Review of effective thermal conductivity models of rock-soil for geothermal energy applications

In recent years, the utilization of geothermal energy through ground buried heat exchanger to provide heat source for buildings has become extremely attractive within China and all over the world. The effective thermal conductivity of the ground, which may affect the heat transfer process between the buried pipes and the surrounding rock-soil significantly, is a key parameter that governs the prediction of the system performance. This article reviews recently proposed methods to determine the effective thermal conductivities of different rock-soil types containing various substances. The applicabilities of various basic methods to include the effects of porosity, water content, material construction and etc. are documented. According to the deficiencies of basic models, the improved models take into account the effects of the temperature, saturation degree, detailed structure of rock-soil and other parameters to improve the accuracy of predictions. For certain types of rock-soil, experimental measurements are needed to improve the accuracy or verify the results of the improved model in the appropriate ranges of porosity and saturation degree. However, these measurements tend to be expensive and time consuming.