Defination

SEMINAR ONMAGNETIC REFRIGERATION

Presented By:- Guru Raj Mechanical 4th year Department of Mechanical Engineering

ObjectiveTo develop more efficient and cost-effective small-scale H2 liquefiers as an alternative to vapour-compression cycles using Magnetic refrigeration (adiabatic magnetization)

To understand the Principle and mechanism for generating cooling effect using the magnet.

Introduction 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.

History Magneto caloric effect was discovered in pure iron in 1881 by E. Warburg.

DeDebye (1926) & Giauque (1927) proposed a improved technique of cooling via adiabatic demagnetization independently.

The cooling technology was first demonstrated experimentally in 1933 by chemist Nobel Laureate William F.Giauque & his colleague Dr.D.P. MacDougall for cryogenic purposes.

In 1997,Prof. Karl A. Gscheidner, Jr. by the lowa State University at Ames Laboratory demonstrated the first near room temperature proof of concept magnetic refrigerator.

Magneto Caloric Effect MCE is a magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to changing magnetic field.

All magnets bears a property called Currie effect i.e. If a temperature of magnet is increased from lower to higher range at certain temperature magnet looses the magnetic field.

Currie temperature Depends on individual property of each material.

As Energy input to the magnet is increased the orientation of the magnetic dipoles in a magnet starts loosing orientation. And vice a versa at curie temperature as magnet looses energy to the media it regains the property.

Working Principle

Thermodynamic Cycle

Details of Thermodynamic Cycle Process is similar to gas compression and expansion cycle as used in regular refrigeration cycle

Steps of thermodynamic Cycle :->

Adiabatic Magnetization

Isomagnetic Enthalpy Transfer

Adiabatic demagnetization

Isomagnetic Entropic Transfer

Adiabatic MagnetizationSubstance 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 + Tad )

Isomagnetic Enthalpy TransferAdded heat removed by a fluid like water or helium (-Q)

Magnetic Field held constant to prevent the dipoles from reabsorbing the heat.

After a sufficient cooling Magnetocalric material and coolant are separated(H=0)

Adiabatic DemagnetizationSubstance returned to another adiabatic(insulated) condition

Entropy remains constant

Magnetic field is decreased

Thermal Energy causes the Magnetic moments to overcome the field and sample cools(adiabatic temperature change)

Energy transfers from thermal entropy to magnetic entropy(disorder of the magnetic dipoles)

Isomagnetic Entropic Transfer Material is placed in thermal contact with the Environment being refrigerated.

Magnetic field held constant to prevent material from heating back up.

Because the working material is cooler than the refrigerated environment, heat energy migrates into the working material (+Q)

Once the refrigerant and refrigerated environment are in thermal equilibrium, the cycle continuous.

Comparison

Construction

Components required for construction :-

Magnets

Hot Heat exchanger

Cold Heat Exchanger

Drive

Magneto caloric wheel

Requirements for Practical Applications Magnetic Materials

Regenerators

Super Conducting Magnets

Active Magnetic Regenerators

Working Materials

Working Materials MCE is an intrinsic property of a magnetic solid

Ease of application and removal of magnetic effect is most desired property of material

Alloys of gadolinium produce 3 to 4 K per tesla of change in magnetic field are used for magnetic refrigeration or power generation purposes.

ferrimagnets, antiferromagnets and spin glass systems are not suitable for this application.

Gd5(SixGe1 x)4, La(FexSi1 x)13Hx and MnFeP1 xAsx alloys are some of the most promising substitutes for Gadolinium and its alloys

RegeneratorsMagnetic refrigeration requires excellent heat transfer to and from the solid magnetic material. Efficient heat transfer requires the large surface areas offered by porous materials. When these porous solids are used in refrigerators, they are referred to as Regenerators

Typical regenerator geometries include:

TubesPerforated platesWire screensParticle beds

Super Conducting MagnetsMost 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

AMRA regenerator that undergoes cyclic heat transfer operations and the magneto 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

Future Applications At the present stage of the development of magnetic refrigerators with permanent 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

AdvantagesRunning costs are 20% less than the conventional chillers.

Thus life cycle cost is much less.

Ozone depleting refrigerants are avoided in this system, hence it more eco-friendly.

Energy conservation and reducing the energy costs are added advantages.

The efficiency of magnetic refrigeration is 60% to 70% as compared to Carnot cycle.

Magnetic refrigeration is totally maintenance free & mechanically simple in construction.

DisadvantagesAs every coin has 2 sides, this technique also posses some drawbacks to be worked on

The initial investment is more as compared with conventional refrigeration.

The magneto caloric materials are rare earth materials hence their availability also adds up an disadvantage in MAGNETIC REFRIGERATION.

ConclusionIt 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 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.

Thank You Any Query ?