Page 338 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

3 14 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

properties for tubular designs are very different from that of planar SOFCs.
Dedicated efforts are required for each individual design.

11.6 System-Level Modelling

In a system-level model, stack models are combined with models of system
components including reformer, contaminant removal unit, compressors,
topping or bottoming turbines, inverters, etc. The principal objective of system
modelling is to determine the energy efficiency and heat/power ratio of the
system. Such a model is also an excellent tool for making initial sketches of
the system design and for initial sizing of components [14,34-381. One example
is the Excel spreadsheet model developed by Keegan et aI. [ 141. It combines models
for gas preheater, reformer with recycle, stack, interface between stack and inlet
gas, external reformer and exit gas, and combustor and uses heat balances and
variable recycle ratio (with reformate composition as a function of recycle ratio)
to evaluate (i) the overall system configuration and connectivity options,
including various other recycle options, heat exchanger types, locations, and
sizes; (ii) required energy transfer, resulting temperatures, and overall system
efficiency, including pumping power and other parasitics, for the different
proposed system designs; (iii) subsystem requirements associated with specific
selected configurations, including required stream mass flows and allowable
branch pressure drops; (iv) system performance at various load conditions; and
(v) dynamic system performance during startup to determine additional
constraining requirements, including allowable thermal mass and required heat
transfer. The model also evaluates the system cycle efficiency, that is, the overall
system performance as a function of the system start-up and shut-down cycling.
Another example is the systematic analysis undertaken by Palsson et uZ. on
combined SOFC and gas turbine cycles [36]. In combination with a robust and
accurate 2-D SOFC model, the system-level model attempts to provide an
unbiased evaluation of performance prospects and operational behaviours of
such systems. The 2-D SOFC model was integrated into a process simulation tool,
Aspen PlusTM, as a user-defined model, whereas other components constituting
the system are modelled as standard unit operation models. Parametric studies
can be carried out to gain knowledge of stack and system behaviour such as the
influence of fuel and air flow rate on the stack performance and the mean
temperature and the effects of cell voltage and compressor pressure on the system
efficiency. The pressure ratio is shown to have a large impact on performance
and electrical efficiencies of higher than 65% are possible at low-pressure ratios.
Extensive system modelling for SOFC systems has been carried out and
published by Winkler et al. [3 7,3 81. Their publications cover the methodology of
system modelling as well as the effect of hardware design variations on efficiency
and cost of integrated SOFC-GT hybrid systems. The market acceptance
ultimately depends on the system cost, which is influenced by the process design,
hardware design, production (materials and handling), and market (production
quantity). Based mainly on the thermodynamics, the process design examines