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"Solid oxide fuel cells consist of these tubular units, composed of concentric electrodes separated by a layer of solid electrolyte," says Chris Cheh of Kinectrics Inc.     |
Nickel magazine, Mar. 02 -- Toronto is poised to become an early entrant into a new-age of power
generators based on fuel cell technology A 250 kilowatt solid oxide fuel cell demonstration power plant using
more than 600 kilograms (kg) of nickel, is scheduled to start up in Toronto, Canada by July 2002. Designed by
Siemens Westinghouse, the plant is being constructed at facilities of Kinectrics Inc., a subsidiary of
Ontario Power Generation Inc. (OPG). The $18-million plant is co-financed by Siemens Westinghouse, OPG, the
U.S. Department of Energy (D.O.E.), and the Canadian government.
Solid oxide fuel cells (SOFCs) are among five types of fuel cells, all of which do the same thing chemically,
but in different ways. Fuel cells electrochemically combine hydrogen and oxygen to produce electricity and
water as a byproduct. Unlike batteries, fuel cells produce electricity as long as fuel is supplied to
them.
SOFCs operate in the region of 1,000°C, and get their name from the fact that they utilize dense zirconium oxide as a solid electrolyte. A single SOFC consists of a vertical ceramic tube, closed on the bottom end, composed of concentric layers -- an inner cathode, an outer anode, and an electrolyte between. The three layers include the following materials:
a doped lanthanum manganese oxide cathode, LaMnO3, on the inside surface;
an yttrium-stabilized zirconia electrolyte, YSZ, in the middle, and
a cermet of nickel metal and YSZ, as an anode, on the tube's outside. The cell is 1.7 metres long with an
inside diameter of 2.2 centimetres (cm). Process air is injected through an alumina tube concentric with the
cell and it flows down to the bottom closed end and back up the annualar space between the two tubes (see
accompanying diagram). Oxygen in the air is ionized as it flows along the inner cathodic surface of the cell,
the oxygen ions pass through the intermediate electrolyte layer and react, at the outer anodic surface, with
hydrogen and carbon monoxide, to produce water, carbon dioxide and electricity. The hydrogen and carbon
monoxide are generated from 'reformed' natural gas, supplied as the fuel to the plant, and are directed along
the outer surface of the tube.
A single SOFC generates about 150 watts of power at about 0.65 volts. For a power plant producing kilowatts
of power, many individual cells must be connected in an array of cell bundles, with 24 cells in each bundle.
For the 250-kW plant in Toronto, for example, the complete cell "stack" consists of 2,300 cells, arranged in
96 bundles. All these cells are interconnected electrically by nickel "felts" of electrolytic-grade nickel.
The stack, in turn, is connected to the output of the generator by DC bus bars of
N02200 and to electrical contact plates, also of N02200. The total nickel requirement of the SOFC stack
for the Toronto plant is about 400 kg.
| Nickel Utilization in a 250 kW SOFC Stack | ||
| Component | Material | Ni Weight, kg |
| Ni Cell anode | Ni powder | 115 |
| Electrical contacts, felts | Electrolytic nickel | 213 |
| DC bus bars | N02200 | 48 |
| Electrical contacts, plates | N02200 | 20 |
| Total Nickel in 250 kW SOFC Stack | 397 | |
| Nickel Utilization for 250 kW SOFC Components | ||
| Preformer recirc. Loop | N06600 | 88 |
| Stack Liner | N06230 | 19 |
| Internal Foil Barriers | N06230 | 22 |
| Fuel Manifolds | N06600 | 85 |
| Total Nickel in 250 kW Stack Components | 214 | |
| Total Nickel in 250 kW SOFC Power Module | 611 | |
Several components directly associated with the stack also use nickel, in the form of N06600 or N06230 alloys. These include the pre-reformer recirculation loop, the stack liner, internal foil barriers, and fuel manifolds. Collectively, these add another 214 kg of nickel to the fuel cell power module.
The fuel cell stack power module is only part of the complete operating power plant. The rest of it, the so-called 'balance of plant,' includes several additional major components such as the fuel processors, compressors, heat exchangers, and power conditioners, all of which utilize nickel as stainless steels and other alloys.
If this particular design proves successful, SOFC power plants will require about 3 kg of nickel per kilowatt of power output. As the market for SOFC power plants is projected to be 240 megawatts per annum by 2010, this translates into a requirement for about 750 tonnes of nickel per year within the next 10 years.
Fuel cells have major advantages over other methods of generating electricity in that they are quiet, odourless, relatively non-polluting and several times more energy-efficient. Conversion of fossil fuels to electrical energy can be as high as 60%, for example, and if byproduct heat, generated by fuel cells, is also utilized, as steam or hot water, overall fuel to energy utilization efficiency can rise to 80% or even higher.
SOFCs are a relatively recent entrant to the fuel cell field. Because fuel cells are not yet in common
commercial use, unit costs for prototypes or pilot production are still relatively high, but they will
diminish to more competitive levels with automated manufacture of production quantities.
By Dr. Gerald Crawford, a Toronto-based consultant to the Nickel Development Institute.
Photo: TOM SKUDRA/NiDI
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Peter J. Schürmann, Vice President |
