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.”