Magnetic refrigeration, a promising technology for the liquefaction of hydrogen

According to the International Energy Agency (1), hydrogen is “an increasingly important piece of the net zero emissions by 2050 puzzle”. Clean hydrogen produced with renewable or nuclear energy, or fossil fuels using carbon capture, can help decarbonise a range of sectors, including long-haul transport, chemicals, iron and steel, where it has proven difficult to reduce emissions. Hydrogen can also support the integration of variable renewables in the electricity system, being one of the few options for storing energy. 

 

However, according to G. F. Peixer et al. (2), this decarbonisation potential has not yet materialised and technical and economic challenges still need to be addressed. The storage and transport of hydrogen are among the most important.

 

In a keynote (3) presented at the 2023 IIR Congress in Paris, Japanese researchers stressed that to make hydrogen an energy source for users, it is necessary to convert it into a compact form that can be used for distribution. Various options such as liquid hydrogen, ammonia and methylcyclohexane have been considered, and ongoing research and development aim to overcome the technical challenges, taking into account the advantages and disadvantages of each option. Among these, liquid hydrogen offers the considerable advantage of significantly reducing the volume of hydrogen gas to 1/800th, but its higher cost compared to other options constitutes a drawback.  

 

To liquefy hydrogen, hydrogen gas needs to be cooled to cryogenic temperatures of around 20 K (-253°C) at a pressure of 1 atmosphere, which requires a considerable amount of electricity. (3) 

 

Hydrogen was first liquefied by Sir James Dewar in 1898. Since then, various technologies and system architectures have been evaluated. Among them, “conventional” technologies based on the Linde, Claude and Brayton cycles are the most prominent. (2) 

 

Over the last decades, more than 50 hydrogen liquefaction plants have been assembled in Europe, North America, and Asia, even though many of them are not still in service. The global hydrogen liquefaction capacity is 350 tonnes per day, with the capacity of the largest liquefier being 32 tonnes per day. Despite numerous developments in recent years, the performance of hydrogen liquefaction systems still needs to be improved, both technically and economically, to ensure that the technology proves to be a viable and efficient option for hydrogen storage and transport. (2) 

 

The efficiency of currently operating refrigeration systems typically falls within the range of 20% to 30%, depending on their capacity. To pursue efficiency enhancements, several ambitious projects are underway, although in the conceptual design phase. (3) 

 

One method that has the potential to greatly enhance liquefaction efficiency is magnetic refrigeration. Compared to conventional gas expansion refrigerators, magnetic refrigeration offers the potential for higher liquefaction efficiencies, with theoretical values exceeding 50%. (3) 

 

Magnetic refrigeration relies on the "magnetocaloric effect". This effect involves aligning or misaligning the magnetic moment (spin entropy) of a magnetic refrigerant through magnetisation and demagnetisation of a magnetic field, resulting in temperature variations of the refrigerant. As magnetic refrigeration operates according to the principles of the Carnot cycle, it offers great potential for improving efficiency. (3) 

 

To enhance the refrigeration capacity of magnetic refrigeration, two key factors are considered: the development of new magnetic refrigerants with larger entropy changes and the utilisation of powerful magnets to maximise magnetic field variations. (3) 

 

Magnetic refrigeration can be primarily categorised into three methods: Adiabatic Demagnetization Refrigeration (ADR), Carnot Magnetic Refrigeration (CMR), and Active Magnetic Regenerative Refrigeration (AMR). Both ADR and CMR have high efficiency but are limited in their temperature ranges due to the Carnot cycle. To overcome this limitation, Active Magnetic Regenerative Refrigeration was proposed by Barclay in 1982 (4). AMR combines the regenerative effect with the magnetocaloric effect, extending the temperature range of CMR that was previously limited to a few Kelvin, to several tens of Kelvin. (3) 

Consideration of AMR for design purposes for hydrogen liquefaction began in the late 20th century. Three teams of researchers from the United States, South Korea and Japan are actively involved in ongoing research and development work. 

 

At the IIR Congress, the Japanese researchers from the National Institute for Materials Science (NIMS) and Kanazawa University (3) announced an important milestone by successfully accomplishing the liquefaction of hydrogen using AMR for the first time. 

 

The development of the AMR hydrogen liquefier should progress in the direction of larger sizes and higher performance to enhance its practicality. Simultaneously, efforts should be made to expand the operating temperature range, making the system more versatile and applicable in various settings. While the development of system elements is crucial, equal attention is given to the development of more advanced magnetic refrigerants making it possible to increase the refrigeration capacity and achieve higher exhaust heat temperatures under the same experimental conditions. (3) 

 

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Sources: 
 

(1) https://www.iea.org/energy-system/low-emission-fuels/hydrogen  

(2) Peixer G. F. et al., Comparison of conventional and emerging technologies for hydrogen liquefaction. Link.

(3) Kamiya K et al, Magnetic refrigerators for hydrogen liquefaction. Link.

(4) Barclay, J.A., Steyert, W.A., 1982. Active magnetic regenerator. U.S. Patent 4,332,135. https://patents.google.com/patent/US4332135A/en