Magnetic refrigeration.

The world's first demonstration of the high-efficiency hydrogen liquefaction by magnetic refrigeration.


 In hydrogen liquefaction by the magnetic refrigeration, the adiabatic demagnetization method to generate temperature below 1 K is extended to the high-temperature region.

 The principles of the magnetic refrigeration and the compressed-gas refrigeration are illustrated in the above figure. The temperature-entropy diagrams of magnetic material and gas as a refrigerant in liquefaction cycle are respectively represented in the above figure. The magnetic refrigeration for hydrogen liquefaction uses an external magnetic field to magnetize and demagnetize a magnetic material in repeated cycles, thus producing low temperatures through the magnetocaloric effect.

 Since the magnetic refrigeration method can ideally realize the reversed Carnot cycle, it is possible to achieve theoretically a higher %Carnot efficiency* in contrast to the compressed-gas refrigeration method as noted in the above figures [4].
 The maximum efficiency is expected to be about 50% in terms of the %Carnot efficiency*, compared to about 38% of the %Carnot efficiency for the world’s largest hydrogen liquefier (liquefaction capacity: 60 ton/day) using the compressed-gas method. Also, given the use of solid magnetic material, which has much greater entropy density than gas, the liquefier can be made compact.

 In the experiment, hydrogen gas evaporated in a liquid hydrogen vessel, as shown in the figure below, was liquefied by using magnetic refrigeration. Gadolinium gallium garnet (GGG: Gd3Ga5O12) was selected as a magnetic material. The magnetic refrigerator maimly consists of a superconducting pulse magnet (the maximum field of 5 Tesla and the maximum magnetization/demagnetization sweep rate of 0.36 T/s), a Gifford-McMahon type refrigerator (UCR31W made by MHI) as a heat rejection source (Th = 25 K), and a heat pipe as a heat absorption switch for hydrogen liquefaction (Tl = 20.3 K) [8, 9]. In the thermal design of the high-performance heat pipe, the Nusselt eq. was applied to evaluate the condensing heat-transfer coefficient of hydrogen as described in "Hydrogen condensation and liquefaction".

 The temperature-entropy diagram of magnetic material (GGG) obtained in the hydrogen liquefaction experiment at 0.35 T/s, the reversed Carnot cycle, and the calculation results using a simulation model are shown in the figure below. The reasons the ideal reversed Carnot cycle indicated as a rectangle cannot be realized are the insufficient heat-transfer performance of the heat absorption and heat rejection switches, and the influence of uncondensed hydrogen gas (continuously evaporating in the liquid hydrogen vessel) around the magnetic material.

 In the hydrogen liquefaction experiment at the fastest magnetization/demagnetization rate of 0.36 T/s, as shown in the figure below, the maximum refrigeration power (at 20.3 K) of 0.4 W (liquefaction rate: 3.55 g/h or 50 cc/h) with the %Carnot efficiency* of 37% and the liquefaction efficiency** of 78% was obtained.

 The achievement of 37% for the %Carnot efficiency in a small-scale liquefaction experiment (3.55 g/h or 50 cc/h) demonstrates the high efficiency of this method.
 The world’s first high-efficiency hydrogen liquefaction using the magnetic refrigeration is also demonstrated [8, 9].
The details of the study and experimental results are given in "Web site" below.

 After our research study mentioned above was published in 1996 and 2000 [8, 9], a Japanese group (National Institute for Materials Science (NIMS) and Kanazawa University) has been reporting repeatedly in their experimental papers*** that they firstly and successfully demonstrated the hydrogen liquefaction by magnetic refrigeration on the basis of evidence only showing that the temperature of magnetic material decreased below hydrogen liquefaction temperature (20 K) by a one-shot demagnetization process***(1, 2, 3). However, as they were not able to measure the hydrogen liquefaction rate***(1, 2, 3), the decreased temperature below 20 K of magnetic material supports no experimental and scientific evidence of hydrogen liquefaction, which shows only the obvious result of an adiabatic demagnetization process. The measured hydrogen liquefaction rate or refrigeration power is crucial in evaluating both the %Carnot efficiency* and the liquefaction efficiency** that are the most important values to estimate the performance of magnetic liquefier or magnetic refrigerator.
 Even though very few experimental data are described in their paper***(4), judging from the figures, their liquefaction cycle was operated at higher temperatures than the hydrogen liquefaction temperature (20.3 K), and consequently, they were not able to measure the hydrogen liquefaction rate. Futhermore, it was found that their liquefaction cycle was different from the reversed Crnot cycle. Although the reversed Carnot cycle consists of four processes: adiabatic magnetization, isothermal magnetization, adiabatic demagnetization and isothermal demagnetization, their cycle consisted of four processes: isomagnetic heating, isothermal magnetization, adiabatic demagnetization and isothermal demagnetization.
 To verify the hydrogen liquefaction by magnetic refrigeration, the demonstration of the liquefiier operated by using the reversed Carnot cycle and the measurement of hydrogen liquefaction rate (g/h) or refrigeration power (W) at 20 K are required. Clearly, their experiments do not satisfy the conditions to verify the hydrogen liquefaction.
 The experiments of NIMS and Kanazawa Univ. have never demonstrated the hydrogen liquefaction by magnetic refrigeration.

 For hydrogen liquefaction, a multistage magnetic refrigerator from room temperature to liquid hydrogen temperature has been proposed, and research and development work is in progress. A method has also been proposed for producing slush hydrogen from liquid hydrogen, using magnetic refrigeration to produce temperature below 14 K.

 * %Carnot efficiency: Ratio of the reversed Carnot work to the actual work per unit mass liquefied. %Carnot efficiency = FOM (Figure of Merit)×100 (%).
 ** Liquefaction efficiency: Ratio of the rerigeration power of the experiment to that of the reversed Carnot cycle at the refrigeration (or liquefaction) temperature.
 *** For example,
(1) Numazawa T. (NIMS), Possibility of hydrogen magnetic refrigeration for incoming hydrogen society. Hydrogen Energy System, Vol 31 (2006), No.2, 2-7. (in Japanese)
http://www.hess.jp/Search/data/31-02-002.pdf
(2) Kamiya K. (NIMS), Takahashi H., Numazawa T. (NIMS), Nozawa H. and Yanagitani T., Hydrogen liquefaction by Magnetic Refrigeration. Cryocoolers, Vol. 14 (2007), 637-644.
https://cryocoolerorg.wildapricot.org/resources/Documents/C14/080.pdf
(3) Numazawa T. (NIMS), Kamiya K. (NIMS), Utaki T. and Matsumoto K. (Kanazawa Univ.), Magnetic refrigerator for hydrogen liquefaction. Superconductivity and Cryogenics, Vol. 15 (2013), No.2, 1-8.
http://dx.doi.org/10.9714/psac.2013.15.2.001
(4) Matsumoto K. (Kanazawa Univ.) and Numazawa T. (NIMS), Magnetic refrigerator for hydrogen liquefaction. J. Cryogenics and Superconductivity Society of Japan, Vol. 50 (2015), No.2, 66-71. (in Japanese)
https://www.jstage.jst.go.jp/article/jcsj/50/2/50_66/_pdf/-char/ja

Temperature-entropy diagram of GGG, refrigeration power and hydrogen liquefaction rate.

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