Scientists from the Dresden High Magnetic Field Laboratory and the Technical University of Darmstadt jointly with engineers from the German company Magnotherm Solutions have created and tested the world’s largest experimental unit for hydrogen liquefaction using magnetic refrigeration. The unit, dubbed HyDRA, is almost 4.6 meters tall and can achieve record-breaking cooling performance with a minimal amount of expensive magnetic materials.
Although hydrogen is considered one of the most promising clean energy sources, its widespread use is currently hampered by the difficulty of storage and transportation. Hydrogen is much easier to transport in liquid form, but this requires cooling to about –253°C. Modern liquefaction technologies, which are based on gas compression and expansion, require enormous amounts of energy, up to a third of the energy contained in the hydrogen.
The researchers have proposed using magnetic refrigeration instead of conventional refrigeration cycles. Its operating principle is based on the magnetocaloric effect: some materials heat up when placed in a strong magnetic field, cooling down when the field is removed. By combining this process with coolant circulation, heat can be gradually transferred from one area to another, achieving ultra-low temperatures without the need for conventional compressor refrigeration systems.
As a magnetocaloric material, the scientists used holmium, a rare earth metal that performs well at ultra-low temperatures. They put it inside a superconducting magnet with a field of up to 19 Tesla. As a point of comparison, Earth’s magnetic field is approximately hundreds of thousands of times weaker. Helium is pumped through the system, transferring heat between the hot and cold areas of the unit. Hydrogen is first cooled with liquid nitrogen to about –196°C, after which the magnetic system further reduces the temperature to a level required for hydrogen liquefaction.
The key element is the so-called active magnetic regenerator. In this special structure, the magnetic material and a helium flow work together, gradually transferring heat from the cold end of the system to the hot end. This results in a temperature difference much greater than the magnetocaloric effect can achieve in a single cycle.
In the course of their experiments, the researchers discovered that the system’s efficiency is highly dependent on the magnetic field and helium pressure. For instance, when the magnetic field increased from 6 Tesla to 9 Tesla, the temperature difference rose from 12.4 Kelvin to 16.4 Kelvin. Increasing the helium pressure also improved the results: the cooling temperature rose from 10.2 Kelvin to 12.7 Kelvin.
The study has also made it possible to identify important aspects of helium’s behavior at ultra-low temperatures. The researchers found that the high compressibility of the gas within the system causes additional efficiency losses. These data will help improve design accuracy in future magnetic refrigerators.
Although HyDRA remains an experimental platform, this technology is seen as a promising way to make hydrogen energy cheaper and more efficient.
