Cooling With Sound: An Effort to Save Ozone Shield
By MALCOLM W. BROWNE
Published: February 25, 1992
AS fears mount that global depletion of the atmospheric ozone layer may lead to catastrophic medical and economic consequences, scientists and engineers are hastening development of new refrigeration technologies that do not depend on ozone-destroying chemicals and may even be more efficient than conventional cooling machines.
One of these technologies abandons the complicated moving parts of conventional refrigerators and uses sound energy blasted into a kind of organ pipe to achieve cooling. Developers of sound-driven refrigerators at Los Alamos National Laboratory, N.M., and the Naval Postgraduate School at Monterey, Calif., say such thermoacoustic devices could chill nitrogen to its liquefaction temperature of minus 320 degrees Fahrenheit, or cool the air in a room or car by only a few degrees. One of these new refrigeration devices was tested aboard last month's space shuttle mission.
Another of the new refrigeration technologies, also under study at Los Alamos, subjects liquids, rather than the gases used in existing refrigerators, to cycles of compression and expansion to move heat away from the objects being cooled.
The principal disadvantage of substituting such novel systems for conventional gas compressors in refrigerators and air-conditioners is cost, scientists and engineers agree. The American economy is currently dependent on devices using conventional refrigeration equipment, which will be extremely expensive to replace.
E. I. du Pont de Nemours & Company, the largest American manufacturer of the chlorofluorocarbon (CFC) compounds that are responsible for ozone depletion, estimates that 5,000 American businesses valued at $28 billion and employing 700,000 workers produce goods or services dependent at least in part on CFC's. If these businesses were abruptly forced to scrap equipment designed to use CFC's, manufacturers say, the expense of investing in entirely new systems could cause staggering economic hardship.
Leaders in the refrigeration industry assert, moreover, that chemical substitutes for CFC's used in conventional refrigerators and air-conditioners will make these machines less efficient, bulkier and shorter-lived. The poorer efficiency of CFC substitutes will make it harder for refrigerator makers to comply with new Federal efficiency standards, they say.
At the same time, new alarms are sounded nearly every day about the danger of continued manufacture and use of CFC's and some substitutes as refrigerants, thermal insulators, solvents and aerosol propellants. Environmental groups argue that the earth's protective ozone shield risks catastrophic depletion, and that even President Bush's recent decision to ban CFC production in the United States by Dec. 31, 1995, falls far short of the needs of the situation.
Underscoring their concern, a report in the current issue of the journal Science confirmed that a 50 percent thinning of the ozone layer over Antarctica in 1990 had far-reaching biological effects. Dr. Raymond C. Smith and Dr. Barbara B. Prezelin of the University of California at Santa Barbara, and their colleagues at the university's San Diego and San Francisco campuses and other institutions, found a direct link between the amount of ultraviolet radiation leaking through the ozone shield and the decline of phytoplankton -- the microscopic organisms that constitute the base of Antarctica's food chain. Every aspect of phytoplankton life was depressed, and the reproduction rate fell as much as 12 percent during the maximum thinning of the ozone layer.
One of the main objections by environmental groups to some of the chemicals scheduled to replace CFC's -- HCFC's and HFC's -- is that they may themselves pose problems and dangers.
The chemical family called HCFC's, like CFC's, contains chlorine in a form that can destroy stratospheric ozone. But HCFC compounds are less stable than CFC's and tend to break down in the lower atmosphere before they can diffuse to high enough altitudes to destroy ozone. Members of a second chemical family, the HFC's, contain only hydrogen, fluorine and carbon atoms, with no chlorine, and are therefore presumed to be harmless to the ozone layer.
But many experts are concerned for other reasons.
"We are not at all confident that even as temporary replacements these chemicals are viable alternatives to CFC's," said Dr. Michael Oppenheimer of the Environmental Defense Fund. "Their chemistry in the atmosphere is not well understood, and it is essential to look farther down the road than these interim stop-gap measures."
Liz Cook of the Friends of the Earth, another environmental organization, said that "HCFC's should be regarded only as a last resort" and that HFC's, while not ozone destroyers, may contribute to global warming and create fire hazards. Research should be directed toward new refrigeration technologies, she said, since "refrigeration and air-conditioning are the main nuts to crack."
Last week the Washington-based Institute for Energy and Environmental Research issued a report charging that HCFC's are "three to five times more damaging than industry and government claim," and that HCFC-22, in particular, should be banned along with the CFC's.
Some experts have proposed returning to the refrigerant gases that were used during the 1930's before the invention of CFC's. These included sulfur dioxide, a toxic gas that liquefies under pressure; ammonia, a gas that irritates the lungs, and various hydrocarbon compounds that posed fire and explosion hazards if released. Many easily liquefied gases that might otherwise serve as refrigerants are undesirable because they cause rapid corrosion of metal parts and seals, destroying the compressors and valves essential to conventional refrigerators. The invention of CFC's was regarded as a breakthrough because they are nontoxic, nonflammable, noncorrosive and practically indestructible in normal use.
Most refrigeration engineers say no new chemicals can ever rival CFC's as refrigerants in conventional refrigerators. But other chemicals would work as well or better than CFC's in some of the new technologies under study. Refrigerator Workings
An ordinary refrigerator or air-conditioner moves heat from the inside of a freezer or bedroom using a gas that readily conducts heat and can be easily liquefied.
The cycle begins when an electrically driven pump compresses the "working gas," thereby heating it. The gas is passed through a system called a heat exchanger, in which heat is removed from the gas and dumped outside the refrigerator or house. Thus cooled, the pressurized gas condenses to a liquid. The pressure is then released so that the liquid can rapidly evaporate, a process that sharply cools it. The chilled liquid, passing through the coils of a freezer or chiller, draws heat from the latter, and the evaporated cool gas returns to the compressor to renew the cycle.
Machines that work on this principle, called Rankine-cycle refrigerators, contain many moving parts, are expensive to build, and typically use CFC's or chemically similar substitutes, which are often vented to the atmosphere when machines are scrapped.
By contrast, the principle of thermoacoustic refrigeration, pioneered at Los Alamos National Laboratory in the 1980's by John C. Wheatley, Thomas Hofler, Steven L. Garrett, Gregory W. Swift and Albert Migliori, is deceptively simple. Sound Moves Heat
The great beauty of a thermoacoustic refrigerator, Dr. Swift said, is that it has no moving parts, aside from the diaphragm of its loudspeaker, and can use environmentally benign working gases. Although the design of these devices is becoming increasingly complex and subtle, the underlying principle is simple: a loudspeaker at the end of a tube emits an extremely loud note that resonates inside the gas-filled tube to form what is called an acoustic "standing wave" in which individual molecules of gas are in rapid oscillating motion while the wave as a whole does not move at all. Organ pipes and other wind instruments set up similar standing-wave resonances.
The gas molecules move back and forth at the same frequency as that of the loudspeaker at the end of the pipe, and as they move they act as microscopic workers in a bucket brigade, carrying heat away from the thinner regions of the standing wave toward its thickest bulge, its antinode. If a metal plate is placed within this antinode, the plate absorbs heat from the gas molecules and can conduct it away from the refrigerator, leaving cooled gas in its place. The cool gas, in turn, can draw heat away from refrigerators or air-conditioners.
"The note played by the thermoacoustic loudspeaker, about in the mid-range of human hearing, is so loud that it would destroy living tissue exposed to it," Dr. Swift said. "But the sound is almost completely confined in the resonating tube. The noise you hear outside is less than the noise of an electric fan."
One thermoacoustic refrigerator, built under a Navy contract by Dr. Garrett and Dr. Hofler, both now at the Naval Postgraduate School, was tested aboard a space shuttle flight last month, where it reduced the temperature of an object under test by 212 degrees Fahrenheit. By modifying the design, said John Adeff, a member of the research group, the device could be made to move much more heat over a smaller temperature range, a requirement of household refrigerators and air conditioners.
Just such a project is already far advanced, Dr. Garrett said. "NASA ruined a $50 million biological experiment in space because of a faulty refrigeration system," he said. "That led to a contract for us. What we're now building for General Electric under a NASA contract is an acoustic space refrigerator that could chill beer as well as experiments. It's virtually a household refrigerator."
Scientists still must probe the basic physics underlying thermoacoustic refrigeration, and its widespread use in commercial appliances is unlikely until the next century. "But there's good reason to believe it will work successfully and without the environmental disadvantages of today's systems," Dr. Swift said. "With $20 million and a few years, we could make thermoacoustic refrigeration a commercial reality." Testing With Liquids
The Los Alamos group is currently working on an entirely different refrigeration system, in which liquids rather than gases are compressed under enormous pressure and used to pump heat away from the interior of refrigerators or rooms.
"We generally think of water and other liquids as incompressible," Dr. Swift said, "but as a matter of fact, many liquids can be compressed under high pressure, and can be even more efficient than compressed gases in moving heat from one place to another."
In one version of the system, which was invented in the 1920's by an American, John Malone, Dr. Swift uses liquid carbon dioxide, an environmentally benign substance, at a pressure of 2,000 pounds per square inch. A Malone refrigerator, Dr. Swift explained, works on much the same principle as conventional refrigerators, except that it must be much more robust to confine the enormous pressure of its working fluid. Compression heats the liquid carbon dioxide as it would a gas, the heat is moved away, and the pressure is released, causing the liquid to cool.
"A Malone-cycle refrigerator must be much heavier and bulkier than ordinary units, and would probably be most useful as a plant refrigerator or for chilling water used to cool entire buildings," he said.
"Whatever happens," Dr. Garrett said, "the ozone depletion crisis has finally forced people and funding agencies to take a fresh look at a fascinating and often overlooked field of physics -- one that may soon begin solving a lot of big environmental problems."
Diagram: "Cool, Cool Sounds" Harnessing the motion of molecules set dancing to sound, scientists are creating "natural" refrigerators. The sound wave forces gas molecules in a tube into a dense, compressed mass at one end, heating them up. A plate in the tube absorbs heat from the molecules as they shuttle along it. This heat can be conducted away from the refrigerator, leaving cooled gas in its place. (Source: Jay Adeff)