Magnetic refrigeration is a relatively rare cooling method. It contrasts with common compression and absorption refrigeration methods and has its advantages, primarily in the aerospace field. With deeper research, this technology has also progressed in civilian ambient temperature refrigeration.
Principle of Magnetic Refrigeration Technology
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect, referring to the process in which the temperature of magnetic materials changes under an applied magnetic field, thereby generating cooling or heating.
Magnetic materials consist of many tiny magnetic particles, each containing a magnetic moment determined by the electron’s spin and orbital angular momentum. When placed in an external magnetic field, the magnetic moments rearrange to adapt to the field, a process known as magnetic moment orientation.
By placing magnetic materials and magnetic field sources in an adiabatic environment and applying an external magnetic field, the magnetic moment orientation changes, and the magnetic moments align in an orderly state. At this time, the internal energy of the magnetic material increases, the temperature rises, and heat is released. After removing the external magnetic field, the magnetic moments return to a disordered state, causing the magnetic material’s temperature to drop, thereby absorbing external heat and generating a cooling effect.
Components of Magnetic Refrigeration
To achieve the magnetic refrigeration process, components such as magnetic materials, magnetic field sources, and heat exchangers are required.
In an adiabatic environment, applying an external magnetic field causes the magnetic material to undergo magnetic moment orientation changes and release heat. The heat is expelled through the heat exchanger from the radiator.
When the magnetic field is removed, the temperature of the magnetic material decreases. Compared to the object being cooled, the temperature of the magnetic material is lower; therefore, the magnetic material absorbs heat from the cooled object, causing its temperature to drop. This cycle continues, creating a closed loop for heat flow. The repeated cycles continuously absorb the heat from the cooled object, achieving a cooling effect.
Magnets Used in Magnetic Refrigeration Technology
Role of Magnets
The role of magnets in magnetic refrigeration technology is to provide a strong magnetic field that encourages the rearrangement of magnetic moments in the magnetic refrigeration material, resulting in temperature changes within the material, thereby achieving the cooling effect.
Types of Magnets
Commonly used magnets in magnetic refrigeration technology mainly include the following:
| Neodymium Magnets: This is a type of man-made permanent magnet with very high magnetic energy product and residual magnetism, widely used in magnetic refrigeration technology.
| Samarium Cobalt Magnets: These materials exhibit excellent magnetic properties at high temperatures and are suitable for high-temperature magnetic refrigeration applications.
Magnetic Materials Used in Magnetic Refrigeration Technology
Finding efficient magnetic refrigeration materials is the key to magnetic refrigeration technology
| Mn-based Metal Compounds: The focus of research on Mn-based metal compounds is on replacing As in the compounds and adding filler atoms. With an increase in X value, the Curie temperature of MnFep1-Xix materials gradually rises, while the thermal hysteresis ΔThys gradually decreases. Under a magnetic field of 0-2T, the maximum magnetic entropy change of the compounds ranges from 7 to 13 J/(kg·K), which is smaller compared to GD metals, but their cooling capacity (RCP) is about 1.5-2.0 times larger.
| GD Metal Compounds: GD metals and their alloy compounds have been widely studied due to their relatively low Curie temperature (293K). For example, the Curie temperature of GD3Ga5O12 (gadolinium gallium garnet) is about 2K, making it suitable for low-temperature refrigeration.
| La-based Metal Compounds: La-based metal compounds, especially the La(Fe, Si)13 series, have a first-order phase transition that belongs to a ferrimagnetic phase transition process, accompanied by isotropic volume changes during the phase transition, which is prone to fracture. Nevertheless, under a 2T magnetic field change, the magnetic entropy change of LaFe11.6Si1.4 alloy can reach 13 J/(kg·K), demonstrating a giant magnetocaloric effect.
| Heusler Alloys: Heusler alloys were proposed by Fritz Heusler, including half-Heusler alloys with a 1:1:1 ratio and full Heusler alloys with a 2:1:1 ratio of intermetallic compounds. Heusler alloys exhibit giant magnetocaloric effects and shape memory effects, making them a research hotspot in the field of magnetic refrigeration.
Advantages of Magnetic Refrigeration Technology
| High efficiency and energy saving: The magnetic refrigeration system operates through changes in magnetic fields rather than mechanically compressing gases like traditional methods. This means magnetic refrigeration can operate at very low temperatures while consuming less energy. The theoretical efficiency of magnetic refrigeration can reach 60-70% of the Carnot cycle efficiency, while gas compression refrigeration generally ranges from 20-40%. Therefore, under the same conditions, the power consumption of magnetic refrigeration is about 50% of that of traditional compression refrigeration refrigerators.
| Eco-friendly solutions: It does not require the use of any harmful chemicals, which is a significant difference from traditional compression refrigeration methods that utilize refrigerants to absorb and release heat. These refrigerants may contain organic fluorocarbons, ammonia, and hydrocarbons, which have detrimental effects on the environment, such as ozone depletion, toxicity, leakage, flammability, and explosiveness.
| Ease of miniaturization: Since magnetic working substances are solids, their entropy density is far greater than that of gases, making magnetic refrigeration devices easier to miniaturize and suitable for various space-constrained environments.
| Stable and reliable: Magnetic refrigeration does not require gas compressors, has fewer moving parts, produces no vibrations or noise, and boasts high reliability and long service life, making maintenance easier.
Challenges of Magnetic Refrigeration Technology
Despite the numerous advantages of magnetic refrigeration technology, its commercialization still faces some challenges.
| Selection of Magnetic Materials: Currently, the magnetic materials used in magnetic refrigeration technology have small magnetic entropy changes, leading to limited temperature differences generated during each magnetic refrigeration cycle. Therefore, developing magnetic materials with higher magnetic entropy changes is key to the advancement of the technology.
| Design of Magnetic Field Sources: To achieve the magnetic refrigeration cycle, a varying magnetic field needs to be provided. Common magnetic field sources include electromagnets, permanent magnets, and superconducting electromagnets, each with its pros and cons. Designing a new type of magnetic field source with high efficiency, low consumption, and controllability is another challenge.
| Optimization of Refrigeration Devices: Besides magnetic materials and magnetic field sources, the structure and parameters of refrigeration devices also need to be considered in terms of their impact on refrigeration efficiency and cost. Selecting an appropriate working cycle, designing suitable cold accumulators and heat exchangers, and controlling the right working frequency and amplitude are all issues that need further research and optimization.
Magnet Manufacturer
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