Improvement of Soil Thermal Conductivity with Graphite-Based Conductive Cement Grouts

Shallow geothermal energy systems (SGES) are a promising technology for contributing to the decarbonization of the energy sector. Soil thermal conductivity (λ) governs the heat transfer process in ground under a steady state; thereby, it is a key parameter for SGES performance. Soil mixing technology has been successful in enhancing the shear strength of soils, but is adopted in this paper for the first time to improve soils as a geothermal energy conductive medium for SGES applications. First, the thermal conductivity of six types of soils was systematically investigated and the key parameters analyzed. Next, graphite-based conductive cement grout was developed and mixed with the six soils in a controlled laboratory setting to demonstrate the significant increase in soil thermal conductivity. For example, the thermal conductivity of a very silty dry sand increased from 0.19 to 2.62 W/m·K (a remarkable 14-fold increase) when mixed with the conductive grout at a soil-to-grout ratio of 6∶1. In addition, the mechanical properties of the cement grouts and cement-mixed soils were examined along with the microstructural analysis, revealing the mechanism behind the thermal conductivity improvement.