New Prototype Magnetic Refrigerators Hold Commercial Promise
Rotating ring (center), roughly the diameter of a compact disk, cycles powdered magnetic material in and out of a gap in the powerful magnet at rear.
Conventional refrigerators work by compressing and expanding a gas as it flows around the cooling unit, but this process is not especially efficient. Refrigeration currently accounts for 25% of residential and 15% of commercial power consumption in the US In the past it has also used gases harmful to the environment.
In contrast, magnetic refrigeration devices have high efficiency even at a small scale, enabling the development of portable, battery-powered products. In fact, Stephen Russek of Aeronautics Corporation, estimates that when magnetic refrigerators are fully developed, they could reduce energy usage by approximately $10 billion per year, along with significant reductions in carbon dioxide emissions. In addition, magnetic refrigeration doesn't use ozone-depleting or global warming gases.
The enabling technology is based upon the magnetocaloric effect, first observed in 1881: an efficient magnetocaloric material warms when placed in a magnetic field and reversibly cools back down when it is removed from the magnetic field.
The first magnetic refrigerator was demonstrated in 1933, and magnetic refrigeration has been used in many laboratories to cool within a thousandth of a degree above absolute zero. Ames Laboratory became involved in 1991, according to senior metallurgist Karl Gschneider, Jr., when Aeronautics asked his group to design less expensive magnetic refrigerants for the liquefaction of hydrogen. They produced materials that were 10% to 30% more efficient than those then in use, and based on this work, Aeronautics demonstrated a prototype unit in November 1996.
A second breakthrough occurred in 1997, when Ames Lab scientists discovered that the giant magnet- ocaloric effect in gadolinium-silicon-germanium alloys was two to 10 times larger than in existing prototype refrigerants. These alloys improve the efficiency of large-scale magnetic refrigerators, but also open the door to new small-scale applications, such as home and automotive air conditioning.
However, initially the process used more expensive high-purity gadolinium and resulted in small quantities of less than 50 grams of the Gd-Si-Ge alloys. Gschneider and his cohorts developed a new process for producing kilogram quantities of the alloy using inexpensive commercial-grade gadolinium, achieving nearly the same magnetocaloric effect as the original discovery. Meanwhile, other Ames Lab researchers have designed a permanent magnet configuration capable of producing a stronger magnetic field, an important advance since the output and efficiency of the device is proportional to the strength of the magnetic field.
Building on its previous demonstration of a room temperature, superconducting-magnet based device, Aeronautics Corporation has now demonstrated the first room temperature, permanent-magnet based rotary magnetic refrigerator. The rotary design consists of a wheel containing gadolinium and a strong permanent magnet. The wheel passes through a gap in the magnet where the magnetic field isconcentrated, and the gadolinium heats up. While still in the field, water is circulated to draw the heat out of the material and reject the heat through the hot heat exchanger. As the material leaves the magnetic field, it cools further. While the material is out of the field, a stream of water is cooled by the material and circulated through the refrigerator's cold heat exchanger, removing heat from the object to be cooled.
Aeronautics is not the only company committed to the development of magnetic refrigeration. Scientists at Japan's Chubu Electric, in cooperation with Toshiba Corporation, have also succeeded in developing a rotating magnetic refrigerator with permanent magnets.
The design schematic is similar to that of the Aeronautics, with an increase in cooling capacity by a factor of 1.5 and a 1/3 decrease in driving power. Chubu's device is also about a twentieth the size of earlier prototype refrigerators employing superconducting magnets. Potential commercial applications of such refrigerators include air conditioning, food preservation, air dehumidification, and beverage dispensing.
However, Russek says that the most likely early applications will be industrial in nature: chilling of process fluids for food, chemicals, industrial gases and pharmaceutical production, as well as refrigerated transport and cooling of electronics. "We firmly believe this could be a great new global business," he says.