Page 247 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

224 High Temperaturr Solid Oxide Fuel Cells: Fundamentals, Drsign and Applications

design is the ease of manifolding and introducing fuel without leaks or
contamination. A microtubular cell can be made long enough to emerge at both
ends of the furnace, so that rubber tubes can be attached in the cold zone to
introduce fuel and to analyse the reaction products as shown in Figure 8.29.
Fuel may be introduced from a gas cylinder or by means of a bubbler/saturator
using a carrier gas such as helium. Other gases such as steam, carbon dioxide or
air can be metered in by flow controllers to give the desired composition. The
microtubular cell is maintained at a constant temperature in the furnace and
oxygen flow through the electrolyte is controlled using a potentiostat. The
output stream can be analysed by mass spectroscopy [63], and carbon deposition
on the anode can be measured after the test by temperature-programmed
oxidation [64,65].

Figure 8.29 Apparatus for measuringjuel reactions ina microtubularcell.

8.5 Summary
This chapter has described the three major cell designs that are dominating
research and development at the present time: these are the planar, the large-
diameter tubular, and the microtubular designs. Planar SOFCs provide very
high areal (W/cm2) and volumetric (W/cm3) power densities and can be
manufactured by low-cost conventional ceramic processing techniques;
however, sealing around the edges of the cells and the control of temperature
gradients which can cause cell cracking remain issues to be resolved. Large-
diameter tubular SOFCs have been the most successful so far. Their main
advantage is the seal-less stack design; the disadvantages are the low power
density, the long start-up times, and the expensive fabrication techniques.
Microtubular SOFCs are especially useful for smaller systems, providing rapid
start-up; the reason for this is the small diameter of the cells and the low wall
thickness which prevent the build-up of damaging thermal stresses. Start-up in
about a minute is possible and leaks can be prevented by bringing the microtubes
through the insulation for sealing in the cold zone. On the negative side, cell
interconnection and assembly issues are significant, and it seems likely that
microtubular systems will mainly be applicable in small systems.