Abstract:
A method is provided for operating a PEM fuel cell plant containing at least one PEM fuel cell block and a speed-controlled compressor upstream of the PEM fuel cell block for supplying air at a volume flow rate. The speed of the compressor is controlled to a desired value for adjusting the electric current of the PEM fuel cell block to a given value. The desired value is derived from the given value of the electric current. This measure ensures a simple control of the air volume flow rate for the PEM fuel cell block with low apparatus requirements. A PEM fuel cell plant is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE98/00037, filed Jan. 7, 1998, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for operating a PEM fuel cell plant, and a PEM fuel cell plant. 
     Fuel cells enable electrical energy to be generated directly from hydrogen (H 2 ) and oxygen (O 2 ) with considerably greater efficiency and significantly lower pollutant emission than conventional measures for generating energy. In addition they operate almost silently. 
     In addition to those basic advantages, the fuel cell with a solid electrolyte of synthetic material (Polymer Electrolyte Membrane or PEM) has further positive features such as a lower operating temperature below 80° C., favorable overload behavior, low voltage degradation, long service life, favorable load and temperature cycle characteristics and the absence of a liquid, corrosive electrolyte. Furthermore, it can be used for operation with air from the surroundings instead of with oxygen (O 2 ). 
     As a result of all of those characteristics, the PEM fuel cell which is operable with air is an almost ideal generator of electrical power, e.g. for the operation of a power-driven vehicle emitting no exhaust gases. 
     PEM fuel cells cannot be operated in isolation. For that reason a PEM fuel cell block including many PEM fuel cells, an operating part and an associated electronic module are combined together to form a PEM fuel cell module. The operating part contains devices for supplying hydrogen (H 2 ) and air, for leading away water which is produced, for dissipation of heat losses, for wetting the reactants and for the separation of gas impurities. 
     Important parameters which characterize the operation with air of a PEM fuel cell plant (with at least one PEM fuel cell module) are the air ratio λ and the air volume flow rate V L . The air volume flow rate V L  is a measure of the quantity of air flowing through the PEM fuel cell block per unit time. The air ratio λ indicates the amount of air required by the reaction if air from the surroundings is used instead of pure oxygen (O 2 ). 
     The control or regulation of the air volume flow rate V L  for a PEM fuel cell plant is complicated. For example, an air supply device for an air-driven fuel cell plant with a compressor is known from German Published, Non-Prosecuted Patent Application DE 43 18 818 A1, corresponding to U.S. Pat. Nos. 5,432,020; 5,434,016; and 5,645,950. In order to adjust the compressor, the air volume flow rate V L  and the electric current I of a PEM fuel cell block are recorded continuously with a flowmeter and an actual current sensor, respectively. Both the air volume flow rate V L  and the electric current I of the PEM fuel cell block are continuously transmitted to a control device. Thus, several parameters are continuously recorded and processed in order to adjust the compressor and therewith the PEM fuel cell block. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method for operating a PEM fuel cell plant with a PEM fuel cell block as well as such a PEM fuel cell plant, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which ensure a simple adjustment of an air volume flow rate V L  for the PEM fuel cell block, with low apparatus requirements and thus at a more reasonable cost than the method known from the state of the art. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating a PEM fuel cell plant having at least one PEM fuel cell block and a speed-controlled compressor disposed upstream of the at least one PEM fuel cell block for supplying air at a volume flow rate V L , which comprises controlling a speed n of the compressor to a desired value n SN  for adjusting an electric current I of the at least one PEM fuel cell block to a given value I SN ; deriving the desired value n SN  from the given value I SN  of the electric current I according to a characteristic curve n=f(I SN ); and simultaneously adjusting the desired value n SN  of the speed to a value according to the characteristic curve when the given value I SN  changes. 
     This method ensures a control with a simple apparatus of the speed n of the speed-controlled compressor and thus of the air volume flow rate V L  for the PEM fuel cell block. This control does not involve measurement, either of the air volume flow rate V L  through the PEM fuel cell block or of the electric current of the PEM fuel cell block. 
     For a given value I SN  of the electric current I of the PEM fuel cell block, the corresponding desired value n SN  for the speed n of the speed-controlled compressor is derived from a predefined characteristic curve for the given air ratio λ. The desired value n SN  is then applied to the speed-controlled compressor. Thus a controller for the speed-controlled compressor is not needed. The method is therefore less costly than the method known from the state of the art. In addition, this method is also very dynamic during changes of load. 
     In accordance with another mode of the invention, the desired value n SN  of the speed n for the speed-controlled compressor is determined by a control unit. 
     In particular, the given value I SN  of the electric current I for a load can be determined by the control unit. 
     In accordance with a further mode of the invention, a valve downstream of the PEM fuel cell block is adjusted in such a way that at a maximum speed n M  of the compressor, air is delivered at a volume flow rate V L  which corresponds to the given air ratio λ. This measure creates optimized operating conditions for the PEM fuel cell plant such as, for example, a suitable operating pressure of operating material in the PEM fuel cell block or maintenance of a constant air ratio λ during changes in the electric current I of the PEM fuel cell block through control of the speed n of the compressor. 
     With the objects of the invention in view, there is also provided a PEM fuel cell plant, comprising at least one PEM fuel cell block; a speed-controlled compressor disposed upstream of the at least one PEM fuel cell block for supplying air at a volume flow rate V L ; and a control unit for adjusting an electric current I of the at least one PEM fuel cell block to a given value I SN , the control unit connected to the compressor for controlling the speed n of the compressor to a desired value n SN  derived from the given value I SN  of the electric current I. 
     In accordance with another feature of the invention, there is provided a valve disposed downstream of the at least one PEM fuel cell block. 
     In accordance with a concomitant feature of the invention, the control unit includes a gas pedal for adjusting the electric current of the at least one PEM fuel cell block. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for operating a PEM fuel cell plant and a PEM fuel cell plant, 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. 
     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 
     FIG. 1 is a schematic and block diagram representing a PEM fuel cell plant according to the invention; and 
     FIG. 2 is a diagram in which an air volume flow rate V L  through a PEM fuel cell block is plotted against an electric current I generated in the PEM fuel cell block for a given air ratio λ. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a PEM fuel cell plant  2  which contains a PEM fuel cell block  4  formed of a combination of a plurality of PEM fuel cells. For example, the PEM fuel cell plant  2  may be part of an electrically powered vehicle, such as a power-driven passenger vehicle, a bus, or a fork-lift truck. 
     The operating materials used for operating the PEM fuel cell plant  2  are hydrogen H 2  and air L from the surroundings. 
     The air L is fed along a feed path  6  into the PEM fuel cell block  4  for electrochemical reaction within the PEM fuel cell block  4 . A compressor  8 , which can be adjusted through its speed, is provided in the feed path  6  upstream of the PEM fuel cell block  4 , in order to feed the air L at a volume flow rate V L  to the PEM fuel cell block  4 . 
     The air which is not used in the electrochemical reaction inside the PEM fuel cell block  4  is led away from the same through an outlet path  10 . A valve  12  is provided in the outlet path  10  downstream of the PEM fuel cell block  4 . This valve  12  in the outlet path  10  is adjusted in such a way that at a maximum speed n M  of the speed-controlled compressor  8  the air volume flow rate V L  which is generated through the PEM fuel cell block corresponds to a given air ratio λ, for example λ=2. Thus the adjustment of the valve  12  determines the value of the air ratio λ for operation of the PEM fuel cell block  2 . 
     An electric current I which is generated through the electrochemical reaction in the PEM fuel cell block  4  is fed into a load  16  over an electrical conductor  14 . The load  16  can, for example, be an electric motor of an electrically powered vehicle. 
     A value I SN  of the electric current I of the PEM fuel cell block  4  is determined with a control unit  18 ,  22 . The control unit  18 ,  22  includes a gas pedal  22  of an electrically driven vehicle. Different positions of the gas pedal  22  correspond to different values I SN  of the electric current I of the PEM fuel cell block  4 . 
     In this method for adjusting the electric current I of the PEM fuel cell block  4  to the given value I SN  a speed n of the compressor  8  is adjusted to a desired value n SN , wherein the desired value n SN  is derived from the given value I SN  of the electric current I. To this end the desired value n SN  of the speed n obtained from the control unit  18 ,  22  is applied over a conductor  20  to the compressor  8 , which is adjustable through its speed n. 
     This method therefore does not require any regulation of the speed-controlled compressor  8  and therewith the air volume flow rate V L . A flowmeter for measuring the air volume flow rate V L  and an actual current value sensor for recording the electric current I of the PEM fuel cell block  4 , can also be dispensed with. Thus the construction of the apparatus for this control system is decisively simplified in comparison with the PEM fuel cell plants known from the state of the art. In addition, a high dynamism is ensured during changes in load. 
     In this method the desired value n SN  is derived from the given value I SN  in dependence on the given air ratio λ. To this end a characteristic curve for the functional connection between the electric current I of the PEM fuel cell block  4  and the speed n of the speed-controlled compressor  8  is provided for the control unit  18 ,  22  for the given air ratio λ, preferably λ=2. It is thus possible through the use of this characteristic curve to directly determine the desired value n SN  of the speed n for the speed-controlled compressor  8  corresponding to the given value I SN  of the electric current I. The compressor  8 , which is then controlled by the desired value n SN , supplies the PEM fuel cell block  4  with air at the volume flow rate V L  that is necessary for generating the given value I SN  of the electric current I. 
     In a diagram shown in FIG. 2, the air volume flow rate V L  of the PEM fuel cell block  4  is plotted against the electric current I generated in the PEM fuel cell block  4  for a given air ratio λ. 
     According to Faraday&#39;s Law, the air volume flow rate V L  and the electric current I of the PEM fuel cell block are related by the following function:            V   L     =     λ   ⋆       0.2091   ⋆   I   ⋆     n   B       0.21     ⋆       T   L                   ,                wherein:                            
     V L  is the air volume flow rate through the PEM fuel cell block 4 in kg/h; 
     λ is the air ratio; 
     0.2091*I*n B  is the volume flow rate through the PEM fuel cell block  4  of oxygen (O 2 ) in kg/h calculated according to Faraday&#39;s Law; 
     I is the electric current in A (amps) generated in the PEM fuel cell block  4 ; 
     n B  is the number of PEM fuel cells in the PEM fuel cell block  4 ; 
     0.21 is the volumetric fraction of oxygen (O 2 ) in the air; and 
     T L  is the temperature in K (Kelvin) of the air flowing through the PEM fuel cell block  4 . 
     Thus, every value of I for the electric current generated in the PEM fuel cell block corresponds to a specific value V L  of the air volume flow rate through the PEM fuel cell block  4  that is necessary for generation of the electric current I. 
     If a constant empirical value is given for the air ratio λ, preferably 2, a linear relationship is obtained between the air volume flow rate V L  and the electric current of the PEM fuel cell block  4 , since all of the other parameters in Faraday&#39;s Law are constants which do not need to be measured separately. 
     Since, in addition, the speed n of the speed-controlled compressor  8  is proportional to the air volume flow rate V L  which it generates, there is a linear relationship between the speed n of the speed-controlled compressor  8  and the electric current I of the PEM fuel cell block  4 . 
     Thus, the desired value n SN  for a given value I SN  of the electric current I can be derived in a simple way through the use of the characteristic curve for the functional relationship between the electric current I of the PEM fuel cell block  4  and the speed n of the speed-controlled compressor  8 .