| dc.description.abstract | Molten carbonate fuel cells (MCFCs) have become an attractive emerging technology for
stationary co-generation of heat and power. From a technical perspective, dynamic operation has
a significant effect on the fuel cell life cycle and, hence, economic viability of the device.
The scope of this thesis is to present an improved understanding of the system behaviour at
transient operation that can be used to design a more robust control system in order to overcome
the cost and the operating lifetime issues. Hence, a comprehensive multi-component multidimensional
transient mathematical model is developed based on the conservation laws of mass,
momentum, species, energy and electric charges coupled through the reaction kinetics. In
essence, this model is a set of partial differential equations that are discretized and solved using
the finite-volume based commercial software, ANSYS FLUENT 12.0.1. The model is validated
with two sets of experimental results, available in open literature, and good agreements are
obtained. The validated model is further engaged in an extensive study. First, the MCFC
behaviour at high current densities or oxidant utilization, when the mass transfer becomes
dominant, is investigated using peroxide and superoxide reaction mechanisms. In brief, both
mechanisms predicted the linear region of the polarization curve accurately. However, none of
these mechanisms showed a downward bent in the polarization curve. A positive exponent for
the carbon-dioxide mole fraction is probably essential in obtaining the downward bent (“knee”)
at high current densities which is in contrast to what has been reported in the literature to date.
Next, a sinusoidal impedance approach is used to examine the dynamic response of the unit
cell to inlet perturbations at various impedance frequencies. This analysis is further used to
determine the phase shifts and time scales of the major dynamic processes within the fuel cell.
Furthermore, numerical simulation is utilized in order to investigate the underlying
electrochemical and transport phenomena without performing costly experiments. Results
showed that the electrochemical reactions and the charge transport process occur under a
millisecond. The mass transport process showed a comparatively larger time scale. The energy
transport process is the slowest process in the cell and takes about an hour to reach its steady
state condition.
Furthermore, the developed mathematical model is utilized as a predictive tool to provide a
three-dimensional demonstration of the transient physical and chemical processes at system startiv
up. The local distribution of field variables and quantities are presented. The results show that
increasing the electrode thickness provides a higher reaction rate, but may lead to larger ohmic
loss which is not desirable. The reversible heat generation and consumption mechanisms of the
cathode and anode are dominant in the first 10 s while the heat conduction from the solid
materials to the gas phase is not considerable. The activation and ohmic heating have the same
impact within the anode and cathode because of their similar electric conductivity and voltage
loss. Increasing the thermal conductivity of the cathode material will facilitate the process of heat
transport throughout the cell. This can also be accomplished by lowering the effects of heat
conduction by means of a cathode material with a smaller thickness.
In addition, a thermodynamic model is utilized to examine energy efficiency, exergy
efficiency and entropy generation of a MCFC. By changing the operating temperature from 883
K to 963 K, the energy efficiency of the unit cell varies from 42.8 % to 50.5 % while the exergy
efficiency remains in the range of 26.8% to 36.3%. Both efficiencies initially rise at lower
current densities up to the point that they attain their maximum values and ultimately decrease
with the increase of current density. With the increase of pressure, both energy and exergy
efficiencies of the cell increase. An increase in this anode/cathode flow ratio lessens the energy
and exergy efficiencies of the unit cell. Higher operating pressure and temperature decrease the
unit cell entropy generation. | en |
