Hydrogen liquefaction by magnetic refrigeration

This paper describes a new method of hydrogen liquefaction with a newly developed magnetic refrigerator. Magnetic refrigeration makes use of the magnetocaloric effect, and is well known as an efficient method, in principal, because its cooling cycle can closely follow the Carnot cycle with appropriate heat switches. A liquefaction principle of our magnetic refrigerator is based on a thermo-siphon method, in which liquid hydrogen is condensed directly onto the surface of magnetic refrigerants and drops downward. Therefore, the magnetic refrigerants are required to have relatively large entropy change at liquefaction temperature of hydrogen and hydrogen resistance. Since most metal compounds don’t satisfy the latter condition, we have developed a new ceramic polycrystal magnetic refrigerant named dysprosium gadolinium aluminum garnet (DGAG). In the liquefaction experiments, we have successfully liquefied hydrogen gas preliminarily cooled to a temperature slightly above the boiling point. Liquefaction was confirmed by the temperature variation of the DGAG, and the resultant condensation efficiency was found to approach 90% of the Carnot efficiency. The maximum cooling power was 14.6 W.