Experimental and Numerical Analysis of a PCM-Supported ...

(i)
(ii)
(iii)
One problem is segregation due to incongruent melting. Segregation is
formation of other hydrates or dehydrated salts that tend to settle out and
reduce the active volume available for heat storage. This problem could be
eliminated by several techniques, such as use of gelled or thickening
agents [102] though this process negatively impacts the heat storage
characteristics of the mixture and the mixture still degrades with time [97],
rotating storage devices for stirring, direct contact heat transfer [101], and
encapsulation of the PCM in a small volume to reduce separation. The
mechanism of such phenomena is that water of crystallization released
during thawing is not sufficient to dissolve the solid present (the resulting
solution is supersaturated at the melting temperature). The solid settles
down at the bottom of the container due to its higher density. During the
reverse process, i.e. crystallization, a substantial part of the settled salt is
unable to come into contact with water required for its crystallization.
Therefore, the compound of the solid is not the same as that of the liquid.
The energy density of the storage reduces after several charge and
discharge cycles due to this phenomenon of incongruent melting.
Supercooling is the second problem common to many salt hydrates.
During cooling, the solidification does not take place at the melting
temperature due to the low rate of crystal formation. Consequently, the
solution has to be supercooled, i.e. cooled below the melting temperature
by several degrees, until a reasonable rate of nucleation is achieved. Due
to supercooling, the PCM does not discharge the stored thermal energy at
the melting temperature as expected. This problem has been avoided in
several salt hydrates through promoting nucleation by one of the following
means: adding small quantities of a nucleating material so that crystals
may start growing around it; mechanical means such as a rough container
walls to promote heterogeneous nucleation; and keeping in the PCM a
cold finger or a region which is allowed to remain cool when the rest of
the PCM has melt. Lane [102] offers a comprehensive listing of nucleating
materials for most common salt hydrates.
Another problem of salt hydrates is their tendency to cause corrosion in
metal containers that are commonly used in thermal storage systems [97].
Compatibility of PCM and container should always be checked before use.
2.6.3 Eutectics
Eutectics are mixtures of two or more salts which have definite melting/freezing
points, and they can be classified as organic, inorganic, and organic-inorganic
eutectics. Their behaviour is analogous to congruent melting salt hydrates, and has
great potential for thermal energy storage applications [101]. Eutectics nearly always
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