Investigation of a Novel Deep Borehole Heat Exchanger for Building Heating and Cooling with Particular Reference to Heat Extraction and Storage

Medium-depth and deep geothermal energy has been widely used because of its abundant resources and supply stability. Recently, attention has been given to the closed-loop heat extraction system using a deep borehole heat exchanger (DBHE), which enables geothermal energy to be harnessed almost everywhere. In this study, a check valve is adopted in a DBHE system in which the whole section of the well is used for heat extraction in winter during building heating and the upper part of the well is used for heat injection in summer during building cooling. The influence of injected water flowrates, water inlet temperatures, depths of the check valve and formation of thermal conductivities on the performance of this novel DBHE system has been investigated. It is found that heat injection through the upper part of the well in summer can improve the heat extraction rates to a certain extent during the heating season. In summer, the inlet temperature of water has a great influence on the heat injection rates. The increase in the depth of the check valve improves the heat injection rates of the novel DBHE system. When the depth of the check valve is 900 m, the heat injection rates in summer can reach 51.03 kW, which is 27.55% of the heat extraction rates in winter. In this case, the heat injection in summer has the greatest effect on the improvement of heat extraction in winter, which is 6.05 kW, accounting for 3.38% of the heat extraction in that year. It is found that the thermal conductivity of the formation has a great influence on the heat extraction rates in winter and heat injection rates in summer. The proposed novel DBHE system can be used to inject the heat discharged from the building in summer and extract geothermal energy for building heating in winter, forming a better heat balance at certain depths and resulting in a sustainable operation for heating and cooling. Another benefit of using this system is that the heat discharged from air conditioning into the air can be reduced in summer and "urban thermal pollution" can be alleviated.