Thermodynamic analysis of the thermocline storage tank with time-varying charging parameters

The thermocline storage tank (TCST) is a cost-effective device, that can provide the peak shaving ability of heating load in the district heating networks. The charging parameters, such as temperature and mass flow rate of charging, are time-varying in actual operating processes, which inevitably influence the thermocline and the thermodynamic performance of the tanks. Therefore, the thermocline evolution phenomenon and the irreversibility of the TCSTs during the time-varying charging parameters processes are studied through numerical simulations. The thermocline evolution with time-varying charging parameters is divided into three stages, namely, the thermocline stabilization phase, thermocline fluctuation phase, and thermocline fully developed phase. The thermocline grows rapidly from 0.16 m to 1.36 m when the variation of charging temperature exceeds 10 °C, and the change in mass flow rate has minimal effect on the thermocline thickness. The exergy efficiency decreases from 96.3 % to 90.0 % when the charging temperature difference decreases from 40 °C to 10 °C, and the instantaneous entropy generation rate increases by 4.3 times. The maximum entropy generation by heat transfer is 27.54 (W/Km⁻³) near the inlet, and the maximum entropy generation by viscous dissipation is 0.1745 (W/Km⁻³) near the outlet. However, the entropy generation by heat transfer dominates the irreversibility of the tanks and is about two orders of magnitude larger than the entropy generation by viscous dissipation in the tanks. When the charging temperature changes within 10 °C, the exergy efficiency changes within 1 %, the thermocline thickness decreases and stabilizes at about 0.10 m, and the entropy generation rate is relatively stable. When the variation of the charging temperature exceeds 10 °C, the thermocline thickness and the instantaneous entropy generation rate increase remarkably, and the exergy efficiency decreases considerably. The findings may provide some guidance for the operation of the TCSTs.