Energy Reports (Nov 2021)
Investigating thermal stratification in a vertical hot water storage tank under multiple transient operations
Abstract
Hot water storage (HWS) tanks are one of the commonly employed sensible energy storage systems that store heat energy using water as the storage medium. Hot water tanks store thermal energy from multiple energy sources and practise several utilization methods, however, identifying the best approach to effectively utilize the stored thermal energy inside the tank from the energy efficiency point of view is critical. Present work focuses on the transient numerical investigation of a domestic, multi-sourced HWS tank. Thermal behaviour of HWS tank is analysed in three dynamic operational modes, namely charging, continuous delivery, and discharging operation. The axis-symmetric, two-dimensional model of the double coiled, vertical HWS tank of cylindrical shape is simulated using ANSYS Fluent. The numerical model is validated against the experiment data in charging mode. Influence of operating modes on the degree of thermal stratification is determined. Axial temperature evolution, dimensionless Stratification, and Richardson numbers are calculated and compared for each operational case. Results show that thermal stratification depends on the location of heating source inside the tank under the given operational mode. The degree of stratification in both the charging and continuous delivery modes is improved when the heat exchanger coil installed at the upper portion of the tank is functioning. The Stratification numbers are 0.28 and 0.32 after 30 min in the charging mode, and continuous delivery mode, respectively. The study compares thermal performance of HWS tank in different conditions and reveals the evolution of the thermal stratification with time, which are essential in designing stratified HWS tanks.
