Development of a novel cement-based grout with enhanced thermal and sealing performance for borehole heat exchangers

Abstract

Filling a borehole with proper grout is important for maintaining the safety and performance of borehole heat exchangers (BHEs). The current grouts have problems of large shrinkage and low thermal conductivity, which may restrain BHE efficiency and pose safety risks. To address this, this study explores solutions to enhance the thermal and sealing performance of cement-based grout, specifically for BHE applications. A novel grout, composed of ordinary Portland cement (OPC)-calcium sulfoaluminate (CSA) cement-gypsum ternary system and graphite, was developed and thoroughly examined. The developed grout exhibited good flowability and volumetric stability, meeting the required compressive strength in one day. It displayed high thermal conductivity, ranging from 1.606 to 2.504 W/mK at oven-dry conditions and 2.173 to 3.532 W/mK at saturated conditions. At last, numerical modeling was conducted to compare the thermal performance of boreholes using the novel grout developed in this study and a conventional grout. Numerical models prove that using developed grout can improve the thermal performance of boreholes and decrease the borehole resistance, thus lessening the required borehole length, further reducing the costs and carbon emissions of borehole construction. Overall, the developed grouts showcased enhanced thermal and sealing performance. They can be applied in BHE systems not only to enhance operational efficiency and safety, but also to reduce costs and carbon footprints.