Industry Market Trends

Getting Closer To Magnetic Refrigeration

July 7, 2010

Imagine a refrigerator that uses 20 to 30 percent less electricity, is quieter, offers more space for groceries, and uses no hydrofluorocarbons. Now, imagine its potential taking into consideration that a single refrigerator chews up about eight percent of the electricity used in a typical household, and that 15 percent of energy use worldwide is related to cooling in some way: refrigeration, freezing, air conditioning or cooling computers or machinery.

Well, for now, all you can do is imagine this, since such a refrigerator hasn't been developed for household use. Yet.

Scientists must first gain a better understanding of a phenomenon called the giant magnetocaloric effect, in which a changing magnetic field in a material causes its temperature to drop precipitously. The effect, first observed in the late nineteenth century by German physicist Emil Warburg, is a magnetic-thermodynamic phenomenon in which a material exposed to a changing magnetic field can experience a precipitous drop in temperature. But not just any material: the material must be one that is particularly suitable to the application of a changing magnetic field. The chemical element gadolinium, a rare-earth mineral, has been used experimentally because of its paramagnetic nature. (For the same reason, the element is also used in intravenous MRI contrast agents.)

Basically, by taking a paramagnetic substance like gadolinium and applying a decreasing externally-applied magnetic field, the material's magnetic domains become "disoriented" from the magnetic field, which causes it to lose its thermal energy. If the material is isolated so that no re-migration of thermal energy back into the material can take place, a steep drop in temperature will occur. (The physics behind the process is complex and involves a brain-twisting thermodynamic process known as adiabatic demagnetization.)

The effect has the potential to produce extremely low temperatures...below one degree Kelvin, which begins to approach absolute zero. Of course, it's doubtful anyone needs to keep their groceries quite this cold, so it shouldn't be much of a stretch to apply the process to the cooling temperatures required for standard refrigeration, freezing, air conditioning and cooling computer and industrial equipment.

The idea is to use the magnet within the refrigerator as the source of cooling, much like old-fashioned ice boxes used a large block of ice in a compartment on the top. The upside is that the refrigerator would be quieter, more compact (leaving more space for leftover Chinese food, beer and varied containers of mysterious, fuzzy gray items) and use 20 to 30 percent less electricity than today's traditional vapor compression refrigerators. Additionally, the units would also not require hydrofluorocarbons, rather nasty substances that contribute greenhouse gases and that are standard in today's refrigerators.

Today's air conditioners and cooling systems for computers are also not very environmentally friendly, and would benefit from the further development of magnetic refrigeration. During the summer months, it's estimated that refrigeration and air conditioning account for roughly half of the United States' energy use.

"It's a very promising concept. But to make it a reality, we first must learn in detail what's happening inside materials as they undergo the giant magnetocaloric effect," said Sujoy Roy, a physicist with Lawrence Berkeley National Laboratory, one of the organizations working to further the technology.

As with many promising concepts, the technique is still impractical for household appliance use, and is currently feasible only in laboratory and experimental settings. Currently, the magnets need to be cooled to the temperatures of liquid nitrogen or liquid helium, which makes it rather cost-prohibitive, as does the use of expensive gadolinium.

The other problem is that rare earth minerals such as gadolinium, which thus far have been necessary to the process of magnetic refrigeration, are...well, rare. The minerals, which are used in a lot of high-tech products, including your iPod, are largely supplied by the Chinese today, which makes a lot of people nervous, particularly since the Chinese have begun limiting production and increasing export tariffs. China is generally believed to hold more than half of all proven rare-earth reserves.

The gadolinium problem was partly solved by researchers at the University of Cambridge when they developed an alloy comprised of less expensive cobalt, manganese, silicon and germanium as the paramagnetic material. As a result, the race is on to develop paramagnetic alloys made from more readily available (and cheaper) substances, which so far hasn't been particularly feasible, though a team of researchers at the Imperial College London has furthered the science greatly by developing a better way of analyzing exactly what happens to different materials at the molecular level when the materials are magnetized and de-magnetized, which has made the search for the best alloys suited for household use of the process faster and more efficient. Researchers believe that this work will ultimately allow them to custom-create a material by building it from the microstructure up that will respond to the magnetocaloric effect for maximum efficiency. Said Imperial College London's researcher Dr. Lesley Cohen, "This is vitally important because finding a low-energy alternative to the fridges and air conditioning systems in our homes and work places is vital for cutting our carbon emissions and tackling climate change."

The next step toward viable consumer use, which was first achieved in 1997 in a proof-of-concept experiment led by metallurgist Karl Gschneidner at the Ames Laboratory at the University of Iowa, is to demonstrate the ability to achieve the effect in a room-temperature setting and without requiring a ridiculous amount of energy.

Currently, the technique is used commercially today only in cryogenics in lab settings, a situation many hope more research - and research funding - will change. This month, the U.S. Department of Energy handed out $76 million in stimulus funding to companies working to further energy efficiency; $1.5 million of the money was given to General Electric to study and hopefully develop magnetic refrigeration materials.

Scientists have also made progress in recent years in furthering refrigeration based on the electrocaloric effect, a phenomenon in which a material displays a temperature change under an applied electric, rather than magnetic, field.

But for now, and probably for the next decade until viable room-temperature solutions using the technology are developed, you'll be stuck keeping your mystery leftovers-turned-science-projects cool the old-fashioned way.

- Tracey E. Schelmetic

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