Thermal conductivity of the geological material plays a critical role in the utilization of the geothermal energy, especially for the performance prediction of the buried heat exchanger. In this research, a packed-sphere model was established in order to study the spatial structure of the three-phase geological material after reviewing the actual components. The cubic cell element in the model was divided into four parallel parts, and the thermal resistance of each part was obtained using the soil physical parameters. To finally acquire the thermal conductivity of the model for certain type of geological material, the calculating method was carried out and realized through coding FORTRAN program. After comparison with previous results on three groups of quartz composed geological materials, the newly proposed model in this work was shown to be successful to forecast the thermal conductivity with around 20% overall relative error except extremely low saturation degree, which indicates that the prediction results of the model can support the performance analysis of the buried heat exchanger.
Geothermal has become a popular renewable energy in recent decades due to its properties of non-polluting, clean and large reserves. The vertical-butted geothermal well is a new structure for geothermal utilization. Two vertical boreholes with a certain distance are connected at bottoms by directional drilling to form a U-shape circulation loop. Due to the existence of the aquifers, the movement of underground water extends the heat exchange range and affects the geothermal system performance. This paper aims to develop an analytical method to solve the soil temperature field around the vertical-butted geothermal well with different flow directions. Taking the interacted effect of the two parallel pipes on the surrounding soil into consideration, the temperature distribution in the heat exchange zone is obtained. It is found that the temperature field changes significantly when the groundwater flow exists. The movement of underground water extends the downstream thermal radii from 4.8 m to 10.7 m, 10.6 m and 11.7 m when the flow directions are 0°, 45° and 90° toward the axis of the two pipes, respectively. The upstream thermal radii reduce from 5.4 m to 1.9 m for all the cases with groundwater movement. In addition, the effect of the underground water flow direction on the temperature field around the butted wells is analyzed. The results show that when the groundwater flow direction is perpendicular to the axis of the two pipes, the temperature field is the most beneficial to the performance of the vertical-butted geothermal well.
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