The ground source heat pump system can be applied to the HVAC system of metro stations. Borehole length design and performance of ground heat exchangers (the GHEs) for ground source heat pumps strongly rely on thermal conductivity of the backfill material. Carbon fiber powders with different contents and fineness were mixed into traditional bentonite grouts, of which thermal conductivities were measured by the guarded heat flow test method. Carbon fiber powders as additive effectively improved thermal conductivity of grouts from 0.969 W/(m·K) to 1.392 ~1.674 W/(m·K). The improvement on heat exchange efficiency with the resulting grouts was calculated theoretically by means of numerical simulation considering the geological conditions of Qingsheng Station of Guangzhou Metro, and found to be 7.53 to 14.52 %, depending on the depth and out tube diameter of the GHEs.

Ground Source Heat Pump System;carbon fiber powder;thermal conductivity;heat transfer efficiency