Bédécarrats et al. [64,65] measured the thermal performance of
an encapsulated PCM (water with a nucleation agent) in spherical
capsules. Fig. 7 shows the scheme of the tank and the spherical
capsules of PCM used. They developed a test plant that permitted
to study the behavior of the tank during the charge mode taking
into account the subcooling and the discharge mode. A simplified
mathematical model, taking the nodules as heat exchangers, confirmed
the experimental results and permitted the detailed analysis
of the charge and the discharge mode. Furthermore, in more
recent studies, they concluded that there was a significant influence
of the subcooling phenomenon during the charging process.
Following the same idea, a numerical and experimental study
was conducted by Ismail and Henriquez [66–68] using spherical
capsules filled with water as PCM. The capsules were placed inside
a cylindrical tank fitted with a working fluid circulation system.
The differential equations describing the system were solved by
the finite difference method and a moving grid inside the spherical
capsules. This model was used then to predict the effect of the
dimensions of the spherical capsules and their shell thickness, shell
material, initial PCM temperature and the external wall temperature
on the solidified mass fraction and time for complete solidification.
Some other researchers have done similar studies than
those mentioned before [69–71] concluding that the solidification
phase front propagates uniformly inwards towards the center of
the sphere and determining some correlation coefficients for the
solidification process. Furthermore, MacPhee and Dincer [72] modeled,
through heat transfer and thermodynamic analysis, the
charging process of an encapsulated ice TES device. With the flow
exergy analysis technique, it was possible to optimize a charging