Research on the development mechanism of thermocline in long-term water pit heat storage

Under the dual‑carbon background, utilizing seasonal heat storage to fully recover the year-round waste heat from power plants and process industries is expected to lead to a low-cost and low-carbon heat supply technology path. Although seasonal heat storage has been successfully applied in some countries such as Denmark, Germany, China, etc., there is still a lack of detailed and in-depth research on the characteristics of the thermocline for the long-term heat storage, especially for such a small height-to-diameter ratio heat storage with limited size in the vertical direction but large enough size in the horizontal direction. In this paper, heat storage experiments were carried out to obtain the dynamic temperature distribution data, which were compared with the CFD simulation results to verify the reliability of the CFD simulation. On this basis, more simulations were carried out for a full-size large-scale heat storage. The generation and development mechanisms of thermocline were revealed. The vorticity field and temperature field were analyzed and discussed, and it was found that vortices could still be formed inside the heat storage at a very low inlet flow velocity (e.g., 0.015 m/s). Vortices were the main cause of the thickening of the thermocline compared to the ideal plug flow, contributing over 80 % to the thermocline. This paper is hopeful to provide support for assessing the heat grade loss in the heat storage, present a reference of the design of disc distributor and give the improvement methods to decrease thickness of the thermocline.