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.sub.2) and oxygen (O.sub.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.degree. 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.sub.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.sub.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 .lambda. and the air volume flow rate V.sub.L. The air volume flow rate V.sub.L is a measure of the quantity of air flowing through the PEM fuel cell block per unit time. The air ratio .lambda. indicates the amount of air required by the reaction if air from the surroundings is used instead of pure oxygen (O.sub.2).
The control or regulation of the air volume flow rate V.sub.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.sub.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.sub.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.