Abstract:
A method for operating a fuel cell system which uses a combustion gas and an oxidant, must take care to ensure sufficient humidification of the combustion gas by evaporating humidifying water. The energy source for the evaporation of the combustion gas and/or oxidant is the heat generated by the coil temperature of an electric motor used to transport the gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation, under 35 U.S.C. § 120, of copending international application No. PCT/DE02/04556, filed Dec. 12, 2002, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 101 61 623.6, filed Dec. 14, 2001; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Field of the Invention  
           [0003]    The invention relates to a method for operating a fuel cell system with a fuel gas and an oxidizing agent, in which it is ensured that the fuel gas and/or the oxidizing agent is/are sufficiently humidified, for which purpose humidification water is evaporated. In addition, the invention also relates to an associated fuel cell system having at least one fuel cell module that is operated with a fuel gas and an oxidizing agent.  
           [0004]    In particular, what are known as air polymer electrolyte membrane (PEM) fuel cells, operated with hydrogen and air, including their method program and the associated functioning are already known in detail from the prior art. The same is true of solid oxide fuel cells (SOFC) that are operated with different fuel gases at high temperatures. In both cases, the oxidizing agent used is advantageously oxygen from the ambient air, and consequently a sufficient quantity of atmospheric oxygen has to be provided for fuel cell operation. For this purpose, it is customary to compress ambient air; suitable compressors are known for this purpose.  
           [0005]    In particular, in the case of PEM fuel cells supplied with air, a sufficient supply of air is important for stable operation that is insensitive to rapid load changes. The supply of air also ensures sufficient humidification of the air in accordance with its respective dew point, with the dew point approximately corresponding to the cooling water outlet temperature or a higher value at the respective pressures and temperatures of the fuel cell module. This is important in particular if the cooling of the fuel cell module is not optimum.  
           [0006]    In the prior art, the energy required to evaporate the water that is used to humidify the air has to be imparted to the water by a sufficient increase in temperature before the water is injected into the compressor. On account of the extremely high evaporation enthalpy of water, this cannot be achieved to a sufficient degree purely by increasing the temperature, for example to 60° C. Alternatively, it would be possible to increase the quantity of water to such an extent that as a result of a drastic excess of water, at, for example 60° C., the required quantity of water would nevertheless be evaporated to a sufficient extent.  
           [0007]    The latter principle is realized in what is known as a liquid ring compressor. However, if a screw-type compressor is used as a technical alternative, it is not possible to inject so much water that a sufficient quantity of water is evaporated.  
         SUMMARY OF THE INVENTION  
         [0008]    It is accordingly an object of the invention to provide a method for operating a fuel cell system that overcomes the above-mentioned disadvantages of the prior art devices of this general type.  
           [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating a fuel cell system with a fuel gas and an oxidizing agent. The method includes the steps of sufficiently humidifying the fuel gas and/or the oxidizing agent by evaporating humidification water, and utilizing heat generated by a winding temperature of an electric motor used to deliver gases as an energy source for evaporating the humidification water to humidify the fuel gas and/or the oxidizing agent.  
           [0010]    The invention makes use of the fact that the water, before it is injected into the compressor, is used to cool the motor of the compressor, and the winding of the motor is constructed in such a manner that the winding temperature, by using suitable insulation, for example a Teflon insulation, can reach temperatures higher than 100° C. As a result, the uptake of heat by the water, if the pressure in the cooling-water line is selected appropriately, is sufficient to take up the evaporation enthalpy for sufficient humidification of gases.  
           [0011]    Therefore, additional cooling of the compressor can also be effected.  
           [0012]    The principle of the invention of utilizing the motor waste heat to evaporate water is advantageously also possible with other motors. For example, if it is used in a vehicle, the method can also be applied in particular to the traction motor. Motors for pumps or the like can also be utilized in accordance with the invention.  
           [0013]    In the fuel cell system according to the invention, the compressor is assigned an electric motor that contains a device for cooling using cooling water, with the cooling water that emerges being fed to the compressor as humidification water. In this case, the cooling-water line may advantageously be a copper tube and can be used directly as coil material for the motor winding. The coil may also have alternating windings of solid material and tube material.  
           [0014]    The invention therefore allows the motor heat to be transferred to the cooling water, so that the process gases are humidified. In particular, the humidification of the oxidizing-agent compressor air is simplified for fuel cell operation. The result is a considerable improvement to efficiency. The fuel gas can also be humidified in the same way.  
           [0015]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0016]    Although the invention is illustrated and described herein as embodied in a method for operating a fuel cell system, and an associated fuel cell system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0017]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a diagrammatic illustration of a fuel cell module which is operated with hydrogen as the fuel gas and compressed ambient air as the oxidizing agent; and  
         [0019]    [0019]FIG. 2 is a fluid circuit diagram for an air compressor for a fuel cell module in accordance with FIG. 1 with an associated electric motor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a fuel cell module  10  which contains a plurality of stacked PEM fuel cells  11 ,  11 ′, . . . with end plates  12 ,  12 ′. A fuel cell stack of this type is also known in the specialist field as just stack for short. To operate the fuel cell module  10 , hydrogen is supplied as a fuel gas through a first line  13  and ambient air is supplied as an oxidizing agent through a second line  14 . There are exit lines  16 ,  18  through which excess fuel and air, respectively, are discharged.  
         [0021]    To provide a sufficient quantity of oxygen as the oxidizing agent for the fuel cell process from the ambient air, the air has to be compressed. To do this, it is known to use, inter alia, ring or screw-type compressors in particular. This also allows the introduction of liquid for humidification of the air. Specifically, a screw-type compressor with injection of liquid is known from German Patent DE 195 43 879 C2. The efficiency of this compressor is good, and the liquid is injected using simple measures.  
         [0022]    In FIG. 2, a compressor  20  of this type, which is fed with ambient air through a line  21  and from which humidified air is discharged through a line  22  which is connected to the entry line  14  of the fuel cell module  10  from FIG. 1.  
         [0023]    The compressor  20  is assigned an electric motor  30  with signal inputs  26 . The electric motor  30  has a rotary axle  31 , through which mechanical power is transmitted to the compressor  20 . This can be realized by the motor axle  31  forming a common axle with the drive of the compressor  20 , which is not illustrated in detail in FIG. 2.  
         [0024]    As an alternative, or in addition, there may be a transmission  35 , which is only indicated in FIG. 2. By way of example, a gearwheel transmission is suitable for this purpose.  
         [0025]    The electric motor  30  has to be cooled. For this purpose, there is a cooling-water line  32  on the entry side. The cooling water for the electric motor  30  is discharged from the motor  20  via a cooling-water exit line  33 , with the line  33  simultaneously serving as an entry line for the compressor  20 . As a result, therefore, the cooling water that has been heated by operation of the motor is simultaneously used as humidification water for the compressor  20 . The humidification water, after it has been cooled, is discharged from the compressor  20  via a line  23 .  
         [0026]    In the configuration shown in FIG. 2, therefore, the water is used to cool the motor  30  of the compressor  20  before it is injected into the compressor  20 . Suitable construction of the motor winding allows the uptake of heat by the water, given a suitable selection of the pressure in the cooling-water line  30 , to be enough to take up the evaporation enthalpy for sufficient humidification of the air in the compressor  20 . It is therefore also possible to perform separate cooling of the compressor  20 .  
         [0027]    The winding of the motor  30  may be constructed in a suitable way, such that the winding temperature, as a result of the use of a Teflon insulation, reaches temperatures of higher than 100° C. At a temperature of this nature, the uptake of heat by the water, given a suitable selection of the pressure, is sufficient to achieve optimum heat transfer. It is advantageous to configure the cooling-water line  32  as a copper tube and for it to be used directly as coil material for the motor  30 . In this case, the coil may be provided with alternating windings of solid material and tube material.  
         [0028]    The principle that has been expounded in detail above on the basis of a fuel cell stack with PEM fuel cells can also be transferred to other fuel cell modules. By way of example in solid oxide fuel cell (SOFC) systems, which operate with a ceramic electrolyte and with standard fuel gas at high temperatures, the oxidizing agent used is likewise atmospheric oxygen, and for this purpose ambient air is prepared by suitable compressors. In this case, the compressed air and, if appropriate, also the fuel gas are likewise humidified, so that in this respect the same relationships as for a polymer electrolyte membrane (PEM) fuel cell result.  
         [0029]    The solution to the problem illustrated in FIGS. 1 and 2 can therefore be used in a corresponding way for SOFC fuel cell systems as well.  
         [0030]    With the structure described, the utilization of the waste heat from the motor winding can be used not only to evaporate the water but also for other applications. Further motor windings, for example those of a traction motor in a motor vehicle, are suitable for this purpose, provided that a PEM fuel cell system is specifically configured for mobile use.  
         [0031]    However, motor windings of pumps in a stationary SOFC fuel cell system or similar configurations can also be used in the context of the invention.