Phase change behaviour of some latent heat storage media based on calcium chloride hexahydrate

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.