Thermodynamic Analysis of a Magnetic Liquefier for Hydrogen

The aim of this paper is to present a thermodynamic analysis of a magnetic liquefier for hydrogen. A hydrogen liquefaction cycle is examined. A magnetic refrigerator, a liquid-nitrogen pre-cooling system, an expansion valve and a liquid-hydrogen separator integrate the system. The magnetic refrigerator consists of two heat exchangers and two stages of beds of magnetic materials. The analysis considered that the system operates with gaseous hydrogen entering at high pressure and ambient temperature. A fraction of this incoming flow is liquefied by the system. One magnetic material GdNi2 is used in the upper stage and another magnetic material GdPd is used in the lower stage. The heat transfer fluid for the magnetic refrigerator is helium. The beds in both stages are periodically magnetized and demagnetized and the fluid flows are arranged to meet the cycle liquefaction requirements. Sensitivity analysis has been performed to study the thermodynamic behavior of the magnetic liquefier cycle. Liquefaction efficiency, the nitrogen boil-off rate per unit mass of hydrogen, the fraction of the hydrogen gas that is liquefied and the magnetic refrigerator performance trends are evaluated. Simulations indicate that the higher the performance of the magnetic refrigerator is the higher the liquefaction efficiency of the system is with the same intermediate temperature. It was also observed that the liquefaction efficiency increases with the decrease in the nitrogen boil-off rate per unit mass of hydrogen. Magnetic liquefier exhibits a great potential by showing a very high efficiency when compared to small and large scale commercial liquefiers for hydrogen.Copyright © 2005 by ASME