Page 90 - Introduction to Transfer Phenomena in PEM Fuel Cells

Charge Transfer Phenomena 79
2.5.6. Optimum operating range
Polarization phenomena that lower the voltage when the current density
increases force the fuel cell outside its limits (very low or very high current
densities). In general, the optimal value is chosen in the linear zone. Given
these phenomena and an external charge (motor, etc.), the actual voltage
varies according to the required intensity, the type of fuel cell and the
operating conditions (temperature, pressure, etc.); it is approximately 0.5 to
1 V. The fuel cell provides a variable voltage depending on the intensity
[BOU 07].

2.6. Modeling of charge transfer

The performance of a steady-state PEMFC is generally expressed by the
polarization curve. A steady-state model can be used to predict the
polarization curve, and thus the fuel cell efficiency. The dynamic
performance of the fuel cell can be modeled by an equivalent circuit that
integrates a general solution of the steady-state model. Charge transfer
models for PEMFCs can be divided into two broad categories, namely
analytical and empirical. Analytical models are based on theory, whereas
empirical models are based on experimentation. Unfortunately, PEMFC cells
are complex systems and they are difficult to model completely in an
analytical way. For this reason, analytical models of fuel cells have a
theoretical basis while still maintaining certain empirical characteristics.
Thirumalai and White [THI 97] supplemented, with a good sensitivity
analysis, an old Nguyen and White [NGU 93] model, which aimed to find
the best operating conditions for the reasonable operation of a PEMFC. They
found that the following factors are of particular importance: gas mass flow,
the operating temperature of PEMFC and the relative humidity of the
reactive gases and more particularly that of hydrogen.

2.7. Overview of analytical models

Analytical models can be classified into two categories, namely simple
and complex. The simple models characterize the operating voltage of the
PEMFC while taking into account the theoretical maximum voltage as well
as the major voltage losses.