Numerical simulation and exergetic performance assessment of charging process in encapsulated ice thermal energy storage system

The solidification process in encapsulated ice thermal energy storage (EITES) system is simulated for water-filled capsules while neglecting storage tank wall effects and heat penetration. Energy and exergy efficiencies were calculated while varying capsule shape, inlet Heat Transfer Fluid (HTF) temperature as well as HTF flow rate. 105 test cases are conducted including seven geometries, five inlet HTF temperatures, and three HTF flow rates. It was found that the energy efficiencies did not accurately reflect system performance, and in all cases, were found to be above 99.96%. However, exergy efficiencies ranged from 78 to 92%, and provided better insight into system losses. The results suggest that an effective way to increase system efficiency is to increase inlet HTF temperature; considerable efficiency gains are possible by setting inlet HTF temperature slightly below solidification temperature. Varying capsule geometry had inconsistent effects on the efficiency, different geometries being optimal in different situations. Surprisingly, viscous dissipation had very little effect on the exergy efficiency and was a source of very little entropy generation. Thus, EITES designers could increase both flow rate and inlet HTF temperature in order to achieve full system charging in an acceptable amount of time.

► Energy analyses resulted in over 99% efficiency in all cases, whereas exergy efficiencies ranged from 78 to 92%.
► Inlet Heat Transfer Fluid (HTF) velocity and capsule geometry have a relatively small effect on system performance.
► Inlet HTF temperature had a substantial effect on exergy efficiency, with higher temperatures being most efficient.
► Entropy generation through viscous dissipation was not a major source of irreversibility.