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

Abstract

The utilization of geothermal energy through ground buried heat exchanger to provide heat source for buildings has become extremely attractive recent years in China and all over the world. As a key parameter that governs the prediction of the system performance, the effective thermal conductivity of the ground significantly affects the heat transfer process between buried pipes and surrounding rock-soil. This paper conducts a review on the methods of determining the effective thermal conductivities of different rock-soil types composed by various substances proposed in recent years. Based on the analysis of theoretical methods and experimental approaches applied to different rock-soil conditions and other relevant parameters such as the porosity, water content and material construction, etc., the applicabilities of various methods have been justified. 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 prediction value. For certain types of rock-soil, the experimental approaches can provide more accurate results but are expensive and time consuming, which have sometimes been used to improve the accuracy or verify the results of improved model in the appropriate range of porosity and saturation degree for certain types of rocks. The concluding remarks would contribute to the estimation of the heat transfer performance of the buried heat exchanger during the prediction process.

Analysis of effective thermal conductivity stability of geothermal heat exchanger according to ambient environmental conditions

“…28,29 In this study, the line-source theory is applied to the TRT for a closed geothermal heat exchanger. The effective thermal conductivity and thermal resistance can be obtained from the TRT data based on [30][31][32][33][34][35] :…”

Novel Experimental Device to Monitor the Ground Thermal Exchange in a Borehole Heat Exchanger

“…The thermal conductivity of a certain material is a constant parameter supposing a homogeneous and isotropic medium. Effective thermal conductivity of the surrounding ground is the foremost parameter for geothermal applications [31]. Thus, experimental models and measurements are required to both understand and verify the results obtained following the theoretical basis, comparing it to commercial measurement instruments.…”

“…From the validation of this model, numerous studies could be carried out. This research considered the estimation of the thermal conductivity of the material used as a surrounding ground as it is a crucial parameter that determines the heat transmission and hence the installation efficiency [31]. Table 6.…”

Comparative Analysis of Geothermal Energy in Korea Based on Closed Borehole and Single- and Two-Well Standing Column Well Geothermal Heat Exchange Systems

“…The line-source theory for calculating the thermal performance of a hermetic heat exchanger is well established, and it was applied to thermal performance testing for decades [57,58]. The theory was extended to comparisons and studies of energy losses in different heat exchanger arrangements [59,60].…”