Assessment of inlet mixing during charge and discharge of a large-scale water pit heat storage

Pit thermal energy storage (PTES) is an efficient renewable energy storage technology widely used in large-scale solar district heating systems. Accurate modeling of mixing in a PTES due to inlet flow is key in calculating heat storage performance. However, the commonly used one-dimensional PTES models fail to consider inlet mixing due to the three-dimensional nature of the mixing flow. This research adopts a three-dimensional model to analyze the dynamic behavior of inlet mixing inside the PTES. The model is validated against measurements of the Dronninglund PTES. To quantify the inlet mixing impact, two performance indicators (i.e., the penetration height (Z) and the energy distribution ratio (ηj)) are proposed. The parametric analysis revealed that Z is more dependent on the Reynold (Re) number than the Froude (Fr) number, while both the Re and Fr numbers influence ηj. According to the dimensional theory, the penetration height Z shows a power-law relation with time. For the energy distribution ratio ηj, a power-law relation with time is seen, although an asymptotic formula is needed in the region of a negative buoyancy jet. Finally, the inflow mixing inside the PTES is characterized under various operating conditions by empirical correlations. The results of this study could be used to improve the current one-dimensional heat storage models in terms of inlet mixing.