heats up considerably faster. 
         [0045]    At the cathode outlet of the fuel cell stack  10 , there is a low-oxygen cathode exhaust gas at temperatures of typically up to 100° C., especially within the range from 60° C. to 100° C., and having a high relative humidity of between 80% and 100%. The humidity content as well as the temperature decrease in the humidifier  38 . Owing to the expansion of the turbine  36 , the cathode exhaust gas continues to cool down and the water partially condenses. After the expansion, depending on the operating point, the cathode exhaust gas is typically at a temperature of about 50° C. at a relative to humidity of 100%. This relatively low temperature is utilized according to the invention in order to further cool down the coolant of the cooling system  40 . As a result, the main cooler  43  is assisted and can be dimensioned smaller. At the same time, the temperature of the cathode exhaust gas in the heat exchanger  39  rises. This prevents additional water from condensing or else water that has already condensed even evaporates. This makes it possible to dispense with the work normally required in the state of the art for purposes of discharging liquid water in order to avoid ice formation in the line system. The evaporation energy of the water also causes further cooling and thus further generation of cooling energy that is utilized by the coolant in the cooling system for itself 
         [0046]    In the embodiment shown in  FIG. 2 , the heat exchanger  39  is in the side branch  41 . 3  of the coolant path  41  and is thus connected in parallel to the fuel cell stack  10 , which is integrated into the main coolant circuit  41 . 1 . Moreover, additional components of the fuel cell system  100 , namely, the electric motor  34  of the electric compressor  33  as well as the power electronics  35 , are integrated into the side branch  41 . 3  of the coolant path  41 . In particular, these components are connected in series, in other words, one after the other, with respect to the flow path of the coolant, whereby the components  34  and  35  are arranged downstream from the heat exchanger  39 . Since the outlet temperature of the coolant out of the heat exchanger  39  is lower than the inlet temperature, the components  34 ,  35  are cooled particularly effectively. Therefore, this arrangement allows components that are especially temperature-sensitive or that generate a great deal of heat to be cooled at very low coolant temperatures. 
         [0047]      FIG. 3  shows another embodiment of a fuel cell system  100  according to the invention. The embodiment shown here differs from that of  FIG. 2  essentially in that the heat exchanger  39  is arranged together with the fuel cell stack  10  in the main circuit  41 . 1  of the coolant path  41 . In the embodiment shown here, the fuel cell stack  10  is situated upstream from the heat exchanger  39  in the main coolant circuit  41 . 1 . In this manner, the coolant heated by the fuel cell stack  10  is pre-cooled in the heat exchanger  39  before it is cooled down again in the main cooler  43 . Therefore, the heat exchanger  39  reduces the temperature of the entire coolant circuit  41 . 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           100  fuel cell system 
           100 ′ conventional fuel cell system 
           10  fuel cell stack 
           11  individual cell 
           12  anode space 
           13  cathode space 
           14  polymer electrolyte membrane 
           20  anode supply source 
           21  anode supply path 
           22  anode exhaust gas path 
           23  fuel tank 
           30  cathode supply source 
           31  cathode supply path 
           32  cathode exhaust gas path 
           33  compressor 
           34  electric motor 
           35  power electronics 
           36  turbine 
           37  heat exchanger/intercooler 
           38  humidifier 
           39  heat exchanger (heat-transferring contact with the coolant/cathode exhaust gas) 
           40  fuel cell cooling system 
           41  coolant path 
           41 . 1  coolant main circuit 
           41 . 2  coolant side branch 
           41 . 3  coolant side branch 
           42  coolant pump 
           43  main cooler 
           44  cooler bypass line 
           45  three-way valve 
           46  heater