Hydrogen liquefaction using a new magnetic cooling technique.
A recent study proposes a technique that could be used for hydrogen liquefaction using magnetic refrigeration at low temperatures.
Magnetic refrigeration is a cooling method that uses magnetic fields. Traditionally, it has been studied for use in near room temperature refrigeration. Recently however, researchers have focused on lower target temperatures. For instance, for hydrogen liquefaction (which is an attractive option for the storage of renewable energy resources) cooling hydrogen gas to a condensation temperature of 20 K is required.
With magnetic refrigeration, cooling is achieved through the changes in magnetic entropy that occur when a magnetic field is applied, a phenomenon known as 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 hydrogen (20.3 K) and nitrogen (77 K) liquefaction temperatures. The use of stronger magnetic fields would therefore be necessary, but at a great 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 a cooling efficiency of a greater magnitude than has been achieved using typical magnetocaloric materials. The authors used holmium, an antiferromagnetic material which exhibits a steep change in magnetization depending on the temperature and magnetic field applied. 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 refrigeration by using materials with steep magnetization variations such as holmium.
Concerning practical applications, previous studies have presented prototype AMR refrigerators using superconducting magnets. In this study, the authors discussed the possible use of a magnetic refrigeration system called active magnetic regenerator (AMR) with holmium, using a small change in magnetic field.
The authors hope that their findings can open a new field of research focused on magnetic refrigeration at a reduced cost using low magnetic field. Indeed, the proposed technique can be implemented using permanent magnets, instead of the superconducting magnets typically used in current research, which require large operational costs. The authors believe that the proposed technique could be a suitable alternative to conventional gas compression cooling for hydrogen liquefaction.
Terada, N., Mamiya, H. High-efficiency magnetic refrigeration using holmium. Nat Commun 12, 1212 (2021). https://doi.org/10.1038/s41467-021-21234-z