Thermal energy storage (TES) has become a key component in combined heat and power (CHP) generation, which enhances the load regulation capability and overall thermal performance. In line with that concept, the present work addresses a numerical study that aims at investigating and predicting the transient thermal behavior of a water thermocline storage tank that integrates into a CHP plant. Investigations are carried out on two distinct two-dimensional models. The first model studies thermocline characteristics of the heat storage system through a single-charge/discharge operation. The influences of various operating parameters such as the inlet flowrate and the inlet temperature of HTF are investigated. Results indicate that the thermocline thickness increases as the inlet flowrate increases, and consequently, the heat storage/release period decreases. The second modified model discusses the technique of simultaneous charging and discharging operation applied in the CHP unit. Two types of operations are analyzing with (a) stable charging with steady discharging and (b) periodic charging with steady discharging. The results ravel out that the mixing ratio of charging and discharging flowrate (Qc/Qd) has a strong influence on the operation performance. When the ratio of Qc/Qd >1, the thermocline forms, and its thickness increases with time. The thermocline thickness remains constant as this ratio declines to 0.625. In the case of periodic charging, both larger charging flowrate and shorter non-charging periods result in maintaining the discharge outlet temperature of HTF stable for the heat-supply net, and discharge performance improved.