23-12-2010, 12:18 PM
Prepared by::ELDHOSE KURIAN
INTRODUCTION
Magnetic refrigeration is a cooling technology based on the magneto caloric effect.
Invented by Emil Warburg in 1880
Used to attain cryogenic temperatures well below 1o K with the help of magnetic fields
PRINCIPLE OF MAGNETIC REFRIGERATION
Magneto caloric effect is the basic principle on which the cooling is achieved.
Magneto caloric effect 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 called adiabatic demagnetisation .
THERMODYNAMIC CYCLE
STEPS OF THERMODYNAMIC CYCLE
Adiabatic magnetisation
Isomagnetic enthalpic transfer
Adiabatic demagnetization
Isomagnetic entropic transfer
REQUIREMENTS FOR PRACTICAL APPLICATIONS
Magnetic Materials
Regenerators
Super Conducting Magnets
Active Magnetic Regenerators(AMR’s)
MAGNETIC MATERIALS
Magnetocaloric effect is an intrinsic property of a substance
Gadolinium and its alloys are the best materials available for magnetic refrigeration
They can reach ultra low temperatures
REGENERATORS
Efficient heat transfer requires large surface areas offered by porous materials
When these porous materials are used in refrigerators they are referred to as regenerators
SUPER CONDUCTING MAGNETS
The magnetic field is created by superconducting magnets
Most practical magnetic refrigerators are based on super conducting magnets operating at cryogenic temperatures
Commonly used is made of a Niobium-Titanium alloy
ACTIVE MAGNETIC REGENERATORS
A regenerator that undergoes cyclic heat transfer operations and magneto caloric effect is called Active Magnetic Regenerator
It should be designed to possess:
high heat transfer rate
high magneto calorific effect
affordable materials
ease of manufacture
APPLICATIONS
Liquefaction purposes in the case of hydrogen,nitrogen and helium
Also focuses on many future applications like magnetic household refrigeration,cooling in transportation,cooling electronic circuits etc.
MERITS
High efficiency
Reduced operating cost
Reliability
CONCLUSIONS
Magnetic refrigeration is a technology that has proven to be environmentally safe
In order to make the Magnetic Refrigerator commercially viable, scientists need to know how to achieve larger temperature swings and also permanent magnets which can produce strong magnetic fields of order 10 tesla
There are still some thermal and magnetic hysteresis problems to be solved for the materials that exhibit the MCE to become really useful
REFERENCES
http://en.wikipediawiki/Magnetic refrigeration
A text book on Refrigeration and Air conditioning by C P Arora and Domkundwar
A text book on Cryogenic Engineering by Thomas M. Flynn
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