Abstract :
Phase change behaviour of pure and some chemically modified calcium chloride hexahydrate materials suggested in the literature as
suitable heat storage media was studied in storage systems with a low degree of mixing in order to evaluate their long-term reversibility.
The influence of gravitation on the phase change properties of the pure system was elaborated by determining local salt concentrations in
the liquid phase and changes in the volume of the system during repeated melting and crystallization, whereby the amounts of the different
phases being present could be assessed. It was found that thermo gravitational effects are of minor importance in generating salt
concentration gradients in the liquid phase when compared to the concentration gradients resulting from the sedimentation of the formed
tetrahydrate and the hexahydrate during the phase change reactions. Due to these effects it was not possible to prevent irreversible tetrahydrate
formation and deterioration in heat storage capability by adding a surplus of water to the hexahydrate materials, if seeds for
the tetrahydrate were present during repeated melting and crystallization. Upon addition of SrCl2 6H2O a solid solution of the two
hexahydrates is formed and as a result the formation of the tetrahydrate is suppressed. Due to irreversible formation of SrCl2 2H2O
a state of true congruent melting seems, however, not possible to reach. Phase segregation effects also in this case lead to irreversible
calcium chloride tetrahydrate formation if seeds for the latter are added during repeated melting and crystallization. If Ca(OH)2 is also
added, the tetrahydrate formation can be further suppressed. But, even in this case, irreversible tetrahydrate formation can not be prevented
if the composition of the liquid phase corresponds to the hexahydrate. On addition of 5 wt% KCl or 5 wt% KCl + 2 wt%
SrCl2 6H2O or 5 wt% KCl + 0.4 wt% NaCl, repeated melting and crystallization in the presence of seeds of the tetrahydrate can, however,
be carried out without irreversible formation of the latter occurs. In the heat storage system CaCl2 6H2O + 5 wt% KCl, formation
of tetrahydrate is observed but its rate of dissolution is found considerably faster than in the pure hexahydrate system. It was concluded
that with those modified calcium chloride hexahydrate systems reversible phase change cycling can be performed and therefore there is
for example no the need for thickener agents to be added to avoid irreversible tetrahydrate formation successively reducing heat storage
capability of systems in those cases.
2008 Elsevier Ltd. All rights reserved.
