30-08-2014, 11:12 AM
Magnetic refrigeration
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INTRODUCTION
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect.This technique can be used to attain extremely low temperatures (well below 1 kelvin), as well as the ranges used in common refrigerators, depending on the design of the system.
This effect, discovered in 1881, is defined as the response of a solid to an applied
magnetic fieldwhich is apparent as a change in its temperature.1 This effect is obeyed by all transition metalsand lanthanide-series elements. When a magnetic field is applied, these metals, known asferromagnets, tend to heat up. As heat is applied, the magnetic moments align. When the field isremoved, the ferromagnet cools down as the magnetic moments become randomly oriented.
Gadolinium, a rare-earth metal, exhibits one of the largest known magnetocaloric effects. It wasused as the refrigerant for many of the early magnetic refrigeration designs. The problem withusing pure gadolinium as the refrigerant material is that it does not exhibit a strongmagnetocaloric effect at room temperature. More recently, however, it has been discovered thatarc-melted alloys of gadolinium, silicon, and germanium are more efficient at roomtemperature
MAGNETO CALORIC 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 affect 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 chosen (magnetocaloric) material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material. If the
WORKING PRINCIPLE
As shown in the figure 2, when the magnetic material is placed in the magnetic field, the thermometer attached to it shows a high temperature as the temperature of it increases.
But on the other side when the magnetic material is removed from the magnetic field, the thermometer shows low temperature as its temperature decreases
3WORKING
The magnetic refrigeration is mainly based on magneto caloric effect according to which some materials change in temperature when they are magnetized and demagnetized.
Near the phase transition of the magnetic materials, the adiabatic application of a magnetic field reduces the magnetic entropy by ordering the magnetic moments.
This results in a temperature increase of the magnetic material. This phenomenon is practically reversible for some magnetic materials; thus, adiabatic removal of the field revert the magnetic entropy to its original state and cools the material accordingly. This reversibility combined with the ability to create devices with inherent work recovery, makes magnetic refrigeration a potentially more efficient process than gas compression and expansion. The efficiency of magnetic refrigeration can be as much as 50% greater than for conventional refrigerators
4PROPER FUNCTIONING
The place we want to cool it, we will apply magnetic field to the material in that place and as its temperature increases, it will absorb heat from that place and by taking the magnetic material outside in the surroundings, we will remove the magnetic material from magnetic field and thus it will loose heat as its temperature decreases and hence the cycle repeats over and again to provide the cooling effect at the desired place
REQUIREMENTS FOR PRATICAL APPLICATIONS
MAGNETIC MATERIALS
Only a limited number of magnetic materials possess a large enough magneto caloric effect to be used in practical refrigeration systems. The search for the "best" materials is focused on rare-earth metals, either in pure form or combined with other metals into alloys and compounds.
The magnetocaloric effect is an intrinsic property of a magnetic solid. This thermal response of a solid to the application or removal of magnetic fields is maximized when the solid is near its magnetic ordering temperature.
The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic order process: the magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems.
Currently, alloys of gadolinium producing 3 to 4 K per tesla of change in a magnetic field can be used for magnetic refrigeration or power generation purposes.
Recent research on materials that exhibit a giant entropy change showed that Gd5(SixGe1 − x)4, La(FexSi1 − x)13Hx and MnFeP1 − xAsx alloys, for example, are some of the most promising substitutes for Gadolinium and its alloys (GdDy, GdTy, etc...). These materials are called giant magneto caloric effect materials (GMCE).
SUPER CONDUCTING MAGNETS
Most practical magnetic refrigerators are based on superconductingmagnets operating at cryogenic temperatures (i.e., at -269 C or 4 K).These devices are electromagnets that conduct electricity with essentiallyno resistive losses. The superconducting wire most commonly used ismade of a Niobium-Titanium alloy
Future Applications
In general, at the present stage of the development of magnetic refrigerators with permanent magnets, hardly any freezing applications are feasible. These results, because large temperature spans occur between the heat source and the heat sink.
An option to realize magnetic freezing applications could be the use of superconducting magnets.
COMPARISON
The magneto caloric effect can be utilized in a thermodynamic cycle to produce refrigeration. Such a cycle is analogous to conventional gas-compression refrigeration
SOCIO-ECONOMIC
Competition in global market:-Research in this field will provide the opportunity so that new industries can be set up which may be capable of competing the global or international market.
Low capital cost:-The technique will reduce the cost as the most inefficient part comp. is not there and hence the initial low capital cost of the equipment.
Key factor to new technologies:-If the training and hard wares are developed in this field they will be the key factor for new emerging technologies in this world
ADVANTAGES OVER VAPOUR COMPRESSION AND VAPOR ABSORPTION CYCLE CYCLES
Magnetic refrigeration performs essentially the same task as traditional compression-cycle gas refrigeration technology. Heat and cold are not different qualities; cold is merely the relative absence of heat. In both technologies, cooling is the subtraction of heat from one place (the interior of a home refrigerator is one commonplace example) and the dumping of that heat another place (a home refrigerator releases its heat into the surrounding air). As more and more heat is subtracted from this target, cooling occurs. Traditional refrigeration systems - whether air-conditioning, freezers or other forms - use gases that are alternately expanded and compressed to perform the transfer of heat. Magnetic refrigeration systems do the same job, but with metallic compounds, not gases. Compounds of the element gadolinium are most commonly used in magnetic refrigeration, although other compounds can also be used.
CONCLUSION AND FUTURE ENHANCEMENTS
Magnetic refrigeration is a technology that has proven to be environmentally safe. Computer models have shown 25% efficiency improvement over vapor compression systems. In order to make the Magnetic Refrigerator commercially viable, scientists need to know how to achieve larger temperature swings. Two advantages to using Magnetic Refrigeration over vapor compressed systems are no hazardous chemicals used and they can be up to 60% efficient.There are still some thermal and magnetic hysteresis problems to be solved for these first-order phase transition materials that exhibit the GMCE to become really useful; this is a subject of current research. This effect is currently being explored to produce better refrigeration techniques, especially for use in spacecraft. This technique is already used to achieve cryogenic temperatures in the laboratory setting (below 10K).
[attachment=67406]
INTRODUCTION
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect.This technique can be used to attain extremely low temperatures (well below 1 kelvin), as well as the ranges used in common refrigerators, depending on the design of the system.
This effect, discovered in 1881, is defined as the response of a solid to an applied
magnetic fieldwhich is apparent as a change in its temperature.1 This effect is obeyed by all transition metalsand lanthanide-series elements. When a magnetic field is applied, these metals, known asferromagnets, tend to heat up. As heat is applied, the magnetic moments align. When the field isremoved, the ferromagnet cools down as the magnetic moments become randomly oriented.
Gadolinium, a rare-earth metal, exhibits one of the largest known magnetocaloric effects. It wasused as the refrigerant for many of the early magnetic refrigeration designs. The problem withusing pure gadolinium as the refrigerant material is that it does not exhibit a strongmagnetocaloric effect at room temperature. More recently, however, it has been discovered thatarc-melted alloys of gadolinium, silicon, and germanium are more efficient at roomtemperature
MAGNETO CALORIC 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 affect 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 chosen (magnetocaloric) material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material. If the
WORKING PRINCIPLE
As shown in the figure 2, when the magnetic material is placed in the magnetic field, the thermometer attached to it shows a high temperature as the temperature of it increases.
But on the other side when the magnetic material is removed from the magnetic field, the thermometer shows low temperature as its temperature decreases
3WORKING
The magnetic refrigeration is mainly based on magneto caloric effect according to which some materials change in temperature when they are magnetized and demagnetized.
Near the phase transition of the magnetic materials, the adiabatic application of a magnetic field reduces the magnetic entropy by ordering the magnetic moments.
This results in a temperature increase of the magnetic material. This phenomenon is practically reversible for some magnetic materials; thus, adiabatic removal of the field revert the magnetic entropy to its original state and cools the material accordingly. This reversibility combined with the ability to create devices with inherent work recovery, makes magnetic refrigeration a potentially more efficient process than gas compression and expansion. The efficiency of magnetic refrigeration can be as much as 50% greater than for conventional refrigerators
4PROPER FUNCTIONING
The place we want to cool it, we will apply magnetic field to the material in that place and as its temperature increases, it will absorb heat from that place and by taking the magnetic material outside in the surroundings, we will remove the magnetic material from magnetic field and thus it will loose heat as its temperature decreases and hence the cycle repeats over and again to provide the cooling effect at the desired place
REQUIREMENTS FOR PRATICAL APPLICATIONS
MAGNETIC MATERIALS
Only a limited number of magnetic materials possess a large enough magneto caloric effect to be used in practical refrigeration systems. The search for the "best" materials is focused on rare-earth metals, either in pure form or combined with other metals into alloys and compounds.
The magnetocaloric effect is an intrinsic property of a magnetic solid. This thermal response of a solid to the application or removal of magnetic fields is maximized when the solid is near its magnetic ordering temperature.
The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic order process: the magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems.
Currently, alloys of gadolinium producing 3 to 4 K per tesla of change in a magnetic field can be used for magnetic refrigeration or power generation purposes.
Recent research on materials that exhibit a giant entropy change showed that Gd5(SixGe1 − x)4, La(FexSi1 − x)13Hx and MnFeP1 − xAsx alloys, for example, are some of the most promising substitutes for Gadolinium and its alloys (GdDy, GdTy, etc...). These materials are called giant magneto caloric effect materials (GMCE).
SUPER CONDUCTING MAGNETS
Most practical magnetic refrigerators are based on superconductingmagnets operating at cryogenic temperatures (i.e., at -269 C or 4 K).These devices are electromagnets that conduct electricity with essentiallyno resistive losses. The superconducting wire most commonly used ismade of a Niobium-Titanium alloy
Future Applications
In general, at the present stage of the development of magnetic refrigerators with permanent magnets, hardly any freezing applications are feasible. These results, because large temperature spans occur between the heat source and the heat sink.
An option to realize magnetic freezing applications could be the use of superconducting magnets.
COMPARISON
The magneto caloric effect can be utilized in a thermodynamic cycle to produce refrigeration. Such a cycle is analogous to conventional gas-compression refrigeration
SOCIO-ECONOMIC
Competition in global market:-Research in this field will provide the opportunity so that new industries can be set up which may be capable of competing the global or international market.
Low capital cost:-The technique will reduce the cost as the most inefficient part comp. is not there and hence the initial low capital cost of the equipment.
Key factor to new technologies:-If the training and hard wares are developed in this field they will be the key factor for new emerging technologies in this world
ADVANTAGES OVER VAPOUR COMPRESSION AND VAPOR ABSORPTION CYCLE CYCLES
Magnetic refrigeration performs essentially the same task as traditional compression-cycle gas refrigeration technology. Heat and cold are not different qualities; cold is merely the relative absence of heat. In both technologies, cooling is the subtraction of heat from one place (the interior of a home refrigerator is one commonplace example) and the dumping of that heat another place (a home refrigerator releases its heat into the surrounding air). As more and more heat is subtracted from this target, cooling occurs. Traditional refrigeration systems - whether air-conditioning, freezers or other forms - use gases that are alternately expanded and compressed to perform the transfer of heat. Magnetic refrigeration systems do the same job, but with metallic compounds, not gases. Compounds of the element gadolinium are most commonly used in magnetic refrigeration, although other compounds can also be used.
CONCLUSION AND FUTURE ENHANCEMENTS
Magnetic refrigeration is a technology that has proven to be environmentally safe. Computer models have shown 25% efficiency improvement over vapor compression systems. In order to make the Magnetic Refrigerator commercially viable, scientists need to know how to achieve larger temperature swings. Two advantages to using Magnetic Refrigeration over vapor compressed systems are no hazardous chemicals used and they can be up to 60% efficient.There are still some thermal and magnetic hysteresis problems to be solved for these first-order phase transition materials that exhibit the GMCE to become really useful; this is a subject of current research. This effect is currently being explored to produce better refrigeration techniques, especially for use in spacecraft. This technique is already used to achieve cryogenic temperatures in the laboratory setting (below 10K).