The Thermodynamics of Magnetocaloric Energy Conversion

Abstract

Magnetocaloric energy conversion is a technology based on the exploitation of the magnetocaloric effect (MCE). The MCE is a physical phenomenon that occurs in magnetic materials under the influence of a varying magnetic field. Is it usually expressed as the adiabatic temperature change or isothermal total entropy change of a material. In a ferromagnetic material the entropy can be, for instance, related to the magnetic part and the part related to the temperature of the system (e.g. the lattice entropy). In the absence of a magnetic field, the magnetic moments in the material are disordered. If a magnetic field is applied to the material, the magnetic moments will be forced to align in a higher order. As a consequence, the magnetic entropy will decrease. In isentropic (adiabatic) conditions, the total entropy will remain constant. Therefore, the decreased magnetic entropy will manifest itself in an increased lattice entropy. The atoms in the material will start to vibrate more intensively, and as the consequence, the temperature of the magnetic material will increase. The opposite occurs when the magnetic field is removed: the magnetic entropy is increased and the temperature decreases. On this basis, it is possible to create energy conversion cycles by applying different thermodynamic processes.In this chapter, the basic magnetocaloric thermodynamic potentials are presented and described. The state of the art gives an overview of the existing theoretical and experimental approaches to magnetocaloric thermodynamic cycles. Different magnetic thermodynamic cycles are described. Besides thermodynamic cycles with conventional simple cycles, an important emphasis is placed on thermodynamic cycles that apply active magnetic regeneration (AMR). Since most of the existing devices apply the AMR principle, a whole chapter (Chap. 4) is dedicated to this topic.

Study of geometries of active magnetic regenerators for room temperature magnetocaloric refrigeration

“…Room temperature magnetocaloric refrigeration (MCR) has been developed significantly in recent years and many prototypes with significant cooling capacity have emerged [1,2,3].…”

“…[1,2,3], the packed particle bed and parallel plate matrix were two commonly used geometries. Due to the advantages of easy construction and good heat transfer performance, the packed particle beds were widely implemented.…”

Magnetocaloric performance of the three-component Ho1-xErxNi2 (x = 0.25, 0.5, 0.75) Laves phases as composite refrigerants

“…Magnetic refrigeration based on the magnetocaloric effect has become an attractive alternative to conventional cooling methods owing to its energy efficiency and ecological safety. Up to now, an intensive search for materials suitable for the use as the working body of magnetocaloric refrigerators is under way 4 – 9 . All magnetic materials intrinsically show MCE, although the intensity of the effect depends on the properties of each material.…”

Efficient and affordable thermomagnetic materials for harvesting low grade waste heat

“…Magnetocaloric materials expand the application range towards the conversion from electrical to thermal energy [1,2]. In these materials, a steep change of magnetisation in the vicinity of room temperature yields a magnetically induced entropy change, which drives a magnetocaloric cooling cycle [3]. The high efficiency of this cycle has triggered intense research on magnetocaloric [4] and related multicaloric materials [5].…”