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Magnetic refrigerator successfully tested

Ames Laboratory develoments help push boundaries
of new refrigeration technology

(left to right) Vitalij
Pecharsky, David Jiles, and Karl Gschneidner are helping push
the advancement of magnetic refrigeration. Earlier this fall,
the first room-temperature, permanent-magnet magnetic
refrigerator was successfully tested by Astronautics
Corporation of America, a research partner in the project with
Ames Laboratory.

Using materials developed at the U.S. Department of Energy's Ames
Laboratory, researchers have successfully demonstrated the world's
first room-temperature, permanent-magnet, magnetic refrigerator. The
refrigerator was developed by Milwaukee-based Astronautics
Corporation of America as part of a cooperative research and
development agreement with Ames Laboratory.

Instead of ozone-depleting refrigerants and energy-consuming
compressors found in conventional vapor-cycle refrigerators, this
new style of refrigerator uses gadolinium metal that heats up when
exposed to a magnetic field, then cools down when the magnetic field
is removed.

"We're witnessing history in the making," Ames Laboratory senior
metallurgist Karl Gschneidner Jr. says of the revolutionary device.
"Previous successful demonstration refrigerators used large
superconducting magnets, but this is the first to use a permanent
magnet and operate at room temperature."

Initially tested in September at the Astronautics Corporation of
America's Technology Center in Madison, Wis., the new refrigerator
is undergoing further testing. The goal is to achieve larger
temperature swings that will allow the technology to provide the
cooling power required for specific markets, such as home
refrigerators, air conditioning, electronics cooling, and fluid
chilling.

According to Gschneidner, who is also an Anson Marston
Distinguished Professor of materials science and engineering at Iowa
State University, the magnetic refrigerator employs a rotary design.
It consists of a wheel that contains segments of gadolinium powder —
supplied by Ames Laboratory — and a high-powered, rare earth
permanent magnet.

The wheel is arranged to pass through a gap in the magnet where
the magnetic field is concentrated. As it passes through this field,
the gadolinium in the wheel exhibits a large magnetocaloric effect —
it heats up. After entering the field, water is circulated to draw
the heat out of the metal. As the gadolinium leaves the magnetic
field, the material cools further as a result of the magnetocaloric
effect. A second stream of water is cooled by the gadolinium. This
water is then circulated through the refrigerator's cooling coils.
The overall result is a compact unit that runs virtually silent and
nearly vibration free, without the use of ozone-depleting gases, a
dramatic change from the vapor-compression-style refrigeration
technology in use today.

"The permanent magnets and the gadolinium don't require any
energy inputs to make them work," Gschneidner says, "so the only
energy it takes is the electricity for the motors to spin the wheel
and drive the water pumps."

Though the test further proves the technology works, two recent
developments at Ames Laboratory could lead to even greater advances
on the magnetic refrigeration frontier. Gschneidner and fellow Ames
Lab researchers Sasha Pecharsky and Vitalij Pecharsky have developed
a process for producing kilogram quantities of
gadolinium-silicon-germanium alloy using commercial-grade
gadolinium. The Gd5(Si2Ge2) exhibits a giant magnetocaloric effect
that offers the promise to outperform the gadolinium powders used in
the current rotary refrigerator.

When the alloy was first discovered in 1996, the process used
high-purity gadolinium and resulted in small quantities (less than
50 grams). However, when lower-quality commercial-grade gadolinium
was used, the magnetocaloric effect was only a fraction of that
found in the original alloy, due mainly to interstitial impurities,
especially carbon. The new process overcomes the deleterious effect
of these impurities, making it viable to use less-expensive
commercial-grade gadolinium to achieve roughly the same
magnetocaloric effect as the original discovery.

At the same time, Ames Lab researchers David Jiles and Seong-Jae
Lee, along with Vitalij Pecharsky and Gschneidner, have designed a
permanent magnet configuration capable of producing a stronger
magnetic field. The new magnet can produce a magnetic field nearly
twice as high as that produced by the magnet used in the initial
refrigerator; this is an important advance since the output and
efficiency of the refrigerator is generally proportional to the
strength of the magnetic field. The group has filed patent
applications on both the gadolinium alloy process and the permanent
magnet.

"These are important advances, but it will require additional
testing to see how much they will enhance refrigeration
capabilities," Gschneidner says. "Progress (in this field) is
measured in small steps, and this is just another of those steps.
However, we've come a long way since first announcing the giant
magnetocaloric alloy five years ago."

###

by Kerry Gibson

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