Pilot-scale experimental investigation of two-tank high-temperature thermal energy storage using liquid sodium

High-temperature thermal energy storage (TES) is attracting increasing interest for industrial heat decarbonization, yet publicly reported experimental data for liquid-sodium TES remain scarce and include no cycle-resolved energy and exergy balance with propagated uncertainties at pilot scale. To address this gap, the Thermal Energy Storage Experimental Verification Test Loop (TESET), a 1.25-MWh liquid-sodium two-tank TES facility, was constructed and operated through a complete charging-standby-discharging cycle. During charging, a distinct thermocline developed in the hot tank despite the high thermal conductivity of liquid sodium, driven by top injection of progressively hotter sodium. During the subsequent 12-h hot-standby period, the thermocline collapsed within approximately 2.5 h, primarily through axial heat conduction enabled by the high thermal diffusivity of liquid sodium. The average sodium temperature decreased at 1.89 °C/h, corresponding to a heat loss rate of 5.27 kW. During discharging, thermal energy was recovered through a sodium-to-air heat exchanger. Energy and exergy analyses were performed using a control-volume approach applied to the hot tank. The resulting thermal inventory retention efficiency and exergy retention efficiency were 92.1% and 89.1%, respectively. These results provide pilot-scale evidence of the thermal retention capability of liquid-sodium TES and support its scale-up for high-temperature energy storage applications.