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The Future of Fuel Cells
by Ben I. Wiens 12Dec1999
8. SOLID OXIDE FUEL CELL (SOFC)

http://www.benweins.com/energy4.html

The Solid Oxide Fuel Cell is considered to be the most desirable fuel cell for generating electricity from hydrocarbon fuels. This is because it is simple, highly efficient,
tolerant to impurities and can internally reform hydrocarbon fuels.
The SOFC runs at a red hot temperature of 800-1000°C. Westinghouse has worked at developing a tubular style of SOFC for many years which operates at 1000°C. These long
tubes have high electrical resistance but are simple to seal. Many companies such as Global Thermoelectric are now working on a planar SOFC composed of thin ceramic sheets which operate at 800°C or even less. Thin sheets have low electrical resistance and possible high efficiencies. Cheaper materials can be used at these lower temperatures. Experts previously
predicted that the SOFC was a long way to becoming commercial reality. Many now believe that these lower temperatures may lead to a quicker solution to these problems.
One of the big advantages of the SOFC over the MCFC is that the electrolyte is a solid. This means that no pumps are required to circulate hot electrolyte. Small planar SOFC of 1
kw could be constructed with very thin sheets and result in a very compact package.
A big advantage of the SOFC is that both hydrogen and carbon monoxide are used in the cell [3]. In the PEFC the carbon monoxide is a poison, while in the SOFC it is a fuel.
This means that the SOFC can readily and safely use many common hydrocarbons fuels such as natural gas, diesel, gasoline, alcohol and coal gas. In the PEFC an external reformer
is required to produce hydrogen gas while the SOFC can reform these fuels into hydrogen and carbon monoxide inside the cell. This results in some of the high temperature waste
thermal-energy being recycled back into the fuel.
Because the chemical reactions in the SOFC are good at the high operating temperatures, air compression is not required. Especially on smaller systems this results in a simpler
system, quiet operation and high efficiencies. Exotic catalysts are not required either.
Many fuel cells such as the PEFC require an expensive liquid cooling system but the SOFC requires none. In fact insulation must be used to maintain the cell temperature on small
systems. The cell is cooled internally by the reforming action of the fuel and by the cool outside air that is drawn into the fuel cell.
Because the SOFC does not produce any power below 650°C, a few minutes of fuel burning is required to reach operating temperature. While the SOFC is also being proposed as an
automotive powerplant, this time delay is considered to be a disadvantage for automotive applications. Because electric powerplants run continuously, this time delay is not a problem.
Still you may be interested to know that the manager for bus development at Ballard, who develop PEFC, is now working for Global, who develop SOFC. The SOFC may well be
suited to at least certain vehicles which run more continuously.
Because of the high temperatures of the SOFC, they may not be practical for sizes much below 1,000 watts or when small to midsize portable applications are involved.
Small SOFC will be about 50% efficient [4] from about 15%-100% power. To achieve even greater efficiency, medium sized and larger SOFC are generally combined with gas
turbines. The fuel cells are pressurized and the gas turbine produces electricity from the extra waste thermal-energy produced by the fuel cell. The resulting efficiency of the medium
SOFC could be 60% and large one's up to 70%.
A SOFC suitable for producing 1-30 kW and using natural gas as it's fuel is shown in Fig 5. On the anode side, natural gas is first ejected into a reforming chamber where it draws
waste thermal-energy from the stack and is converted into hydrogen and carbon monoxide. It then flows into the anode manifold where most of the hydrogen and carbon monoxide is
oxidized into water and carbon dioxide. This gas stream is then partly recycled to the reforming chamber where the water is used in the reforming chamber. On the cathode side, air
is first blown into a heat exchanger where it reaches nearly operating temperature. The air is brought up to the operating temperature of 800°C by combustion of the remaining
hydrogen and carbon monoxide gas from the anode. The oxygen in the cathode manifold is converted into an oxygen ion which travels back to the anode.