中深层套管式换热器可持续供热性能及优化设计研究

Medium-deep coaxial heat exchanger (MDBHE) has the characteristic of “geothermal utilization without water extraction”, which is one of the important techniques to safely promote the medium-deep geothermal heating in northern China. As the heating performance of a MDBHE is unknown during long-term heat extraction, a numerical heat transfer model is proposed in this study to analyze the evolution of the fluid temperature and performance coefficient with the operating year based on the sustainable heating operation characteristics. Subsequently, a design optimization research was pertinently carried out. The results show that during the sustainable heating operation period, the fluid temperature of a MDBHE declines in the first five years and it barely changes after 20 years. The rock-soil thermal conductivity largely impacts the decline of the MDBHE seasonal performance factor. Under the condition of 1.5 W/(m·K), the heat source seasonal performance factor (SPF₁) and heat pump unit seasonal performance factor (SPF₂) reduce by 11.50% and 10.56%, respectively. In contrast, under the condition of 3.0 W/(m·K), the SPF₁ and SPF₂ only reduce by 4.73% and 4.23%, respectively. The decline is more pronounced under the condition of low rock-soil thermal conductivity. According to the sustainable heating performance evolution, the MDBHE design using heating performance in the “quasi-steady characteristics year” as a benchmark is proposed. Subsequently, the operating condition could be optimized by comprehensively considering the requirements of both operation and energy conservation. Consequently, the optimal operating velocities and heating loads of the MDBHE under the conditions of various geological parameters and pipe lengths were clarified, which have practical guiding significance for high-efficiency and sustainable heating design of MDBHE.