09-03-2012, 04:55 PM
i need magnetic refrigeration full report for giving seminar on this topic
i am the finel year student of mechanical branch...plz send it quickly with latest technology
09-03-2012, 04:55 PM
i need magnetic refrigeration full report for giving seminar on this topic i am the finel year student of mechanical branch...plz send it quickly with latest technology
11-07-2012, 10:20 AM
to get information about the topic "magnetic refrigeration" full report ppt and related topic refer the link bellow
https://seminarproject.net/Thread-magnet...ull-report https://seminarproject.net/Thread-magnet...rigeration
12-07-2013, 04:51 PM
MAGNETIC REFRIGERATION MAGNETIC REFRIGERATION.pdf (Size: 311.89 KB / Downloads: 60) Introduction:- Refrigeration can be defined simply as ‘the process of removing heat from a body to maintain the temperature of the body below that of that of its surrounding’. The science of refrigeration utilizes several methods for providing low temperatures. Everybody is familiar with the vapour compression cycle, which is to date the most popular cycle, used for refrigeration, both for industrial & commercial purpose. However there are various limitations in using vapour compression system. The major drawback of the vapour compression system is that it requires a compressor to compressor to compress a large volume of refrigerant vapour which requires a large power for its operation. In addition it has poor COP as compared with the Carnot cycle, environmental hazards like Global warming, limit to the lowest temperature reached as its drawbacks. Hence we have to continuously look for alternative methods for refrigeration. A large research is going on non-conventional refrigeration systems to produce very low temperatures which includes Thermo-electric refrigeration, Pulse tube refrigeration, Vortex tube refrigeration etc. Principle behind Magnetic Refrigeration:- Magnetic refrigeration is based on the "Magnetocaloric Effect"; the ability of some metals to heat when magnetized and cool when removed from the magnetic field. Using these materials as refrigerants provides an environmentally friendly alternative to the volatile liquid chemicals, such as chlorofluorocarbons and hydrochlorofluorocarbons, which are used in traditional vapour-cycle cooling systems. MAGNETO-CALORIC EFFECT :- Magnetocaloric effect is defined as the response of a solid to an applied magnetic field which is apparent as a change in its temperature. This effect is obeyed by all transition metals and lanthanide-series elements. When a magnetic field is applied, these metals, known as ferromagnets, tend to heat up. As heat is applied, the magnetic moments align. When the field is removed, the ferromagnet cools down as the magnetic moments become randomly oriented. When a strong magnetic field is applied to the magnetocaloric material, the magnetic moments of its atoms become aligned, making the system more ordered.When the strong magnetic field is removed, the party is forced to cool down. The magnetic moments return to their random directions, entropy increases and the material cools. Upon the removal of a magnetic field from a material, the resulting reduction in magnetic spin alignment represents an increase in the material's spin entropy (delta S). If the field reduction is performed adiabatically so that the total entropy change is zero, then the increased spin entropy is offset by an equal decrease in lattice entropy, as reflected by a decrease in the temperature of the material. This delta T is called the ‘magnetocaloric effect’ ORDERING TEMPERATURE: The temperature at which most of the change in magnetic entropy occurs is known as the material's ordering temperature or its Curie point. This is the point where the material changes from being ferromagnetic to paramagnetic, and the farther away from this point the weaker the magnetocaloric effect. The useful portion of the magnetocaloric effect usually spans about 25 degrees C (77 F) on either side of the material's Curie temperature. Therefore, in order to span a wide temperature range, a refrigerator must contain several different coolants arranged according to their differing ordering temperatures Hence it is important to know whether we can adjust the useful range of the magnetocaloric effect to create a particular temperature. In other words similar to the conventional system, where given a particular evaporator temperature we select a suitable refrigerant, here also we need to have materials with varying temperature range of the magnetocaloric effect. GADOLINIUM AND ITS ALLOYS: Since the discovery of the magnetocaloric effect in pure iron by E.Warburg in 1881, it has been measured experimentally on many magnetic metals and compounds. Gadolinium, a rare-earth metal, exhibits one of the largest known magnetocaloric effects. It was used as the refrigerant for many of the early magnetic refrigeration designs. The problem with using pure gadolinium as the refrigerant material is that it does not exhibit a strong magnetocaloric effect at room temperature. More recently, however, it has been discovered that arc-melted alloys of gadolinium, silicon, and germanium are more efficient at room temperature. Gschneidner and Pecharsky found that they could tune the operating temperature (gradually lower the Curie point) of a gadolinium silicide compound (Gd5Si4) by substituting germanium (Ge) for silicon. This resulted in a new compound, Gd5Si2Ge2, which has a magnetocaloric effect about twice as large as gadolinium alone. Magnetic Refrigeration System:- With this background of the principal behind the system, let's take a look at the schematic diagram of the theoretical magnetic refrigeration system & its vapour compression counterpart. The conventional vapour compression system makes use of a compressor, two heat exchangers- evaporator & condenser, a throttling device. The refrigerant picks up heat from the space to be refrigerated in the evaporator where it is converted into vapour state. This vapour then passes through the compressor where its pressure & temperature is increased. Refrigerant then gives out its heat in a condenser & gets converted into a liquid. The throttling device is used to reduce the pressure of the refrigerant to the evaporator pressure. As compared with this the magnetic system does away with the compressor. Instead it makes use of magnets, either permanent or superconducting, to effect a change in magnetic field. The CFC or HFC refrigerant in the conventional system is replaced by a working substance i.e. a magneto-caloric material. The two heat exchangers are off course still present to effect heat exchange between working material & a heat transfer fluid. High Thermodynamic Efficiency:- Another important advantage offered by magnetic system is the high thermodynamic efficiency as compared with the conventional system. Fig. 3 shows the T-S diagram for both the conventional & magnetic system. As seen from the fig. the magnetic system approximates a reversed Carnot cycle much better than a conventional system. The system has practically very little entropy generation or irreversibility. Due to this the work input required to produce a desired cooling effect is much less. As against this the conventional system involves a finite amount of entropy generation or irreversibility due to various reasons such as friction, heat exchange of hot refrigerant with the atmosphere & walls of compressor etc. Even with the best of the designs it is not possible to reduce this entropy generation to zero. Subsequently the work input required is higher to overcome all these irreversibilities. Naturally the COP of conventional system is much lower as compared to the magnetic system. With the magnetic system a COP of 15 has been reached in the setup developed by Ames Lab. & ACA. Lowest Temperature That Can Be Reached:- Leaving aside the environmental & cost savings, the area where the magnetic refrigeration system leaves the conventional system far behind is the lowest temperature that can be reached. This is particularly important in applications such as liquefaction of gases such as Hydrogen, Nitrogen. With the continuing shortage of fossil fuels energy sources such as liquid Hydrogen & Nitrogen are becoming increasingly important. However the liquefaction of this gases requires maintaining quite low temperature such as 20 K. Now in a conventional vapour compression system maintaining a particular temperature is governed by the boiling point of the refrigerant. With the vapour compression system we can surely have a refrigerant which can boil at 20 K & extract heat at that temperature. However it is not possible to have a single refrigeration cycle operating between 20 K & room temperature. Hence we have to go for cascading of systems to achieve this low temperature. Thus today it requires sometimes as many as 15 stages to achieve a temperature of 20 K. With the inherent low efficiency of the conventional system this means considerable wastage of energy & the system can not economically produce less than 5 tons/day of Hydrogen. Another drawback of the conventional system is that even with cascading of systems the lowest possible temperature in conventional system is restricted to 1 K. Conclusion:- Magnetic Refrigeration is a clean, environmentally friendly technology, which replaces the environmentally hazardous refrigerants in a vapour compression system with a magnetocaloric substance & a heat transfer fluid, which are environmentally friendly. With the ever increasing concern about environmental hazards it promises to be a technology of the future. However, before the widespread use of magnetic refrigerators can begin in both industrial & commercial application, the technology has to cross a few technical hurdles & prove it's worth. But it won't be long before we will see magnetic refrigerators take over from the conventional vapour compression system in all the fields of application. |
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