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Sounds that cool spark hot ideas.

The developing technology of thermoacoustics is showing that sound energy offers a relatively simple and environmentally friendly means to drive refrigerators, air conditioners, and other cooling systems.

A sound system has been catching the ear of refrigeration people, physicists, engineers, and business people the world over. No, it's not the stereo of the future. It's a system that employs a loudspeaker to power a refrigerator. Based on an approach called thermoacoustic refrigeration, it makes use of the energy in sound waves to provide cooling.

Most of us rely on cooling systems to preserve food and to provide comfort control at work and at home, as well as in cars, planes, and trains. There are numerous other applications, ranging from cooling tiny computer chips to large, industrial machines. While these are important applications, there is concern that the already enormous energy demand for cooling purposes will only increase as nations raise their level of technological development. In addition, there is the concern that many established cooling systems operate, with refrigerants that, if leaked to the atmosphere, may contribute to ozone depletion and global warming.

In response to these concerns, new technologies that are both energy efficient and environmentally friendly are being sought. Impetus in this direction is also being driven by international bodies that have initiated agreements to phase out the production of refrigerants with high ozone-depletion potential (ODP). After the expenditure of hundreds of millions of dollars, new refrigerants have been found that have zero ODP, but they all have nonzero global-warming potential (GWP), suggesting that they, too, may be phased out in the future. A suitable new refrigerant must satisfy numerous additional criteria related to such factors as the temperatures desired, toxicity, flammability, materials compatibility, and cost. The list of potential refrigerants becomes very short when all these factors are considered, making it clear that other technologies should be explored.

In this light, the use of sound energy to drive cooling systems is attractive because it is inherently clean and simple, working with environmentally friendly fluids and materials (having zero ODP and zero GWP). A variety of prototypes have shown that this approach works in real systems, though typically not yet as well as computer models predict. One day, refrigerators, air conditioners, and ice chests might run on sound produced by electrical power or by heat from the sun.

Looking back

People first recorded their observations of naturally occurring thermoacoustic effects almost 150 years ago, when they noticed that a cool glass tube would often sing when brought in contact with a hot bulb This effect, wherein heat energy is converted to sound energy, is the reverse of a thermoacoustic cooling process. Researchers began investigating these effects in the 1960s, and a Swiss scientist, named Nicholas Rott developed most of the theory that is now used to model thermoacoustic systems.

The development of thermoacoustics for practical refrigeration began in the early 1980s at Los Alamos National Laboratory (LANL) in Los Alamos, New Mexico. The first carefully instrumented thermoacoustic cooler system was completed in 1985 as part of Tom Hofler's doctoral work at the University of San Diego, with guidance from LANL researchers Gregory Swift and the late John Wheatley. It achieved temperatures near -80 [degrees] C (-112 [degrees] F).

Since then, several generations of prototypes have been built, including the first domestic refrigerator (in the early 1990s) at the Naval Postgraduate School (NPS) in Monterey, California. Each new prototype achieved some new performance targets and provided a wealth of information used in developing the next. At NPS, early efforts focused primarily on prototypes providing low-temperature, low-capacity cooling, while at LANL, much of the work involved making large, thermoacoustic engines, using heat to generate sound, and sound to generate electrical power [see "Cooling With Sound," The World & I, September 1992, p. 292].

The development of thermoacoustics for commercial applications has accelerated since researchers at LANL made software available that performs calculations and predicts the behavior of thermoacoustic systems, enabling an increase in the number of newcomers participating in the field. Thermoacoustic refrigerators have been or are now being developed for a variety of purposes, such as cooling medical supplies, tropical fruit cargo, and seismic instruments in the earth's crust, as well as for liquefying natural gas.

About sound

About 300 years ago, Newton surmised that the transmission of sound through a fluid was characterized by "pressure pulses" moving through particles of the fluid. He was right: Longitudinal pressure waves travel through an elastic medium at the speed of sound, jostling fluid elements back and forth (at fluctuating velocities much slower than the speed of sound) in the direction of the wave motion. (This differs from transverse waves in a string, where string motion is perpendicular to wave propagation.) The local changes in pressure, which are small compared with the total pressure in the fluid, lead to changes in local density and temperature, and these changes enable thermoacoustic phenomena.

There are different kinds of sound-wave fields. Waves that are confined to small spaces are reflected at the boundaries, leading to the formation of "standing" waves. On the other hand, if most of the sound energy is absorbed at the ends and reflection is negligible, the result is a "traveling" wave field. The timing (or "phasing") between pressure fluctuations and displacement of fluid particles depends upon the type of wave. This phasing is important in systems that employ sound for useful thermodynamics. Thermoacoustic systems primarily employ standing waves, while conventional Stirling systems (see Heating and Cooling Cycles) employ the phasing of traveling waves.

Sound does not travel indefinitely--it dissipates. The dissipation is quickest near solid boundaries, because of(a) friction between fluid layers, and (b) heat transfer caused by temperature fluctuations of fluid elements next to the solid. In designing thermoacoustic systems, we seek to reduce sound dissipation by the former mechanism and to enhance it by the latter. Useful heat pumping can be done only near solid boundaries in thermoacoustic systems.

A simple thermoacoustic heat pump has an acoustic driver (loudspeaker-like power source) attached to one end of a rigid tube that is sealed at the other end. The tube, whose diameter varies along its length, contains (a) a stack of thin, parallel plates, which functions as a heat-pumping element, and (b) hot and cold heat exchangers, perhaps with finned tubes. The sealed tube also contains the working fluid, which is normally an inert gas such as helium or argon or their mixture. The gas is pressurized to about 300 pounds per square inch (20 atmospheres).

People often think they could solve some noise problems by putting such sounds to useful work. But thermoacoustic systems use pure tone (single-frequency) sound, often (but not always) around 200 hertz (near middle C). This tone is called the system resonance frequency--the frequency at which the response is highest for the given power input. If more tones are added, the performance degrades. The whole system acts as a resonator. By using damping techniques, however, the exterior noise level will be below that of conventional systems.

As the driver operates at the resonance frequency, it produces relatively large pressure fluctuations that oscillate the fluid back and forth. Only the fluid near the stack participates in the relevant thermoacoustic effects. Each acoustically driven set of fluid particles close to a surface in the stack undergoes a thermodynamic cycle (see sidebar), absorbing work while repeatedly mow ing a bundle of heat energy from a cooler location in the stack to a warmer location. When many gas particles contribute to the process, significant amounts of heat can be moved, making one side of the stack cold and the other side hot.

The thermoacoustic system described above has a fairly simple appearance, but designing a machine raises many questions. For instance, what should the frequency of sound in the pump be? What gas should be used, and at what pressure? What would be the best designs for the stack, the heat exchanger, and the container tube? How should the acoustic system interact with the mechanical and electrical systems of the driver? These and other questions have complicated answers. To design a useful system, computer modeling is required to figure out the many trade-offs.

Some current sound efforts

Thermoacoustic projects are ongoing at various laboratories around the world, and some research teams have produced fully operational prototypes. For instance, a partnership between Denver's Cryenco, a large-scale manufacturer of equipment for gas liquefaction and transportation, and LANL (funded in part by the U.S. Department of Energy) has developed a 40-hertz acoustic cooler for liquefying natural gas. This process requires lowering temperatures to near -250 [degrees] F. The conventional process involves large, expensive, nonportable equipment. By contrast, the acoustic cooler, which uses both thermoacoustic (heat-driven) and Stirling technologies, has no moving parts, no exotic materials, and no tight tolerances, making it extremely reliable and low cost. It is also much smaller, relatively portable, and well suited for low- and high-capacity systems.

The liquefaction process makes it easier to purify and transport natural gas, and it can also be applied to recover landfill gas, other biowaste gases, and gases that escape from oil wells and other facilities. Cryenco and LANL have proved the technology with a 100-plus gallons/day system, and they are working to develop a second-generation prototype liquefier that will burn about 20 percent of the gas (compared with 10-15 percent in conventional systems) to liquefy the other 80 percent, at a rate of about 10,000 gallons/day (a refrigeration power of about 140 kilowatts).

The U.S. Navy has funded much of the thermoacoustics research in the United States. At NPS, scientists and engineers have made several thermoacoustic machines with innovative designs to support systems on the space shuttle Discovery and the warship USS Deyo. One of their significant contributions involves the resonator shape: By introducing certain changes in the cross section, they showed that shock waves characteristic of nonlinear systems can be suppressed, In building a prototype called TADTAR (thermoacoustically driven thermoacoustic refrigerator), a team led by Hofler used heat rather than electricity as the input power source, which was converted to acoustic energy. NPS has built some coolers that a large temperature drop, including temperatures below -150 [degrees] F.

Another group of researchers, at Pennsylvania State University, is led by Steven Garrett, who was originally at NPS. Garrett's lab (funded in part by the U.S. Navy) is designing, fabricating, and testing refrigeration systems for military and commercial applications. It has produced operational prototypes that provide cooling capacities ranging from a few watts to hundreds of watts. One chiller called TRITON is designed to provide auxiliary cooling for Navy ships. It has a cooling capacity of three tons (about 10 kilowatts), meaning that it can convert three tons of water at 32 [degrees] F to ice at the same temperature in one day. Garrett expects this first-ever large-scale thermoacoustic cooler to have an efficiency within 30 percent of that of conventional systems (and become better in future generations) and to have comparable size and weight. The Penn State team has also built a solar-powered thermoacoustic engine that produces 120 decibels of sound, which may drive a thermoacoustic ice maker in the near future, and another unit that looks and operates much like a domestic refrigerator.

For my doctoral work at Purdue University, I have been working under the guidance of mechanical engineering professors James Braun and Luc Mongeau. Our research efforts have aimed at developing an optimizing design tool (software) and a low-cost prototype. This research seeks to validate modeling and to prove the technology for a light air conditioning system. While the current prototype has achieved some cooling, it is not yet fully operational.

The applications of thermoacoustics are being extended in various directions. At the University of Utah, systems have been built for use in cooling electronic microchips. Several other laboratories are working on innovative improvements to heat exchangers, electrodynamic drivers, and system configuration. They include research teams at Ford Motor Company, the University of Mississippi, and Johns Hopkins University.

Sound ideas

Is cooling by sound a sound idea? It's certainly appealing for a variety of reasons. Besides being environmentally friendly, sound can be used to make a refrigerator with no moving parts, except perhaps for a moving coil and a piston or diaphragm vibrating at low amplitudes. No lubricants or sliding seals are required, promising high reliability. A significant performance advantage is that there is no on/off operation of a compressor--to lower the temperature, just "crank up the volume." The pressure changes experienced in the system are actually quite a bit lower than those experienced in conventional vapor compression systems, helping reduce noise problems.

On the other hand, thermoacoustic systems have an important disadvantage relative to vapor compression systems: They typically require extra heat exchangers and a secondary fluid for heat transfer between the gas in the system and the air outside. Moreover, designing the driver, the stack, and the heat exchangers presents a number of nontrivial challenges. For instance, the electrical and mechanical characteristics of the driver must be "tuned" to cooperate well with the acoustic system. Garett's team at Penn State, in conjunction with a company called CFIC Inc. (of Troy, New York), has developed a sophisticated driver at about 88 percent efficiency, but the prototype is still fairly expensive.

Computer models (verified in some cases) suggest that thermoacoustic systems can be made to operate efficiently and can be competitive in several markets, including domestic refrigeration. But these predictions have not yet been realized with the few existing prototypes. In addition, most prototypes have been costly and have involved incremental changes from previous systems. Because of the simplicity of these systems, however, it is reasonable to expect that when production moves to larger scales, they will become cost competitive with conventional systems.

Thermoacoustic technology is in its infancy, and the research monies spent to develop it are pennies compared with the amount spent on other refrigeration technologies. The investments already made in existing technologies tend to make industry more inclined to intensify efforts on current system improvements than to spend resources on new and uncertain technologies. The best opportunities for development therefore tend to come from new cooling ideas for niche markets.

How long will it be before thermoacoustics is commonly used in a commercial product? That is unclear, but it will likely take several years of significant research and development to bring the technology to the mainstream. It is a matter of obtaining adequate financial resources to develop the technology and, perhaps just as important, a matter of attracting talented people trained in a wide array of disciplines. While these are substantial hurdles, the potential of thermoacoustic technology suggests that not only can it help with existing cooling needs but it will probably offer solutions for cooling, heating, and energy-recovery applications yet to be identified.

RELATED ARTICLE: Heating and Cooling Cycles

The technologies used in most of the world's cooling machinery are based on cycles (called thermodynamic cycles) of heating and cooling what is called a "working fluid." In each cycle, the fluid (which may be a gas or a liquid) undergoes several steps:(1) compression, which raises its pressure and temperament;(2) heat rejection (expulsion);(3) expansion, which lowers its pressure and temperature; and (4) heat uptake from the material to be cooled. Work is done on the fluid to make it move heat energy from a region at a lower temperature to one at a higher temperature.

When the system operates, the working fluid may either remain in the gaseous form or undergo phase changes ("vapor cycles") from liquid to vapor to liquid again. In the latter case, the fluid is referred to as a refrigerant. During evaporation, the refrigerant absorbs heat; during condensation, it loses heat. An advantage of using refrigerants is that during phase changes, heat transfer occurs at constant temperature. Moreover, the energy transferred per unit mass of liquid is high. Such vapor cycles have long been employed in most domestic refrigeration and air-conditioning systems.

Conventional systems using gas as the working fluid include those that operate on what are called Brayton and Stirling cycles. (Modern Stirling systems include pulse-tube systems.) Thermoacoustic systems also use gas. Brayton cycles, which often use air as the working fluid, are employed in airplanes and modern European trains, where reliability and weight are more critical than cost and efficiency. Stirling system have been around for a long time and offer high efficiency by a high cost, and they have some reliability problems. They are widely used in low-temperature applications, such as gas liquefaction.

Stirling and thermoacoustic systems both use sound energy. But while the former use sound in the form of traveling waves (see main text) and require large piston displacements, the latter use standing waves and need only small (or no) piston displacements. The compression and expansion processes occur at near-constant temperature in Stirling systems but not in thermoacoustic systems. Brayton systems circulate all of the working fluid from cold to hot temperatures, so the fluid experiences large pressure changes, and expensive equipment is needed to achieve efficient compression and expansion processes. In thermoacoustic systems, each fluid element operates between temperature and pressure ratios that are much lower, making compression and expansion processes simpler and more efficient.

Brian L. Minner is a doctoral student in mechanical engineering at Purdue University. His field of research is thermoacoustics.
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Author:Minner, Brian L.
Publication:World and I
Date:Jul 1, 1998
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