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Development and Modelling of a Polymer Electrolyte Fuel Cell Stack of the
5 kW Class
by Thorsten Wüster
Abstract:
Polymer electrolyte fuel cells (PEFCs) require considerable optimization before they can
compete with technologies already established on the market. The approaches described in this
thesis contribute to the optimized design of PEFCs of the 5 kW class. The approaches concern
the mutual interactions of individual components of the cell stack and place these interactions
on a new model basis. They comprise, amongst other aspects, water and heat management,
knowledge of the expected flow and current density distribution as well as the dependence of
cell performance on the operating parameters. The generalized results can be transferred to the
design and optimization of fuel cell stacks of different power classes.
Experimental investigations of single cells and short stacks serve as the design basis for the
5 kW cell stack. Local changes in cell power caused by varying the operating parameters are
determined by measuring the current density distribution. Two innovative measuring methods
are used for this purpose. The first procedure is based on local in situ resistance
measurements. The second non-invasive method of magnetic tomography permits conclusions
to be drawn about the current density distribution inside the cell stack from measurements of
magnetic flux density outside the cell stack.
A novel, quasi-one-dimensional model is derived for characterizing PEFC performance in the
form of voltage-current density curves. The application of the model shows that with decreasing
operating pressure and rising current density the recommended maximum cathodic reactant
conversion is shifted towards a higher oxygen stoichiometry coefficient. An effective rise in
performance can be achieved by an enlarged catalyst surface area and increased catalyst
activity.
An analytical one-dimensional calculation approach determines that the media are uniformly
distributed among the individual cells in a cell stack. This approach is generalized by combining
the factors of influence into suitable dimensionless numbers. For any cell stack geometries, the
uniformity of the media distribution is presented as a function of these numbers in a graphical
and analytical form.
Depending on the respective application, an operating pressure of 1.1 to 2.2 bar proves to be
particularly suitable for operation of the 5 kW stack. A water-based cooling concept is selected
for removing the reaction heat from the cell stack. The humidifier cells on the anode side are
integrated into the fuel cell stack. The cooling water is fed into the humidifier cells after having
been heated by flowing through the cell stack. The use of a capillary or hollow-fibre humidifier
module is recommended in order to moisten the oxidant.
Based on the results, a fuel cell stack consisting of 55 cells has been constructed and put into
operation. An initial performance characterization shows that the envisaged power of 5 kW has
been achieved.