Magnetic cooling: how cool is that!

By Dean Williams on February 17, 2017

More than 15 % of energy is spent worldwide on various forms of cooling. In addition to the energy bill, the compressor in cooling units can bring a rich palette of sounds to your home: think of all this clanking, cranking, humming. Have you ever wondered whether there are refrigerators that are quieter, more environmentally friendly than the conventional fridges and on top of that, help you save energy?

Refrigerators employing magnetic cooling technology could be the answer? Magnetic cooling systems are expected to provide up to 50 % savings versus traditional systems, with a Carnot efficiency of more than 60 %. Magnetic cooling offers a lower environmental impact: there are no hazardous fluids – as heat transfer in fluid water at ambient pressure can be used – and there are only a few moving parts resulting in low noise pollution and limited vibration.

Fig. 1. Magnetic cooling principle demonstrated in terms of the spin and phonon systems of a solid. The application of the magnetic field leads to the alignment of spins and reduces the magnetic entropy Smag. Under adiabatic conditions (the total entropy is constant), the reduction in the magnetic entropy is compensated by an increase in the phonon part of the entropy Sph, i.e. by intensified lattice vibration (see a largely exaggerated phonon wave in step I). As a result of the increased lattice vibration the temperature rises. The evolved heat is removed from the system to the environment (step II). On thermal isolation of the solid from the environment and removal of the magnetic field, the spins randomise causing an increase in Smag and the corresponding reduction in Sph (step III). This step leading to the temperature reduction forms the basis of the magnetic cooling. Image taken from Julia Lyubina 2017 J. Phys. D: Appl. Phys. 50 053002, © IOP Publishing, All Rights Reserved.

The magnetic cooling technology is based on the magnetocaloric effect, which is the emission or absorption of heat in a magnetic material under magnetic field variation (Fig. 1). The magnetocaloric effect is present in all the magnetic materials. It is particularly large at magnetic phase transitions. In a special class of materials exhibiting a first order magnetic phase transition, this effect can be of considerable size.

A practical implementation of the magnetocaloric effect was demonstrated several decades ago and has been used ever since in low temperature physics for cooling to temperatures close to absolute zero. Use of the magnetocaloric effect for cooling near room temperature has only recently been seriously considered. Although a first industrial prototype refrigerated by a magnetic heat pump was demonstrated in 2015, magnetic refrigeration has not yet been commercialised. It has been recognised that in order for this technology to disrupt the existing vapour compression-dominated market, considerable research activity needs to continue in terms of both providing the fundamental knowledge on how to design the most effective magnetocaloric materials and how to engineer efficient solid-state magnetic cooling systems.

This Topical Review focuses on fundamentals of magnetic materials with practical magnetic cooling application in mind. It concentrates on the progress made in the field of magnetocaloric materials and brings up key issues to the research community’s attention. A part of the review is dedicated to fundamental aspects of phase transitions arising as a result of complex interactions in the solid state, with extended analysis of first order transitions and nomenclature proposal for particular transition types according to the relation of the thermal energy to the height of the energy barrier between magnetic states. The discussion of phenomena emerging at phase transitions, such as hysteresis, volume expansion, and their influence on material performance can be treated in analogous ways to other fields, from ferroelectricity to energy storage, and thus will be of interest to a broader scientific community.

The full review is available now on IOPscience.

About the author

Dr Julia Lyubina received her BS and MS in Physics from National University of Science and Technology “MISIS” in Moscow, Russia. During this time she was a visiting researcher at IFW Dresden, Germany and in 2002 she joined IFW Dresden to prepare her PhD thesis in the field of permanent magnet materials. After obtaining her PhD in Physics from TU Dresden in 2006, she focused her research activities at IFW Dresden on magnetocaloric materials. In 2010 she continued on to Imperial College London, UK as Marie Curie Fellow, studying novel synthesis routes for magnetocaloric materials. Since 2012 she has been working for Evonik Industries AG focusing on new product and new business development in the field of functional materials. Her research was distingushed by Deutsche Bank Young Scientist Award of IFW and Georg Helm Award. Since 2012 she is a Honorary Senior Research Fellow at Imperial College London.