Fuel sells if
it is cheap, clean, and carbon-free. That may become a maxim of this
millennium and an argument in favor of fuel cells. These devices may
be used to produce electricity in homes and cars using oxygen from air
and hydrogen from natural gas. Their chief waste product is harmless
water vapor.
At ORNL, Tim Armstrong
of the Metals and Ceramics (M&C) Division is leading the effort
to develop solid-oxide fuel cells and components using advanced materials.
This type of fuel cell is flexible in the fuels it can use; for example,
it can use natural gas in a process to convert chemical energy to electrical
energy. How is the system made and how does it work?
Hydrogen from
natural gas is passed over an anode (negative electrode) made of nickel
and yttria-stabilized zirconia (YSZ). The hydrogen atoms break apart
into positively charged ions and electrons. The electrons travel through
an external circuit to a cathode (positive electrode) made of the rare-earth
oxide lanthanum strontium manganate (LaSrMnO3).
Oxygen from the
air or carbon monoxide is collected at the cathode where the gas accepts
electrons to form negatively charged oxygen ions, which are passed through
a YSZ electrolyte separating the anode and cathode. The electrolyte
is heated to 600 to 1000°C by an electric heater to start the electrochemical
reaction. On arrival at the anode, each oxygen ion is discharged by
reacting with two hydrogen ions to form water. The heat from the electrochemical
reaction maintains the cell temperature, allowing the electric heater
to be turned off.
Iron aluminide
alloys developed in the M&C Division by C. T. Liu, Claudette McKamey,
and Vinod Sikka are candidates for solid-oxide fuel cell containment
vessels. ORNL Fossil Fuel Energy Program Manager Rod Judkins and Sikka
worked with Siemens Westinghouse to confirm the efficacy of iron aluminide
in the fuel cell application. Compared with the stainless steels currently
used for containment, the iron aluminide alloys are stronger and more
resistant to the simultaneous oxidizing and reducing conditions to which
containment vessels are exposed. Thus, iron aluminide containments are
expected to be more reliable and to last much longer. Mike Santella
of the M&C Division is working with industrial partners to develop the
technology for fabricating iron aluminide containment vessels.
"This solid-oxide
fuel cell can also provide high-quality waste heat that can be used
to warm the home or provide refrigeration and air conditioning," Armstrong
says. "Its only emissions are steam, trace amounts of nitrogen oxides
and sulfur oxides, and a small amount of carbon dioxide."
For powering many
homes at once while eliminating carbon dioxide emissions, the M&C group
has designed a power plant using a solid-oxide fuel cell and gas turbine
and incorporating ORNL's novel activated-carbon and membrane-separation
technologies and heat-exchange technologies, such as the carbon foam
that rapidly transports heat. The most efficient gas turbines available
today produce electricity using 60% of the energy in the fuel gas. Siemens-Westinghouse
has designed a solid-oxide fuel cell that is 60% efficient and a hybrid
fuel cell–microturbine plant in which waste heat from the fuel cell
is used to drive the microturbine. This combined-cycle power plant is
70% efficient.
"Based on the
results of our computer model, the ORNL design for a similar combined-cycle
power plant is 80% efficient," Armstrong says. "The reason is that we
combine our efficient heat-exchange technologies with membrane technologies
for separating hydrogen and carbon monoxide from natural gas for use
in the fuel cell." In addition, the power plant allows carbon sequestration
because ORNL's carbon fiber composite molecular sieve technology (see
Capturing Carbon the ORNL Way article)
can capture the carbon dioxide leaving the fuel cell as a waste product.
This gas can then be collected and used for enhanced oil recovery or
sequestered in geological formations.
Cars powered by electricity
from hydrogen fuel cells are being designed because they will eliminate
discharges of carbon dioxide, nitrogen oxides, and particulate emissions.
Such a "zero emissions" vehicle is a goal of the Partnership for a New
Generation of Vehicles, which involves the automobile industry and the
Department of Energy.
A proton exchange
membrane (PEM) fuel cell is the technology of choice in the automobile
industry for future electric cars because of its low-temperature operation
and rapid startup. PEM fuel cells have been plagued with problems, but
recent developments at ORNL may make this technology more feasible and
affordable.
The problem with
using today's PEM fuel cells to power cars is that their bipolar plates
(positive and negative electrodes), which are made of machined graphite,
are too heavy, too brittle, and too costly for use in automobiles. ORNL's
solution is to make bipolar plates from a carbon-fiber composite, which
is lighter, tougher, and cheaper than machined graphite.
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ORNL's
carbon-composite bipolar plate may be used for fuel cells in electric
cars.
|
Ted Besmann, James
Klett, Tim Burchell, and John J. Henry, Jr., all of the M&C Division,
have developed a method for making composite plates that includes chemical
vapor infiltration. Basically, carbon fibers are molded to make an electrode,
and methane is flowed over the plate at high temperatures to deposit
carbon that seals its surface pores. Because a fuel cell is a stack
of bipolar plates with electrolytes between, the porous plate surfaces
must be sealed to prevent leakage of hydrogen and oxygen from one cell
to another—a showstopper for fuel cells.
ORNL researchers have
shown that carbon-fiber composite plates not only can be made to perform
as well as graphite plates but also are half as heavy, may cost one-fifth
as much, are more conductive and corrosion resistant, and are easier
to manufacture.
Thanks to ORNL's
progress in this area, the fuel-cell car may be just around the bend.
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