Thermodynamic Analysis of Freezing and Melting Processes in a Bed of Spherical PCM Capsules

The solidification and melting processes in a spherical geometry are investigated in this study. The capsules considered are filled with de-ionized water, so that a network of spheres can be thought of as being the storage medium for an encapsulated ice storage module. ANSYS GAMBIT and FLUENT 6.0 packages are used to employ the present model for heat transfer fluid (HTF) past a row of such capsules, while varying the HTF inlet temperature and flow rate, as well as the reference temperatures. The present model agrees well with experimental data taken from literature and was also put through rigorous time and grid independence tests. Sufficient flow parameters are studied so that the resulting solidification and melting times, exergy and energy efficiencies, and exergy destruction could be calculated. All energy efficiencies are found to be over 99%, though viscous dissipation was included. Using exergy analysis, the exergetic efficiencies are determined to be about 75% to over 92%, depending on the HTF scenario. When the HTF flow rate is increased, all efficiencies decrease, due mainly to increasing heat losses and exergy dissipation. The HTF temperatures, which stray farther from the solidification temperature of water, are found to be most optimal exergetically, but least optimal energetically. The main reason for this, as well as the main mode of loss exergetically, is due to entropy generation accompanying heat transfer, which is responsible for over 99.5% of exergy destroyed in all cases. The results indicate that viewing the heat transfer and fluid flow phenomena in a bed of encapsulated spheres, it is of utmost importance to assess the major modes of entropy generation; in this case from heat transfer accompanying phase change.