A comprehensive experimental analysis of gadolinium active magnetic regenerators

The main goal of this study was an experimental comparison of six different active magnetic regenerators (AMRs) with gadolinium as the magnetocaloric material. The analysis was carried out for three different parallel-plate AMRs (with different porosities and different orientations of the plates in the magnetic field) and three different packed-bed AMRs (filled with spheres, powders and cylinders). Since the operation of an AMR is strongly affected by the operating conditions, the experiments were performed at different mass-flow rates and at different operating frequencies. These were required in order to define the optimum corresponding operating conditions for the analyzed AMRs. As comparative criteria the maximum temperature span, the cooling capacity and the experimentally predicted COP were taken into consideration. The experimental analysis was performed on a new prototype of magnetic refrigerator designed as an experimental device. Its operation is based on the linear movement of a permanent-magnet assembly over a static AMR. The magnet assembly provides a measured magnetic field of about 1.15 T. The results reveal that the geometry of the AMR (the form of the magnetocaloric material) has a crucial impact on the performance of the magnetic refrigerator. The best overall cooling characteristics (temperature span, cooling capacity and COP) were obtained for the parallel-plate AMR with the smallest porosity (similar to 25%) and the orientation of the plates parallel to the magnetic field. This particular AMR generated a temperature span of 20 K, which is also the largest, so-far measured and published temperature span with a parallel-plate AMR for a given magnetic field change generated by permanent magnets. With respect to the comparison of the experimentally predicted COP values, the parallel-plate AMRs show higher efficiencies than the packed-bed AMRs. (C) 2013 Elsevier Ltd. All rights reserved.