Abstract
The heat released by buried pipes of the soil thermal storage (STS) system will inevitably aggravate the coupling effect of heat and moisture migration in the surrounding soil, which may affect soil thermal properties and thermal storage characteristics. However, there is no conclusive evidence. A series of model tests were conducted to investigate the potential effect of the heat and moisture coupling migration on the heat transfer characteristics of buried pipes. A three-dimensional model considering the temperature dependence of soil thermal conductivity was established and verified. The effects of heat storage temperature and initial soil moisture on the heat transfer characteristics of buried pipes were further evaluated using the proposed model. Test results show that the thermal conductivity of lateritic clay increases with increasing soil temperature due to the latent heat transfer of vapor. For lateritic clay with an initial saturation of 13.42 %, the thermal conductivity increases to 7.3 times that of 5 °C at 90 °C. The driving force of moisture migration is generated from both temperature and humidity gradient. Soil moisture near to the buried pipe varies significantly during the first 3 ∼ 12 h due to the hysteresis of moisture migration caused by the temperature gradient driving. The soil moisture variations on the one-dimensional conditions are more significant than those under three-dimensional conditions due to the effects of gravity and paths on the moisture migration. Model validation results show that ignoring the moisture migration and temperature dependence of soil thermal conductivity will lead to the lower prediction value of the numerical model. Simulation results show that the influence of the initial soil moisture on the heat and moisture coupling migration should be taken into account when accurately evaluating the performance of STS system.
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