It is extremely relevant to create currently a compact, environmentally friendly, energy-efficient and highly reliable refrigerator operating in the room’s temperature range. This task is due to a number of serious claims to the existing cooling systems. It is known that when operating refrigerators currently in use, there may be the leaks of working gases (refrigerants) that cause such serious environmental problems as ozone depletion and global warming are possible. Among a variety of alternative technologies that could be used in the cooling devices, more and more attention of researchers around the world is attracting to the technology of magnetic cooling. Intensive work on magnetic cooling is leading in many laboratories and universities in Europe, USA, Canada, China and Russia.
A magnetic refrigerator is an environmentally friendly device and allows to significantly reducing power consumption. The latter circumstance is extremely important given the truly huge number of refrigeration units used by man in various fields of his activity. The magnetic cooling technology is based on the ability of any magnetic material to change its temperature and entropy under the influence of a magnetic field. This ability manifests itself when compressing or expanding gas or steam in traditional refrigerators. Such a change in the temperature or entropy of the magnetic material due to a change in the strength of the magnetic field is called the magnetocaloric effect (hereinafter referred to as MCE).
The change in the temperature of the magnetic material is the result of the redistribution of the internal energy of the magnetic substance between the system of magnetic moments of its atoms and crystal lattice. The maximum value of MCE is obtained in magnetically ordered materials, such as ferromagnets, antiferromagnets, etc., located at temperatures of magnetic phase transitions (temperatures of magnetic ordering - Curie, Neel, etc.).
The main advantage of devices for magnetic cooling is associated with a high density of heat transfer material - a solid body compared to the steam or gas density. The change of entropy per unit volume in the solid magnetic materials is seven times higher than in a gas. This allows making it possible to design refrigerators that are more compact. The magnetic working medium itself serves as an analogue of refrigerants used in traditional combined-cycle refrigerating plants. Moreover, the demagnetization-magnetization process is analogous to compression-expansion cycles. The efficiency of any refrigerator is mainly determined by the amount of irreversible work done during the cycle - for the effective devices, it should be as low as possible. In a gas-heated refrigerator, there are devices that produce a significant amount of irreversible work - a regenerator, a compressor and heat exchangers.
Much of irreversible work is done in the heat exchangers. It is directly proportional to the adiabatic change in the temperature of the working fluid. It is much larger in a gas than in a magnetic material. Due to this, the most efficient heat dissipation is performed with magnetic, especially in the regenerative, refrigeration cycle. The special design of the heat exchanger and the use of a regenerator with a large surface area make it possible to achieve a small part of irreversible work during the magnetic cooling. The effectiveness of the magnetic regenerative cooling cycle in the temperature range from 4.5 to 300 0K can be from 38 to 60% of the Carnot cycle (about 52% efficiency in the temperature range from 20 to 150 0K, and about 85% in the range from 150 to 300 0K). Herewith, at all stages of the cycle, the heat transfer conditions will be the best known for today. In addition, magnetic refrigerators include a small number of moving parts, operate at low frequencies, which allow minimizing the wear of refrigerator and increase its operating time.
The Chronology of This Problem. Basic Principles of Magnetic Cooling
- Warburg discovered the MCE relatively long ago, in 1881. He observed how, under the action of a magnetic field, the iron sample heated up, or cooled. This scientist concluded that the temperature changing of the sample is a consequence of the change in the internal energy of a substance, having a magnetic structure, under the influence of a magnetic field.
However, it was still far away before the practical use of this phenomenon. Langevin (in 1905) was the first who demonstrated that the magnetization change of a paramagnet leads to a reversible changing in the temperature of a sample. The magnetic cooling itself was proposed almost 50 years after discovering of MCE, by two American scientists, Peter Debye (in 1926) and William Giauque (in 1927), independently of each other, as a mode of achieving temperatures below liquid helium boiling point. Jiok and McDougall were the first to demonstrate the simplest experiment in the magnetic refrigeration in 1933. A little later it was also done by de Haas (in 1933) and Kurti (in 1934). In the course of this experiment, it was possible to reach a temperature of 0.25 0
K. In addition, as the heat-transferring substance, the pumped liquid helium was used for at a temperature of 1.5 0
The pill with the magnetic salt was being in a thermal equilibrium state with the heat-sinking material, while there was a strong magnetic field in the solenoid. Whenever the solenoid discharged, the magnetic pill has become thermally insulated and its temperature lowered. Such technique, called cooling by the adiabatic demagnetization, is a standard laboratory technique, used to obtain the ultra-low temperatures. However, the capacity of such a refrigerator and its operating temperature range are too small for the industrial applications. More complex methods, with the thermal regeneration and cyclic changes in the magnetic field, were proposed in the 60s of the last century. In 1976, J. Brown (from NASA) demonstrated a regenerative magnetic refrigerator, operating at a working temperature range of 50 0
K already at the range of room's temperature. However, the power of the refrigerator and its efficiency were still low in this case since the temperature gradient needed to be maintained by mixing the fluid heat sink, and the time required for charging and discharging the magnet was too great.
The small low-power refrigeration devices were built in the 80s and 90s in several research centres: Los Alamos National Lab, Navy Lab at Annapolis, Oak Ridge National Lab, Astronautics (all USA), Toshiba (Japan). At present, several NASA research centres fund works with the compact magnetic refrigerators for space applications on the principle of adiabatic demagnetization operations. Astronautics Corporation of America (USA, Wisconsin) and University of Victoria (Canada) is conducting studies of the magnetic refrigerators possibilities for commercial applications. Research of materials for a working solid body of magnetic refrigerators from an applied point of view, is intensively currently carrying out by the ‘Ames Laboratory’ (Ames, Iowa), the ‘University of Three Rivers’ in Quebec (Canada), NIST (Gaithersburg, MD) and the company 'Advanced Magnetic Technologies and Consulting’ (AMT&C).
In 1997, the ‘Astronautics Corporation of America’ demonstrated a relatively powerful (600-Watt) magnetic refrigerator operating at near room temperature. The efficiency of this refrigerator was already comparable with the efficiency of conventional Freon refrigerators. This device, using an active magnetic regenerator (in which the functions of a thermal regenerator and a working medium are combined), worked for more than 1500 hours at the room temperature range, a power of 600 watts. The efficiency was about 35% with respect to the Carnot cycle at the magnetic field of five Tesla. In this device, it was used as a superconducting solenoid and, as the working solid body, the rare earth metal of gadolinium (Gd). A pure gadolinium was used in this capacity not only by Astronautics, but also by NASA, Navy and other laboratories, which is due to its magnetic properties, namely - a suitable Curie temperature (about 293 0
K) and a presence of the rather significant magnetocaloric effect. The MCE magnitude, and therefore the efficiency of the cooling process in a magnetic refrigerator, is determined by the properties of magnetic working bodies.
In 1997, the Ames Research Center reported the discovery of four giant magnetocaloric effects in the Gd5 (Si2Ge1-X) compounds. The magnetic ordering temperature of these materials can vary over a wide range from 20 0
K to room temperature due to a change in the ratio of silicon (Si) and germanium (Ge). The most promising for using as working solid bodies are currently the gadolinium metal, a number of intermetallic compounds based on the rare-earth elements, a system of silicide-germanide compounds Gd5 (Ge-Si) 4, and also La (Fe-Si) 13. Use of these materials allows the refrigerator working temperature range to be extended and its economic indicators significantly improved. It must be noted, that the pioneering works on the search for effective alloys for magnetic refrigerators working solid bodies were done out several years earlier at the Physics Department of Moscow University. The most complete results of these researchers are set forth in the doctoral dissertation of the leading research associate of the physics faculty of the Moscow State University, A.M. Tishina, in 1994.
In the course of this work, numerous possible combinations of rare-earth and magnetic metals and other materials have been analyzed from the point of view of searching for optimal alloys for the realization of magnetic cooling in the different temperature ranges. It was found, in particular, that among materials with high magnetocaloric properties, the compound Fe49Rh51 (iron-rhodium alloy) has the largest specific (ie, per unit magnetic field) magnetocaloric effect. The specific MCE for this compound is several times larger than in the silicides-germanides compounds. This alloy cannot be used in practice because of its high cost and the absence of significant hysteresis effects in it. However, it can serve as a kind of standard with which to compare the magnetocaloric properties of the materials under study. At last, Science News (v.161, n.1, p.4, 2002) reported the creation of the world's first refrigerator appliance (that is applicable not only for the scientific purposes but also for the household purposes). A working model of such refrigerator was manufactured jointly by Astronautics Corporation of America and Ames Laboratory and was first demonstrated at the G8 Conference in Detroit in May 2002. The working prototype of the proposed household magnetic refrigerator operates in the room temperatures range and uses a permanent magnet as the field source. This device was received the high appraisal by experts and the U.S. Energy Secretary. Estimates show that the use of magnetic refrigerators will reduce the total energy consumption in the USA by 5%. It is planned, that magnetic cooling can be used in various fields of human activity, for example, in:
- hydrogen liquefiers,
- cooling devices for high-speed computers and devices based on the SQUIDs,
- air conditioners for the residential and industrial premises,
- cooling systems for the vehicles,
- household and industrial refrigerators, etc.
It should be noted, that the magnetic refrigerators works have funded by the U.S. Department of Energy for 20 years already.
Refrigerator Construction Structure
In the created prototype of the magnetic refrigerator, a rotating wheel construction arrangement is used. It consists of a wheel containing segments with the gadolinium powder, as well as the powerful permanent magnet.
This construction is designed in such a way that the wheel scrolled through the working gap of the magnet, in which the magnetic field is concentrated. When a segment with gadolinium enters the magnetic field of gadolinium, a magnetocaloric effect arises - it heats up. This heat is removed by a water-cooled heat exchanger. When gadolinium leaves the magnetic field zone, a magnetocaloric effect of the opposite sign arises and the material is further cooled, cooling the heat exchanger with the second water flow circulating therein. This flow, in fact, is used to freeze the cooling chamber of the magnetic refrigerator. Such a device is compact and operates virtually noiselessly and without vibrations, which distinguishes it from the currently used refrigerators with a steam-gas cycle. For the first time, this technology was approved back to September 2001. Currently, work is underway to expand its capabilities further: the technological process of commercial production of pure gadolinium and its necessary compounds are being improved, which will allow achieving a greater value of the MCE at a lower cost. Simultaneously, the Ames Laboratory staff constructed a permanent magnet, capable of creating a strong magnetic field. The new magnet creates a field twice as strong as the magnet in the previous construction of the magnetic refrigerator (in 2001). It is very important because the magnitude of magnetic field determines such parameters as efficiency and output power of the refrigerator. Patent applications for the preparation of a compound for the working substance Gd5 (Si2Ge2) and the construction of a permanent magnet have been filed.
Advantages, Disadvantages and Applications
All magnetic refrigerators can be divided into two classes according to the type of used magnets:
- systems using superconducting magnets;
- systems on the permanent magnets.
The first of them have a wide range of operating temperatures and a relatively high output power. They can be used, for example, in air conditioning systems in the large premises and for the food storage equipment. The permanent magnet cooling systems have a relatively limited temperature range (no more than 303 °K per cycle) and, in principle, can be used in the devices with an average power (up to 100 watts). For instance, as a car cooler or a portable picnic refrigerator are. However, they both have a number of advantages over the traditional combined-cycle refrigerating systems:
- Low environmental hazard. The working body is solid and can be easily isolated from the environment. The lanthanide metals used as working bodies are low in toxicity and can be reused after disposing of the device. The heat-eliminating medium must have only a low viscosity and sufficient thermal conductivity, which corresponds well to the properties of water, helium, or air. They are well compatible with the environment.
- High efficiency. Magnetocaloric heating and cooling are practically reversible thermodynamic processes, in contrast to the process of vapour compression in the working cycle of a combined-cycle refrigerator. Theoretical calculations and experimental studies show that the magnetic cooling units are characterized by higher efficiency and In particular, in the field of room temperatures, the magnetic refrigerators are potentially 20-30% more effective than those operating in the gas-vapour cycle are. The technology of magnetic cooling in the future can be very effective, which will significantly reduce the cost of such installations.
- Long service life. The technology involves the use of a small number of moving parts and a few operating frequencies in the cooling devices, which significantly reduces their wear and tear.
- The flexibility of technology. It is possible to use the different designs of magnetic refrigerators depending on the purpose.
- Useful properties of freezing. Magnetic technology allows cooling and freezing of various substances (water, air, chemicals) with minor changes in each case. In contrast, an efficient combined-cycle cooling cycle requires many segregated stages or a mixture of different working coolants for the same procedure.
- Rapid progress in the development of superconductivity and the improvement of the magnetic properties of permanent magnets are. Currently, a whole number of well-known commercial companies are successfully engaged in improving the properties of NdFeB magnets (the most efficient permanent magnets) and are working on their constructions. Along with the known progress in the field of superconductivity, this allows hoping for an improvement in the quality of magnetic refrigerators and their simultaneous cheapening.
Disadvantages of magnetic cooling
- Need for shielding of a magnetic source;
- Relatively high current price of magnetic field sources;
- Limited temperature range in one cooling cycle in the permanent magnet systems (not more than 303 °K).