Development and Validation of a Analytical Model for Large-Scale Water Pit Thermal Energy Storage System

The use of large-scale water-pit thermal energy storage (PTES) systems can increase the share of renewable energy sources in district heating systems. Currently, the available models for PTES systems are mostly based on numerical models that are not amenable to fast calculation, lacking accurate analytical models. Accordingly, this study proposes a analytical model suitable for PTES. The model solves for the water and soil domains separately and then couples them through the pit sidewall, bottom temperature, and boundary heat fluxes. For water domain heat transfer analysis, a "three-zone" model is proposed, dividing the cross-section into central, transition, and edge zones. Compared to simple one-dimensional models, this "three-zone" model considers horizontal water flow, thereby providing a more accurate dynamic simulation of water temperature. In the heat transfer analysis of the soil domain, the finite cylindrical source model used in the field of ground-source heat pumps was improved to make it suitable for modeling PTES. Furthermore, the semi-analytical model was validated using 10 years of measured data from a 60000 m³ PTES in Denmark. The average temperature errors at the top, upper-middle, middle, lower-middle, and bottom positions were 0.233%, 0.44%, 0.445%, 0.316%, and 1.27%, respectively, all less than 1.5%, indicating that the model exhibits high accuracy and reliability.