Thermal performance and influencing range of underground inclined medium-deep geothermal heat exchangers

Abstract

Geothermal energy is one of the most competitive renewable energies. For large-scale projects, multiple medium-deep geothermal heat exchangers (MGHEs) with depths of 2000–3000 m are often used to satisfy the building heating. Limited by occupied area, thermal influencing radius is an important parameter reflecting the MGHE influencing range in surrounding strata. This paper presented a novel investigation on the impact of inclination on MGHE thermal performance and influencing range. A finite volume method (FVM) based algorithm coded by MATLAB was established and validated by 28 days’ project operating data, before it was used to study the influencing range affected by the depth of kick-off point (H), angle of inclination (α) and working fluid flow rate. It was concluded that for a conventional vertical MGHE with a 2500 m burial depth, the maximum influencing radius increases from 8.06 m to 31.84 m after 10 years of operation. For the inclined MGHE with the same length, increasing α decreases the maximum thermal influencing radius position (TIRP). The relationships among heat extraction rate, TIRP and MGHE parameters were established through non-linear regression. When α increases from 0° to 10°, TIRP can be reduced to a minimum value of 5.3 m when H = 500 m. A smaller H conduces to reducing TIRP without affecting the thermal performance. The proposed methods and results will be conducive to the MGHE array design and efficient use of geothermal energy.