Abstract
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 were systematically investigated and the key
parameters analysed. 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.