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  2. 2.  Introduction Basics of Refrigeration Basics of Thermoacoustic Refrigeration Thermoacoustics Main Parts Case Study Advantages, Disadvantages and Applications of TAR Conclusion 2
  3. 3. INTRODUCTION Over the past two decades, physicists and engineers have been working on a class of heat engines and compression-driven refrigerators that use no oscillating pistons, oil seals or lubricants. Thermo acoustic devices take advantage of sound waves reverberating within them to convert a temperature differential into mechanical energy or mechanical energy into a temperature differential. 3
  4. 4. STEVEN L. GARRETT Leading Researcher United Technologies Corporation Professor of Acoustics The Pennsylvania State University. He invented the thermoacoustic refrigerator in the year 1992 and that TAR was used in the space shuttle Discovery(STS-42). 4
  5. 5. A refrigerant is a compound used in a heatcycle that undergoes a phase change from agas to a liquid and back.For example, let us assume that the refrigerantbeing used is pure ammonia, which boils at-27 degrees F. And this is what happens tokeep the refrigerator cool: 5
  6. 6. 1. The compressor compresses the ammonia gas. The compressed gas heats up as it is pressurized (orange).2. The coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. The ammonia gas condenses into ammonia liquid (dark blue) at high pressure.3. The high-pressure ammonia liquid flows through the expansion valve. The expansion valve is a small hole. On one side of it, is high-pressure ammonia liquid. On the other side of the hole is a low- pressure area.4. The liquid ammonia immediately boils and vaporizes (light blue), its temperature dropping to -27 F. This makes the inside of the refrigerator cold.5. The cold ammonia gas is sucked up by the compressor, and the cycle repeats. 6
  7. 7. D I S A DVA N TAG E S O F C O N V E N T I O N A L R E F R I G E R AT O R Uses harmful refrigerants like ammonia, CFC‟s and HFC‟s Refrigerants if leaked causes the depletion in the ozone layers. Refrigerants are costly. The moving parts like the compressors require lubrication. Leakage of refrigerant 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 vector borne diseases. 7
  8. 8.  The principle can be imagined as a loud speaker creating high amplitude sound waves that can compress refrigerant allowing heat absorption The researches have exploited the fact that sound waves travel by compressing and expanding the gas they are generated in. Suppose that the above said wave is traveling through a tube. Now, a temperature gradient can be generated by putting a stack of plates in the right place in the tube, in which sound waves are bouncing around. 8
  9. 9.  Some plates in the stack will get hotter while the others get colder. All it takes to make a refrigerator out of this is to attach heat exchangers to the end of these stacks. 9
  10. 10.  Acoustic or sound waves can be utilized to produce cooling. The pressure variations in the acoustic wave are accompanied by temperature variations due to compressions and expansions of the gas. For a single medium, the average temperature at a certain location does not change. When a second medium is present in the form of a solid wall, heat is exchanged with the wall. An expanded gas parcel will take heat from the wall, while a compressed parcel will reject heat to the wall. 10
  11. 11.  As expansion and compression in an acoustic wave is inherently associated with a displacement, a net transport of heat results. To fix the direction of heat flow, a standing wave pattern is generated in an acoustic resonator. The reverse effect also exists: when a large enough temperature gradient is imposed to the wall, net heat is absorbed and an acoustic wave is generated, so that heat is converted to work. 11
  12. 12.  Thermoacoustics combines the branches of acoustics and thermodynamics together to move heat by using sound. While acoustics is primarily concerned with the macroscopic effects of sound transfer like coupled pressure and motion oscillations, thermoacoustics focuses on the microscopic temperature oscillations that accompany these pressure changes. Thermoacoustics takes advantage of these pressure oscillations to move heat on a macroscopic level. This results in a large temperature difference between the hot and cold sides of the device and causes refrigeration. 12
  13. 13. CARNOT CYCLE  The most efficient cycle of thermodynamics.  The Carnot cycle uses gas in a closed chamber to extract work from the system. 13
  14. 14.  The figure traces the basic thermoacoustic cycle for a packet of gas, a collection of gas molecules that act and move together. Starting from point 1, the packet of gas is compressed and moves to the left. As the packet is compressed, the sound wave does work on the packet of gas, providing the power for the refrigerator. Figure 4.3 THERMOACOUSTIC REFRIGERATION CYCLE (From Reference 2) 14
  15. 15.  As the packet is compressed, the sound wave does work on the packet of gas, providing the power for the refrigerator. When the gas packet is at maximum compression, the gas ejects the heat back into the stack since the temperature of the gas is now higher than the temperature of the stack. This phase is the refrigeration part of the cycle, moving the heat Figure 4.3 THERMOACOUSTIC REFRIGERATION CYCLE (From Reference 2) farther from the bottom of the tube. 15
  16. 16.  In the second phase of the cycle, the gas is returned to the initial state. As the gas packet moves back towards the right, the sound wave expands the gas. Although some work is expended to return the gas to the initial state, the heat released on the top of the stack is greater than the work expended to return the gas to the initial state. This process results in a net transfer of heat to the left side of the stack. Figure 4.3 THERMOACOUSTIC REFRIGERATION CYCLE (From Reference 2) 16
  17. 17.  Finally, in step 4, the packets of gas reabsorb heat from the cold reservoir. Ant the heat transfer repeats and hence the thermoacoustic refrigeration cycle. Figure 4.3 THERMOACOUSTIC REFRIGERATION CYCLE (From Reference 2) 17
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  19. 19. Two main parts are in the TAR 1. Driver Houses the Loudspeaker 2. Resonator Houses the gas The hot and cold heat exchangers Houses the Stack 19
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  21. 21.  A loudspeaker (or "speaker") is an electroacoustic transducer that produces sound in response to an electrical audio signal input. It was invented in the mid 1820‟s by the scientist Johann Philipp Reis. It is powered by electricity. The magnet or the coil in the speaker vibrates to produce the waves of required frequency. 21
  22. 22.  It is also called as regenerator. The most important piece of a thermoacoustic device is the stack. The stack consists of a large number of closely spaced surfaces that are aligned parallel to the to the resonator tube. In a usual resonator tube, heat transfer occurs between the walls of cylinder and the gas. 22
  23. 23.  However, since the vast majority of the molecules are far from the walls of the chamber, the gas particles cannot exchange heat with the wall and just oscillate in place, causing no net temperature difference. The purpose of the stack is to provide a medium where the walls are close enough so that each time a packet of gas moves, the temperature differential is transferred to the wall of the stack. Most stacks consist of honeycombed plastic spacers that do not conduct heat throughout the stack but rather absorb heat locally. With this property, the stack can temporarily absorb the heat transferred by the sound waves. 23
  24. 24.  The spacing of these designs is crucial. If the holes are too narrow, the stack will be difficult to fabricate, and the viscous properties of the air will make it difficult to transmit sound through the stack. If the walls are too far apart, then less air will be able to transfer heat to the walls of the stack, resulting in lower efficiency. The different materials used in the Stack are  Paper  Alluminium  Lexan  Foam 24
  25. 25.  Heat exchangers are devices used to transfer heat energy from one fluid to another. 25
  26. 26.  A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. 26
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  28. 28.  The Space Thermo acoustic Refrigerator (STAR) was designed and built by a team at the Naval Postgraduate School led by Steve Garrett. It has the ability to move about 50 Watts of heat. The Space Thermo Acoustic Refrigerator was the first electrically- driven thermo acoustic chiller designed to operate autonomously outside a laboratory. It was launched on the Space Shuttle Discovery (STS-42) on January 22, 1992. It was not a very efficient thermoacoustic refrigerator. And hence did not refrigerate for many years. 28
  29. 29.  The refrigerator is driven by a modified compression driver that is coupled to a quarter-wavelength resonator using a single-convolution electroformed metal bellow. The resonator contains the heat exchangers and the stack. The stack is 3.8 cm in diameter and 7.9 cm in length. It was constructed by rolling up polyester film (Mylar™) using fishing line as spaces placed every 5 mm. The device was filled with a 97.2% Helium and 2.7% Xenon gas mixture at a pressure of 10 bars. 29
  30. 30.  Length of the tube is 35 cm. Diameter of the tube is 3.9 cm Length of the stack is 7.9 cm Diameter of the stack is 3.8 cm Gas used is 97.2% Helium and 2.7% Xenon Heat pumping capacity is 50 Watts. Refrigeration Temperature is 12ᴼC Commercial Loudspeaker is used. Speaker operates at 135 Hz and 100 W. 30
  31. 31.  No moving parts for the process, so very reliable and a long life span. Environmentally friendly working medium (air, noble gas). The use of air or noble gas as working medium offers a large window of applications because there are no phase transitions. Use of simple materials with no special requirements, which are commercially available in large quantities and therefore relatively cheap. On the same technology base a large variety of applications can be covered. 31
  32. 32.  Out of these, the two distinct advantages of thermo acoustic refrigeration are that the harmful refrigerant gases are removed. The second advantage is that the number of moving parts is decreased dramatically by removing the compressor. Also sonic compression or „sound wave refrigeration‟ uses sound to compress refrigerants which replace the traditional compressor and need for lubricants. The technology could represent a major breakthrough using a variety of refrigerants, and save up to 40% in energy. Thermo acoustic refrigeration works best with inert gases such as helium and argon, which are harmless, nonflammable, nontoxic, non-ozone depleting or global warming and is judged inexpensive to manufacture. 32
  33. 33.  Efficiency: Thermo acoustic refrigeration is currently less efficient than the traditional refrigerators. Lack of suppliers producing customized components. Lack of interest and funding from the industry due to their concentration on developing alternative gases to CFCs. Talent Bottleneck: There are not enough people who have expertise on the combination of relevant disciplines such as acoustic, heat exchanger design etc. 33
  34. 34. In order to overcome the drawbacks, some improvements were made. In order to improve the efficiency, regenerators are used. The function of a regenerator is to store thermal energy during part of the cycle and return it later. This component can increase the thermodynamic efficiency to impressive levels. The extra stress given in using standing waves also paved to be fruitful. This increased the level of temperature gradient setup thereby providing more refrigeration effect. 34
  35. 35.  Liquefaction of natural gas: Burning natural gas in a thermo acoustic engine generates acoustic energy. This acoustic energy is used in a thermo acoustic heat pump to liquefy natural gas. Chip cooling: In this case a piezoelectric element generates the sound wave. A thermo acoustic heat pump cools the chip. 35
  36. 36.  Electronic equipment cooling on naval ships: In this application, a speaker generates sound waves. Again a thermo acoustic pump is used to provide the cooling.  Electricity from sunlight: Concentrated thermal solar energy generates an acoustic wave in a heated thermo acoustic engine. A linear motor generates electricity from this. Upgrading industrial waste heat: Acoustic energy is created by means of industrial waste heat in a thermo acoustic engine. In a thermo acoustic heat pump this acoustic energy is used to upgrade the same waste heat to a useful temperature level. 36
  37. 37.  Thermo acoustic engines and refrigerators were already being considered a few years ago for specialized applications, where their simplicity, lack of lubrication and sliding seals, and their use of environmentally harmless working fluids were adequate compensation for their lower efficiencies. In future let us hope these thermo acoustic devices which promise to improve everyone‟s standard of living while helping to protect the planet might soon take over other costly, less durable and polluting engines and pumps. The latest achievements of the former are certainly encouraging, but there are still much left to be done. 37
  38. 38. •• Daniel A. Russell and Pontus Weibull, “Tabletop thermoacoustic refrigerator for demonstrations,” Am. J. Phys. 70 (12), December 2002.• G. W. Swift, “Thermoacoustic engines and refrigerators,” Phys. Today 48, 22-28 (1995).••• Chilling at Ben & Jerry‟s: Cleaner, Greener.” Ken Brown. Available:• 17 July 2006.• S. L. Garrett and S. Backhaus, „„The power of sound,‟‟ Am. Sci. 88, 516–• 525 (2000). 38
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