Bathinda, , India
Physics, Internet Expert, General Knowledge
0 टिप्पणी करें | 72 लोगो ने देखा है | 21 अक्तूबर 13  | Harjinder Kumar
Magnetic refrigeration technique
ABSTRACT The objective of this effort is to determine the feasibility of designing, fabricating and testing a sensor cooler, which uses solid materials as the refrigerant. These materials demonstrate the unique property known as the magneto caloric effect, which means that they increase and decrease in temperature when magnetized/demagnetized. This effect has been observed for many years and was used for cooling near absolute zero. Recently, materials are being developed which have sufficient temperature and entropy change to make them useful for a wide range of temperature applications. The proposed effort includes magneto caloric effect material selection, analyses, design and integration of components into a preliminary design. Benefits of this design are lower cost, longer life, lower weight and higher efficiency because it only requires one moving part - the rotating disk on which the magneto caloric material is mounted. The unit uses no gas compressor, no pumps, no working fluid, no valves, and no ozone-destroying chlorofluorocarbons/hydro chlorofluorocarbons CFC39s/HCFC39s. Potential commercial applications include cooling of electronics, super conducting components used in telecommunications equipment cell phone base stations, home and commercial refrigerators, heat pumps, air conditioning for homes, offices and automobiles, and virtually any place that refrigeration is needed. emsp INTRODUCTION Magnetic refrigeration is based on a fundamental thermodynamic property of magnetic materials the so-called magneto caloric effect, which causes a temperature change if the material is subject to an applied magnetic field under adiabatic conditions. The magneto caloric effect was discovered in 1881 in iron by the German physicist Emil Warburg. Usually the temperature increases when the field is applied and decreases when the field is removed and the process is reversible. The magneto caloric effect can qualitatively be understood as an interaction between the entropy which is a measure of the disorder associated with the magnetic moments of the atoms spins in a material and the entropy associated with the heat motion of the same atoms. If an external magnetic field is applied it will tends to order the spins, thus decreasing the magnetic entropy. If the material is isolated from its surroundings i.e. its entropy is constant, the decrease in magnetic entropy must be compensated by an increase of the entropy of the heat motion and therefore an increase in temperature. The magneto caloric effect is most pronounced in the vicinity of a magnetic phase transition of the material, e.g. from a non-ordered paramagnetic to a ferromagnetic state. A magneto caloric material can be used as the active element in a refrigeration apparatus. The apparatus can for instance be operated in a four step cycle Magnetic refrigeration is an emerging technology using solid, non-volatile magnetic materials as the active components and water or alcohol as the medium for heat transport. It holds great potential for low energy consumption and environmentally friendly cooling at a competitive price. Traditional refrigeration technology, as found in, e.g., household refrigerators, relies on compressors to achieve a cooling cycle consisting of the liquefaction and evaporation of a gaseous refrigerant. This is a mature and reliable technology but it has a number of drawbacks ndash in particular, that the most widely used refrigerants are greenhouse gases and that small-scale compressors are inherently inefficient. Magnetic refrigeration is based on a fundamental thermodynamic property of magnetic materials, the so-called magneto caloric effect which causes a temperature change if the material is subject to a magnetic field applied under adiabatic conditions. The effect occurs both in metals and in ceramic materials. The fact that the effect is reversible allows a higher efficiency to be achieved. A magneto caloric material can be used as the active element in a refrigeration apparatus, by having it periodically experience a magnetic field while transferring heat to and from it. The Department of Energy Conversion and Storage has worked in the field of ceramic magneto caloric materials since 2000. The advantages of using ceramics are that they are very stable and that their active temperature range can be finely tuned. We cover all aspects of magnetic refrigeration, including materials research and characterization, advanced numerical modeling, magnet design, and systems design and construction. A major research project, funded by the Danish Council for Strategic Research and carried out in collaboration with industrial partners, has led to the demonstration of a prototype in 2010. Like other cooling technologies, magnetic refrigeration can be used lsquoin reversersquo as a heat pump. 1 The magneto caloric material is magnetized by a magnet and the temperature increases. 2 The material cools by giving off heat to the surroundings through a heat exchanger. 3 The magnetic field is removed and the temperature of the material drops further. Such a magnetic refrigerator has a number of advantages compared to conventional refrigerators, e.g. environmentally hazardous refrigeration gasses such as HFC hydrofluoric carbons or ammonia are avoided, and higher efficiencies are possible. emsp THE MAGNETOCALORIC EFFECT The Magneto caloric effect MCE, from magnet and calorie is a magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to a changing magnetic field. This is also known as adiabatic demagnetization by low temperature physicists, due to the application of the process specifically to effect a temperature drop. In that part of the overall refrigeration process, a decrease in the strength of an externally applied magnetic field allows the magnetic domains of a chosenmagneto caloric material to become disoriented from the magnetic field by the agitating action of the thermal energy phonons present in the material. If the material is isolated so that no energy is allowed to e migrate into the material during this time i.e. an adiabatic process, the temperature drops as the domains absorb the thermal energy to perform their reorientation. The randomization of the domains occurs in a similar fashion to the randomization at the Curie temperature, except that magnetic dipoles overcome a decreasing external magnetic field while energy remains constant, instead of magnetic domains being disrupted from internal ferromagnetism as energy is added. One of the most notable examples of the magneto caloric effect is in the chemical element gadolinium and some of its alloys. Gadolinium39s temperature is observed to increase when it enters certain magnetic fields. When it leaves the magnetic field, the temperature returns to normal. The effect is considerably stronger for the gadolinium alloy Gd5 Si2Ge2. Praseodymium alloyed with nickel PrNi5 has such a strong magneto caloric effect that it has allowed scientists to approach. MAGNETIC THERMODYNAMIC CYCLE emsp Magnetic Refrigeration Cycle The cycle is performed as a refrigeration cycle, analogous to the Carnot cycle, and can be described as a starting point whereby the chosen working substance is introduced into a magnetic fieldi.e. the magnetic flux density is increased.The working material is the refrigerant, and starts in thermal equilibrium with the refrigerated environment. Adiabatic magnetization The substance is placed in an insulated environment. The increasing external magnetic field H causes the magnetic dipoles of the atoms to align, thereby decreasing the material39s magnetic entropy and heat capacity. Since overall energy is not lost yet and therefore total entropy is not reduced according to thermodynamic laws, the net result is that the item heats up T DeltaTad. Isomagnetic enthalpic transfer This added heat can then be removed by a fluid like water or helium for example -Q. The magnetic field is held constant to prevent the dipoles from reabsorbing the heat. Once sufficiently cooled, the magneto caloric material and the coolant are separated H0. Adiabatic demagnetization The substance is returned to another adiabatic insulated condition so the total entropy remains constant. However, this time the magnetic field is decreased, the thermal energy causes the domains to overcome the field, and thus the sample cools i.e. an adiabatic temperature change. Energy and entropy transfers from thermal entropy to magnetic entropy disorder of the magnetic dipoles. Isomagnetic entropic transfer The magnetic field is held constant to prevent the material from heating back up. The material is placed in thermal contact with the environment being refrigerated. Because the working material is cooler than the refrigerated environment by design, heat energy migrates into the working material Q.Once the refrigerant and refrigerated environment is in thermal equilibrium, the cycle begins anew. Magnetic household refrigeration appliances - Household refrigerator without freezer - Wine/beverage refrigerator. b Magnetic cooling and air conditioning in buildings and houses - Magnetic RAC, window, wall or ceiling mounted - Magnetic split system e.g. single outside heat rejection unit, multiple inner cooling units. c Central cooling system - Magnetic water cooled water or brine chiller water/water, brine/brine - Magnetic air cooled water or brine chiller water/air, brine/air. Both units may be used for fan coils, ceiling cooling or in the air-conditioning system. dRefrigeration in medicine- Blood plasma storage refrigerators, chromatography and other laboratory refrigerators- Walk in rooms refrigeration, not freezing. e Cooling in food industry and storage - Food production Refrigerated silos, vessels or blenders, e.g. in diary industry Wine and beer fermenters Beverage carbonation. - Food processing for storage Hydro cooling of vegetables and fruits by immersing Forced air cooling of vegetables and flowers Spray chilling or brine cooling of meat Dry air coolers for meat. - Food storage Cold storage of fruits, vegetables and flowers Short term storage of meat products Refrigerated walk in rooms Cold storage rooms with temperatures above freezing. ADVANTAGES AND DRABACKS Advantages Green technology, no use of conventional refrigerants noiseless technology no compressor. This is an advantage in certain contexts such as medical applications. Higher energy efficiency. Thermodynamic cycles close to Carnot process are possible due to the reversibility of the MCE. Simple design of machines, e.g. rotary porous heat exchanger refrigerator low maintenance costs. Low atmospheric pressure. This is an advantage in certain applications such as in air-conditioning and refrigeration units in automobiles. Disadvantages Protection of electronic components from magnetic fields. But notice that they are static, of short range and may be shielded. Permanent magnets have limited field strength. Electromagnets and Super conducting magnets are too expensive. Temperature changes are limited. Multi-stage machines lose efficiency through the heat transfer between the stages moving machines need high precision to avoid magnetic field reduction due to gaps between the magnets and the magneto caloric material. GMCE materials need to be developed to allow higher frequencies of rectilinear and rotary magnetic refrigerators. emsp HISTORY The effect was discovered in pure ironin 1881 by E. Warburg. Originally, the cooling effect varied between 0.5 to 2 K/ T. Major advances first appeared in the late 1920s when cooling via adiabatic demagnetization was independently proposed by two scientists Debye1926 and Giauque 1927.This cooling technology was first demonstrated experimentally by chemist Nobel Laureate Giauque and his colleague Dr. D.P. MacDougallin 1933 for cryogenic purposes when they reached 0.25 K. Between 1933 and 1997, a number of advances in utilization of the MCE for cooling occurred. See Reviews In 1997, the first near room temperature proof of concept magnetic refrigerator was demonstrated by Prof. Karl A. Gs chneidner, Jr. by the Iowa State University at Ames Laboratory. This event attracted interest from scientists and companies worldwide who started developing new kinds of room temperature materials and magnetic refrigerator designs. Refrigerators based on the magneto caloric effect have been demonstrated in laboratories, using magnetic fields starting at 0.6 T up to 10teslas. Magnetic fields above 2 T are difficult to produce with permanent magnets and are produced by a super conducting magnet 1 tesla is about 20,000 times the Earth39s magnetic field. emsp CONCLUSION Nevertheless, it is notable that main work is concentrated on investigations of magnetic materials, lack of experimental explorations of magnetic refrigerator. From the former results achieved by researchers, it can be seen that there is still a great performance difference between magnetic refrigerator and vapor compression refrigerator in terms of cooling capacity and temperature span. At the end we can say that- 1. Large MCE of magnetic material is investigated for room temperature magnetic cooling application. 2. Strong magnetic field is required. 3. Room temperature magnetic refrigeration is anew highly efficient. 4. It can be use household refrigerator, central cooling systems, room air conditioners and super market refrigeration applications. 5. It is environmentally protective technology. 6. This technology must be universalized worldwide. emsp REFERENCES 1. Gschneidner, Karl, Vitalij Pecharsky and Carl Zimm, ldquoMagnetic Cooling for Appliances,rdquo International Appliance Technical Conference Proceedings, p. 144, May, 1999. 2. Gschneidner, Karl, and Vitalij Pecharsky ldquoThe Giant Magneto caloric Effect in Gd5 SixGe1-x 4 Materials for Magnetic Refrigerationrdquo Advances in Cryogenic Engineering, Plenum Press, New York, p. 1729, 1998. 3. www.sciencedirect.com 4. Long, Robert A. Langersquos Handbook of Chemistry McGraw Hill, New York, p. 10-75. 5. Wikipedia http//en.wikipedia.org/wiki/EmilWarburg, 2007. 6. Warburg E. G., 1881, Magnetische Untersuchungen uumlber einige Wirkungen der Coerzitivkraft, Ann. Phys.13, p. 141-164. 7. Kitanovski A., Egolf P.W., 2006. The Thermodynamics of Magnetic Refrigeration. Review Article of the Int. J. Refr. 29, p. 3-21.

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