Thermal performance enhancement of borehole heat exchanger by thermally induced groundwater convection in fractured rock

Abstract

Thermal performance enhancement of Borehole Heat Exchanger (BHE) was a key to sustain the high effectiveness of this technology. Many measures have been taken to improve heat transfer efficiency of BHE in the past decades. However, all these changes leaded to slight thermal improvement due to restriction of heat conduction dominated heat transfer process in subsurface. To achieve an alternatively higher heat transfer mode of BHE, this study proposes a novel approach to induce groundwater thermal convection in fractured rock. A borehole was backfilled by gravels to sustain free groundwater flow in fracture network. A series of Thermal Response Tests (TRTs) show the fluid temperature of a gravel-filled BHE is 2.0–––2.4 °C lower than a sand-bentonite mixture grouted BHE, indicating the higher cooling efficiency. Two main factors including fracture dip angle and aperture, determine the flow behavior of a fracture network. The flow speed increases linearly with enlarging dip angle and shows a cubic polynomial relation with fracture aperture. Physical model tests show that the BHE induces groundwater convection flow by forming a circuit which couples with heat convection, mitigates the thermal stagnation around a borehole. As a result, thermal performance of a BHE was improved effectively.