THERMAL CONDUCTIVITY MEASUREMENT OF GROUTING MATERIALS FOR GROUND HEAT EXCHANGER BOREHOLE

The geothermal heat pump (GHP) system, which uses the heat energy of the earth, is a preferable and alternative energy system in Korea. According to the Environmental Protection Agency, GHP is one of the most energy-efficient and cost-effective space conditioning systems available in the market. For the GHP system, a high density polythylene (HDPE) U-loop pipe connected to the heat pump is placed in the borehole for exchanging the heat energy with the ground, through circulation of water or antifreeze in the pipe. The borehole is backfilled with grouting materials, such as bentonite and bentonite mixtures, which act as a heat transfer medium and also control groundwater movement to prevent contamination of groundwater. So the thermal conductivity of grouting materls is a key factor for the design of GHPs. In this study, the thermal conductivity of neat bentonite and bentonite-sand grouts were investigated. Mix proportions adopted in this study were 20% bentonite, adding 30% and 50% quartzite sand of the total weight. Mixed grout specimens were prepared in rectangular parallelepiped shapes (11cm×6cm×4cm). Thermal conductivity of saturated samples was measured using two different types of line-source commercial devices. One is QTM-500 (Kyoto Electronics Manufacturing Co.) with a half-space type box probe. The other is KD2 Pro (Decagon Devices) with a full-space type needle probe. In the case of KD2 Pro, thermal conductivity was measured using TR-1 probe (single-needle type) and SH-1 probe (dual-needle type). The results showed that thermal conductivity of samples increased as the amounts of quartzite sand and bentonite increased. However, measured values of thermal conductivity were apparently dependent on the probe type used. Thermal conductivity measured using QTM-500 was the highest, followed by TR-1 and then SH-1. It seemed that this phenomenon occurred due to differences in accuracy of probes, size of measurement area, etc. The result of this study can be used as preliminary data for designing the GHP system.