KOSEN 한인과학기술자네트워크

오존층을 파괴하는 프레온 냉매 대신에 화학적으로 안정된 헬륨이나 알곤개스를 이용한 열음향 냉동기는 몇가지 기술적인 문제로 열효율이 떨어지나 환경친화적이고 에너지 절약형이어서 많은 관심의 대상이 되고 있다고 생각됩니다. 이를 활발히 연구하고 있는 연구소는 현재 미국 펜실바니아 주립대학(PSU) 과 로스알라모스 국립연구소 두곳 정도이며 PSU과학자들이 중심이된 벤처기술회사가 하나 있읍니다. 열음향 냉동기 기술 개발 히스토리 http://www.lanl.gov/thermoacoustics/ehistory.pdf 열음향 냉동기 관련학회논문 http://www.thermoacousticscorp.com/pdf/10.pdf 열음향 냉동기 시제품 http://www.thermoacousticscorp.com/news/index.cfm/ID/6.htm 열음향 냉동기 특허정보 http://www.thermoacousticscorp.com/news/index.cfm/ID/1.htm 펜실바니아 주립대학 열음향 연구실 http://www.arl.psu.edu/capabilities/rap_acous_refrig.html 로스알라모스 연구소 열음향 연구실 http://www.lanl.gov/projects/thermoacoustics/ http://www.acoustics.org/press/147th/thermoacoustics.htm 열음향 냉동기 벤처기술 회사 http://www.thermoacousticscorp.com/ >열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다. >열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다.

Thermoacoustic Refrigeration 개론입니다. >열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다.

첨부자료를 참조하십시오 >열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다.

>열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다. Sounds Cool! Thermoacoustic Refrigeration At Purdue University (W. Lafayette, Ind.), researchers are trying to optimize a technology that has been around for over a decade: thermoacoustic refrigeration. Basically, the technology involves using the energy in sound waves to provide cooling. The Purdue team is working towards an efficient and cost effective thermoacoustic device that uses computer models to verify performance. Using sound waves to cool was a theory originally developed in the 1960s. The first thermoacoustic cooler was built by a post graduate student 12 years ago at San Diego State University. Since then, researchers have constructed a couple of variations. One produces temperatures similar to a basic refrigerator, while another becomes extremely cold. Work has long been underway to improve efficiency and fabrication of the systems, but the Purdue team is trying something different: a methodology which uses computer modeling and optimization. This means the researchers would be able to predict the best performance of the technology for specific applications, and aid in the development of prototype designs. What's New at Purdue The Purdue device is comprised of a hollow tube with a diameter that varies in length. At one end is an acoustic driver, which vibrates a diaphragm or a piston. On the other end is a cap, and in the middle are heat-pumping components. As the diaphragm vibrates, gas particles of helium and argon pressurized to 20 atmospheres oscillate back and forth inside the enclosure. Pressure fluctuations inside the cavity are accompanied by fluctuations in temperature. When you compress a gas quickly, it becomes warmer, and when you decompress it quickly, it becomes cooler, explains Luc Mongau, assistant professor of mechanical thermoacoustic design and team member. The gas particles within the device become alternately hot and cold at a frequency of 200 oscillations per second. Temperature changes, combined with particle displacements and the close proximity of a solid material that enables heat pumping to occur, mean the particles transfer heat to and from a piece of porous material called a stack, located near the acoustic driver. The result is that heat is pumped in the same direction by all the particles, such that one end of the stack gets warm and the other cool. Like a conventional refrigeration system, the thermoacoustic system would require coolants to circulate through pipes. One coolant loop would remove heat from the cooled space and bring it to the cooled side of the stack while another would remove heat from the hot side and discard it into the surroundings. Environmentally Benign Mongeau says that the pressure fluctuations in a thermoacoustic system propagate as sound waves that are extremely loud. If you were inside, it would be unbearable to listen to, he says. But a thermoacoustic refrigerator wouldn't be any louder than a conventional compressor, due to noise control technology. The coolants required would include water and glycol mixtures. Unlike systems that use phase-change refrigerants, all of the elements used for thermoacoustic cooling would be environmentally benign, including the coolants and inert gases, such as helium and argon, inside the device. And, since there are no moving parts or sliding seals, no lubrication would be required, so maintenance would be low. Aside from domestic applications, the researchers envision other uses for the technology, including space shuttle cooling, ship cooling, cooling computer and seismic instruments. For further information, contact Purdue's Ray Herrick Laboratories at 765-494-2132, fax: 765-494-0787, or e-mail: RHLB@ECN.Purdue.edu.

>열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다. REFRIGERATION & AIR CONDITIONING: Sound Power Thermoacoustic refrigeration uses a loudspeaker to keep things cool. By Karen Buscemi Posted on: 04/01/2005 The Penn State team behind the Sounds Cool thermoacoustic refrigeration project includes (from left) Matt Poese,Steven Garrett, and Bob Smith. Thermoacoustics could be the sound of things to come in alternative refrigerants. Steven Garrett, United Technologies Corp. professor of acoustics at Penn State University and a team of researchers built a prototype thermoacoustic chiller for Ben & Jerry’s, now part of Unilever. The chiller was attached to a standard ice cream freezer cabinet in a Ben & Jerry’s store in New York City, where it successfully kept the merchandise chilled. The prototype has sufficient cooling capacity to maintain ice cream at the proper serving temperature of –18˚C in a 200 L storage cabinet. Garrett says he expects the new approach to be used in applications that are difficult for chemical refrigeration, such as beverage vending machines, cooling microprocessor chips in computers and ice cream cabinets. To better understand how thermoacoustics work, it is important to first understand how conventional vapor compression refrigeration works. A vapor compression machine has four main parts. First, there is a compressor. At the inlet, it sucks in low pressure gaseous refrigerant. It takes this gas, and work is done by the compressor to compress this gas. When the gas is compressed, it gets hot. The gas then goes to the condenser, where it gives up its heat and cools off. This heat is transferred to outside air in the case of an air conditioning system. With a household refrigerator, the heat is released into the kitchen. As this high-pressure gas cools, it condenses into a liquid, and in this process, it loses even more heat. Next the liquid goes to a throttling device, basically a valve, or a long thin capillary, that has lots of flow resistance. Liquid refrigerant moves through this expansion device. As the liquid moves from a high-pressure region to a low-pressure region, the liquid gas evaporates in another heat exchanger called the evaporator, where the process of evaporation produces a cooling effect, inside the refrigerator. The process is much like what happens with water on the skin when one gets out of a swimming pool. The pressure in the evaporator is kept low, in the presence of all of this liquid evaporating back into gas, because the compressor is constantly drawing gas out of it, and so the loop is a closed circuit. There are two essential things happening. First, every refrigeration device has the job of trying to suck heat out of something that is already cold, and exhausting it someplace hot, doing exactly the opposite of what heat would like to do naturally. Second, the vapor compression cycle uses a gas, which goes from a liquid to vapor state, and back, in a closed loop. Robert Smith, research engineer for Penn State’s Applied Research Laboratory, says there is nothing really essential about having a phase transition to produce cooling, however, when a gas is compressed it always heats up, and when it is expanded, it always cools. “In a thermoacoustic machine, the gas is expanded and compressed, and at the same time, the gas is moved back and forth by a passive mechano-acoustic phasing network,” Smith says. “In this way, warm gas moves toward an internal exhaust heat exchanger, during the part of the cycle when the gas is compressed, and moves the gas toward an internal cold heat exchanger when the gas is expanding.” What is different, is that a secondary cooling fluid is used to transport the cooling from the inside of the machine to some useful place like a refrigerator. “The fluid does not have to be anything special,” Smith says. “It is possible to use ordinary automotive antifreeze, but for the Ben & Jerry’s project we chose ethanol for the cold side, because ethanol doesn't freeze or get really viscous at ice-cream temperatures, but lots of things will work, and lots of cooling fluids are designed to do this that exist.” Big sound Ordinary sound also produces temperature fluctuations, but even at 94 dB, the temperature fluctuation is only about 1/100,000th of a degree C. This represents a pressure fluctuation of about 1 part in 100,000. This is because ears are extraordinarily sensitive. In the thermoacoustic refrigerator, the acoustic pressure fluctuations are about 5 parts in 100. This produces a temperature fluctuation that is on the scale of a few degrees. “It’s an impressive sound level inside the machine (which can’t be heard on the outside), but not really a knock-your-socks off temperature fluctuation,” Smith says. “It isn't really enough by itself to cool things over refrigerator-like temperature spans, like the 38˚F cold-side to, say, 85˚F kitchen. What is needed is a second material, called a regenerator, which will permit cold gas to stay in some part of the machine at an average lower temperature, and let another part stay relatively warm. And all of these regions compress, and expand simultaneously. So, gas in the cold region can vary up and down a few degrees, and push heat up the regenerator.” Smith says that all along the regenerator there is movement and pressure fluctuation, and this permits a “bucket-brigade“ to occur. “Heat is pumped up to a high temperature but each person in the bucket brigade only has to move heat up a few degrees,” Smith says. “Nonetheless, even though the potential temperature fluctuation is locally small, heat is moved all the way up the ladder through a span larger than what one would get in a single gas parcel, undergoing pressure changes, without communication of heat to another surface.” Heat is exchanged between the gas and the regenerator, a porous material with very fine structure that can exchange heat very easily and quickly with the gas. “For the Ben & Jerry's machine, this was thick layers of very fine mesh screen material.” Helium was the gas used in the Ben & Jerry’s machine due to its properties: It is relatively inexpensive — only 15c worth was used to charge the Ben & Jerry’s machine. It is non-flammable and non-toxic. It has no environmental effects. In fact, these so-called noble gasses really can't react with anything else chemically, they are the loners of the chemical world. It has very good thermal conductivity for a gas. Noble gasses (Helium, Neon, Argon, Krypton and Xenon) all produce the largest temperature fluctuation, for a given pressure change. “The pressure at which we work is about 10-20 atmospheres or 150-300 psi, which actually is about the same range as the high side pressure for conventional VC refrigeration,” Smith says. “So it doesn't represent anything of a hazard.” The prototype During operation, the machine is inverted from the orientation shown in the photo to suppress buoyancy-driven thermal convection. As the objective of the prototype is to cool to a 200 L ice cream cabinet, the prototype can deliver 120 W of effective cooling capacity at a cold load temperature of –24.6˚C and an exhaust temperature of 34˚C. These design conditions make it possible to combine the thermoacoustic prototype with secondary distribution systems (e.g. pumped glycol loop), and keep the ice cream in the cabinet at a maximum product temperature of –18˚C in an ambient air condition of 25˚C. “The two most novel features of the design that enabled execution of a thermoacoustic-Stirling cycle in a compact and efficient way were the bellows bounce resonator and the vibromechanical multiplier,” Garrett says. “The resonator utilized a moving-magnet linear motor to create pressure oscillation without use of sliding seals or lubrication. The vibromechanical multiplier allowed efficient recycling or the acoustic power, which is key to the Stirling cycle’s efficiency, without the use of kinematic linkages or a displacer piston.” Bellows bounce resonator The moving-magnet loudspeaker is operated at the acousto-mechanical resonance frequency, ƒd, set by the stiffness of the gas contained inside the bellows and the moving mass of the loudspeaker, bellows and bellows piston. The optimum operating frequency to maximize the fatigue life of the bellows is about 100 Hz. This frequency corresponds to the 1/4 l standing wave resonance frequency for compressional waves in the metal bellows. Once the machine has been constructed, the acousto-mechanical resonance frequency can be tuned to near 100 Hz by adjusting either the mean pressure of the helium gas or by the addition/subtraction of mass on the piston. The wavelength in room temperature helium gas at this frequency is about 10 M, and the effective bellows length is about 17 cm, which makes this “lumped-element” acousto-mechanical resonator no more than 1/50 of a wavelength. Because of this compactness, the pressure within the bellows volume is spatially uniform. It is called a bellows bounce resonator because the moving mass, comprised of the piston, the moving mass of the motor and the moving mass of the bellows, bounces against stiffness provided by the pressurized helium contained within the bellows. The use of a solid material mass instead of hydrodynamic mass eliminates the nonlinear dissipative effects of turbulence and entry/exit loses present in most pure acoustic resonators driven at high amplitudes. Vibromechanical multiplier The feedback network is comprised of an inertial element and a compliant element that together create a Helmholtz resonator. In this incarnation, the feedback network is called a vibromechanical multiplier. The compliance is provided by the volume of gas enclosed within the multiplier and the inertial element is provided by a conventional 5-1/4-in. loudspeaker cone intended for audio playback application. This cone is attached to one end of the cylindrical multiplier wall by the outside rim of the Santoprene™ surround that comes with the cone. The resonance frequency of the vibromechanical multiplier containing 20 g of helium at 10 atmospheres of static pressure is approximately 330 Hz, a factor of three greater than the driving frequency. The pressure enhancement inside of the vibromechanical multiplier compared to the outside is 8.5 percent. Since the multiplier is driven far below its resonance frequency, the phase difference between the pressure inside compared to the pressure outside is nearly zero. It is the small dynamic pressure enhancement inside of the multiplier that drives gas through the regenerator with a velocity that is substantially in phase with the oscillating pressure inside of the bellows created by the oscillation of the linear motor. Inside of the regenerator, the gas undergoes a modified Stirling cycle and moves a net amount of heat from the cold heat exchanger to the ambient heat exchanger. Commercialization According to the Penn State Acoustics department, the largest hurdle to commercialization is lack of talent. As thermoacoustic technology is relatively new, there are few people with the combination of expertise in acoustics, transduction, heat exchanger design and instrumentation required to produce thermoacoustic cooling systems. Also, there is a lack of a supplier base to mass-produce the components. Garrett, Smith and fellow colleague Matt Poese have created a company called ThermoAcoustics Corp, which is currently funding development of a commercial refrigeration unit, but are not releasing any details about the new machine at this time. AUTHOR INFORMATION Karen Buscemi is associate editor of Appliance Design Magazine. E-mail Karen at: buscemik@bnpmedia.com

>열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다. Frequently Asked Questions about Thermoacoustics 1. Since the acoustic amplitudes are so large within these devices, won't it be loud outside too? Not if it is designed properly! The pressure amplitudes within the thermoacoustic resonator are only a small fraction (5%) of the static internal pressures which are approximately 20 atmospheres. Given the relatively small acoustic pressure amplitudes, a pressure vessel which is strong enough to safely contain the static pressure cannot yield enough under the acoustic pressure variations to radiate much sound to the environment. If there is any perceptible acoustic radiation at all, it is usually due to some imbalance in the electroacoustic driver. In the SETAC fridge, there were two drivers which created a small oscillatory torsional moment that caused the large, flat insulation panels surrounding the cold portions of the refrigerator to radiate. In that device, the sound level due to the radiation of the insulation panels, though preceptible, was well below that of the noise produced by the pumps which circulated the transport fluids and the fan which removed the exhaust heat. 2. How efficient are thermoacoustic refrigerators? At the present time, the efficiency of thermoacoustic refrigerators is 20-30% lower than their vapor compression counterparts. Part of that lower efficiency is due to the intrinsic irreversibilities of the thermoacoustic heat transport process. These intrinsic irreversibilities are also the favorable aspects of the cycle, since they make for mechanical simplicity, with few or no moving parts. A greater part of the inefficiency of current thermoacoustic refrigerators is simply due to technical immaturity. With time, improvements in heat exchangers and other sub-systems should narrow the gap. It is also likely that the efficiency in many applications will improve due only to the fact that thermoacoustic refrigerators are well suited to proportional control. One can easily and continuously control the cooling capacity of a thermoacoustic refrigerator so that its output can be adjusted accurately for varying load conditions. This could lead to higher efficiencies than conventional vapor compression chillers which have constant displacement compressors and are therefore only capable of binary (on/off) control. Proportional control avoids losses due to start-up surges in conventional compressors and reduces the inefficiencies in the heat exchangers, since the proportional systems can operate over smaller temperature gaps between the coolant fluid and the heat load. 2a. Could you say more about proportional control? The second law of thermodynamics sets an absolute limit on the performance (“efficiency“) of a refrigerator of any design. The larger the temperature difference which a refrigerator must produce, the less efficient it can be, even if it is perfectly designed and built. One feature of thermoacoustic devices which may allow them to overcome some of the inefficiency of the cycle is that they can use proportional control. Proportional control means that the output of the device may be turned up or down gradually depending on conditions. A dimmer switch on a lamp is an example of this kind of control. In contrast, an ordinary light switch is an example of binary control-it is either on or off, with no in-between. A vapor compression refrigerator uses binary control: it comes on for a while, then it goes off. If the conditions require more output, the unit comes on more frequently, but it is never partially on. A thermoacoustic cooler, on the other hand, can be partially on. The advantage to this is that the less hard a refrigerator is working, the more efficient it becomes. When producing maximum output, a vapor compression refrigerator is more efficient than a thermoacoustic fridge of the same capacity, but when less output is needed (which is most of the time), the thermoacoustic device increases in efficiency, but the vapor compression fridge does not. There are other advantages to proportional control. You can imagine that it would be nicer if your home air conditioner would keep the house at a constant cool temperature rather than cycling between somewhat too hot and somewhat too cold. Similarly, the performance and lifetime of some types of electronics could be increase by the steadier temperatures available through proportional control. Proportional control also eliminates the electronics-damaging “power surges“ that occur throughout the electrical system when the compressor in a conventional chiller turns on or off. 3. How large/heavy are thermoacoustic refrigerators compared to their vapor compression counterparts? For all thermodynamic devices, there will always be a trade-off between efficiency and power density. For the small power devices built thus far (less than 1,400 Btu/hr = 400 W thermal) and the larger devices currently under construction (36,000 Btu/hr = 10 kW thermal), the size and weight are similar to their vapor compression equivalents. The cooling capacity of vapor compression units depends upon operating pressure and the amount of phase-change fluid. The size of a thermoacoustic device is determined (roughly) by its operating frequency. If small size is important, higher frequency operation may be required. 4. What are the possible applications for thermoacoustic technology? Are they useful at all temperatures? At this point, we do not see any cooling application which is not suited to thermoacoustics. Conventional, single-stage, electrically operated thermoacoustic refrigerators can reach cold-side temperatures which are two-thirds to three-quarters of ambient, so they are not well suited to cryogenic applications (T < -40 C = -40 F). Thermoacoustically driven pulse-tube style refrigerators can reach the cryogenic temperatures required to liquefy air or natural gas. In its early commercial stages, thermoacoustic refrigerators will probably be limited to niche applications such as in military systems which are required to operate in closed environments and food merchandising where toxicity is an important issue. As global environmental legislation, such as the Montreal Protocols on Substances which Deplete Stratospheric Ozone and the Berlin Mandate on Global Warming Gases become more restrictive, we expect the scope of thermoacoustic applications to expand both domestically and in emerging markets. 5. How soon will we be able to purchase commercial thermoacoustic refrigerators and air conditioners? The answer depends upon the availability of funds for research and development and the severity of the restrictions which will be placed on conventional vapor compression chemical refrigerants. CFCs were banned internationally ten years after the signing of the Montreal Protocols in 1986. It is not yet clear what will be the fate of the CFC substitutes (e.g., HFCs and HCFCs) which have very large global warming potentials and unknown toxicity. Those regulations are currently being debated and are due to be ratified in Tokyo in December, 1997. In any case, we expect that it will be at least 3-5 more years before thermoacoustic refrigerators start appearing in “specialty“ applications and probably 10 more years before they start to appear in appliance stores. 6. When thermoacoustic refrigerators and air conditioners become commercially available, will they cost more than their conventional vapor compression equivalents? There are no intrinsically expensive components in thermoacoustic refrigerators. They operate at pressures which are similar to vapor compression refrigerators. Thermoacoustic refrigerators do not require any exotic materials and do not depend upon close tolerances nor do they require lubrication, since they have no sliding seals. Unlike vapor compression refrigeration, which use the “working fluid“ as the heat transport fluid, all of the thermoacoustic refrigerators which provide more that 10 to 20 W thermal of cooling capacity have used secondary heat transport fluids. This could increase the cost, since additional heat exchangers and fluid pumps are required. On the other hand, the separation of the working fluid and the heat transport fluids allow each to be optimized independently. This could lead to more uniform thermal distribution and higher efficiency which might increase acquisition cost but reduce life-cycle costs. This is not a cost factor in many applications which currently use secondary heat transport fluids. 7. Although my home and car air conditioners require costly periodic maintenance, my home refrigerator seems to work for ten to fifteen years without any maintenance. Will there be maintenance problems with thermoacoustic refrigerators and air conditioners? Thermoacoustic refrigerators will be at least as trouble-free as current home refrigerators. Thermoacoustic refrigerators and air conditioners use inert gases which will never be controlled substances and will always be readily available (The atmosphere is 1% argon. The atmospheric concentration of CO2 is only 0.03%). Since they have no sliding seals, they do not require lubrication. At the present time, we have not been able to identify what will be the possible failure modes for thermoacoustic refrigerators and air conditioners but the leading candidate is metal fatigue in the elastic suspension. It appears that proper design of these springs can lead to “infinite“ lifetimes. The reason your home refrigerator is so trouble-free is that it uses CFCs and has a hermetically sealed compressor. CFCs are compatible with hydrocarbon lubricants (oil) and do not decompose when exposed to electrical discharge. The new substitute chemicals are less stable than CFCs, so that they won't travel up to the stratosphere and destroy the ozone. This decreased chemical stability makes them incompatible with hydrocarbon lubricants, so the compressors are far more difficult to lubricate. Don't expect your new refrigerators, which will use HFCs, such as R-134a, to be as robust. Both fixed and mobile air conditioners will also be experiencing more maintenance problems now that CFCs are very expensive and will eventually become unavailable at any cost. 8. Who is working on developing thermoacoustic refrigerators and air conditioners? It is difficult to tell exactly how many groups are working on developing thermoacoustic technology, either domestically or internationally. Commercial refrigeration and air conditioning manufacturers do not “advertise“ their new product development efforts. Ford Motor Company is the only industrial laboratory which has published their research in this area, although IBM, Macrosonix (an acoustic compressor manufacturer) and Modine Manufacturing (a heat exchanger manufacturer) have been issued patents on thermoacoustic technology and Cryenco has a working device which uses a thermoacoustically-driven pulse-tube refrigerator to liquefy natural gas. In Japan, there is a association of 100 researchers from industry and academia who are working on thermoacoustic refrigeration (including pulse-tube refrigerators). There are several academic institutions and government laboratories which are doing varying amounts of research on issues surrounding thermoacoustic heat transport (e.g., U. Mississippi, Johns Hopkins, Ohio U., U. Utah, NIST, U. Nevada - Desert Research Institute, etc.) but only Los Alamos National Laboratory, the Naval Postgraduate School, Penn State University and (soon) Purdue University have complete working thermoacoustic cooling systems. Outside the United States, there are academic and/or industrial efforts in at least France, England, Argentina, Bangladesh and South Africa. 9. What are the hurdles to commercialization of thermoacoustic technology? The largest hurdle to commercialization is the “talent bottleneck.“ Due to the novelty of thermoacoustic technology, there are very few people who have the combination of expertise in acoustics, transduction, heat exchanger design, and instrumentation required to produce complete thermoacoustic cooling systems. There are not even people with that combination of expertise outside of the thermoacoustic community who can be called upon to provide an independent assessment of the current state of the technology or the prospects for future improvements. There is also no existing supplier base or commercial infrastructure which is currently producing components such as high-power, high-efficiency (narrow bandwidth) loudspeakers (linear motors) or heat exchangers optimized for high-frequency oscillatory flow of compressed gases. Until there are component suppliers, it will be difficult to create a group of systems assemblers who will market thermoacoustic devices. 10. What are the other alternative refrigeration techniques which are as environmentally benign as thermoacoustics? Prior to the commercial introduction of CFCs in the 1940's, ammonia was used as a working fluid for vapor compression refrigeration in homes. It was abandoned due to its toxicity, but is still in widespread use for industrial and agricultural applications. In Europe, and particularly in Germany, hydrocarbons, such as propane and butane, are used in small quantities for domestic refrigerators. Due to their flammability and explosive potential, they may not be suitable for applications requiring larger cooling capacities, such as air conditioners. (The hydrocarbon issues are related to saftey, not global warming.) Stirling cycle refrigerators also work best with inert gases and have efficiencies which are equal to or better than thermoacoustics. Their drawback is that they are far more complicated and therefore much more expensive to produce and maintain. It is also possible to produce refrigerators which are based on solid-state thermoelectric materials. At the present time, they are far less efficient than thermoacoustic or Stirling cycle refrigerators and the best thermoelectric materials are both brittle and hydroscopic. 11. What are the outstanding research issues which should ba addressed to understand how to improve thermoacoustic refrigerator performance in the future? The reason that thermoacoustic technology has progressed so rapidly during the past decade is that there has been an excellent theoretical understanding of the thermoacoustic heat pumping process which was developed by N. Rott in the late 1960's and early 1970's, and by J. Wheatley and G. Swift in the 1980's and G. Swift in the 1990's. Unfortunately, that understanding has been limited to a fairly small portion of the available “parameter space.“ In particular, existing models have been limited to fairly low acoustic Mach Numbers (Mac < 3% or p1/pm < 5%), due to the one-dimensional nature of the equations, the limitations of linear acoustics, the absence of mean flow, and the assumption of a stable laminar boundary layer. Since the power density of thermoacoustic devices depends upon (p1/pm)^2, there is quite a strong motivation to understand thermoacoustics at higher amplitudes. Progress in this direction will require the construction of thermoacoustic refrigerators which can achieve higher acoustic Mach Numbers and theoretical advances which could require a solution to the full non-linear thermo-hydrodynamic equtions in two- or three-dimensions. It would also be useful to study new structures for components such as stacks, resonators, heat exchangers and electroacoustic driver mechanisms. At the present time, there are no models for the stack/heat exchanger interface. There are no models for heat transport between the thermoacoustically oscillating gas and the heat exchanger surfaces which could be used to suggest what geometries would optimize the useful transfer of heat on and off of the stack. All electrically-driven thermoacoustic refrigerators to date have employed electrodynamic drive mechanisms (moving coil or moving magnet). There are several “solid-state“ materials, such as piezoelectric and magnetostrictive compounds, which have high energy densities and low losses, but which have not been adapted to thermoacoustic loads. Most of the world's machines are powered by rotary motors. What is the best way to incorporate such rotary drive mechanisms in thermoacoustic devices? The above is only a small subset of the possibilities which could lead to a more compltete understanding and better devices. With an increase in the number of working devices and motivated investigators, the rate at which thermoacoustics will progress should increase steadily for many more years.

>열음향 냉동기에 관한 해외의 최근 동향자료를 구하고 있습니다. >관련학회나 보고서, 시제품 등 정보를 알려주시면 고맙겠습니다. Rock 'n' Roll Refrigerator Ginger Pinholster Someday, household refrigerators and air conditioners might be powered by loudspeakers blasting sound thousands of times more intense than the Rolling Stones in concert. “Thermoacoustic“ refrigerators now under development use sound waves strong enough to make your hair catch fire, inventor Steven L. Garrett notes. But don't worry--the noise is safely contained in a pressurized tube. If the tube shattered, the noise would instantly dissipate to harmless levels. Because it conducts heat, such intense acoustic power is a clean, dependable replacement for cooling systems that use ozone-destroying chlorofluorocarbons (CFCs), which will be banned after 31 December 1995, says Garrett, a physics professor at the Naval Postgraduate School in Monterey, California. Already, Garrett and NPS Research Assistant Professor Tom Hofler have developed a thermoacoustic refrigerator offering 200 watts of cooling power--a level comparable to existing CFC-based refrigerators. Their “rock 'n' roll refrigerator“ is cold enough to freeze ice or “simply keep beer chilled.“ Hofler is also developing supercold “cryocoolers“ capable of temperatures as low as -135°F (180°K). He hopes to achieve -243°F (120°K) because such cryogenic temperatures would keep electronic components cool in space or speed the function of new microprocessors. Skeptics say current thermoacoustic designs are inefficient compared to conventional refrigeration systems. But Garrett continues to improve his invention, which requires only one moving part in the form of a loudspeaker and therefore may be more dependable than CFC-type refrigerators. It's also more environmentally friendly, promising a route to “leap-frog over this whole chemical dependency problem,“ says Garrett, a 1993 winner of the Rolex Foundation awards for enterprise in the applied sciences and invention, exploration, and discovery in the environment. A Simple Design How does it work? First, customized loudspeakers are attached to cylindrical chambers filled with inert, pressurized gases such as xenon and helium. At the opposite end of the tubes are tightly wound “jelly rolls“ made of plastic film glued to ordinary fishing line. When the loudspeakers blast sound at 180 decibels, an acoustic wave resonates in the chambers. As gas molecules begin dancing frantically in response to the sound, they are compressed and heated, with temperatures reaching a peak at the thickest point of the acoustic wave. That's where the superhot gas molecules crash into the plastic rolls. After transferring their heat to the stack, the sound wave causes the molecules to expand and cool. “Each one of these oscillating molecules acts as a member of a 'bucket brigade,' carrying heat toward the source of the sound,“ says Garrett. Cold temperatures can then be tapped for chilling refrigerators, bedrooms, cars, or electronic components on satellites and inside computers, according to Garrett. Someday, he says, turning up the air-conditioner could be accomplished by adjusting a volume-control knob. In contrast, inside conventional refrigerators and air conditioners, CFC gas is compressed and heated by an electrically driven pump, then cooled and condensed by a heat exchanger in a process known as a “Rankine cycle.“ When the liquefied gas is depressurized, it evaporates and drops to a much cooler temperature. Moving through the freezer coils of a food compartment, the cold fluid picks up heat, starting the cycle all over again. Before World War II, ammonia and sulfur dioxide were commonly used in refrigerators, explains Gregory W. Swift, a thermoacoustics expert at Los Alamos National Laboratory in New Mexico. But these substances were soon replaced with CFCs, which are noncorrosive, nonflammable, and relatively nontoxic, Swift says. Unfortunately, he adds, CFCs leak from cooling systems, destroying the atmospheric ozone that protects the earth's surface from ultraviolet radiation. Damage to the ozone shield may result in adverse human health effects including cancers, cataracts, immune system deficits, and respiratory effects, as well as diminish food supplies and promote increases in vectorborne diseases. The Sound-Heat Connection The relationship between sound and heat was recognized more than 100 years ago, when glassblowers heard the tone generated by a hot glass bulb attached to a cool tube, Swift says. Thermoacoustic devices simply reverse this phenomenon, using sound to move heat. As part of his doctoral thesis for the University of California at San Diego, Hofler used thermoacoustics to make a simple yet surprisingly powerful heat-moving tube under the direction of the late John C. Wheatley and others at Los Alamos. In 1986, Hofler joined the NPS faculty and helped Garrett's research team design the “Space ThermoAcoustic Refrigerator“ (STAR), a fully autonomous device weighing less than 200 pounds. Launched on the Space Shuttle Discovery in January 1992, STAR demonstrated the feasibility of thermoacoustic refrigerators. But it provided only 5 watts of cooling power, and therefore fell far short of the power requirements for household refrigerators. Since STAR's debut, Garrett reported at the February meeting of the American Association for the Advancement of Science that NPS researchers have increased the cooling power of their original design by a factor of 40, making it feasible for use in full-scale refrigerators. Dubbed “TALSR“ for ThermoAcoustic Life Sciences Refrigerator, STAR's successor was originally designed to keep biological samples cool in space, where zero-gravity creates problems such as the migration of lubricants inside vapor-compression refrigerators. When the National Aeronautics and Space Administration withdrew its support for TALSR because of a funding shortage, Garrett jokingly renamed his invention HOTAR, for “Homeless Orphan ThermoAcoustic Refrigerator.“ The U.S. Navy is now funding TALSR as a method for cooling shipboard electronics. Meanwhile, Garrett hopes a third design not yet unveiled will provide another factor of 40 increase in cooling power--enough for air-conditioning systems. A handful of research laboratories and major corporations, including Ford Motor Company and Modine Manufacturing of Racine, Wisconsin, have now joined Garrett in developing thermoacoustic refrigeration systems. At Ford's Scientific Research Lab, research scientist George Mozurkewich continues to study thermoacoustics, though he says it probably won't be useful for car air-conditioning systems anytime soon. Car air-conditioning units require at least 5000 watts of cooling power, and existing thermoacoustic designs provide only 200 watts, he notes. Current thermoacoustic systems are also too bulky and heavy for car air conditioning, he says. Still, thermoacoustic refrigeration may prove useful for “niche applications,“ such as cooling satellite sensors or super-fast computers, Mozurkewich adds. Peavey Electronics of Meridian, Mississippi, and Cardinal Research Corporation of Richmond, Virginia, hope to use Garrett's patented design in thermoacoustic refrigerators for fishing boats and other marine vessels. Refrigerating the day's catch or keeping a yacht cool has always been problematic, explains Cardinal President Jeremy Crews. Conventional refrigerators require a 120-volt alternating current, which must be generated at sea by an on-board diesel power plant. Unfortunately, Crews says, diesel generators tend to be noisy, smelly, and unreliable. A battery-powered thermoacoustic refrigerator might be suitable for use on commercial or pleasure vessels, he adds. Thermoacoustic air conditioning systems for homes and office buildings are planned by Cool Sound Industries of Port St. Lucie, Florida. CSI President Frank Wighard says the technology “could be brought to market in less than 2 years with the proper funding.“ In rural Bangladesh, where electricity is scarce and ice must be hauled long distances, researchers are developing simple kerosene-driven thermoacoustic refrigerators to keep life-saving medical supplies cool. And at Los Alamos, Swift's team is collaborating with Ray Radebaugh of the National Institute of Standards and Technology and Tektronix Corporation to develop coolers based on pulse-tube technology for electronic components. A technological cousin of thermoacoustic refrigerators, pulse-tube coolers use a traveling acoustic wave (instead of a standing wave) and offer greater efficiency at lower temperatures. Los Alamos researchers are also working with Radebaugh and Cryenco Company to make pulse-tube coolers that liquefy natural gas at derelict well sites, perhaps for use in fleet vehicles. Bucket brigade. A gas parcel is compressed and heated by the sound wave and deposits some of its heat to the stack. The sound wave then expands and cools the gas parcel so that the gas can absorb heat from the stack and cool it. The parcels absorb heat from the cold exchanger and pass it along the stack. Technical Challenges Because they use low-cost components and require only one moving part, Garrett's refrigerator could lead to inexpensive, maintenance-free systems that don't destroy the ozone. But researchers must overcome a number of technical challenges. The efficiency of thermoacoustic refrigerators, for instance, has repeatedly been called into question. Since they use electricity to drive a pump that moves working gas, conventional refrigerators represent 6% of the nation's annual electricity consumption, Swift notes. Similarly, the loudspeakers inside a thermoacoustic refrigerator also must be activated by electrical power. Swift says the best thermoacoustic coolers built thus far use “twice as much electricity as conventional refrigerators.“ Though much greater efficiency is theoretically possible, Swift doubts that thermoacoustic refrigerators will ever catch up with traditional Rankine-cycle designs. Complex physical factors such as the friction generated by gas molecules churning back and forth inside a chamber place fundamental limits on the efficiency of thermoacoustic refrigerators, according to Swift. Losses also occur because of acoustic distortions generated at levels above 155 decibels, says Rick Weisman, vice president of applied technologies at the Harman International Acoustics Company of Northridge, California. “We can't afford to replace the old CFC-based technology with one that's less efficient, because then we're using more fossil fuels to run those devices,“ Swift says. But Garrett points out that thermoacoustic technologies still have plenty of room to grow. “Year after year, improvements are made and breakthroughs are achieved,“ he says. Unfortunately, he adds, progress has been stymied by a lack of support from the U.S. refrigeration industry, which is focusing on chemical CFC substitutes. “The vapor-compression guys are doing very well, and they're going to be hard to beat because they have the money to keep pushing improvements. But thermoacoustic technologies are young and moving fast.“ Garrett also argues that thermoacoustic refrigeration systems can be more precisely controlled than vapor-compression coolers, and therefore waste less energy. With a conventional refrigerator, “it's either fully on or off,“ Garrett says. “So, when the thermostat turns it on, it gets too cold and when it is off, it heats up too much. With an acoustic refrigerator, it's much like your home stereo. It's got a volume control so it can be set by the thermostat to precisely the appropriate level for the required temperature and heat load at all times.“ Ford's Mozurkewich says the efficiency of electric-to-acoustic conversion in thermoacoustic refrigerators “isn't particularly good.“ Yet, he admires Garrett's “unsuppressible enthusiasm,“ and points out that NPS researchers have already made significant design improvements. Garrett's collaborator at Peavey Electronics, Mike O'Neill, director of transducer engineering, says the efficiency of thermoacoustic refrigerators might not be so critical for certain niche applications such as space cooling. The Search for CFC Replacements The nation's largest CFC manufacturer, the DuPont company of Wilmington, Delaware, has invested nearly $500 million in CFC alternatives thus far, reports staffer Sharon Gidumal, an environmental specialist. Much of the research has emphasized chemical CFC substitutes such as hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). “We are focusing mainly on chemical compounds to replace CFCs,“ Gidumal says. “Our goal was to minimize economic disruption and develop compounds that perform as closely to the originals as possible.“ According to DuPont, CFC production employs an estimated 700,000 workers at 5,000 American businesses worth $28 billion. Complete retooling of existing vapor-compression equipment and factories would have dire economic consequences, Gidumal notes. Unfortunately, the safety of HCFCs has been questioned. Although they're believed to decompose before reaching ozone altitudes, HCFCs clearly destroy ozone, Hofler says. Consequently, HCFCs will be banned in developed countries by the year 2030 in keeping with the Montreal Protocol, an international treaty. A DuPont spokeswoman says HFCs, which contain no chlorine and therefore don't destroy ozone, appear to be a safe, viable alternative to CFCs. Swift and some environmentalists aren't completely convinced. “[HFCs] are, as far as everyone knows, perfectly safe in terms of their interaction with stratospheric ozone,“ Swift says. “However, they are greenhouse gases. Whether or not the greenhouse gases are a problem is still a matter of debate. But some people wouldn't be surprised if [HFCs] are eventually regulated.“ Swift believes that propane and butane are far more feasible as replacements for CFCs inside vapor-compression refrigerators. These hydrocarbons are highly flammable, but he claims they could be used safely and would offer greater efficiency than thermoacoustic refrigerators. Carbon dioxide is also being studied as an environmentally benign substitute for CFCs. Despite his skepticism about the efficiency of thermoacoustic refrigerators, Swift respects Garrett's determination. “If the optimistic point of view is right,“ Swift says, “then the scientists that think about these things might come up with something new to improve efficiency. Who knows?“ Garrett, meanwhile, is doggedly pursuing his dream of a thermoacoustic future. “Ultimately, the answer is going to be given in the basement of Sears in the year 2000,“ he says. “I'm certainly not reducing my efforts.“ Ginger Pinholster Ginger Pinholster is a freelance writer in Wilmington, Delaware. Suggested Reading Acoustical heat pumping engine. U.S. patent no. 4,398,398. Washington, DC:U.S. Patents Office, 16 August 1983. Garrett S. Thermoacoustic life sciences refrigerator. NASA technical report no. LS-10114. Houston, TX:Johnson Space Center, 1991. Garrett SI, Adeff JA, Hofler TJ. Thermoacoustic refrigeration for space applications. J Thermophys Heat Transfer 7(3): (1993). Intrinsically irreversible heat engine. U.S. patent no. 4,489,553. Washington, DC:U.S. Patents Office, 25 December 1984. Swift GW. Thermoacoustic engines. J Acoust Soc Am 84:1145-1180 (1988).