By Mark Bowman

Define the perfect fuel and it would most likely be one that burns cleanly, poses no harm to the environment and, above all, is renewable or in limitless supply.

Liquid hydrogen could prove to be close to a perfect fuel, but first scientists and engineers must jump a few technological hurdles.

One of the biggest hurdles, an efficient method of liquefying hydrogen, has been eliminated by recent developments at Ames Laboratory. Scientists have developed a highly efficient magnetocaloric material that makes magnetic refrigeration technology efficient enough to cheaply produce liquid hydrogen, very likely one of the first major commercial uses of magnetic refrigerators.

Conventional production methods for liquid hydrogen begin by using liquid nitrogen to lower hydrogen gas to minus 196 degrees Celsius (320.8 degrees Fahrenheit). A gas-compression system, similar to the one in your refrigerator at home, is then used to further reduce the temperature to minus 253 C (423.4 F). One drawback to this method, however, is that the inefficiency of the gas-compression cycle cannot economically produce less than five tons of liquefied gas a day. This limits the production sources of liquid hydrogen to large plants that are few and far between.

Karl Gschneidner Jr. hopes that this is where magnetic refrigeration could eventually fill in. Gschneidner is a senior metallurgist at Ames Lab and an Anson Marston distinguished professor of materials science and engineering at Iowa State University. Early this decade, Gschneidner, Ames Lab Associate Scientist Vitalij Pecharsky and fellow researchers began developing intermetallic compounds specifically for application in magnetic refrigerators.

Their latest discovery is a new class of alloys with significantly more cooling power than the best existing materials. The new materials are based on gadolinium, an element with two to three times the magnetocaloric effect of a typical ferromagnetic iron and a popular choice for low-temperature ranges. "This is a tremendous breakthrough," says Gschneidner. "I think it's going to put magnetic refrigeration in the market."

Magnetic refrigeration technology takes advantage of the magnetocaloric effect, the remarkable ability of a magnetic material to heat up in the presence of a magnetic field and cool when the field is removed. Magnetocaloric materials store heat energy in the way the atoms vibrate and in the way in which electrons spin within each atom. More heat energy increases the vibrations and also makes the spins more random. In other word, when the party heats up, things get a little crazy. Scientists refer to this "craziness" as entropy, which is a measure of thermodynamic disorder. When a strong magnetic field is applied to the coolant material, the magnetic moments of its atoms become aligned, making the system more ordered. The more ordered material has a lower entropy and compensates for the loss by heating up.

But when the strong magnetic field is removed, the party is forced to cool down. The magnetic moments return to their random directions, entropy increases and the material cools. Typically, the temperature of a material can drop by about 10 to 15 degrees C (52 to 59 F), depending on the magnetic field strength.

The temperature at which most of the change in magnetic entropy occurs is known as the material's ordering temperature or its Curie point. This is the point where the material changes from being ferromagnetic to paramagnetic, and the farther away from this point the weaker the magnetocaloric effect. The useful portion of the magnetocaloric effect usually spans about 25 degrees C (77 F) on either side of the material's Curie temperature. Therefore, in order to span a wide temperature range, a refrigerator must contain several different coolants arranged according to their differing ordering temperatures.

Gschneidner and Pecharsky found that they could tune the operating temperature (gradually lower the Curie point) of a gadolinium silicide compound (Gd5Si4) by substituting germanium (Ge) for silicon. This resulted in a new compound, Gd5Si2Ge2, which has a magnetocaloric effect about twice as large as gadolinium alone.

Additional work has revealed that Gd5Si2Ge2 is one of a family of compounds that exhibits a giant magnetocaloric effect and whose ordering temperature can be tuned from 30 Kelvin (-405.4 F) to near room temperature (290 K or 62.6 F) by adjusting the ratio of silicon to germanium.

"There are very few systems that will give you that temperature span," says Gschneidner. "It's unheard of."

That is music to the ears of Carl Zimm. Zimm is chief scientist at Astronautics Corporation of America in Madison, WI. He and his colleagues have been working with the researchers at Ames Lab and other Department of Energy research facilities since the early 1990s to develop magnetic refrigeration technology to the point where it can compete with conventional gas-cycle systems, and in some cases make them obsolete. "There are many places in the United States that produce smaller amounts of hydrogen as a byproduct of some other industrial process," says Zimm. "An efficient liquefaction system could turn those amounts into fuel instead of having them go to waste." Earlier this year, Astronautics unveiled an active magnetic refrigerator with unprecedented efficiency.

Zimm and other researchers nationwide see more than just the liquefaction of hydrogen and other gases on the horizon for magnetic refrigeration technology. Although the history of development for gas-compression cycle refrigerators has given it a head start, the efficiency of the new coolants makes magnetic refrigeration truly competitive with conventional gas-compression technology for the first time. Large-scale applications, says Zimm, could soon be developed. Examples include supermarket-sized refrigerators and freezers, air conditioning for large buildings, industrial chemical processing, and waste separation and treatment.

Also, the new coolants may eliminate the need for the superconducting magnets associated with earlier cryocoolers. This opens the way for small-scale applications of this technology, such as car and home air conditioners.

Another factor helping to heat up the development of magnetic refrigeration technology is the recent ban on chlorofluorocarbons (CFCs) and other environmentally harmful substances. Magnetic refrigeration doesn't use CFCs and, in the case of the Astronautics model, water is used as the heat transfer fluid. Only antifreeze is added to allow the Astronautics unit to reach temperatures below 273 K, the freezing point of water, and down to about 225 K (-54.4 F). Below that, helium gas is used as the heat transfer fluid.

Despite all its promise, magnetic refrigeration technology still has hurdles to overcome if it is to ever give conventional vapor-based technology a run for the money. For example, when it comes to a small temperature span, such as the range of temperature in cooling a home or car, the old standard still leads the race. Only for large temperature spans, such as those associated with liquefying gases, do small increases in efficiency make a big money-saving difference.

Continuing research into even more efficient coolant materials could narrow the gap, says Gschneidner. "The new knowledge will allow us to improve existing materials and point the way to new and better ones, which will ensure the success of magnetic refrigeration as a viable energy-saving and environmentally safe technology in the next century," Gschneidner says. "The limitations of magnetic refrigeration are only in the minds of scientists and engineers."

For more information:
Karl Gschneidner Jr., 515-294-7931 cagey@ameslab.gov
Carl Zimm, 608-221-9001 Zimm%astroatc.UUCP@cs.wisc.edu

Current research funded by:
DOE Advanced Energy Projects Program

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Last revision: 4/17/98 sd

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