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Solid Oxide Fuel Cells
The essay below outlines the technology and
history of solid oxide fuel cells. If you have artifacts,
photos, documents, or other materials that would help to improve
our understanding of these devices be sure to respond to the
questionnaire:
Solid
Oxide Fuel Cell Technology
A solid oxide fuel
cell (SOFC) uses a hard ceramic electrolyte instead of a liquid
and operates at temperatures up to 1,000 degrees C (about
1,800 degrees F). A mixture of zirconium oxide and calcium
oxide form a crystal lattice, though other oxide combinations
have also been used as electrolytes. The solid electrolyte
is coated on both sides with specialized porous electrode
materials.
Fuel flows
over this tubular solid oxide fuel cell; air flows through
the center.
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At these high operating temperature, oxygen ions (with a negative
charge) migrate through the crystal lattice. When a fuel gas
containing hydrogen is passed over the anode, a flow of negatively
charged oxygen ions moves across the electrolyte to oxidize
the fuel. The oxygen is supplied, usually from air, at the
cathode. Electrons generated at the anode travel through an
external load to the cathode, completing the circuit and supplying
electric power along the way. Generating efficiencies can
range up to about 60 percent.
In one configuration, the SOFC consists of
an array of tubes (see image below). Another variation includes
a more conventional stack of disks. Since SOFCs operate
at such high temperatures, a reformer is not required to
extract hydrogen from the fuel. Some demonstration units
have capacities up to 100 kilowatts.
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Solid
Oxide Fuel Cell History
Tubular
solid oxide fuel cells arranged in bundles.
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Both solid oxide
and molten carbonate fuel cells are high temperature devices.
The technical history of both cells seems to be rooted in
similar lines of research until the late 1950s.
Swiss scientist Emil Baur and his colleague
H. Preis experimented with solid oxide electrolytes in the
late 1930s, using such materials as zirconium, yttrium,
cerium, lanthanum, and tungsten. Their designs were not
as electrically conductive as hoped and reportedly experienced
unwanted chemical reactions between the electrolytes and
various gases, including carbon monoxide.
In the 1940s, O. K. Davtyan of Russia added
monazite sand to a mix of sodium carbonate, tungsten trioxide,
and soda glass "in order to increase the conductivity and
mechanical strength." Davtyan's designs, however, also experienced
unwanted chemical reactions and short life ratings.
By the late 1950s, research into solid oxide
technology began to accelerate at the Central Technical
Institute in The Hague, Netherlands, Consolidation Coal
Company, in Pennsylvania, and General Electric, in Schenectady,
New York. A 1959 discussion of fuel cells noted that problems
with solid electrolytes included relatively high internal
electrical resistance, melting, and short-circuiting due
to semiconductivity. It seems that many researchers began
to believe that molten carbonate fuel cells showed more
short-term promise.
Not all gave up on solid oxide, however. The
promise of a high-temperature cell that would be tolerant
of carbon monoxide and use a stable solid electrolyte continued
to draw modest attention. Researchers at Westinghouse, for
example, experimented with a cell using zirconium oxide
and calcium oxide in 1962. More recently, climbing energy
prices and advances in materials technology have reinvigorated
work on SOFCs, and a recent report noted about 40 companies
working on these fuel cells.
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Solid
Oxide Fuel Cell Applications
Like molten carbonate
fuel cells, solid oxide cells require high operating temperatures,
and their most common application is in large, stationary
power plants. The high temperatures open the opportunity for
"cogeneration"–using waste heat to generate steam for
space heating, industrial processing, or in a steam turbine
to make more electricity.
Solid oxide fuel cells, like most other types,
produce little pollution. Although they require inverters
to change their direct current to alternating current, they
can be manufactured in relatively small, modular units.
The compact size and cleanliness of SOFCs make them especially
attractive for urban settings like Tokyo, where 25 kw units
are already on line.
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Siemens Westinghouse tubular
solid oxide fuel cell. |
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In April 2000,
the U.S. Department of Energy announced that a SOFC-microturbine
cogeneration unit will be evaluated by the National Fuel Cell
Research Center and Southern California Edison. The fuel cell
was built by Siemens Westinghouse and the microturbine by
Northern Research and Engineering Corporation. According to
Siemens Westinghouse, the 220 kw SOFC operated for nearly
3400 hours, and achieved an electrical efficiency of about
53%.
Other companies working on SOFC technology
include Fuel Cell Energy (which acquired Global Thermoelectric's
Fuel Cell Division in late 2003). Cermatec is continuing
work on units for mobile power generation.
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©2004
Smithsonian Institution
(Copyright Statement)
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