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.