Impact of inlet fluctuations on the stratification of a sensible thermal energy storage

Hot water thermal energy storage systems are an established standard with a wide range of applications, from industrial processes to district heating. They are commonly paired with solar thermal plants, a combination that is prevalent due to their efficiency. However, this pairing is often affected by intermittency and fluctuations in output thermal power and delivered temperature. In this study, a two-dimensional finite element model is employed to evaluate the impact of these fluctuations, which are modelled using periodic functions to examine their effects on the system's ability to establish or maintain thermal stratification. Both the amplitude and frequency of temperature oscillations are analysed, along with the co-dependent impact of inlet velocity fluctuations. Results demonstrate that temperature fluctuations at the tank inlet significantly degrade stratification, with small variations (2.5 °C) increasing the MIX number by 102% and larger fluctuations causing up to a 312% increase compared to the storage's initial state. These effects are independent of mean inlet temperature and velocity variations. A correlation between MIX number and temperature fluctuation amplitude is provided. The findings offer practical insights for enhancing TES performance, particularly through advanced control strategies like Model Predictive Control to mitigate stratification degradation under dynamic operating conditions.