Heat transfer modeling of a pit thermal energy storage with an embankment and the effects of structural parameters on thermal storage performance

Large-scale pit thermal energy storage (PTES) is widely applied in solar district heating. In practice, PTES are typically constructed with a truncated-pyramid geometry. To balance earthwork and address groundwater constraints, embankment is often introduced, resulting in a partly buried configuration. However, most simplified nodal PTES models deviate from these as-built features, limiting their applicability. This study refines the geometric representation of conventional models and develops a partly buried truncated-pyramid PTES model with an embankment. Model accuracy is evaluated based on field measurements from the Langkazi PTES project in Xizang, China, with additional supplementary validation using data from the Dronninglund PTES project in Denmark. Based on the validated model, a coupled solar-driven short-term thermal storage heating framework is established to quantify the effects of key structural parameters on thermal storage performance under short-term operating conditions.The results show that: (1) The model agrees well with field measurements, with relative errors below 4% for average water temperature and below 7% for soil temperature; (2) Thermal stratification strengthens with increasing pit height and side slope. (3) Reducing embankment width from 6 m to 1 m decreases annual heat loss from 886.98 to 881.26 MWh (0.65%), indicating a minor effect; (4) Increasing embankment height intensifies heat loss and reduces efficiency; at 6 m, annual heat loss increases by 8.92% and thermal storage efficiency decreases by 2.45% relative to the fully buried case; (5) Thermal storage efficiency decreases with increasing surface-area-to-volume ratio (SA/V). A fitted SA/V–efficiency relationship is provided for short-term operation, and design charts for selecting pit height and slope are developed based on SA/V minimization. This study broadens the applicability of existing PTES heat transfer models, making them more representative of practical engineering conditions, and provides theoretical support for the design of PTES structural parameters.