International. A recent study proposes a technique that could be used for hydrogen liquefaction by magnetic cooling at low temperatures.
Magnetic cooling is a cooling method that uses magnetic fields. It has traditionally been studied for use in refrigeration almost at room temperature. However, recently, researchers have focused on lower target temperatures. For example, for hydrogen liquefaction (which is an attractive option for the storage of renewable energy resources) it is required to cool hydrogen gas to a condensation temperature of 20 K.
With magnetic cooling, cooling is achieved through changes in magnetic entropy that occur when a magnetic field is applied, a phenomenon known as the magnetocaloric effect. So far, most research has used high magnetic fields (at least 5 Teslas) to obtain a large entropy change, which requires a superconducting magnet and, therefore, a large energy cost. For hydrogen liquefaction, the required magnetic field is more than several Teslas in the temperature range between the liquefaction temperatures of hydrogen (20.3 K) and nitrogen (77 K). Therefore, it would be necessary to use stronger magnetic fields, but with a high energy cost to generate the magnetic field.
A recent study has proposed a more efficient magnetic cooling technique, in which small variations in the magnetic field can achieve cooling efficiency of a magnitude greater than has been achieved using typical magnetocaloric materials. The authors used holmium, an antiferromagnetic material that presents a pronounced change in magnetization according to temperature and applied magnetic field. It should be noted that the authors did not find a significant conventional magnetocaloric effect. They found that a small variation of the magnetic field allows for very efficient magnetic cooling by using materials with strong magnetization variations, such as holmium.
As for practical applications, previous studies have presented prototypes of AMR refrigerators using superconducting magnets. In this study, the authors discussed the possible use of a magnetic cooling system called an active magnetic regenerator (AMR) with holmium, using a small change in the magnetic field.
The authors hope their findings can open up a new field of research focused on magnetic cooling at a reduced cost using a low magnetic field. In fact, the proposed technique can be implemented using permanent magnets, rather than the superconducting magnets normally used in current research, which require large operating costs. The authors believe that the proposed technique could be a suitable alternative to conventional gas compression cooling for hydrogen liquefaction.
Source: Nature.
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