Experimental study on thermocline behavior and management in phase change material-enhanced thermal energy storage: Implications for charging and discharging cycles

• Quantify thermal efficiency and temperature distribution in a vertical tank with bulk PCM. • Optimize tank efficiency by regulating and minimizing thermocline expansion. • Use aluminum cylinders to boost PCM surface area, speeding charge/discharge and improving utilization factor. • Examine PCM’s impact on thermocline thickness, energy storage, and energy extraction in the tank model. • Enhance thermocline by adding paraffin wax to a water-PCM storage tank. The increasing costs of designing and constructing a thermal energy storage (TES) tank have led many researchers to seriously consider the factors affecting thermocline deterioration inside the tank during charging and discharging cycles. It is crucial to regulate the thermocline, which is the boundary between the hot and cold heat transfer fluid (HTF) zones, to minimize its expansion and optimize the operating efficiency of the storage tanks. The study focuses on various experimental aspects of the hot water storage tank (HWST). It also shows the effect of phase change material (PCM) on the thermocline, the total energy storage capacity, and the actual energy extracted from the tank model during the charging and discharging processes. The HWST with a TES tank model was constructed for experimental purposes. The TES tank model used a solid phase change material of 54 kg (15 %) of cylindrical paraffin wax, specifically of the RT42 classification. Additionally, 18 aluminum cylinders were employed to increase the surface area of the phase change material, accelerate the charging and discharging processes, and improve the utilization factors. The incorporation of paraffin wax into the water-PCM storage tank improved the thermocline layer during the discharging process, reducing its thickness by 50 %, 50 %, and 34 % after 30 min at extracted water temperatures of 48 °C, 53 °C, and 56 °C, respectively, with a mass flow rate of 0.13 kg/s. The maximum utilization factors reached 72.2 % with total energy storage increasing by 30.5 % and extracted energy by 68.8 % during charging and discharging cycles, respectively.