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