A New Analytical Model for the Estimation of Three-phase Rock-soil Thermal Conductivity for Geothermal Utilization

As a kind of clean energy resource, geothermal energy is widely utilized in many fields, especially for heating and ventilation through heat exchangers buried underground. The use of geothermal heat exchange system will contribute to energy saving as well as building sustainable, therefore it has become more and more popular in recent years. Rock-soil thermal conductivity plays a noticeable role in the performance of ground buried heat exchanger. In the present study, a new analytical model was proposed to describe the spatial structure of the multiphase rock-soil and relative position of solid, liquid and gas phase. Through analyzing the relationship among different phases and coding FORTRAN program, the model structure parameters can be obtained for the calculation of parallel thermal resistances. The expression of the thermal conductivity derived from the model was then applied to obtain the thermal conductivity of the Tripoli sand from North Africa, before the results were compared with previous tests. After comparative analysis, the newly proposed model in this study was proved accurate in predicting the thermal conductivity of the chosen soil with around 20% averaged relative error, which will contribute to the prediction of rock-soil thermal properties as well as the design of ground buried heat exchanger.