A high-performance magnetic refrigerator is expected to serve as an alternative technology for air-conditioners in electric vehicles. The high power consumption of conventional air-conditioners in electric vehicles considerably decreases the cruising distance. In the background, the demands of cooling power density, temperature difference between hot and cold sides, COP and transient properties must be high. We have devised a novel system that doesn't require heat-transfer fluid where heat is transported from the cold side to the hot side via well-controlled thermal switches. The heat transfer mechanism has been verified both experimentally and numerically. Furthermore, the superiority of the refrigerating capacities have also been verified by numerical simulation, with specifications of the main constituent parts varied over a wide range. However, several issues remain to be solved in order to draw out even higher potential. Among the issues, a thermal switch with liquid metal driven by electrowetting-on-dielectric (EWOD) has been introduced as an example solution for the thermal switch.
To introduce a recent situation of materials research for mangetocaloric effect at toom temperature, various magnetocaloric materials are compared in terms of the industrial advantage. Some case studies for La(Fe,Si)13-based materials are also introduced with tailoring of magnetic properties in view of system load.
Hydrogen promises to be one of the most important energy sources in the near future. Liquid hydrogen can be utilized for infrastructure construction consisting of storage and transportation. The figure of merit (FOM) must be larger than 0.57 for a hydrogen liquefier when comparing the consuming energy of hydrogen liquefaction with high-pressure (70 MPa) hydrogen gas. Magnetic refrigeration using the magneto-caloric effect has the potential to realize not only a higher liquefaction efficiency >50%, but also to be environmentally friendly and cost effective. Our hydrogen magnetic refrigeration system consists of a Carnot cycle for the liquefaction stage and active magnetic regenerators (AMR) for the precooling stages. Various magnetic materials were studied for candidate refrigerants. We developed a highly efficient liquefaction stage with >80% liquefaction efficiency and confirmed the AMR effect for the precooling stage.
This paper discusses the system performance of a magnetic heat pump with an active magnetic regenerator. The heat-transfer coefficient and pressure drop in the AMR are performed analytically. Measurements on the experimental magnetic heat pump test device are performed using distilled water as the working fluid. The coupling of a one-dimensional thermal model is presented. The thermal model takes into consideration the magneto-caloric effect as a source term and the energy conservation between a solid and a fluid. At cyclic steady states, the temperatures of the fluid on both sides of the regenerator are calculated numerically and compared with the measurement results. A reasonable agreement between simulations and experiments confirms the validity of the proposed model.
Railway Technical Research Institute has been studying a magnetic refrigerator for train air-conditioners. Hydrochlorofluorocarbons (HCFCs) are used in the conventional air-conditioners. The development of HCFC-free systems or the usage of substances which have minimal greenhouse effect is required because the problem with HCFCs is that they cause global warming. Magnetic refrigeration technology has the potential of high efficiency without the use of Freon gases. A prototype magnetic refrigerator has been developed. It consists of fixed active magnetic regenerator (AMR) beds and Halbach-arrayed NdFeB magnets with a peak field of 1.5 Tesla rotating over the beds. Each bed is packed with sphere gadolinium (Gd) or Gdbased compounds. The system has a maximum cooling capacity of 1.4 kW. Several cascade patterns of the packed materials in the AMR beds were evaluated and a maximum temperature span of 21.3 K was obtained.
The development of magnetic refrigerants and basic research on active magnetic regenerative (AMR) refrigeration at room temperature were performed. Temperature differences between the hot and cold ends of the AMR unit of more than 45 degrees, and achieving the lowest temperature of minus 10 degrees were obtained by operating the AMR cycle with a NdFeB Halbach-type permanent magnet and spherical-shaped GdY magnetic refrigerants. In the case of the La(Fe,Co,Si)13 compound, the temperature difference between the ends of the AMR unit reached only 21 degrees. This is mainly attributed to the fact that larger specific heat provides smaller temperature changes in the magnetocaloric effect even if the La(Fe,Co,Si)13 compound shows larger magnetic entropy change than Gd alloys. At the same time, large specific heat is effective for load characteristics. Model calculations indicated that multi-layered magnetic refrigerants whose magnetic transition temperatures are tailored to the temperature gradient in the AMR unit improve both temperature difference and load characteristics. Ideal design for achieving optimized heat flow and regeneration leads to the room-temperature application.
Adiabatic demagnetization refrigerator (ADR) does not use working fluids contrary to conventional refrigerators that make use of the fluid density difference, which leads to superiority of the ADR under weak gravitational conditions. In this study, we developed a continuous ADR system to provide constant cooling temperatures ~ 100 mK. The system consists of four stages of magnetic materials and magnets cascaded with heat switches. The magnetic materials, CPA and GdLiF4, are used for three stages between 0.1 - 1.4 K, and a single stage between 1.4 - 4 K, respectively. Passive heat switches are used for three stages > 0.3 K, and a superconducting heat switch is used for the continuous stage at ~ 0.1 K. A Gifford-McMahon cycle cryocooler is used to cool the ADR and cryostat shieldings. The total mass of the flight model is less than 60 kg. Cooling tests with a transition edge sensor (TES) on the ground showed that the ADR provided continuous cooling temperatures from 105 - 120 mK, and it successfully operated the TES. Airborne flight experiments confirmed the ability of the cooling system under milli-gravity conditions. The experimental results showed that the ADR could provide stable temperature under a weak gravity. In conclusion, a continuous ADR system will be useful for many applications that require higher efficiency and compactness.