Appliance Design - April 2008 - (Page 16)

COOLING TECHNOLOGIES Thermoacoustic Engine In a pure thermoacoustic engine, fluid oscillates back and forth in a standing wave. The pressure oscillations are maximum at either end, and the gas motion is maximum at the middle. When a carefully-spaced heat-exchange surface is located between these peaks (spaced so the moving gas only has time to make good contact at the ends of its oscillating travel) and a temperature difference is applied, then the local gas goes through a cycle that pumps it to higher oscillation intensity by capturing some thermal energy. That captured energy can be harvested as mechanical power by a reciprocating alternator (like a highpower microphone). < fluid is alternately compressed and expanded. By providing a secondary space connected to the compression/expansion cylinder through the heat exchangers and a narrow duct, a second resonant tuning is established. This second tuning is chosen to be different from the motor’s motion, so that the fluid flow lags the pressure wave imposed by the motor. It is this timephased relation between pressure and volume flow that produces the cycle. (See Fig. 1.) As the piston comes forward, compressing the gas, the gas begins to flow away toward the colder space. By the time the piston has reached its uppermost, that flow is at its maximum and the fluid is concentrated toward the cold end. Then the piston retreats, even while the flow continues away, though slowing. The falling pressure expands the gas concentrated at the cold side, allowing it to receive heat from the surrounding load. By the time the piston returns to its original mid-stroke position, the flow away has stopped, its inertia finally overcome by the adverse pressure gradient. As the piston heads toward its bottom extreme, the flow reverses and begins rushing toward the warm end. Then the piston reverses and returns toward its starting place again, even while the flow is at full speed toward the warm end. The gas between, now concentrated at that warm end, is compressed and its heat rejected. The cycle is complete in less than 20 milliseconds. Without complicated linkages and rubbing seals, acoustic-Stirling devices can operate reliably at frequencies of 50 or 60 cycles per second, which are conveniently provided by the power company. With acoustic-Stirling, unlike a conventional vapor cycle, each particle carries much less enthalpy from cold to hot (with no 16 applianceDESIGN April 2008 phase change). However, it does so much more often, making the size of the machine competitive with pumped-flow systems. In actual coolers, the motors are used in pairs for balance, and a buffer tube is added so that the heat of the high-speed oscillating flow in the inertia tube is rejected away from the cold zone. (See Fig. 2.) But, the basic principle is unchanged, and the mechanism is as simple, even though the theory is complex. The first acoustic-Stirling products are cryocoolers, typically rated at 77 K, the temperature of liquid nitrogen, with capacities ranging from watts to kilowatts. At those temperatures and scales, no other approach is more efficient. Because the speaker is flexure supported, there are no wear or contact surfaces and therefore no lubricant required. As a result, the method is reliable and maintenance-free. Acoustic-Stirling cryocoolers have been identified by both the U.S. Department of Energy and the International Energy Agency as the most promising option for practical superconductor cooling. They are the preferred technology for liquefying pilots’ oxygen on new U.S. Navy carriers, and they are being applied in science laboratories worldwide in place of trucked-in liquid cryogens. The next, most obvious question is whether acoustic-Stirling can be used for for cooling at less severe temperatures in applications such as freezers, refrigerators, or air-conditioners. The answer is a qualified, but definite yes. Because these nearambient coolers must have coefficients of performance (COPs) greater than 1 to be competitive, the amount of heat rejected in the inertial flow is much higher and must be recovered as work to maintain efficiency. Also, the size of heat exchangers, relative to the driving motors is much larger, requiring some clever engineering to couple the cycle to the environment without secondary fluid loops. An initial practical acoustic-Stirling cooling appliance has been built by CFICQdrive for the U.S. Army. The unit is a food-storage refrigerator and has completed its first year of continuous duty. This rugged unit is a single-compartment refrigerator with about 18 cu. ft. capacity. The foam-insulated polymer cabinet is a mobile cold-storage unit used for Army foodservice support. The acoustic cooling equipment is mounted in the compartment on top for easy demonstration. The application is field kitchen duty, where rough handling on the way to the service site is a normal event and the ruggedness of acoustic-Stirling equipment is a key feature. While still a prototype that has not yet been fully optimized, the unit has nonetheless matched the efficiency of a www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - April 2008 Contents Editorial Shipments/Forecasts News Watch A Variant of Thermoacoustic Refrigeration Known as Acoustic-Stirling is Expanding Its Range Outside Cryocooling Into More Conventional Applications Such As Refrigeration and Air Conditioning Free-Piston, Stirling Coolers Are Now Showing Up in Mainstream Applications As Costs Come Down and Heat Transport Solutions Are Devised Heat Exchanger Innovations Are Helping To Improve the Overall Efficiency of Vapor Compression Systems Environmental Concerns Have Once Again Ignited a Debate Over Refrigerants A New Glass Ceramic Material for Induction Cooktops Can Be Decorated In a Variety of Glimmering Metallic, 3-D Looks That Can Employ a Number of Shades, Patterns, Styles, Colors, Intensities, and Textures Colorful, Reflective Coatings for Glass Create a Variety of Design Options To Help Designers Establish a Unique Look for Their Products Appliance Designers Faced With Different Motor and Drive Options Need To Understand the Differences and Tradeoffs Before Choosing A New MEMS Valve Offers a Number of Advantages Over Solenoids In Certain Applications, Including the Opportunity To Cut Energy Use The AHR Expo Set Attendance Records and Showcased Thousands of Innovative New Products Classifieds Design Marts Advertiser’s Index Association Report: CEA

Appliance Design - April 2008

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