In the present study, using a computational fluid dynamics code, we numerically investigated the effect of the aspect ratio of rock caverns for underground thermal energy storage (UTES) on thermal stratification during the standby mode of operation without energy flows due to charging and discharging processes. The numerical investigation was based on the Lyckebo storage cavern in Sweden. This cavern has a unique toroidal shape and is the first large-scale rock cavern for UTES. Heat transfer simulations were carried out for different aspect ratios ranging from 1.0 to 4.0, and the heating of the surrounding rock due to longterm TES was taken into account. In these simulations, the variation of the thermal stratification with respect to time was quantitatively examined based on indices for the degree of thermal stratification. The results of the numerical simulations showed that the thermal stratification in rock caverns varied depending on the aspect ratio and the duration of the standby mode. It was also demonstrated that better thermal stratification can be obtained by increasing the aspect ratio, the effect of which was significant when this ratio was below 3.5.

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