Thermal stratification characteristics during simultaneous charging and discharging for different storage tank geometries with immersed discharging coil

The geometrical shapes of the thermal energy storage and the configurations of immersed discharging coils dictate the efficacy of low-to-medium temperature hot water applications. This study uses a three-dimensional numerical model to investigate the thermal characteristics of three storage configurations for simultaneous charging and discharging. The same tank volume and height, as well as the same coil tube dimensions, are considered for three different configurations -cylindrical tank with a helical discharging coil, as well as circular truncated cone-shaped and paraboloidal tanks with conical discharging coils. The discharging flow rate is varied for a deeper understanding of the realistic interplay between energy demand and supply. Results obtained for the cylindrical tank are found to have good agreement with the observations from the in-house experiments. The energy stored, discharging coil outlet temperature, and the extent of thermal stratification decrease with increasing coil flow rate, whilst discharging efficiency and primary thermocline thickness increase. The parab-oloidal tank equipped with a conical discharging coil exhibited the highest cumulative energy stored (8821 kJ) just before the commencement of dynamic dual operation, owing to the minimal thermal losses attributable to the lowest surface-area-to-volume ratio. Thermocline thickness is found to expand with time for all the cases, and the highest thermocline thickness expansion rate of 0.208 mm/s is found for the cylindrical tank indicating a higher degree of energy degradation. The peak values for the cumulative energy discharged (9788 kJ), and average discharging efficiency (0.36) occurred for the paraboloidal tank with the conical discharging coil configuration. Based on the detailed analyses, the paraboloidal storage tank configuration is found to exhibit better system reliability and energy management, leading to more efficient heat dispatch controllability due to the enhanced energy harnessing features.

Automated summary

The geometrical shapes of thermal energy storage and the configurations of immersed discharging coils have a significant impact on the effectiveness of low-to-medium temperature hot water applications. This study uses a three-dimensional numerical model to analyze the thermal characteristics of three storage configurations for simultaneous charging and discharging. Results show that the paraboloidal tank with a conical discharging coil configuration exhibits the best system reliability and energy management, leading to more efficient heat dispatch controllability.