Magnetic refrigerant materials are one of the key factors for governing the cooling performance of magnetic refrigeration systems. In particular, not only the physical properties such as magnetocaloric effects, but also the practical form of the magnetic material are of crucial importance to active magnetic regenerative refrigeration （AMRR）. In this JST-mirai project, the development of several types of magnetic materials that show the large magnetocaloric effects in the temperature range from 20 K to 80 K and fabrication of spherical particles of the materials were conducted. The magnetic properties and magnetocaloric effects between the mother-alloy and spherical particles were compared for HoAl₂, ErCo₂, and HoB₂. Moreover, thermal property and electrical property as well as the shape and size of the materials are discussed from the perspective of their application to the AMRR system for hydrogen liquefaction.

For the supply chain of hydrogen, as the liquefaction cost may occupy 1/3 of the total supply price, developing a highly efficient hydrogen liquefier is one of the most important technology issues for hydrogen society. Magnetic refrigeration using the magnetocaloric effect has the potential to realize liquefaction efficiency higher than 50 %, and also to be environmentally friendly and cost effective. A hybrid refrigeration cycle consisting of the precooling cycle and magnetic active regenerator cycle has been proposed and is estimated to achieve a liquefaction capacity of >100 kg/day with liquefaction efficiency of >50 %.

Our new project is committed to developing 1）such a highly efficient hydrogen liquefier and also, 2）a compact and energy saving re-condensation refrigerator to realize zero boil-off in the liquid hydrogen storage.

The article summarizes the research and development of magnetic refrigeration for hydrogen liquefaction, and the Active Magnetic Regenerative Refrigeration (AMRR) developed by National Institute for Materials Science (NIMS). The NIMS-AMR consists of optimized AMR beds, a superconducting solenoid with correction coils, and a dedicated liquid hydrogen vessel with a liquid level sensor. This article also reports the first successful demonstration of hydrogen liquefaction with the AMRR.

From the COP （Coefficient of Performance） equation, there are two ways to improve the cooling efficiency of cryocoolers: （1） increasing the cooling capacity, and （2） decreasing the electrical input power. In recent years, we have focused on the possibility of method （2） for a 4 K-GM （Gifford-McMahon） cryocooler. In this paper, a 7 kW class compressor, which is commonly connected to a 4 K-GM cold head, was replaced with a low power consumption （2 kW class） compressor. The experiments achieved a measured cooling capacity of 0.95 W at 4.2 K with a cold head reciprocation speed of 36 rpm and a compressor electrical input of 1.9 kW. The calculated COP and the relative Carnot efficiency were 5.0×10^-4 and 3.5 %, respectively. A low reciprocating speed was selected to secure sufficient expansion work in the second expansion space. This is an effective method to improve the cooling capacity at 4.2 K by a low power consumption compressor.