Medium Rock-Soil Temperature Distribution Characteristics at Different Time Scales and New Layout Forms in the Application of Medium-Deep Borehole Heat Exchangers

Abstract

Medium-deep borehole heat exchangers (MBHEs) have received increasing attention with respect to building heating. To avoid the thermal interference of adjacent MBHEs, the temperature distribution characteristics of medium-deep rock soil were investigated in this work. The evolution of the maximum rock-soil thermal affected radius (MTAR) over a full lifecycle was analyzed. The results showed that the rock-soil thermal affected area (RTAA) continuously expanded in both the radial and vertical directions when the MBHE continuously extracted geothermal energy during a heating season. The factors of the thermal extraction load, fluid velocity, geothermal gradient, and pipe length, impacted the RTAA in the vertical direction, while rock-soil thermal conductivity affected the RTAA in both the radial and vertical directions. Furthermore, the thermal affected radius (TAR) in deeper formations was larger, reaching even 96 m, such that thermal interference between adjacent MBHEs was more likely to occur. The MTAR in shallow formations was limited to 20 m. Consequently, a new layout form, achieved by inclining the borehole, was proposed to increase the distance between adjacent MBHEs in deep formations. The recommended incline angle was equal to or larger than four times the TAR angle. This work provides a scientific reference for promoting the application of multiple MBHE arrays.