Soil mixed with graphite-based conductive grout for improved thermal conductivity

Shallow geothermal energy systems (SGESs) are a promising technology for contributing to the decarbonisation of the energy sector. Soil thermal conductivity (λ) governs heat transfer process in ground under 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 representing commonly encountered ground were systematically investigated and the key parameters analysed. Next, graphite-based conductive cement grout was developed and mixed with the six soils to demonstrate the significant increase in soil thermal conductivity. For example, the thermal conductivity of a very silty 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.