Magnetic Refrigeration (MR)

Definition

MR is an emerging technology based on Magneto Caloric Effect (MCE)

MCE is caused due to varying magnetic entropy of magnetic materials in varying magnetic field

MCE concept is used because in traditional vapor compression refrigeration (VCR)

High cost of electricity to operator compressor

The refrigerant gas is costing

High cost of compressor

The refrigerant gas toxic and flamable

Principle of MCE

A heating or cooling of Magneto Caloric Material when it is magnetized and demagnetized respectively where pressure has no influence

Focus on the 2nd Law of Thermodynamics because it is related to energy and entropy

Entropy is the degree of randomness

  • High randomness
  • High energy
  • High disorder

and vice versa

Magnetic Entropy f(H,T)

Thermal Entropy f(T)

Theory of Magneto Caloric Effect

1) Adiabatic Magnetization (H is high)

2) Adiabatic Demagnetization (H is low)

Q=0 -> Stotal = CONSTANT

  • H (magnetic filed) is high, Sm (magnetic entropy) is reduced to compensate it high St (thermal entropy)

Temperture of material increases because :

  • Electron spin within material are forced to align with magnetic field.
  • Thus magnetic field does not work at higher order state.
  • Q=0, by the first law of thermodynamic internal energy increase, temperature increases (Eint;inc , T;inc).
  • It's magnetic entropy (Sm) reduced and thermal entropy (St) in vibrational lattice increases to compensate total entropy (Stotal).

Q=0 -> Stotal = CONSTANT

  • H is low, Sm increased to compensate it low St

Temperature of the material decreases because :

  • The electron spin returns to original alignment (no push factor) as St decreased (vibration of molecule decreases)
  • Sm is increased and St is decreased to compensate Stotal.

Using the concept of Refrigeration Cycle

1) Adiabatic Magnetization
2) Isomagnetic Cooling
3) Adiabatic Demagnetization
4) Isomagnetic Heating

Necessary part of Magnetic Refrigerator

Hot heat excanger

Heat transfer fluid

Cold heat exchnager

Motor, pump & valve

Magnet

Power supply

Magneto Caloric material

Glycol water

Ethanol water

Properties of Magneto Caloric Materials

  • Show increase in Magneto Caloric effect (Tadiabatic increase)
  • Low in specific heat - ratio of the quantity of heat required to raise temperature of a body degree.
  • High thermal conductivity - generate from non electricity to electricity lead to the thermal conductivity.
  • non toxic - environmental friendly
  • Resistance to corrosion - bcs it will reduce the quality of refrigeration.
  • High electrical resistance - high power.
  • Low manufacturing cost.
  • Low environmental impact.

Example of material

Gadolinium and some alloys

Advatages

Disadvantages

Safety cooling and less environmental effect

Low pressure operation

Low cost

No refrigerant

Achieving very low temperature nearly 4 Kelvin

No compressor

Initial cost of refrigerator is high (due to Gadolinium)

Highly reversable

Limitation

Idea of magneto caloric material still in research process

Magnetic fridge usage still in research

Application

Cryogenic engine

Magnetic air conditioner in building and houses