Modeling of a two-phase CO2-filled vertical borehole for geothermal heat pump applications

A numerical model has been developed to study the complex thermal behavior of a CO2-filled vertical geothermal borehole consisting of a long copper U-tube embedded in a solid material (grout). The model accounts for combinations of single-phase and two-phase fluid flow which may occur along the U-tube length. An explicit solution for fully coupled steady-state conservation equations of mass, momentum and energy is applied. The borehole is divided into small volume elements due to relatively significant changes in the thermophysical properties of the fluid. Borehole wall temperature variations as well as thermal interaction between tube sections are taken into account and the model predicts the fluid temperature, pressure and two-phase quality profiles.