Relationship between the thermal properties and degree of saturation of cementitious grouts used in vertical borehole heat exchangers

Abstract

Although the thermal properties of soil and rock are strongly affected by moisture content, a number of previous experimental studies on the grouting materials used in vertical borehole heat exchangers (BHEs) have assessed their thermal properties under saturated and dried conditions. These studies have focused primarily on measurement and improvement of thermal conductivity. However, recent numerical studies on vertical BHEs reported that when a vertical BHE is under intermittent operation both the thermal conductivity of the grouting material and its high specific-heat capacity have a positive effect on the performance of the BHE. Moreover, because both the specific-heat capacity and thermal conductivity of the grouting material are essential parameters when numerically simulating vertical BHEs, they should be investigated together. In this study, cementitious grout specimens were prepared for use in vertical BHEs with different water/cement (w/c) and sand/cement (s/c) ratios, and their thermal conductivity and specific-heat capacity under saturated, air-dried, and partially saturated conditions were measured. Furthermore, the relationships of the mixing ratio of cementitious grout, degree of saturation, and thermal properties were analyzed. The thermal conductivity and specific-heat capacity of cementitious grout decreased by 15.56–38.30% and 11.79–22.34%, respectively, under air-dried rather than saturated conditions. Moreover, under partially saturated conditions, the thermal conductivity and specific-heat capacity of cementitious grout decreased linearly with the degree of saturation. The thermal conductivity of the cementitious grout was more significantly affected by variations in the s/c ratio than in the w/c ratio, while the specific-heat capacity was affected by the amount of water in voids. The results were used to derive empirical equations to predict the thermal properties of cementitious grouts with various mixing ratios, which are expected to be useful for further studies on vertical BHEs.