A magnetocaloric mixture based refrigerator: From material to system level analysis

Magnetic refrigeration (MR) is considered a potential alternative to vapour compression refrigeration technology due to its relatively higher energy efficiency and its use of environment-friendly materials. The refrigerant in MR is a magnetocaloric material which changes its temperature under varying magnetic fields. A typical MR system consists of a moving permanent magnet, and a stationary porous magnetocaloric material through which a heat transfer liquid passes and circulates in the system with help of a mechanical pump/actuator. To realize a MR with no moving parts, the present thesis considers a mixture of magnetocaloric material and heat transfer liquid as the refrigerant. This mixture is circulated between hot and cold heat exchangers using a magnetic pump, while the permanent magnet is stationary. This is analysed by first considering the magnetic pump design, followed by material analysis of the mixture, and finally a system level analysis of the MR under various operating conditions.

Magnetic pump design: In existing works on magnetic or ferrohydrodynamic pumps, pumping is obtained by utilizing travelling wave magnetic fields with frequencies in the range of 100 to 1000 Hz. Such high frequencies when utilized in the proposed refrigerator could cause heating. Hence, a novel pump design with inclined pipes that uses pulsed magnetic fields is presented. It achieves similar flow rates as the previously discussed magnetic field pumps, but at frequencies in the range of 1 Hz.

Material Analysis: For the mixture, utilizing liquid metals as heat transfer liquid aids in increasing the thermal performance of the refrigerator. Unlike Hg, and NaK, gallium, indium, and tin - GaInSn based liquid metal is non-toxic and nonflammable, but is known to be reactive with certain elements. So a mixture of GaInSn with LaFeMnSiH, and MnFePSi, which are promising magnetocaloric materials for commercial applications, is analysed for changes in their magnetocaloric effect.

System Level Analysis: The magnetic refrigerator performance is numerically studied by considering a broader choice of magnetocaloric materials (LaFeMnSiH, MnFePSi, and Gd) with heat transfer liquids (GaInSn liquid metal, and water). The system performance is analysed and compared in terms of following parameters: temperature drop across the hot and cold heat exchangers (Δ𝑇 ), cooling power, magnetic work, frictional work, and second-law efficiency.