Zoning Operation Method For Retarding The Cold/Heat Accumulation Of Large Multi-borehole Ground Heat Exchangers

Ground source heat pump (GSHP) is a typical renewable energy technology which promotes the low rank thermal energy to the high rank thermal energy for the air conditioning, and it plays a significant role in the energy conservation. The borehole heat exchanger (BHE) is an important part of the GSHP system, and the energy efficiency of BHE is the key to the promotion of GSHP technology. As well known, some large GSHP systems are in need of large number of buried pipes, and always are in matrix arrangement if the land resource is adequate. At present, the borehole heat exchangers (BHEs) mostly operate with full size, i.e., all buried pipes are used in summer and winter. Nevertheless, the ground temperature of the BHE buried area, especially in the central part of the field, will continuously increase or decrease after long-term running in cases with unbalanced heating and cooling workloads. However, continuous ground temperature increasing or deceasing will induce performance decline of a GSHP system, and even some buried pipes may fail to work due to extreme heat or cool accumulation in the bigger load season. Therefore, especially for large multi-borehole ground heat exchangers(LMBGHE), retarding or inhibiting the thermal accumulation of BHE, has important engineering significance.The temperature distribution of the underground field after 20 years is simulated by finite line source model of LMBGHE using C language programming. When heat extraction and injection of a borehole heat exchangers is not seasonally balanced, a zoning operation strategy is adopted in this article to alleviate the thermal accumulation, of which only the relatively central part of the BHE runs during the low load season. By analyzing the case that the heat injected into the ground in summer is greater than that extracted from the ground in winter, it found that, compared with the full operation mode, the highest and average field temperature significantly decreases when only the central part of the BHE runs in winter, and the total ground enthalpy decreases by 20%. The temperature variation with time of some selected representative sites and the temperature distribution where different distance from the center site are analyzed, and the results show that the thermal accumulation can be effectively inhibited by the BHE zoning operation. The study also indicates that the zoning operation method is more effective when the ground has smaller thermal conductivity or smaller ground volume heat capacity based on the comparison of different thermal conductivity and different ground volume heat capacity.Considering the importance of the moving groundwater for GSHP system, the temperature distribution of the underground field after 20 years is studied by moving finite line source mode of LMBGHE with the case that heat extraction and injection of a borehole heat exchanger is not seasonally balanced. The ground temperature distribution in the zoning operation mode is compared with that in the non-zoning operation mode and the temperature distribution is studied in the condition of different porosity and different Darcy’s velocity. This article proposes a transitional region in the buried pipe area, both exiting in the zoning operation mode and in the non-zoning operation mode. The results show that the faster Darcy’s velocity is, the bigger transitional region is. On the other hand, the temperature variation is tending towards stability as running time goes on. In addition, it’s indicated that the thermal accumulation in the downstream can be effectively inhibited by the BHE zoning operation, the system is more reliable in this method.