Numerical Simulation to Investigate Thermal Stratification in an Energy Storage System Under No-Flow Condition

Thermal energy storage (TES) systems enhance the reliability of intermittent energy sources. The present work examines the thermal stratification in a thermocline storage system during charging and discharging with no fluid flows. With this goal, numerical simulations are performed on a storage tank consisting of a constant temperature heat source and sink. Molten nitrate salts mixture (60 % NaNO3+ 40 % KNO3) is used as storage material as it is thermally stable at higher temperatures (up to 565 °C). The thermocline behaviour of the numerical model is assessed during charging and discharging using constant temperature heating and cooling sources. In the present study, molten salts' lower and upper-temperature limits are 300 °C and 500 °C, respectively. The qualitative behaviour of thermocline formation and propagation during charging and discharging is analysed by temperature contours and axial temperature profiles. Quantitatively, the thermocline behaviour is analysed by two non-dimensional parameters: Stratification number (Str) and Mix number (MIX). It is observed that with a constant temperature heat source and sink, the tank remains stratified throughout the charging and discharging. The charging and discharging rates have been determined, and the results will be helpful in designing a lab-scale thermocline demonstration setup and further study.