49 research outputs found
From a magnet to a heat pump
The magnetocaloric effect (MCE) is the thermal response of a magnetic material to an applied magnetic field. Magnetic cooling is a promising alternative to conventional vapor compression technology in near room temperature applications and has experienced significant developments over the last five years. Although further improvements are necessary before the technology can be commercialized.Researchers were mainly focused on the development of materials and optimization of a flow system in order to increase the efficiency of magnetic heat pumps. The project, presented in this paper, is devoted to the improvement of heat pump and cooling technologies through simple tests of prospective regenerator designs. A brief literature review and expected results are presented in the paper. It is mainly focused on MCE technologies and provides a brief introduction to the magnetic cooling as an alternative for conventional vapor compression technology
Magneto-elastic coupling in La(Fe, Mn, Si)<sub>13</sub>H<i>y</i> within the Bean-Rodbell model
First order magnetic phase transition materials present a large magnetocaloric effect around the transition temperature, where these materials usually undergo a large volume or structural change. This may lead to some challenges for applications, as the material may break apart during field change, due to high internal stresses. A promising magnetocaloric material is La(Fe, Mn, Si)13Hy, where the transition temperature can be controlled through the Mn amount. In this work we use XRD measurements to evaluate the temperature dependence of the unit cell volume with a varying Mn amount. The system is modelled using the Bean-Rodbell model, which is based on the assumption that the spin-lattice coupling depends linearly on the unit cell volume. This coupling is defined by the model parameter η, where for η > 1 the material undergoes a first order transition and for η  ≤ 1 a second order transition. We superimpose a Gaussian distribution of the transition temperature with a standard deviation
σ
T
0
, in order to model the chemical inhomogeneity. Good agreement is obtained between measurements and model with values of η  ∼ 1.8 and σ(T0) = 1.0 K
Comparing superconducting and permanent magnets for magnetic refrigeration
We compare the cost of a high temperature superconducting (SC) tape-based solenoid with a permanent magnet (PM) Halbach cylinder for magnetic refrigeration. Assuming a five liter active magnetic regenerator volume, the price of each type of magnet is determined as a function of aspect ratio of the regenerator and desired internal magnetic field. It is shown that to produce a 1 T internal field in the regenerator a permanent magnet of hundreds of kilograms is needed or an area of superconducting tape of tens of square meters. The cost of cooling the SC solenoid is shown to be a small fraction of the cost of the SC tape. Assuming a cost of the SC tape of 6000 /kg, the superconducting solenoid is shown to be a factor of 0.3-3 times more expensive than the permanent magnet, for a desired field from 0.5-1.75 T and the geometrical aspect ratio of the regenerator. This factor decreases for increasing field strength, indicating that the superconducting solenoid could be suitable for high field, large cooling power applications
Magnetic levitation by rotation
A permanent magnet can be levitated simply by placing it in the vicinity of
another permanent magnet that rotates in the order of 200 Hz. This surprising
effect can be easily reproduced in the lab with off-the-shelf components. Here
we investigate this novel type of magnetic levitation experimentally and
clarify the underlying physics. Using a 19 mm diameter spherical NdFeB magnet
as rotor magnet, we capture the detailed motion of levitating, spherical NdFeB
magnets, denoted floater magnets. We find that as levitation occurs, the
floater magnet frequency-locks with the rotor magnet, and, noticeably, that the
magnetization of the floater is oriented close to the axis of rotation and
towards the like pole of the rotor magnet. This is in contrast to what might be
expected by the laws of magnetostatics as the floater is observed to align its
magnetization essentially perpendicular to the magnetic field of the rotor.
Moreover, we find that the size of the floater has a clear influence on the
levitation: the smaller the floater, the higher the rotor speed necessary to
achieve levitation, and the further away the levitation point shifts. We verify
that magnetostatic interactions between the rotating magnets are responsible
for creating the equilibrium position of the floater. Hence, this type of
magnetic levitation does not rely on gravity as a balancing force to achieve an
equilibrium position. Based on theoretical arguments and a numerical model, we
show that a constant, vertical field and eddy-current enhanced damping is
sufficient to produce levitation from rest. This enables a gyroscopically
stabilised counter-intuitive steady-state moment orientation, and the resulting
magnetostatically stable, mid-air equilibrium point. The numerical model
display the same trends with respect to rotation speed and the floater magnet
size as seen in the experiments.Comment: 15 pages, 6 figures, 10 videos + 8 pages supplementary material.
Videos available at
https://youtube.com/playlist?list=PLOfbFSFa_WoK4PgYQXhuNucS_WcIxXDE
High performance magnetocaloric perovskites for magnetic refrigeration
et al.We have applied mixed valance manganite perovskites as magnetocaloric materials in a magnetic refrigeration device. Relying on exact control of the composition and a technique to process the materials into single adjoined pieces, we have observed temperature spans above 9 K with two materials. Reasonable correspondence is found between experiments and a 2D numerical model, using the measured magnetocaloric properties of the two materials as input. © 2012 American Institute of Physics.The authors would like to acknowledge the support of the Programme Commission on Energy and Environment (EnMi) (Contract No. 2104-06-0032) which is part of the Danish Council for Strategic Research.Peer Reviewe