Improvement Of Thermal Conductivity Of Grout Mixture For Geothermal Heat Pump Systems

A vertical geothermal probe consists of two or four polyethylene tubes U-shaped, used for circulating the coolant between the heat pump and the ground. It is installed in a deep borehole and sealed in a special grout, a kind of liquid cement, used to seal the borehole, and avoid the presence of air bags between the probe and the ground. Moreover, the grout provides a heat transfer medium between the geothermal probe and surrounding. The main cost in a geothermal installation is drilling itself. Reducing the heat exchanger length, and thus reducing the depth of the borehole is an interesting solution to reduce the cost of the installation. The conventional cement grout has low thermal conductivity varying between 0,8 and 1 W/m.K, which leads to high heat exchanger length. Increasing this thermal conductivity, permit the improvement the heat transfer between the soil and the probe, thus reducing the heat exchanger length. Laboratory studies were undertaken to determine the thermal conductivity of various grout mixture. Such as cementitious grout with bentonite, silica sand and graphite. A thermal modeling of the soil, the grout and the heat exchanger was conducted. Experimental results shows that thermal conductivity of the grout can be increased up to 4 W/m.K. The simulation results shows that the reduction in required borehole length was predicted theoretically for an example heat pump system and found to reach 20%, depending on the soil thermal conductivity and borehole diameter.