Magnetic Refrigeration

NON-CONVENTIONAL REFRIGERATION (MAGNETIC REFRIGERATION)

A Technical Paper K.Anil Kumar.

L.Subba rao

[email protected]

[email protected]

Contact no :9866686300 (IV/IV B.Tech Mechanical Engineering) RVR & JC College of engineering, Guntur.

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 (CFC's/HCFC's). 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.

INTRODUCTION:

Refrigeration: Definition: Refrigeration is the process of reducing the temperature of any substance below that of the surrounding temperature using some working medium called refrigerants. Initially refrigeration was used in the preservation of foodstuff by preventing bacterial action and this technology was further developed and extended its use in industrial applications. For example cool cutting oil helps in machining operations by lowering the temperature of work piece to prevent overheating, Quenching baths for heat treating operations, pharmaceutical field, etc are some of the industrial applications. Conventional Refrigeration Vs Non-conventional (Magnetic) Refrigeration : In conventional refrigeration system we need a medium for the removal of heat from the refrigerator to the surrounding atmosphere. This medium may be a solid, liquid or a gas. Some of the refrigerants which were used initially are ammonia (NH3), carbon dioxide (CO2), sulphur dioxide (SO2), etc. There are some drawbacks in the use of these refrigerants so refrigerants like F-11, F-12, F-22, F-113, etc are being used which are both economical as well as efficient. Minimum temperature that can be obtained by these refrigerants is 0.71oK by boiling liquid helium under the smallest pressure obtainable. Temperatures below this range can be obtained only by the use of Non-Conventional refrigeration system. Magnetic

refrigeration

is

the

method

of

refrigeration

based

on

MAGNETOCALORIC EFFECT, which is defined as the response of a solid to an applied magnetic field, which is apparent as a change in its temperature.

Instead of ozone-depleting refrigerants and energy-consuming compressors found in conventional vapor-cycle refrigerators, this new style of refrigerator uses iron ammonium alum that heats up when exposed to a magnetic field, then cools down when the magnetic field is removed. NON-CONVENTIONAL REFRIGERATION : TYPES INCLUDE : 1. Thermo Electric Refrigeration. 2. Acoustic Refrigeration. 3. Magnetic Refrigeration.

MAGNETIC REFRIGERATION: PRINCIPLE: Magnetic refrigerants heat up when they are subjected to a magnetic field because the second law of thermodynamics states that the entropy - or disorder - of a closed system must increase with time. This is because the electron spins in the atoms of the material are aligned by the magnetic field, which reduces entropy. To compensate for this, the motion of the atoms becomes more random, and the material heats up. In a magnetic refrigerator, this heat would be carried away by water or by air. When the magnetic field is turned off, the electron spins become random again and the temperature of the material falls below that of its surroundings. This allows it to absorb more unwanted heat, and the cycle begins again.

Producing very low temperature through the process of adiabatic demagnetization can do refrigeration. The paramagnetic salt is suspended by a thread in a tube containing a low pressure of gaseous helium to provide thermal communication with the surrounding bath of pumped helium. In operation the liquid helium bath is cooled by pumping to the lowest practical pressure, usually achieving a temperature in the neighborhood of 1oK. The temperature of the paramagnetic salt approaches that of the helium bath by conduction through the exchange gas. Next the magnetic field is turned on, causing heating of the salt and a decrease in entropy of the magnetic ions by virtue of their partial alignment in the direction of the applied field. The heat produced is conducted to the surrounding bath of liquid helium so that the temperature again approaches 1oK. If the magnetic field is increased slowly the heat can flow out, as it is generated-the magnetization being almost isothermal. Next the exchange gas surrounding the sample is removed by pumping, and now, with the salt thermally isolated, the magnetic field is turned off.

The temperature of the sample decreases markedly as a consequence of the adiabatic demagnetization, which allows the magnetic ions to regain some of their entropy at the expense of the lattice energy of the salt. The iron ammonium alum salt, originally in zero field (H=0,S=S1), is magnetized isothermally at the temperature T1, by increasing the magnetic field to H=H1.This magnetization, by orienting the magnetic ions of the salt and thus decreasing their disorder, causes a reduction in entropy from S1 to S2. Now the salt is isothermally isolated from its surroundings and thus when the magnetic field is reduced to zero the process follows the horizontal isentropic line and the temperature falls to 10K.The great decrease in temperature and the close approach zero is a consequence of the peculiar shape of the entropy-temperature relation WORKING The process flow diagram for the Magnetic Refrigeration system is show in the figure below. The mixture of water and ethanol serves as the heat transfer fluid for the system. The fluid first passes through the hot heat exchanger, which uses air to transfer heat to the atmosphere. The fluid then passes through the copper plates attached to the non-magnetized cooler Magneto caloric beds and loses heat.

CYCLE FOR MAGNETIC REFRIGERATION IN POSITION 1:

Driving Shaft

Magneto caloric Bed Copper Plates Pump

To Atmosph ere

Hot Heat Exchan ger

Electro magnets

Cold Heat Exchan ger

To Fa n

Vessel A fan blows air past this cold fluid into the freezer to keep the freezer temperature at approximately 0°F. The heat transfer fluid then gets heated up to 80°F as it passes through the copper plates adjoined by the magnetized warmer Magneto caloric Beds, where it continues to cycle around the loop. However, the Magneto caloric beds simultaneously move up and down, into and out of the magnetic field.

CYCLE FOR MAGNETIC REFRIGERATION IN POSITION 2 :

Driving Shaft

Magnetocalori c Bed Copper Plates Pump

To Atmosph ere

Hot Heat Exchan ger

Electro Magnets

Cold Heat Exchan ger

To Fa n

Vessel

Figure below, shows how the cold air from the freezer is blown into the refrigerator by the freezer fan. The temperature of the refrigerator section is kept around 39°F. The typical household refrigerator has an internal volume of 21 cu.ft, where the freezer represents approximately 30% of this volume. Freezers are designed to maintain a temperature of 0oF. Refrigerators maintain a temperature of 39oF. The

refrigerator will be insulated with polyurethane foam, one of the most common forms of insulation available. The refrigerator is kept cool by forcing cold air from the freezer into the refrigerator by using a small fan. The control system for machining the desired internal temperatures consists of two thermostats with on/off switches.

Driving Shaft

Magneto caloric Bed

Copper Plates Electro Magnets Freeze r fan

Pump

Freeze r Refriger 0°F ator fan

To Fa n

To Atmosph ere

Refrigerat or 39°F

ROOM AIR Vessel FIG : REFRIGERATOR WITH A FREEZER FAN

The freezer thermostat regulates the temperature by turning the compressor off when the temperature gets below 0oF. A second thermostat regulates the fan that cools the refrigerator to 39oF.

RESULTS OBTAINED FOR VARIOUS MAGNETIC FIELDS :

FIG : ENTROPY-TEMPERATURE DIAGRAM FOR IRON AMMONIUM ALUM The above figure is an entropy-temperature diagram for iron ammonium alum for various magnetic fields; on it is super imposed the refrigeration cycle ABCDA. From A to B the working salt is magnetized isothermally and heat is absorbed by the liquid alcohol-water bath. From B to C the salt is demagnetized isentropically, causing a substantial decrease of temperature. From C to D the salt is demagnetized isothermally, extracting heat from the experimental region. From D to A the salt is remagnetized to starting condition. ADVANTAGES : 1. Very low temperatures of the order of 001K can be obtained. 2. Required pressures are obtained without the aid of a compressor. 3. Does not produce toxic gases and chloro-fluoro carbons, thus reducing ozone layer depletion. 4. Efficiency and compactness are increased where as power consumption is reduced. 5. Larger temperature swings that will allow the technology to provide the

Cooling power required for specific markets, such as home refrigerators, air conditioning, electronics cooling, and fluid chilling can be obtained. 6. The unit runs virtually silent and is vibration free. 7. The magnetic material in the regenerator bed will ever need to be replaced when changing refrigerant to achieve a different temperature range. 8. When a better magnetic material is developed, the refrigerator will not need to be redesigned.

APPLICATIONS: 1. It is used in large-scale refrigeration, food processing, heating& airconditioning, liquor distilling, grain drying, waste separation and treatment systems. 2. Magnetic refrigerator can be utilized in actual engineering applications, such as cooling sensitive electronics and optical devices on board spacecraft.

REFERENCES: 1. A textbook on “CRYOGENIC ENGIMEERING” by V.J. Johnson. 2. “Refrigeration and Air-conditioning” by Arora & Domkundwar.