Simultaneous charging and discharging operations of thermal energy storages render effective energy-harnessing features. However, it leads to thermocline formation due to the dynamic interplay between energy input, energy extraction, and losses. Reliable retention of good thermodynamic quality of energy is realized by minimizing the energy degradation and thermocline thickness. Near real-time tracking of heat content degradation is a tedious task for real-sized storage systems, which involves accurate quantification of the temporal evolution of thermocline thickness. A few simplified, one-dimensional energy-based models are available for tracking the movement of the thermocline layer inside the thermal energy storage tank. However, these models largely assume the thermocline layer to be a thin horizontal plane within the tank, without quantifying the thermocline thickness. In this study, a reduced-order realistic stratification (RS) model has been developed for the vertical cylindrical tank equipped with an immersed helical discharging coil, working under dual-dynamic mode. For evaluating the thermocline thickness, the prediction equations for the average temperatures in the upper and the lower regions of the tank are formed using a machine-learning-based technique, considering a reasonably wide range of operating and geometrical parameters. The developed model utilizes approximate formulations for the average temperatures of the upper and lower regions of fluid within the storage tank, coil outlet temperature, and the overall average temperature of working fluid. For the larger storage tank (155 L), the thermocline thickness predicted by the new RS model is 738.26 mm at the end of simultaneous charging and discharging operation, whereas that estimated by detailed numerical simulation is 798.12 mm. This improved model reduces the computational time by about 80 % owing to effective approximations and can be used for a reasonably accurate rapid assessment of thermal degradation in the storages deployed for low-temperature solar thermal applications, including domestic hot water systems.
