Magnetic refrigeration was the first method developed for cooling below 2K. It is a low-temperature cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators, depending on the design of the system.
The magnetocaloric effect (MCE) is an intrinsic property of a magnetic solid. It is a magneto-thermodynamic phenomenon in which a change in temperature of a suitable material is caused by exposing the material to a changing magnetic field. This is also known as adiabatic demagnetization.
The working principle of magnetic refrigeration. When the magnetic material is placed in the magnetic field, the thermometer attached to it shows a high temperature as the temperature of it increases (material magnetized). But on the other side when the magnetic material is removed from the magnetic field, the thermometer shows low temperature as its temperature decreases (material demagnetized).
Magnetic refrigeration is mainly based on the magnetic caloric effect according to which some materials change in temperature when they are magnetized and demagnetized.
Near the phase transition of the magnetic materials, the adiabatic application of a magnetic field reduces the magnetic entropy by ordering the magnetic moments. This results in a temperature increase of the magnetic material. This phenomenon is practically reversible for some magnetic materials which cool the material accordingly. This reversibility combined with the ability to create devices with inherent work recovery makes magnetic refrigeration a potentially more efficient process than gas compression and expansion. The efficiency of magnetic refrigeration can be as much as 50% greater than that of conventional refrigerators.
The process is performed as a refrigeration cycle, analogous to the Carnot cycle, and can be described at a starting point whereby the chosen working substance is introduced into a magnetic field (ie. the magnetic flux density is increased). The working material is the refrigerant and starts in thermal equilibrium with the refrigerated environment.
- Adiabatic magnetization: A magnetocaloric substance is placed in an insulated environment. The increased external magnetic field (+H) results in heating (T+Tad).
- Isomagnetic enthalpic transfer: The added heat can then be removed (-Q) by a fluid or gas. The magnetic field is held constant to prevent the dipoles from reabsorbing the heat. Once sufficiently cooled, the magnetocaloric substance and the coolant are separated (H=0).
- Adiabatic demagnetization: The substance is returned to another adiabatic (insulated) condition so the total entropy remains constant. However, this time the magnetic field is decreased, the thermal energy causes the magnetic moments to overcome the field, and thus the sample cools, ie.., an adiabatic temperature change.
- Isomagnetic entropic transfer: The magnetic field is held constant to prevent the material from heating back up. The material is placed in thermal contact with the environment being refrigerated. 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 begins again.
This refrigeration could be used in any possible application where cooling, heating, or power generation is required. Since it is only at an early stage of development, there are several technical and efficiency issues that should be analyzed. The magnetocaloric refrigeration system is composed of pumps, electric motors, magnets, and magnetic materials. These processes are greatly affected by irreversibilities and should be adequately considered.
The technology is clearly not cost-and energy-efficient for home appliances, but for experimental, laboratory, and industrial use only.
Comparison between magnetic refrigeration and conventional refrigeration.
The thermodynamic cycle of magnetic refrigeration is analogous to a conventional gas-compression refrigeration system.
- High efficiency
- Reduced operating cost
- Competition in the global market
- Low capital cost
- Magnetic household refrigeration appliances
- Magnetic cooling and air conditioning in buildings and houses
- Central cooling system
- Refrigeration in medicine
- Cooling in the food industry and storage
- Cooling in transportation
- Cooling of electronics