Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger

• Published in: International journal of energy research Vol. 43; no. 12; pp. 6337 - 6352
• Main Authors: Liu, Jun, Wang, Fenghao, Cai, Wanlong, Wang, Zhihua, Wei, Qingpeng, Deng, Jiewen
• Format: Journal Article
• Language: English
• Published: Bognor Regis Hindawi Limited 10-10-2019
• Subjects: Boreholes; deep borehole heat exchanger; Design parameters; Energy conservation; Exact solutions; Flow rates; Flow velocity; geothermal; Heat exchangers; Heat loss; Heat pumps; Heat transfer; heat transfer performance; Inlet flow; Load distribution; Mathematical models; Measuring instruments; numerical modeling; Numerical models; Optimization; Parameters; Pipes; Soil; Velocity
• ISSN: 0363-907X; 1099-114X
• DOI: 10.1002/er.4357

Summary
Deep borehole heat exchanger (DBHE) is attracting attention intensively owing to much more geothermal extraction, higher efficiency for heat pumps, and lesser land demand compared with shallow borehole heat exchanger. DBHE is usually dipped into several thousand meters in the subsurface, having a complicated heat transfer with surrounding rock–soil. However, the heat transfer characteristics below surface under different conditions are rarely studied. In this study, a numerical model considering the comprehensive effects of geothermal gradients and heat loss from inner pipe was proposed. The model was validated with experimental data and Beier analytical solution. Based on the model, the effects of primary design parameters on the heat transfer performance below surface along the pipe were investigated. The results indicate that temperature at pipe bottom increases with inlet flow rate decreasing, while the heat load cannot be extracted fully to the surface because of the heat loss of inner pipe. When the inlet flow rates decrease from 41.39 to 4.52 m3/h, the heat loss ratio increases from 25.5% to 63.7%. It is an effective way of insulating inner pipe to reduce heat loss under low inlet flow rates. Increasing the velocity in inner pipe by lessening the inner pipe diameter can also decline the heat loss well. While by this way, the increasing pumping power resulting from the higher velocity in inner pipe has to be considered. This study is significant to effective optimization of DBHE and energy conservation of buildings.
A new deep borehole heat exchanger (DBHE) numerical model with logarithmic discretization in radial direction was proposed, which can account the small distances between the DBHE and the surrounding rock–soil. The developed model considers the comprehensive effects of geothermal gradients and heat loss from inner pipe. The heat transfer along the pipe can be reflected by the temperature profiles and the specific heat load distributions, which is solved on the basis of the finite volume method.