In the present study, a computational fluid dynamics approach has been developed with the aim of replicating the thermo-fluid dynamics behavior of the high-temperature thermal energy storage (TES) system integrated into the world’s first underground advanced adiabatic compressed-air energy storage (AA-CAES) pilot plant. A 120 m long section of an unused tunnel (4.9 m diameter) under the Swiss Alps (Canton of Ticino) was exploited as air reservoir. A 12 MWhth packed rock bed TES system, directly integrated into the pressure chamber, was used to store and release the thermal energy produced during air compression. Several experimental tests were performed operating the pilot plant under different charge/discharge cycles with air temperature up to 550°C and a maximum pressure of 7 bars gauge. The numerical model developed was satisfactorily validated against experimental data demonstrating its accuracy in replicating the thermo-fluid dynamics behavior of the experimental TES unit. The performance of the TES unit were also evaluated in terms of energy and exergy efficiencies resulting to be in the range of 0.77-0.91 and 0.72-0.89 respectively.