Development of the low temperature active magnetic regenerative refrigerator with conduction cooled high temperature superconducting magnet

Abstract

Magnetic refrigeration is a cooling technique that uses the magnetocaloric effect (MCE). The MCE is based on the phenomenon that the temperature or entropy of a material changes due to variation of magnetic field. Since the MCE is a reversible change in temperature of entropy of magnetic material under the varying external magnetic field, the reversible processes in the magnetic refrigeration cycle such as a magnetization and demagnetization process can provide an opportunity to develop a more effective refrigeration system than the compression-expansion cycle. An active magnetic regenerative refrigerator (AMRR), a type of magnetic refrigerator, utilizes magnetic materials as a regenerator, so called an active magnetic regenerator (AMR). The characteristics of an AMR is that it functions as both the working refrigerant and the heat transfer medium. Its temperature gradient, which allows for a large temperature span, develops along the length of the AMR. The AMRR which can be considered as multi-stage cascade system can be useful if the heat sink temperature is significantly apart from the heat source temperature. The advantage of AMRR is well-recognized and often emphasized in the case of having to cope with large temperature span. Magnetic cooling technique was first conceived as an enabling technology to reach sub-Kelvin tempera-ture where no other conventional gas-expansion method is any longer effective to lower temperature. Further-more, the magnetic refrigeration has been extensively explored at room temperatures for past several decades. However, only few research efforts for magnetic refrigeration for the temperature range between 20 K and 77 K has been conducted. Several experimental investigations on the magnetic refrigerator operating between 20 K and 80 K are introduced. All of them employ low temperature superconducting (LTS) magnets to drive the AMR cycle. Since LTS magnets require very low temperature for operation, it is inherently ineffective for cooling at temperatures of interest for the study. Moreover, an AMRR with direct current superconducting magnet needs relative movement between the magnet and the AMR for the magnetic field variation. Because the mechanical movement in a cryogenic environment can impact the reliability of the system, many researchers adopt the al-ternating current operation for the magnetic field change. For these reasons, the conduction cooled HTS magnet with AC operation is utilized as a magnetic field generator in this work. This dissertation presents design and development of the AMRR operating between 20 K and 77 K. Four different rare-earth intermetallic compounds ($GdNi_2, Dy_{0.85}Er_{0.15}Al_2, Dy_{0.5}Er_{0.5}Al_2, and Gd_{0.1}Dy_{0.9}Ni_2$) are utilized according to their favorable temperature regions. The magnetic refrigerator is composed of two stages each of that is made of two different AMRs. Each stage of AMR is designed by one-dimensional numerical simulation to maximize its performance. Magnetic field alternation throughout the AMR is generated by the HTS magnet. The maximum magnetic field and the ramp rate are set to 3 T and 1 T/s, respectively, taking into account a stable operation. Helium gas was employed as a working fluid and its oscillating flow in the AMR is controlled in accordance with the magnetic field variation. The performance of the AMRR is analyzed by observing internal temperature variations at cyclic steady state. Numerical estimation of the cooling capacity and the temperature variation of the AMR are examined and compared with the experimental results.