Numerical analysis of thermocline evolution during charging phase in a stratified thermal energy storage tank

Abstract Thermocline thickness (TLT) is the best parameter to quantify the thermal performance of stratified thermal energy storage (TES) tanks as it defines the inactive part of a storage medium. A detailed literature review reveals that there is no consensus in the community on the temperature band where the TLT is quantified. In this study, a new criterion for quantifying TLT is proposed that addresses its dynamic nature and is based on 1 K below T m a x and 1 K above T m i n inside a TES tank. Additionally, an ideal charging diffuser is defined that can attribute to the minimum theoretical TLT at a certain TES tank height to diameter ratio and charging mass flow rate. Therefore, the effectiveness of any enhancement strategies for improving TES thermal performance in a real working condition can be compared against the ideal case. The proposed criterion is applied during the charging process of a cylindrical TES tank through computational fluid dynamic (CFD) simulation by solving the coupled hydrodynamic (continuity and momentum) and energy equations in a 2D axisymmetric computational domain for a real TES tank. The accuracy of the obtained results is validated against available experimental data and a good agreement is observed. The main observation is that the ideal minimum TLT evolves proportional to the square root of dimensionless charging time divided by Peclet number, however, with the actual diffusers when heat loss to the ambient air and mixing effect are present, TLT undergoes three formation stages, namely, mixing dominated, developing, and fully developed regions. The evolution of TLT in the dynamics of the TES charging process is reported at different working conditions.