Model-based energy performance assessment of the world largest underground seasonal thermal energy storage in a pilot district heating system in Chifeng City

District heating systems play an important role in supporting energy transition by using and storing energy delivered by renewable and other low-grade energy sources such as industrial waste heat. However, this low-grade heat is not always able to satisfy the heating demand, including space heating and domestic hot water (DHW). This mismatch is even more evident when looking at it seasonally. A proven solution to resolve this discrepancy is to implement seasonal thermal energy storage (STES) technology. This research focuses on the performance assessment of district heating (DH) systems powered by low-grade energy sources with large-scale, STES technology. A pilot DH system, located in Chifeng, China that integrates a 0.5 million m3 borehole thermal energy storage system, an on-site solar thermal plant and excess heat from a copper plant nearby to avoid additional fossil fuel-based heating supply is presented. It is, to date, the world largest (in volume) underground seasonal thermal energy storage. The research in this paper proposes a modeling and simulation method to analyze the energy performance of the STES for two district heating system configurations. Several performance indicators are selected to assess the STES system performance. Results show that a lower STES discharge temperature leads to a better energy performance. A sensitivity analysis of the site properties illustrates that the soil thermal conductivity is the most sensitive parameter among the chosen performance indicators. The long-term performance of the STES is also discussed in this paper to explore the time needed to reach a stable operation condition under different scenarios.