Evaluation of Thermal Stratification of an Air-Based Thermocline TES with Low-cost Filler Material

In the present work, a computational fluid dynamics (CFD) approach was followed to evaluate the extent of thermal stratification of an industrial-scale thermal energy storage (TES) system, based on a packed bed of river pebbles The TES is integrated into a reference concentrating solar power plant which uses air as heat transfer fluid. The transient evolution of thermal stratification was qualitatively evaluated according to the dimensionless MIX number based on the so-called moment of energy, or height-weighted energy, into the packed bed. The resulting stratification efficiency ranges between 0 and 1 with the theoretical threshold values given by the moment of energy of fully mixed and ideally stratified TES respectively. The 30 consecutive cycles analyzed were characterized by 12 hours of charging followed by 12 hours of discharging. The results obtained showed that the TES system reached a stable working condition after 20-22 cycles with an average stratification efficiency of about 0.95. The CFD simulations were performed with Fluent 14.5 code from ANSYS.