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VIII SEM ME
Department of Mechanical Engineering
To develop more efficient and cost-effective small-scale H2 liquefiers asan alternative to vapour-compression cycles using Magneticrefrigeration (adiabatic magnetization)
To understand the Principle and mechanism for generating coolingeffect using the magnet.
Magnetic refrigeration is a cooling technology based on the magneto caloric 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.
It is a physical process that exploits the magnetic properties of certain solid materials
to produce refrigeration.
The refrigerant is often a paramagnetic salt, such as cerium magnesium nitrate.
It gives cooling nearest to absolute zero than any other method hence it made
liquidification of gases easier.
At the same time it does not emit any CFC or HCFC compounds hence it never
affects our environment specially OZONE layer.
Magneto caloric effect was discovered in pure iron in 1881 by E.Warburg.
DeDebye (1926) & Giauque (1927) proposed a improved techniqueof cooling via adiabatic demagnetization independently.
The cooling technology was first demonstrated experimentally in1933 by chemist Nobel Laureate William F.Giauque & his colleagueDr.D.P. MacDougall for cryogenic purposes.
In 1997,Prof. Karl A. Gscheidner, Jr. by the lowa State University atAmes Laboratory demonstrated the first near room temperatureproof of concept magnetic refrigerator.
MCE is a magneto-thermodynamic phenomenon in which a reversiblechange in temperature of a suitable material is caused by exposing thematerial to changing magnetic field.
All magnets bears a property called Currie effect i.e. If a temperature ofmagnet is increased from lower to higher range at certain temperaturemagnet looses the magnetic field.
Currie temperature Depends on individual property of each material.
As Energy input to the magnet is increased the orientation of themagnetic dipoles in a magnet starts loosing orientation. And vice a versaat curie temperature as magnet looses energy to the media it regains theproperty.
• Process is similar to gascompression and expansioncycle as used in regularrefrigeration cycle
• Steps of thermodynamic Cycle :->
Isomagnetic Enthalpy Transfer
Isomagnetic Entropic Transfer
Substance placed in insulated environment
Magnetic field +H increased
This causes the magnetic dipoles of the atoms to align
The net result is that total Entropy of the item is not reduced and item heats up (T + ΔTad )
Added heat removed by a fluid like water or helium (-Q)
Magnetic Field held constant to prevent the dipoles fromreabsorbing the heat.
After a sufficient cooling Magnetocalric material and coolant areseparated(H=0)
Substance returned to another adiabatic(insulated) condition
Entropy remains constant
Magnetic field is decreased
Thermal Energy causes the Magnetic moments to overcome thefield and sample cools(adiabatic temperature change)
Energy transfers from thermal entropy to magnetic entropy(disorderof the magnetic dipoles)
Material is placed in thermal contact with the Environment beingrefrigerated.
Magnetic field held constant to prevent material from heating backup.
Because the working material is cooler than the refrigeratedenvironment, heat energy migrates into the working material (+Q)
Once the refrigerant and refrigerated environment are in thermal
equilibrium, the cycle continuous.
Components required for construction :-
Hot Heat exchanger
Cold Heat Exchanger
Magneto caloric wheel
Super Conducting Magnets
Active Magnetic Regenerators
MCE is an intrinsic property of a magnetic solid
Ease of application and removal of magnetic effect is most desiredproperty of material
Alloys of gadolinium produce 3 to 4 K per tesla of change inmagnetic field are used for magnetic refrigeration or powergeneration purposes.
ferrimagnets, antiferromagnets and spin glass systems are notsuitable for this application.
Gd5(SixGe1 − x)4, La(FexSi1 − x)13Hx and MnFeP1 − xAsx alloys are some ofthe most promising substitutes for Gadolinium and its alloys
Magnetic refrigeration requires excellent heat transfer to and from thesolid magnetic material. Efficient heat transfer requires the large surfaceareas offered by porous materials. When these porous solids are used inrefrigerators, they are referred to as “Regenerators”
Typical regenerator geometries include:
• Perforated plates
• Wire screens
• Particle beds
Most practical magnetic refrigerators are based on superconducting magnets operating at cryogenic temperatures (i.e., at -269 C or 4 K)
These devices are electromagnets that conduct electricity with essentially no resistive losses.
The superconducting wire most commonly used is made of a Niobium-Titanium alloy
A regenerator that undergoes cyclic heat transfer operations and themagneto caloric effect is called an Active Magnetic Regenerator.
An AMR should be designed to possess the following attributes:-
High heat transfer rate High magneto caloric effect Sufficient structural integrity Low thermal conduction in the direction of fluid flow Affordable materials Ease of manufacture
• High Efficiency
• Reduced Operating Cost
• Competition in Global Market
• Low Capital Cost
• Key Factor to new technologies
At the present stage of the development of magnetic refrigerators withpermanent magnets, hardly any freezing applications are feasible.
Some of the future applications are:-
Magnetic household refrigeration appliances
Magnetic cooling and air conditioning in buildings and houses
Central cooling system
Refrigeration in medicine
Cooling in food industry and storage
Cooling in transportation
Cooling of electronic equipments
Purchase cost may be high, but running costs are 20% less than theconventional chillers.
Thus life cycle cost is much less.
Ozone depleting refrigerants are avoided in this system, hence it moreeco-friendly.
Energy conservation and reducing the energy costs are added advantages.
The efficiency of magnetic refrigeration is 60% to 70% as compared toCarnot cycle.
Magnetic refrigeration is totally maintenance free & mechanically simplein construction.
As every coin has 2 sides, this technique also posses some drawbacks to be worked on
The initial investment is more as compared with conventionalrefrigeration.
The magneto caloric materials are rare earth materials hence theiravailability also adds up an disadvantage in MAGNETIC REFRIGERATION.
It is a technology that has proven to be environmentally safe. Computermodels have shown 25% efficiency improvement over vapor compressionSystems.
In order to make the magnetic refrigerator commercially Viable, scientistsneed to know how to achieve larger temperature swings and alsopermanent magnets which can produce strong magnetic fields of order10 tesla.
There are still some thermal and magnetic hysteresis problems to beSolved for the materials that exhibit the MCE to become really useful.