Thermal performance response and heat load redistribution mechanism of a deep U-type borehole heat exchanger in heating systems

Deep closed-loop borehole heat exchanger systems have gained significant attention in recent years for extracting geothermal energy to effectively heat buildings, e.g. by integrating them into district heating systems. In this work, a 3D numerical model of the pilot Deep U-type Borehole Heat Exchanger (DUBHE) system that was recently implemented in Xi’an, China, is established based on the OpenGeoSys software. The model is fully validated by 2-months of monitoring data from the pilot project. Then, a thermodynamic heat pump model is further coupled to investigate the transient thermal response of the DUBHE to the heat pump’s off-design performance. Subsequently, dynamic operations in a district heating system are simulated to evaluate the flexibility of the DUBHE-couple heat pump system. For the first time, the mechanism of heat load distribution by the heat pump and the behavior of heat load redistribution during operation are clarified between the subsurface DUBHE and the heat pump. The maximum sustainable heating power of the whole system is found to be around 780 kW in 120-day operation with the working fluid R410A and the required feed flow temperature of 65°C in the heat pump. With increasing operation time, the heat load distributed to the DUBHE decreases by more than 21% in 120 days due to the decrease in the heat pump performance. The R600 heat pump has the best performance and efficiency among four different working fluids, but leads to a 3.4°C reduction in the outflow temperature of the DUBHE compared to the R410A heat pump. This over-extracted performance of the DUBHE poses a challenge to its sustainable operation in terms of the circulation fluid temperature of the DUBHE. In the two operation patterns of integrating into the district heating system, the subsurface DUBHE can provide both around 70% of the total heat power to the district heating system. The average annual COP is 0.2 higher with low feed flow temperature to the district heating system and more frequent shutdown operation, showing significant flexibility in integrating the DUBHE-coupled heat pump system into the district heating system.