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 shown in the above figure. The temperature-entropy diagrams of magnetic material and gas as a refrigerant in liquefaction cycle are respectively shown in the above figure. The magnetic refrigeration for hydrogen liquefaction uses an external magnetic field to magnetize and demagnetize a paramagnetic 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 high liquefaction 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 (at20.3 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 (ratio of the rerigeration power of the experiment to that of the ideal cycle) 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 magnetic refrigeration is also demonstrated [8, 9].
The details of the study and experimental results are given in "Web site" below.

 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: The ratio of the Carnot work to actual work per unit mass liquefied.

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

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