Page 33 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

size that has been successfully fabricated is about 30 by 30 cm, far smaller than
that possible with polymer membranes. The second problem is the gas sealing
around the edges of the planar cells: this can be achieved with metal or glass seals
but the required tolerance of around 10 pm in the membrane dimensions causes
high cost. Sulzer avoided this issue by having discs without seals at the outer
circumference. Of course, any rigid bonding together of a large ceramic structure
also exacerbates the thermal shock issue. Monolithic designs have not been
successful for that reason.
The Westinghouse tubular design is ingenious because 1.5-2 m long cells
could be manufactured and handled as a result of the inherent strength of the
tube structure. A 100 kWe generator could then be built from 1152 such cells.
Moreover, the sealing problem was eliminated by inserting an air feeder tube
down the cell tube. Although the Westinghouse tubular design is large and
expensive, it did demonstrate several important features which have lent
credence to the SOFC technology:

0 The cells can run for long periods without much deterioration
0 The efficiency can be impressive, around 50%
0 Methane can be used as fuel after desulphurising and pre-reforming
0 The SOFC exhaust can drive a gas turbine
0 Emissions are low

In order to understand and predict the performance of such complex stack
structures, various mathematical models have been developed, as described in
Chapter 11. The most fundamental model starts from the reaction diffusion
equations, assuming constant temperature conditions, and calculates the
gradient of reactants, products and potentials along a tube or plate of electrolyte
[30]. This gives a very sharp reaction front under normal operating conditions
if the tube or plate is open ended. The chemical gradient along the SOFC can also
be predicted as oxide ions permeate through the electrolyte [31]. Another
important model sets out to calculate temperature and current distributions in a
stack of cells [32]. Many such models for different geometries including planar
and tubular have been published.

1.9 SOFC Power Generation Systems
Typically 2 5% of the volume of a fuel cell system is made up of the cell stack. The
rest of the reactor is the balance of plant (BOP) which includes thermal
insulation, pipework, pumps, heat exchangers, heat utilisation plant, fuel
processors, control system, start-up heater and power conditioning, as described
in Chapter 13. Arguably, this BOP is the dominant part of the system and should
be treated with some concern. One of the major problems of the original
Westinghouse design for a 100 kWe cogenerator was its large 16 m2 footprint
and huge weight of 9.3 te [33]. This was not competitive with a standard diesel
engine combined heat and power unit.