Optimization of continuous heat discharge from a thermal storage reservoir under extreme weather conditions using a cascaded water-intake strategy

Given the inherent intermittency of solar energy, maintaining high-quality thermal stratification during continuous discharge is pivotal for enhancing the supply reliability of Pit Thermal Energy Storage (PTES) systems, particularly under extreme weather with insufficient heat collection. However, the multi-physical coupling mechanisms involving truncated pyramid geometry, soil thermal feedback, and outlet flow dynamics remain poorly understood. In this study, a field-validated 3D transient numerical model was developed to systematically investigate the thermocline evolution and thermodynamic performance of a truncated pyramid pit. The limitations of the conventional discharge mode were quantified, and an optimized cascaded stratified withdrawal strategy was proposed. This strategy was rigorously evaluated using a comprehensive framework integrating dimensionless exergy and thermocline thickness. Results indicate that in the conventional mode, jet mixing and geometric effects drive nonlinear thermocline expansion, creating a stagnant thermal “dead zone" at the pit top that resists extraction. In contrast, the optimized withdrawal strategy effectively suppresses early-stage diffusion, stabilizing the dimensionless thermocline thickness at approximately 0.17. Crucially, this strategy achieves effective heat displacement from the top dead zone during the late discharge stage. Although the exergy extraction rate slightly decreases by 2.23% due to the energy level dilution effect, the heat discharge efficiency increases by 10.29%, extending the total effective discharge duration by 41.2%. This research provides critical operational strategies for maximizing energy utilization in large-scale district heating applications.