On thermal characteristics of coarse grained and fine grained soils in ground heat exchanger applications

Abstract

The thermal properties of soils and rocks is an important property for the design of thermally active ground structures such as geothermal energy foundations and heat exchange systems. This paper presents the results of a laboratory study thermal diffusivity (α) of two different soils namely, sandy and clayey soils. These soils were tested under varying water content to investigate the effects of water content on the thermal properties. From complete dryness, the soil water content of the specimens were varied to eight different percent of water content (0, 4, 8, 12, 16, 20, 24, and 28 %). In addition, at each water content, the thermal properties variations were measured using a KD2 pro analyzer. Additionally, the water retention characteristics of both soils were established using a WP4C chilled-mirror dew-point hygrometer. Test results revealed that in general, water-sorbed specimens have improved thermal characteristics than dry specimens of the same soil. Furthermore, it was noted that the soil containing higher percentage of clay particles had lower thermal characteristics as compared to soil having higher percentage of quartz content at the same percentage of water content. Test results also showed that the thermal characteristics increased with increasing water content. Although the water retention characteristics for coarse grained soil appears to be limited, a greater percentage of quartz is more effective in transferring heat than soil containing higher percentage of clay having greater water content.