Comparative Analysis of Thermal Energy Storage Performance in District Heating Networks: Evaluating the Impact of Different Injection Systems

A primary objective of contemporary district heating (DH) networks is to minimize the use of primary energy, especially fossil fuels, for meeting the heating demands of grid customers. In this context, thermal energy storages (TESs) serve as crucial devices, facilitating the decoupling of grid demand from heat generation. This study presents an experimental comparison of three large-scale TESs, each employing distinct injection and extraction systems. The performance of these were examined based on data collected over a consistent two-month operational period, enabling a quantitative comparison. The TESs under scrutiny, located in the DH networks of Milan and Brescia in Northern Italy, each have a capacity of a few thousand cubic meters of water and differ in their injection system and shape ratio. Notably, the evolution and thickness of the thermocline and the percentage of energy waste were examined to discern the impact of the injection system, specifically the presence of a flow-straightening device, and the shape ratio on the performance of the TES systems. These characteristics were found to significantly influence energy waste in heat storage, which ranged from 1.56% to 6.50% of the total stored energy, depending on the specific TES tank under consideration.