Optimal design of underground facilities considering coupled ground thermal characteristics

Abstract

Research on the design of underground facilities such as ground-coupled pump systems or high-level radioactive waste repositories should accurately consider the heat exchange mechanism between the underground facilities and surrounding ground. In this study, a numerical study was conducted on the thermal behavior of ground-coupled heat pump systems and high-level radioactive waste repositories considering coupled relationships, which refer to the variation in thermal properties of geomaterials.

First, a numerical model that considers the variation of ground thermal properties was established for a ground coupled heat pump system based on the finite element method. The numerical model was validated through laboratory thermal response tests of a horizontal ground heat exchanger, showing that the numerical model is in good agreement with the experimental results. The thermal conductivity and ground temperature were detected as significant design factors requiring coupled relationships through a sensitivity analysis. These factors were coupled using analytical method. In addition, considering the coupled relationship, the sensitivity analysis was additionally performed to investigate the significance of design factors in the ground coupled heat pump systems. As a result, the ground thermal conductivity, installation depth, and inlet temperature were determined as major design factors. However, the order of significance of the major factors was dependent on regional climate factors. Furthermore, the thermal performance evaluation, which takes into account the variation in the thermal conductivity due to the ground temperature distribution, was conducted on a horizontal spiral-coil type ground heat exchanger. The results revealed that the thermal performance was enhanced when the coupled relationship was considered, and an optimum installation depth of the horizontal ground heat exchanger, of which there was no statistical significance, was identified.

Meanwhile, a numerical model was also constructed for a high-level radioactive waste repository, taking into account thermal and hydraulic properties of a bentonite buffer. The model was verified by considering the temperature variations of the buffer obtained by an applicable analytical solution. The sensitivity analysis was performed to determine properties of the buffer requiring coupled relationships. The relationships were set up by a multiple regression analysis using experimental data. Considering the coupled relationship, the sensitivity analysis was additionally conducted to evaluate the significance of design factors of a buffer. As a result, thermal conductivity, saturated permeability, and viscosity were identified as key factors. The results also showed that the degree of significance of the design factors on the thermal behavior depends on the coupled relationship among the physical properties of the reference buffer material. In addition, an optimum initial condition for compacted bentonite blocks to achieve the minimum peak temperature was determined, which is the main design criterion of repositories, by applying a firefly algorithm optimization technique in consideration with the coupled relationship. The suggested optimum initial condition was validated through a parametric study.