Magnetic Refrigeration: An environment friendly cooling technology

Why does it need to look for an alternate cooling technology?

Cooling appliances based on thermodynamic cycle till 1995 used to produce many toxic components Chloroflurocarbon CFC), Hydrochloroflurocarbon (HCFC), potent greenhouse gases (GHG) and other ozone depleting substances (ODC). After gradual prohibition of such gases, the cooling appliances till date producing HFCs which have no impact on ozone layer but still contain potential GHGs. The GHGs are found to have major impact on atmospheric warming. Moreover, a refrigerator damaged and thrown to garbage release up to 3.7 metric tons of CO2 equivalent to the emission of a car covered a distance of approximately 17,000 Km.

What is Magnetocaloric effect (MCE) in Magnetic refrigeration?

Magnetic refrigeration is a cooling technology based on magnetocaloric effect (MCE). MCE is characterized by a heating or cooling effect exhibited by some magnetic materials when exposed to changing external magnetic field. The alignment of magnetic domains in these materials use energy and the consumed energy will be at the cost of heat and it appear as a cooling effect in these materials. The magnetic refrigeration exploits such magnetic properties of solids to produce refrigeration. Worth mentioning, it doesn’t emit any CFC or HCFC with potential harm to environment.

History

  • 1881: E. Warburg discovered MCE in pure iron.
  • 1926 & 1927: Debye and Giauque independently proposed a cooling method based on adiabatic demagnetization.
  • 1933: First experimental demonstration of the cooling technology based on MCE effect by Nobel Laureate William. F. Giauque & Dr. D. P. MacDougall.
  • 1997: Room temperature magnetic refrigeration was proven from the work published by Prof. Karl. A. Gscheidner.

Components of a magnetic refrigerator

  • Magneto caloric material
  • Magnet
  • Heat exchangers
  • Heat transfer medium
  • Motor
  • Pump
  • Valves

Working principle

The working principle of magnetic refrigeration is based on MCE usually perceived as an adiabatic temperature variation or isothermal entropy change. The conventional refrigeration system comprised of a compressor, two heat exchangers (cooling and heating), evaporator, condenser and a throttling device. In such a system, the refrigerant is a fluid such as CFC or HCFC which absorbs heat from the space to be refrigerated. The refrigerant fluid then undergoes a process of phase change by being evaporated by absorbing heat from the surrounding and get condensed by leaving heat out to the surrounding. This can be achieved by controlling the pressure inside the evaporator. The evaporator is fitted inside the fridge at low pressure to make the passing refrigerant fluid to evaporate. The compressor is maintained under high pressure to liquify the evaporated refrigerant. The throttling device reduce the pressure of the refrigerant to the pressure level of evaporator. The liquid under high pressure from compressor is injected into the low-pressure evaporator via a capillary tube. The liquid gets evaporated at the evaporator making the surrounding cool. The evaporated refrigerant gas then goes back to the compressor. The condenser cools the hot refrigerant gas leaves out of the high-pressure compressor and turns it into liquid and the cycle repeats. The condenser achieves this cooling effect by moving the heat through the condenser wall in touch with the surroundings.

Here, in the magnetic refrigeration system, the conventional CFC or HCFC refrigerant is replaced by a magneto-caloric material. The refrigerant picks up heat from the space to be refrigerated via the cold heat exchanger.

Magnetic refrigeration cycle

To begin with, the refrigerant (here a chosen MCE material ), which is in thermal equilibrium with the refrigerating environment is exposed to an external magnetic field. The cycle is analogous to the Carnot cycle in vapor compression type cooling systems. The major difference is, the cycle follows the variations in the external magnetic field strength as compared to that of the increase or decrease in pressure followed in Carnot cycle of vapor compression systems.

The refrigeration cycle is as follows:

Adiabatic magnetization: An MCE material is exposed to an external magnetic field in insulating (adiabatic) environment. The magnetic dipoles get aligned with increase in field strength. This causes a decrease in magnetic entropy and heat capacity of the material. The net result will be an effective increase in temperature and so the material gets heated.

Isomagnetic enthalpic transfer: In this cycle the heat gained in adiabatic magnetization process is removed by gaseous or liquid medium called coolant. The magnetic field is held constant throughout this cycle to avoid any reabsorption/emission of heat with spin alignment.

Adiabatic demagnetization: During this cycle, the magnetic domains of the material become disoriented with the decrease in applied magnetic field strength. The disorientation occurs by agitating the phonons present in the material. As it is restricted as an adiabatic process in which no energy is allowed to enter or go out from the material, the magnetic domain reorientation happen at the cost of agitating phonons (thermal energy). The temperature drops as the magnetic domains take this thermal energy for their reorientation resulting to the cooling effect.

Isomagnetic entropic transfer: The material is placed in thermal contact with the environment to be refrigerated by keeping the external magnetic field constant. As the refrigerant material is cooler than the environment, heat transfers from the surrounding to the material resulting to refrigeration effect.

To be concise, MCE is an intrinsic property of some magnetic solids and magnetic refrigeration make use of the the thermal response of such materials to the applied magnetic field.

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