Environmentally Friendly Fridges - A new magnetic-cooling system could lead to more-energy-efficient refrigerators.

Technology Review
September 14, 2007

http://www.technologyreview.com/Infotech/19380/?a=f

By Prachi Patel-Predd

Modern coolers and fridges may not cause holes in the ozone layer like their pre-1994 counterparts, but they still use greenhouse gases that are warming the planet. Their compressors also consume a lot of energy: air conditioners and refrigerators used about 340 billion kilowatt hours in 2005--nearly 30 percent of the total energy used in U.S. homes.

Researchers at the Risoe National Laboratory, in Roskilde, Denmark, are now one step closer to building a magnetic-cooling system that promises energy-efficient, environmentally friendly, and completely silent fridges. Temperatures in conventional fridges swing between −20 and 20 ºC. Achieving this 40 ºC temperature span is one of the most significant challenges with magnetic refrigeration. The Danish researchers have built a refrigerator that can vary temperature by almost 9 ºC.

This is an important step toward practical temperature spans of 40 ºC, says Nini Pryds, a senior scientist at Risoe who is leading the work. The research team is now working with Danfoss, one of the largest compressor manufacturers in the world, to build a commercial prototype; the company says that it should be ready by 2010.

Magnetic-cooling technology exploits materials that heat up when exposed to a magnetic field and cool down when the magnetic field is removed. As the material cools down, it pulls heat out of its surroundings. The larger the difference between the hottest and coldest temperatures achieved under the influence of a magnetic field, the better the material is at cooling.

Magnetic coolers have been used for years in laboratories for cryogenic temperatures tens of degrees below zero. In 1995, Ames Laboratory, in Iowa, demonstrated the first magnetic refrigerator that cooled contents in a room-temperature environment. The company used the metal gadolinium.

Since then, researchers have found many other materials that work at room temperature. The problem is that the temperature swings in all these substances is only a few degrees. "Achieving a large change of temperature is easy if you use a superconducting magnet," Pryds says. But superconducting magnets are large and require cooling themselves, making them impractical for everyday appliances such as household fridges and air conditioners. For these applications, he says, "the only way to go is a permanent magnet." Ideally, it should be a small, cheap magnet with a field of less than one tesla.

Getting large temperature spans with a permanent magnet calls for some clever engineering. Typically, it means using cooling liquids such as water. The material, with water circulating around it, is alternately placed in and out of a magnetic field. When it's in the field, it heats up. The circulating water draws heat from the material and transfers it to a heat sink. Then the magnetic field is removed, and the material, which was already being cooled by the water, cools down even more. As it cools, it absorbs heat from the water, making it cold enough to be used as the refrigerator. This hot-cold cycle is repeated over and over.

Putting the different pieces--material, magnets, liquid cooling--together in a practical magnetic refrigerator is tough. Researchers need to design a system that gets at least a 40 ºC temperature change and enough cooling power--fridges currently have powers of as much as 150 watts--using a permanent magnet with a magnetic field less than one tesla. That requires a delicate balance between the system's parameters. For instance, as researchers expand the temperature span, the cooling power might go down, or the system may need more energy. "It's an engineering nightmare," says Ames Laboratory researcher Karl Gschneidner, a pioneer in magnetic cooling.

But the rewards will be plenty. Magnetic refrigerators will be much more energy efficient than conventional fridges because they only need energy to circulate the water. "The energy consumption of magnetic refrigerators [should] be as much as 60 percent lower than traditional refrigeration," Pryds says. Also, unlike conventional fridges, magnetic systems do not need refrigerants such as hydrofluorocarbons, which are potent greenhouse gases.

Pryds is confident that his group's work will lead to commercial magnetic fridges. Like other research teams, the Risoe group is using the water-cooling design. But while most research teams are using gadolinium powder, the Danish researchers use plates made from a ceramic material containing lanthanum, strontium, calcium, and manganese. Pryds says that "ceramics are chemically stable; they don't corrode in corroding fluids such as water." The ceramic plates should also be easier to manufacture on a large scale. The combination of ceramic material and the researchers' final refrigerator design--which is not yet public--could lead to practical success, he says.

The researchers face some tough competitors, though. Ames Laboratory researchers, working with Milwaukee-based Aeronautics Corporation of America, have made systems with temperature spans of 25 ºC and 95 watts of cooling power using 1.5-tesla magnets. Andrew Rowe and his colleagues at the University of Victoria, in Canada, have made 15-watt cooling systems with temperature spans of 30 ºC. Meanwhile, researchers at Chubu Electric Power and Toshiba, in Japan, have gone down to about 0.8 teslas to get a 10 ºC span.

Things are looking up, Gschneidner says, and in another 5 to 10 years, magnetic fridges should be on the market. Many research groups are now working on magnetic refrigerators, making better materials and coming up with better system designs. Also, adds Rowe, permanent magnets are getting smaller and cheaper. "The basic principles have been shown and demonstrated," he says. "Magnetic refrigeration works. Now we need some hard thinking [and] good designs, and hopefully these things will come together."