Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 433

device. However, as was emphasized in the paper, the concept was only used to
illustrate the barocaloric refrigeration process and many technical problems should
be solved before building a real prototype [ 90 ].

10.3 Elastocaloric Energy Conversion

In this subsection, the elastocaloric effect, which is associated with shape-memory
and superelastic effect, is discussed. The fundamental elastocaloric (superelastic)
behaviour is shown and the basic equations for the estimation of the elastocaloric
effect are introduced. Finally, so far evaluated elastocaloric materials and its
properties are presented and the some design concepts of the elastocaloric cooling
device are reviewed.

10.3.1 Introduction to the Elastocaloric Effect

The elastocaloric effect (EsCE), sometimes referred to as the thermoelastic effect, is
a physical phenomenon associated with the entropy and/or temperature changes of
certain materials subjected to an external mechanical stress. The EsCE is closely
related to the well-known shape-memory or, better, superelastic effect [ 91 ]. In
general, we distinguish between shape-memory alloys and polymers.
Shape-memory alloys undergo a reverse martensitic transformation between the
austenitic (cubic) and the martensitic (monoclinic) phases (see Fig. 10.24 ). The
transformation can be induced by a temperature change or by the application of an
external stress. The temperature-induced transformation (shape-memory effect) is
observed by cooling the material from the austenitic to the (twinned) martensitic
phase, where it becomes softer and easier to deform. When reheated back it
undergoes the reverse martensite-austenite temperature-induced transformation and
is restored to its original shape, acting as though it has memory. Similarly, a stress-
induced transformation (superelastic effect) is observed when the material is loaded
in the austenitic phase. During loading, at a certain critical stress, the transformation
to the martensitic phase begins, which causes large strains in the material at almost
constant stress, until it fully transforms to a de-twinned martensite. Upon unloading,
reverse martensite-austenite transformations occur and the material returns to its
original state. Both transformations are associated with a hysteresis in the trans-
formation region. A schematic representation of the shape-memory and superelastic
effects is shown in Fig. 10.24 , together with the typical shape-memory (down-left)
and superelastic (down-right) behaviours.
In order to generate the EsCE a stress-induced transformation is required. The
stress-induced forward austenitic-martensitic transformation is exothermic, while
the reverse transformation is endothermic. In some alloys, the latent heat released
(absorbed) during the transformation can be as high as 20 Jg − 1

[ 92 ]. If the