Experimental and numerical investigation of thermal properties of cement-based grouts used for vertical ground heat exchanger

In this study, the thermal conductivities and specific heat capacities of cement-based grouts used for vertical ground heat exchanger (GHE) were investigated in a laboratory experiment. Nine different mix proportions with different water/cement and silica sand/cement ratios were scrutinized. Comparing the dried condition with the saturated condition, the specimens' thermal conductivity and specific heat capacity decreased. As the sand/cement (s/c) ratio increased and water/cement (w/c) ratio decreased, under the saturated condition the thermal conductivities of the specimens increased, whereas the specific heat capacities decreased. However, increasing s/c ratio had greater influence on the thermal conductivity improvement of the cement-based grouts than did decreasing w/c ratio. Also, due to the effect of the water's high specific heat capacity, the higher the water absorption rate of the cement-based grout was, the higher the specific heat capacity was. Additionally, in order to evaluate the effects of the thermal properties of the cement-based grouts on GHE performance, a series of numerical simulations were carried out. In the results, in the continuous operation mode, only thermal conductivity of the grout material had a positive effect on the GHE performance, whereas in the intermittent operation mode, both thermal conductivity and specific heat capacity did. This result indicates that in order to further improve GHE performance, further research related to the thermal properties of grout material will have to consider not only its high thermal conductivity but also its high specific heat capacity. The experimental and numerical results obtained in this study will prove constructive to subsequent GHE performance improvement efforts.