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.