Numerical analysis of the packed bed TES system integrated into the first parabolic trough CSP pilot-plant using air as heat transfer fluid

Single-tank thermal energy storage (TES) systems represent not only a valuable alternative to the most commonly exploited two-tank configuration in conventional concentrating solar power (CSP) plants, but also the most suitable solution for air-based solar systems. The first parabolic trough CSP pilot plant (3.9 MWth peak), which uses air as heat transfer fluid (HTF), has been constructed in Ait-Baha Morocco. An important characteristic of this plant is the integration of a 100 MWhth TES unit based on a packed bed of gravel as storage medium. The TES unit is designed for daily operating conditions of 10 h of charging followed by 4.5 h of discharging, at higher HTF mass flow rate, and 9.5 h of idle. The nominal charging and discharging temperatures are 570°C and 270°C respectively. A previously validated computational fluid dynamics (CFD) approach was followed to evaluate the thermo-fluid dynamics behavior of the TES unit under investigation subjected to a total of 5 pre-charging cycles, to reduce the initial transients of the system, followed by 60 consecutive cycles. The TES performance was also evaluated on the basis of the first- and the second-law of thermodynamics. According to the simulation results, despite a relatively weak thermal stratification was observed into the packed bed, the TES performance showed an increase with cycles from 52% and 63% up to stable values of 90% and 88% for the energy and the exergy efficiencies respectively.