Experimental study of thermocline behavior under high propagation velocities in a water based thermal energy storage

This experimental study investigates the development of the thermocline under varied operating conditions and particularly at high propagation velocities ranging from 0.5 mm.s −1 up to 8 mm.s −1 and temperature differences (ΔT) of 10 °C, 30 °C and 65 °C. For this purpose, a new experimental setup, designed to investigate thermocline formation, propagation, and withdrawal is implemented. The setup meets essential requirements for acquiring controlled validation data for CFD models and enables the collection of novel high-velocity flow data. The storage system features extensive instrumentation, including temperature measurements at curved tank bottoms and radial positions. Dynamic experiments validated literature findings, demonstrating stronger stratification with slower axial propagation velocities and larger temperature differences. High propagation velocities remain feasible with adequate ΔT, i.e. with a Richardson number based on the penetration height of the inlet fluid, R i 0 , h i ≥ 0.25 ensuring a thin thermocline for this test section. Variation of the extraction distribution system revealed the influence of fluid withdrawal on thermocline thickness, underscoring the need to consider outlet dynamics in distributor design. These findings establish the relevance of R i 0 , h i for predictive performance correlations and provide a foundation for optimizing thermal energy storage systems through refined distributor designs. Finally, an original definition of thermocline thickness was introduced, based on the slope of the thermocline, offering an alternative to the classical boundary-temperature approach and limiting the dependence on temperature limits. • New set-up is used for the investigation of the thermocline formation, propagation and withdrawal in water thermocline thermal energy storage • New data, not available in the literature, has been obtained, notably at high fluid velocity • The Richardson number based on the penetration height of the inlet fluid is a relevant parameter to characterize the quality of the axial stratification • The results confirm the threshold on the Richardson parameter for good fluid distribution and axial stratification, i.e. R i 0 , h i ≥ 0.25 • An original definition of thermocline thickness, based on the slope of the thermocline, is introduced