Influence of inlet structure on thermal stratification in a heat storage tank with PCMs: CFD and experimental study

Abstract

Heat storage tanks are one of the key components in solar thermal utilisation systems. In this study, the effect of the inlet stratifier on the thermal stratification in a heat storage tank with phase-change materials (PCMs) was investigated. A heat storage tank with a volume of 60 L and aspect ratio of 1.68 was developed based on the phase-change temperature of 331.15 K for sodium acetate trihydrate. The thermodynamic characteristics of the heat storage tank were measured at an initial temperature of 353.15 K and inlet water temperature of 278.15 K. Moreover, a computational fluid dynamics (CFD) model of the heat storage tank was established to simulate the discharge process. In addition, the CFD model was verified with experimental data. The impact of the PCM position on thermal stratification was thoroughly analysed for various flow rates. Furthermore, performance parameters including the Fill Efficiency (FE), Richardson number (Ri), and MIX number were considered. The results show that the equalizer enhances the thermal stratification effectively in the phase-change heat storage tank and stabilises the heat output characteristics of the water tank. Furthermore, in the water discharge process (t* = 0.1–0.7), the distances between the isothermal surfaces (279.15 K) and isothermal surfaces (352.15 K) in PCM4, PCM3, PCM2, and PCM1 increase by 6.56, 7.2, 8.98, and 12.34 cm, respectively. Thus, the mixing of hot and cold water strengthens with higher PCM position, which improves the thermal stratification in a heat storage tank. The half-life of the PCMs (melting rate reaches 50%) is prolonged with increasing inlet flow rate. For an inlet flow rate of 1 L/min, the half-life of PCM4 is t* = 0.5. For an inlet flow rate of 5 L/min, the half-life of PCM4 is t* = 0.95. Moreover, the simulated results of the FE and Ri are slightly higher than the experimental values, whereas the simulated MIX number results are below the experimental values. Finally, the simulated and experimental root mean-squared error results increase with lower PCM positions and increasing inlet flow rates.