Practice and Optimization of Seasonal/Daily Composite Thermal Energy Storage Technology Based on Lakes/Rivers

Heat pump technology has become an essential solution in the field of medium-and low-temperature heating due to its superior efficiency in converting electrical energy into thermal energy. However, owing to the lack of stable heat sources during winter in northern China, it is necessary to store the heat sources within the urban area throughout the seasons to ensure the stability of the heat provided by the heat pump system. In this study, based on the project of seasonal/daily thermal storage of river water in Guantao, a simulation model was constructed using the Transient System Simulation Tool (TRNSYS). A sensitivity analysis of the thermal energy storage unit parameters was conducted under the constraint of the fixed site area. The configurations and strategies of the daily thermal energy storage were optimized. The results show that the temperature rise of the thermal energy storage unit reaches 3.2 ℃ after 10 years of operation, effectively preventing the soil heat imbalance. The distance between the buried pipes and the depth had the greatest influence on the performance of the system. The total length of the pipes could be reduced by approximately 23% with a spacing of 4 m and a depth of 150 m. The daily thermal energy storage system should be matched to the valley power storage heat and heat release load to avoid wasting the valley power. If the storage heat power is large, the volume of the storage tank can be increased, and the flat power heat release is considered. The optimized system improves the utilization rate of valley power and reduces annual operating costs by 11.2%.