Heat Transfer in Soft Clay: Pilot-Scale Experiment Using Solar Collectors

This work introduces a pilot-scale experiment conducted for 18 months to study the soil heating performance of solar collectors in Japan for borehole thermal energy storage and thermal consolidation applications. In this experiment, water was heated using a solar collector and then circulated in three heat exchangers inserted into a $2.63\text{-}{\mathrm{m}}^3$ box of kaolin clay installed in sandy ground. The solar collector had a heat collection area of $4{\mathrm{m}}^2$ and was attached to a 200-L water tank. The tank containing the clay was internally lined by a 6-cm-thick box of polyethylene foam to reduce heat exchange with the environment. The average clay temperature reached a maximum value of 41.1°C in the summer and a minimum of 14.9°C in the winter. The circulation water temperature varied seasonally between a daytime maximum of 77.5°C in summer and a daytime minimum of 22.5°C in winter. The experiment was simulated using a finite-element analysis model based on thermal conduction. The model used the Dittus–Boelter equation to model the heat exchangers’ energy output and adopted an environmental heat exchange boundary condition using ambient temperature, wind speed, and total daily solar flux data. The numerical results accurately predicted the ground and average kaolin temperature within a 2°C–3°C margin of error. The numerical and experimental spatial distributions of kaolin temperature suggest that gravity and temperature-induced moisture transport was sufficient to measurably change the kaolin water content. Additionally, the numerical results of the sandy ground temperature revealed its sensitivity to variation of moisture content induced by evapotranspiration. Last, changing the boundary condition of the kaolin box to thermal insulation revealed that environmental heat loss accounted for a 13.5°C–19.5°C difference in the average kaolin temperature between the numerical and experimental results, emphasizing the importance of adequate thermal insulation in ground heating applications.