Comparative performance of two full-scale Danish pit thermal energy storages with contrasting geometries under seasonal and short-term operation

Pit thermal energy storage (PTES) enhances the flexibility of district-heating systems by buffering the mismatch between heat supply and demand. As PTES applications expand from seasonal storage toward short-term operation, full-scale pits increasingly exhibit different geometries and operating regimes, whereas the applicability of validated models to full-scale PTES with different operating modes and geometry regularization requirements remains insufficiently examined. To address this gap, this study developed a coupled TRNSYS modelling framework that integrated Type 1536 for the water domain with Type 1301 for the soil and lid-insulation domain. The developed PTES model was validated against the monitoring data from two Danish PTES with different functions and shapes: the standard Dronninglund PTES with the shape of an inverted truncated pyramid and the slim Høje Taastrup PTES with a large aspect ratio. The validated model was used to compare temperature profiles, charging/discharging behaviour, heat loss partitioning, energy and exergy performance, and thermal stratification in the two PTES. Sensitivity analyses were conducted to quantify the combined effects of soil thermal properties and middle diffuser elevation on both PTES systems, and the combined effects of top-surface aspect ratio and annual storage cycle number on the Høje Taastrup PTES. The simulations reproduced measured temperatures and annual energy indicators with good accuracy. The short-term Høje Taastrup PTES had about four times as many annual storage cycles as the Dronninglund PTES, while its annual charge and discharge energies, and heat loss were about three times higher. Nevertheless, the two PTES showed similar annual storage efficiencies of about 92 % and similar annual exergy efficiencies of about 78 % under the system-specific conditions. The Høje Taastrup PTES also maintained stronger annual stratification, reflected by a lower mean MIX number and a higher normalized stratification coefficient. Sensitivity analyses showed that higher conductive soil lowered storage efficiency and increased mixing, and that the effect of middle diffuser elevation on stratification was non-monotonic and system specific. For the Høje Taastrup PTES, lowering the top-surface aspect ratio slightly reduced heat loss and mixing within the constrained geometry set considered, but its influence remained weaker than that of annual storage cycle number. These results demonstrated the value of using rare full-scale monitoring data to assess seasonal and short-term PTES operation under real conditions. The coupled TRNSYS framework reproduced the thermal behaviour of the two Danish PTES systems under their respective operating conditions, and provided case-based insights for diffuser arrangement, geometry regularization, and the design of the seasonal and short-term PTES applications.