Toward efficient numerical modeling and analysis of large-scale thermal energy storage for renewable district heating

The buildings sector is a main player in the decarbonization pathway as it contributes with a share of 40% of the total energy use in which space heating and domestic hot water are responsible for a considerable portion. A key lever to overcome the challenges in the buildings sector related to today's extensive utilization of fossil fuels is the introduction of renewables-based district heating systems. Yet, most renewables fluctuate based on seasonal and hourly patterns. This pinpoints the significance of large-scale seasonal thermal energy storage (TES) systems. Yet, such large-scale systems require a thorough planning in order to avoid the high investment cost. Consequently, numerical models gain importance as an alternative. Accordingly, this work develops numerical finite element models for large-scale tanks and pits. To obtain credibility in the approach, the models are then validated against measured data from the Dronninglund pit TES in Denmark. The outcomes exemplify that the simulation method is suitable and the models can be calibrated very well. Next, the work examines pit TES performance considering two energetic efficiency indicators and two stratification quality measures. The performance evaluation shows that the Dronninglund pit achieved an efficiency of 90%, whereas only 76% of the pit energy capacity was effectively utilized for the year 2015. Further, the pit maintained a moderate quality of stratification for longer periods. The work later demonstrates the influence of TES geometry on stratification quality by comparing the MIX number between Dronninglund PTES and a corresponding cylindrical TES.