Design of an optimized two-stage active magnetic regenerator (AMR) for hydrogen liquefaction

An Active Magnetic Regenerator (AMR) system designed for hydrogen liquefaction is presented. It is shown that because the specific heat capacity of magnetocaloric materials decreases sharply towards zero at cryogenic temperatures, it is not possible to construct a single AMR spanning the temperature range from liquid nitrogen (77 K) to the temperature of hydrogen liquefaction (20 K). A numerical model, accounting for the compressibility of the heat transfer fluid, was used to identify the optimal Curie temperature of the magnetocaloric materials, the fractional composition of the layers within the regenerator bed and the mass flow rate of the heat transfer fluid for a two-stage active magnetic regenerator system that can span 77-20 K. The optimal design of the two-stage system comprises a three-layer regenerator for the warmer portion of the span and a single-layer regenerator for the colder portion. The resulting configuration provides an overall coefficient of performance of 0.082 and a second-law efficiency of 23.4 %. With regenerator beds with an individual volume of  ∼ 6 L and an applied magnetic field of 5 T, the hydrogen liquefaction capacity would be 14 kg/day.