Environmental Engineering Reference
In-Depth Information
Chapter 1
The Thermodynamics of Magnetocaloric
Energy Conversion
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 eld. 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
eld, the magnetic moments in the
material are disordered. If a magnetic
eld 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
eld 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 mag-
netic 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.
1.1 Introduction
The earliest thermodynamic studies of the magnetocaloric effect near or above room
temperature began in the 1950s and 1960s. In addition to a number of cryogenic
applications, this work was initially focused on the development of heat engines for
the generation of useful power. Researchers were investigating different magnetic
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